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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2016	Feb. 24, 2017	Jun. 30, 2016
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-K
Document Period End Date	Dec. 31, 2016
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 3,889,471,650
Entity Common Stock, Shares Outstanding		4,561,057
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	287.0	211.2
Common equity securities, at fair value	474.3	1,113.9
Other long-term investments	323.3	315.8
Total investments	5,334.8	4,271.1
Cash (restricted $0.0 and $5.8)	149.8	173.0
Reinsurance recoverable on unpaid losses	172.9	186.0
Reinsurance recoverable on paid losses	6.6	7.5
Insurance premiums receivable	229.9	220.3
Deferred acquisition costs	106.9	107.6
Deferred Income Tax Assets	126.7	112.8
Ceded unearned insurance premiums	44.2	29.5
Accrued investment income	26.1	13.9
Accounts receivable on unsettled investment sales	6.2	41.9
Goodwill and intangible assets	55.9	55.4
Other assets	274.6	273.2
Assets held for sale	10.1	4,790.4
Total assets	6,544.7	10,282.6
Liabilities
Loss and loss adjustment expense reserves	1,365.6	1,389.8
Unearned insurance premiums	658.0	610.5
Debt	285.9	337.6
Accrued incentive compensation	140.0	140.7
Funds held under insurance contracts	153.0	137.8
Other liabilities	199.9	250.8
Liabilities held for sale	5.1	3,047.4
Total liabilities	2,807.5	5,914.6
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 4,563,814 and 5,623,735 shares	4.6	5.6
Paid-in surplus	806.1	972.6
Retained earnings	2,797.2	3,084.9
Accumulated other comprehensive loss, after-tax:
Net unrealized foreign currency translation losses	(1.4)	(145.6)
Pension liability and other	(3.2)	(4.3)
Total White Mountains’s common shareholders’ equity	3,603.3	3,913.2
Non-controlling interests
Non-controlling interests	133.9	454.8
Total equity	3,737.2	4,368.0
Total liabilities and equity	$ 6,544.7	$ 10,282.6

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Restricted cash	$ 0.0	$ 5.8
Common shares, par value per share (in dollars per share)	$ 1.00	$ 1.00
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	4,563,814	5,623,735
Common shares, outstanding shares	4,563,814	5,623,735

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gain (loss) from sale of other discontinued operations, net of tax	$ 415.1	$ 18.2	$ (1.6)
Revenues
Earned insurance premiums	1,114.0	1,188.2	1,185.0
Net investment income	86.8	60.8	59.5
Net realized and unrealized investment gains	10.3	225.4	78.5
Other revenue	149.6	147.3	88.1
Total revenues	1,360.7	1,621.7	1,411.1
Expenses
Loss and loss adjustment expenses	664.0	708.9	824.0
Insurance acquisition expenses	211.6	220.1	206.2
Other underwriting expenses	209.5	218.6	179.6
General and administrative expenses	299.9	302.2	216.5
Interest expense on debt	16.1	14.6	14.2
Total expenses	1,401.1	1,464.4	1,440.5
Pre-tax (loss) income from continuing operations	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	45.4	0.2	14.8
Net income (loss) from continuing operations	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(0.3)	78.7	260.6
Income before equity in earnings of unconsolidated affiliates	419.8	254.4	244.4
Equity in earnings of unconsolidated affiliates, net of tax	0.0	25.1	45.6
Net income	419.8	279.5	290.0
Net (income) loss attributable to non-controlling interests	(7.3)	18.1	22.2
Net income attributable to White Mountains’s common shareholders	412.5	297.6	312.2
Other comprehensive income, net of tax:
Change in equity in net unrealized investment (losses) gains in Symetra common shares, net of tax	0.0	(34.9)	75.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax	0.3	(0.5)	(10.7)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax		0.0	0.0
Comprehensive income	558.1	197.2	207.3
Comprehensive (income) loss attributable to non-controlling interests	(0.3)	0.0	3.3
Comprehensive income attributable to White Mountains’s common shareholders	$ 557.8	$ 197.2	$ 210.6
Basic earnings per share
Continuing operations	$ (0.47)	$ 34.12	$ 8.70
Discontinued operations	82.71	16.48	42.43
Total consolidated operations	82.24	50.60	51.13
Diluted earnings per share
Continuing operations	(0.47)	34.12	8.70
Discontinued operations	82.66	16.48	42.43
Total consolidated operations	82.19	50.60	51.13
Dividends declared and paid per White Mountains’s common share	$ 1.00	$ 1.00	$ 1.00
Sirius Group
Gain (loss) from sale of other discontinued operations, net of tax	$ 363.2
Revenues
Net realized and unrealized investment gains	3.7	$ 205.0	$ 2.7
Tranzact
Gain (loss) from sale of other discontinued operations, net of tax	51.9
Esurance [Member]
Gain (loss) from sale of other discontinued operations, net of tax	0.0	17.9	3.2
Other Discontinued Operations [Member]
Gain (loss) from sale of other discontinued operations, net of tax	0.0	0.3	(4.8)
Sale of Sirius Group [Member]
Other comprehensive income, net of tax:
OCI, Change in foreign currency transaction and translation adjustment, net of tax	32.0	$ (65.0)	$ (169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	$ 113.3

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCL, after-tax	Parent	Non-controlling Interests	Total Equity includes Noncontrolling Interest [Member]	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	Foreign Currency Gain (Loss) AOCL, after-tax	Foreign Currency Gain (Loss) Parent	Foreign Currency Gain (Loss) Non-controlling Interests	Foreign Currency Gain (Loss) Total Equity includes Noncontrolling Interest [Member]	Pension liability and other accumulated comprehensive items Common shares and paid-in surplus	Pension liability and other accumulated comprehensive items Retained earnings	Pension liability and other accumulated comprehensive items AOCL, after-tax	Pension liability and other accumulated comprehensive items Parent	Pension liability and other accumulated comprehensive items Non-controlling Interests	Pension liability and other accumulated comprehensive items Total Equity includes Noncontrolling Interest [Member]	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling Interests	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity includes Noncontrolling Interest [Member]	Prospector Turtle Fund Common shares and paid-in surplus	Prospector Turtle Fund Retained earnings	Prospector Turtle Fund AOCL, after-tax	Prospector Turtle Fund Parent	Prospector Turtle Fund Non-controlling Interests	Prospector Turtle Fund Total Equity includes Noncontrolling Interest [Member]	Buzzmove [Member] Common shares and paid-in surplus	Buzzmove [Member] Retained earnings	Buzzmove [Member] AOCL, after-tax	Buzzmove [Member] Parent	Buzzmove [Member] Non-controlling Interests	Tranzact Non-controlling Interests	Sirius Group Non-controlling Interests	Sirius Group Total Equity includes Noncontrolling Interest [Member]	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent	Sale of Sirius Group [Member]	Sale of Sirius Group [Member] Common shares and paid-in surplus	Sale of Sirius Group [Member] Retained earnings	Sale of Sirius Group [Member] AOCL, after-tax	Sale of Sirius Group [Member] Parent	Tranzact Non-controlling Interests	Tranzact Total Equity includes Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2013		$ 1,051.1	$ 2,801.9	$ 52.1	$ 3,905.1	$ 491.7	$ 4,396.8
Increase (Decrease) in Shareholders' Equity
Net income	$ 290.0	0.0	312.2	0.0	312.2	(22.2)	290.0
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	75.3	0.0	0.0	75.3	75.3	0.0	75.3
Other comprehensive income (loss), after-tax														$ 0.0	$ 0.0	$ (8.7)	$ (8.7)	$ (3.0)	$ (11.7)
OCI, Change in foreign currency transaction and translation adjustment, net of tax	(10.7)							$ 0.0	$ 0.0	$ (168.2)	$ (168.2)	$ (0.3)	$ (168.5)																											$ (169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	0.0
Comprehensive income	207.3	0.0	312.2	(101.6)	210.6	(25.5)	185.1
Dividends declared on common shares		0.0	(6.2)	0.0	(6.2)	0.0	(6.2)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(40.7)	(40.7)
Issuances of common shares		4.8	0.0	0.0	4.8	0.0	4.8
Repurchases and retirements of common shares	(134.5)	(37.1)	(97.4)	0.0	(134.5)	0.0	(134.5)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0.0	0.0	0.0	0.0	8.3	8.3
Noncontrolling Interest, Increase from Business Combination							122.1																												$ 122.1
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests							(14.0)																					$ (14.0)
Amortization of restricted share and option awards		15.9	0.0	0.0	15.9	0.8	16.7
Ending balance at Dec. 31, 2014		1,034.7	3,010.5	(49.5)	3,995.7	542.7	4,538.4
Increase (Decrease) in Shareholders' Equity
Net income	279.5	0.0	297.6	0.0	297.6	(18.1)	279.5
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	(34.9)	0.0	0.0	(34.9)	(34.9)	0.0	(34.9)
Other comprehensive income (loss), after-tax														$ 0.0	$ 0.0	0.3	0.3	0.0	0.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax	(0.5)							0.0	0.0	(65.8)	(65.8)	0.0	(65.8)																											(65.0)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	0.0
Comprehensive income	197.2	0.0	297.6	(100.4)	197.2	(18.1)	179.1
Dividends declared on common shares		0.0	(6.0)	0.0	(6.0)	0.0	(6.0)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(51.1)	(51.1)
Issuances of common shares		0.9	0.0	0.0	0.9	0.0	0.9
Repurchases and retirements of common shares	(284.2)	(67.0)	(217.2)	0.0	(284.2)	0.0	(284.2)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0.0	0.0	0.0	0.0	17.5	17.5
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(5.3)	0.0	0.0	(5.3)	(2.7)	(8.0)																	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (31.5)	$ (31.5)
Amortization of restricted share and option awards		14.9	0.0	0.0	14.9	(2.0)	12.9
Ending balance at Dec. 31, 2015	3,913.2	978.2	3,084.9	(149.9)	3,913.2	454.8	4,368.0
Increase (Decrease) in Shareholders' Equity
Net income	419.8	0.0	412.5	0.0	412.5	7.3	419.8
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0
Other comprehensive income (loss), after-tax																$ 0.6	$ 0.6	$ 0.3	$ 0.9
OCI, Change in foreign currency transaction and translation adjustment, net of tax	0.3							$ 0.0	$ 0.0	$ 31.4	$ 31.4	$ 0.0	$ 31.4																											32.0
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax																				$ 0.0	$ 0.0	$ 0.0	$ 113.3															$ 113.3	$ 113.3	$ 113.3
Comprehensive income	558.1	0.0	412.5	145.3	557.8	7.6	565.4
Dividends declared on common shares		0.0	(5.4)	0.0	(5.4)	0.0	(5.4)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(22.7)	(22.7)
Issuances of common shares		9.1	0.0	0.0	9.1	0.0	9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)	0.0	(887.2)	0.0	(887.2)
Noncontrolling Interest, Decrease from Deconsolidation																																				$ (250.0)	$ (250.0)				$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (78.4)	$ (78.4)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance						27.3	27.3
Noncontrolling Interest, Increase from Business Combination							3.3																							$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 3.3
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(2.7)	0.0	0.0	(2.7)	(8.8)	(11.5)
Amortization of restricted share and option awards		18.5	0.0	0.0	18.5	0.8	19.3
Ending balance at Dec. 31, 2016	$ 3,603.3	$ 810.7	$ 2,797.2	$ (4.6)	$ 3,603.3	$ 133.9	$ 3,737.2

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operations:
Net income	$ 419.8	$ 279.5	$ 290.0
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains	(10.3)	(225.4)	(78.5)
Net gain on sale of consolidated and unconsolidated affiliates	(0.5)	(16.1)	0.0
Amortization and depreciation	64.5	46.4	35.3
Deferred income tax expense (benefit)	2.9	5.6	(22.5)
Undistributed equity in earnings from unconsolidated affiliates, after-tax	0.0	(25.1)	(45.6)
Net loss (income) from discontinued operations	0.3	(78.7)	(260.6)
Net (gain) loss on sale of discontinued operations	(415.1)	(18.2)	1.6
Other operating items:
Net change in loss and loss adjustment expense reserves	25.2	(45.8)	(282.2)
Net change in reinsurance recoverable on paid and unpaid losses	14.2	(20.1)	(83.2)
Net change in unearned insurance premiums	(46.5)	4.1	(57.6)
Net change in ceded unearned insurance premiums	(14.7)	(11.6)	(3.5)
Net change in insurance premiums receivable	(8.1)	17.8	(8.8)
Net change in deferred acquisition costs	0.7	(0.4)	(1.8)
Net change in funds held under insurance contracts	15.3	56.7	17.7
Net change in restricted cash	5.8	17.9	32.4
Net change in other assets and liabilities, net	(226.7)	49.7	(142.4)
Net cash (used for) provided from continuing operations	(130.6)	119.7	69.9
Net cash (used for) provided from discontinued operations	(24.7)	56.0	48.9
Net cash (used for) provided from operations	(155.3)	175.7	118.8
Cash flows from investing activities:
Net change in short-term investments	(70.1)	140.0	(138.0)
Sales of fixed maturity and convertible investments	3,428.7	1,281.7	2,210.8
Maturities, calls and paydowns of fixed maturity and convertible investments	726.9	335.0	452.4
Sales of common equity securities	1,200.8	461.4	609.8
Distributions and redemptions of other long-term investments	28.9	84.7	64.5
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	2,646.2	24.0	0.0
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries	(141.6)	0.0	0.0
Purchases of other long-term investments	(39.5)	(78.0)	(114.7)
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries	(402.0)	17.5	(88.1)
Purchases of common equity securities	(538.3)	(409.9)	(289.7)
Purchases of fixed maturity and convertible investments	(5,802.9)	(1,867.2)	(2,456.1)
Purchases of consolidated and unconsolidated affiliates (net of cash acquired)	(13.4)	(2.6)	(205.0)
Net change in unsettled investment purchases and sales	35.7	(4.6)	(44.7)
Net (acquisitions) dispositions of property and equipment	(4.3)	34.7	(4.5)
Net cash provided from (used for) investing activities — continuing operations	1,055.1	16.7	(3.3)
Net cash provided from investing activities — discontinued operations	221.7	(5.4)	51.2
Net cash provided from investing activities	1,276.8	11.3	47.9
Cash flows from financing activities:
Draw down of debt and revolving line of credit	352.5	171.5	133.6
Repayment of debt and revolving line of credit	(404.6)	(76.1)	(65.2)
Change in capital lease obligation	(1.7)	(5.4)	(5.3)
Cash dividends paid to the Company’s common shareholders	(5.4)	(6.0)	(6.2)
Payments to Acquire Additional Interest in Subsidiaries	0.0	(9.1)	0.0
(Contributions to) distributions from discontinued operations	(3.0)	(7.6)	12.1
Common shares repurchased	(881.3)	(268.6)	(128.2)
OneBeacon Ltd. common shares repurchased and retired	(11.5)	(3.7)	(1.7)
Proceeds from issuances of common shares	3.7	0.0	0.0
Capital contributions from non-controlling interest of consolidated LPs	0.0	0.0	4.9
Redemptions paid to non-controlling interest of consolidated LPs	0.0	0.0	(5.5)
Distributions to non-controlling interest shareholders	(21.1)	(27.2)	(19.8)
Payments to contingent considerations related to purchases of consolidated subsidiaries	(7.8)	0.0	0.0
Capital contributions from BAM members	38.0	29.2	16.2
Net cash used for financing activities — continuing operations	(942.2)	(203.0)	(65.1)
Net cash used for financing activities — discontinued operations	(0.2)	(11.6)	(64.5)
Net cash used for financing activities	(942.4)	(214.6)	(129.6)
Effect of exchange rate changes on cash (excludes $0.0, $(4.5) and $(14.3) related to discontinued operations)	0.0	0.0	0.0
Net change in cash during the period - continuing operations	(17.7)	(66.6)	1.5
Less: cash held for sale at the end of period	1.2	2.3	0.0
Add: cash held for sale at the beginning of period	0.9	1.2	2.3
Cash balance at beginning of year	167.2	232.7	233.5
Cash balance at end of year	$ 149.8	$ 167.2	$ 232.7

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Cash Flows [Abstract]
Restricted cash balances, beginning of period	$ 5.8	$ 23.7	$ 56.1
Restricted cash balances, end of period	0.0	5.8	23.7
Discontinued operations, beginning of period	150.2	116.8	93.2
Discontinued operations, end of period	0.9	150.2	116.8
Effect of exchange rate on cash	$ 0.0	$ (4.5)	$ (14.3)

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and and other entities required to be consolidated under GAAP. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, HG Global/BAM and Other Operations.

As discussed further in the Company’s consolidated financial statements in Note 2 — “Significant Transactions”, on April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. Also, on July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. For the year ended December 31, 2016, Sirius Group and Tranzact have been presented as discontinued operations in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. The assets and liabilities of Sirius Group and Tranzact are classified as held for sale in the balance sheet as of December 31, 2015. See Note 22 — “Held for Sale and Discontinued Operations”.

The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies and Split Rock Insurance, Ltd. (“Split Rock”), a Bermuda based reinsurance company (collectively “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States primarily through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of December 31, 2016 and 2015, White Mountains owned 76.1% and 75.5% of OneBeacon Ltd.’s outstanding common shares. As discussed further in Note 2 — “Significant Transactions”, in December 2014, OneBeacon completed the sale of its of runoff business consisting of assets, liabilities and capital related principally to OneBeacon’s non-specialty business, including the vast majority of its asbestos and environmental reserves (“Runoff Business”) to an affiliate of Armour Group Holdings Limited (the “Runoff Transaction”). Accordingly, OneBeacon’s runoff business is presented as discontinued operations in the statement of operations and comprehensive income. In the second quarter of 2015, OneBeacon completed the sale of its building in Canton, MA for $58.0 million.

The HG Global/BAM segment consists of White Mountains’s investment in HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is the first and only mutual bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities, and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of December 31, 2016 and 2015, White Mountains owned 96.9% of HG Global's preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM. However, GAAP requires White Mountains to consolidate BAM’s results in its financial statements. BAM’s results are attributed to non-controlling interests.

White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”) and certain consolidated and unconsolidated private capital investments. The consolidated private capital investments consist of QL Holdings LLC (“MediaAlpha”), Wobi Insurance Agency Ltd. (“Wobi”), Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and Removal Stars Ltd. (“Buzzmove”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE. As a result, White Mountains is required to consolidate SSIE in its GAAP financial statements. SSIE’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and the SSIE Surplus Notes to K2 Insurance Services LLC. As a result, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015. White Mountains’s Other Operations segment also includes its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”).

All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Significant Accounting Policies

Investment Securities

As of December 31, 2016, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments, common equity securities and other long-term investments held for general investment purposes are classified as trading securities and are reported at fair value as of the balance sheet date. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment.

White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common equity securities and other long-term investments, such as interests in hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes issued in connection with the Runoff Transaction. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs.

As of December 31, 2016 and 2015, approximately 94% and 91% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, short-term investments, which include U.S. Treasury Bills and common equity securities. Investments valued using Level 2 inputs are comprised of fixed maturity investments including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government, agency and provincial obligations and preferred stocks. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include certain debt securities, including asset-backed securities, where quoted market prices are unavailable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services used by White Mountains have indicated that if no observable inputs are available for a security, they will not provide a price. In those circumstances, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark interest rates, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs.

White Mountains performs procedures to validate the market prices obtained from the outside pricing sources. Such procedures, which cover substantially all of its fixed maturity investments include, but are not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control processes and procedures on at least an annual basis, comparison of our invested asset market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices, and review of the underlying assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Prices that have not changed from period to period and prices that have trended unusually compared to market conditions are also considered outliers.

In circumstances where the results of White Mountains’s review process do not appear to support the market price provided by the pricing services, White Mountains challenges the price. During the past year, nine securities fell outside White Mountains’s expected results, thereby triggering the challenge with the pricing service. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The fair values of such securities are considered to be Level 3 measurements.

White Mountains’s investments in debt securities, including mortgage and asset-backed securities, are generally valued using pricing evaluation techniques that considers relevant market information, relevant credit information, perceived market movements and sector news. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life.

Short-term investments consist of interest bearing money market funds, certificates of deposit, time deposits and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized cost, which approximated fair value as of December 31, 2016 and 2015.

Other Long-term Investments

Other long-term investments consist primarily of hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes. See Note 5 — “Investment Securities”. White Mountains has taken the fair value option for most of its investments in hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes. For the investments for which White Mountains has taken the fair value option, changes in fair value are reported in revenues on a pre-tax basis. For those long-term investments for which White Mountains has not made the fair value election, White Mountains accounts for its interests under the equity method.

Derivative Financial Instruments

White Mountains occasionally holds a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income.

Warrants

From time to time, White Mountains holds warrants that it has received in the restructuring (e.g., securities received from bankruptcy proceedings) of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives.

Derivatives—Variable Annuity Reinsurance

White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan through its wholly owned subsidiary, WM Life Re. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability.

Guaranteed minimum accumulation benefits (“GMABs”) are paid to an annuitant for any shortfall between accumulated account value at the end of the accumulation period and the annuitant’s total deposit, less any withdrawal payments made to the annuitant during the accumulation period. GMABs meet the definition of a derivative for accounting purposes. Therefore, GMABs are carried at fair value, with changes thereon recognized in income in the period of the change. The liability for the reinsured GMAB contracts has been determined using internal valuation methodologies that use assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates.

If an annuitant dies during the accumulation period of an annuity contract, guaranteed minimum death benefits (“GMDBs”) are paid to the annuitant’s beneficiary for shortfalls between accumulated account value at the time of an annuitant’s death and the annuitant’s total deposit, less any living benefit payments or withdrawal payments previously made to the annuitant. White Mountains elected to measure its GMDB liabilities at fair value.

The valuation of these liabilities involves significant judgment and is subject to change based upon changes in capital market assumptions and emerging surrender and mortality experience of the underlying contracts in force.

WM Life Re has completed its run-off as all of its contracts matured as of June 30, 2016. WM Life Re entered into derivative contracts that were designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. The derivatives included futures and over-the-counter option contracts on interest rates, major bond and equity indices, and foreign currencies. All WM Life Re’s derivative instruments were recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments did not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value were recognized in the appropriate period as gains or losses in the income statement within other revenues.

WM Life Re included the effect of counterparty credit risk when determining the fair value of its derivative contracts and its GMAB and GMDB liabilities.

Cash

Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash. See Note 9 — “Derivatives”.

Insurance and Reinsurance Operations

White Mountains accounts for insurance and reinsurance policies that it writes in accordance with ASC 944. Premiums written are recognized as revenues and are earned ratably over the term of the related policy or reinsurance treaty. Unearned premiums represent the portion of premiums written that are applicable to future insurance and reinsurance coverage provided by policies or treaties in force. White Mountains charges fees on certain of its insurance policies. Refundable fees are classified with premiums and recognized in earnings over the policy term. Fees that represent a reimbursement of expenses, such as installment fees, are recorded as a reduction of underwriting expenses.

Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Amortization of deferred acquisition costs are presented within insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency.

Losses and LAE are charged against income as incurred. Unpaid insurance losses and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance losses and LAE are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any resulting adjustments are reflected in current operations. The process of estimating loss and LAE involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements.

White Mountains’s insurance and reinsurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. Such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies White Mountains for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. White Mountains has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro-rata basis. The amount of each risk ceded by White Mountains is subject to maximum limits which vary by line of business and type of coverage.

Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectability of reinsurance recoverables is also subject to the solvency of the reinsurers. White Mountains is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with a strong financial condition, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis.

Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly. Funds held by ceding companies represent amounts due to White Mountains in connection with certain assumed reinsurance agreements in which the ceding company retains a portion of the premium to provide security against future loss payments. The funds held by ceding companies are generally invested by the ceding company and a contractually agreed interest amount is credited to the Company and recognized as investment income. Funds held under insurance and reinsurance contracts represent contractual payments due to White Mountains that have been retained to secure such obligations. Such amounts are recorded as liabilities in the consolidated financial statements.

Accruals for contingent commission liabilities are established for reinsurance contracts that provide for the stated commission percentage to increase or decrease based on the loss experience of the contract. Changes in the estimated liability for such arrangements are recorded as contingent commissions. Accruals for contingent commission liabilities are determined through the review of the contracts that have these adjustable features and are estimated based on expected loss and LAE.

Municipal Bond Insurance

All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods.

Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income.

Funds Held

Funds held under reinsurance contracts primarily represent amounts due to White Mountains in connection with the Standard Reinsurance Agreement (“SRA”) with the Federal Crop Insurance Corporation (“FCIC”), which is managed by an agency of the U.S. Department of Agriculture. The SRA governs the relationship, including the exchange of funds, between private insurance companies, including White Mountains, and the FCIC relating to our MPCI crop insurance business. Funds held under insurance contracts represents unrestricted collateral held by White Mountains primarily relating to the surety business.

Mandatory Shared Market Mechanisms

As a condition to its licenses to do business in certain states, White Mountains’s insurance operations must participate in various mandatory shared market mechanisms commonly referred to as “residual” or “involuntary” markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer’s market share of voluntary business written within that state. In certain cases, White Mountains is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms is accounted for as direct insurance business or as assumed reinsurance depending upon the structure of the mechanism.

OneBeacon’s market assignments are typically required to be written in the current period, however, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated.

Insurance-related Assessments

Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains records guaranty fund assessments when it is probable that an assessment will be made and the amount can be reasonably estimated.

Deferred Software Costs

White Mountains capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll-related costs. White Mountains begins amortization of these costs once the project is completed and ready for its intended use. Amortization is on a straight-line basis and over a useful life of eighteen months to five years.

Costs related to software developed for sale to third parties are expensed until technological feasibility has been established. Once technological feasibility has been established, software development costs are capitalized and reported at their net realizable value. Upon product release, the amortization of software development costs is determined annually as the greater of the amount calculated using the ratio of current gross revenues to the total of current and expected gross revenues for the product or the straight-line method over the estimated economic life, which is generally between 18 to 36 months.

As of December 31, 2016 and 2015, White Mountains had unamortized deferred software costs of $16.4 million and $15.8 million. Amortization expense of software development costs of $6.3 million, $4.4 million and $2.9 million were recognized for the years ended December 31, 2016, 2015 and 2014, respectively.

Commission and Other Revenue Recognition

White Mountains recognizes agent commissions and other revenues when it has fulfilled all of its obligations necessary to earn the revenue and when it can reliably estimate both the amount of revenue, net of any amounts expected to be uncollectible, and any amounts associated with expected cancellations.

Federal and Foreign Income Taxes

A significant portion of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States.

Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.

Foreign Currency Exchange

The U.S. dollar is the functional currency for all of White Mountains’s businesses except for Sirius International and certain other smaller international activities. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss.

Assets and liabilities relating to foreign operations are translated into the functional currency using current exchange rates; revenues and expenses are translated into the functional currency using the weighted average exchange rate for the period. The resulting exchange gains and losses are reported as a component of net income in the period in which they arise. As of December 31, 2016 and 2015, White Mountains had unrealized foreign currency translation losses of $1.4 million and $145.6 million recorded in accumulated other comprehensive income on its consolidated balance sheet.

The following rates of exchange for the U.S. dollar have been used for the most significant operations:



Currency	Opening Rate 2016	Closing Rate 2016	Opening Rate 2015	Closing Rate 2015
Israeli Shekels	3.9051	3.8476	3.8865	3.9051
British Pound Sterling	0.6757	0.8074	0.6426	0.6757
Euro	0.9189	0.9479	0.8245	0.9189
Swedish Kronor ⁽¹⁾	8.4247	N/A	7.7737	8.4247

⁽¹⁾On April 18, 2016 White Mountains completed its sale of Sirius Group.

Goodwill and Other Intangible Assets

Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses.

Goodwill is not amortized, but rather is evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill is performed during the fourth quarter of each year. White Mountains initially evaluates goodwill using a qualitative approach (“step zero”) to determine whether it is more likely than not that the fair value of goodwill is greater than its carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill exceeds its fair value, White Mountains, performs the two-step quantitative test for impairment.

Other intangible assets with finite lives are measured at their acquisition date fair values, are amortized over their economic lives and presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are evaluated for impairment at least annually and when events or changes in circumstances indicate that it is more likely than not that the asset is impaired.

White Mountains evaluated the recoverability of goodwill and other intangible assets and did not recognize any impairment losses for any of the years ended December 31, 2016, 2015 and 2014. See Note 6 — “Goodwill and Other Intangible Assets”.

Non-controlling Interest

Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries, and are presented separately on the balance sheet. The portion of comprehensive income attributable to non-controlling interests is presented net of related income taxes in the statement of operations and comprehensive income. See Note 14 — “Common Shareholders’ Equity and Non-controlling Interests”.

Variable Interest Entities

White Mountains consolidates a variable interest entity (“VIE”) when it has both the power to direct the activities of the VIE that most significantly impact its economic performance and either the obligation to absorb losses or the right to receive returns from the VIE that could potentially be significant to the VIE. See Note 18 — “Variable Interest Entities”.

Recently Adopted Changes in Accounting Principles

Short-Duration Contracts

Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnote disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”.

Business Combinations - Measurement Period Adjustments

Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments (ASC 805), which requires adjustments to provisional amounts recorded in connection with a business combination that are identified during the measurement period to be recorded in the reporting period in which the adjustment amounts are determined, rather than as retroactive adjustments to prior periods. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended December 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption.

Fair Value Measurements

On January 1, 2016, White Mountains adopted ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820), which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value (“NAV”) using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures.

Amendments to Consolidation Analysis

On January 1, 2016, White Mountains adopted ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a VIE. ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments.

Share-Based Compensation Awards

On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718). The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures.

Debt Issuance Costs

On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of December 31, 2016, the unamortized debt issuance costs included in debt is $1.6 million.

Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity

Effective January 1, 2015, White Mountains adopted ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity (ASC 205 and ASC 360), which limits discontinued operations treatment to disposals that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. ASU 2014-08 also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. As discussed further in Note 2, White Mountains completed its sale of Sirius Group on April 18, 2016. Sirius Group has been presented as discontinued operations for all periods presented.

Qualified Affordable Housing Projects

Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.4 million. The retrospective adoption resulted in an increase of $1.7 million and $1.9 million to net investment income and a net increase of $2.3 million and $2.2 million to income tax expense for the years ended December 31, 2014 and 2013. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above.

Pushdown Accounting

ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014. The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections. White Mountains has not had any acquisitions for which it has elected to apply pushdown accounting since ASU 2014-17 became effective.

Recently Issued Accounting Pronouncements

Cash Flow Statement

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230). Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented.

The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance.

Credit Losses

In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. The types of assets included in the scope of the new guidance includes premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures financial assets at fair value with changes therein recognized in current period earnings and accordingly, does not expect adoption to have a significant impact on its financial statements.

Stock Compensation

In March 2016, the Board issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which is intended to simplify certain aspects of the accounting for share-based compensation, including forfeiture assumptions, net settlement of equity awards for withholding taxes and accounting for excess tax benefits. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. The new ASU also permits net settlement of equity awards for withholding taxes up to the maximum statutory rate and requires such amounts to be classified as a financing activities in the statement of cash flows. In addition, the new guidance changes the accounting for excess tax benefits and deficiencies by requiring recognition in the income statement, with treatment of the tax effects as discrete items in determining a reporting entity’s effective rate in the period in which exercise or vesting of awards occurs. The new guidance became effective on January 1, 2017 for White Mountains and did not have a significant effect upon adoption.

Leases

In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the impact of this new guidance and available adoption methods.

Financial Instruments - Recognition and Measurement

In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings accordingly, does not expect the adoption of ASU 2016-01 to have a significant impact on its financial statements.

Revenue Recognition

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is to be recognized at an amount that reflects the consideration to which an entity expects to be entitled in exchange for goods or services transferred to customers. The new guidance sets forth the steps to be followed to recognize revenue: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. Subsequently, the FASB issued additional ASUs clarifying the guidance in and providing implementation guidance for ASU 2014-09. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, which delays the effective date of ASU 2014-09 and all related ASUs to annual and interim reporting periods beginning after December 15, 2017. Most of White Mountains’s revenue from customers relates to insurance contracts, which are excluded from the scope of ASU 2014-09, as are investment income and investments gains and losses. However, the new guidance is applicable to some of White Mountains’s revenue streams, including certain fee arrangements as well as commissions and other non-insurance revenues. White Mountains is evaluating the new guidance, but does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s non-insurance revenues from customers.

Significant Transactions

12 Months Ended

Dec. 31, 2016

Significant Transactions [Abstract]

Significant Transactions

Acquisitions

The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The total assets acquired and liabilities assumed have been measured at their acquisition date fair values.

Buzzmove

On August 4, 2016, White Mountains acquired a 70.9% ownership share in Buzzmove for a purchase price of GBP 6.1 million (approximately $8.1 million based upon the foreign exchange spot rate at the date of acquisition). White Mountains recognized total assets acquired related to Buzzmove of $11.5 million, including $7.6 million of goodwill and $1.1 million of intangible assets, and total liabilities assumed of $0.1 million, reflecting acquisition date fair values.

MediaAlpha

On March 14, 2014, White Mountains acquired 60.0% of the outstanding Class A common units of MediaAlpha. White Mountains paid an initial purchase price of $28.1 million. The purchase price was subject to adjustment equal to 62.5% of the 2015 gross profit in excess of the 2013 gross profit. On February 26, 2016, White Mountains paid $7.8 million in

settlement of the final purchase adjustment. After adjustment for the estimated contingent purchase price adjustment, White Mountains recognized total assets acquired related to MediaAlpha of $70.1 million, including $18.3 million of goodwill and $38.5 million of other intangible assets, and total liabilities assumed of $10.0 million, reflecting acquisition date fair values.

On January 15, 2016, MediaAlpha acquired certain assets from Oversee.net for a purchase price of $3.9 million. The majority of assets acquired, which are included in other intangible assets, consists of customer relationships, a customer contract, a non-compete agreement from the seller, domain names and technology.

Wobi

On February 19, 2014, White Mountains acquired 54.0% of the outstanding common shares of Wobi for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition). During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $9.0 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. As of the acquisition date, White Mountains recognized total assets acquired related to Wobi of $13.4 million, including $5.5 million of goodwill and $2.9 million of other intangible assets; and total liabilities assumed of $0.7 million at their estimated acquisition date fair values.

During 2015, White Mountains purchased NIS 79.6 million (approximately $20.7 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. In addition, during 2015 White Mountains also purchased NIS 11.8 million (approximately $3.1 million based upon the foreign exchange spot rate at the date of acquisition) of common shares of Wobi.

On February 23, 2015, Wobi acquired 56.2% of the outstanding share capital of Tnuva Finansit Ltd. (“Cashboard”) for NIS 9.5 million (approximately $2.4 million). The acquisition of Cashboard accelerated Wobi’s development of its pension products comparison service. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million, including $0.3 million of goodwill and $2.8 million of other intangible assets; and total liabilities assumed of $1.2 million at their estimated acquisition date fair values. During 2015, Wobi purchased the remaining share capital of Cashboard for NIS 26.4 million (approximately $6.5 million).

During 2016, White Mountains purchased NIS 35.9 million (approximately $9.6 million based upon the foreign exchange spot rates at the dates of acquisitions) of convertible preferred shares of Wobi. As of December 31, 2016 and 2015, White Mountains’s ownership share was 95.0% and 96.1%.

Star & Shield

On January 31, 2014, White Mountains acquired certain assets and liabilities of Star & Shield Holdings LLC, including SSRM, the attorney-in-fact for SSIE, for a purchase price of $1.8 million.

During 2015 and 2014, White Mountains also purchased $4.0 million and $17.0 million of SSIE Surplus Notes. Principal and interest on the SSIE Surplus Notes are payable to White Mountains only with approval from the Florida Office of Insurance Regulation.

On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in the SSIE Surplus Notes which resulted in a $21.0 million total decrease to net income attributable to White Mountain’s common shareholders and a corresponding increase to net income attributable to non-controlling interests. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services LLC. As a result, White Mountains has presented Star & Shield's and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015.

On December 30, 2016, SSIE entered into a Consent Order with the Florida Office of Insurance Regulation governing its wind down and the surrender and termination of SSIE’s Certificate of Authority as a domestic reciprocal in the State of Florida. Pursuant to the Consent Order, it is anticipated that SSIE will non-renew its last policy in August 2017.

SSIE is a Florida-domiciled reciprocal insurance exchange providing private passenger auto insurance to the public safety community and their families. SSIE is a VIE. SSRM’s role as the attorney-in-fact for SSIE gives it the power to direct the significant economic activities of SSIE and therefore, White Mountains is required to consolidate SSIE. See Note 18 — “Variable Interest Entities”.

Dispositions

Sale of Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion. $161.8 million of this amount was used to purchase certain assets to be retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. During 2016, White Mountains recorded $363.2 million of gain from sale of Sirius Group in discontinued operations and $113.3 million in other comprehensive income from discontinued operations.

The transactions to purchase the investments in OneBeacon and the other investments held by Sirius Group prior to the closing are presented in the statement of cash flows as net settlement of investment cash flows with discontinued operations. See Note 22 — “Held for Sale and Discontinued Operations”.

Symetra

On February 1, 2016, White Mountains sold its investment in Symetra Financial Corporation (“Symetra”) for proceeds of $658.0 million, or $32.00 per share. See Note 17 — “Investments in Unconsolidated Affiliates”.

Sale of Tranzact

On July 21, 2016, White Mountains completed the sale of Tranzact to Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million at closing. In connection with the sale of Tranzact, the purchaser directly repaid $56.3 million for the portion of Tranzact's debt attributable to White Mountains's common shareholders. On October 5, 2016, White Mountains received additional proceeds of $1.2 million following the release of the post-closing purchase price adjustment escrow.

White Mountains recorded a $51.9 million gain from the sale of Tranzact in discontinued operations, which included a $30.2 million tax expense for the reversal of a tax valuation allowance that is offset by a tax benefit recorded in continuing operations. See Note 8 — “Income Taxes”. The increase to White Mountains’s book value from the sale of Tranzact was $82.1 million. A reconciliation of the gain reported in discontinued operations to the impact to White Mountains's book value is as follows:



Millions
Gain from sale of Tranzact reported in discontinued operations	$	51.9
Add back reclassification from continuing operations for the release of a tax valuation allowance	30.2
Comprehensive income from sale of Tranzact	$	82.1

Through July 21, 2016, Tranzact’s results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains's GAAP financial statements. See Note 22 — “Held for Sale and Discontinued Operations”.

OneBeacon Crop Business

On July 31, 2015, OneBeacon exited its multiple peril crop insurance (“MPCI”) and its related crop-hail business (collectively, “Crop Business”) as its exclusive managing general agency, Climate Crop Insurance Agency (“CCIA”), exited the business through a sale of the agency to an affiliate of AmTrust. As a result of the transaction, OneBeacon and CCIA agreed to an early termination of the existing five-year agreement. During the year ended December 31, 2015, in connection with the termination of the agreement, OneBeacon received a payment of $3.0 million recorded in other revenues. Also related to the transaction, OneBeacon withdrew its 2016 Plan of Operations, which previously authorized it to write MPCI for the 2016 Reinsurance Year, and affiliates of AmTrust agreed to reinsure the Company’s remaining net Crop Business exposure for the 2015 Reinsurance Year under a related 100% quota share reinsurance agreement which, coupled with other transfer and assignment agreements as well as communications with policyholders and agents, had the effect of assumption reinsurance.

Sale of OneBeacon Runoff Business

On December 23, 2014, OneBeacon completed the sale of its Runoff Business to Trebuchet US Holdings, Inc., a wholly-owned subsidiary of Armour Group Holdings Limited (“Armour”). Financing was provided in the form of the OneBeacon Surplus Notes of $101.0 million that had a fair value of $64.9 million on the date of sale. Subsequent to closing, the OneBeacon Surplus Notes are included in OneBeacon’s investment portfolio, categorized within other long-term investments. See Note 5 - “Investment Securities”. The difference of $36.1 million between the par value and the fair value of the OneBeacon Surplus Notes at the date of sale is included in the loss from sale of discontinued operations. See Note 22 — “Held for Sale and Discontinued Operations”.

Sale of Essentia Insurance Company

Effective January 1, 2013, OneBeacon completed the sale of Essentia Insurance Company (“Essentia”), an indirect wholly-owned subsidiary which wrote the collector cars and boats business, to Markel Corporation. Concurrently therewith, OneBeacon and Hagerty Insurance Agency (“Hagerty”) terminated their underwriting arrangement with respect to the collector cars and boats business. OneBeacon recognized a pre-tax gain on sale of $23.0 million ($15.0 million after tax) in the first quarter of 2013. During the year ended December 31, 2015, OneBeacon recognized in other revenues a $3.7 million negative adjustment to the pre-tax gain on sale of Essentia in connection with an assessment from the Michigan Catastrophic Claims Association (“MCCA”) payable to Markel Corporation pursuant to the indemnification provisions in the stock purchase agreement governing the sale of Essentia.

Reserves for Unpaid Losses and Loss Adjustment Expenses

12 Months Ended

Dec. 31, 2016

Insurance Loss Reserves [Abstract]

Reserves for Unpaid Losses and Loss Adjustment Expenses

Insurance Operations

White Mountains establishes loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. The process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain.

Loss and LAE reserves are typically comprised of (1) case reserves for claims reported and (2) IBNR reserves, which include a provision for expected future development on case reserves and for losses that have occurred but for which claims have not yet been reported. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made.

Ultimate loss and LAE are generally determined by extrapolation of claim emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE with respect to any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. White Mountains’s own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its reserves. External data, available from organizations such as statistical bureaus, consulting firms and reinsurance companies, is sometimes used to supplement or corroborate White Mountains’s own experience. External data can be especially useful for estimating costs on newer lines of business (e.g., Programs and Surety).

For some lines of business, such as “long-tail” coverages discussed below, claims data reported in the most recent accident or report year or years is often too limited to provide a meaningful basis for analysis due to the typical delay in reporting and settling of claims. For this type of business, White Mountains uses an expected loss ratio method for the initial accident year or years. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends, for that line of business.

Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and eventually settled. This time lag is sometimes referred to as the “claim-tail”. The claim-tail for most property coverages is typically short (usually a few days up to a few months). The claim-tail for casualty coverages, such as automobile liability, general liability including the liability portion of multiple peril coverage, workers compensation, and professional liability can be especially long as claims are often reported and ultimately paid or settled years, even decades, after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior accident years, as well as about actual claims and trends may become known that cause White Mountains to adjust its reserves. If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted.

In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claim costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services, and legislated changes in statutory benefits, as well as by the particular, unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claim costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE and resulting IBNR reserves is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected more reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain.

White Mountains’s actuaries use several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. Management places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories:

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Historical paid loss development methods: These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company’s claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to approach the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past.

•

Historical incurred loss development methods: These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters’ evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods.

•

Expected loss ratio methods: These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing, and are multiplied by the total amount of premiums earned to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tailed lines of business, when little or no paid or incurred loss information is available.

•

Bornhuetter-Ferguson methods: These methods are a blend of the expected loss ratio and loss development methods. The percent of incurred (or paid) loss to ultimate loss implied by the selected development pattern from the incurred (or paid) loss development method is used to determine the percentage of ultimate loss yet to be developed. Inception to date losses are added to losses yet to be developed, yielding an estimate of ultimate for each accident year.

•

Adjusted historical paid and incurred loss development methods: These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claim payments or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes.

White Mountains performs an actuarial review of its recorded reserves each quarter. As part of that review, White Mountains’s actuaries compare the previous quarter’s projections of incurred, paid and case reserve activity, including amounts incurred but not reported, to amounts experienced in the quarter. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE should be relied upon to a greater or lesser extent than it had been in the past. While some variance is expected each quarter due to the inherent uncertainty in loss and LAE, persistent or large variances would indicate that prior assumptions and/or reliance on certain reserving methods may need to be revised going forward.

Upon completion of each quarterly review, White Mountains’s actuaries select indicated reserve levels based on the results of the actuarial methods described previously, which are the primary consideration in determining management's best estimate of required reserves. However, in making its best estimate, management also considers other qualitative factors that may lead to a difference between held reserves and the actuarial central estimate of reserves. Typically, these factors exist when management and our actuaries conclude that there is insufficient historical incurred and paid loss information or that trends included in the historical incurred and paid loss information are unlikely to repeat in the future. Such factors include, among others, recent entry into new markets or new products, improvements in the claims department that are expected to lessen future ultimate loss costs, legal and regulatory developments, or other volatilities that may arise.

OneBeacon Reserve Estimation by Line of Business

The process of establishing loss and LAE reserves, including amounts incurred but not reported, is complex and imprecise as it must consider many variables that are subject to the outcome of future events. As a result, informed subjective estimates and judgments as to the ultimate exposure to losses are an integral component of the loss and LAE reserving process. OneBeacon categorizes and tracks insurance reserves by “line of business”, which are summarized herein as either property short-tailed lines, casualty long-tailed lines, or other (accident, surety, and credit) lines.

OneBeacon regularly reviews the appropriateness of reserve levels at the line of business level, considering the variety of trends that impact the ultimate settlement of claims for the subsets of claims in each particular line of business.

For loss and allocated LAE reserves, the key assumption as of December 31, 2016 was that the impact of the various reserving factors, as described below, on future paid losses would be similar to the impact of those factors on the historical loss data with the exception of severity trends. Severity trends have been relatively stable over the relevant historical period. The actuarial methods used would project losses assuming continued stability in severity trends. OneBeacon has considered a range of assumptions regarding future increases in loss severity trends, including the impact of inflation, in making its reserve selections.

The major causes of material uncertainty (“reserving factors”) generally will vary for each line, as well as for each separately analyzed component of the line. The following section details reserving factors by product line. Also, reserving factors can have offsetting or compounding effects on estimated reserves. For example, in workers compensation, the use of expensive medical procedures that result in medical cost inflation may enable workers to return to work faster, thereby lowering indemnity costs. Thus, in almost all cases, it is impossible to discretely measure the effect of a single reserving factor and construct a meaningful sensitivity expectation. Actual results will likely vary from expectations for each of these assumptions, resulting in an ultimate claim liability that is different from that being estimated currently.

Additional causes of material uncertainty exist in most product lines and may impact the types of claims that could occur within a particular underwriting unit or book of business. Examples where reserving factors, within an underwriting unit or book of business, are subject to change include changing types of insured (e.g. type of insured vehicle, size of account, industry insured, jurisdiction, etc.), changing underwriting standards, or changing policy provisions (e.g. deductibles, policy limits, or endorsements).

Following is a detailed description of the reserve factors and consideration for each of the major product lines.

Property

The property short-tailed lines represent lines for which the payout of the claim liability occurs shortly after the occurrence of the loss. Reserving for property lines generally involves less uncertainty given the short-tailed nature of these lines.

Property lines, including the property portion of the multiple peril coverage and Inland Marine, cover losses to a business' premises, inventory and equipment as a result of weather, fire, theft and other causes. Claims associated with property coverage generally take a relatively short period of time to close. The reserve risk is driven by occasional catastrophic events or large single losses. The relatively high attachment points and insured values of the property policies underwritten in the Specialty Property underwriting units present a potentially longer tail and greater uncertainty than our standard property business. Property also includes the comprehensive and collision coverages of automobile policies characterized by low severity payments that are made quickly. Additionally, Property includes all of the Ocean Marine products. Ocean Marine has property and liability exposure related to commercial hull, marine, and cargo products. The exposure is generally low severity and short to medium tailed.

Casualty

The casualty long-tailed lines represent lines for which the loss may not be paid, or even reported, until well after the loss occurred. As a result, the amount of liability may not be known at the date of the loss. Reserving for casualty lines generally involves more uncertainty given these factors.

Casualty lines cover a variety of losses associated with policies that cover general liability, the liability portion of commercial multiple peril, professional liability, workers compensation, and commercial automobile liability lines. Losses associated with casualty coverage generally take a longer period of time to close claims. Most of the general reserving factors for casualty are applicable across these underwriting units, while certain underwriting units have additional reserving factors.

Casualty policies can generally be written on either a claims made or occurrence form. Most professional liability, management liability, and medical professional policies are written on a claims made basis, under which the trigger of loss is based on the date the loss is discovered or the loss is reported to OneBeacon. Professional liability policies cover the defense expenses and damages related to negligence claims brought against the insured professional services firm or government entity. The coverage focuses on damages resulting from an alleged failure to perform, error or omission in the product or service provided by the policyholder. OneBeacon liability policies cover the defense expenses and damages related to alleged wrongful acts committed by the directors and officers of the insured organization. Medical professional liability policies cover the defense expenses and damages related to negligence claims brought against the insured health care institution or provider. The coverage focuses on damages resulting from an alleged failure to perform, error or omission in the service provided by the policyholder.

Most general liability policies are written on an occurrence basis under which the trigger of loss is based on the date the loss happened. They cover businesses for any liability resulting from bodily injury and property damage arising from general business operations, accidents on business premises and the products manufactured or sold. Reserves for these policies generally include two components: bodily injury and property damage. Bodily injury payments reimburse the claimant for damages pertaining to physical injury as a result of the policyholder's legal obligation arising from non-intentional acts such as negligence, subject to the insurance policy provisions. In some cases the damages can include future wage loss, which is a function of future earnings power and wage inflation, and future medical treatment costs. Property damage payments result from damages to the claimant’s private property arising from the policyholder's legal obligation for non-intentional acts. In most cases, property damage losses are a function of costs as of the loss date or soon thereafter. Defense costs are also a part of the insured costs covered by liability policies and can be significant, sometimes greater than the cost of the actual paid claims, though for some products this risk is mitigated by policy language such that the insured portion of defense costs erodes the amount of policy limit available to pay the claim.

Casualty coverages are generally considered long-tail line business, as it takes a relatively long period of time to finalize and settle claims from a given accident year. The speed of claim reporting and claim settlement is a function of the specific coverage provided and the jurisdiction, among other factors. There are numerous components underlying these product lines. Some of these have relatively moderate payment patterns, with most of the claims for a given accident year closed within 5 to 7 years, while others can have extreme lags in both reporting and payment of claims (e.g., a reporting lag of a decade for construction defect claims).

Examples of common reserving factors across casualty lines that can change and, thus, affect estimated casualty reserves include:


•	Changes in claim handling philosophies (e.g., case reserving standards), including the use of third party claims administrators


•	Changes in the pattern of underlying claims, including frequency and severity


•	Changes in policy provisions or court interpretations of such provisions


•	New theories of liability (e.g., cyber related claims)


•	Trends in litigation or jury awards, including the propensity to sue


•	Changes in statutes of limitations


•	Shifts in lawsuit mix between federal and state courts


•	Changes in tort or case law


•	Distortions from losses resulting from large single accounts or single issues


•	Subrogation opportunities

While these reserving factors are applicable to most casualty reserving product lines, there are certain underwriting units within the casualty major product line that have additional unique reserving factors. These include commercial automobile liability and workers compensation.

Commercial automobile liability coverage insures relatively short-tailed property damage liability claims and longer-tailed, more difficult to estimate, bodily injury claims. In general, claim reporting lags are minor, claim complexity is not a major issue, and the line is viewed as high frequency, low to moderate severity. In addition to the examples of common reserving factors related to casualty described above, other examples that affect estimated commercial automobile liability reserves include:


•	Frequency of claims with payment capped by policy limits


•	Change in average severity of accidents, or proportion of severe accidents


•	Frequency of visits to health providers


•	Number of medical procedures given during visits to health providers


•	Types of health providers used


•	Types of medical treatments received


•	Changes in cost of medical treatments


•	Degree of patient responsiveness to treatment

Workers compensation covers an employer's liability for injuries, disability or death of employees, without regard to fault, as prescribed by state workers compensation law and other statutes. Workers compensation is a long-tail coverage as it takes a relatively long period of time to finalize claims from a given accident year. While certain payments such as initial medical treatment or temporary wage replacement for the injured worker are made quickly, some other payments are made over the course of several years, such as awards for permanent partial injuries. In addition, some payments can run as long as the injured worker's life, such as permanent disability benefits and ongoing medical care. Despite the possibility of long payment tails, the reporting lags are generally short, settlements are generally not complex, and most of the claims can be considered high frequency with moderate severity. The largest reserve risk generally comes from the low frequency, high severity claims providing lifetime coverage for medical expenses arising from a worker's injury. Examples of common reserving factors that can change and, thus, affect the estimated workers compensation reserves include:


•	Changes in the type, frequency of utilization or cost of medical treatments (e.g. changes in the use of pharmaceutical drugs, types of health providers used, use of preferred provider networks and other medical cost containment practices)


•	Availability of new medical processes and equipment


•	Degree of patient responsiveness to treatment


•	Mortality trends of injured workers with lifetime indemnity and medical treatment benefits


•	Degree of cost shifting between workers compensation and health insurance


•	Time required to recover from the injury and return to regular or transitional work


•	Future wage inflation for states that index benefits

Other lines

Other lines represent lines that do not fall into either the property or casualty definition. These lines have unique characteristics that are taken into consideration during the reserving process.

Accident includes accidental death and dismemberment, occupational accident, sports accident, non-truckers liability and other accident coverages. It has a short to medium tailed development pattern and moderate severity profile.

Credit represents primarily the Tuition Reimbursement underwriting operating segment which provides insurance protection for schools and parents from the financial consequences of a student's withdrawal or dismissal. The reserve risk for this line is relatively low given the extremely short-tailed and low severity nature.

Surety, which also has a significant credit risk component, is a short-tailed but high severity coverage. As a newer business, lack of historical data means external data is heavily relied upon where available and applicable. Surety reserving factors that can change and, thus, affect estimated surety reserves include size of payment (severity) which is impacted by the bond limit, the ability of the principal (insured) or OneBeacon to mitigate the loss, or amount and collectability of assets or other collateral available to mitigate loss.

Cumulative Number of Reported Claims

OneBeacon counts a claim for each unique combination of individual claimant and major coverage (e.g. auto liability - bodily injury, auto liability - property damage, auto physical damage, etc.). The claim is counted only if, net of any applicable deductibles, a payment has been made or a case reserve has been recorded. As a point of clarification, it is counted if a case reserve is established and the claim is later closed with no payment. For a relatively small amount of bulk-coded assumed reinsurance and pools & association losses there are no claim counts available.

Discount

OneBeacon discounts its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using the statutory rate (2.5% as of both December 31, 2016 and 2015). For the years ended December 31, 2016 and 2015, reserves for unpaid loss and LAE were reduced by $1.6 million and $1.1 million. Changes in loss and LAE reserves resulting from a change in the average long-term workers compensation discount rate is recorded within incurred loss and LAE expense.

Loss and Loss Adjustment Expense Reserve Summary

The following table summarizes the loss and LAE reserve activities of White Mountains’s insurance subsidiaries for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
Millions	2016			2015			2014
Gross beginning balance	$	1,389.8		$	1,342.2		$	1,054.3
Less beginning reinsurance recoverable on unpaid losses	(186.0		)	(161.6		)	(80.2		)
Net loss and LAE reserves	1,203.8			1,180.6			974.1

Loss and LAE reserves consolidated - SSIE	—			—			13.6

Add: SSIE reserves held for sale at the beginning of the period⁽¹⁾	5.5			7.7			—

Losses and LAE incurred relating to:
Current year losses	649.2			712.9			732.0
Prior year losses	14.8			(4.0		)	92.0
Total incurred losses and LAE	664.0			708.9			824.0

Loss and LAE paid relating to:
Current year losses	(192.8		)	(208.8		)	(202.6		)
Prior year losses	(483.1		)	(479.1		)	(420.8		)
Total loss and LAE payments	(675.9		)	(687.9		)	(623.4		)

Less: SSIE reserves held for sale at the end of the period⁽¹⁾	4.7			5.5			7.7

Net ending balance	1,192.7			1,203.8			1,180.6
Plus ending reinsurance recoverable on unpaid losses	172.9			186.0			161.6
Gross ending balance	$	1,365.6		$	1,389.8		$	1,342.2

⁽¹⁾See Note 22 - “Held for Sale and Discontinued Operations”.

Loss and LAE development —2016

During the year ended December 31, 2016, White Mountains experienced $14.8 million of net unfavorable loss reserve development, which consisted of $15.4 million of net unfavorable loss reserve development at OneBeacon partially offset by $0.6 million of net favorable loss reserve development at SSIE. The net unfavorable loss reserve development at OneBeacon was primarily due to unfavorable net loss reserve development in the Healthcare ($40.7 million) and Programs ($13.3 million) underwriting units, partially offset by favorable net loss reserve development in the Accident ($16.1 million), Entertainment ($9.4 million), Technology ($9.3 million) and Financial Services ($8.2 million) underwriting units.

The $40.7 million unfavorable net loss reserve development from OneBeacon’s Healthcare business was principally related to the complex risk business, which provides professional liability coverage to hospitals, physicians, and physician groups as well as physicians’ extended reporting period coverage, and the senior living business, which provides medical malpractice and general liability insurance for extended care facilities, including assisted living, memory care and continuing care facilities. As a result of the continuing loss activity experienced in these areas, OneBeacon management conducted in-depth claim file and actuarial reviews and determined that increased frequency in the more recent prior accident years and adverse loss trends in the more high-risk categories of business within the senior living business warranted an increase to its best estimate of prior accident year loss reserves. OneBeacon management also increased its loss provisions for the current accident year based on the updated actuarial indications. In addition, also within OneBeacon’s Healthcare business, there were two large claims within the managed care errors and omissions business related to unexpected outcomes from mediation and extended costs associated with claim defense, which contributed to the unfavorable development in prior accident years.

Despite the reserve actions taken through the first three quarters of 2016, case incurred loss activity continued to exceed expectations during the fourth quarter of 2016. The adverse development was driven by recent prior accident years spread across the complex risks, extended care, managed care errors and omissions, and medical excess businesses.

Loss and LAE development —2015

During the year ended December 31, 2015, White Mountains experienced $4.0 million of net favorable loss reserve development, which consisted of $1.8 million of net favorable loss reserve development at OneBeacon and $2.2 million of net favorable loss reserve development at SSIE. The OneBeacon net favorable loss reserve development was primarily attributable to favorable loss reserve development in Technology, Collector Cars and Boats, Specialty Property and Financial Services businesses, offset by unfavorable net loss reserve development from the Entertainment and Ocean Marine businesses.

Loss and LAE development —2014

During the year ended December 31, 2014, White Mountains experienced $92.0 million of net unfavorable loss reserve development, which consists of $89.8 million of net unfavorable loss reserve development at OneBeacon, of which $75.5 million related to the 2014 fourth quarter reserve increase described below, and $2.2 million of net unfavorable loss reserve development at SSIE.

OneBeacon 2014 Fourth Quarter Loss and LAE Reserve Increase

Through the first nine months of 2014, OneBeacon recorded $14.3 million of unfavorable loss and LAE reserve development, driven by greater-than-expected large losses in several underwriting units, primarily in the professional and management liability lines within Professional Insurance. In 2015, Professional Insurance was reorganized into Other Professional Lines, Management Liability, Financial Services and Healthcare. This large loss activity, which occurred mostly during the second and third quarters of 2014, also impacted the current accident year loss and LAE estimates. Additionally, OneBeacon incurred higher-than-usual claim coverage determination costs, a component of LAE expenses, during the first nine months of 2014. Other underwriting units also reported increased claim activity, including the Entertainment, Government Risks, and Accident underwriting units.

Since the increased level of loss and LAE activity continued into the early part of the fourth quarter of 2014, the high level of activity in the second and third quarters no longer seemed to be isolated occurrences. As such, during the fourth quarter of 2014, OneBeacon enhanced its actuarial and claims review in several areas. OneBeacon isolated the recent large loss activity in each of its underwriting units and examined the emergence of large losses relative to the timing and amounts of expected large losses. OneBeacon also conducted additional analyses in the lawyers’ professional liability line within Professional Insurance. These new analyses included a claim level review and the application of additional actuarial methods and loss development assumptions. The results of these analyses indicated that the assumed tail risk included in the loss development patterns used to record IBNR reserves for this line were insufficient and needed to be increased for remaining long-tail exposures. OneBeacon’s claims and actuarial staff also conducted an in-depth review of coverage determination, litigation and other claim-specific adjusting expenses as a result of an emerging trend of increased expenses in these areas over recent quarters, particularly coverage determination expenses. This review concluded that the ultimate costs of these loss adjustment expenses were larger than previously estimated, causing management to record an increase in estimated LAE expenses, primarily in Professional Insurance. Finally, OneBeacon also recorded unfavorable prior year development in other underwriting units, including Entertainment and Government Risks. The unfavorable loss development in Entertainment and Government Risks resulted from heavier than expected claim activity during the fourth quarter, predominantly in the general liability and commercial auto liability lines.

In order to fully reflect these recent trends, OneBeacon recorded a $109.2 million increase in loss and LAE reserves, which included a $75.5 million increase in prior accident year loss and LAE reserves and a $33.7 million increase in the current accident year loss and LAE reserves recorded at September 30, 2014.

The components of the 2014 fourth quarter loss and LAE reserve increase and the net loss and LAE development for the full year are provided below:



Underwriting Unit	2014 Fourth Quarter Reserve Increases								Full Year 2014
Millions	Current Accident Year			Prior Accident Year			Total		Net Prior Year Development
Professional Insurance ⁽¹⁾	$	22.9		$	46.4		$	69.3	$	59.1
Specialty Property	(1.1		)	5.7			4.6		1.1
Crop Business	3.8			—			3.8		—
Other	2.8			(.4		)	2.4		1.6
Specialty Products	28.4			51.7			80.1		61.8
Entertainment	1.5			11.6			13.1		13.5
Government Risks	1.2			7.1			8.3		8.5
Accident	—			3.5			3.5		6.0
Other	2.6			1.6			4.2		—
Specialty Industries	5.3			23.8			29.1		28.0
Total	$	33.7		$	75.5		$	109.2	$	89.8

⁽¹⁾Professional Insurance includes Other Professional Lines, Management Liability, Financial Services and Healthcare.

As noted above, OneBeacon increased its provision for current accident year losses and LAE by $33.7 million in the fourth quarter of 2014. In making its loss and LAE reserve picks for the 2014 accident year, OneBeacon considered the results of the enhanced actuarial and claim review and the fact that reported large claims were approaching estimated ultimate held reserves for large losses sooner than originally expected. $3.8 million of the increase is related to higher-than-expected reports of Crop Business losses that emerged in the fourth quarter. The remaining $29.9 million of the increase reflects an increase in management’s best estimate of current losses and LAE as of December 31, 2014 from those recorded in the first nine months of 2014. This increase primarily affected the Professional Insurance underwriting unit, which represented $22.9 million of the total provision.

Reconciliation of liabilities for unpaid loss and LAE

The following table reconciles loss and LAE reserves determined on a statutory basis to loss and LAE reserves determined in accordance with GAAP as of December 31, 2016 and 2015 as follows:



	December 31,
Millions	2016		2015
Statutory reserves (unaudited)	$	1,192.7	$	1,203.8
Reinsurance recoverable on unpaid losses and LAE ⁽¹⁾	172.9		186.0
GAAP reserves	$	1,365.6	$	1,389.8


⁽¹⁾	Represents adjustments made to add back reinsurance recoverables on unpaid losses and LAE included with the presentation of reserves under statutory accounting.

The following table summarizes the ending liabilities for unpaid loss and LAE, net of reinsurance for each of OneBeacon’s major lines of business by underwriting group as of December 31, 2016:



Liabilities for unpaid claims and claims adjustment expenses, net of reinsurance Millions	As of December 31, 2016
Specialty Products - Property	$	30.2
Specialty Industries - Property	108.5
Specialty Products - Casualty	568.7
Specialty Industries - Casualty	360.0
Specialty Products - Other	38.2
Specialty Industries - Other	47.4
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	1,153.0
Less: Discount on workers compensation reserves	(1.6		)
Total unpaid loss and allocated LAE reserves, net of reinsurance	1,151.4
Plus: Unallocated LAE	41.3
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7
Plus: Reinsurance recoverables on unpaid losses
Specialty Products - Property	40.9
Specialty Industries - Property	13.2
Specialty Products - Casualty	64.0
Specialty Industries - Casualty	19.9
Specialty Products - Other	16.0
Specialty Industries - Other	.6
Investing, Financing and Corporate ⁽¹⁾	18.3
Plus: Total Reinsurance recoverables on unpaid losses	172.9
Total unpaid loss and LAE reserves	$	1,365.6

⁽¹⁾OneBeacon’s subsidiary, Atlantic Specialty Insurance Company (“ASIC”) , entered into a 100% quota share reinsurance agreement with OBIC, an entity sold to Armour, as part of the sale of the Runoff Business. See Note 22 — “Held for Sale and Discontinued Operations”. As of December 31, 2016, $18.3 is included in reinsurance recoverables on unpaid losses within Investing, Financing, and Corporate resulting from that agreement.

The following six table groupings, reflecting one table each for the Property, Casualty and Other major lines of business for each of the Specialty Products and Specialty Industries underwriting groups and include three sections.

The first table (top section) presents, for each of the previous 10 accident years (1) cumulative total undiscounted incurred loss and allocated LAE, net of reinsurance, as of each of the previous 10 year-end evaluations, (2) total IBNR plus expected development on reported claims as of December 31, 2016, and (3) the cumulative number of reported claims as of December 31, 2016.

The second table (middle section) presents cumulative paid loss and allocated LAE, net of reinsurance, for each of the previous 10 accident years as of each of the previous 10 year-end evaluations. Also included in this table is a calculation of the liability for loss and allocated LAE as of December 31, 2016 which is then included the reconciliation to the consolidated balance sheet presented above. The liability as of December 31, 2016 is calculated as the cumulative incurred loss and allocated LAE from the first table less the cumulative paid loss and allocated from the second table, plus any outstanding liabilities from accident years prior to 2007.

The third table (bottom section) is supplementary information about the average historical claims duration as of December 31, 2016. It shows the weighted average annual percentage payout of incurred loss and allocated LAE by accident year as of each age. For example, the first column is calculated as the incremental paid loss and allocated LAE in the first calendar year for each given accident year (e.g. calendar year 2008 for accident year 2008, calendar year 2009 for accident year 2009) divided by the cumulative incurred loss and allocated LAE as of December 31, 2016 for that accident year. The resulting ratios are weighted together using cumulative incurred loss and allocated LAE as of December 31, 2016.



Specialty Products - Property
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	12.0	$	11.1	$	9.1	$	7.6	$	7.6	$	7.7	$	8.7	$	8.6	$	8.6	$	8.6	$	—	250
2008	—		31.3		29.2		25.4		25.6		26.6		26.8		26.8		26.8		26.8		—		3,466
2009	—		—		55.2		52.1		52.9		53.3		52.9		52.6		52.5		51.6		—		8,768
2010	—		—		—		51.0		49.5		55.6		56.7		55.9		55.8		55.8		—		10,170
2011	—		—		—		—		58.7		59.8		60.2		60.6		60.8		60.7		.2		11,121
2012	—		—		—		—		—		79.9		86.7		92.9		92.2		92.2		.2		12,405
2013	—		—		—		—		—		—		36.4		27.6		27.6		27.7		.3		5,267
2014	—		—		—		—		—		—		—		37.5		32.1		31.6		.4		1,227
2015	—		—		—		—		—		—		—		—		28.3		25.7		1.1		2,186
2016	—		—		—		—		—		—		—		—				24.5		5.2		1,825
																	Total		$	405.2



Specialty Products - Property
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	1.8	$	3.7	$	6.8	$	7.0	$	7.0	$	7.0	$	8.7	$	8.6	$	8.6	$	8.6
2008	—		15.5		23.5		25.2		25.5		26.0		26.8		26.8		26.8		26.8
2009	—		—		41.5		47.3		51.2		51.9		51.9		51.6		51.6		51.6
2010	—		—		—		43.1		47.9		53.3		55.2		55.7		55.7		55.7
2011	—		—		—		—		51.8		59.0		59.7		60.1		60.4		60.5
2012	—		—		—		—		—		58.8		72.5		78.3		79.0		78.8
2013	—		—		—		—		—		—		24.3		26.9		27.1		27.4
2014	—		—		—		—		—		—		—		19.8		28.2		31.2
2015	—		—		—		—		—		—		—		—		15.0		22.1
2016	—		—		—		—		—		—		—		—		—		12.3
																	Total		375.0
All outstanding liabilities before 2007, net of reinsurance																			—
Liabilities for loss and allocated LAE, net of reinsurance																			$	30.2



Specialty Products - Property
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	70.1%	14.6%	5.9%	1.1%	0.3%	0.2%	0.4%	—%	—%	—%



Specialty Industries - Property
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	110.0	$	104.8	$	101.3	$	94.9	$	95.1	$	95.3	$	95.1	$	95.1	$	95.1	$	94.9	$	—	4,828
2008	—		147.3		137.6		129.6		129.3		132.0		132.7		130.0		129.8		129.7		.1		6,603
2009	—		—		124.5		118.6		118.2		117.9		118.8		118.6		118.7		118.6		.1		6,718
2010	—		—		—		131.4		131.5		127.2		130.0		129.7		131.4		131.2		—		8,617
2011	—		—		—		—		138.5		141.5		141.1		143.5		143.2		142.5		.3		9,704
2012	—		—		—		—		—		126.9		123.8		126.7		130.4		130.1		.8		9,697
2013	—		—		—		—		—		—		125.1		127.0		142.9		143.6		1.9		8,085
2014	—		—		—		—		—		—		—		123.9		123.5		130.1		3.9		8,352
2015	—		—		—		—		—		—		—		—		142.3		138.9		10.2		8,607
2016	—		—		—		—		—		—		—		—		—		120.3		32.8		7,429
																	Total		$	1,279.9



Specialty Industries - Property
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	42.2	$	70.9	$	82.7	$	87.6	$	91.7	$	93.4	$	94.2	$	94.5	$	94.5	$	94.6
2008	—		62.5		101.4		116.8		123.4		129.5		129.1		129.2		129.3		129.3
2009	—		—		50.4		85.9		105.4		111.0		115.5		117.2		118.0		118.4
2010	—		—		—		69.0		106.2		118.2		122.8		125.9		129.3		130.8
2011	—		—		—		—		72.5		117.4		130.7		138.0		139.5		141.4
2012	—		—		—		—		—		64.2		103.1		116.4		128.5		131.9
2013	—		—		—		—		—		—		59.8		101.5		126.1		137.7
2014	—		—		—		—		—		—		—		56.5		94.6		111.6
2015	—		—		—		—		—		—		—		—		68.6		114.4
2016	—		—		—		—		—		—		—		—		—		61.3
																	Total		$	1,171.4
All outstanding liabilities before 2007, net of reinsurance																			—
Liabilities for loss and allocated LAE, net of reinsurance																			$	108.5



Specialty Industries - Property
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	47.4%	27.3%	9.9%	4.1%	1.8%	0.7%	0.2%	0.1%	—%	—%



Specialty Products - Casualty
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	138.3	$	120.2	$	97.3	$	90.7	$	70.5	$	70.1	$	61.8	$	58.3	$	57.1	$	56.5	$	2.5	1,236
2008	—		145.6		128.6		118.2		100.8		98.3		95.5		95.3		97.3		96.8		3.3		2,199
2009	—		—		169.5		186.2		194.9		193.2		192.0		194.9		195.0		195.5		4.9		4,002
2010	—		—		—		211.5		225.4		226.2		225.0		232.3		234.9		240.0		7.8		6,029
2011	—		—		—		—		194.9		189.8		205.4		214.2		214.8		214.9		9.4		6,661
2012	—		—		—		—		—		213.5		220.1		246.1		248.8		249.4		14.4		6,794
2013	—		—		—		—		—		—		205.2		233.1		229.6		247.9		20.3		5,866
2014	—		—		—		—		—		—		—		240.5		235.9		258.9		44.4		6,504
2015	—		—		—		—		—		—		—		—		206.9		213.4		78.5		9,710
2016	—		—		—		—		—		—		—		—		—		202.8		142.8		9,093
																	Total		$	1,976.1



Specialty Products - Casualty
$ in millions
	Cumulative Paid Losses and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	6.8	$	24.8	$	32.4	$	41.7	$	44.4	$	50.7	$	51.9	$	53.6	$	54.0	$	54.0
2008	—		8.8		32.3		50.8		65.4		77.6		83.1		85.3		88.1		91.5
2009	—		—		26.5		81.8		124.0		147.8		161.4		169.9		178.3		187.8
2010	—		—		—		32.0		107.0		157.8		180.5		196.3		213.6		220.3
2011	—		—		—		—		25.8		88.9		131.3		163.7		186.2		192.5
2012	—		—		—		—		—		25.8		86.7		157.7		193.6		219.8
2013	—		—		—		—		—		—		26.4		86.3		143.1		194.2
2014	—		—		—		—		—		—		—		34.3		95.2		156.7
2015	—		—		—		—		—		—		—		—		19.9		78.4
2016	—		—		—		—		—		—		—		—		—		16.5
																	Total		1,411.7
All outstanding liabilities before 2007, net of reinsurance																			4.3
Liabilities for loss and allocated LAE, net of reinsurance																			$	568.7



Specialty Products - Casualty
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	11.3%	24.0%	17.8%	9.6%	4.7%	2.2%	0.9%	0.7%	0.2%	—%



Specialty Industries - Casualty
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	24.9	$	24.3	$	21.7	$	34.0	$	34.9	$	35.1	$	35.1	$	36.0	$	37.1	$	36.9	$	.3	1,770
2008	—		26.3		26.2		30.4		26.4		25.8		25.2		23.7		22.2		22.1		.5		1,835
2009	—		—		51.0		51.1		51.5		49.6		54.8		53.6		51.6		51.0		1.0		2,982
2010	—		—		—		80.8		72.4		69.3		69.4		73.3		75.5		77.2		3.5		4,791
2011	—		—		—		—		88.1		87.5		90.2		100.5		111.8		112.2		3.8		5,254
2012	—		—		—		—		—		118.8		103.4		102.0		101.8		98.8		7.9		6,298
2013	—		—		—		—		—		—		121.9		131.5		133.6		138.2		18.0		7,107
2014	—		—		—		—		—		—		—		135.6		127.4		124.2		35.7		7,633
2015	—		—		—		—		—		—		—		—		146.3		127.7		61.8		7,315
2016	—		—		—		—		—		—		—		—		—		129.9		98.4		5,701
																	Total		$	918.2



Specialty Industries - Casualty
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	2.8	$	5.5	$	7.9	$	13.8	$	27.9	$	29.9	$	32.4	$	32.9	$	34.3	$	35.9
2008	—		3.2		6.9		11.5		14.2		17.3		19.9		21.2		20.8		20.9
2009	—		—		5.1		16.6		25.0		34.5		42.9		46.3		46.3		47.4
2010	—		—		—		8.7		21.1		37.3		51.4		57.5		65.3		68.9
2011	—		—		—		—		11.1		34.2		53.7		69.8		88.9		96.4
2012	—		—		—		—		—		12.9		31.9		50.6		67.8		79.7
2013	—		—		—		—		—		—		18.6		45.5		72.7		96.6
2014	—		—		—		—		—		—		—		16.6		40.2		61.4
2015	—		—		—		—		—		—		—		—		13.6		39.3
2016	—		—		—		—		—		—		—		—		—		11.8
																	Total		$	558.3
All outstanding liabilities before 2007, net of reinsurance																			.1
Liabilities for loss and allocated LAE, net of reinsurance																			$	360.0



Specialty Industries - Casualty
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	11.4%	16.2%	12.9%	9.7%	6.8%	2.5%	0.8%	0.1%	0.2%	0.2%



Specialty Products - Other
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	26.8	$	26.7	$	26.8	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	—	5,073
2008	—		31.7		31.5		31.2		31.2		31.2		31.2		31.2		31.2		31.2		—		5,192
2009	—		—		33.8		31.4		31.3		30.8		30.9		30.9		30.9		30.9		—		5,138
2010	—		—		—		34.7		34.3		32.5		32.6		32.5		32.5		32.5		—		5,182
2011	—		—		—		—		33.2		35.5		35.4		35.4		35.4		35.4		—		5,306
2012	—		—		—		—		—		36.6		36.9		36.9		36.9		37.0		—		5,324
2013	—		—		—		—		—		—		39.6		40.6		39.2		39.2		—		5,510
2014	—		—		—		—		—		—		—		96.6		90.8		92.9		—		9,971
2015	—		—		—		—		—		—		—		—		75.5		73.2		1.8		10,704
2016	—		—		—		—		—		—		—		—		—		77.1		33.8		4,306
																	Total		$	478.0



Specialty Products - Other
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	19.8	$	28.4	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6
2008	—		21.3		31.1		31.2		31.2		31.2		31.2		31.2		31.2		31.2
2009	—		—		21.6		30.8		30.8		30.9		30.9		30.9		30.9		30.9
2010	—		—		—		22.4		32.4		32.5		32.5		32.5		32.5		32.5
2011	—		—		—		—		25.2		35.2		35.4		35.4		35.4		35.4
2012	—		—		—		—		—		26.1		36.8		36.9		36.9		37.0
2013	—		—		—		—		—		—		27.4		39.0		39.2		39.2
2014	—		—		—		—		—		—		—		46.2		88.5		90.3
2015	—		—		—		—		—		—		—		—		41.7		70.6
2016	—		—		—		—		—		—		—		—		—		44.2
																	Total		$	439.9
All outstanding liabilities before 2007, net of reinsurance																			.1
Liabilities for loss and allocated LAE, net of reinsurance																			$	38.2



Specialty Products - Other
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	61.9%	29.5%	0.6%	—%	—%	—%	—%	—%	—%	—%



Specialty Industries - Other
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	4.3	$	4.6	$	4.5	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	—	291
2008	—		10.4		10.8		12.2		11.9		11.9		11.8		12.1		12.1		12.3		—		975
2009	—		—		19.1		19.2		19.0		19.3		20.9		21.4		21.5		21.7		.2		1,763
2010	—		—		—		25.6		26.6		27.1		27.9		28.2		28.4		28.4		—		2,449
2011	—		—		—		—		35.1		36.0		37.7		38.5		38.2		38.2		.2		3,638
2012	—		—		—		—		—		42.0		39.6		40.1		41.1		41.6		.3		3,861
2013	—		—		—		—		—		—		41.1		41.2		42.2		42.5		.7		4,245
2014	—		—		—		—		—		—		—		42.6		40.4		40.6		1.9		3,497
2015	—		—		—		—		—		—		—		—		46.6		31.9		4.9		3,301
2016	—		—		—		—		—		—		—		—		—		33.2		19.4		2,222
																	Total		$	294.8



Specialty Industries - Other
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	1.2	$	4.0	$	4.3	$	4.3	$	4.3	$	4.3	$	4.3	$	4.4	$	4.4	$	4.4
2008	—		3.8		9.0		11.1		11.7		11.8		11.7		12.0		12.0		12.2
2009	—		—		6.3		14.6		17.7		18.3		19.2		21.2		21.4		21.4
2010	—		—		—		8.3		22.9		25.1		26.9		27.6		28.0		28.1
2011	—		—		—		—		13.5		29.4		34.6		36.5		37.2		37.3
2012	—		—		—		—		—		13.2		30.7		35.8		38.3		39.1
2013	—		—		—		—		—		—		13.0		29.3		37.2		39.6
2014	—		—		—		—		—		—		—		12.6		28.9		34.8
2015	—		—		—		—		—		—		—		—		9.8		22.2
2016	—		—		—		—		—		—		—		—		—		8.0
																	Total		$	247.1
All outstanding liabilities before 2007, net of reinsurance																			(.3		)
Liabilities for loss and allocated LAE, net of reinsurance																			$	47.4



Specialty Industries - Other
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	30.4%	37.0%	10.8%	3.3%	1.1%	0.9%	0.2%	—%	0.1%	—%

Third Party Reinsurance

12 Months Ended

Dec. 31, 2016

Reinsurance Disclosures [Abstract]

Third-Party Reinsurance

In the normal course of business, White Mountains’s insurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third-party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. The effects of reinsurance on White Mountains’s insurance subsidiaries’ written and earned premiums and on losses and LAE were as follows (see Note 10 — “Municipal Bond Guarantee Insurance” for balances related to White Mountains financial guarantee business):



	Year ended December 31, 2016
Millions	OneBeacon			HG/BAM ⁽¹⁾		Other			Total
Written premiums:
Direct	$	1,193.3		$	38.6	$	13.3		$	1,245.2
Assumed	28.0			—		—			28.0
Gross written premiums	1,221.3			38.6		13.3			1,273.2
Ceded	(120.6		)	—		(6.8		)	(127.4		)
Net written premiums	$	1,100.7		$	38.6	$	6.5		$	1,145.8
Earned premiums:
Direct	$	1,177.0		$	5.9	$	15.2		$	1,198.1
Assumed	29.4			—		—			29.4
Gross earned premiums	1,206.4			5.9		15.2			1,227.5
Ceded	(105.8		)	—		(7.7		)	(113.5		)
Net earned premiums	$	1,100.6		5.9		$	7.5		$	1,114.0
Losses and LAE:
Direct	$	679.5		$	—	$	14.3		$	693.8
Assumed	21.2			—		—			21.2
Gross losses and LAE	700.7			—		14.3			715.0
Ceded	(44.7		)	—		(6.3		)	(51.0		)
Net losses and LAE	$	656.0		$	—	$	8.0		$	664.0

⁽¹⁾During 2016, BAM ceded $27.2 in written premiums and $21.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.



	Year ended December 31, 2015
Millions	OneBeacon				HG/BAM ⁽¹⁾		Other			Total
Written premiums:
Direct	$	1,279.9			$	25.9	$	19.9		$	1,325.7
Assumed	36.0				—		—			36.0
Gross written premiums	1,315.9				25.9		19.9			1,361.7
Ceded	(179.3		)	⁽²⁾	—		(9.8		)	(189.1		)
Net written premiums	$	1,136.6			$	25.9	$	10.1		$	1,172.6
Earned premiums:
Direct	$	1,298.0			$	3.3	$	20.7		$	1,322.0
Assumed	45.9				—		—			45.9
Gross earned premiums	1,343.9				3.3		20.7			1,367.9
Ceded	(167.7		)	⁽²⁾	—		(12.0		)	(179.7		)
Net earned premiums	$	1,176.2			3.3		$	8.7		$	1,188.2
Losses and LAE:
Direct	$	783.0			$	—	$	19.5		$	802.5
Assumed	55.7				—		—			55.7
Gross losses and LAE	838.7				—		19.5			858.2
Ceded	(138.0		)	⁽²⁾	—		(11.3		)	(149.3		)
Net losses and LAE	$	700.7			$	—	$	8.2		$	708.9

⁽¹⁾During 2015, BAM ceded $19.3 in written premiums and $16.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.

⁽²⁾During 2015, OneBeacon recorded ceded $33.3 in written premiums, $33.3 in earned premiums and $33.4 in loss and LAE as a result of the exit of the Crop Business due to the 100% quota share reinsurance agreement with AmTrust.



	Year ended December 31, 2014
Millions	OneBeacon			HG/BAM ⁽¹⁾		Other⁽²⁾			Total
Written premiums:
Direct	$	1,257.5		$	16.2	$	22.6		$	1,296.3
Assumed	65.9			—		—			65.9
Gross written premiums	1,323.4			16.2		22.6			1,362.2
Ceded	(106.5		)	—		(16.7		)	(123.2		)
Net written premiums	$	1,216.9		$	16.2	$	5.9		$	1,239.0
Earned premiums:
Direct	$	1,209.1		$	1.8	$	22.6		$	1,233.5
Assumed	70.9			—		—			70.9
Gross earned premiums	1,280.0			1.8		22.6			1,304.4
Ceded	(102.9		)	—		(16.5		)	(119.4		)
Net earned premiums	$	1,177.1		$	1.8	$	6.1		$	1,185.0
Losses and LAE:
Direct	$	778.7		$	—	$	24.1		$	802.8
Assumed	115.7			—		—			115.7
Gross losses and LAE	894.4			—		24.1			918.5
Ceded	(79.3		)	—		(15.2		)	(94.5		)
Net losses and LAE	$	815.1		$	—	$	8.9		$	824.0

⁽¹⁾During 2014, BAM ceded $12.3 in written premiums and $1.4 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.

⁽²⁾During 2014, SSIE ceded $16.0 in written premiums, $15.7 in earned premiums, and $16.9 in loss and LAE to OneBeacon, which have been eliminated in consolidation.

OneBeacon

The timing and size of catastrophe losses are unpredictable and the level of losses experienced in any year could be material to OneBeacon’s operating results and financial condition. Examples of catastrophes include losses caused by earthquakes, wildfires, hurricanes and other types of storms and terrorist acts. The extent of losses caused by a catastrophic event is a function of severity and the amount and type of insured exposure in the affected area. In the normal course of business, OneBeacon's insurance subsidiaries seek to limit losses that may arise from catastrophes or other events through individual risk selection, imposing deductibles and limits, limiting its concentration of insurance in catastrophe-prone areas, such as coastal regions, and reinsuring with third-party reinsurers.

OneBeacon uses models (primarily AIR Worldwide Touchstone version 4.1) to estimate potential losses from catastrophes. OneBeacon uses this model output in conjunction with other data to manage its exposure to catastrophe losses based on a probable maximum loss forecast to quantify its exposure to an extreme catastrophe event.

OneBeacon utilizes a general catastrophe reinsurance treaty with third-party reinsurers to manage its exposure to large catastrophe losses. Effective May 1, 2016, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2017. The program provides coverage for OneBeacon’s property business as well as certain acts of terrorism. Under the program, the first $20.0 million of losses resulting from any single catastrophe are retained and 100% of the next $110.0 million of losses resulting from the catastrophe are reinsured. Any part of a catastrophe loss in excess of $130.0 million would be retained in full. In the event of a catastrophe, OneBeacon’s property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium.

OneBeacon’s current third party reinsurance programs provide varying degrees of coverage for terrorism events. The Company's overall terrorism exposure is impacted by the Terrorism Risk Insurance Program (the “Terrorism Act”), which is a federal program administered by the Department of the Treasury that provides for a shared system of public and private compensation for commercial property and casualty losses resulting from events that reach the threshold for losses ($140.0 million in 2017 and increasing $20.0 million in subsequent years until the threshold becomes $200.0 million in 2020) and are certified as an act of terrorism by the U.S. Secretary of the Treasury, in concurrence with the Secretary of Homeland Security and the Attorney General of the United States. The Terrorism Act limits the industry's aggregate liability for losses from certified terrorist acts by requiring the federal government to share a set amount of losses (83% in 2017 and decreasing 1% annually in subsequent years until it reaches a floor of 80% in 2020) once a company meets a specific retention or deductible as determined by its prior year's direct written premiums. It also limits the aggregate liability to be paid by the government and industry without further action by Congress to $100.0 billion. In exchange for this “backstop,” primary insurers are required to make coverage available to commercial insureds for losses from acts of terrorism as specified in the Terrorism Act. The following types of coverage are excluded from the program: commercial automobile, burglary and theft, surety, farmowners multi-peril and all professional liability coverage except directors and officers coverage.

All losses that result from a nuclear, biological, chemical or radiological terrorist attack are excluded from OneBeacon’s current third party reinsurance program. OneBeacon’s property catastrophe treaty also excludes acts of terrorism certified pursuant to the Terrorism Act and committed by an individual or individuals acting on behalf of any foreign person or foreign interest. OneBeacon's casualty clash treaty provides coverage for losses that result from certified and non-certified acts of terrorism, on an aggregated basis, subject to a maximum of one full treaty limit. OneBeacon's property per risk, casualty and workers compensation treaties each provide full coverage for certified acts of terrorism on behalf of a non-foreign person or interest, but are sublimited to one full treaty limit for certified acts of terrorism committed on behalf of any foreign person or foreign interest. OneBeacon’s healthcare treaty is sublimited to one full treaty limit of coverage for all acts of terrorism.

OneBeacon estimates its individual retention level for commercial policies subject to the Terrorism Act to be approximately $140.0 million in 2017. The federal government will pay 83% of covered terrorism losses that exceed OneBeacon’s or the industry’s retention levels in 2017, up to a total of $100.0 billion. As indicated above, OneBeacon’s 17% copay will increase annually beginning in 2017 by 1% until it reaches a limit of 20% in 2020.

In addition to the corporate catastrophe reinsurance protection, OneBeacon also purchases dedicated reinsurance protection for certain lines of business. The following table summarizes the reinsurance coverage currently in effect as of December 31, 2016:



$ in millions
Coverage	Contract Type	Renewal Date	First-Dollar Retention		Per Risk Limit Purchased		Maximum Retention
Corporate Property Catastrophe - Property and Inland Marine	Excess of Loss	5/1	$	20.0	$	110.0	$	20.0
Property Per Risk - Property and Inland Marine	Excess of Loss	5/1	3.0		100.0		3.0
Specialty Property - Excess and Surplus Property Catastrophe	Excess of Loss	5/1	6.0		34.0		6.0
Medical Excess - HMO/Provider Excess	Excess of Loss	1/1	5.0		Unlimited		5.0
Ocean and Inland Marine	Excess of Loss	4/1	2.5		57.5		7.0
Surety	Excess of Loss	10/1	5.0		45.0		5.0
Film Completion Bonds	Excess of Loss	6/1	2.0		38.0		2.0
Casualty Clash/Workers Compensation Catastrophe	Excess of Loss	6/1	6.0		34.0		6.0
Workers Compensation Catastrophe	Excess of Loss	6/1	40.0		20.0		6.0
Financial Institutions - Professional Liability	Quota Share	6/1	N/A		10.0		5.0
Combined Healthcare/Casualty 2nd Layer - Various lines	Excess of Loss	6/1	10.0 / 11.0		10.0		3.0
Casualty Per Policy - Various lines	Excess of Loss	6/1	3.0		8.0		3.0
Workers Compensation Per Occurrence	Excess of Loss	6/1	2.0		8.0		2.0
Healthcare Professional Liability	Excess of Loss	6/1	3.0		7.0		3.0

As of December 31, 2016, OneBeacon had $6.6 million and $172.9 million of reinsurance recoverables on paid and unpaid losses. As reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders, collectability of balances due from reinsurers is important to OneBeacon’s financial strength. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength of its reinsurers on an ongoing basis. Uncollectible amounts historically have not been significant.

The following table summarizes A.M. Best Company, Inc. (“A.M. Best”) ratings for OneBeacon’s reinsurers.



A.M. Best’s Rating⁽¹⁾
$ in millions	Balance at December 31, 2016		% of Total
A+ or better	$	73.9	41	%
A- to A	79.7		44	%
B, Not rated and other⁽²⁾	25.9		15	%
Total	$	179.5	100	%

⁽¹⁾A.M. Best’s ratings as detailed above are “A+ or better” (Superior), “A- to A” (Excellent) and “B” (Fair).

⁽²⁾ Includes reinsurance recoverable on unpaid losses of $18.3 million related to OBIC, an unrated entity sold to Armour as part of the sale of the Runoff Business.

- Definition

The entire disclosure pertaining to the existence, magnitude and information about insurance that has been ceded to or assumed from another insurance company, including the methodologies and assumptions used in determining recorded amounts.

+ References + Details

Investment Securities

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Investment Securities

White Mountains’s invested assets consist of investment securities and other long-term investments held for general investment purposes. The portfolio of investment securities includes fixed maturity investments, short-term investments, common equity securities, and other-long term investments which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Net realized and unrealized investment gains (losses) on trading securities are reported in pre-tax revenues.

White Mountains’s fixed maturity investments are generally valued using industry standard pricing methodologies. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life.

Realized investment gains (losses) resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of December 31, 2016 and December 31, 2015.

Other long-term investments consist primarily of hedge funds, private equity funds, non-controlling interests in private capital investments and the OneBeacon Surplus Notes.

Net Investment Income

White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments and dividend income from its common equity securities and other long-term investments.

Pre-tax net investment income for 2016, 2015 and 2014 consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Investment income:
Fixed maturity investments	$	77.3		$	52.6		$	51.3
Short-term investments	1.1			.2			.1
Common equity securities	7.5			10.1			16.6
Other long-term investments	4.0			3.3			4.4
Total investment income	89.9			66.2			72.4
Third-party investment expenses	(3.1		)	(5.4		)	(12.9		)
Net investment income, pre-tax	$	86.8		$	60.8		$	59.5

Net Realized and Unrealized Investment Gains (Losses)

Net realized and unrealized investment gains (losses) consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Net realized investment gains, pre-tax	$	280.3		$	77.3		$	166.8
Net unrealized investment (losses) gains, pre-tax	(270.0		)	148.1			(88.3		)
Net realized and unrealized investment gains, pre-tax	10.3			225.4			78.5
Income tax expense attributable to net realized and unrealized investment gains	(8.8		)	(35.2		)	(18.0		)
Net realized and unrealized investment gains, after tax	$	1.5		$	190.2		$	60.5

Net realized investment gains

Net realized investment gains for 2016, 2015 and 2014 consisted of the following:



	Year ended December 31, 2016
Millions	Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(2.1	)	$	.4	$	(1.7	)
Short-term investments	.4			—		.4
Common equity securities	280.7			—		280.7
Other long-term investments	.9			—		.9
Net realized investment gains, pre-tax	279.9			.4		280.3
Income tax expense attributable to net realized investment gains	(49.5		)	—		(49.5		)
Net realized investment gains, after-tax	$	230.4		$	.4	$	230.8



	Year ended December 31, 2015
Millions	Net realized gains			Net foreign exchange gains		Total net realized gains reflected in earnings
Fixed maturity investments	$	1.9		$	—	$	1.9
Common equity securities	64.4			.4		64.8
Other long-term investments	10.6			—		10.6
Net realized investment gains, pre-tax	76.9			.4		77.3
Income tax expense attributable to net realized investment gains	(22.8		)	—		(22.8		)
Net realized investment gains, after-tax	$	54.1		$	.4	$	54.5



	Year ended December 31, 2014
Millions	Net realized gains			Net foreign exchange gains		Total net realized gains reflected in earnings
Fixed maturity investments	$	5.8		$	—	$	5.8
Common equity securities	138.0			—		138.0
Other long-term investments	23.0			—		23.0
Net realized investment gains, pre-tax	166.8			—		166.8
Income tax expense attributable to net realized investment gains	(28.4		)	—		(28.4		)
Net realized investment gains, after-tax	$	138.4		$	—	$	138.4

Net unrealized investment gains (losses)

The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value:



	Year ended December 31, 2016
Millions	Net unrealized losses			Net foreign exchange gains (losses)			Total net unrealized losses reflected in earnings
Fixed maturity investments	$	(12.1	)	$	2.1		$	(10.0	)
Common equity securities	(254.6		)	(3.3		)	(257.9		)
Other long-term investments	(.6		)	(.3		)	(.9		)
Forward contracts	—			(1.2		)	(1.2		)
Net unrealized investment losses, pre-tax	(267.3		)	(2.7		)	(270.0		)
Income tax benefit attributable to net unrealized investment losses	40.7			—			40.7
Net unrealized investment losses, after-tax	$	(226.6	)	$	(2.7	)	$	(229.3	)



	Year ended December 31, 2015
Millions	Net unrealized (losses) gains			Net foreign exchange losses			Total net unrealized (losses) gains reflected in earnings
Fixed maturity investments	$	(15.6	)	$	—		$	(15.6	)
Common equity securities	207.6			(3.7		)	203.9
Other long-term investments	(39.1		)	(1.1		)	(40.2		)
Net unrealized investment gains (losses), pre-tax	152.9			(4.8		)	148.1
Income tax expense attributable to net unrealized investment gains (losses)	(12.3		)	(.1		)	(12.4		)
Net unrealized investment gains (losses), after-tax	$	140.6		$	(4.9	)	$	135.7



	Year ended December 31, 2014
Millions	Net unrealized gains (losses)			Net foreign exchange losses			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	11.3		$	—		$	11.3
Common equity securities	(83.0		)	(7.7		)	(90.7		)
Other long-term investments	(7.6		)	(1.3		)	(8.9		)
Net unrealized investment losses, pre-tax	(79.3		)	(9.0		)	(88.3		)
Income tax benefit attributable to net unrealized investment losses	9.9			.5			10.4
Net unrealized investment losses, after-tax	$	(69.4	)	$	(8.5	)	$	(77.9	)

White Mountains recognized gross realized investment gains of $315.5 million, $112.9 million and $194.0 million and gross realized investment losses of $35.2 million, $35.6 million and $27.2 million on sales of investment securities during 2016, 2015 and 2014.

The following table summarizes the amount of total gains (losses) included in earnings attributable to net unrealized investment gains (losses) for Level 3 investments for the years ended December 31, 2016, 2015 and 2014.



	Year Ended December 31,
Millions	2016		2015			2014
Fixed maturity investments	$	.1	$	(1.1	)	$	1.9
Common equity securities	—		(9.0		)	5.8
Other long-term investments	6.1		(13.0		)	—
Total net unrealized investment gains (losses), pre-tax - Level 3 investments	$	6.2	$	(23.1	)	$	7.7

The components of White Mountains’s net realized and unrealized investment gains (losses), after-tax, as recorded on the statements of operations and comprehensive income were as follows:



	Year Ended December 31,
Millions	2016		2015			2014
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$	—	$	(39.2	)	$	81.2
Income tax benefit (expense)	—		2.9			(5.9		)
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	—		(36.3		)	75.3
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	—		1.4			—
Total investment (losses) gains through accumulated other comprehensive income	—		(34.9		)	75.3
Net realized and unrealized investment gains, after-tax	1.5		190.2			60.5
Total investment gains recorded during the period, after-tax	$	1.5	$	155.3		$	135.8

Investment Holdings

The cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains, and carrying values of White Mountains’s fixed maturity investments as of December 31, 2016 and 2015, were as follows:



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
US Government and agency obligations	$	281.7	$	.1	$	(3.5	)	$	—	$	278.3
Debt securities issued by corporations	1,512.6		8.4		(13.7		)	2.1		1,509.4
Municipal obligations	308.8		1.9		(1.7		)	—		309.0
Mortgage and asset-backed securities	2,141.7		2.6		(11.4		)	—		2,132.9
Foreign government, agency and provincial obligations	12.9		.3		—			—		13.2
Preferred stocks	8.3		5.7		—			—		14.0
Total fixed maturity investments	4,266.0		19.0		(30.3		)	2.1		4,256.8
Fixed maturity investments reclassified to assets held for sale related to SSIE										(6.6		)
Total fixed maturity investments										$	4,250.2



	December 31, 2015
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains (losses)		Carrying value
US Government and agency obligations	$	160.4	$	—	$	(.4	)	$	—	$	160.0
Debt securities issued by corporations	1,001.0		4.3		(5.3		)	—		1,000.0
Municipal obligations	227.8		2.2		(1.2		)	—		228.8
Mortgage and asset-backed securities	1,170.6		2.0		(5.6		)	—		1,167.0
Foreign government, agency and provincial obligations	1.0		.2		—			—		1.2
Preferred stocks	78.3		4.4		—			—		82.7
Total fixed maturity investments	2,639.1		13.1		(12.5		)	$	—	2,639.7
Fixed maturity investments reclassified to assets held for sale related to SSIE										(9.5		)
Total fixed maturity investments										$	2,630.2

The weighted average duration of White Mountains’s fixed income portfolio was approximately 2.8 years, including short-term investments, and approximately 3.0 years excluding short-term investments as of December 31, 2016.

The cost or amortized cost and carrying value of White Mountains’s fixed maturity investments as of December 31, 2016 is presented below by contractual maturity. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties.



	December 31, 2016
Millions	Cost or amortized cost		Carrying value
Due in one year or less	$	212.8	$	213.3
Due after one year through five years	1,272.6		1,270.2
Due after five years through ten years	462.8		458.4
Due after ten years	167.8		168.0
Mortgage and asset-backed securities	2,141.7		2,132.9
Preferred stocks	8.3		14.0
Total	$	4,266.0	$	4,256.8

⁽¹⁾Includes carrying value of $6.6 in fixed maturity investments that are classified as assets held for sale related to SSIE.

The cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of White Mountains’s common equity securities and other long-term investments as of December 31, 2016 and 2015 were as follows:



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	440.8	$	35.9	$	(2.4	)	$	—		$	474.3
Other long-term investments	$	314.9	$	40.3	$	(28.0	)	$	(3.9	)	$	323.3



	December 31, 2015
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	822.5	$	302.8	$	(11.4	)	$	—		$	1,113.9
Other long-term investments	$	304.5	$	32.0	$	(18.4	)	$	(2.3	)	$	315.8

Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $5.4 billion, $2.2 billion and $3.3 billion for the years ended December 31, 2016, 2015 and 2014.

Investments Held on Deposit or as Collateral

As of December 31, 2016 and 2015, investments of $165.9 million and $143.6 million, were held in trusts required to be maintained in relation to various reinsurance agreements. White Mountains’s insurance subsidiaries are required to maintain deposits with certain insurance regulatory agencies in order to maintain their insurance licenses. The fair value of such deposits which are included within total investments totaled $93.5 million and $88.0 million as of December 31, 2016 and 2015.

As of December 31, 2016 and 2015, OneBeacon held unrestricted collateral from its customers, which is included in cash and invested assets, relating to its surety business of $153.0 million and $137.7 million. The obligation to return these funds is included in funds held under insurance and reinsurance contracts in the consolidated balance sheets.

Fair value measurements as of December 31, 2016

Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity's internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets or liabilities have the highest priority (“Level 1”), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (“Level 2”) and unobservable inputs, including the reporting entity's estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”).

As of December 31, 2016 and December 31, 2015 White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 94% and 91% of its investment portfolio. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, short-term investments, which include U.S. Treasury Bills and common equity securities. Investments valued using Level 2 inputs are primarily comprised of fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government, agency and provincial obligations and preferred stocks. Investments valued using Level 2 inputs also include certain passive exchange traded funds (“ETFs”) that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges and that management values using the fund manager’s published NAV to account for the difference in market close times. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include White Mountains’s investments in the OneBeacon Surplus Notes, as well as certain investments in fixed maturity investments, common equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Transfers between levels are based on investments held as of the beginning of the period.

White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention.

White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control processes and procedures on at least an annual basis, a comparison of its invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these assessment procedures are considered outliers. Also considered outliers are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question.

The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follow:

Debt securities issued by corporations: The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications.

Mortgage and asset-backed securities: The fair value of mortgage and asset-backed securities is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications.

Municipal obligations: The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications.

Foreign government, agency and provincial obligations: The fair value of foreign government, agency and provincial obligations is determined from a pricing evaluation technique that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications.

Preferred stocks: The fair value of preferred stocks is determined from a pricing evaluation technique that calculates the appropriate spread over a comparable security for each issue. Key inputs include exchange prices (underlying and common stock of same issuer), benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications.

Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2.

White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for its other long-term investments, including obtaining and reviewing periodic and audited annual financial statements of hedge funds and private equity funds and discussing each fund’s pricing with the fund manager throughout the year. However, since the fund managers do not provide sufficient information to evaluate the pricing inputs and methods for each underlying investment, the inputs are considered to be unobservable. The fair value of White Mountains’s investments in hedge funds and private equity funds has generally been determined using the fund manager’s NAV. In the event White Mountains believes that its estimate of NAV of a hedge fund or private equity fund differs from that reported by the fund manager due to illiquidity or other factors, White Mountains will adjust the reported NAV to more appropriately represent the fair value of its interest in the hedge fund or private equity fund investment. As of December 31, 2016 and 2015, White Mountains recorded negative adjustments of $5.0 million and $2.4 million to the reported NAV of certain investments in private equity funds.

Fair Value Measurements by Level

The following tables summarize White Mountains’s fair value measurements for investments as of December 31, 2016 and 2015 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage and asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and common equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg Barclays U.S. Intermediate Aggregate and S&P 500 indices. The fair value measurements for derivative assets associated with White Mountains’s variable annuity business are presented in Note 9.



	December 31, 2016
Millions	Fair value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	278.3	$	268.8	$	9.5	$	—
Debt securities issued by corporations:
Consumer	385.6		—		385.6		—
Health Care	244.2		—		244.2		—
Utilities	180.3		—		180.3		—
Financials	176.0		—		176.0		—
Industrial	146.4		—		146.4		—
Communications	131.4		—		131.4		—
Materials	102.6		—		102.6		—
Technology	89.4		—		89.4		—
Energy	53.5		—		53.5		—
Total debt securities issued by corporations:	1,509.4		—		1,509.4		—

Mortgage and asset-backed securities	2,132.9		—		2,132.9		—
Municipal obligations	309.0		—		309.0		—
Foreign government, agency and provincial obligations	13.2		.6		12.6		—
Preferred stocks	14.0		—		14.0		—
Total fixed maturity investments ⁽⁴⁾	4,256.8		269.4		3,987.4		—

Short-term investments ⁽⁴⁾⁽⁵⁾	287.1		274.4		12.7		—

Common equity securities:

Exchange traded funds ⁽¹⁾	321.6		270.4		51.2		—
Health Care	20.9		20.9		—		—
Consumer	12.9		12.9		—		—
Financials	11.6		11.6		—		—
Technology	11.0		11.0		—		—
Communications	10.5		10.5		—		—
Energy	3.7		3.7		—		—
Industrial	2.2		2.2		—		—
Other	79.9		—		79.9		—
Total common equity securities	474.3		343.2		131.1		—
Other long-term investments ⁽²⁾⁽³⁾	177.7		—		—		177.7
Total investments ^(2)(3)(4)	$	5,195.9	$	887.0	$	4,131.2	$	177.7

⁽¹⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽²⁾ Excludes carrying value of $3.5 associated with other long-term investment limited partnerships accounted for using the equity method and $(1.2) related to foreign currency forward contracts. Excludes carrying value of $12.3 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

⁽³⁾Excludes carrying value of $131.0 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Includes carrying value of $6.6 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.

⁽⁵⁾Short-term investments are measured at amortized cost, which approximates fair value.



	December 31, 2015
Millions	Fair value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	160.0	$	133.4	$	26.6	$	—
Debt securities issued by corporations:
Consumer	253.3		—		253.3		—
Financials	175.9		—		175.9		—
Health Care	151.3		—		151.3		—
Industrial	135.6		—		135.6		—
Energy	82.0		—		82.0		—
Utilities	61.5		—		61.5		—
Technology	60.0		—		60.0		—
Communications	49.2		—		49.2		—
Materials	31.2		—		31.2		—
Total debt securities issued by corporations:	1,000.0		—		1,000.0		—

Mortgage and asset-backed securities	1,167.0		—		1,167.0		—
Municipal obligations	228.8		—		228.8		—
Foreign government, agency and provincial obligations	1.2		.6		.6		—
Preferred stocks	82.7		—		12.7		70.0
Total fixed maturity investments ⁽⁴⁾	2,639.7		134.0		2,435.7		70.0

Short-term investments ⁽⁴⁾⁽⁵⁾	211.3		211.3		—		—

Common equity securities:
Financials	653.2		653.2		—		—
Exchange traded funds⁽¹⁾	183.3		162.0		21.3		—
Consumer	70.0		70.0		—		—
Communications	43.7		43.7		—		—
Health Care	35.7		35.7		—		—
Technology	27.0		27.0		—		—
Industrial	26.6		26.6		—		—
Other	74.4		—		74.4		—
Total common equity securities	1,113.9		1,018.2		95.7		—
Other long-term investments ⁽²⁾⁽³⁾	169.5		—		—		169.5
Total investments ^(2)(3)(4)	$	4,134.4	$	1,363.5	$	2,531.4	$	239.5

⁽¹⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽²⁾Excludes carrying value of $3.8 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $14.7 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

⁽³⁾Excludes carrying value of $127.8 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Includes carrying value of $9.5 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.

⁽⁵⁾Short-term investments are measured at amortized cost, which approximates fair value.

Debt securities issued by corporations

The following table summarizes the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2016 and 2015:



	Fair Value at December 31,
Millions	2016		2015
AA	$	100.9	$	95.2
A	381.9		397.7
BBB	786.5		507.1
BB	214.0		—
B	26.1		—
Debt securities issued by corporations⁽¹⁾	$	1,509.4	$	1,000.0


⁽¹⁾	Credit ratings are assigned based on the following hierarchy: 1) Standard & Poors Financial Services LLC (“Standard & Poor’s”) and 2) Moody’s Investor Services (“Moody’s”)

Mortgage and Asset-backed Securities

White Mountains purchases commercial mortgage-backed securities (“CMBS”) and residential mortgage-backed securities (“RMBS”) with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelf registrations or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics).

White Mountains categorizes mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. White Mountains’s non-agency RMBS portfolio is generally moderate-term and structurally senior. White Mountains does not own any collateralized loan obligations. White Mountains does not own any collateralized debt obligations, with the exception of $30.2 million of non-agency residential mortgage resecuritization tranches, each a senior tranche in its own right and each collateralized by a single earlier vintage Super Senior or Senior non-agency RMBS.

The following table summarizes the carrying value of White Mountains’s mortgage and asset-backed securities as of December 31, 2016 and December 31, 2015:



	December 31, 2016						December 31, 2015
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	283.9	$	283.9	$	—	$	265.5	$	265.5	$	—
FNMA	278.3		278.3		—		42.2		42.2		—
FHLMC	89.8		89.8		—		22.8		22.8		—
Total Agency ⁽¹⁾	652.0		652.0		—		330.5		330.5		—
Non-agency:
Residential	205.3		205.3		—		133.2		133.2		—
Commercial	127.5		127.5		—		140.4		140.4		—
Total Non-agency	332.8		332.8		—		273.6		273.6		—

Total mortgage-backed securities	984.8		984.8		—		604.1		604.1		—
Other asset-backed securities:
Vehicle receivables	479.5		479.5		—		269.7		269.7		—
Credit card receivables	438.3		438.3		—		217.7		217.7		—
Other	230.3		230.3		—		75.5		75.5		—
Total other asset-backed securities	1,148.1		1,148.1		—		562.9		562.9		—
Total mortgage and asset-backed securities	$	2,132.9	$	2,132.9	$	—	$	1,167.0	$	1,167.0	$	—


⁽¹⁾	Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).

Non-agency Mortgage-backed Securities

The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2016 are as follows:



							Security Issuance Year
Millions	Fair Value		2004		2005		2006		2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
Non-agency RMBS	$	205.3	$	19.4	$	5.7	$	3.0	$	—	$	2.7	$	—	$	7.4	$	9.9	$	5.0	$	15.3	$	53.3	$	48.3	$	35.3
Non-agency CMBS	127.5		—		—		—		—		—		—		4.3		—		18.1		11.5		23.4		44.4		$	25.8
Total	$	332.8	$	19.4	$	5.7	$	3.0	$	—	$	2.7	$	—	$	11.7	$	9.9	$	23.1	$	26.8	$	76.7	$	92.7	$	61.1

Non-agency Residential Mortgage-backed Securities

The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of December 31, 2016:



Millions	Fair Value		Super Senior⁽¹⁾		Senior⁽²⁾		Subordinate⁽³⁾
Prime	$	205.3	$	150.6	$	54.7	$	—
Non-prime	—		—		—		—
Sub-prime	—		—		—		—
Total non-agency RMBS	$	205.3	$	150.6	$	54.7	$	—


⁽¹⁾	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings (“Fitch”) and were senior to other “AAA” or “Aaa” bonds.


⁽²⁾	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds.


⁽³⁾	At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.

Non-agency Commercial Mortgage-backed Securities

White Mountains’s non-agency CMBS portfolio is generally short-term and structurally senior, with more than 25 points of seniority on average for both fixed rate CMBS and floating rate CMBS as of December 31, 2016. In general, seniority represents the percentage principal loss on the underlying collateral that would be absorbed by other securities lower in the capital structure before the more senior security incurs a loss. As of December 31, 2016, none of the underlying loans of the non-agency CMBS held by White Mountains were reported as non-performing.

The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of December 31, 2016:



Millions	Fair Value		Super Senior⁽¹⁾		Senior⁽²⁾		Subordinate⁽³⁾
Fixed rate CMBS	$	116.0	$	8.7	$	59.1	$	48.2
Floating rate CMBS	11.5		—		—		11.5
Total non-agency CMBS	$	127.5	$	8.7	$	59.1	$	59.7


⁽¹⁾	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to other “AAA” or “Aaa” bonds.


⁽²⁾	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds.


⁽³⁾	At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.

Other Long-term Investments

Other long-term investments consist of the following as of December 31, 2016 and 2015:



	Carrying Value at
Millions	December 31, 2016		December 31, 2015
Hedge funds and private equity funds, at fair value ⁽¹⁾	$	131.0	$	127.8
Private equity securities and limited liability companies, at fair value ⁽¹⁾	72.0		82.1
OneBeacon Surplus Notes, at fair value⁽¹⁾	71.9		51.5
Private convertible preferred securities, at fair value ⁽¹⁾	30.6		32.7
Tax advantaged federal affordable housing development fund ⁽²⁾	12.3		14.7
Partnership investments accounted for under the equity method	3.5		3.8
Other	2.0		3.2
Total other-long term investments	$	323.3	$	315.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾Fund accounted for using the proportional amortization method.

Hedge Funds and Private Equity Funds

White Mountains holds investments in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments has generally been estimated using the NAV of the funds. As of December 31, 2016, White Mountains held investments in 5 hedge funds and 23 private equity funds. The largest investment in a single fund was $36.5 million as of December 31, 2016 and $22.7 million as of December 31, 2015. The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2016 and 2015:



	December 31, 2016				December 31, 2015
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	36.5	$	—	$	12.8	$	—
Long/short equity REIT	19.9		—		20.6		—
Other	3.4		—		3.6		—
Total hedge funds	59.8		—		37.0		—

Private equity funds
Aerospace/Defense/Government	19.4		25.9		19.8		30.3
Manufacturing/Industrial	15.9		22.4		24.9		2.5
Energy infrastructure & services	14.1		3.2		20.7		3.4
Multi-sector	11.4		2.0		14.8		2.1
Healthcare	3.5		.4		3.8		.4
Private equity secondaries	3.0		2.1		4.4		2.1
Direct lending/Mezzanine debt	1.8		35.7		—		—
Financial Services	1.0		5.0		—		—
Insurance	.8		41.3		2.0		41.3
Real estate	.3		.1		.4		.1
Total private equity funds	71.2		138.1		90.8		82.2
Total hedge and private equity funds included in other long-term investments	$	131.0	$	138.1	$	127.8	$	82.2

Redemption of investments in certain hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. As of December 31, 2016, one hedge fund with a fair value of $21.5 million was subject to a lock-up period that expires on September 1, 2018. The following summarizes the December 31, 2016 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:



Millions	Notice Period
Redemption frequency	30-59 days notice		60-89 days notice		90-119 days notice		Total
Monthly	$	—	$	—	$	—	$	—
Quarterly	15.9		—		—		15.9
Semi-annual	21.6		19.9		—		41.5
Annual	—		—		2.4		2.4
Total	$	37.5	$	19.9	$	2.4	$	59.8

Certain of White Mountains’s investments in hedge funds are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund’s underlying investments are liquidated. As of December 31, 2016, the fair value of hedge funds in liquidation was $1.0 million. The actual amount of the final distribution remittances remain subject to market fluctuations. The date at which such remittances will be received is not determinable as of December 31, 2016.

White Mountains has also submitted redemption requests for certain of its investments in active hedge funds. As of December 31, 2016, redemptions of $2.4 million are outstanding that would be subject to market fluctuations. The date at which such redemptions will be received is not determinable as of December 31, 2016. Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations.

Investments in private equity funds are generally subject to a lock-up period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds provide an option to extend the lock-up period at either, the sole discretion of the fund manager or upon agreement between the fund and the investors.

As of December 31, 2016, investments in private equity funds were subject to lock-up periods as follows:



Millions	1-3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private Equity Funds — expected lock-up period remaining	$	14.8	$	3.5	$	50.0	$	2.9	$	71.2

OneBeacon Surplus Notes

In the fourth quarter of 2014, in conjunction with OneBeacon's sale of its runoff business to an affiliate of Armour Group Holdings Limited (the “OneBeacon Runoff Transaction”), OneBeacon provided financing in the form of the OneBeacon Surplus Notes with a par value of $101.0 million, which had a fair value of $71.9 million and $51.5 million as of December 31, 2016 and December 31, 2015. Subsequent to closing, the OneBeacon Surplus Notes are included in OneBeacon’s investment portfolio, classified within other long-term investments.

The internal valuation model used to estimate the fair value is based on a discounted expected cash flow model using information as of the measurement date. The estimated fair value of the OneBeacon Surplus Notes is sensitive to changes in public debt credit spreads, as well as changes in estimates with respect to other variables including a discount to reflect the private nature of the notes (and the related lack of liquidity), the credit quality of the notes, based on the financial performance of the Issuer relative to expectations, and the timing, amount, and likelihood of interest and principal payments on the notes, which are subject to regulatory approval and therefore may vary from the contractual terms. Anticipating a change in the assumed timeline, amounts, and process necessary to obtain regulatory approval, OneBeacon made a change in estimate as of December 31, 2016 with regard to the timing of regulatory approval of principal and interest payments on the notes. For the purpose of estimating fair value at December 31, 2016, OneBeacon assumed that all accrued but unpaid interest on the seller priority note since the date of issuance is paid in 2020, with regular annual interest payments on both the seller priority note and pari passu note beginning thereafter, all accrued but unpaid interest on the pari passu note since the date of issuance is paid in 2025, and principal repayments begin on a graduated basis in 2030 for the seller priority note and 2035 for the pari passu note. Previously, including as of December 31, 2015, OneBeacon had assumed for the purpose of estimating fair value that interest payouts, including all accrued but unpaid amounts, began in 2020 for both notes and that principal repayments began on a graduated basis in 2025 for the seller priority note and 2030 for the pari passu note. Although these variables involve considerable judgment, OneBeacon does not currently expect any resulting changes in the estimated value of the OneBeacon Surplus Notes to be material to its financial position.

Below is a table illustrating the valuation adjustments taken to arrive at the estimated fair value of the OneBeacon Surplus Notes as of December 31, 2016 and 2015:



	Type of Surplus Note						Total as of December 31, 2016			Total as of December 31, 2015
Millions	Seller Priority			Pari Passu			Total as of December 31, 2016			Total as of December 31, 2015
Par Value	$	57.9		$	43.1		$	101.0		$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments⁽¹⁾	6.2			(1.1		)	5.1			(15.1		)
Regulatory approval ⁽²⁾	(.2		)	(15.4		)	(15.6		)	(24.2		)
Liquidity adjustment ⁽³⁾	(12.8		)	(5.8		)	(18.6		)	(10.2		)
Total adjustments	(6.8		)	(22.3		)	(29.1		)	(49.5		)
Fair value⁽⁴⁾	$	51.1		$	20.8		$	71.9		$	51.5

⁽¹⁾Represents the value of the surplus notes, at current market yields on comparable publicly traded debt, and assuming issuer is allowed to make principal and interest payments when its financial capacity is available, as measured by statutory capital in excess of a 250% RBC score under the NAIC’s risk-based capital standards for property and casualty companies. The favorable year-over-year change in impact is due principally to the narrowing of non-investment grade credit spreads as well as the time value of money benefit from moving one year closer to modeled cash receipts.

⁽²⁾Represents anticipated delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer's statutory surplus. The monetary impact of the anticipated delay is measured based on credit spreads of public securities with roughly equivalent percentages of discounted payments missed. The favorable year-over-year change in impact is driven primarily by the narrowing of non-investment grade credit spreads, which causes the anticipated delay in securing regulatory approval to be less punitive, partially offset by the change in estimates and assumptions regarding the timeline, amounts, and process necessary to obtain regulatory approval for principal and interest payments described below.

⁽³⁾Represents impact of liquidity spread to account for OneBeacon’s sole ownership of the OneBeacon Surplus Notes, lack of a trading market and unique nature of the ongoing regulatory approval process. The unfavorable year-over-year change in impact is due largely to the increased fair value of the notes as well as a higher effective duration resulting from a lower overall yield to maturity; there was no change in the magnitude of the liquidity spread in 2016.

⁽⁴⁾The increase in the fair value of the OneBeacon Surplus Notes during the year ended December 31, 2016 was driven primarily by the narrowing of non-investment grade credit spreads, partially offset by the impact of a change in estimates and assumptions regarding the timing of regulatory approval of principal and interest payments on the notes.

Rollforward of Fair Value Measurements by Level

White Mountains uses quoted market prices where available as the inputs to estimate fair value for its investments in active markets. Such measurements are considered to be either Level 1 or Level 2 measurements, depending on whether the quoted market price inputs are for identical securities (Level 1) or similar securities (Level 2). Level 3 measurements for fixed maturity investments, common equity securities and other long-term investments as of December 31, 2016 and 2015 consist of securities for which the estimated fair value has not been determined based upon quoted market price inputs for identical or similar securities.

The following tables summarize the changes in White Mountains’s fair value measurements by level for the years ended December 31, 2016 and 2015:



							Level 3 Investments
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Common equity securities		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2016	$	1,152.2		$	2,531.4		$	70.0		$	—	$	169.5		$	127.8		$	4,050.9		^(1)(2)(4)
Total net realized and unrealized investment gains (losses)	11.3			.2			.1			—		6.1			(6.1		)	11.6
Amortization/Accretion	.1			(19.4		)	—			—		—			—			(19.3		)
Purchases	2,243.7			3,979.8			120.8			—		2.2			35.2			6,381.7
Sales	(2,794.8		)	(2,491.5		)	(72.9		)	—		(.1		)	(25.9		)	(5,385.2		)
Transfers in	—			118.0			—			—		—			—			118.0
Transfers out	—			—			(118.0		)	—		—			—			(118.0		)
Balance at December 31, 2016	$	612.5		$	4,118.5		$	—		$	—	$	177.7		$	131.0		$	5,039.7		^(1)(2)(4)

⁽¹⁾

Excludes carrying value of $3.5 and $3.8 as of December 31, 2016 and January 1, 2016 associated with other long-term investments accounted for using the equity method and $(1.2) related to foreign currency forward contracts. Excludes carrying value of $12.3 and $14.7 at December 31, 2016 and January 1, 2016 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.


⁽²⁾	Excludes carrying value of $287.1 and $211.3 as of December 31, 2016 and January 1, 2016 classified as short-term investments, of which $0.1 and $0.1 is classified as held for sale at December 31, 2016 and January 1, 2016.


⁽³⁾	Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Summary of Significant Accounting Policies”.


⁽⁴⁾	Includes carrying value of $6.6 and $9.5 of fixed maturity investments at December 31, 2016 and January 1, 2016 that is classified as assets held for sale related to SSIE.



							Level 3 Investments
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Common equity securities			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾				Total
Balance at January 1, 2015	$	550.6		$	2,372.9		$	76.4		$	39.5		$	125.9		$	178.3		⁽¹⁾	$	3,343.6		⁽¹⁾⁽²⁾
Total net realized and unrealized investment gains (losses)	263.0			(13.7		)	(1.1		)	7.8			(20.0		)	(9.2		)		226.8			⁽³⁾⁽⁴⁾
Amortization/Accretion	—			(19.6		)	—			—			—			—				(19.6		)
Purchases	814.5			1,436.0			35.3			—			76.5			14.8				2,377.1
Sales	(825.7		)	(1,286.0		)	—			(43.7		)	(12.9		)	(41.8		)		(2,210.1		)
Symetra transfer	394.5			—			—			—			—			—				394.5
Effect of redemption of Prospector hedge funds	(43.5		)	—			—			(3.6		)	—			(14.3		)		(61.4		)
Transfers in	—			41.8			—			—			—			—				41.8
Transfers out	(1.2		)	—			(40.6		)	—			—			—				(41.8		)
Balance at December 31, 2015	$	1,152.2		$	2,531.4		$	70.0		$	—		$	169.5		$	127.8		⁽¹⁾	$	4,050.9		⁽¹⁾⁽²⁾

⁽¹⁾

Excludes carrying value of $3.8 and $5.2 as of December 31, 2015 and January 1, 2015 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $14.7 and $16.8 at December 31, 2015 and January 1, 2015 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.


⁽²⁾	Excludes carrying value of $211.3 and $376.8 as of December 31, 2015 and January 1, 2015 classified as short-term investments.


⁽³⁾	Excludes $0.8 of net realized and unrealized investment losses associated with the Prospector Funds and consolidation of investment-related liabilities.


⁽⁴⁾	Includes unrealized investment gains of $258.8 associated with the Symetra transfer from investments in unconsolidated affiliates to common equity securities.

⁽⁵⁾ Includes carrying value of $10.1 and $9.5 of fixed maturity investments at January 1, 2015 and December 31, 2015 that is classified as assets held for sale related to SSIE.

Fair Value Measurements — transfers between levels

Transfers between levels are recorded using the fair value measurement as of the end of the quarterly period in which the event or change in circumstance giving rise to the transfer occurred.

During 2016, there were three fixed maturity investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements because quoted market prices for similar securities that were considered reliable and could be validated against an alternative source were available at December 31, 2016. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $118.0 million for the period ended December 31, 2016.

During 2015, there were six fixed maturity investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements because quoted market prices for similar securities that were considered reliable and could be validated against an alternative source were available as of December 31, 2015. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $40.6 million for the period ended December 31, 2015.

Significant Unobservable Inputs

The following summarizes significant unobservable inputs used in estimating the fair value of investment securities, other than hedge funds and private equity funds, classified within Level 3 as of December 31, 2016 and December 31, 2015. The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds.



Description	December 31, 2016
$ in millions, except share price	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Discounted cash flow	$22.1	Discount rate	-	25.0%
Private equity security	Share price of most recent transaction	$3.2	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$3.6	EBITDA multiple	-	6.00
Private convertible preferred security	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community reinvestment vehicle	Member share of GAAP net equity	$14.3	GAAP net equity		$14.3
Private equity security	Discounted cash flow/ Option pricing method	$9.3	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.00%
OneBeacon Surplus Notes:
- Seller priority	Discounted cash flow	$51.1	Discount rate ⁽²⁾	-	9.6%
		Timing of interest payments ⁽⁴⁾		-	2020
		Timing of principal payments ⁽⁴⁾		-	2030
- Pari passu	Discounted cash flow	$20.8	Discount rate ⁽³⁾	-	15.0%
		Timing of interest payments ⁽⁵⁾		-	2021
		Timing of principal payments ⁽⁵⁾		-	2035

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

⁽²⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread increased by an additional 250 basis points to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note.

⁽³⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread increased by an additional 250 basis points to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note.

⁽⁴⁾As of December 31, 2016, OneBeacon has assumed for the purpose of estimating fair value that all accrued but unpaid interest on the seller priority note since the date of issuance is paid in 2020, with regular annual interest payments beginning thereafter. Principal repayments are assumed to begin on a graduated basis in 2030.

⁽⁵⁾As of December 31, 2016, OneBeacon has assumed for the purpose of estimating fair value that regular annual interest payments on the pari passu note begin in 2021. All accrued but unpaid interest since the date of issuance is assumed to be paid in 2025. Principal repayments are assumed to begin on a graduated basis in 2035.



Description	December 31, 2015
$ in millions, except share price	Valuation Technique(s)	Fair Value⁽¹⁾	Unobservable Input
Preferred Stock	Par value ⁽²⁾	$70.0	Issuer’s intent to call	-	$70.0
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Share price of most recent transaction	$33.8	Share price	-	$1.03
Private equity security	Share price of most recent transaction	$3.0	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$5.7	EBITDA multiple	-	6.00
Private convertible preferred security	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community reinvestment vehicle	Member share of GAAP net equity	$14.3	GAAP net equity		$14.3
Private equity security	Option pricing method	$9.6
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	60.0%
		Risk free rate		-	1.15%
OneBeacon Surplus Notes ⁽⁷⁾:
- Seller priority	Discounted cash flow	$38.0	Discount rate ⁽³⁾	-	13.0%
		Timing of interest payments ⁽⁵⁾		-	2020
		Timing of principal payments ⁽⁵⁾		-	2025
- Pari passu	Discounted cash flow	$13.5	Discount rate ⁽⁴⁾	-	22.4%
		Timing of interest payments ⁽⁶⁾		-	2020
		Timing of principal payments ⁽⁶⁾		-	2030

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

⁽²⁾Valuation based on the issuer’s intent as of December 31, 2015 to call the security in the near term.

⁽³⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread increased by an additional 250 basis points to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note.

⁽⁴⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread increased by an additional 250 basis points to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note.

⁽⁵⁾As of December 31, 2015, OneBeacon assumed for the purpose of estimating fair value that all accrued but unpaid interest on the seller priority note since the date of issuance was paid in 2020, with regular annual payments beginning thereafter. Principal repayments were assumed to begin on a graduated basis in 2025.

⁽⁶⁾As of December 31, 2015, OneBeacon assumed for the purpose of estimating fair value that all accrued but unpaid interest on the pari passu note since the date of issuance was paid in 2020, with regular annual payments beginning thereafter. Principal repayments were assumed to begin on a graduated basis in 2030.

⁽⁷⁾The decrease in the fair value of the OneBeacon Surplus Notes during the twelve months ended December 31, 2015 was primarily due to widening of non-investment grade credit spreads.

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Debt and Standby Letter of Credit Facilities

12 Months Ended

Dec. 31, 2016

Debt Disclosure [Abstract]

Debt and Standby Letter of Credit Facilities

Debt

White Mountains’s debt outstanding as of December 31, 2016 and 2015 consisted of the following:



	December 31,			Effective	December 31,			Effective
Millions	2016			Rate ⁽¹⁾	2015			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	50.0		3.9%
OneBeacon Bank Facility	—			N/A	—			N/A
OBH Senior Notes, at face value	275.0			4.7%	275.0			4.7%
Unamortized original issue discount and debt issuance costs	(1.8		)		(2.1		)
OBH Senior Notes, carrying value	273.2				272.9
MediaAlpha Bank Facility	12.9			5.7%	15.0			5.5%
Unamortized issuance cost	(.2		)		(.3		)
MediaAlpha Bank Facility, carrying value	12.7				14.7
Total debt	$	285.9			$	337.6

⁽¹⁾Effective rate considers the effect of the debt issuance costs.

A schedule of contractual repayments of White Mountains’s debt as of December 31, 2016, follows:



Millions	December 31, 2016
Due in one year or less	$	5.0
Due in two to three years	7.9
Due in four to five years	—
Due after five years	275.0
Total	$	287.9

WTM Bank Facility

On August 14, 2013, White Mountains entered into a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A. which has a total commitment of $425.0 million and has a maturity date of August 14, 2018 (the “WTM Bank Facility”). The WTM Bank Facility replaced White Mountains’s previous revolving credit facility, which had a total commitment of $375.0 million. During 2016, White Mountains borrowed a total of $350.0 million and repaid a total of $400.0 million under the WTM Bank Facility at a blended interest rate of 3.85%. During 2015, White Mountains borrowed a total of $125.0 million and repaid a total of $75.0 million under the WTM Bank Facility at a blended interest rate of 3.74%. As of December 31, 2016, the WTM Bank Facility was undrawn. White Mountains recorded $1.2 million, $0.1 million, $0.3 million of interest expense on the WTM Bank Facility for the years ended December 31, 2016, 2015 and 2014.

The WTM Bank Facility contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. These covenants can restrict White Mountains in several ways, including its ability to incur additional indebtedness.

OneBeacon Bank Facility

On September 29, 2015, OneBeacon Ltd. and OBH, as co-borrowers and co-guarantors, entered into a revolving credit facility administered by U.S. Bank N.A. and also including BMO Harris Bank N.A., which has a total commitment of $65.0 million and has a maturity date of September 29, 2019 (the “OneBeacon Bank Facility”). As of December 31, 2016, the OneBeacon Bank Facility was undrawn.

The OneBeacon Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. These covenants can restrict White Mountains in several ways, including its ability to incur additional indebtedness.

OBH Senior Notes

In November 2012, OneBeacon U.S. Holdings, Inc. (“OBH”), an intermediate holding company of OneBeacon, issued $275.0 million face value of senior unsecured notes (“OBH Senior Notes”) through a public offering, at an issue price of 99.9% and received $272.9 million of proceeds. The OBH Senior Notes bear an annual interest rate of 4.6% payable semi-annually in arrears on May 9 and November 9, until maturity on November 9, 2022, and are fully and unconditionally guaranteed as to the payment of principal and interest by OneBeacon Ltd. Taking into effect the amortization of the original issue discount and all underwriting and issuance expenses, the OBH Senior Notes have an effective yield to maturity of approximately 4.7% per annum.

In December 2012, the proceeds from the OBH Senior Notes were utilized to repurchase the remaining $269.8 million of outstanding senior notes issued in 2003 by OBH with an annual interest rate of 5.875% for $275.9 million. The repurchase resulted in a $6.3 million loss, including transaction fees and the write-off of the remaining $0.2 million in unamortized deferred costs and original issue discount at the time of repurchase.

OBH recorded $13.1 million, $13.0 million and $13.0 million in interest expense on the OBH Senior Notes for the years ended December 31, 2016, 2015 and 2014.

The OBH Senior Notes were issued under indentures that contain restrictive covenants which, among other things, limit the ability of OneBeacon Ltd., OBH, and their respective subsidiaries to create liens and enter into sale and leaseback transactions and limits the ability of OneBeacon Ltd. and OBH and their respective subsidiaries to consolidate, merge or transfer their properties and assets. The indentures do not contain any financial ratios or specified levels of net worth or liquidity to which OneBeacon Ltd. or OBH must adhere.

MediaAlpha Bank Facility

On July 23, 2015, MediaAlpha entered into a credit facility with Opus Bank, which has a total commitment of $20.0 million and has a maturity date of July 23, 2019 (the “MediaAlpha Bank Facility”). The MediaAlpha Bank Facility consists of a $15.0 million term loan facility, which had an outstanding balance of $12.9 million as of December 31, 2016, and a $5.0 million revolving credit facility, which was undrawn as of December 31, 2016. During 2016, MediaAlpha repaid a total of $2.1 million under the term loan facility and borrowed and repaid $2.5 million under the revolving loan facility. The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.5% as of December 31, 2016.

The MediaAlpha Bank Facility is secured by the intellectual property and the common stock of MediaAlpha's subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a maximum leverage ratio.

Debt Covenants

As of December 31, 2016, White Mountains was in compliance with all of the covenants under all of its debt facilities.

Interest

Total interest expense incurred by White Mountains for its indebtedness was $16.1 million, $14.6 million and $14.2 million in 2016, 2015 and 2014. Total interest paid by White Mountains for its indebtedness was $14.8 million, $12.8 million, and $13.0 million in 2016, 2015 and 2014.

Goodwill and Other Intangible Assets

12 Months Ended

Dec. 31, 2016

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Other Intangible Assets

White Mountains has recognized goodwill and other identifiable intangible assets at the acquisition date fair values in connection with its purchases of subsidiaries. The following table shows the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, for each company acquired:



$ in millions	Weighted Average Economic life (in years)	December 31, 2016						December 31, 2015
$ in millions	Weighted Average Economic life (in years)	Acquisition date fair value		Accumulated amortization		Net carrying value		Acquisition date fair value		Accumulated amortization		Net carrying value
Goodwill:
MediaAlpha	N/A	$	18.3	$	—	$	18.3	$	18.3	$	—	$	18.3
Wobi	N/A	5.8		—		5.8		5.8		—		5.8
Buzzmove	N/A	7.6		—		7.6		—		—		—
Total goodwill		31.7		—		31.7		24.1		—		24.1

Other intangible assets:
MediaAlpha
Customer relationships	3	10.0		6.2		3.8		6.5		2.9		3.6
Information technology	5	32.4		17.9		14.5		32.0		11.2		20.8
Subtotal		42.4		24.1		18.3		38.5		14.1		24.4

Wobi
Trademark	8	2.1		.8		1.3		2.1		.5		1.6
Information technology	7	3.6		1.2		2.4		3.6		.7		2.9
Subtotal		5.7		2.0		3.7		5.7		1.2		4.5

Buzzmove
Trademark	7	.6		.1		.5		—		—		—
Information technology	5	.5		—		.5		—		—		—
Subtotal		1.1		.1		1.0		—		—		—

OneBeacon	10	9.4		8.2		1.2		9.4		7.0		2.4

Total other intangible assets		58.6		34.4		24.2		53.6		22.3		31.3
Total goodwill and other intangible assets⁽¹⁾		$	90.3	$	34.4	$	55.9	$	77.7	$	22.3	$	55.4

⁽¹⁾Excludes amounts related to Star & Shield. See Note 22 — “Held for Sale and Discontinued Operations”.

The goodwill recognized for the above acquisitions is attributed to expected future cash flows. The acquisition date fair values of other intangible assets with finite lives are estimated using income approach techniques, which use future expected cash flows to develop a discounted present value amount.

The multi-period-excess-earnings method estimates fair value using the present value of the incremental after-tax cash flows attributable solely to the intangible asset over its remaining life. This approach was used to estimate the fair value of intangible assets associated with trademarks, brand names, customer relationships and contracts and information technology.

The relief-from-royalty method was used to estimate fair value for intangible assets that relate to rights that could be obtained via a license from a third-party owner. Under this method, the fair value is estimated using the present value of license fees avoided by owning rather than leasing the asset. This technique was used to estimate the fair value of domain names.

The with-or-without method estimates the fair value of an intangible asset that provides an incremental benefit. Under this method, the fair value of the intangible asset is calculated by comparing the value of the entity with and without the intangible asset. This approach was used to estimate the fair value of favorable lease terms, which is included in other.

The following table shows the change in goodwill and other intangible assets:



	December 31,
	2016					2015
Millions	Goodwill		Other intangible assets			Goodwill		Other intangible assets
Beginning balance	$	24.1	$	31.3		$	23.8	$	38.7
Add: Star & Shield amounts held for sale at beginning of the period ⁽¹⁾	—		.4			—		.8
Acquisitions of businesses	7.6		1.1			.3		2.8
Acquisitions of other intangible assets	—		3.9			—		—
Amortization, including foreign currency translation	—		(12.5		)	—		(10.6		)
Less: Star & Shield amounts held for sale at end of the period⁽¹⁾	—		—			—		.4
Ending balance	$	31.7	$	24.2		$	24.1	$	31.3

⁽¹⁾See Note 22 — “Held for Sale and Discontinued Operations”.

Amortization expense was $12.5 million, $10.6 million and $8.3 million for the years ended December 31, 2016, 2015 and 2014.

White Mountains expects to recognize amortization expense in each of the next five years as follows:



Millions	Amortization expense
2017	$	10.9
2018	8.3
2019	2.3
2020	.7
2021	.9
Total	$	23.1

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Gibraltar, Ireland, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States.

The total income tax benefit for the years ended December 31, 2016, 2015 and 2014 consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Current tax benefit (expense):
U.S. federal	$	50.7		$	9.1		$	(4.3	)
State	(1.6		)	(1.8		)	(2.0		)
Non-U.S.	(.8		)	(1.5		)	(1.4		)
Total current tax benefit (expense)	48.3			5.8			(7.7		)
Deferred tax (expense) benefit:
U.S. federal	(2.3		)	(6.7		)	22.8
State	(.6		)	1.1			(.3		)
Total deferred tax (expense) benefit	(2.9		)	(5.6		)	22.5
Total income tax benefit	$	45.4		$	.2		$	14.8

Effective Rate Reconciliation

A reconciliation of taxes calculated using the 35% U.S. statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income follows:



	Year Ended December 31,
Millions	2016			2015			2014
Tax benefit (expense) at the U.S. statutory rate	$	14.1		$	(55.1	)	$	10.3
Differences in taxes resulting from:
Tax reserve adjustments	13.7			(1.7		)	5.2
Change in valuation allowance	9.1			(19.4		)	(35.7		)
Non-U.S. earnings, net of foreign taxes	6.9			76.7			37.8
Tax rate changes	(3.5		)	—			—
Tax exempt interest and dividends	2.5			2.6			2.5
Withholding tax	(.5		)	(1.2		)	(2.4		)
Other, net	3.1			(1.7		)	(2.9		)
Total income tax benefit on pre-tax income	$	45.4		$	.2		$	14.8

The non-U.S. component of pre-tax income was $16.3 million, $208.5 million and $60.4 million for the years ended December 31, 2016, 2015 and 2014.

Tax Payments and Receipts

Net income (receipts from) tax payments to national governments (primarily the United States) totaled $(12.8) million, $(6.5) million, and $2.7 million for the years ended December 31, 2016, 2015 and 2014.

Deferred Tax Inventory

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes.

An outline of the significant components of White Mountains’s deferred tax assets and liabilities follows:



	December 31,
Millions	2016			2015
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$	189.3		$	191.0
Incentive compensation	42.5			49.0
Unearned premiums	37.8			37.5
Non-U.S. net operating loss carryforwards	29.5			33.8
Tax credit carryforwards	20.7			16.9
Loss reserve discount	18.1			26.8
Runoff Transaction	12.6			12.6
Deferred compensation	5.7			6.2
Accrued interest	2.2			1.3
Other items	4.0			8.6
Total gross deferred income tax assets	362.4			383.7
Less: valuation allowances	(146.6		)	(165.4		)
Total net deferred income tax assets	215.8			218.3
Deferred income tax liabilities related to:
Deferred acquisition costs	33.6			34.9
Members surplus contributions	30.0			19.0
SSIE Surplus Notes	7.1			—
Pension and benefit accruals	6.4			3.5
Capitalized software	4.7			4.5
Investment basis difference	4.1			15.2
Net unrealized investment gains	2.9			28.4
Other items	.3			—
Total deferred income tax liabilities	89.1			105.5
Net deferred tax asset	$	126.7		$	112.8

White Mountains’s deferred tax assets are net of U.S. federal, state, and non-U.S. valuation allowances and, to the extent they relate to non-U.S. jurisdictions, they are shown at year-end exchange rates.

Valuation Allowance

White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense.

Of the $146.6 million valuation allowance as of December 31, 2016, $117.3 million relates to deferred tax assets on net operating losses in the United States and other federal and state deferred tax benefits, $24.4 million relates to deferred tax assets on net operating losses in Luxembourg subsidiaries, $4.6 million relates to net operating losses in the Israel subsidiaries and $0.3 million relates to net operating losses in the United Kingdom subsidiary. Of the $165.4 million valuation allowance as of December 31, 2015, $131.5 million relates to deferred tax assets on U.S. losses and other federal and state deferred tax benefits, $28.3 million relates to deferred tax assets on net operating losses in Luxembourg subsidiaries, $5.5 million relates to net operating losses in the Israel subsidiaries and $0.1 million relates to net operating losses in the Netherlands subsidiary.

United States

During 2016 and 2015, White Mountains recorded tax expense of $17.1 million and $9.1 million to establish a valuation allowance against deferred tax assets of Guilford Holdings, Inc. and subsidiaries (“Guilford”), as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets. Guilford consists of service companies and certain other investments that are included in the Other Operations segment. Also during 2016, Guilford has income in discontinued operations that is available to offset its loss from continuing operations. However, ASC 740 includes an exception to the general principle of intra-period tax allocations that requires a consolidated tax group, such as Guilford, with a current period loss within continuing operations to consider income recorded in other categories, including discontinued operations, in determining the tax benefit that is allocated to continuing operations. As a result of Guilford’s current period losses within continuing operations, White Mountains recorded a tax benefit of $21.4 million in continuing operations, with an offsetting tax expense in discontinued operations.

During 2016 and 2015, White Mountains recorded tax expense of $3.6 million and $6.3 million to establish valuation allowances against deferred tax assets of BAM, as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. Also during 2016 and 2015, BAM has income in other comprehensive income that is available to offset its loss from continuing operations, as a result, BAM recorded a tax benefit of $11.0 million and $8.7 million, in continuing operations, with an offsetting tax expense in other comprehensive income. However, since BAM is a mutual insurance company that is owned by its members, its results do not affect White Mountains’s common shareholders’ equity, as they are attributable to non-controlling interests.

During 2016, Houston General Insurance Exchange (“HGIE”) recorded tax expense of $0.1 million as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. During 2015, HGIE recorded a tax benefit of $0.5 million to reduce a valuation allowance, primarily due to the restructuring of the surplus note issued by HGIE (“HGIE Surplus Notes”).

During 2016, SSIE recorded a tax benefit of $6.9 million to reduce a valuation allowance, primarily due to the write down of the SSIE Surplus Notes. During 2015, SSIE recorded tax expense of $1.2 million, as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. SSIE is a reciprocal, which is included in the White Mountains’s consolidated results as a variable interest entity. See Note 18 — “Variable Interest Entities”.

Non-U.S. jurisdictions

During 2016, White Mountains recorded tax expense of $2.8 million for a reduction to deferred tax assets, and a corresponding reduction to the valuation allowance, which primarily related to a decrease in the corporate income tax rate for Luxembourg-domiciled subsidiaries. During 2015, White Mountains recorded tax expense of $0.3 million to establish a valuation allowance against deferred tax assets at certain Luxembourg-domiciled subsidiaries, as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets.

During 2016 and 2015, White Mountains recorded tax expense of $1.1 million and $3.0 million to establish a valuation allowance against deferred tax assets at certain Israeli-domiciled subsidiaries, as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets.

During 2016, White Mountains recorded tax expense of $0.1 million to establish a valuation allowance against deferred tax assets at certain United Kingdom-domiciled subsidiaries, as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets.

Net Operating Loss and Capital Loss Carryforwards

Net operating loss and capital loss carryforwards as of December 31, 2016, the expiration dates and the deferred tax assets thereon are as follows:



	December 31, 2016
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2017-2021	$	.8		$	—		$	—		$	—		$	.8
2022-2026	1.6			—			—			—			1.6
2027-2036	556.3			—			—			—			556.3
No expiration date	—			93.8			3.2			19.8			116.8
Total	$	558.7		$	93.8		$	3.2		$	19.8		$	675.5
Gross deferred tax asset	189.3			24.4			.5			4.6			218.8
Valuation allowance	(115.4		)	(24.4		)	(.5		)	(4.6		)	(144.9		)
Net deferred tax asset	$	73.9		$	—		$	—		$	—		$	73.9

Included in the U.S. net operating loss carryforwards are losses of $4.2 million subject to an annual limitation on utilization under Internal Revenue Code Section 382. These loss carryforwards will begin to expire in 2025. Also included in the U.S. net operating loss carryforwards are losses of $9.9 million due to additional deductions related to equity compensation. These loss carryforwards begin to expire in 2030. As of December 31, 2016, there are U.S. foreign tax credit carryforwards available of $0.4 million, which begin to expire in 2023. As of December 31, 2016, there are U.S. alternative minimum tax credit carryforwards of $2.3 million which do not expire. As of December 31, 2016, there are U.S. low income housing credit carryforwards of $18.0 million, which begin to expire in 2031.

Uncertain Tax Positions

Recognition of the benefit of a given tax position is based upon whether a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more-likely-than-not recognition threshold, White Mountains must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:



Millions	Permanent Differences⁽¹⁾			Temporary Differences⁽²⁾			Interest and Penalties⁽³⁾			Total
Balance at January 1, 2014	$	8.4		$	16.6		$	8.7		$	33.7
Changes in prior year tax positions	(2.2		)	(.8		)	(1.9		)	(4.9		)
Tax positions taken during the current year	—			7.3			—			7.3
Lapse in statute of limitations	(.8		)	—			(.3		)	(1.1		)
Balance at December 31, 2014	5.4			23.1			6.5			35.0
Changes in prior year tax positions	—			(12.4		)	1.7			(10.7		)
Balance at December 31, 2015	5.4			10.7			8.2			24.3
Changes in prior year tax positions	—			(5.5		)	.1			(5.4		)
Settlements with tax authorities	(5.4		)	—			(8.3		)	(13.7		)
Balance at December 31, 2016	$	—		$	5.2		$	—		$	5.2


⁽¹⁾	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.


⁽²⁾	Represents the amount of unrecognized tax benefits that, if recognized would create a temporary difference between the reported amount of an item in White Mountains’s Consolidated Balance Sheet and its tax basis.


⁽³⁾	Net of tax benefit.

If White Mountains determines in the future that its reserves for unrecognized tax benefits on temporary differences are not needed, the reversal of $5.2 million of such reserves as of December 31, 2016 would not impact the effective tax rate due to deferred tax accounting but would accelerate the payment of cash to the taxing authority. The vast majority of White Mountains’s reserves for unrecognized tax benefits on temporary differences relate to deductions for loss reserves where the timing of the deductions is uncertain.

White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2016, 2015 and 2014, White Mountains recognized $(8.2) million, $1.7 million, $(2.2) million in net interest (income) expense. There was no accrued interest as of December 31, 2016. The accrued interest as of December 31, 2015 is $8.2 million, net of any tax benefit.

Tax Examinations

With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2012.

On May 27, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax years 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded a tax benefit of $3.5 million in the second quarter of 2016 related to tax years 2010, 2011 and 2012.

On January 19, 2016, White Mountains received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Overassessment) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved with the Joint Committee on Taxation. As the receipt of the Form 870-AD described above represents formal settlement, White Mountains recorded a tax benefit of $12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009.

White Mountains recorded a tax benefit of $5.0 million in the first quarter of 2014 relating to the examination of tax years 2005 and 2006 for certain U.S. subsidiaries of OneBeacon.

On June 22, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax year 2012 for Guilford Holdings, Inc. and subsidiaries. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded an increase to the deferred tax asset on net operating loss carryforwards of $0.6 million, and a corresponding increase to the valuation allowance, related to tax year 2012.

Derivatives

12 Months Ended

Dec. 31, 2016

Dec. 31, 2015

Derivative [Line Items]

Derivatives

Variable Annuity Reinsurance

Derivative [Line Items]

Derivatives

Variable Annuity Reinsurance

White Mountains entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. During the third quarter of 2015, the variable annuity contracts reinsured by WM Life Re began to mature and were fully runoff by June 30, 2016. The reinsurance agreement was commuted in December 2016.

The following table summarizes the pre-tax operating results of WM Life Re for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
Millions	2016			2015			2014
Fees, included in other revenue	$	1.2		$	9.3		$	18.6
Change in fair value of variable annuity liability, included in other revenue	(.3		)	(.4		)	52.9
Change in fair value of derivatives, included in other revenue	(2.0		)	(8.8		)	(72.4		)
Foreign exchange, included in other revenue	1.3			(1.3		)	(3.2		)
Other investment income and losses	—			(.4		)	(1.4		)
Total revenues	.2			(1.6		)	(5.5		)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	—			—			.6
Death benefit claims paid, included in general and administrative expenses	(.3		)	(.1		)	(.1		)
General and administrative expenses	(2.6		)	(4.0		)	(4.3		)
Pre-tax loss	$	(2.7	)	$	(5.7	)	$	(9.3	)

During 2013, the ratio of annuitants’ aggregate account values to the aggregate guarantee value provided by WM Life Re increased, and as a result, annuitants surrendered their policies at higher rates than WM Life Re observed in the past. In response to this trend, WM Life Re adjusted the projected surrender assumptions used in the valuation of its variable annuity reinsurance liability upward. In 2014, surrender rates continued to outpace assumptions and WM Life Re again adjusted the projected surrender assumptions. For the year ended December 31, 2014, the change in the fair value of the variable annuity liability included $0.2 million of losses associated with changes in projected surrender assumptions. There was no change in projected surrender assumptions in 2015 or 2016.

The following summarizes net realized and unrealized derivative gains (losses) recognized in other revenue for the years ended December 31, 2016, 2015 and 2014 and the carrying values, included in other assets, as of December 31, 2016 and 2015 by type of instrument:



										Carrying Value
	Year Ended December 31,									December 31,
Millions	2016			2015			2014			2016		2015
Fixed income/interest rate	$	1.8		$	6.4		$	(33.7	)	$	—	$	.5
Foreign exchange	(4.8		)	(7.3		)	(1.3		)	—		14.8
Equity	1.0			(7.9		)	(37.4		)	—		4.8
Total	$	(2.0	)	$	(8.8	)	$	(72.4	)	$	—	$	20.1

The following table summarizes the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2016, 2015 and 2014:



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2016	$	.3			$	2.7		$	16.5		$	.9		$	20.1
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.3		)	⁽⁵⁾	2.9			(.7		)	(4.2		)	(2.0		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.6		)	(15.8		)	3.3			(18.1		)
Balance at December 31, 2016	$	—			$	—		$	—		$	—		$	—



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2015	$	.7			$	18.9		$	33.8		$	3.7		$	56.4
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.4		)	⁽⁵⁾	(9.7		)	(7.5		)	8.4			(8.8		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(6.5		)	(9.8		)	(11.2		)	(27.5		)
Balance at December 31, 2015	$	.3			$	2.7		$	16.5		$	.9		$	20.1



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2014	$	(52.8	)		$	63.4		$	4.7		$	1.1		$	69.2
Purchases	—				—			—			—			—
Net realized and unrealized gains (losses)	53.5			⁽⁵⁾	(38.6		)	(71.0		)	37.2			(72.4		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.9		)	100.1			(34.6		)	59.6
Balance at December 31, 2014	$	.7			$	18.9		$	33.8		$	3.7		$	56.4


⁽¹⁾	Consists of over-the-counter instruments.


⁽²⁾	Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.


⁽³⁾	Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.


⁽⁴⁾	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $0.0, $5.8 and $33.2 as of December 31, 2016, 2015 and 2014 posted as collateral to its reinsurance counterparties.


⁽⁵⁾	There was no changes in the fair value of variable annuity death benefit liabilities for the years ended December 31, 2016 and December 31, 2015. In 2014, $0.6 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.


⁽⁶⁾	Consists of foreign currency options and equity options.

In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties. The total collateral includes the following:



	December 31,
Millions	2016		2015
Cash	$	—	$	5.8
Fixed maturity investments	—		—
Total	$	—	$	5.8

Collateral in the form of fixed maturity securities consisted of Government of Japan Bonds, recorded at fair value. Collateral in the form of short-term investments consisted of money-market instruments, carried at amortized cost, which approximated fair value.

All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements. The fair value of White Mountains’s variable annuity reinsurance liabilities were estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, were generally unobservable inputs and significantly impacted the fair value estimates. Generally, the liabilities associated with these guarantees increased with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains used derivative instruments to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policy holder behavior and risk margins, were generally the same as those used to estimate the fair value of variable annuity liabilities. There were no open derivatives instruments remaining as of December 31, 2016.

WM Life Re entered into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee. In the case of OTC derivatives, WM Life Re had exposure to credit risk for amounts that were uncollateralized by counterparties. WM Life Re’s internal risk management guidelines established net counterparty exposure thresholds that took into account OTC counterparties’ credit ratings. The OTC derivative contracts were subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements. In the case of exchange traded instruments, WM Life Re had exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re had master netting agreements with certain of its counterparties whereby the collateral provided (held) was calculated on a net basis. The following summarizes amounts offset under master netting agreements:



	December 31, 2015
Millions	Gross asset amounts before offsets⁽¹⁾		Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other Assets
Interest rate contracts
OTC	$	2.4	$	(2.1	)	$	.3
Exchange traded	.1		(.1		)	—
Foreign exchange contracts
OTC	15.0		—			15.0
Exchange traded	.1		(.3		)	(.2		)
Equity contracts
OTC	4.4		(.6		)	3.8
Exchange traded	1.2		—			1.2
Total⁽²⁾	$	23.2	$	(3.1	)	$	20.1

⁽¹⁾Amount equal to fair value of instrument as recognized in other assets.

⁽²⁾ All derivative instruments held by WM Life Re were subject to master netting arrangements.

There were no open derivative instruments as of December 31, 2016 and no exposure to credit losses on OTC and exchanged traded derivatives as of that date. The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets as of December 31, 2015:



	December 31, 2015
Millions	Net amount of assets reflected in Balance Sheet		Collateral provided to counterparty - Cash		Collateral provided to counter-party - Financial Instruments		Net amount of exposure after effect of collateral provided		Excess collateral provided to counter-party- Cash		Excess collateral provided - Financial Instruments		Counter-party collateral held by WM Life Re- Cash		Net amount of exposure to counter-party		Standard & Poor's Rating⁽¹⁾

JP Morgan	$	8.5	$	—	$	—	$	8.5	$	—	$	—	$	5.5	$	3.0	A	+
Bank of America	.7		$	—	—		.7		$	—	—		—		.7		A
Citigroup - OTC	9.9		—		—		9.9		—		—		1.4		8.5		A
Citigroup - Exchange Traded	1.0		—		—		1.0		5.8		—		—		6.8		A
Total	$	20.1	$	—	$	—	$	20.1	$	5.8	$	—	$	6.9	$	19.0


⁽¹⁾	Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings),“A” (which is the sixth highest of twenty-three creditworthiness ratings), “A-” (which is the seventh highest of twenty-three creditworthiness ratings), and BBB+ (which is the eighth highest of twenty-three creditworthiness ratings).

- Definition

The entire disclosure for derivative instruments and hedging activities including, but not limited to, risk management strategies, non-hedging derivative instruments, assets, liabilities, revenue and expenses, and methodologies and assumptions used in determining the amounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 30
-Section 45
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-URI http://asc.fasb.org/extlink&oid=6441202&loc=d3e80720-113993

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 815
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579240-113959

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624177-113959

Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41641-113959

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624258-113959

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
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-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5708775-113959

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5708773-113959

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959

Reference 16: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624181-113959

Reference 17: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41638-113959

Reference 19: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Municipal Bond Guarantee Insurance

12 Months Ended

Dec. 31, 2016

Insurance [Abstract]

Municipal Bond Guarantee Insurance

In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of December 31, 2016, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding on a per policy basis through HG Re, which had statutory capital and surplus of $467.2 million and $452.9 million as of December 31, 2016 and 2015. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. At December 31, 2016 and 2015, the collateral trusts held assets of $465.4 million and $441.0 million, which included $300 million of BAM Surplus Notes. As of December 31, 2016 and 2015, HG Global had pre-tax income of $22.2 million and $17.9 million, which included $17.8 million and $15.8 million of interest income on the BAM Surplus Notes. As of December 31, 2016 and 2015, BAM had a pre-tax loss of $48.9 million and $47.3 million that was recorded in net loss attributable to non-controlling interests, which included $17.8 million and $15.8 million of interest expense on the BAM Surplus Notes. As of December 31, 2016, HG Global has accrued $108.0 million of interest receivable on the BAM Surplus Notes.

The following table provides a schedule of BAM’s insured obligations:



	December 31, 2016		December 31, 2015
Contracts outstanding	4,807		3,103
Remaining weighted average contract period (in years)	10.8		12.8
Contractual debt service outstanding (in millions):
Principal	$	33,057.3	$	22,556.0
Interest	$	16,396.6	$	11,984.4
Gross unearned insurance premiums	$	83.0	$	50.2

The following table is a schedule of BAM’s expected premiums to be collected as of December 31, 2016:



Millions	December 31, 2016
January 1, 2017 - March 31, 2017	$	1.9
April 1, 2017 - June 30, 2017	1.8
July 1, 2017 - September 30, 2017	1.8
October 1, 2017 - December 31, 2017	1.8
	7.3

2018	7.0
2019	6.6
2020	6.3
2021	5.9
2022 and thereafter	49.9
Total	$	83.0

Earnings Per Share

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Earnings Per Share

Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2016, 2015 and 2014. See Note 22 — “Held for Sale and Discontinued Operations” for earnings per share amounts for discontinued operations.



	Year Ended December 31,
	2016			2015			2014
Basic and diluted earnings per share numerators (in millions):
Net income from continuing operations attributable to White Mountains’s common shareholders	$	(2.3	)	$	200.7		$	53.2
Allocation of income for unvested restricted common shares⁽¹⁾	—			(2.3		)	(0.7		)
Dividends declared on participating restricted common shares⁽¹⁾	—			(.1		)	(.1		)
Total allocation to restricted common shares	—			(2.4		)	(0.8		)
Net income attributable to White Mountains’s common shareholders, net of restricted share amounts	$	(2.3	)	$	198.3		$	52.4
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	(2.3	)	$	198.3		$	52.4
Dividends declared net of restricted common share amounts⁽¹⁾	(5.4		)	(5.9		)	(6.1		)
Total undistributed net earnings, net of restricted common share amounts	$	(7.7	)	$	192.4		$	46.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	5,014.9			5,879.2			6,104.9
Average unvested restricted common shares⁽²⁾	(64.8		)	(68.0		)	(78.9		)
Basic earnings per share denominator	4,950.1			5,811.2			6,026.0
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	5,018.1			5,879.2			6,104.9
Average unvested restricted common shares⁽²⁾	(64.8		)	(68.0		)	(78.9		)
Diluted earnings per share denominator⁽³⁾	4,953.3			5,811.2			6,026.0
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$	(0.47	)	$	34.12		$	8.70
Dividends declared and paid	(1.00		)	(1.00		)	(1.00		)
Undistributed (loss) earnings	$	(1.47	)	$	33.12		$	7.70
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$	(0.47	)	$	34.12		$	8.70
Dividends declared and paid	(1.00		)	(1.00		)	(1.00		)
Undistributed (loss) earnings	$	(1.47	)	$	33.12		$	7.70


⁽¹⁾	Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.


⁽²⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Incentive Compensation Plans”.

⁽³⁾

The diluted earnings (loss) per share denominator for the year ended December 31, 2016, includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which results in 3,217 incremental shares outstanding over the period. Prior periods do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Retirement and Postretirement Plans

12 Months Ended

Dec. 31, 2016

Compensation and Retirement Disclosure [Abstract]

Retirement and Postretirement Plans

OneBeacon previously sponsored the OneBeacon qualified pension plan (the “Qualified Plan”). During the year ended December 31, 2016, the Qualified Plan finalized its termination by purchasing group annuity contracts from the Principal Financial Group (“Principal”), and making lump sum distributions to Qualified Plan participants electing such payments, which eliminated the remaining Qualified Plan liability and also ceased administratively paying benefits. During the year ended December 31, 2016, OneBeacon recognized an estimated pre-tax pension settlement gain of $3.0 million, primarily resulting from a refund from Principal related to the final settlement of the Qualified Plan. As of December 31, 2016, OneBeacon no longer has a projected benefit obligation with respect to the Qualified Plan. During the year ended December 31, 2016, OneBeacon transferred $47.1 million of excess invested assets from the Qualified Plan into the trust supporting the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (“KSOP”), which OneBeacon determined to be the Qualified Replacement Plan (“QRP”). See Note 13 — “Employee Share-Based Incentive Compensation Plans”. As of December 31, 2016, an additional $13.0 million of excess invested assets remain in the Qualified Plan trust for potential post-termination obligations of that plan, as approved by a March 2016 private letter ruling from the IRS. The invested assets related to both the Qualified Plan and the QRP are included in other assets and are accounted for at fair value with related income recognized in net other revenues (expenses).

OneBeacon continues to sponsor a non-qualified, non-contributory, defined benefit pension plan (“Non-qualified Plan”) covering certain employees who were employed as of December 31, 2001 and former employees who had met the eligibility requirements, as well as retirees. The Non-qualified Plan was frozen and curtailed in 2002, resulting in the pension benefit obligation being equal to the accumulated benefit obligation. The benefits are based primarily on years of service and employees’ compensation through December 31, 2002. OneBeacon’s funding policy is generally to contribute amounts to satisfy actual disbursements for the calendar year. There are no regulatory funding requirements that apply to the Non-qualified Plan.

The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans as of December 31, 2016 and 2015:



	Pension Benefits December 31,
Millions	2016			2015
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$	109.5		$	119.7
Service cost	.2			.9
Interest cost	1.5			4.6
Settlement loss (gain) on projected benefit obligation⁽¹⁾⁽²⁾	1.2			(1.8		)
Assumption changes	(.9		)	(3.5		)
Actuarial loss	.4			.9
Annuity contract purchases related to termination of Qualified Plan ⁽³⁾	(69.0		)	—
Benefits, including lump sum payments, and expenses paid with plan assets⁽³⁾	(17.0		)	(13.7		)
Benefits paid directly by OneBeacon	(2.1		)	(2.2		)
Remeasurement due to plan termination ⁽⁴⁾	—			4.6
Projected benefit obligation at end of year	$	23.8		$	109.5
Change in plan assets:
Fair value of plan assets at beginning of year	$	139.8		$	146.0
Actual return on plan assets	6.3			7.5
Annuity contract purchases related to termination of Qualified Plan⁽³⁾	(69.0		)	—
Transfer of assets to the QRP⁽⁵⁾	(47.1		)	—
Benefits and expenses paid	(17.0		)	(13.7		)
Fair value of plan assets at end of year	$	13.0		$	139.8
(Unfunded) over funded status at end of year	$	(10.8	)	$	30.3

⁽¹⁾During the year ended December 31, 2016, OneBeacon recognized a $1.2 million settlement loss primarily resulting from final settlement of the Qualified Plan.

⁽²⁾During the fourth quarter of 2015, OneBeacon triggered settlement accounting for the Qualified Plan as the total lump sum payments exceeded the service plus interest costs, resulting in a $1.8 adverse effect on accumulated other comprehensive income.

⁽³⁾ During the year ended December 31, 2016, the Qualified Plan finalized its termination by purchasing group annuity contracts for $69.0 million from Principal and making lump sum distributions, included in the $17.0 million above, to Qualified Plan participants electing such payments.

⁽⁴⁾ As of December 31, 2015, the projected benefit obligation was valued on a plan termination basis.

⁽⁵⁾ During the year ended December 31, 2016, OneBeacon transferred excess invested assets from the Qualified Plan after its termination into the trust supporting the QRP.

The unfunded status of the consolidated pension plans as of December 31, 2016 was $10.8 million, which includes $13.0 million of excess invested assets remaining in the Qualified Plan trust for potential post-termination obligations of that plan and an under-funding of $23.8 million related to the Non-qualified Plan. The Non-qualified Plan, which is unfunded, does not hold any assets. OneBeacon has set aside $20.6 million in an irrevocable rabbi trust for the benefit of Non-qualified Plan participants. Assets held in the rabbi trust are not reflected in the unfunded status of the consolidated pension plans as presented but are included in other assets in the consolidated balance sheet.

During the year ended December 31, 2016, OneBeacon transferred $47.1 million of excess invested assets from the Qualified Plan into the QRP. Amounts recognized in the financial statements as of December 31, 2016 and 2015 consist of:



	December 31,
Millions	2016			2015
Net balance sheet asset recorded in other assets	$	13.0		$	55.8
Net balance sheet liability recorded in other liabilities	(23.8		)	(25.5		)
Net amount recognized in the financial statements	$	(10.8	)	$	30.3

Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows:



	December 31,
Millions	2016		2015
Projected benefit obligation	$	23.8	$	25.5
Accumulated benefit obligation	$	23.8	$	25.5
Fair value of plan assets	$	—	$	—

During the year ended December 31, 2016, information for the Qualified Plan, which had accumulated benefit obligations less than plan assets until its termination, was finalized. Information for the Qualified Plan was as follows:



	December 31,
Millions	2016		2015
Projected benefit obligation	$	—	$	84.0	⁽¹⁾
Accumulated benefit obligation	$	—	$	84.0	⁽¹⁾
Fair value of plan net assets	$	13.0	$	139.8

⁽¹⁾ Measured on a plan termination basis

The amounts recognized in accumulated other comprehensive loss on a pre-tax basis and before non-controlling interest for the years ended December 31, 2016 and 2015 were as follows:



	December 31,
Millions	2016			2015
Accumulated other comprehensive loss beginning balance	$	(8.0	)	$	(8.0	)
Increase (decrease) in accumulated other comprehensive loss:
Amortization of net actuarial gains recognized during the year	1.3			1.3
Net actuarial gains (losses) occurring during the year ⁽¹⁾	3.2			(1.3		)
Qualified Plan termination impact ⁽²⁾	(3.0		)	—
Accumulated other comprehensive loss ending balance	$	(6.5	)	$	(8.0	)


⁽¹⁾	Net actuarial gains in 2016 resulted from investment returns and demographic experience different than assumed. The 2015 net actuarial losses reflect the valuation of the Qualified Plan on a termination basis.


⁽²⁾	During the year ended December 31, 2016, OneBeacon recognized a $3.0 million gain resulting from the final settlement of the Qualified Plan.

The amount in accumulated other comprehensive loss, on a pre-tax basis, that has not yet been recognized as a component of net periodic benefit cost for the year ended December 31, 2016 is attributable to net losses. During 2017, OneBeacon expects $0.9 million will be amortized from accumulated other comprehensive loss into net periodic benefit cost.

The components of net periodic benefit cost (income) for the years ended December 31, 2016, 2015 and 2014 were as follows:



	Year Ended December 31,
Millions	2016			2015			2014
Service cost	$	.2		$	.9		$	.6
Interest cost	1.5			4.6			4.7
Expected return on plan assets	(1.0		)	(8.7		)	(8.5		)
Amortization of net actuarial losses	1.3			1.3			.3
Net periodic pension (cost) income before special termination benefits	2.0			(1.9		)	(2.9		)
Settlement gain ⁽¹⁾	(3.0		)	—			—
Special termination benefits expense ⁽²⁾	—			—			.3
Total net periodic benefit income	$	(1.0	)	$	(1.9	)	$	(2.6	)

⁽¹⁾ Represents the impact of the termination of the Qualified Plan during the year ended December 31, 2016.

⁽²⁾ Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.

Assumptions

The weighted average discount rate assumptions used to determine benefit obligations was 3.78% and 3.22% as of December 31, 2016 and 2015. The weighted average assumptions used to determine net periodic benefit cost included a 3.88% discount rate and 3.00% expected long-term rate of return on plan assets for the year ended December 31, 2016. The weighted average assumptions used to determine net periodic benefit cost included a 3.91% discount rate and 6.00% expected long-term rate of return on plan assets for the year ended December 31, 2015.

OneBeacon’s discount rate assumptions used to account for the Plans reflect the rates at which the benefit obligations could be effectively settled. In addition to consideration of published yields for high quality long-term corporate bonds, U.S. Treasuries and insurance company annuity contract pricings, consideration was given to cash flow matching analyses.

OneBeacon performed an analysis of expected long-term rates of return based on the allocation of its Qualified Plan assets as of December 31, 2015 and 2014 to develop expected rates of return for 2016 and 2015 for each significant asset class or economic indicator. A range of returns was developed based both on forecasts and on broad market historical benchmarks for expected return, correlation, and volatility for each asset class. During 2015, OneBeacon changed the asset allocation to more fixed income and shorter duration in anticipation of the plan termination, which resulted in a decrease in this assumption as compared to the prior year.

Plan Assets

The remaining assets of the Qualified Plan, which are managed internally by OneBeacon and consist substantially of common equity securities tracking the S&P 500 and Russell 1000 indices, are held for potential post-termination obligations of the plan, as approved by a March 2016 private letter ruling from the IRS. The Qualified Plan's investments are stated at fair value.

The fair value of the Qualified Plan’s assets and their related inputs as of December 31, 2016 and 2015 by asset category were as follows:



	December 31, 2016								December 31, 2015
Millions	Fair Value⁽¹⁾		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs		Fair Value		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs
Common equity securities	$	12.8	$	12.8	$	—	$	—	$	—	$	—	$	—	$	—
Fixed maturity investments	—		—		—		—		132.4		132.4		—		—
Short-term investments	—		—		—		—		7.0		7.0		—		—
Total	$	12.8	$	12.8	$	—	$	—	$	139.4	$	139.4	$	—	$	—

⁽¹⁾Excludes cash and short-term investments

There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2016 and 2015.

The Qualified Plan’s asset allocations as of December 31, 2016 and 2015, by asset category were as follows:



	Plan Assets at December 31,
Asset Category	2016		2015
Common equity securities	98.5	%	—	%
Fixed maturity investments	—		95.0
Cash and short-term investments	1.5		5.0
Total	100.0	%	100.0	%

Cash Flows

OneBeacon anticipates contributing $2.1 million to the Non-qualified Plan in 2017, for which OneBeacon has assets held in a rabbi trust.

The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid:



Millions	Expected Benefit Payments
2017	$	2.1
2018	2.1
2019	2.0
2020	2.0
2021	1.9
2022 - 2026	8.4

Other Benefit Plans

OneBeacon sponsors a defined contribution plan, the OneBeacon 401(k) Savings and KSOP, covering the majority of its employees. The contributory plan provides qualifying employees with matching contributions of 50% up to the first 6% of salary (subject to U.S. federal limits on allowable contributions in a given year). During 2016, the matching contribution of the contributory plan was replaced with a fixed 3% of salary employer contribution (subject to federal limits on allowable contributions in a given year). The total expense for employer contributions to the plan was $3.2 million, $3.0 million and $2.7 million in 2016, 2015 and 2014.

The employee stock ownership component of the KSOP provides all of its participants with an annual base contribution in common shares equal to 3% of their salary, up to the applicable Social Security wage base ($118,500 for 2016). Additionally, those participants not otherwise eligible to receive certain other OneBeacon benefits can earn a variable contribution of up to 6% of their salary, subject to the applicable IRS annual covered compensation limits ($265,000 for 2016) and contingent upon OneBeacon’s performance. White Mountains recorded $5.5 million, $5.7 million and $4.4 million in compensation expense to pay benefits and allocate common shares to participant’s accounts for the years ended 2016, 2015 and 2014.

OneBeacon had a post-employment benefit liability, which primarily relates to disability and health benefits available to former employees, of $3.1 million and $4.1 million as of December 31, 2016 and 2015.

OneBeacon also had a post-employment benefit liability related to death benefits to beneficiaries of former executives of $12.8 million and $12.5 million as of December 31, 2016 and 2015. OneBeacon has set aside funds to satisfy its obligation in a rabbi trust of $29.3 million and $33.9 million as of December 31, 2016 and 2015. During 2016, OneBeacon withdrew $5.5 million from the rabbi trust in accordance with the trust agreement, which remains overfunded.

Employee Share-Based Incentive Compensation Plans

12 Months Ended

Dec. 31, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Employee Share-Based Incentive Compensation Plans

White Mountains’s share-based incentive compensation plans are designed to incentivize key employees and service providers to maximize shareholder value over long periods of time. White Mountains believes that this is best pursued by utilizing a pay-for-performance program that closely aligns the financial interests of management with those of its shareholders. White Mountains accomplishes this by emphasizing highly variable long-term compensation that is contingent on performance over a number of years rather than entitlements. White Mountains expenses all its share-based compensation. As a result, White Mountains’s calculation of its owners’ returns includes the expense of all outstanding share-based compensation awards.

Incentive Compensation Plans

White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees and service providers of White Mountains. The WTM Incentive Plan was adopted by the Board, was approved by the Company’s sole shareholder in 1985 and was subsequently amended by its shareholders in 1995, 2001, 2003, 2005, 2010 and 2013. Share-based incentive awards that may be granted under the plan include performance shares, restricted shares, incentive stock options and non-qualified stock options (“Non-Qualified Options”). Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. Performance shares earned under the WTM Incentive Plan are typically paid in cash but may be paid in common shares. Compensation expense is recognized on a pro rata basis over the vesting period of the awards.

The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based incentive awards, including performance shares, restricted shares, restricted stock units and Non-Qualified Options.

White Mountains offers certain types of share-based compensation under qualified retirement plans. The defined contribution plan sponsored by OneBeacon, the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (“KSOP”), offers U.S. domiciled participants the ability to invest their balances in several different investment options, including the common shares of the Company or OneBeacon.

The employee stock ownership component of the KSOP provides all participants with an annual base contribution in common shares of the Company or OneBeacon, depending on their employer, equal to 3% of their salary, up to the applicable Social Security wage base ($118,500 for 2016). Additionally, those participants not otherwise eligible to receive certain other benefits can earn a variable contribution of up to 6% of their salary, subject to the applicable IRS annual covered compensation limits ($265,000 for 2016) and contingent upon the Company’s or OneBeacon’s performance.

Performance Shares

Performance shares are designed to reward company-wide performance. The level of payout ranges from zero to two times the number of shares initially granted, depending on White Mountains’s financial performance. Performance shares become payable at the conclusion of a performance cycle (typically three years) if pre-defined financial targets are met.

The principal performance measure used for determining performance share payouts is after-tax growth in White Mountains’s intrinsic business value per share. The Compensation Committee historically has considered the growth in intrinsic business value per share to be based equally on the growth of economic value per share and growth in adjusted book value per share, both inclusive of dividends. Economic value is calculated by adjusting the GAAP book value per share for differences between the GAAP carrying values of certain assets and liabilities and White Mountains’s estimate of their underlying economic values (for example, the time value discount in loss reserves).

The following summarizes performance share activity for the years ended December 31, 2016, 2015 and 2014 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	93,654		$	57.7		111,257		$	44.4		108,605		$	54.9
Shares paid or expired⁽¹⁾	(36,294	)	(41.0		)	(42,959	)	(30.8		)	(33,730	)	(24.5		)
New grants	22,615		—			29,195		—			39,590		—
Assumed forfeitures and cancellations⁽²⁾	378		.5			(3,839	)	(.3		)	(3,208	)	(.1		)
Expense recognized	—		25.2			—		44.4			—		14.1
End of period ⁽³⁾	80,353		$	42.4		93,654		$	57.7		111,257		$	44.4


⁽¹⁾	WTM performance share payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target. WTM performance shares payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance shares payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.


⁽²⁾	Amounts include changes in assumed forfeitures, as required under GAAP.


⁽³⁾	Outstanding performance share awards as of December 31, 2016, 2015 and 2014 exclude 7,315, 10,826 and 12,607 unvested performance shares awards for employees of Sirius Group.

For the 2013-2015 performance cycle, the Company issued common shares for 5,000 performance shares earned and all other performance shares earned were settled in cash. For the 2012-2014 performance cycle, all performance shares earned were settled in cash. For 2011-2013 performance cycle, the Company issued 3,570 performance shares earned and all performance shares earned were settled in cash.

If the outstanding WTM performance shares had vested on December 31, 2016, the total additional compensation cost to be recognized would have been $23.1 million, based on accrual factors as of December 31, 2016 (common share price and payout assumptions).

Performance shares granted under the WTM Incentive Plan

The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2016 for each performance cycle:



$ in millions	Target WTM Performance Shares Outstanding		Accrued Expense
Performance cycle:
2016 – 2018	19,615		$	5.4
2015 – 2017	31,795		19.1
2014 – 2016	30,167		18.6
Sub-total	81,577		43.1
Assumed forfeitures	(1,224	)	(.7		)
Total at December 31, 2016	80,353		$	42.4

For the 2016-2018 performance cycle, the Compensation Committee will use its judgment to evaluate management’s performance by taking into account all facts the Compensation Committee deems relevant including the Company’s growth in intrinsic business value per share to determine the payout of the performance shares on a 0%-200% scale.

For the 2015-2017 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel is an 8% growth in intrinsic business value per share. Growth of 2% or less would result in no payout and growth of 14% or more would result in a payout of 200%.

For the 2014-2016 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel is a 9% growth in intrinsic business value per share. Growth of 3% or less would result in no payout and growth of 15% or more would result in a payout of 200%.

For investment personnel, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is based one-third on growth in intrinsic business value per share (as described above), one-third on achieving a total return on invested assets as measured against metrics based on U.S. Treasury Note returns and one-third on achieving a total return on invested assets as measured against metrics based on the Bloomberg Barclays U.S. Intermediate Aggregate Index returns.

Restricted Shares

The following outlines the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	70,675		$	15.7		83,314		$	14.3		94,130		$	17.0
Issued	25,365		20.2			23,640		15.7			23,440		13.1
Vested	(24,620	)	—			(36,279	)	—			(33,205	)	—
Forfeited	(800	)	(.3		)	—		—			(1,051	)	(.5		)
Expense recognized	—		(15.9		)	—		(14.3		)	—		(15.3		)
End of period ⁽¹⁾	70,620		$	19.7		70,675		$	15.7		83,314		$	14.3


⁽¹⁾	Outstanding restricted share awards as of December 31, 2016, 2015 and 2014 include 5,235, 9,205, and 12,477 unvested restricted shares for employees of Sirius Group.

During 2016, White Mountains issued 24,615 restricted shares that vest on January 1, 2019 and 750 restricted shares that vest on January 1, 2018. During 2015, White Mountains issued 23,640 restricted shares that vest on January 1, 2018. During 2014, White Mountains issued 23,440 restricted shares that vest on January 1, 2017. The unrecognized compensation cost as of December 31, 2016 is expected to be recognized ratably over the remaining vesting periods.

Stock Options

Non-Qualified Options

As of December 31, 2016 and 2015, the Company’s Chairman and CEO held 40,000 and 125,000 Non-Qualified Options, which are exercisable at a fixed price of $742 per common share and expire on January 20, 2017. During 2016, 5,000 Non-Qualified Options, with an intrinsic value of $0.4 million, were exercised at $742 per common share and 80,000 Non-Qualified Options, with an intrinsic value of $8.4 million, were exercised in exchange for 9,930 common shares with an equal total market value. Intrinsic value represents the difference between the market price of the Company’s common shares at the date of exercise less the strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011.

Share-Based Compensation Based on OneBeacon Ltd. Common Shares

The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based and non share-based incentive awards. OneBeacon’s share-based incentive awards include OneBeacon performance shares and restricted shares.

OneBeacon Performance Shares

The following summarizes activity for the years ended December 31, 2016, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	449,435		$	1.4		517,470		$	3.4		493,421		$	4.0
Payments and deferrals⁽¹⁾	(167,300	)	(.6		)	(181,290	)	(1.5		)	(142,138	)	(1.0		)
New awards	163,150		—			154,887		—			165,800		—
Forfeitures and cancellations⁽²⁾	7,234		—			(41,632	)	(.1		)	387		—
Expense recognized	—		.8			—		(.4		)	—		.4
End of period	452,519		$	1.6		449,435		$	1.4		517,470		$	3.4


⁽¹⁾	OneBeacon performance share payments in 2016 for the 2013-2015 performance cycle were at 24.3% of target. OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target.


⁽²⁾	Amounts include changes in assumed forfeitures, as required under GAAP.

The following summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan as of December 31, 2016 for each performance cycle:



$ in millions	Target OneBeacon Performance Shares Outstanding	Accrued Expense
Performance cycle:
2016 – 2018	163,150	$	.9
2015 – 2017	146,659	.7
2014 – 2016	142,710	—
Total at December 31, 2016	452,519	$	1.6

If the outstanding OneBeacon performance shares had been vested on December 31, 2016, the total additional compensation cost to be recognized would have been $2.1 million, based on December 31, 2016 accrual factors (common share price, accumulated dividends and payout assumptions).

The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2016 is growth in book value per share of 12%. At a growth in book value per share of 5% or less, no performance shares would be earned and at a growth in book value per share of 19% or more, 200% of performance shares would be earned.

The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2015 is growth in book value per share of 13%. At a growth in book value per share of 6% or less, no performance shares would be earned and at a growth in book value per share of 20% or more, 200% of performance shares would be earned.

The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2014 is growth in book value per share of 14%. At a growth in book value per share of 7% or less, no performance shares would be earned and at a growth in book value per share of 21% or more, 200% of performance shares would be earned.

OneBeacon Restricted Shares

The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the years ended December 31, 2016, 2015 and 2014.



	Year Ended December 31,
	2016					2015					2014
$ in millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Beginning of period	382,722		$	2.5		612,500		$	3.5		915,000		$	6.5
Issued	170,650		2.3			75,950		1.1			—		—
Vested	(157,500	)	—			(296,000	)	—			(300,000	)	—
Forfeited	—		—			(9,728	)	(.1		)	(2,500	)	—
Expense recognized	—		(2.7		)	—		(2.0		)	—		(3.0		)
End of period	395,872		$	2.1		382,722		$	2.5		612,500		$	3.5

On February 24, 2016, OneBeacon issued 170,650 shares of restricted stock, of which 92,500 restricted shares vest on February 24, 2018 and 78,150 vest on January 1, 2019. On February 24, 2015, OneBeacon issued to certain employees 75,950 shares of restricted stock, which are scheduled to cliff vest on January 1, 2018.

On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments beginning on February 22, 2014, 2015, 2016 and 2017. Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent 5 years have been or will also be reduced by 35,000 shares. The restricted shares contain dividend participation features, and therefore, are considered participating securities.

The unrecognized compensation cost as of December 31, 2016 is expected to be recognized ratably over the remaining vesting periods.

OneBeacon Restricted Stock Units

During the year ended December 31, 2016, 233,461 RSUs were issued, with 214,506 RSUs outstanding as of December 31, 2016 that are scheduled to cliff vest on December 31, 2018. During the year ended December 31, 2015, 226,778 RSUs were issued, with 173,042 RSUs outstanding as of December 31, 2016 that are scheduled to cliff vest on December 31, 2017. At vesting the RSUs will be paid out in cash or common shares at the discretion of the Compensation Committee. Compensation expense associated with the RSUs, which is recognized ratably over the three year vesting period, was $2.1 million and $0.8 million for the year ended December 31, 2016 and 2015.

If 100% of the outstanding RSUs had been vested on December 31, 2016, the total additional compensation cost to be recognized would have been $3.9 million, based on current accrual factors (common share price and accumulated dividends) as of December 31, 2016.

Share-based Compensation Under Qualified Retirement Plans

Contributions to the KSOP with respect to the years ended 2016, 2015 and 2014 were made with either the Company’s or OneBeacon Ltd.’s common shares, dependent on the employer. The variable contribution amounts for eligible participants of the KSOP constituted approximately 3%, 3% and 2% of salary for the years ended 2016, 2015 and 2014. White Mountains recorded $5.5 million, $5.7 million and $4.4 million in compensation expense to pay benefits and allocate common shares to participant’s accounts for the years ended 2016, 2015 and 2014. As of December 31, 2016 and 2015, the plans owned less than 3% or less of either of the Company’s or OneBeacon Ltd.’s total common shares outstanding. All White Mountains common shares held by the KSOP are considered outstanding for earnings (loss) per share computations.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Common Shareholders’ Equity and Non-controlling Interests

12 Months Ended

Dec. 31, 2016

Stockholders' Equity Note [Abstract]

Common Shareholder's Equity

Common Shareholders’ Equity and Non-controlling Interests

Common Shares Repurchased and Retired

During the past several years, White Mountains’s board of directors authorized the Company to repurchase its common shares, from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. The repurchase authorizations do not have a stated expiration date. As of December 31, 2016, White Mountains may repurchase an additional 878,130 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through tender offers that were separately approved by its board of directors.

During 2016, the Company repurchased 1,106,145 common shares for $887.2 million at an average share price of $802, which were comprised of 1,098,123 common shares repurchased under the board authorization for $881.4 million at an average share price of $803 and 8,022 common shares repurchased pursuant to employee benefit plans. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.

During 2015, the Company repurchased 387,495 common shares for $284.2 million at an average share price of $733, which were comprised of 361,839 common shares repurchased under the board authorization for $267.4 million at an average share price of $739 and 10,802 common shares repurchased pursuant to employee benefit plans.

During 2014, the Company repurchased 217,879 common shares for $134.5 million at an average share price of $617, which were comprised of 207,404 common shares repurchased under the board authorization for $128.2 million at an average share price of $618 and 10,475 common shares repurchased pursuant to employee benefit plans.

Common Shares Issued

During 2016, the Company issued a total of 47,030 common shares, which consisted of 25,365 restricted shares to key personnel, 14,930 shares issued to the Company’s Chairman and CEO as a result of exercised options, 5,000 shares issued in satisfaction of performance shares and 1,735 shares issued to directors of the Company.

During 2015, the Company issued a total of 25,016 common shares, which consisted of 23,640 restricted shares issued to key management personnel and 1,376 shares issued to directors of the Company.

During 2014, the Company issued a total of 28,405 common shares, which consisted of 23,440 restricted shares to key personnel, 3,570 shares issued in satisfaction of performance shares and 1,395 shares issued to directors of the Company.

Dividends on Common Shares

For the years ended December 31, 2016, 2015 and 2014, the Company declared and paid cash dividends totaling $5.4 million, $6.0 million and $6.2 million (or $1.00 per common share).

Non-controlling Interests

Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated entities and are presented separately on the balance sheet. The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of December 31, 2016 and 2015:



	December 31, 2016					December 31, 2015
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon Ltd.	23.9	%	$	244.6		24.5	%	$	245.6
SIG Preference Shares	—		—			100.0		250.0

Other, excluding mutuals and reciprocals
HG Global	3.1		16.6			3.1		17.1
MediaAlpha	40.0		11.7			40.0		14.4
Dewar	18.8		3.9			19.0		3.7
Buzzmove	29.1		2.9			—		—
Wobi	5.0		.7			3.9		.6
Tranzact	—		—			36.8		79.4
Total other, excluding mutuals and reciprocals			35.8					115.2

Mutuals and reciprocals
BAM	100.0		(150.9		)	100.0		(140.0		)
SSIE	100.0		4.4			100.0		(16.0		)
Total mutuals and reciprocals			(146.5		)			(156.0		)
Total non-controlling interests			$	133.9				$	454.8

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

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Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Statutory Capital and Surplus	12 Months Ended
Statutory Capital and Surplus	Dec. 31, 2016
Insurance [Abstract]
Statutory Capital and Surplus	Statutory Capital and Surplus White Mountains’s insurance operations are subject to regulation and supervision in each of the jurisdictions where they are domiciled and licensed to conduct business. Generally, regulatory authorities have broad supervisory and administrative powers over such matters as licenses, standards of solvency, premium rates, policy forms, investments, security deposits, methods of accounting, form and content of financial statements, reserves for unpaid loss and LAE, reinsurance, minimum capital and surplus requirements, dividends and other distributions to shareholders, periodic examinations and annual and other report filings. In general, such regulation is for the protection of policyholders rather than shareholders. The NAIC uses risk-based capital (“RBC”) standards for U.S. property and casualty insurers as a means of monitoring certain aspects affecting the overall financial condition of insurance companies. As of December 31, 2016, White Mountains’s active U.S. insurance operating subsidiaries exceeded their respective RBC requirements. The Insurance Act 1978 of Bermuda and related regulations, as amended (“Insurance Act”), regulates the insurance business of Bermuda-domiciled insurers. Under the Insurance Act, insurers are required to maintain available statutory capital and surplus at a level equal to or in excess of its enhanced capital requirement which is established by reference to either a Bermuda Solvency Capital Requirement (“BSCR”) model or an approved internal capital model. Generally, the Bermuda Monetary Authority (“BMA”) has broad supervisory and administrative powers over such matters as licenses, standards of solvency, investments, methods of accounting, form and content of financial statements, minimum capital and surplus requirements, and annual and other report filings. OneBeacon: OneBeacon’s U.S. combined statutory surplus was $624.8 million and $622.3 million as of December 31, 2016 and 2015. OneBeacon’s combined U.S. statutory net income (loss) for the years ended December 31, 2016, 2015 and 2014 was $36.7 million, $44.5 million and $(14.2) million. The minimum policyholders' surplus necessary to satisfy OneBeacon’s top tier regulated U.S. insurance subsidiary, ASIC, regulatory requirements was $107.5 million as of December 31, 2016, which equals the authorized control level of the NAIC risk-based capital of ASIC’s policyholders’ surplus. Split Rock’s statutory capital and surplus was $270.2 million and $249.5 million as of December 31, 2016 and 2015, which met Bermuda’s statutory capital and surplus requirements. Split Rock reported $42.4 million, $45.5 million and $46.2 million of statutory net income for the years ended December 31, 2016, 2015 and 2014. The principle differences between OneBeacon’s combined U.S. and Split Rock’s statutory reported amounts, and the amounts reported in accordance with GAAP include deferred acquisition costs, deferred taxes, and market value adjustments for debt securities. OneBeacon’s insurance subsidiaries' statutory policyholders’ surplus as of December 31, 2016 was in excess of the minimum requirements of relevant state and Bermuda insurance regulations. HG Global/BAM: HG Re is a Special Purpose Insurer under Bermuda insurance regulations and is subject to regulation and supervision by the BMA. As of December 31, 2016, HG Re had statutory capital and surplus of $467.2 million. As a Special Purpose Insurer, HG Re does not have minimum regulatory capital requirements. BAM is domiciled in New York and is subject to regulation by the New York State Department of Financial Services (“NYDFS”). New York financial guarantee insurance law establishes single risk and aggregate limits with respect to insured obligations insured by financial guarantee insurers. BAM’s statutory net loss for the years ended December 31, 2016, 2015 and 2014 was $32.7 million, $32.0 million and $31.8 million. BAM’s members’ surplus, as reported to regulatory authorities as of December 31, 2016 was $431.5 million, which exceeds the minimum members’ surplus necessary for BAM to maintain its New York State financial guarantee insurance license of $66.0 million. Other Operations: SSIE’s policyholders’ surplus, as reported to regulatory authorities as of December 31, 2016 and 2015, was $4.4 million and $5.0 million. SSIE’s statutory net income (loss) for the year ended December 31, 2016 and 2015 was $20.4 million and $(3.5) million. The minimum policyholders’ surplus necessary to satisfy SSIE’s regulatory requirements was $0.4 million as of December 31, 2016, which equals the authorized control level of the NAIC risk-based capital based on SSIE’s policyholders’ surplus. On December 30, 2016, SSIE entered into a Consent Order with the Florida Office of Insurance Regulation governing its wind down and the surrender and termination of SSIE’s Certificate of Authority as a domestic reciprocal in the State of Florida. Pursuant to the Consent Order, it is anticipated that SSIE will non-renew its last policy in August 2017. Dividend Capacity There are no restrictions under Bermuda law or the law of any other jurisdiction on the payment of dividends from retained earnings by White Mountains, provided that after the payment of any dividend, the Company would continue to be able to pay its liabilities as they become due and the realizable value of the Company’s assets would remain greater that its liabilities. However, under the insurance laws of the states and jurisdictions under which White Mountains’s insurance operating subsidiaries are domiciled, an insurer is restricted with respect to the timing and the amount of dividends it may pay without prior approval by regulatory authorities. As of December 31, 2016, White Mountains’s top tier insurance subsidiaries have approximately $562.5 million of GAAP shareholders’ equity (net of $177.1 million of non-controlling interest at OneBeacon), $60.0 million of which can be distributed to White Mountains without prior regulatory approval. As a result, as of December 31, 2016, $502.5 million of White Mountains’s GAAP shareholders’ equity held in its insurance subsidiaries was not available for the payment of dividends without prior regulatory approval, and approximately $2.3 billion of White Mountains’s retained earnings is unrestricted with respect to the payment of dividends to White Mountains’s common shareholders. When determining whether to make distributions from its insurance subsidiaries, White Mountains also considers factors such as internal capital targets and rating agency capital requirements. Accordingly, there can be no assurance regarding the amount of such dividends that may be paid by such subsidiaries in the future. Following is a description of the dividend capacity of White Mountains’s insurance operating subsidiaries: OneBeacon: On December 23, 2014, OneBeacon Insurance Company (subsequently renamed Bedivere) distributed ASIC to its immediate parent at a value of $700.5 million as part of the Runoff Transaction. OBIC also distributed $151.2 million of cash and other securities to its immediate parent in accordance with the prescribed minimum capital to be included in the company at the time of its sale to Armour, as approved by the PID. OneBeacon’s top-tier regulated U.S. insurance subsidiary, ASIC, has the ability to pay dividends to its immediate parent without the prior approval of regulatory authorities in an amount set by formula based on the lesser of (i) adjusted net investment income, as defined by statute, or (ii) 10% of statutory surplus, in both cases as most recently reported to regulatory authorities, subject to the availability of earned surplus. Based upon the formula above, ASIC currently has the ability to pay $11.4 million of dividends without prior approval of regulatory authorities. As of December 31, 2016, ASIC had $624.8 million of statutory surplus and $69.0 million of earned surplus. During 2016, ASIC paid $26.5 million of dividends to its immediate parent. In 2017, Split Rock has the ability to distribute statutory capital without the prior approval of the BMA, provided it does not reduce its total statutory capital, as shown in the previous year’s statutory financial statements, by 15% or more. In addition, Split Rock has the ability to pay dividends without the prior notification of regulatory authorities of up to 25% of its previous financial year’s total statutory capital and surplus, subject to meeting all appropriate liquidity and solvency requirements as specified in the Insurance Act of 1978 and the Companies Act of 1981. As of December 31, 2016, Split Rock had $210.1 million of statutory capital and $60.1 million of statutory surplus for total statutory capital and surplus of $270.2 million. Based upon the limitations described above, Split Rock currently has the ability to distribute up to $31.5 million of statutory capital and pay up to $60.1 million of dividends during 2017 without the prior approval of regulatory authorities. During 2016, Split Rock paid $25.0 million of dividends to its immediate parent and paid no such dividends during 2015. Also, during 2015, OneBeacon Ltd., through an intermediary holding company, contributed $85.0 million to Split Rock as statutory capital. During 2016 and 2015, OneBeacon’s unregulated insurance subsidiaries paid $5.7 million and $5.3 million of dividends to their immediate parent. As of December 31, 2016, OneBeacon’s unregulated insurance subsidiaries had $56.4 million of net unrestricted cash, short-term investments and fixed maturity investments and $71.9 million of other long-term investments, consisting of the OneBeacon Surplus Notes. During 2016 and 2015, OneBeacon Ltd. paid $79.2 million and $80.0 million of regular quarterly dividends to its common shareholders. For both 2016 and 2015, White Mountains received $60.3 million of these dividends. As of December 31, 2016, OneBeacon Ltd. and its intermediate holding companies subsidiaries had $64.5 million of net unrestricted cash, short-term investments and fixed maturity investments and $11.9 million of common equity securities outside of its regulated and unregulated insurance subsidiaries. HG Global/BAM: As of December 31, 2016, HG Global had $619.0 million face value of preferred shares outstanding, of which White Mountains owned 96.9%. Holders of the HG Global preferred shares receive cumulative dividends at a fixed annual rate of 6.0% on a quarterly basis, when and if declared by HG Global. HG Global did not declare or pay any preferred dividends in 2016. As of December 31, 2016, HG Global has accrued $186.2 million of dividends payable to holders of its preferred shares, $180.2 million of which is payable to White Mountains and eliminated in consolidation. HG Re is a Special Purpose Insurer subject to regulation and supervision by the BMA, but does not require regulatory approval to pay dividends. However, HG Re’s dividend capacity is limited to amounts held outside of the collateral trusts pursuant to the first loss reinsurance treaty (“FLRT”) with BAM. As of December 31, 2016, HG Re had statutory capital and surplus of $467.2 million, of which $465.4 million was held in the collateral trusts pursuant to the FLRT with BAM. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 from a fixed rate of 8.0% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which was 3.54% for 2016 and is 3.78% for 2017. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8.0%. BAM is required to seek regulatory approval to pay interest and principal on the BAM Surplus Notes only when adequate capital resources have accumulated beyond BAM’s initial capitalization and a level that continues to support its outstanding obligations, business plan and ratings. BAM did not pay any interest on the BAM Surplus Notes in 2016, 2015 or 2014. Other Operations: During 2016, WM Advisors did not pay any dividends to its immediate parent. As of December 31, 2016, WM Advisors held $17.8 million of net unrestricted cash and short-term investments. During 2016, White Mountains paid a $5.4 million common share dividend. As of December 31, 2016, the Company and its intermediate holding companies held $1,655.0 million of net unrestricted cash, short-term investments and fixed maturity investments, $285.6 million of common equity securities and $66.9 million of other long-term investments included in its Other Operations segment.

Segment Information

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

Segment Information

White Mountains has determined that its reportable segments are OneBeacon, HG Global/BAM and Other Operations. As a result of the Sirius Group and Tranzact sale, the results of operations for Sirius Group and Tranzact have been classified as discontinued operations and are now presented as such, net of related income taxes, in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. See Note 22 — “Held for Sale and Discontinued Operations”.

White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors.

OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products through independent agencies, regional and national brokers, wholesalers and managing general agencies.

The HG Global/BAM segment consists of White Mountains’s investment in HG Global and the consolidated results of BAM. BAM is a municipal bond insurer domiciled in New York that was established to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of BAM Surplus Notes. HG Global also provides 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. BAM's results are attributed to non-controlling interests.

Other Operations consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, WM Advisors, WM Life Re, WM Capital Investments, which includes the consolidated results of MediaAlpha, Wobi, Buzzmove and Star & Shield, and various other investments. In 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes. As a result, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015. SSIE’s results are attributed to non-controlling interests.

Significant intercompany transactions among White Mountains’s segments have been eliminated herein.

Financial information for White Mountains’s segments follows:



			HG Global/BAM
Millions	OneBeacon		HG Global		BAM⁽¹⁾			Other Operations				Total
Year Ended December 31, 2016
Earned insurance and reinsurance premiums	$	1,100.6	$	4.4	$	1.5		$	7.5			$	1,114.0
Net investment income	50.6		2.2		6.8			27.2				86.8
Net investment income (loss) - BAM Surplus Note	—		17.8		(17.8		)	—				—
Net realized and unrealized investment gains (losses)	37.7		.1		.6			(28.1		)		10.3
Other revenue	5.5		—		1.1			143.0			⁽⁴⁾	149.6
Total revenues⁽³⁾	1,194.4		24.5		(7.8		)	149.6				1,360.7
Losses and LAE	656.0		—		—			8.0				664.0
Insurance and reinsurance acquisition expenses	206.0		.9		2.5			2.2				211.6
Other underwriting expenses	209.0		—		.4			.1				209.5
General and administrative expenses	13.0		1.4		38.2			234.8			⁽⁵⁾	287.4
Amortization of intangible assets	1.2		—		—			11.3				12.5
Interest expense	13.1		—		—			3.0				16.1
Total expenses	1,098.3		2.3		41.1			259.4				1,401.1
Pre-tax income (loss)	$	96.1	$	22.2	$	(48.9	)	$	(109.8	)		$	(40.4	)



				HG Global/BAM
Millions	OneBeacon			HG Global			BAM⁽¹⁾			Other Operations			Total
Year Ended December 31, 2015
Earned insurance and reinsurance premiums	$	1,176.2		$	2.5		$	.8		$	8.7		$	1,188.2
Net investment income	45.9			1.9			4.2			8.8			60.8
Net investment income (loss) - BAM Surplus Note	—			15.8			(15.8		)	—			—
Net realized and unrealized investment (losses) gains	(35.1		)	(.3		)	.9			259.9		⁽²⁾	225.4
Other (loss) revenue	(.6		)	—			.7			147.2		⁽⁴⁾	147.3
Total revenues⁽³⁾	1,186.4			19.9			(9.2		)	424.6			1,621.7
Losses and LAE	700.7			—			—			8.2			708.9
Insurance and reinsurance acquisition expenses	213.8			.6			2.3			3.4			220.1
Other underwriting expenses	218.2			—			.4			—			218.6
General and administrative expenses	14.1			1.4			35.4			240.7		⁽⁵⁾	291.6
Amortization of intangible assets	1.3			—			—			9.3			10.6
Interest expense	13.0			—			—			1.6			14.6
Total expenses	1,161.1			2.0			38.1			263.2			1,464.4
Pre-tax income (loss)	$	25.3		$	17.9		$	(47.3	)	$	161.4		$	157.3



			HG Global/BAM
Millions	OneBeacon		HG Global		BAM⁽¹⁾			Other Operations				Total
Year Ended December 31, 2014
Earned insurance and reinsurance premiums	$	1,177.1	$	1.4	$	.4		$	6.1			$	1,185.0
Net investment income	43.4		1.4		5.7			9.0				59.5
Net investment income (loss) - BAM Surplus Note	—		15.7		(15.7		)	—				—
Net realized and unrealized investment gains	40.4		1.7		6.6			29.8				78.5
Other revenue	5.8		—		.6			81.7			⁽⁴⁾	88.1
Total revenues⁽³⁾	1,266.7		20.2		(2.4		)	126.6				1,411.1
Losses and LAE	815.1		—		—			8.9				824.0
Insurance and reinsurance acquisition expenses	203.3		.3		1.8			0.8				206.2
Other underwriting expenses	179.2		—		.4			—				179.6
General and administrative expenses	12.4		1.6		35.9			158.3			⁽⁵⁾	208.2
Amortization of intangible assets	1.4		—		—			6.9				8.3
Interest expense	13.0		—		—			1.2				14.2
Total expenses	1,224.4		1.9		38.1			176.1				1,440.5
Pre-tax income (loss)	$	42.3	$	18.3	$	(40.5	)	$	(49.5	)		$	(29.4	)


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services.


⁽²⁾	Net realized and unrealized investment gains (losses) in the Other Operations segment includes the unrealized investment gain of $258.8 related to the investment in Symetra common shares, representing the difference between the carrying value under the equity method at November 5, 2015 and the fair value at December 31, 2015. See Note 17 — “Investments in Unconsolidated Affiliates”.


⁽³⁾	Total revenues includes both revenues from customers, as well as, investment performance.


⁽⁴⁾	Includes $116.5 from MediaAlpha for the year ended December 31, 2016; $105.5 from MediaAlpha for the year ended December 31, 2015; $65.3 from MediaAlpha for the year ended December 31, 2014.


⁽⁵⁾	Includes $109.6 from MediaAlpha for the year ended December 31, 2016; and $99.0 from MediaAlpha for the year ended December 31, 2015; $60.6 from MediaAlpha for the year ended December 31, 2014.



			HG Global/BAM
Selected Balance Sheet Data Millions	OneBeacon		HG Global		BAM				Other Operations		Eliminations			Held for Sale			Total
December 31, 2016:
Total investments	$	2,620.4	$	161.6	$	468.1			$	2,084.7	$	—		$	—		$	5,334.8
Reinsurance recoverable on paid and unpaid losses	179.5		—		—				—		—			—			179.5
Assets held for sale	—		—		—				—		—			10.1			10.1
Total assets	3,589.4		787.0		(78.9		)	⁽¹⁾	2,417.3		(180.2		)	10.1			6,544.7
Loss and LAE reserves	1,365.6		—		—				—		—			—			1,365.6
Liabilities held for sale	—		—		—				—		—			5.1			5.1
Total liabilities	2,564.2		247.6		72.0				98.8		(180.2		)	5.1			2,807.5
Total White Mountains’s common shareholders’ equity	780.6		522.8		—				2,294.9		—			5.0			3,603.3
Non-controlling interest	244.6		16.6		(150.9		)		23.6		—			—			133.9
December 31, 2015:
Total investments	$	2,591.4	$	136.2	$	460.3			$	1,083.2	$	—		$	—		$	4,271.1
Reinsurance recoverable on paid and unpaid losses	193.5		—		—				—		—			—			193.5
Assets held for sale	—		—		—				—		—			4,790.4			4,790.4
Total assets	3,600.8		739.0		(91.1		)	⁽¹⁾	1,378.8		(135.3		)	4,790.4			10,282.6
Loss and LAE reserves	1,389.8		—		—				—		—			—			1,389.8
Liabilities held for sale	—		—		—				—		—			3,047.4			3,047.4
Total liabilities	2,596.3		181.2		48.9				176.1		(135.3		)	3,047.4			5,914.6
Total White Mountains’s common shareholders’ equity	755.2		540.7		—				1,124.3		—			1,493.0			3,913.2
Non-controlling interest	249.3		17.1		(140.0		)		78.4		—			—		⁽²⁾	204.8


⁽¹⁾	BAM total assets reflect the elimination of $503.0 in the BAM Surplus Notes issued to HG Global and its subsidiaries, and $108.0 and $90.2 in accrued interest related to the BAM Surplus Notes as of December 31, 2016 and 2015.

⁽²⁾ Excludes $250.0 related to SIG Preference Shares. See Note 14 — “Common Shareholders’ Equity and Non-controlling Interests”.

As of December 31, 2016 and 2015, HG Global has accrued $180.2 million and $135.3 million of preferred dividends payable to Lone Tree Holdings, which is included in the Other Operations segment and eliminated in consolidation. As of December 31, 2016 and 2015, the total amount of capital White Mountains has committed to HG Global/BAM, comprised of total common shareholders’ equity and the preferred dividends receivable at Lone Tree Holdings, is $703.0 million and $676.0 million.

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 34
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Paragraph 41
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8380-108599

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 26
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8844-108599

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
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-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8984-108599

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e9031-108599

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e9054-108599

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
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Reference 12: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8924-108599

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8906-108599

+ Details

Investments in Unconsolidated Affiliates

12 Months Ended

Dec. 31, 2016

Equity Method Investments and Joint Ventures [Abstract]

Investments in Unconsolidated Affiliates

White Mountains’s investments in unconsolidated affiliates represent investments in other companies in which White Mountains has a significant voting and economic interest but does not control the entity.

Symetra

In 2004, White Mountains, Berkshire Hathaway, Inc. and several other private investors capitalized Symetra in order to purchase the life and investment operations of Safeco Corporation for $1.35 billion, during which White Mountains invested $195 million in Symetra. As of February 1, 2016 and December 31, 2015, White Mountains owned 20,562,379 common shares of Symetra, a 17.7% common share ownership. On February 1, 2016, Symetra closed its merger agreement with Sumitomo Life Insurance Company (“Sumitomo Life”) and White Mountains received proceeds of $658 million, or $32 per common share.

In August 2015, Symetra announced it had entered into a merger agreement with Sumitomo Life pursuant to which Sumitomo Life would acquire all of the outstanding shares of Symetra. Following the announcement and Symetra shareholders’ November 5, 2015 meeting to approve the transaction, White Mountains relinquished its representation on Symetra’s board of directors. As a result, White Mountains changed its accounting for Symetra common shares from the equity method to fair value. During the fourth quarter of 2015, White Mountains recognized $258.8 million ($241.1 million after tax) of unrealized investment gains through net income, representing the difference between the carrying value of Symetra common shares under the equity method at the date of change and fair value at December 31, 2015. During the first quarter of 2016, White Mountains recognized an additional $4.7 million of unrealized investment gains through net income.

As of December 31, 2011, White Mountains concluded that its investment in Symetra common shares was other-than-temporarily impaired and wrote down the GAAP book value of the investment to its estimated fair value of $261.0 million or $15 per share. This impairment, as well as the effect of Symetra capital transactions, resulted in a basis difference between the GAAP carrying value of White Mountains’s investment in Symetra common shares under the equity method and the amount derived by multiplying the percentage of White Mountains common share ownership by Symetra’s total GAAP equity. As of November 5, 2015, the pre-tax unamortized basis difference was $159.9 million, of which $32.6 million is attributable to equity in earnings of unconsolidated affiliates and $127.3 million is attributable to equity in net unrealized investment gains from investments in unconsolidated affiliates. The pre-tax basis differences were to be amortized over a 30-year period, based on estimated future cash flows associated with Symetra’s underlying assets and liabilities to which the basis differences were attributed. White Mountains recognized the amortization of the basis differences through equity in earnings of unconsolidated affiliates and equity in net unrealized investment gains (losses) from investments in unconsolidated affiliates consistent with the original attribution of the basis differences between equity in earnings and equity in net unrealized investment gains (losses). For the period ended November 5, 2015, White Mountains recognized after-tax amortization of $2.2 million through equity in earnings of unconsolidated affiliates and $8.9 million through equity in net unrealized investment gains from investments in unconsolidated affiliates. For 2014, White Mountains recognized after-tax amortization of $2.8 million through equity in earnings of unconsolidated affiliates and $11.7 million through equity in net unrealized investment gains from investments in unconsolidated affiliates.

The following table summarizes amounts recorded by White Mountains under the equity method relating to its investment in Symetra through November 5, 2015:



Millions
Equity method carrying value of investment in Symetra as of December 31, 2014 ⁽¹⁾	$	411.4
Equity in earnings ^(2)(3)(4)	25.1
Equity in net unrealized investment losses from Symetra’s fixed maturity portfolio ⁽⁵⁾⁽⁶⁾	(39.2		)
Distribution from Prospector Offshore Fund ⁽⁷⁾	12.4
Dividends received	(16.9		)
Equity method carrying value of investment in Symetra as of November 5, 2015⁽¹⁾⁽⁸⁾	$	392.8


⁽¹⁾	Includes White Mountains’s equity in net unrealized investment (losses) gains from Symetra’s fixed maturity portfolio of $(1.6) and $37.6, as of November 5, 2015 and December 31, 2014, which excludes tax benefit (expense) of $0.2 and $(2.7).


⁽²⁾	Equity in earnings for the period ended November 5, 2015 excludes tax expense of $1.6.


⁽³⁾	Equity in earnings for the period ended November 5, 2015 includes an increase of $2.3 relating to the pre-tax amortization of the Symetra common share basis difference.


⁽⁴⁾	Equity in earnings for the period ended November 5, 2015 includes losses of $(0.1) from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra


⁽⁵⁾	Net unrealized investment losses for the period ended November 5, 2015 includes an increase of $9.4 relating to the pre-tax amortization of the Symetra common share basis difference.


⁽⁶⁾	Net unrealized investment losses from Symetra’s fixed maturity portfolio excludes a tax benefit of $2.9 for the period ended November 5, 2015.


⁽⁷⁾	During 2015, pursuant to the redemption of White Mountains's investments in the Prospector Funds, 513,500 common shares of Symetra were distributed to White Mountains.


⁽⁸⁾	The aggregate value of White Mountains’s investment in common shares of Symetra was $651.2 based upon the quoted market price of $31.67 per share as of November 5, 2015.

The following table summarizes financial information for Symetra as of September 30, 2015:



	September 30,
Millions	2015
Symetra balance sheet data:
Total investments	$	32,409.2
Separate account assets	885.9
Total assets	34,962.8
Policyholder liabilities	29,492.0
Long-term debt	697.5
Separate account liabilities	885.9
Total liabilities	31,836.7
Common shareholders’ equity	3,126.1

The following table summarizes financial information for Symetra for the nine months ended September 30, 2015 and year ended December 31, 2014:



	Nine months ended September 30,			Year ended December 31,
Millions	2015			2014
Symetra income statement data:
Net premiums earned	$	539.3		$	629.1
Net investment income	994.3			1,320.5
Total revenues	1,605.9			2,182.4
Policy benefits	1,143.7			1,399.7
Total expenses	1,543.6			1,882.5
Net income	89.6			254.4
Comprehensive net (loss) income	(234.1		)	397.0

Hamer

On May 27, 2015, White Mountains sold its interest in Hamer LLC, a small manufacturing company that White Mountains received in 2012 in connection with the liquidation of a limited partnership fund, which resulted in a gain of $20.0 million recorded in other revenue. Prior to the sale, White Mountains recorded equity in earnings of $1.6 million for the six months ended June 30, 2015 and $1.9 million for the year ended December 31, 2014.

Variable Interest Entities	12 Months Ended
Variable Interest Entities	Dec. 31, 2016
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities BAM As a mutual insurance company, BAM is owned by its members. BAM charges an insurance premium on each municipal bond insurance policy it insures, a portion of which is contributed to BAM’s qualified statutory capital and conveys to the issuer certain interests in BAM. During 2012, White Mountains formed and capitalized HG Global to fund the startup of BAM through the purchase of $503.0 million of BAM Surplus Notes. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional subordinated financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE. BAM and HG Global, through its wholly-owned subsidiary, HG Re, entered into a first loss reinsurance treaty (“FLRT”), under which HG Re will provide first loss protection up to 15% of par outstanding on each bond insured by BAM in exchange for 60% of the premium, net of a ceding commission, charged by BAM. HG Re’s obligations under the FLRT are satisfied by the assets in two collateral trusts: a Regulation 114 Trust and a Supplemental Trust. Losses required to be reimbursed to BAM by HG Re are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, HG Global has the right to designate two directors for election to BAM’s board of directors. White Mountains is required to consolidate the results of BAM. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. Reciprocals Reciprocals are policyholder-owned insurance carriers organized as unincorporated associations. Each policyholder insured by the reciprocal shares risk with the other policyholders. Policyholders share profits and losses in the same proportion as the amount of insurance purchased but are not subject to assessment for net losses of the reciprocal. Houston General Insurance Exchange Houston General Management Company, a wholly-owned indirect subsidiary of OneBeacon provides management services for a fee to a reciprocal, HGIE. OneBeacon holds HGIE Surplus Notes, the remaining balance of which is $5.0 million as of December 31, 2016. The principal and interest on the HGIE Surplus Notes is repayable to OneBeacon only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. OneBeacon has no ownership interest in the reciprocal. OneBeacon has determined that HGIE qualifies as a VIE. Furthermore, OneBeacon has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates HGIE. Consolidated amounts related to HGIE included total assets of $5.1 million and total liabilities of $5.0 million as of both December 31, 2016 and 2015. As of December 31, 2016, the net amount of capital at risk is equal to the HGIE Surplus Notes of $5.0 million less the accumulated losses of $0.5 million which includes accrued interest on the HGIE Surplus Notes of $0.6 million that eliminates in consolidation. Star & Shield Insurance Exchange SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE. Furthermore, White Mountains has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal. Thus, SSIE qualifies as a VIE and is required to consolidate SSIE in its GAAP financial statements. As of December 31, 2015, White Mountains held $21.0 million of SSIE Surplus Notes. On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in the SSIE Surplus Notes which resulted in a $21.0 million decrease to net income attributable to White Mountains’s common shareholders and a corresponding increase to net income attributable to non-controlling interests. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to National General Holdings Corp., which was subsequently terminated on December 2, 2016. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services LLC. Consequently, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015. As of December 31, 2016 and 2015, SSIE had total assets of $9.2 million and $14.2 million and total liabilities of $4.8 million and $30.3 million. Prospector Offshore Fund In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. of which White Mountains’s owned 67.6% prior to the redemption on June 30, 2015. The consolidated results of the Prospector Offshore Fund are included in Other Operations from January 1, 2015 through June 30, 2015, at which point the results of the Prospector Offshore Fund were no longer consolidated by White Mountains. Prior to the redemption, White Mountains determined that the Prospector Offshore Fund was a VIE of which White Mountains was the primary beneficiary and was required to consolidate.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

White Mountains accounts for its financial instruments at fair value with the exception of the OBH Senior Notes and MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount. The following table summarizes the fair values and book values as of December 31, 2016 and 2015:



	December 31, 2016				December 31, 2015
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
OBH Senior Notes	$	274.2	$	273.2	$	276.4	$	272.9
MediaAlpha Bank Facility	13.0		12.7		15.0		14.7

The fair value estimate for the OBH Senior Notes has been determined using observable inputs and is considered a Level 2 measurement. The fair value estimate for the MediaAlpha Bank Facility has been determined based on a discounted cash flows approach and is considered to be Level 3 measurement.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Transactions with Related Persons	12 Months Ended
Transactions with Related Persons	Dec. 31, 2016
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie, the founder and Managing Member of Prospector, retired from the WTM Board of Directors in May 2015. Until June 30, 2015, Prospector served as a discretionary adviser with respect to specified assets, primarily common equity securities and convertible fixed maturity investments, managed directly or through WM Advisors on behalf of White Mountains and other clients of WM Advisors. At that time, the investment management agreements between WM Advisors and Prospector and OneBeacon and Prospector and the Consulting Agreement described below were terminated. Pursuant to an investment management agreements with WM Advisors (the “WMA Agreement”) and OneBeacon (the “OneBeacon Agreement”), Prospector charged WM Advisors and OneBeacon fees based on the following schedule: 100 basis points on the first $200 million of assets under management; 50 basis points on the next $200 million; and 25 basis points on amounts over $400 million. During 2015 and 2014, Prospector earned $2.1 million and $6.5 million in total fees pursuant to the investment management agreements with WM Advisors and OneBeacon. Prospector also had a separate investment management agreement with Symetra that was terminated in the fourth quarter of 2015. Prospector also advised White Mountains on matters including capital management, asset allocation, private equity investments and mergers and acquisitions. Pursuant to a Consulting Agreement for those services, Prospector was granted 6,250 performance shares for the 2014-2016 cycle and 7,000 performance shares for the 2013-2015 cycle. Under the terms of the Consulting Agreement, Prospector earned a prorated portion of the outstanding performance share grants at the time of the termination of the Consulting Agreement and was paid $5.5 million in respect thereof. Pursuant to a pre-existing revenue sharing agreement (the “Revenue Agreement”), Prospector paid White Mountains 6% of the annual revenues in excess of $500,000 of certain of Prospector’s funds in return for White Mountains having made a founding investment in 1997. White Mountains and Prospector mutually agreed to terminate the Revenue Agreement as of June 30, 2015. During 2014, White Mountains earned $0.4 million under the Revenue Agreement. No amount was earned under the Revenue Agreement in 2015. Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector (the “Funds”). For the years ended December 31, 2015 and 2014, White Mountains and OneBeacon incurred $0.4 million and $1.1 million in management fees and $0.1 million and $0.7 million in incentive fees. Other Relationships and Transactions WM Advisors provided investment advisory and management services to Symetra through the fourth quarter of 2016. In August 2015, Symetra announced it had entered into a merger agreement with Sumitomo Life pursuant to which Sumitomo Life would acquire all of the outstanding shares of Symetra. Following the announcement and Symetra shareholders’ November 5, 2015 meeting to approve the transaction, White Mountains relinquished its representation on Symetra’s board of directors and Symetra was no longer a related party to White Mountains. During 2015 and 2014, WMA earned $8.0 million and $18.4 million in fees from Symetra. During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 WTM common shares for $807.00 per share, the market price at the time the agreement was reached. On September 15, 2016, the Company repurchased 305,000 WTM common shares for $820.00 per share, the market price at the time the agreement was reached. During 2015, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company, in two transactions. On June 1, 2015, the Company repurchased 19,688 WTM common shares for $650.03 per share, the market price at the time the agreement was reached. On September 17, 2015, the Company repurchased 26,300 WTM common shares for $761.50, the market price at the time the agreement was reached.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office space under non-cancellable operating leases that expire on various dates through 2021. Rental expense for all of White Mountains’s locations was $14.1 million, $14.0 million and $12.9 million for the years ended December 31, 2016, 2015 and 2014. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $11.4 million, $9.5 million, $8.0 million, and $32.6 million for the years 2017, 2018, 2019 and 2020 and thereafter. White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $138.1 million, do not have fixed funding dates. Capital Lease In December 2016, OneBeacon’s lease agreement with Fifth Third expired and OneBeacon purchased the leased assets under the agreement. In December 2016, the lease agreement with US Bancorp expired and OneBeacon’s leased assets under the agreement with a remaining book value of $1.3 million transferred back to OneBeacon. As of December 31, 2015, OneBeacon had a capital lease obligation of $1.7 million, included within other liabilities, and a capital lease asset of $2.4 million, included within other assets. As of December 31, 2016, OneBeacon does not have capital lease assets or obligations. Assigned Risks As a condition of its license to do business in certain states, White Mountains’s insurance operations are required to participate in mandatory shared market mechanisms. Each state dictates the types of insurance and the level of coverage that must be provided. The total amount of business an insurer is required to accept is based on its market share of voluntary business in the state. In certain cases, White Mountains is obligated to write business from mandatory shared market mechanisms at some time in the future based on the market share of voluntary policies it is currently writing. Underwriting results related to assigned risk plans are typically adverse and are not subject to the predictability associated with White Mountains’s voluntarily written business. Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains accrues any significant insolvencies when the loss is probable and the assessment amount can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years. As of December 31, 2016, the reserve for such assessments totaled $8.0 million. Legal Contingencies White Mountains, and the insurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3 - “Reserves for Unpaid Losses and Loss Adjustment Expenses”. White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or it there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of December 31, 2016: Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011. Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) losses of the Transferred Subsidiaries arising from extra-contractual claims and claims in excess of policy limits, (b) certain corporate reorganizations effected to remove entities from the Transferred Subsidiaries that were not being sold in the transaction, and (c) certain tax matters, including certain net operating losses being less than stated levels. Tribune Company In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the U.S. District Court for the Southern District of New York (the “SDNY”). Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon received approximately $32.0 million and Sirius Group, which is now included in discontinued operations, received $6.1 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs’ appealed. On March 29, 2016, a three judge panel of the U.S Second Circuit Court of Appeals affirmed the dismissal of the Noteholder Actions. On July 22, 2016, the Plaintiff's petition to the Second Circuit for reconsideration or for a rehearing en banc was denied in full. On September 9, 2016 the Plaintiffs filed for a writ of certiorari, seeking review in the U. S. Supreme Court. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the SDNY and was stayed pending the motion to dismiss in the Noteholder Actions. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014 and the motion was granted on January 6, 2017. The plaintiff has requested permission to move the SDNY to certify the decision as a final judgment capable of immediate appeal. No amount has been accrued in connection with this matter as of December 31, 2016, as the amount of loss, if any, cannot be reasonably estimated. OneTitle White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OTH”) and has provided a $10.0 million surplus note facility under which OTH’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may, under certain circumstances, draw funds. At December 31, 2016, no funds had been drawn on the surplus note facility.

Discontinued Operations

12 Months Ended

Dec. 31, 2016

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Discontinued Operations

Held for Sale and Discontinued Operations

Sirius Group

On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion. $161.8 million of this amount was used to purchase certain assets to be retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. During 2016, White Mountains recorded $363.2 million of gain from sale of Sirius Group in discontinued operations in the statement of operations and $113.3 million in other comprehensive income from discontinued operations.

Through April 18, 2016, Sirius Group’s results are reported as discontinued operations and assets and liabilities held for sale within White Mountains’s GAAP financial statements. Assets held for sale did not include White Mountains’s investment in OneBeacon and certain other investments that were held in the Sirius Group legal entities. As of December 31, 2015, the value of these investments, net of related tax effects, was $686.2 million, of which $528.6 million related to Symetra. Net income (loss) from discontinued operations does not include White Mountains’s net investment income and net realized and unrealized investment gains (losses) associated with these investments. For the years ended December 31, 2016, 2015 and 2014, $3.7 million, $205.0 million, of which $200.8 million is related to Symetra, and $2.7 million of net investment income and net realized and unrealized investment gains (losses), net of related tax effects, that were included in the Sirius Group legal entities have been excluded from net income (loss) from discontinued operations. For the years ended December 31, 2016, 2015 and 2014, White Mountains recorded $(4.3) million, $81.1 million and $262.0 million of net (loss) income from discontinued operations and $32.0 million, $(65.0) million, and $(169.5) million of other comprehensive income (loss) from Sirius Group.

Tranzact

On June 9, 2016, White Mountains announced that it had entered into an agreement for the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC. On July 21, 2016, White Mountains completed the sale of Tranzact and received net proceeds of $221.3 million at closing. On October 5, White Mountains received additional proceeds of $1.2 million following the release of the post-closing purchase price adjustment escrow. For the year ended December 31, 2016, White Mountains recorded $51.9 million of gain from the sale of Tranzact in discontinued operations in the statement of operations. See Note 2 - “Significant Transactions”.

Through July 21, 2016, Tranzact's results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains's GAAP financial statements. For the year ended December 31, 2016, 2015, and 2014, White Mountains recorded net income (loss) from discontinued operations of $6.1 million, $(1.9) million and $0.4 million from Tranzact.

White Mountains recognized a $21.4 million tax benefit in continuing operations related to the reversal of a valuation allowance that resulted from the gain on the sale of Tranzact recognized within discontinued operations. This tax benefit was recorded in continuing operations with an offsetting amount of net tax expense recorded in discontinued operations, $30.2 million of tax expense was recorded to gain from sale of Tranzact in discontinued operations and a $8.8 million tax benefit was recorded to net income from discontinued operations.

Star & Shield

On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in SSIE Surplus Notes which resulted in a $21.0 million total decrease to net income attributable to White Mountain’s common shareholders and a corresponding increase to net income attributable to non-controlling interests. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services LLC. As a result, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015.

OneBeacon Runoff

In December 2014, OneBeacon completed the Runoff Transaction. The results of operations for the Runoff Business have been classified as discontinued operations and are presented, net of related income taxes, in the statement of operations and comprehensive income. The amounts classified as discontinued operations exclude investing and financing activities that are conducted on an overall consolidated level and, accordingly, there were no separately identifiable investments associated with the Runoff Business. The vast majority of the gross and net run-off unpaid loss and LAE reserves were included in the entities that were transferred to Armour. As of December 31, 2016 and 2015, gross loss and LAE reserves of $18.3 million and $20.4 million related to the Runoff Business were ceded to OBIC, an indirect wholly-owned subsidiary of Armour, beginning December 23, 2014, by a 100% quota share agreement with ASIC.

For the year ended December 31, 2015, White Mountains recorded $0.3 million to the gain from sale of discontinued operations related to an adjustment on the estimated loss on sale, which included the final settlement of certain post-closing items. For the year ended December 31, 2015, the net loss from discontinued operations relating to the Runoff Business, net of tax, was $0.5 million.

Esurance

For the years ended December 31, 2015 and 2014, White Mountains recorded a gain from sale of Esurance and AFI in discontinued operations of $17.9 million and $3.2 million, which primarily related to a payments from Allstate for favorable development on loss reserves. Since the closing of the transaction through December 31, 2016, White Mountains has received a net amount of $28.3 million from Allstate, primarily related to the favorable development on loss reserves. See Note 21 - “Commitments and Contingencies”.

Fireman's Fund

During 2014, White Mountains recorded a gain in discontinued operations of $14.0 million from a payment received from Allianz, the purchaser of White Mountains's former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991.

Summary of Reclassified Balances and Related Items

Net Assets Held for Sale

The following summarizes the assets and liabilities associated with business classified as held for sale. At December 31, 2016, the amounts presented relate to Star and Shield and SSIE. At December 31, 2015, amounts presented relate to Sirius Group, Tranzact, Star and Shield and SSIE.



	December 31,
Millions	2016			2015
Assets held for sale
Fixed maturity investments, at fair value	$	6.6		$	2,383.5
Short-term investments, at amortized cost (which approximates fair value)	.2			352.1
Common equity securities, at fair value	—			174.4
Other long-term investments	—			72.2
Total Investments	6.8			2,982.2
Cash	.9			150.2
Reinsurance recoverable on unpaid losses	.3			283.6
Reinsurance recoverable on paid losses	—			10.2
Insurance and reinsurance premiums receivable	1.5			326.6
Deferred acquisition costs	—			74.6
Deferred tax asset	—			303.1
Ceded unearned insurance and reinsurance premiums	—			87.7
Accounts receivable on unsettled investment sales	—			29.0
Goodwill and other intangible assets	—			330.5
Other assets	.6			212.7
Total assets held for sale	$	10.1		$	4,790.4
Liabilities held for sale
Loss and loss adjustment expense reserves	$	5.0		$	1,650.4
Unearned insurance and reinsurance premiums	1.2			344.3
Debt	—			506.4
Deferred tax liability	—			270.6
Accrued incentive compensation	—			64.2
Funds held under reinsurance treaties	—			52.9
Other liabilities	(1.1		)	158.6
Total liabilities held for sale	5.1			3,047.4
Net assets held for sale	$	5.0		$	1,743.0

Net Income (Loss) from Discontinued Operations

The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations. For the year ended December 31, 2016, the amounts presented relate to Sirius Group and Tranzact. For the years ended December 31, 2015 and 2014, the amounts presented relate to Sirius Group, Tranzact, Esurance, and the OneBeacon's runoff business. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains.



	Year Ended December 31, 2016
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	240.1		$	—		$	240.1
Net investment income	14.4			—			14.4
Net realized and unrealized investment losses	(1.5		)	—			(1.5		)
Other revenue	.6			119.6			120.2
Total revenues	253.6			119.6			373.2
Expenses
Loss and loss adjustment expenses	154.9			—			154.9
Insurance and reinsurance acquisition expenses	59.0			—			59.0
Other underwriting expenses	30.9			—			30.9
Interest expense on debt	7.9			3.2			11.1
General and administrative expenses	8.3			118.8			127.1
Total expenses	261.0			122.0			383.0
Pre-tax loss	(7.4		)	(2.4		)	(9.8		)
Income tax benefit	3.1			6.4			9.5
Net (loss) income from discontinued operations	(4.3		)	4.0			(.3		)
Gain from sale of Sirius Group, net of tax	363.2			—			363.2
Gain from sale of Tranzact, net of tax	—			51.9			51.9
Total income from discontinued operations	358.9			55.9			414.8
Change in foreign currency translation and other from discontinued operations, net of tax	32.0			—			32.0
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	113.3			—			113.3
Comprehensive income from discontinued operations	$	504.2		$	55.9		$	560.1



	Year Ended December 31, 2015
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	847.0		$	—		$	847.0
Net investment income	40.7			—			40.7
Net realized and unrealized investment gains	15.1			—			15.1
Other (loss) revenue	(20.6		)	186.2			165.6
Total revenues	882.2			186.2			1,068.4
Expenses
Loss and loss adjustment expenses	422.7			—			422.7
Insurance and reinsurance acquisition expenses	189.8			—			189.8
Other underwriting expenses	107.9			—			107.9
Interest expense on debt	26.6			4.0			30.6
General and administrative expenses	27.0			185.3			212.3
Total expenses	774.0			189.3			963.3
Pre-tax income (loss)	108.2			(3.1		)	105.1
Income tax (expense) benefit	(27.1		)	.7			(26.4		)
Net income (loss) from discontinued operations	81.1			(2.4		)	78.7
Gain from sale of OneBeacon, net of tax	—			.3			.3
Gain from sale of Esurance and AFI, net of tax	—			17.9			17.9
Total income from discontinued operations	81.1			15.8			96.9
Change in foreign currency translation and other from discontinued operations, net of tax	(65.0		)	—			(65.0		)
Comprehensive income from discontinued operations	$	16.1		$	15.8		$	31.9



	Year Ended December 31, 2014
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	873.9		$	.1		$	874.0
Net investment income	40.9			—			40.9
Net realized and unrealized investment gains	205.4			—			205.4
Other (loss) revenue	(62.4		)	43.0			(19.4		)
Total revenues	1,057.8			43.1			1,100.9
Expenses
Loss and loss adjustment expenses	345.3			(.7		)	344.6
Insurance and reinsurance acquisition expenses	193.6			.1			193.7
Other underwriting expenses	129.7			3.5			133.2
Interest expense on debt	26.3			1.4			27.7
General and administrative expenses	30.5			41.2			71.7
Total expenses	725.4			45.5			770.9
Pre-tax income (loss)	332.4			(2.4		)	330.0
Income tax (expense) benefit	(70.4		)	1.0			(69.4		)
Net income (loss) from discontinued operations	262.0			(1.4		)	260.6
Gain from sale of FFIC, net of tax	—			14.0			14.0
Loss from sale of OneBeacon, net of tax	—			(18.8		)	(18.8		)
Gain from sale of Esurance and AFI, net of tax	—			3.2			3.2
Total income (loss) from discontinued operations	262.0			(3.0		)	259.0
Change in foreign currency translation and other from discontinued operations, net of tax	(169.5		)	—			(169.5		)
Comprehensive income (loss) from discontinued operations	$	92.5		$	(3.0	)	$	89.5

Net Change in Cash from Discontinued Operations

The following summarizes the net change in cash associated with the businesses classified as discontinued operations:



	Year ended
	December 31,
Millions	2016			2015			2014
Net cash (used for) provided from operations	$	(24.7	)	$	56.0		$	48.9
Net cash provided from investing activities	221.7			(5.4		)	51.2
Net cash used for financing activities	(.2		)	(11.6		)	(64.5		)
Effect of exchange rate changes on cash	—			(4.5		)	(14.3		)
Net change in cash during the period	196.8			34.5			21.3
Cash balances at beginning of period	150.2			116.8			93.2
Net change in cash held for sale	(.3		)	(1.1		)	2.3
Cash sold as part of sale of consolidated subsidiaries	345.8			—			—
Cash balances at end of period	$	.9		$	150.2		$	116.8

Earnings Per Share

Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016			2015			2014
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	414.8		$	96.9		$	259.0
Allocation of income for participating unvested restricted common shares ⁽¹⁾	(5.4		)	(1.1		)	(3.3		)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts ⁽²⁾	$	409.4		$	95.8		$	255.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	5,014.9			5,879.2			6,104.9
Average unvested restricted common shares ⁽³⁾	(64.8		)	(68.0		)	(78.9		)
Basic earnings per share denominator	4,950.1			5,811.2			6,026.0
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	5,018.1			5,879.2			6,104.9
Average unvested restricted common shares⁽³⁾	(64.8		)	(68.0		)	(78.9		)
Diluted earnings per share denominator⁽⁴⁾	4,953.3			5,811.2			6,026.0
Basic earnings per share (in dollars):	$	82.71		$	16.48		$	42.43
Diluted earnings per share (in dollars):	$	82.66		$	16.48		$	42.43


⁽¹⁾	Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.


⁽²⁾	Net income (loss) attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2016, 2015 and 2014.


⁽³⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Incentive Compensation Plans”.


⁽⁴⁾	The diluted earnings (loss) per share denominator for the years ended December 31, 2016, includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which results in 3,217 incremental shares outstanding over the period.

Sirius Group

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Discontinued Operations

Fair Value of Financial Instruments

The following table summarizes the fair value and carrying value of the Sirius Group financials instruments, the SIG Senior Notes and the SIG Preference Shares, and the Tranzact Bank Facility, which were carried at face value less unamortized issuance costs, as of December 31, 2015:



	December 31, 2015
Millions	Fair Value		Carrying Value
SIG Senior Notes	$	410.0	$	399.8
SIG Preference Shares	255.0		250.0
Tranzact Bank Facility	102.8		102.9

The fair value estimates for the SIG Senior Notes and the SIG Preference Shares have been determined based on indicative broker quotes and are considered to be Level 3 measurements. The fair value estimate for the Tranzact Bank Facility was determined based on a discount cash flows approach and is considered to be Level 3 measurements.

- Definition

The entire disclosure related to a disposal group. Includes, but is not limited to, a discontinued operation, disposal classified as held-for-sale or disposed of by means other than sale or disposal of an individually significant component.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721665-107760

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5C
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64862545&loc=SL51724579-110230

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721663-107760

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721659-107760

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721671-107760

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5D
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721677-107760

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1474-107760

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721683-107760

+ Details

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Selected Quartely Financial Data (Unaudited)

SELECTED QUARTERLY FINANCIAL DATA

(Unaudited)

Selected quarterly financial data for 2016 and 2015 is shown in the following table. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. As a result of the sale of Tranzact, Sirius, and Esurance and the Runoff Business, the results of operations for Tranzact, Sirius, Esurance and the Runoff Business have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. See Note 22 — “Held for Sale and Discontinued Operations”.



	2016 Three Months Ended											2015 Three Months Ended
Millions, except per share amounts	Dec. 31			Sept. 30		June 30			Mar. 31			Dec. 31			Sept. 30			June 30			Mar. 31
Revenues	$	268.2		$	365.0	$	358.2		$	369.3		$	602.6		$	296.1		$	368.7		$	354.3
Expenses	335.0			345.0		360.3			360.8			356.3			383.0			373.8			351.3
Pre-tax (loss) income	(66.8		)	20.0		(2.1		)	8.5			246.3			(86.9		)	(5.1		)	3.0
Tax benefit (expense)	23.0			6.7		6.0			9.7			1.0			1.6			2.2			(4.6		)
(Loss) income from continuing operations	(43.8		)	26.7		3.9			18.2			247.3			(85.3		)	(2.9		)	(1.6		)
(Loss) income from discontinued operations, net of tax	(6.8		)	61.9		358.6			1.1			19.6			6.5			(.5		)	71.3
Non-controlling interest in consolidated subsidiaries	17.3			3.1		(21.4		)	(6.3		)	(6.1		)	16.0			.9			7.3
Equity in earnings of unconsolidated affiliates	—			—		—			—			7.1			3.9			6.8			7.3
(Loss) income attributable to White Mountains’s common shareholders	$	(33.3	)	$	91.7	$	341.1		$	13.0		$	267.9		$	(58.9	)	$	4.3		$	84.3
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(5.81	)	$	6.12	$	(3.44	)	$	2.51		$	43.82		$	(11.10	)	$	0.80		$	1.76
Discontinued operations	(1.49		)	12.72		70.37			(.17		)	3.46			1.09			(.08		)	12.33
Total consolidated operations	$	(7.30	)	$	18.84	$	66.93		$	2.34		$	47.28		$	(10.01	)	$	0.72		$	14.09
Diluted
Continuing operations	$	(5.81	)	$	6.11	$	(3.43	)	$	2.51		$	43.79		$	(11.10	)	$	0.80		$	1.76
Discontinued operations	(1.49		)	12.69		70.22			(.17		)	3.46			1.09			(.08		)	12.33
Total consolidated operations	$	(7.30	)	$	18.80	$	66.79		$	2.34		$	47.25		$	(10.01	)	$	0.72		$	14.09

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES

12 Months Ended

Dec. 31, 2016

Summary of Investments, Other than Investments in Related Parties [Abstract]

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUMMARY OF INVESTMENTS—OTHER THAN

INVESTMENTS IN RELATED PARTIES

At December 31, 2016



Millions	Cost		Carrying Value		Fair Value
Fixed maturity investments:
Bonds:
U.S. Government and government agencies and authorities	$	281.7	$	278.3	$	278.3
Debt securities issued by corporations	1,512.6		1,509.4		1,509.4
States, municipalities and political subdivisions	308.8		309.0		309.0
Mortgage and asset-backed securities	2,141.7		2,132.9		2,132.9
Foreign governments	12.9		13.2		13.2
Preferred stocks	8.3		14.0		14.0
Total fixed maturity investments ⁽¹⁾	4,266.0		4,256.8		4,256.8
Short-term investments ⁽¹⁾	287.1		287.1		287.1
Common equity securities:
Exchange traded funds	316.4		321.6		321.6
Banks, trust and insurance companies	11.2		11.6		11.6
Public utilities	—		—		—
Industrial, miscellaneous and other	113.2		141.1		141.1
Total common equity securities	440.8		474.3		474.3
Other long-term investments ⁽²⁾	314.9		323.3		323.3
Total investments	$	5,308.8	$	5,341.5	$	5,341.5


⁽¹⁾	Includes carrying value of $6.6 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.


⁽²⁾	Includes carrying value of $3.5 associated with other long-term investment limited partnerships accounted for using the equity method, $(1.2) related to foreign currency forward contracts, and $12.3 for the carrying value of tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

12 Months Ended

Dec. 31, 2016

Condensed Financial Information of Parent Company Only Disclosure [Abstract]

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT

CONDENSED BALANCE SHEETS



	December 31,
Millions	2016			2015
Assets:
Cash	$	3.1		$	.3
Fixed maturity investments, at fair value	80.0			—
Other long-term investments ⁽¹⁾	(1.2		)	—
Short-term investments, at amortized cost	12.5			23.4
Other assets	1.4			6.3
Investments in consolidated and unconsolidated affiliates	4,898.2			4,350.9
Total assets	$	4,994.0		$	4,380.9
Liabilities:
Revolving line of credit	$	—		$	50.0
Payable to subsidiary ⁽²⁾	1,387.2			400.0
Other liabilities	3.5			17.7
Total liabilities	1,390.7			467.7
White Mountains’s common shareholders’ equity	3,603.3			3,913.2
Total liabilities and equity	$	4,994.0		$	4,380.9

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME



	Year Ended December 31,
Millions	2016			2015			2014
(Loss) revenues (including realized gains and losses)	$	(1.0	)	$	5.0		$	.3
Expenses	68.2			59.9			32.3
Pre-tax loss	(69.2		)	(54.9		)	(32.0		)
Income tax (expense) benefit	(.5		)	—			.9
Net loss	(69.7		)	(54.9		)	(31.1		)
Net gain from discontinued operations, net of tax	—			—			13.9			⁽³⁾
Equity in earnings from consolidated and unconsolidated affiliates	482.2			352.5			329.4
Net income attributable to White Mountains’s common shareholders	412.5			297.6			312.2
Other comprehensive income (loss) items, after-tax	145.3			(100.4		)	(101.6		)
Comprehensive income attributable to White Mountains’s common shareholders	$	557.8		$	197.2		$	210.6
Computation of net income available to common shareholders:
Net income available to common shareholders	$	412.5		$	297.6		$	312.2

⁽¹⁾Other investments includes $(1.2) related to foreign currency forward contracts. See Note 9 — “Derivatives”.

⁽²⁾ During 2016 and 2015, the Company used cash proceeds received from the issuance of debt from its direct wholly-owned subsidiary, Lone Tree Holdings, Ltd., primarily to fund repurchases of its common shares.

⁽³⁾During 2014, the Company received a payment from Allianz, the purchaser of White Mountains’s former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991. See Note 22 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE II

(continued)

CONDENSED STATEMENTS OF CASH FLOWS



	Year Ended December 31,
Millions	2016			2015			2014
Net income attributable to White Mountains’s common shareholders	$	412.5		$	297.6		$	312.2
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment gains (losses) on sales of investments	1.1			—			(.2		)
Undistributed current earnings from subsidiaries	(482.2		)	(352.5		)	(329.4		)
Net gain on sale of other discontinued operations ⁽¹⁾	—			—			(13.9		)
Other non-cash reconciling items including restricted share and option amortization	17.9			(.4		)	12.9
Net change in other assets and liabilities ⁽²⁾	(11.4		)	21.4			(7.6		)
Net cash used for continuing operations	(62.1		)	(33.9		)	(26.0		)
Net cash provided from discontinued operations ⁽¹⁾	—			—			13.9
Net cash used for operations	(62.1		)	(33.9		)	(12.1		)
Cash flows from investing activities:
Net decrease in short-term investments	10.9			7.6			(29.6		)
Purchases of investment securities	—			—			(7.6		)
Sales and maturities of investment securities	—			—			39.4
Issuance of debt from subsidiaries ⁽⁴⁾	992.0			271.0			144.6
Repayment of debt to subsidiaries	(5.0		)	(35.0		)	(30.0		)
Receipt of cash flows from discontinued operations ⁽¹⁾	—			—			13.9
Distributions from subsidiaries⁽³⁾	—			15.0			29.7
Net cash provided from investing activities — continuing operations	997.9			258.6			160.4
Net cash used for investing activities — discontinued operations	—			—			(13.9		)
Net cash provided from investing activities	997.9			258.6			146.5
Cash flows from financing activities:
Draw down of revolving line of credit ⁽⁵⁾	350.0			125.0			65.0
Repayment of revolving line of credit ⁽⁵⁾	(400.0		)	(75.0		)	(65.0		)
Proceeds from issuances of common shares	3.7			—			—
Repurchases and retirement of common shares⁽⁴⁾	(881.3		)	(268.6		)	(128.2		)
Dividends paid on common shares	(5.4		)	(6.0		)	(6.2		)
Net cash used for financing activities — continuing operations	(933.0		)	(224.6		)	(134.4		)
Net cash (used for) provided from financing activities — discontinued operations	—			—			—
Net cash used for financing activities	(933.0		)	(224.6		)	(134.4		)
Net decrease in cash during the year	2.8			.1			—
Cash balance at beginning of year	.3			.2			.2
Cash balance at end of year	$	3.1		$	.3		$	.2
Supplemental cash flow information:
Interest paid	$	(1.2	)	$	—		$	(.3	)


⁽¹⁾	During 2014, the Company received a payment from Allianz, the purchaser of White Mountains’s former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991. See Note 22 — “Held for Sale and Discontinued Operations”.


⁽²⁾	For 2016, 2015 and 2014, net change in other assets and liabilities included a $0.2, $2.4, and 0.7 payable to the Company’s subsidiaries.


⁽³⁾	During 2016, the Company received a non-cash distribution of $80.0 from Lone Tree Holdings, Ltd., a direct wholly-owned subsidiary. The distribution was completed through the transfer of fixed maturity investments. During 2015 and 2014, the Company received cash distributions of $15.0 and $29.7 from Lone Tree Holdings, Ltd.


⁽⁴⁾	During 2016, 2015 and 2014, the Company used cash proceeds received from the issuance of debt from Lone Tree Holdings, Ltd. primarily to fund repurchases of its common shares.


⁽⁵⁾	The WTM Bank Facility presented in Note 7 — “Debt” is a direct obligation of the Registrant.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

- Definition

The entire disclosure for condensed financial information, including the financial position, cash flows, and the results of operations of the registrant (parent company) as of the same dates or for the same periods for which audited consolidated financial statements are being presented. Alternatively, the details of this disclosure can be reported by the specific parent company taxonomy elements, indicating the appropriate date and period contexts in an instance document.

+ References + Details

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION

12 Months Ended

Dec. 31, 2016

Supplementary Insurance Information [Abstract]

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTARY INSURANCE INFORMATION ⁽³⁾

(Millions)



Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H		Column I		Column J		Column K
Segment Millions	Deferred acquisition costs		Future policy benefits, losses, claims and loss expenses		Unearned premiums		Other policy claims and benefits payable		Premiums earned		Net investment income⁽¹⁾		Benefits, claims, losses, and settlement expenses		Amortization of deferred policy acquisition costs		Other operating expenses		Premiums written
Years ended:
December 31, 2016
OneBeacon	$	96.3	$	1,365.6	$	575.1	$	—	$	1,100.6	$	50.6	$	656.0	$	206.0	$	209.0	$	1,100.7
HG Global/BAM	10.6		—		82.9		—		5.9		9.0		—		3.4		.4		38.6
Other operations ⁽²⁾	—		5.0		1.2		—		7.5		.2		8.0		2.2		.1		6.5
December 31, 2015
OneBeacon	$	100.7	$	1,389.8	$	560.3	$	—	$	1,176.2	$	45.9	$	700.7	$	213.8	$	218.2	$	1,136.6
HG Global/BAM	6.9		—		50.2		—		3.3		6.1		—		2.9		.4		25.9
Other operations ⁽²⁾	—		6.0		2.1		—		8.7		.2		8.2		3.4		—		10.1
December 31, 2014
OneBeacon	$	103.2	$	1,342.2	$	588.3	$	—	$	1,177.1	$	43.4	$	815.1	$	203.3	$	179.2	$	1,216.9
HG Global/BAM	4.0		—		27.6		—		1.8		7.1		—		2.1		.4		16.2
Other operations ⁽²⁾	—		7.8		.8		—		6.1		.2		8.9		.8		—		5.9

⁽¹⁾ The amounts shown exclude net investment income relating to non-insurance operations of $27.0, $8.6 and $8.8 for the twelve months ended December 31, 2016, 2015 and 2014, respectively.

⁽²⁾The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests.

⁽³⁾Schedule excludes activity related to Sirius Group for all periods presented. See Note 22 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE IV REINSURANCE

12 Months Ended

Dec. 31, 2016

Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]

SCHEDULE IV REINSURANCE

WHITE MOUNTAINS INSURANCE GROUP, LTD.

REINSURANCE ⁽³⁾

(Millions)



Column A	Column B		Column C			Column D		Column E		Column F
Premiums earned $ in millions	Gross amount		Ceded to other companies			Assumed from other companies		Net amount		Percentage of amount assumed to net
Years ended:
December 31, 2016
OneBeacon	$	1,177.0	$	(105.8	)	$	29.4	$	1,100.6	2.7	%
HG/BAM	5.9		—			—		5.9		—	%
Other operations ⁽²⁾	15.2		(7.7		)	—		7.5		—
December 31, 2015
OneBeacon ⁽¹⁾	$	1,298.0	$	(167.7	)	$	45.9	$	1,176.2	3.9	%
HG/BAM	3.3		—			—		3.3		—	%
Other operations ⁽²⁾	20.7		(12.0		)	—		8.7		—
December 31, 2014
OneBeacon ⁽¹⁾	$	1,209.1	$	(102.9	)	$	70.9	$	1,177.1	6.0	%
HG/BAM	1.8		—			—		1.8		—	%
Other operations ⁽²⁾	$	22.6	$	(16.5	)	$	—	$	6.1	—	%


⁽¹⁾	The amounts shown excludes balances reclassified to discontinued operations in the consolidated income statement related to the Runoff Transaction as of December 31, 2015 and 2014. See Note 22 — “Held for Sale and Discontinued Operations”.


⁽²⁾	The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests.


⁽³⁾	Schedule excludes activity related to Sirius Group for all periods presented. See Note 22 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS

12 Months Ended

Dec. 31, 2016

Valuation and Qualifying Accounts [Abstract]

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS

WHITE MOUNTAINS INSURANCE GROUP, LTD.

VALUATION AND QUALIFYING ACCOUNTS ⁽³⁾



Column A	Column B		Column C						Column D			Column E
			Additions (subtractions)
Millions	Balance at beginning of period		Charged to costs and expenses			Charged to other accounts ⁽¹⁾			Deductions described ⁽²⁾			Balance at end of period
Years ended:

December 31, 2016
Reinsurance recoverable on paid losses:
Allowance for reinsurance balances	$	.7	$	.2		$	—		$	—		$	.9
Property and casualty insurance and reinsurance premiums receivable:
Allowance for uncollectible accounts	2.0		(.3		)	(.4		)	—			1.3

December 31, 2015
Reinsurance recoverable on paid losses:
Allowance for reinsurance balances	$	2.2	$	(.1	)	$	—		$	(1.4	)	$	.7
Property and casualty insurance and reinsurance premiums receivable:
Allowance for uncollectible accounts	1.9		.8			(.7		)	—			2.0

December 31, 2014
Reinsurance recoverable on paid losses:
Allowance for reinsurance balances	$	14.3	$	(.5	)	$	—		$	(11.6	)	$	2.2
Property and casualty insurance and reinsurance premiums receivable:
Allowance for uncollectible accounts	3.1		(.2		)	(1.0		)	—			1.9


⁽¹⁾	Amount credited to other accounts represents a reduction in the allowance for uncollectible premiums receivable which was offset by a corresponding reduction in gross premiums receivable. There was no impact to premiums receivable as presented in the consolidated balance sheet.


⁽²⁾	Represents net collections (charge-offs) of balances receivable.


⁽³⁾	Schedule excludes activity related to Sirius Group and Tranzact for all periods presented. See Note 22 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS

12 Months Ended

Dec. 31, 2016

Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS ⁽³⁾

(Millions)



Column A	Column B		Column C		Column D			Column E		Column F		Column G		Column H					Column I			Column J		Column K
Affiliation with registrant	Deferred acquisition costs		Reserves for Unpaid Claims and Claims Adjustment Expenses		Discount, if any, deducted in Column C			Unearned Premiums		Earned Premiums		Net investment income		Claims and Claims Adjustment Expenses Incurred Related to					Amortization of deferred policy acquisition costs			Paid Claims and Claims Adjustment Expenses		Premiums written
Millions	Deferred acquisition costs		Reserves for Unpaid Claims and Claims Adjustment Expenses		Discount, if any, deducted in Column C			Unearned Premiums		Earned Premiums		Net investment income		Current Year ⁽¹⁾		Prior Year			Amortization of deferred policy acquisition costs			Paid Claims and Claims Adjustment Expenses		Premiums written
OneBeacon:
2016	$	96.3	$	1,365.6	$	1.6	⁽¹⁾	$	575.1	$	1,100.6	$	50.6	$	640.6	$	15.4		$	(206.0	)	$	667.1	$	1,100.7
2015	100.7		1,389.8		1.1		⁽¹⁾	560.3		1,176.2		45.9		702.5		(1.8		)	(213.8		)	677.5		1,136.6
2014	103.2		1,342.2		1.0		⁽¹⁾	588.3		1,177.1		43.4		725.3		89.8			(203.3		)	608.6		1,216.9
Other operations⁽²⁾:
2016	$	—	$	5.0	$	—		$	1.2	$	7.5	$	.2	$	8.6	$	(0.6	)	$	—		$	8.8	$	6.5
2015	—		6.0		—			2.1		8.7		.2		10.4		(2.2		)	—			10.4		10.1
2014	—		7.8		—			.8		6.1		.2		6.7		2.2			—			14.8		5.9

⁽¹⁾

The amounts shown represent OneBeacon’s discount on its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using a discount rate which is determined based on the facts and circumstances applicable at the time the claims are settled (2.5% as of December 31, 2016, 2015 and 2014).


⁽²⁾	The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests. SSIE’s assets and liabilities are presented held for sale. See Note 22 — “Held for Sale and Discontinued Operations”.


⁽³⁾	Schedule excludes amounts related to Sirius Group for all periods presented. See Note 22 — “Held for Sale and Discontinued Operations”.

Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2016
Accounting Policies [Abstract]
Revenue Recognition, Services, Commissions [Policy Text Block]	Commission and Other Revenue Recognition White Mountains recognizes agent commissions and other revenues when it has fulfilled all of its obligations necessary to earn the revenue and when it can reliably estimate both the amount of revenue, net of any amounts expected to be uncollectible, and any amounts associated with expected cancellations.
Investment securities	Investment Securities As of December 31, 2016, White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments, common equity securities and other long-term investments held for general investment purposes are classified as trading securities and are reported at fair value as of the balance sheet date. Changes in net unrealized investment gains (losses) are reported pre-tax in revenues. Realized investment gains (losses) are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common equity securities and other long-term investments, such as interests in hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes issued in connection with the Runoff Transaction. In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs. As of December 31, 2016 and 2015, approximately 94% and 91% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, short-term investments, which include U.S. Treasury Bills and common equity securities. Investments valued using Level 2 inputs are comprised of fixed maturity investments including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government, agency and provincial obligations and preferred stocks. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include certain debt securities, including asset-backed securities, where quoted market prices are unavailable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services used by White Mountains have indicated that if no observable inputs are available for a security, they will not provide a price. In those circumstances, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark interest rates, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs. White Mountains performs procedures to validate the market prices obtained from the outside pricing sources. Such procedures, which cover substantially all of its fixed maturity investments include, but are not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control processes and procedures on at least an annual basis, comparison of our invested asset market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices, and review of the underlying assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Prices that have not changed from period to period and prices that have trended unusually compared to market conditions are also considered outliers. In circumstances where the results of White Mountains’s review process do not appear to support the market price provided by the pricing services, White Mountains challenges the price. During the past year, nine securities fell outside White Mountains’s expected results, thereby triggering the challenge with the pricing service. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The fair values of such securities are considered to be Level 3 measurements. White Mountains’s investments in debt securities, including mortgage and asset-backed securities, are generally valued using pricing evaluation techniques that considers relevant market information, relevant credit information, perceived market movements and sector news. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Short-term investments consist of interest bearing money market funds, certificates of deposit, time deposits and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized cost, which approximated fair value as of December 31, 2016 and 2015.
Other long-term investments	Other Long-term Investments Other long-term investments consist primarily of hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes. See Note 5 — “Investment Securities”. White Mountains has taken the fair value option for most of its investments in hedge funds, private equity funds, various non-controlling interests in private capital investments and the OneBeacon Surplus Notes. For the investments for which White Mountains has taken the fair value option, changes in fair value are reported in revenues on a pre-tax basis. For those long-term investments for which White Mountains has not made the fair value election, White Mountains accounts for its interests under the equity method.
Derivative financial instruments	Derivative Financial Instruments White Mountains occasionally holds a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income.
Warrants	Warrants From time to time, White Mountains holds warrants that it has received in the restructuring (e.g., securities received from bankruptcy proceedings) of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives.
Derivatives - Variable annuity reinsurance	Derivatives—Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan through its wholly owned subsidiary, WM Life Re. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. Guaranteed minimum accumulation benefits (“GMABs”) are paid to an annuitant for any shortfall between accumulated account value at the end of the accumulation period and the annuitant’s total deposit, less any withdrawal payments made to the annuitant during the accumulation period. GMABs meet the definition of a derivative for accounting purposes. Therefore, GMABs are carried at fair value, with changes thereon recognized in income in the period of the change. The liability for the reinsured GMAB contracts has been determined using internal valuation methodologies that use assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates. If an annuitant dies during the accumulation period of an annuity contract, guaranteed minimum death benefits (“GMDBs”) are paid to the annuitant’s beneficiary for shortfalls between accumulated account value at the time of an annuitant’s death and the annuitant’s total deposit, less any living benefit payments or withdrawal payments previously made to the annuitant. White Mountains elected to measure its GMDB liabilities at fair value. The valuation of these liabilities involves significant judgment and is subject to change based upon changes in capital market assumptions and emerging surrender and mortality experience of the underlying contracts in force. WM Life Re has completed its run-off as all of its contracts matured as of June 30, 2016. WM Life Re entered into derivative contracts that were designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. The derivatives included futures and over-the-counter option contracts on interest rates, major bond and equity indices, and foreign currencies. All WM Life Re’s derivative instruments were recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments did not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value were recognized in the appropriate period as gains or losses in the income statement within other revenues. WM Life Re included the effect of counterparty credit risk when determining the fair value of its derivative contracts and its GMAB and GMDB liabilities.
Cash	Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash. See Note 9 — “Derivatives”.
Insurance and reinsurance operations	Insurance and Reinsurance Operations White Mountains accounts for insurance and reinsurance policies that it writes in accordance with ASC 944. Premiums written are recognized as revenues and are earned ratably over the term of the related policy or reinsurance treaty. Unearned premiums represent the portion of premiums written that are applicable to future insurance and reinsurance coverage provided by policies or treaties in force. White Mountains charges fees on certain of its insurance policies. Refundable fees are classified with premiums and recognized in earnings over the policy term. Fees that represent a reimbursement of expenses, such as installment fees, are recorded as a reduction of underwriting expenses. Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Amortization of deferred acquisition costs are presented within insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. Losses and LAE are charged against income as incurred. Unpaid insurance losses and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance losses and LAE are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any resulting adjustments are reflected in current operations. The process of estimating loss and LAE involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. White Mountains’s insurance and reinsurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. Such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies White Mountains for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. White Mountains has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro-rata basis. The amount of each risk ceded by White Mountains is subject to maximum limits which vary by line of business and type of coverage. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectability of reinsurance recoverables is also subject to the solvency of the reinsurers. White Mountains is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with a strong financial condition, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis. Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly. Funds held by ceding companies represent amounts due to White Mountains in connection with certain assumed reinsurance agreements in which the ceding company retains a portion of the premium to provide security against future loss payments. The funds held by ceding companies are generally invested by the ceding company and a contractually agreed interest amount is credited to the Company and recognized as investment income. Funds held under insurance and reinsurance contracts represent contractual payments due to White Mountains that have been retained to secure such obligations. Such amounts are recorded as liabilities in the consolidated financial statements. Accruals for contingent commission liabilities are established for reinsurance contracts that provide for the stated commission percentage to increase or decrease based on the loss experience of the contract. Changes in the estimated liability for such arrangements are recorded as contingent commissions. Accruals for contingent commission liabilities are determined through the review of the contracts that have these adjustable features and are estimated based on expected loss and LAE.
Municipal Bond Insurance	Municipal Bond Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income.
Funds Held Under Reinsurance Contracts Policy [Policy Text Block]	Funds Held Funds held under reinsurance contracts primarily represent amounts due to White Mountains in connection with the Standard Reinsurance Agreement (“SRA”) with the Federal Crop Insurance Corporation (“FCIC”), which is managed by an agency of the U.S. Department of Agriculture. The SRA governs the relationship, including the exchange of funds, between private insurance companies, including White Mountains, and the FCIC relating to our MPCI crop insurance business. Funds held under insurance contracts represents unrestricted collateral held by White Mountains primarily relating to the surety business.
Mandatory Shared Market Mechanisms	Mandatory Shared Market Mechanisms As a condition to its licenses to do business in certain states, White Mountains’s insurance operations must participate in various mandatory shared market mechanisms commonly referred to as “residual” or “involuntary” markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer’s market share of voluntary business written within that state. In certain cases, White Mountains is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms is accounted for as direct insurance business or as assumed reinsurance depending upon the structure of the mechanism. OneBeacon’s market assignments are typically required to be written in the current period, however, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated.
Insurance-Related Assessments	Insurance-related Assessments Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains records guaranty fund assessments when it is probable that an assessment will be made and the amount can be reasonably estimated.
Deferred Software Costs	Deferred Software Costs White Mountains capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll-related costs. White Mountains begins amortization of these costs once the project is completed and ready for its intended use. Amortization is on a straight-line basis and over a useful life of eighteen months to five years. Costs related to software developed for sale to third parties are expensed until technological feasibility has been established. Once technological feasibility has been established, software development costs are capitalized and reported at their net realizable value. Upon product release, the amortization of software development costs is determined annually as the greater of the amount calculated using the ratio of current gross revenues to the total of current and expected gross revenues for the product or the straight-line method over the estimated economic life, which is generally between 18 to 36 months.
Federal and foreign income taxes	Federal and Foreign Income Taxes A significant portion of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.
Foreign currency exchange	Foreign Currency Exchange The U.S. dollar is the functional currency for all of White Mountains’s businesses except for Sirius International and certain other smaller international activities. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the weighted average exchange rates for the period. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss. Assets and liabilities relating to foreign operations are translated into the functional currency using current exchange rates; revenues and expenses are translated into the functional currency using the weighted average exchange rate for the period. The resulting exchange gains and losses are reported as a component of net income in the period in which they arise.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses. Goodwill is not amortized, but rather is evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill is performed during the fourth quarter of each year. White Mountains initially evaluates goodwill using a qualitative approach (“step zero”) to determine whether it is more likely than not that the fair value of goodwill is greater than its carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill exceeds its fair value, White Mountains, performs the two-step quantitative test for impairment. Other intangible assets with finite lives are measured at their acquisition date fair values, are amortized over their economic lives and presented net of accumulated amortization on the balance sheet. Other intangible assets with finite lives are evaluated for impairment at least annually and when events or changes in circumstances indicate that it is more likely than not that the asset is impaired. White Mountains evaluated the recoverability of goodwill and other intangible assets and did not recognize any impairment losses for any of the years ended December 31, 2016, 2015 and 2014. See Note 6 — “Goodwill and Other Intangible Assets”.
Noncontrolling Interest	Non-controlling Interest Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries, and are presented separately on the balance sheet. The portion of comprehensive income attributable to non-controlling interests is presented net of related income taxes in the statement of operations and comprehensive income. See Note 14 — “Common Shareholders’ Equity and Non-controlling Interests”.
Variable Interest Entities	Variable Interest Entities White Mountains consolidates a variable interest entity (“VIE”) when it has both the power to direct the activities of the VIE that most significantly impact its economic performance and either the obligation to absorb losses or the right to receive returns from the VIE that could potentially be significant to the VIE. See Note 18 — “Variable Interest Entities”.
Recent Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Short-Duration Contracts Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnote disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments (ASC 805), which requires adjustments to provisional amounts recorded in connection with a business combination that are identified during the measurement period to be recorded in the reporting period in which the adjustment amounts are determined, rather than as retroactive adjustments to prior periods. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended December 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. Fair Value Measurements On January 1, 2016, White Mountains adopted ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820), which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value (“NAV”) using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures. Amendments to Consolidation Analysis On January 1, 2016, White Mountains adopted ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a VIE. ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718). The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of December 31, 2016, the unamortized debt issuance costs included in debt is $1.6 million. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity Effective January 1, 2015, White Mountains adopted ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity (ASC 205 and ASC 360), which limits discontinued operations treatment to disposals that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. ASU 2014-08 also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. As discussed further in Note 2, White Mountains completed its sale of Sirius Group on April 18, 2016. Sirius Group has been presented as discontinued operations for all periods presented. Qualified Affordable Housing Projects Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.4 million. The retrospective adoption resulted in an increase of $1.7 million and $1.9 million to net investment income and a net increase of $2.3 million and $2.2 million to income tax expense for the years ended December 31, 2014 and 2013. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above. Pushdown Accounting ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014. The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections. White Mountains has not had any acquisitions for which it has elected to apply pushdown accounting since ASU 2014-17 became effective.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Schedule of rates of exchange for the U.S. dollar that have been used for the most significant operations

The following rates of exchange for the U.S. dollar have been used for the most significant operations:



Currency	Opening Rate 2016	Closing Rate 2016	Opening Rate 2015	Closing Rate 2015
Israeli Shekels	3.9051	3.8476	3.8865	3.9051
British Pound Sterling	0.6757	0.8074	0.6426	0.6757
Euro	0.9189	0.9479	0.8245	0.9189
Swedish Kronor ⁽¹⁾	8.4247	N/A	7.7737	8.4247

Reserves for Unpaid Losses and Loss Adjustment Expenses (Tables)

12 Months Ended

Dec. 31, 2016

Insurance Loss Reserves [Abstract]

Loss and LAE reserve activities

The following table summarizes the ending liabilities for unpaid loss and LAE, net of reinsurance for each of OneBeacon’s major lines of business by underwriting group as of December 31, 2016:



Liabilities for unpaid claims and claims adjustment expenses, net of reinsurance Millions	As of December 31, 2016
Specialty Products - Property	$	30.2
Specialty Industries - Property	108.5
Specialty Products - Casualty	568.7
Specialty Industries - Casualty	360.0
Specialty Products - Other	38.2
Specialty Industries - Other	47.4
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	1,153.0
Less: Discount on workers compensation reserves	(1.6		)
Total unpaid loss and allocated LAE reserves, net of reinsurance	1,151.4
Plus: Unallocated LAE	41.3
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7
Plus: Reinsurance recoverables on unpaid losses
Specialty Products - Property	40.9
Specialty Industries - Property	13.2
Specialty Products - Casualty	64.0
Specialty Industries - Casualty	19.9
Specialty Products - Other	16.0
Specialty Industries - Other	.6
Investing, Financing and Corporate ⁽¹⁾	18.3
Plus: Total Reinsurance recoverables on unpaid losses	172.9
Total unpaid loss and LAE reserves	$	1,365.6

The following table reconciles loss and LAE reserves determined on a statutory basis to loss and LAE reserves determined in accordance with GAAP as of December 31, 2016 and 2015 as follows:



	December 31,
Millions	2016		2015
Statutory reserves (unaudited)	$	1,192.7	$	1,203.8
Reinsurance recoverable on unpaid losses and LAE ⁽¹⁾	172.9		186.0
GAAP reserves	$	1,365.6	$	1,389.8


⁽¹⁾	Represents adjustments made to add back reinsurance recoverables on unpaid losses and LAE included with the presentation of reserves under statutory accounting.

The following table summarizes the loss and LAE reserve activities of White Mountains’s insurance subsidiaries for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
Millions	2016			2015			2014
Gross beginning balance	$	1,389.8		$	1,342.2		$	1,054.3
Less beginning reinsurance recoverable on unpaid losses	(186.0		)	(161.6		)	(80.2		)
Net loss and LAE reserves	1,203.8			1,180.6			974.1

Loss and LAE reserves consolidated - SSIE	—			—			13.6

Add: SSIE reserves held for sale at the beginning of the period⁽¹⁾	5.5			7.7			—

Losses and LAE incurred relating to:
Current year losses	649.2			712.9			732.0
Prior year losses	14.8			(4.0		)	92.0
Total incurred losses and LAE	664.0			708.9			824.0

Loss and LAE paid relating to:
Current year losses	(192.8		)	(208.8		)	(202.6		)
Prior year losses	(483.1		)	(479.1		)	(420.8		)
Total loss and LAE payments	(675.9		)	(687.9		)	(623.4		)

Less: SSIE reserves held for sale at the end of the period⁽¹⁾	4.7			5.5			7.7

Net ending balance	1,192.7			1,203.8			1,180.6
Plus ending reinsurance recoverable on unpaid losses	172.9			186.0			161.6
Gross ending balance	$	1,365.6		$	1,389.8		$	1,342.2

Loss and LAE Reserve and Net Loss and LAE Development

The components of the 2014 fourth quarter loss and LAE reserve increase and the net loss and LAE development for the full year are provided below:



Underwriting Unit	2014 Fourth Quarter Reserve Increases								Full Year 2014
Millions	Current Accident Year			Prior Accident Year			Total		Net Prior Year Development
Professional Insurance ⁽¹⁾	$	22.9		$	46.4		$	69.3	$	59.1
Specialty Property	(1.1		)	5.7			4.6		1.1
Crop Business	3.8			—			3.8		—
Other	2.8			(.4		)	2.4		1.6
Specialty Products	28.4			51.7			80.1		61.8
Entertainment	1.5			11.6			13.1		13.5
Government Risks	1.2			7.1			8.3		8.5
Accident	—			3.5			3.5		6.0
Other	2.6			1.6			4.2		—
Specialty Industries	5.3			23.8			29.1		28.0
Total	$	33.7		$	75.5		$	109.2	$	89.8

⁽¹⁾Professional Insurance includes Other Professional Lines, Management Liability, Financial Services and Healthcare.

Incurred Loss, Cumulative Paid Loss, and Allocated LAE, Net of Reinsurance



Specialty Products - Casualty
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	138.3	$	120.2	$	97.3	$	90.7	$	70.5	$	70.1	$	61.8	$	58.3	$	57.1	$	56.5	$	2.5	1,236
2008	—		145.6		128.6		118.2		100.8		98.3		95.5		95.3		97.3		96.8		3.3		2,199
2009	—		—		169.5		186.2		194.9		193.2		192.0		194.9		195.0		195.5		4.9		4,002
2010	—		—		—		211.5		225.4		226.2		225.0		232.3		234.9		240.0		7.8		6,029
2011	—		—		—		—		194.9		189.8		205.4		214.2		214.8		214.9		9.4		6,661
2012	—		—		—		—		—		213.5		220.1		246.1		248.8		249.4		14.4		6,794
2013	—		—		—		—		—		—		205.2		233.1		229.6		247.9		20.3		5,866
2014	—		—		—		—		—		—		—		240.5		235.9		258.9		44.4		6,504
2015	—		—		—		—		—		—		—		—		206.9		213.4		78.5		9,710
2016	—		—		—		—		—		—		—		—		—		202.8		142.8		9,093
																	Total		$	1,976.1



Specialty Products - Casualty
$ in millions
	Cumulative Paid Losses and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	6.8	$	24.8	$	32.4	$	41.7	$	44.4	$	50.7	$	51.9	$	53.6	$	54.0	$	54.0
2008	—		8.8		32.3		50.8		65.4		77.6		83.1		85.3		88.1		91.5
2009	—		—		26.5		81.8		124.0		147.8		161.4		169.9		178.3		187.8
2010	—		—		—		32.0		107.0		157.8		180.5		196.3		213.6		220.3
2011	—		—		—		—		25.8		88.9		131.3		163.7		186.2		192.5
2012	—		—		—		—		—		25.8		86.7		157.7		193.6		219.8
2013	—		—		—		—		—		—		26.4		86.3		143.1		194.2
2014	—		—		—		—		—		—		—		34.3		95.2		156.7
2015	—		—		—		—		—		—		—		—		19.9		78.4
2016	—		—		—		—		—		—		—		—		—		16.5
																	Total		1,411.7
All outstanding liabilities before 2007, net of reinsurance																			4.3
Liabilities for loss and allocated LAE, net of reinsurance																			$	568.7



Specialty Industries - Other
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	4.3	$	4.6	$	4.5	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	4.4	$	—	291
2008	—		10.4		10.8		12.2		11.9		11.9		11.8		12.1		12.1		12.3		—		975
2009	—		—		19.1		19.2		19.0		19.3		20.9		21.4		21.5		21.7		.2		1,763
2010	—		—		—		25.6		26.6		27.1		27.9		28.2		28.4		28.4		—		2,449
2011	—		—		—		—		35.1		36.0		37.7		38.5		38.2		38.2		.2		3,638
2012	—		—		—		—		—		42.0		39.6		40.1		41.1		41.6		.3		3,861
2013	—		—		—		—		—		—		41.1		41.2		42.2		42.5		.7		4,245
2014	—		—		—		—		—		—		—		42.6		40.4		40.6		1.9		3,497
2015	—		—		—		—		—		—		—		—		46.6		31.9		4.9		3,301
2016	—		—		—		—		—		—		—		—		—		33.2		19.4		2,222
																	Total		$	294.8



Specialty Industries - Other
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	1.2	$	4.0	$	4.3	$	4.3	$	4.3	$	4.3	$	4.3	$	4.4	$	4.4	$	4.4
2008	—		3.8		9.0		11.1		11.7		11.8		11.7		12.0		12.0		12.2
2009	—		—		6.3		14.6		17.7		18.3		19.2		21.2		21.4		21.4
2010	—		—		—		8.3		22.9		25.1		26.9		27.6		28.0		28.1
2011	—		—		—		—		13.5		29.4		34.6		36.5		37.2		37.3
2012	—		—		—		—		—		13.2		30.7		35.8		38.3		39.1
2013	—		—		—		—		—		—		13.0		29.3		37.2		39.6
2014	—		—		—		—		—		—		—		12.6		28.9		34.8
2015	—		—		—		—		—		—		—		—		9.8		22.2
2016	—		—		—		—		—		—		—		—		—		8.0
																	Total		$	247.1
All outstanding liabilities before 2007, net of reinsurance																			(.3		)
Liabilities for loss and allocated LAE, net of reinsurance																			$	47.4



Specialty Industries - Property
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	110.0	$	104.8	$	101.3	$	94.9	$	95.1	$	95.3	$	95.1	$	95.1	$	95.1	$	94.9	$	—	4,828
2008	—		147.3		137.6		129.6		129.3		132.0		132.7		130.0		129.8		129.7		.1		6,603
2009	—		—		124.5		118.6		118.2		117.9		118.8		118.6		118.7		118.6		.1		6,718
2010	—		—		—		131.4		131.5		127.2		130.0		129.7		131.4		131.2		—		8,617
2011	—		—		—		—		138.5		141.5		141.1		143.5		143.2		142.5		.3		9,704
2012	—		—		—		—		—		126.9		123.8		126.7		130.4		130.1		.8		9,697
2013	—		—		—		—		—		—		125.1		127.0		142.9		143.6		1.9		8,085
2014	—		—		—		—		—		—		—		123.9		123.5		130.1		3.9		8,352
2015	—		—		—		—		—		—		—		—		142.3		138.9		10.2		8,607
2016	—		—		—		—		—		—		—		—		—		120.3		32.8		7,429
																	Total		$	1,279.9



Specialty Industries - Property
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	42.2	$	70.9	$	82.7	$	87.6	$	91.7	$	93.4	$	94.2	$	94.5	$	94.5	$	94.6
2008	—		62.5		101.4		116.8		123.4		129.5		129.1		129.2		129.3		129.3
2009	—		—		50.4		85.9		105.4		111.0		115.5		117.2		118.0		118.4
2010	—		—		—		69.0		106.2		118.2		122.8		125.9		129.3		130.8
2011	—		—		—		—		72.5		117.4		130.7		138.0		139.5		141.4
2012	—		—		—		—		—		64.2		103.1		116.4		128.5		131.9
2013	—		—		—		—		—		—		59.8		101.5		126.1		137.7
2014	—		—		—		—		—		—		—		56.5		94.6		111.6
2015	—		—		—		—		—		—		—		—		68.6		114.4
2016	—		—		—		—		—		—		—		—		—		61.3
																	Total		$	1,171.4
All outstanding liabilities before 2007, net of reinsurance																			—
Liabilities for loss and allocated LAE, net of reinsurance																			$	108.5



Specialty Products - Other
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	26.8	$	26.7	$	26.8	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	—	5,073
2008	—		31.7		31.5		31.2		31.2		31.2		31.2		31.2		31.2		31.2		—		5,192
2009	—		—		33.8		31.4		31.3		30.8		30.9		30.9		30.9		30.9		—		5,138
2010	—		—		—		34.7		34.3		32.5		32.6		32.5		32.5		32.5		—		5,182
2011	—		—		—		—		33.2		35.5		35.4		35.4		35.4		35.4		—		5,306
2012	—		—		—		—		—		36.6		36.9		36.9		36.9		37.0		—		5,324
2013	—		—		—		—		—		—		39.6		40.6		39.2		39.2		—		5,510
2014	—		—		—		—		—		—		—		96.6		90.8		92.9		—		9,971
2015	—		—		—		—		—		—		—		—		75.5		73.2		1.8		10,704
2016	—		—		—		—		—		—		—		—		—		77.1		33.8		4,306
																	Total		$	478.0



Specialty Products - Other
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	19.8	$	28.4	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6	$	28.6
2008	—		21.3		31.1		31.2		31.2		31.2		31.2		31.2		31.2		31.2
2009	—		—		21.6		30.8		30.8		30.9		30.9		30.9		30.9		30.9
2010	—		—		—		22.4		32.4		32.5		32.5		32.5		32.5		32.5
2011	—		—		—		—		25.2		35.2		35.4		35.4		35.4		35.4
2012	—		—		—		—		—		26.1		36.8		36.9		36.9		37.0
2013	—		—		—		—		—		—		27.4		39.0		39.2		39.2
2014	—		—		—		—		—		—		—		46.2		88.5		90.3
2015	—		—		—		—		—		—		—		—		41.7		70.6
2016	—		—		—		—		—		—		—		—		—		44.2
																	Total		$	439.9
All outstanding liabilities before 2007, net of reinsurance																			.1
Liabilities for loss and allocated LAE, net of reinsurance																			$	38.2



Specialty Products - Property
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	12.0	$	11.1	$	9.1	$	7.6	$	7.6	$	7.7	$	8.7	$	8.6	$	8.6	$	8.6	$	—	250
2008	—		31.3		29.2		25.4		25.6		26.6		26.8		26.8		26.8		26.8		—		3,466
2009	—		—		55.2		52.1		52.9		53.3		52.9		52.6		52.5		51.6		—		8,768
2010	—		—		—		51.0		49.5		55.6		56.7		55.9		55.8		55.8		—		10,170
2011	—		—		—		—		58.7		59.8		60.2		60.6		60.8		60.7		.2		11,121
2012	—		—		—		—		—		79.9		86.7		92.9		92.2		92.2		.2		12,405
2013	—		—		—		—		—		—		36.4		27.6		27.6		27.7		.3		5,267
2014	—		—		—		—		—		—		—		37.5		32.1		31.6		.4		1,227
2015	—		—		—		—		—		—		—		—		28.3		25.7		1.1		2,186
2016	—		—		—		—		—		—		—		—				24.5		5.2		1,825
																	Total		$	405.2



Specialty Products - Property
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	1.8	$	3.7	$	6.8	$	7.0	$	7.0	$	7.0	$	8.7	$	8.6	$	8.6	$	8.6
2008	—		15.5		23.5		25.2		25.5		26.0		26.8		26.8		26.8		26.8
2009	—		—		41.5		47.3		51.2		51.9		51.9		51.6		51.6		51.6
2010	—		—		—		43.1		47.9		53.3		55.2		55.7		55.7		55.7
2011	—		—		—		—		51.8		59.0		59.7		60.1		60.4		60.5
2012	—		—		—		—		—		58.8		72.5		78.3		79.0		78.8
2013	—		—		—		—		—		—		24.3		26.9		27.1		27.4
2014	—		—		—		—		—		—		—		19.8		28.2		31.2
2015	—		—		—		—		—		—		—		—		15.0		22.1
2016	—		—		—		—		—		—		—		—		—		12.3
																	Total		375.0
All outstanding liabilities before 2007, net of reinsurance																			—
Liabilities for loss and allocated LAE, net of reinsurance																			$	30.2



Specialty Industries - Casualty
$ in millions
	Incurred Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,																				As of December 31, 2016
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016		Total IBNR plus expected development on reported claims		Cumulative number of reported claims
	Unaudited
2007	$	24.9	$	24.3	$	21.7	$	34.0	$	34.9	$	35.1	$	35.1	$	36.0	$	37.1	$	36.9	$	.3	1,770
2008	—		26.3		26.2		30.4		26.4		25.8		25.2		23.7		22.2		22.1		.5		1,835
2009	—		—		51.0		51.1		51.5		49.6		54.8		53.6		51.6		51.0		1.0		2,982
2010	—		—		—		80.8		72.4		69.3		69.4		73.3		75.5		77.2		3.5		4,791
2011	—		—		—		—		88.1		87.5		90.2		100.5		111.8		112.2		3.8		5,254
2012	—		—		—		—		—		118.8		103.4		102.0		101.8		98.8		7.9		6,298
2013	—		—		—		—		—		—		121.9		131.5		133.6		138.2		18.0		7,107
2014	—		—		—		—		—		—		—		135.6		127.4		124.2		35.7		7,633
2015	—		—		—		—		—		—		—		—		146.3		127.7		61.8		7,315
2016	—		—		—		—		—		—		—		—		—		129.9		98.4		5,701
																	Total		$	918.2



Specialty Industries - Casualty
$ in millions
	Cumulative Paid Loss and Allocated LAE, Net of Reinsurance
	For the Years Ended December 31,
Accident Year	2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
	Unaudited
2007	$	2.8	$	5.5	$	7.9	$	13.8	$	27.9	$	29.9	$	32.4	$	32.9	$	34.3	$	35.9
2008	—		3.2		6.9		11.5		14.2		17.3		19.9		21.2		20.8		20.9
2009	—		—		5.1		16.6		25.0		34.5		42.9		46.3		46.3		47.4
2010	—		—		—		8.7		21.1		37.3		51.4		57.5		65.3		68.9
2011	—		—		—		—		11.1		34.2		53.7		69.8		88.9		96.4
2012	—		—		—		—		—		12.9		31.9		50.6		67.8		79.7
2013	—		—		—		—		—		—		18.6		45.5		72.7		96.6
2014	—		—		—		—		—		—		—		16.6		40.2		61.4
2015	—		—		—		—		—		—		—		—		13.6		39.3
2016	—		—		—		—		—		—		—		—		—		11.8
																	Total		$	558.3
All outstanding liabilities before 2007, net of reinsurance																			.1
Liabilities for loss and allocated LAE, net of reinsurance																			$	360.0

Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance



Specialty Products - Property
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	70.1%	14.6%	5.9%	1.1%	0.3%	0.2%	0.4%	—%	—%	—%



Specialty Products - Other
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	61.9%	29.5%	0.6%	—%	—%	—%	—%	—%	—%	—%



Specialty Industries - Other
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	30.4%	37.0%	10.8%	3.3%	1.1%	0.9%	0.2%	—%	0.1%	—%



Specialty Industries - Property
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	47.4%	27.3%	9.9%	4.1%	1.8%	0.7%	0.2%	0.1%	—%	—%



Specialty Industries - Casualty
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	11.4%	16.2%	12.9%	9.7%	6.8%	2.5%	0.8%	0.1%	0.2%	0.2%



Specialty Products - Casualty
	Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance
	Unaudited
Years	1	2	3	4	5	6	7	8	9	10
	11.3%	24.0%	17.8%	9.6%	4.7%	2.2%	0.9%	0.7%	0.2%	—%

Third Party Reinsurance (Tables)

12 Months Ended

Dec. 31, 2016

Reinsurance Disclosures [Abstract]

Reinsurance Retention Policy

The following table summarizes the reinsurance coverage currently in effect as of December 31, 2016:



$ in millions
Coverage	Contract Type	Renewal Date	First-Dollar Retention		Per Risk Limit Purchased		Maximum Retention
Corporate Property Catastrophe - Property and Inland Marine	Excess of Loss	5/1	$	20.0	$	110.0	$	20.0
Property Per Risk - Property and Inland Marine	Excess of Loss	5/1	3.0		100.0		3.0
Specialty Property - Excess and Surplus Property Catastrophe	Excess of Loss	5/1	6.0		34.0		6.0
Medical Excess - HMO/Provider Excess	Excess of Loss	1/1	5.0		Unlimited		5.0
Ocean and Inland Marine	Excess of Loss	4/1	2.5		57.5		7.0
Surety	Excess of Loss	10/1	5.0		45.0		5.0
Film Completion Bonds	Excess of Loss	6/1	2.0		38.0		2.0
Casualty Clash/Workers Compensation Catastrophe	Excess of Loss	6/1	6.0		34.0		6.0
Workers Compensation Catastrophe	Excess of Loss	6/1	40.0		20.0		6.0
Financial Institutions - Professional Liability	Quota Share	6/1	N/A		10.0		5.0
Combined Healthcare/Casualty 2nd Layer - Various lines	Excess of Loss	6/1	10.0 / 11.0		10.0		3.0
Casualty Per Policy - Various lines	Excess of Loss	6/1	3.0		8.0		3.0
Workers Compensation Per Occurrence	Excess of Loss	6/1	2.0		8.0		2.0
Healthcare Professional Liability	Excess of Loss	6/1	3.0		7.0		3.0

Schedule of direct assumed and ceded amounts

The effects of reinsurance on White Mountains’s insurance subsidiaries’ written and earned premiums and on losses and LAE were as follows (see Note 10 — “Municipal Bond Guarantee Insurance” for balances related to White Mountains financial guarantee business):



	Year ended December 31, 2016
Millions	OneBeacon			HG/BAM ⁽¹⁾		Other			Total
Written premiums:
Direct	$	1,193.3		$	38.6	$	13.3		$	1,245.2
Assumed	28.0			—		—			28.0
Gross written premiums	1,221.3			38.6		13.3			1,273.2
Ceded	(120.6		)	—		(6.8		)	(127.4		)
Net written premiums	$	1,100.7		$	38.6	$	6.5		$	1,145.8
Earned premiums:
Direct	$	1,177.0		$	5.9	$	15.2		$	1,198.1
Assumed	29.4			—		—			29.4
Gross earned premiums	1,206.4			5.9		15.2			1,227.5
Ceded	(105.8		)	—		(7.7		)	(113.5		)
Net earned premiums	$	1,100.6		5.9		$	7.5		$	1,114.0
Losses and LAE:
Direct	$	679.5		$	—	$	14.3		$	693.8
Assumed	21.2			—		—			21.2
Gross losses and LAE	700.7			—		14.3			715.0
Ceded	(44.7		)	—		(6.3		)	(51.0		)
Net losses and LAE	$	656.0		$	—	$	8.0		$	664.0

⁽¹⁾During 2016, BAM ceded $27.2 in written premiums and $21.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.



	Year ended December 31, 2015
Millions	OneBeacon				HG/BAM ⁽¹⁾		Other			Total
Written premiums:
Direct	$	1,279.9			$	25.9	$	19.9		$	1,325.7
Assumed	36.0				—		—			36.0
Gross written premiums	1,315.9				25.9		19.9			1,361.7
Ceded	(179.3		)	⁽²⁾	—		(9.8		)	(189.1		)
Net written premiums	$	1,136.6			$	25.9	$	10.1		$	1,172.6
Earned premiums:
Direct	$	1,298.0			$	3.3	$	20.7		$	1,322.0
Assumed	45.9				—		—			45.9
Gross earned premiums	1,343.9				3.3		20.7			1,367.9
Ceded	(167.7		)	⁽²⁾	—		(12.0		)	(179.7		)
Net earned premiums	$	1,176.2			3.3		$	8.7		$	1,188.2
Losses and LAE:
Direct	$	783.0			$	—	$	19.5		$	802.5
Assumed	55.7				—		—			55.7
Gross losses and LAE	838.7				—		19.5			858.2
Ceded	(138.0		)	⁽²⁾	—		(11.3		)	(149.3		)
Net losses and LAE	$	700.7			$	—	$	8.2		$	708.9

⁽¹⁾During 2015, BAM ceded $19.3 in written premiums and $16.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.



	Year ended December 31, 2014
Millions	OneBeacon			HG/BAM ⁽¹⁾		Other⁽²⁾			Total
Written premiums:
Direct	$	1,257.5		$	16.2	$	22.6		$	1,296.3
Assumed	65.9			—		—			65.9
Gross written premiums	1,323.4			16.2		22.6			1,362.2
Ceded	(106.5		)	—		(16.7		)	(123.2		)
Net written premiums	$	1,216.9		$	16.2	$	5.9		$	1,239.0
Earned premiums:
Direct	$	1,209.1		$	1.8	$	22.6		$	1,233.5
Assumed	70.9			—		—			70.9
Gross earned premiums	1,280.0			1.8		22.6			1,304.4
Ceded	(102.9		)	—		(16.5		)	(119.4		)
Net earned premiums	$	1,177.1		$	1.8	$	6.1		$	1,185.0
Losses and LAE:
Direct	$	778.7		$	—	$	24.1		$	802.8
Assumed	115.7			—		—			115.7
Gross losses and LAE	894.4			—		24.1			918.5
Ceded	(79.3		)	—		(15.2		)	(94.5		)
Net losses and LAE	$	815.1		$	—	$	8.9		$	824.0

⁽¹⁾During 2014, BAM ceded $12.3 in written premiums and $1.4 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment.

⁽²⁾During 2014, SSIE ceded $16.0 in written premiums, $15.7 in earned premiums, and $16.9 in loss and LAE to OneBeacon, which have been eliminated in consolidation.

Ceded Credit Risk

The following table summarizes A.M. Best Company, Inc. (“A.M. Best”) ratings for OneBeacon’s reinsurers.



A.M. Best’s Rating⁽¹⁾
$ in millions	Balance at December 31, 2016		% of Total
A+ or better	$	73.9	41	%
A- to A	79.7		44	%
B, Not rated and other⁽²⁾	25.9		15	%
Total	$	179.5	100	%

⁽¹⁾A.M. Best’s ratings as detailed above are “A+ or better” (Superior), “A- to A” (Excellent) and “B” (Fair).

⁽²⁾ Includes

Investment Securities (Tables)

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Par Value to Fair Value Reconciliation of Surplus Notes [Table Text Block]

Below is a table illustrating the valuation adjustments taken to arrive at the estimated fair value of the OneBeacon Surplus Notes as of December 31, 2016 and 2015:



	Type of Surplus Note						Total as of December 31, 2016			Total as of December 31, 2015
Millions	Seller Priority			Pari Passu			Total as of December 31, 2016			Total as of December 31, 2015
Par Value	$	57.9		$	43.1		$	101.0		$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments⁽¹⁾	6.2			(1.1		)	5.1			(15.1		)
Regulatory approval ⁽²⁾	(.2		)	(15.4		)	(15.6		)	(24.2		)
Liquidity adjustment ⁽³⁾	(12.8		)	(5.8		)	(18.6		)	(10.2		)
Total adjustments	(6.8		)	(22.3		)	(29.1		)	(49.5		)
Fair value⁽⁴⁾	$	51.1		$	20.8		$	71.9		$	51.5

Pre-tax net investment income

Pre-tax net investment income for 2016, 2015 and 2014 consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Investment income:
Fixed maturity investments	$	77.3		$	52.6		$	51.3
Short-term investments	1.1			.2			.1
Common equity securities	7.5			10.1			16.6
Other long-term investments	4.0			3.3			4.4
Total investment income	89.9			66.2			72.4
Third-party investment expenses	(3.1		)	(5.4		)	(12.9		)
Net investment income, pre-tax	$	86.8		$	60.8		$	59.5

Net Realized and Unrealized Investment Gains and Losses

Net realized and unrealized investment gains (losses) consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Net realized investment gains, pre-tax	$	280.3		$	77.3		$	166.8
Net unrealized investment (losses) gains, pre-tax	(270.0		)	148.1			(88.3		)
Net realized and unrealized investment gains, pre-tax	10.3			225.4			78.5
Income tax expense attributable to net realized and unrealized investment gains	(8.8		)	(35.2		)	(18.0		)
Net realized and unrealized investment gains, after tax	$	1.5		$	190.2		$	60.5

Net unrealized investment gains (losses) for Level 3 investments



	Year Ended December 31,
Millions	2016		2015			2014
Fixed maturity investments	$	.1	$	(1.1	)	$	1.9
Common equity securities	—		(9.0		)	5.8
Other long-term investments	6.1		(13.0		)	—
Total net unrealized investment gains (losses), pre-tax - Level 3 investments	$	6.2	$	(23.1	)	$	7.7

Net realized and unrealized investment gains, after-tax, as recorded on the statements of operations and comprehensive income (losses)

The components of White Mountains’s net realized and unrealized investment gains (losses), after-tax, as recorded on the statements of operations and comprehensive income were as follows:



	Year Ended December 31,
Millions	2016		2015			2014
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$	—	$	(39.2	)	$	81.2
Income tax benefit (expense)	—		2.9			(5.9		)
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	—		(36.3		)	75.3
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	—		1.4			—
Total investment (losses) gains through accumulated other comprehensive income	—		(34.9		)	75.3
Net realized and unrealized investment gains, after-tax	1.5		190.2			60.5
Total investment gains recorded during the period, after-tax	$	1.5	$	155.3		$	135.8

Realized Gain (Loss) on Investments

Net realized investment gains for 2016, 2015 and 2014 consisted of the following:



	Year ended December 31, 2016
Millions	Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(2.1	)	$	.4	$	(1.7	)
Short-term investments	.4			—		.4
Common equity securities	280.7			—		280.7
Other long-term investments	.9			—		.9
Net realized investment gains, pre-tax	279.9			.4		280.3
Income tax expense attributable to net realized investment gains	(49.5		)	—		(49.5		)
Net realized investment gains, after-tax	$	230.4		$	.4	$	230.8



	Year ended December 31, 2015
Millions	Net realized gains			Net foreign exchange gains		Total net realized gains reflected in earnings
Fixed maturity investments	$	1.9		$	—	$	1.9
Common equity securities	64.4			.4		64.8
Other long-term investments	10.6			—		10.6
Net realized investment gains, pre-tax	76.9			.4		77.3
Income tax expense attributable to net realized investment gains	(22.8		)	—		(22.8		)
Net realized investment gains, after-tax	$	54.1		$	.4	$	54.5



	Year ended December 31, 2014
Millions	Net realized gains			Net foreign exchange gains		Total net realized gains reflected in earnings
Fixed maturity investments	$	5.8		$	—	$	5.8
Common equity securities	138.0			—		138.0
Other long-term investments	23.0			—		23.0
Net realized investment gains, pre-tax	166.8			—		166.8
Income tax expense attributable to net realized investment gains	(28.4		)	—		(28.4		)
Net realized investment gains, after-tax	$	138.4		$	—	$	138.4

Unrealized Gain (Loss) on Investments

The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value:



	Year ended December 31, 2016
Millions	Net unrealized losses			Net foreign exchange gains (losses)			Total net unrealized losses reflected in earnings
Fixed maturity investments	$	(12.1	)	$	2.1		$	(10.0	)
Common equity securities	(254.6		)	(3.3		)	(257.9		)
Other long-term investments	(.6		)	(.3		)	(.9		)
Forward contracts	—			(1.2		)	(1.2		)
Net unrealized investment losses, pre-tax	(267.3		)	(2.7		)	(270.0		)
Income tax benefit attributable to net unrealized investment losses	40.7			—			40.7
Net unrealized investment losses, after-tax	$	(226.6	)	$	(2.7	)	$	(229.3	)



	Year ended December 31, 2015
Millions	Net unrealized (losses) gains			Net foreign exchange losses			Total net unrealized (losses) gains reflected in earnings
Fixed maturity investments	$	(15.6	)	$	—		$	(15.6	)
Common equity securities	207.6			(3.7		)	203.9
Other long-term investments	(39.1		)	(1.1		)	(40.2		)
Net unrealized investment gains (losses), pre-tax	152.9			(4.8		)	148.1
Income tax expense attributable to net unrealized investment gains (losses)	(12.3		)	(.1		)	(12.4		)
Net unrealized investment gains (losses), after-tax	$	140.6		$	(4.9	)	$	135.7



	Year ended December 31, 2014
Millions	Net unrealized gains (losses)			Net foreign exchange losses			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	11.3		$	—		$	11.3
Common equity securities	(83.0		)	(7.7		)	(90.7		)
Other long-term investments	(7.6		)	(1.3		)	(8.9		)
Net unrealized investment losses, pre-tax	(79.3		)	(9.0		)	(88.3		)
Income tax benefit attributable to net unrealized investment losses	9.9			.5			10.4
Net unrealized investment losses, after-tax	$	(69.4	)	$	(8.5	)	$	(77.9	)

Investment holdings, fixed maturity investments



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
US Government and agency obligations	$	281.7	$	.1	$	(3.5	)	$	—	$	278.3
Debt securities issued by corporations	1,512.6		8.4		(13.7		)	2.1		1,509.4
Municipal obligations	308.8		1.9		(1.7		)	—		309.0
Mortgage and asset-backed securities	2,141.7		2.6		(11.4		)	—		2,132.9
Foreign government, agency and provincial obligations	12.9		.3		—			—		13.2
Preferred stocks	8.3		5.7		—			—		14.0
Total fixed maturity investments	4,266.0		19.0		(30.3		)	2.1		4,256.8
Fixed maturity investments reclassified to assets held for sale related to SSIE										(6.6		)
Total fixed maturity investments										$	4,250.2



	December 31, 2015
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains (losses)		Carrying value
US Government and agency obligations	$	160.4	$	—	$	(.4	)	$	—	$	160.0
Debt securities issued by corporations	1,001.0		4.3		(5.3		)	—		1,000.0
Municipal obligations	227.8		2.2		(1.2		)	—		228.8
Mortgage and asset-backed securities	1,170.6		2.0		(5.6		)	—		1,167.0
Foreign government, agency and provincial obligations	1.0		.2		—			—		1.2
Preferred stocks	78.3		4.4		—			—		82.7
Total fixed maturity investments	2,639.1		13.1		(12.5		)	$	—	2,639.7
Fixed maturity investments reclassified to assets held for sale related to SSIE										(9.5		)
Total fixed maturity investments										$	2,630.2

Schedule of contractual maturities of fixed maturities



	December 31, 2016
Millions	Cost or amortized cost		Carrying value
Due in one year or less	$	212.8	$	213.3
Due after one year through five years	1,272.6		1,270.2
Due after five years through ten years	462.8		458.4
Due after ten years	167.8		168.0
Mortgage and asset-backed securities	2,141.7		2,132.9
Preferred stocks	8.3		14.0
Total	$	4,266.0	$	4,256.8

Investment holdings, equity securities, convertible fixed maturities and other long-term investments



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	440.8	$	35.9	$	(2.4	)	$	—		$	474.3
Other long-term investments	$	314.9	$	40.3	$	(28.0	)	$	(3.9	)	$	323.3



	December 31, 2015
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	822.5	$	302.8	$	(11.4	)	$	—		$	1,113.9
Other long-term investments	$	304.5	$	32.0	$	(18.4	)	$	(2.3	)	$	315.8

Fair value measurements by level, investment securities



	December 31, 2016
Millions	Fair value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	278.3	$	268.8	$	9.5	$	—
Debt securities issued by corporations:
Consumer	385.6		—		385.6		—
Health Care	244.2		—		244.2		—
Utilities	180.3		—		180.3		—
Financials	176.0		—		176.0		—
Industrial	146.4		—		146.4		—
Communications	131.4		—		131.4		—
Materials	102.6		—		102.6		—
Technology	89.4		—		89.4		—
Energy	53.5		—		53.5		—
Total debt securities issued by corporations:	1,509.4		—		1,509.4		—

Mortgage and asset-backed securities	2,132.9		—		2,132.9		—
Municipal obligations	309.0		—		309.0		—
Foreign government, agency and provincial obligations	13.2		.6		12.6		—
Preferred stocks	14.0		—		14.0		—
Total fixed maturity investments ⁽⁴⁾	4,256.8		269.4		3,987.4		—

Short-term investments ⁽⁴⁾⁽⁵⁾	287.1		274.4		12.7		—

Common equity securities:

Exchange traded funds ⁽¹⁾	321.6		270.4		51.2		—
Health Care	20.9		20.9		—		—
Consumer	12.9		12.9		—		—
Financials	11.6		11.6		—		—
Technology	11.0		11.0		—		—
Communications	10.5		10.5		—		—
Energy	3.7		3.7		—		—
Industrial	2.2		2.2		—		—
Other	79.9		—		79.9		—
Total common equity securities	474.3		343.2		131.1		—
Other long-term investments ⁽²⁾⁽³⁾	177.7		—		—		177.7
Total investments ^(2)(3)(4)	$	5,195.9	$	887.0	$	4,131.2	$	177.7

⁽¹⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Excludes carrying value of $131.0 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Includes carrying value of $6.6 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.

⁽⁵⁾Short-term investments are measured at amortized cost, which approximates fair value.



	December 31, 2015
Millions	Fair value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	160.0	$	133.4	$	26.6	$	—
Debt securities issued by corporations:
Consumer	253.3		—		253.3		—
Financials	175.9		—		175.9		—
Health Care	151.3		—		151.3		—
Industrial	135.6		—		135.6		—
Energy	82.0		—		82.0		—
Utilities	61.5		—		61.5		—
Technology	60.0		—		60.0		—
Communications	49.2		—		49.2		—
Materials	31.2		—		31.2		—
Total debt securities issued by corporations:	1,000.0		—		1,000.0		—

Mortgage and asset-backed securities	1,167.0		—		1,167.0		—
Municipal obligations	228.8		—		228.8		—
Foreign government, agency and provincial obligations	1.2		.6		.6		—
Preferred stocks	82.7		—		12.7		70.0
Total fixed maturity investments ⁽⁴⁾	2,639.7		134.0		2,435.7		70.0

Short-term investments ⁽⁴⁾⁽⁵⁾	211.3		211.3		—		—

Common equity securities:
Financials	653.2		653.2		—		—
Exchange traded funds⁽¹⁾	183.3		162.0		21.3		—
Consumer	70.0		70.0		—		—
Communications	43.7		43.7		—		—
Health Care	35.7		35.7		—		—
Technology	27.0		27.0		—		—
Industrial	26.6		26.6		—		—
Other	74.4		—		74.4		—
Total common equity securities	1,113.9		1,018.2		95.7		—
Other long-term investments ⁽²⁾⁽³⁾	169.5		—		—		169.5
Total investments ^(2)(3)(4)	$	4,134.4	$	1,363.5	$	2,531.4	$	239.5

⁽¹⁾ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement.

⁽³⁾Excludes carrying value of $127.8 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Includes carrying value of $9.5 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.

⁽⁵⁾Short-term investments are measured at amortized cost, which approximates fair value.

Debt securities issued by corporations, credit ratings

The following table summarizes the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2016 and 2015:



	Fair Value at December 31,
Millions	2016		2015
AA	$	100.9	$	95.2
A	381.9		397.7
BBB	786.5		507.1
BB	214.0		—
B	26.1		—
Debt securities issued by corporations⁽¹⁾	$	1,509.4	$	1,000.0


⁽¹⁾	Credit ratings are assigned based on the following hierarchy: 1) Standard & Poors Financial Services LLC (“Standard & Poor’s”) and 2) Moody’s Investor Services (“Moody’s”)

Mortgage-backed, asset-backed securities

The following table summarizes the carrying value of White Mountains’s mortgage and asset-backed securities as of December 31, 2016 and December 31, 2015:



	December 31, 2016						December 31, 2015
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	283.9	$	283.9	$	—	$	265.5	$	265.5	$	—
FNMA	278.3		278.3		—		42.2		42.2		—
FHLMC	89.8		89.8		—		22.8		22.8		—
Total Agency ⁽¹⁾	652.0		652.0		—		330.5		330.5		—
Non-agency:
Residential	205.3		205.3		—		133.2		133.2		—
Commercial	127.5		127.5		—		140.4		140.4		—
Total Non-agency	332.8		332.8		—		273.6		273.6		—

Total mortgage-backed securities	984.8		984.8		—		604.1		604.1		—
Other asset-backed securities:
Vehicle receivables	479.5		479.5		—		269.7		269.7		—
Credit card receivables	438.3		438.3		—		217.7		217.7		—
Other	230.3		230.3		—		75.5		75.5		—
Total other asset-backed securities	1,148.1		1,148.1		—		562.9		562.9		—
Total mortgage and asset-backed securities	$	2,132.9	$	2,132.9	$	—	$	1,167.0	$	1,167.0	$	—


⁽¹⁾	Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).

Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities

The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2016 are as follows:



							Security Issuance Year
Millions	Fair Value		2004		2005		2006		2007		2008		2009		2010		2011		2012		2013		2014		2015		2016
Non-agency RMBS	$	205.3	$	19.4	$	5.7	$	3.0	$	—	$	2.7	$	—	$	7.4	$	9.9	$	5.0	$	15.3	$	53.3	$	48.3	$	35.3
Non-agency CMBS	127.5		—		—		—		—		—		—		4.3		—		18.1		11.5		23.4		44.4		$	25.8
Total	$	332.8	$	19.4	$	5.7	$	3.0	$	—	$	2.7	$	—	$	11.7	$	9.9	$	23.1	$	26.8	$	76.7	$	92.7	$	61.1

Non-agency residential mortgage securities, collateral quality and tranche levels

The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of December 31, 2016:



Millions	Fair Value		Super Senior⁽¹⁾		Senior⁽²⁾		Subordinate⁽³⁾
Prime	$	205.3	$	150.6	$	54.7	$	—
Non-prime	—		—		—		—
Sub-prime	—		—		—		—
Total non-agency RMBS	$	205.3	$	150.6	$	54.7	$	—


⁽¹⁾	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings (“Fitch”) and were senior to other “AAA” or “Aaa” bonds.


⁽²⁾	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds.


⁽³⁾	At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.

Non-agency commercial mortgage securities, type of interest rate and tranche levels

The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of December 31, 2016:



Millions	Fair Value		Super Senior⁽¹⁾		Senior⁽²⁾		Subordinate⁽³⁾
Fixed rate CMBS	$	116.0	$	8.7	$	59.1	$	48.2
Floating rate CMBS	11.5		—		—		11.5
Total non-agency CMBS	$	127.5	$	8.7	$	59.1	$	59.7


⁽¹⁾	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to other “AAA” or “Aaa” bonds.


⁽²⁾	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds.


⁽³⁾	At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds

Other long-term investments

The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2016 and 2015:



	December 31, 2016				December 31, 2015
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	36.5	$	—	$	12.8	$	—
Long/short equity REIT	19.9		—		20.6		—
Other	3.4		—		3.6		—
Total hedge funds	59.8		—		37.0		—

Private equity funds
Aerospace/Defense/Government	19.4		25.9		19.8		30.3
Manufacturing/Industrial	15.9		22.4		24.9		2.5
Energy infrastructure & services	14.1		3.2		20.7		3.4
Multi-sector	11.4		2.0		14.8		2.1
Healthcare	3.5		.4		3.8		.4
Private equity secondaries	3.0		2.1		4.4		2.1
Direct lending/Mezzanine debt	1.8		35.7		—		—
Financial Services	1.0		5.0		—		—
Insurance	.8		41.3		2.0		41.3
Real estate	.3		.1		.4		.1
Total private equity funds	71.2		138.1		90.8		82.2
Total hedge and private equity funds included in other long-term investments	$	131.0	$	138.1	$	127.8	$	82.2

Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds

The following summarizes the December 31, 2016 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:



Millions	Notice Period
Redemption frequency	30-59 days notice		60-89 days notice		90-119 days notice		Total
Monthly	$	—	$	—	$	—	$	—
Quarterly	15.9		—		—		15.9
Semi-annual	21.6		19.9		—		41.5
Annual	—		—		2.4		2.4
Total	$	37.5	$	19.9	$	2.4	$	59.8

Fair Value of private equity funds subject to lock-up periods

As of December 31, 2016, investments in private equity funds were subject to lock-up periods as follows:



Millions	1-3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private Equity Funds — expected lock-up period remaining	$	14.8	$	3.5	$	50.0	$	2.9	$	71.2

Rollforward of fair value investments by level

The following tables summarize the changes in White Mountains’s fair value measurements by level for the years ended December 31, 2016 and 2015:



							Level 3 Investments
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Common equity securities		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2016	$	1,152.2		$	2,531.4		$	70.0		$	—	$	169.5		$	127.8		$	4,050.9		^(1)(2)(4)
Total net realized and unrealized investment gains (losses)	11.3			.2			.1			—		6.1			(6.1		)	11.6
Amortization/Accretion	.1			(19.4		)	—			—		—			—			(19.3		)
Purchases	2,243.7			3,979.8			120.8			—		2.2			35.2			6,381.7
Sales	(2,794.8		)	(2,491.5		)	(72.9		)	—		(.1		)	(25.9		)	(5,385.2		)
Transfers in	—			118.0			—			—		—			—			118.0
Transfers out	—			—			(118.0		)	—		—			—			(118.0		)
Balance at December 31, 2016	$	612.5		$	4,118.5		$	—		$	—	$	177.7		$	131.0		$	5,039.7		^(1)(2)(4)

⁽¹⁾


⁽²⁾	Excludes carrying value of $287.1 and $211.3 as of December 31, 2016 and January 1, 2016 classified as short-term investments, of which $0.1 and $0.1 is classified as held for sale at December 31, 2016 and January 1, 2016.


⁽³⁾	Investments for which fair value is measured at NAV using the practical expedient are no longer classified within the fair value hierarchy. See Note 1 — “Summary of Significant Accounting Policies”.


⁽⁴⁾	Includes carrying value of $6.6 and $9.5 of fixed maturity investments at December 31, 2016 and January 1, 2016 that is classified as assets held for sale related to SSIE.



							Level 3 Investments
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Common equity securities			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾				Total
Balance at January 1, 2015	$	550.6		$	2,372.9		$	76.4		$	39.5		$	125.9		$	178.3		⁽¹⁾	$	3,343.6		⁽¹⁾⁽²⁾
Total net realized and unrealized investment gains (losses)	263.0			(13.7		)	(1.1		)	7.8			(20.0		)	(9.2		)		226.8			⁽³⁾⁽⁴⁾
Amortization/Accretion	—			(19.6		)	—			—			—			—				(19.6		)
Purchases	814.5			1,436.0			35.3			—			76.5			14.8				2,377.1
Sales	(825.7		)	(1,286.0		)	—			(43.7		)	(12.9		)	(41.8		)		(2,210.1		)
Symetra transfer	394.5			—			—			—			—			—				394.5
Effect of redemption of Prospector hedge funds	(43.5		)	—			—			(3.6		)	—			(14.3		)		(61.4		)
Transfers in	—			41.8			—			—			—			—				41.8
Transfers out	(1.2		)	—			(40.6		)	—			—			—				(41.8		)
Balance at December 31, 2015	$	1,152.2		$	2,531.4		$	70.0		$	—		$	169.5		$	127.8		⁽¹⁾	$	4,050.9		⁽¹⁾⁽²⁾

⁽¹⁾


⁽²⁾	Excludes carrying value of $211.3 and $376.8 as of December 31, 2015 and January 1, 2015 classified as short-term investments.


⁽³⁾	Excludes $0.8 of net realized and unrealized investment losses associated with the Prospector Funds and consolidation of investment-related liabilities.


⁽⁴⁾	Includes unrealized investment gains of $258.8 associated with the Symetra transfer from investments in unconsolidated affiliates to common equity securities.

⁽⁵⁾ Includes carrying value of $10.1 and $9.5 of fixed maturity investments at January 1, 2015 and December 31, 2015 that is classified as assets held for sale related to SSIE.

Schedule of significant unobservable inputs used in estimating the fair value of investment securities



Description	December 31, 2016
$ in millions, except share price	Valuation Technique(s)	Fair Value ⁽¹⁾	Unobservable Input
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Discounted cash flow	$22.1	Discount rate	-	25.0%
Private equity security	Share price of most recent transaction	$3.2	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$3.6	EBITDA multiple	-	6.00
Private convertible preferred security	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community reinvestment vehicle	Member share of GAAP net equity	$14.3	GAAP net equity		$14.3
Private equity security	Discounted cash flow/ Option pricing method	$9.3	Discount rate	-	21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.00%
OneBeacon Surplus Notes:
- Seller priority	Discounted cash flow	$51.1	Discount rate ⁽²⁾	-	9.6%
		Timing of interest payments ⁽⁴⁾		-	2020
		Timing of principal payments ⁽⁴⁾		-	2030
- Pari passu	Discounted cash flow	$20.8	Discount rate ⁽³⁾	-	15.0%
		Timing of interest payments ⁽⁵⁾		-	2021
		Timing of principal payments ⁽⁵⁾		-	2035

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.



Description	December 31, 2015
$ in millions, except share price	Valuation Technique(s)	Fair Value⁽¹⁾	Unobservable Input
Preferred Stock	Par value ⁽²⁾	$70.0	Issuer’s intent to call	-	$70.0
Private equity security	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	Share price of most recent transaction	$33.8	Share price	-	$1.03
Private equity security	Share price of most recent transaction	$3.0	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$5.7	EBITDA multiple	-	6.00
Private convertible preferred security	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community reinvestment vehicle	Member share of GAAP net equity	$14.3	GAAP net equity		$14.3
Private equity security	Option pricing method	$9.6
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	60.0%
		Risk free rate		-	1.15%
OneBeacon Surplus Notes ⁽⁷⁾:
- Seller priority	Discounted cash flow	$38.0	Discount rate ⁽³⁾	-	13.0%
		Timing of interest payments ⁽⁵⁾		-	2020
		Timing of principal payments ⁽⁵⁾		-	2025
- Pari passu	Discounted cash flow	$13.5	Discount rate ⁽⁴⁾	-	22.4%
		Timing of interest payments ⁽⁶⁾		-	2020
		Timing of principal payments ⁽⁶⁾		-	2030

⁽¹⁾Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

⁽²⁾Valuation based on the issuer’s intent as of December 31, 2015 to call the security in the near term.

⁽⁷⁾The decrease in the fair value of the OneBeacon Surplus Notes during the twelve months ended December 31, 2015 was primarily due to widening of non-investment grade credit spreads.

Other Investments Not Readily Marketable [Table Text Block]

Other long-term investments consist of the following as of December 31, 2016 and 2015:



	Carrying Value at
Millions	December 31, 2016		December 31, 2015
Hedge funds and private equity funds, at fair value ⁽¹⁾	$	131.0	$	127.8
Private equity securities and limited liability companies, at fair value ⁽¹⁾	72.0		82.1
OneBeacon Surplus Notes, at fair value⁽¹⁾	71.9		51.5
Private convertible preferred securities, at fair value ⁽¹⁾	30.6		32.7
Tax advantaged federal affordable housing development fund ⁽²⁾	12.3		14.7
Partnership investments accounted for under the equity method	3.5		3.8
Other	2.0		3.2
Total other-long term investments	$	323.3	$	315.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾Fund accounted for using the proportional amortization method.

Debt and Standby Letter of Credit Facilities (Tables)

12 Months Ended

Dec. 31, 2016

Debt Disclosure [Abstract]

Schedule of debt outstanding

White Mountains’s debt outstanding as of December 31, 2016 and 2015 consisted of the following:



	December 31,			Effective	December 31,			Effective
Millions	2016			Rate ⁽¹⁾	2015			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	50.0		3.9%
OneBeacon Bank Facility	—			N/A	—			N/A
OBH Senior Notes, at face value	275.0			4.7%	275.0			4.7%
Unamortized original issue discount and debt issuance costs	(1.8		)		(2.1		)
OBH Senior Notes, carrying value	273.2				272.9
MediaAlpha Bank Facility	12.9			5.7%	15.0			5.5%
Unamortized issuance cost	(.2		)		(.3		)
MediaAlpha Bank Facility, carrying value	12.7				14.7
Total debt	$	285.9			$	337.6

Schedule of contractual repayments of debt

A schedule of contractual repayments of White Mountains’s debt as of December 31, 2016, follows:



Millions	December 31, 2016
Due in one year or less	$	5.0
Due in two to three years	7.9
Due in four to five years	—
Due after five years	275.0
Total	$	287.9

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

+ Details

Goodwill and Other Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2016

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Intangible Assets and Goodwill

The following table shows the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, for each company acquired:



$ in millions	Weighted Average Economic life (in years)	December 31, 2016						December 31, 2015
$ in millions	Weighted Average Economic life (in years)	Acquisition date fair value		Accumulated amortization		Net carrying value		Acquisition date fair value		Accumulated amortization		Net carrying value
Goodwill:
MediaAlpha	N/A	$	18.3	$	—	$	18.3	$	18.3	$	—	$	18.3
Wobi	N/A	5.8		—		5.8		5.8		—		5.8
Buzzmove	N/A	7.6		—		7.6		—		—		—
Total goodwill		31.7		—		31.7		24.1		—		24.1

Other intangible assets:
MediaAlpha
Customer relationships	3	10.0		6.2		3.8		6.5		2.9		3.6
Information technology	5	32.4		17.9		14.5		32.0		11.2		20.8
Subtotal		42.4		24.1		18.3		38.5		14.1		24.4

Wobi
Trademark	8	2.1		.8		1.3		2.1		.5		1.6
Information technology	7	3.6		1.2		2.4		3.6		.7		2.9
Subtotal		5.7		2.0		3.7		5.7		1.2		4.5

Buzzmove
Trademark	7	.6		.1		.5		—		—		—
Information technology	5	.5		—		.5		—		—		—
Subtotal		1.1		.1		1.0		—		—		—

OneBeacon	10	9.4		8.2		1.2		9.4		7.0		2.4

Total other intangible assets		58.6		34.4		24.2		53.6		22.3		31.3
Total goodwill and other intangible assets⁽¹⁾		$	90.3	$	34.4	$	55.9	$	77.7	$	22.3	$	55.4

Schedule of Goodwill and Intangible Assets Rollforward

The following table shows the change in goodwill and other intangible assets:



	December 31,
	2016					2015
Millions	Goodwill		Other intangible assets			Goodwill		Other intangible assets
Beginning balance	$	24.1	$	31.3		$	23.8	$	38.7
Add: Star & Shield amounts held for sale at beginning of the period ⁽¹⁾	—		.4			—		.8
Acquisitions of businesses	7.6		1.1			.3		2.8
Acquisitions of other intangible assets	—		3.9			—		—
Amortization, including foreign currency translation	—		(12.5		)	—		(10.6		)
Less: Star & Shield amounts held for sale at end of the period⁽¹⁾	—		—			—		.4
Ending balance	$	31.7	$	24.2		$	24.1	$	31.3

Schedule of Finite-Lived Intangible Assets, Future Amortization Expense

White Mountains expects to recognize amortization expense in each of the next five years as follows:



Millions	Amortization expense
2017	$	10.9
2018	8.3
2019	2.3
2020	.7
2021	.9
Total	$	23.1

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Schedule of total income tax benefit (expense)

The total income tax benefit for the years ended December 31, 2016, 2015 and 2014 consisted of the following:



	Year Ended December 31,
Millions	2016			2015			2014
Current tax benefit (expense):
U.S. federal	$	50.7		$	9.1		$	(4.3	)
State	(1.6		)	(1.8		)	(2.0		)
Non-U.S.	(.8		)	(1.5		)	(1.4		)
Total current tax benefit (expense)	48.3			5.8			(7.7		)
Deferred tax (expense) benefit:
U.S. federal	(2.3		)	(6.7		)	22.8
State	(.6		)	1.1			(.3		)
Total deferred tax (expense) benefit	(2.9		)	(5.6		)	22.5
Total income tax benefit	$	45.4		$	.2		$	14.8

Reconciliation of the U.S. federal statutory income tax rate and actual effective tax rate on pre-tax income



	Year Ended December 31,
Millions	2016			2015			2014
Tax benefit (expense) at the U.S. statutory rate	$	14.1		$	(55.1	)	$	10.3
Differences in taxes resulting from:
Tax reserve adjustments	13.7			(1.7		)	5.2
Change in valuation allowance	9.1			(19.4		)	(35.7		)
Non-U.S. earnings, net of foreign taxes	6.9			76.7			37.8
Tax rate changes	(3.5		)	—			—
Tax exempt interest and dividends	2.5			2.6			2.5
Withholding tax	(.5		)	(1.2		)	(2.4		)
Other, net	3.1			(1.7		)	(2.9		)
Total income tax benefit on pre-tax income	$	45.4		$	.2		$	14.8

Schedule of components of deferred income tax assets and liabilities

An outline of the significant components of White Mountains’s deferred tax assets and liabilities follows:



	December 31,
Millions	2016			2015
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$	189.3		$	191.0
Incentive compensation	42.5			49.0
Unearned premiums	37.8			37.5
Non-U.S. net operating loss carryforwards	29.5			33.8
Tax credit carryforwards	20.7			16.9
Loss reserve discount	18.1			26.8
Runoff Transaction	12.6			12.6
Deferred compensation	5.7			6.2
Accrued interest	2.2			1.3
Other items	4.0			8.6
Total gross deferred income tax assets	362.4			383.7
Less: valuation allowances	(146.6		)	(165.4		)
Total net deferred income tax assets	215.8			218.3
Deferred income tax liabilities related to:
Deferred acquisition costs	33.6			34.9
Members surplus contributions	30.0			19.0
SSIE Surplus Notes	7.1			—
Pension and benefit accruals	6.4			3.5
Capitalized software	4.7			4.5
Investment basis difference	4.1			15.2
Net unrealized investment gains	2.9			28.4
Other items	.3			—
Total deferred income tax liabilities	89.1			105.5
Net deferred tax asset	$	126.7		$	112.8

Schedule of net operating and capital loss carryforwards by expiration dates and the deferred tax assets thereon

Net operating loss and capital loss carryforwards as of December 31, 2016, the expiration dates and the deferred tax assets thereon are as follows:



	December 31, 2016
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2017-2021	$	.8		$	—		$	—		$	—		$	.8
2022-2026	1.6			—			—			—			1.6
2027-2036	556.3			—			—			—			556.3
No expiration date	—			93.8			3.2			19.8			116.8
Total	$	558.7		$	93.8		$	3.2		$	19.8		$	675.5
Gross deferred tax asset	189.3			24.4			.5			4.6			218.8
Valuation allowance	(115.4		)	(24.4		)	(.5		)	(4.6		)	(144.9		)
Net deferred tax asset	$	73.9		$	—		$	—		$	—		$	73.9

Reconciliation of changes in the amount of unrecognized tax benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:



Millions	Permanent Differences⁽¹⁾			Temporary Differences⁽²⁾			Interest and Penalties⁽³⁾			Total
Balance at January 1, 2014	$	8.4		$	16.6		$	8.7		$	33.7
Changes in prior year tax positions	(2.2		)	(.8		)	(1.9		)	(4.9		)
Tax positions taken during the current year	—			7.3			—			7.3
Lapse in statute of limitations	(.8		)	—			(.3		)	(1.1		)
Balance at December 31, 2014	5.4			23.1			6.5			35.0
Changes in prior year tax positions	—			(12.4		)	1.7			(10.7		)
Balance at December 31, 2015	5.4			10.7			8.2			24.3
Changes in prior year tax positions	—			(5.5		)	.1			(5.4		)
Settlements with tax authorities	(5.4		)	—			(8.3		)	(13.7		)
Balance at December 31, 2016	$	—		$	5.2		$	—		$	5.2


⁽¹⁾	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.


⁽²⁾	Represents the amount of unrecognized tax benefits that, if recognized would create a temporary difference between the reported amount of an item in White Mountains’s Consolidated Balance Sheet and its tax basis.


⁽³⁾	Net of tax benefit.

Derivatives (Tables)

12 Months Ended

Dec. 31, 2016

Dec. 31, 2015

Derivative [Line Items]

Derivatives

Variable Annuity Reinsurance

Derivative [Line Items]

Derivatives

Variable Annuity Reinsurance

The following table summarizes the pre-tax operating results of WM Life Re for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
Millions	2016			2015			2014
Fees, included in other revenue	$	1.2		$	9.3		$	18.6
Change in fair value of variable annuity liability, included in other revenue	(.3		)	(.4		)	52.9
Change in fair value of derivatives, included in other revenue	(2.0		)	(8.8		)	(72.4		)
Foreign exchange, included in other revenue	1.3			(1.3		)	(3.2		)
Other investment income and losses	—			(.4		)	(1.4		)
Total revenues	.2			(1.6		)	(5.5		)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	—			—			.6
Death benefit claims paid, included in general and administrative expenses	(.3		)	(.1		)	(.1		)
General and administrative expenses	(2.6		)	(4.0		)	(4.3		)
Pre-tax loss	$	(2.7	)	$	(5.7	)	$	(9.3	)



										Carrying Value
	Year Ended December 31,									December 31,
Millions	2016			2015			2014			2016		2015
Fixed income/interest rate	$	1.8		$	6.4		$	(33.7	)	$	—	$	.5
Foreign exchange	(4.8		)	(7.3		)	(1.3		)	—		14.8
Equity	1.0			(7.9		)	(37.4		)	—		4.8
Total	$	(2.0	)	$	(8.8	)	$	(72.4	)	$	—	$	20.1

The following table summarizes the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2016, 2015 and 2014:



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2016	$	.3			$	2.7		$	16.5		$	.9		$	20.1
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.3		)	⁽⁵⁾	2.9			(.7		)	(4.2		)	(2.0		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.6		)	(15.8		)	3.3			(18.1		)
Balance at December 31, 2016	$	—			$	—		$	—		$	—		$	—



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2015	$	.7			$	18.9		$	33.8		$	3.7		$	56.4
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.4		)	⁽⁵⁾	(9.7		)	(7.5		)	8.4			(8.8		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(6.5		)	(9.8		)	(11.2		)	(27.5		)
Balance at December 31, 2015	$	.3			$	2.7		$	16.5		$	.9		$	20.1



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2014	$	(52.8	)		$	63.4		$	4.7		$	1.1		$	69.2
Purchases	—				—			—			—			—
Net realized and unrealized gains (losses)	53.5			⁽⁵⁾	(38.6		)	(71.0		)	37.2			(72.4		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.9		)	100.1			(34.6		)	59.6
Balance at December 31, 2014	$	.7			$	18.9		$	33.8		$	3.7		$	56.4


⁽¹⁾	Consists of over-the-counter instruments.


⁽²⁾	Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.


⁽³⁾	Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.


⁽⁴⁾	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $0.0, $5.8 and $33.2 as of December 31, 2016, 2015 and 2014 posted as collateral to its reinsurance counterparties.


⁽⁵⁾	There was no changes in the fair value of variable annuity death benefit liabilities for the years ended December 31, 2016 and December 31, 2015. In 2014, $0.6 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.


⁽⁶⁾	Consists of foreign currency options and equity options.



	December 31,
Millions	2016		2015
Cash	$	—	$	5.8
Fixed maturity investments	—		—
Total	$	—	$	5.8



	December 31, 2015
Millions	Gross asset amounts before offsets⁽¹⁾		Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other Assets
Interest rate contracts
OTC	$	2.4	$	(2.1	)	$	.3
Exchange traded	.1		(.1		)	—
Foreign exchange contracts
OTC	15.0		—			15.0
Exchange traded	.1		(.3		)	(.2		)
Equity contracts
OTC	4.4		(.6		)	3.8
Exchange traded	1.2		—			1.2
Total⁽²⁾	$	23.2	$	(3.1	)	$	20.1

⁽¹⁾Amount equal to fair value of instrument as recognized in other assets.

⁽²⁾ All derivative instruments held by WM Life Re were subject to master netting arrangements.



	December 31, 2015
Millions	Net amount of assets reflected in Balance Sheet		Collateral provided to counterparty - Cash		Collateral provided to counter-party - Financial Instruments		Net amount of exposure after effect of collateral provided		Excess collateral provided to counter-party- Cash		Excess collateral provided - Financial Instruments		Counter-party collateral held by WM Life Re- Cash		Net amount of exposure to counter-party		Standard & Poor's Rating⁽¹⁾

JP Morgan	$	8.5	$	—	$	—	$	8.5	$	—	$	—	$	5.5	$	3.0	A	+
Bank of America	.7		$	—	—		.7		$	—	—		—		.7		A
Citigroup - OTC	9.9		—		—		9.9		—		—		1.4		8.5		A
Citigroup - Exchange Traded	1.0		—		—		1.0		5.8		—		—		6.8		A
Total	$	20.1	$	—	$	—	$	20.1	$	5.8	$	—	$	6.9	$	19.0


⁽¹⁾	Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings),“A” (which is the sixth highest of twenty-three creditworthiness ratings), “A-” (which is the seventh highest of twenty-three creditworthiness ratings), and BBB+ (which is the eighth highest of twenty-three creditworthiness ratings).

Pre-tax operating results

The following table summarizes the pre-tax operating results of WM Life Re for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
Millions	2016			2015			2014
Fees, included in other revenue	$	1.2		$	9.3		$	18.6
Change in fair value of variable annuity liability, included in other revenue	(.3		)	(.4		)	52.9
Change in fair value of derivatives, included in other revenue	(2.0		)	(8.8		)	(72.4		)
Foreign exchange, included in other revenue	1.3			(1.3		)	(3.2		)
Other investment income and losses	—			(.4		)	(1.4		)
Total revenues	.2			(1.6		)	(5.5		)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	—			—			.6
Death benefit claims paid, included in general and administrative expenses	(.3		)	(.1		)	(.1		)
General and administrative expenses	(2.6		)	(4.0		)	(4.3		)
Pre-tax loss	$	(2.7	)	$	(5.7	)	$	(9.3	)

Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table

The following table summarizes the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2016, 2015 and 2014:



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2016	$	.3			$	2.7		$	16.5		$	.9		$	20.1
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.3		)	⁽⁵⁾	2.9			(.7		)	(4.2		)	(2.0		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.6		)	(15.8		)	3.3			(18.1		)
Balance at December 31, 2016	$	—			$	—		$	—		$	—		$	—



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2015	$	.7			$	18.9		$	33.8		$	3.7		$	56.4
Purchases	—				—			—			—			—
Net realized and unrealized (losses) gains	(.4		)	⁽⁵⁾	(9.7		)	(7.5		)	8.4			(8.8		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(6.5		)	(9.8		)	(11.2		)	(27.5		)
Balance at December 31, 2015	$	.3			$	2.7		$	16.5		$	.9		$	20.1



	Variable Annuity (Liabilities)				Derivative Instruments
Millions	Level 3				Level 3⁽¹⁾⁽⁶⁾			Level 2⁽¹⁾⁽²⁾			Level 1⁽³⁾			Total⁽⁴⁾
Balance at January 1, 2014	$	(52.8	)		$	63.4		$	4.7		$	1.1		$	69.2
Purchases	—				—			—			—			—
Net realized and unrealized gains (losses)	53.5			⁽⁵⁾	(38.6		)	(71.0		)	37.2			(72.4		)
Transfers in (out)	—				—			—			—			—
Sales/settlements	—				(5.9		)	100.1			(34.6		)	59.6
Balance at December 31, 2014	$	.7			$	18.9		$	33.8		$	3.7		$	56.4


⁽¹⁾	Consists of over-the-counter instruments.


⁽²⁾	Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.


⁽³⁾	Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.


⁽⁴⁾	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $0.0, $5.8 and $33.2 as of December 31, 2016, 2015 and 2014 posted as collateral to its reinsurance counterparties.


⁽⁵⁾	There was no changes in the fair value of variable annuity death benefit liabilities for the years ended December 31, 2016 and December 31, 2015. In 2014, $0.6 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.


⁽⁶⁾	Consists of foreign currency options and equity options.

Schedule of collateral



	December 31,
Millions	2016		2015
Cash	$	—	$	5.8
Fixed maturity investments	—		—
Total	$	—	$	5.8

Offestting Assets and Liabilities



	December 31, 2015
Millions	Gross asset amounts before offsets⁽¹⁾		Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other Assets
Interest rate contracts
OTC	$	2.4	$	(2.1	)	$	.3
Exchange traded	.1		(.1		)	—
Foreign exchange contracts
OTC	15.0		—			15.0
Exchange traded	.1		(.3		)	(.2		)
Equity contracts
OTC	4.4		(.6		)	3.8
Exchange traded	1.2		—			1.2
Total⁽²⁾	$	23.2	$	(3.1	)	$	20.1

⁽¹⁾Amount equal to fair value of instrument as recognized in other assets.

⁽²⁾ All derivative instruments held by WM Life Re were subject to master netting arrangements.

Offsetting Assets



	December 31, 2015
Millions	Net amount of assets reflected in Balance Sheet		Collateral provided to counterparty - Cash		Collateral provided to counter-party - Financial Instruments		Net amount of exposure after effect of collateral provided		Excess collateral provided to counter-party- Cash		Excess collateral provided - Financial Instruments		Counter-party collateral held by WM Life Re- Cash		Net amount of exposure to counter-party		Standard & Poor's Rating⁽¹⁾

JP Morgan	$	8.5	$	—	$	—	$	8.5	$	—	$	—	$	5.5	$	3.0	A	+
Bank of America	.7		$	—	—		.7		$	—	—		—		.7		A
Citigroup - OTC	9.9		—		—		9.9		—		—		1.4		8.5		A
Citigroup - Exchange Traded	1.0		—		—		1.0		5.8		—		—		6.8		A
Total	$	20.1	$	—	$	—	$	20.1	$	5.8	$	—	$	6.9	$	19.0


⁽¹⁾	Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings),“A” (which is the sixth highest of twenty-three creditworthiness ratings), “A-” (which is the seventh highest of twenty-three creditworthiness ratings), and BBB+ (which is the eighth highest of twenty-three creditworthiness ratings).

Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument



										Carrying Value
	Year Ended December 31,									December 31,
Millions	2016			2015			2014			2016		2015
Fixed income/interest rate	$	1.8		$	6.4		$	(33.7	)	$	—	$	.5
Foreign exchange	(4.8		)	(7.3		)	(1.3		)	—		14.8
Equity	1.0			(7.9		)	(37.4		)	—		4.8
Total	$	(2.0	)	$	(8.8	)	$	(72.4	)	$	—	$	20.1

Forward Contracts [Member]

Derivative [Line Items]

Derivatives

The following table summarizes the notional amount and the uncollateralized balance associated with the forward currency contract:



	December 31, 2016
Millions	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾
Barclays Bank PLC	$	184.6	$	(1.2	)	A-

⁽¹⁾Standard & Poor’s ratings “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).

Municipal Bond Guarantee Insurance (Tables)

12 Months Ended

Dec. 31, 2016

Insurance [Abstract]

Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Table Text Block]

The following table is a schedule of BAM’s expected premiums to be collected as of December 31, 2016:



Millions	December 31, 2016
January 1, 2017 - March 31, 2017	$	1.9
April 1, 2017 - June 30, 2017	1.8
July 1, 2017 - September 30, 2017	1.8
October 1, 2017 - December 31, 2017	1.8
	7.3

2018	7.0
2019	6.6
2020	6.3
2021	5.9
2022 and thereafter	49.9
Total	$	83.0

Schedule of Insured Obligations

The following table provides a schedule of BAM’s insured obligations:



	December 31, 2016		December 31, 2015
Contracts outstanding	4,807		3,103
Remaining weighted average contract period (in years)	10.8		12.8
Contractual debt service outstanding (in millions):
Principal	$	33,057.3	$	22,556.0
Interest	$	16,396.6	$	11,984.4
Gross unearned insurance premiums	$	83.0	$	50.2

Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Computation of earnings per share

The following table outlines the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2016, 2015 and 2014. See Note 22 — “Held for Sale and Discontinued Operations” for earnings per share amounts for discontinued operations.



	Year Ended December 31,
	2016			2015			2014
Basic and diluted earnings per share numerators (in millions):
Net income from continuing operations attributable to White Mountains’s common shareholders	$	(2.3	)	$	200.7		$	53.2
Allocation of income for unvested restricted common shares⁽¹⁾	—			(2.3		)	(0.7		)
Dividends declared on participating restricted common shares⁽¹⁾	—			(.1		)	(.1		)
Total allocation to restricted common shares	—			(2.4		)	(0.8		)
Net income attributable to White Mountains’s common shareholders, net of restricted share amounts	$	(2.3	)	$	198.3		$	52.4
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	(2.3	)	$	198.3		$	52.4
Dividends declared net of restricted common share amounts⁽¹⁾	(5.4		)	(5.9		)	(6.1		)
Total undistributed net earnings, net of restricted common share amounts	$	(7.7	)	$	192.4		$	46.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	5,014.9			5,879.2			6,104.9
Average unvested restricted common shares⁽²⁾	(64.8		)	(68.0		)	(78.9		)
Basic earnings per share denominator	4,950.1			5,811.2			6,026.0
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	5,018.1			5,879.2			6,104.9
Average unvested restricted common shares⁽²⁾	(64.8		)	(68.0		)	(78.9		)
Diluted earnings per share denominator⁽³⁾	4,953.3			5,811.2			6,026.0
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$	(0.47	)	$	34.12		$	8.70
Dividends declared and paid	(1.00		)	(1.00		)	(1.00		)
Undistributed (loss) earnings	$	(1.47	)	$	33.12		$	7.70
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$	(0.47	)	$	34.12		$	8.70
Dividends declared and paid	(1.00		)	(1.00		)	(1.00		)
Undistributed (loss) earnings	$	(1.47	)	$	33.12		$	7.70


⁽¹⁾	Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.


⁽²⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Incentive Compensation Plans”.

⁽³⁾

Retirement and Postretirement Plans (Tables)

12 Months Ended

Dec. 31, 2016

Compensation and Retirement Disclosure [Abstract]

Schedule of changes in projected benefit obligations and fair value of plan assets and funded status of plan assets

The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans as of December 31, 2016 and 2015:



	Pension Benefits December 31,
Millions	2016			2015
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$	109.5		$	119.7
Service cost	.2			.9
Interest cost	1.5			4.6
Settlement loss (gain) on projected benefit obligation⁽¹⁾⁽²⁾	1.2			(1.8		)
Assumption changes	(.9		)	(3.5		)
Actuarial loss	.4			.9
Annuity contract purchases related to termination of Qualified Plan ⁽³⁾	(69.0		)	—
Benefits, including lump sum payments, and expenses paid with plan assets⁽³⁾	(17.0		)	(13.7		)
Benefits paid directly by OneBeacon	(2.1		)	(2.2		)
Remeasurement due to plan termination ⁽⁴⁾	—			4.6
Projected benefit obligation at end of year	$	23.8		$	109.5
Change in plan assets:
Fair value of plan assets at beginning of year	$	139.8		$	146.0
Actual return on plan assets	6.3			7.5
Annuity contract purchases related to termination of Qualified Plan⁽³⁾	(69.0		)	—
Transfer of assets to the QRP⁽⁵⁾	(47.1		)	—
Benefits and expenses paid	(17.0		)	(13.7		)
Fair value of plan assets at end of year	$	13.0		$	139.8
(Unfunded) over funded status at end of year	$	(10.8	)	$	30.3

⁽¹⁾During the year ended December 31, 2016, OneBeacon recognized a $1.2 million settlement loss primarily resulting from final settlement of the Qualified Plan.

⁽⁴⁾ As of December 31, 2015, the projected benefit obligation was valued on a plan termination basis.

⁽⁵⁾ During the year ended December 31, 2016, OneBeacon transferred excess invested assets from the Qualified Plan after its termination into the trust supporting the QRP.

Schedule of amounts recorded in financial statements

Amounts recognized in the financial statements as of December 31, 2016 and 2015 consist of:



	December 31,
Millions	2016			2015
Net balance sheet asset recorded in other assets	$	13.0		$	55.8
Net balance sheet liability recorded in other liabilities	(23.8		)	(25.5		)
Net amount recognized in the financial statements	$	(10.8	)	$	30.3

Information for the Non-qualified Plan, which has accumulated benefit obligations in excess of plan assets

Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows:



	December 31,
Millions	2016		2015
Projected benefit obligation	$	23.8	$	25.5
Accumulated benefit obligation	$	23.8	$	25.5
Fair value of plan assets	$	—	$	—

Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets

nformation for the Qualified Plan, which had accumulated benefit obligations less than plan assets until its termination, was finalized. Information for the Qualified Plan was as follows:



	December 31,
Millions	2016		2015
Projected benefit obligation	$	—	$	84.0	⁽¹⁾
Accumulated benefit obligation	$	—	$	84.0	⁽¹⁾
Fair value of plan net assets	$	13.0	$	139.8

⁽¹⁾ Measured on a plan termination basis

Schedule of amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis and before noncontrolling interest

The amounts recognized in accumulated other comprehensive loss on a pre-tax basis and before non-controlling interest for the years ended December 31, 2016 and 2015 were as follows:



	December 31,
Millions	2016			2015
Accumulated other comprehensive loss beginning balance	$	(8.0	)	$	(8.0	)
Increase (decrease) in accumulated other comprehensive loss:
Amortization of net actuarial gains recognized during the year	1.3			1.3
Net actuarial gains (losses) occurring during the year ⁽¹⁾	3.2			(1.3		)
Qualified Plan termination impact ⁽²⁾	(3.0		)	—
Accumulated other comprehensive loss ending balance	$	(6.5	)	$	(8.0	)


⁽¹⁾	Net actuarial gains in 2016 resulted from investment returns and demographic experience different than assumed. The 2015 net actuarial losses reflect the valuation of the Qualified Plan on a termination basis.


⁽²⁾	During the year ended December 31, 2016, OneBeacon recognized a $3.0 million gain resulting from the final settlement of the Qualified Plan.

Schedule of components of net periodic benefit cost (income)

The components of net periodic benefit cost (income) for the years ended December 31, 2016, 2015 and 2014 were as follows:



	Year Ended December 31,
Millions	2016			2015			2014
Service cost	$	.2		$	.9		$	.6
Interest cost	1.5			4.6			4.7
Expected return on plan assets	(1.0		)	(8.7		)	(8.5		)
Amortization of net actuarial losses	1.3			1.3			.3
Net periodic pension (cost) income before special termination benefits	2.0			(1.9		)	(2.9		)
Settlement gain ⁽¹⁾	(3.0		)	—			—
Special termination benefits expense ⁽²⁾	—			—			.3
Total net periodic benefit income	$	(1.0	)	$	(1.9	)	$	(2.6	)

⁽¹⁾ Represents the impact of the termination of the Qualified Plan during the year ended December 31, 2016.

⁽²⁾ Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.

Schedule of fair value of the Plan assets and their related inputs

The fair value of the Qualified Plan’s assets and their related inputs as of December 31, 2016 and 2015 by asset category were as follows:



	December 31, 2016								December 31, 2015
Millions	Fair Value⁽¹⁾		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs		Fair Value		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs
Common equity securities	$	12.8	$	12.8	$	—	$	—	$	—	$	—	$	—	$	—
Fixed maturity investments	—		—		—		—		132.4		132.4		—		—
Short-term investments	—		—		—		—		7.0		7.0		—		—
Total	$	12.8	$	12.8	$	—	$	—	$	139.4	$	139.4	$	—	$	—

⁽¹⁾Excludes cash and short-term investments

There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2016 and 2015.

The Qualified Plan’s asset allocations as of December 31, 2016 and 2015, by asset category were as follows:



	Plan Assets at December 31,
Asset Category	2016		2015
Common equity securities	98.5	%	—	%
Fixed maturity investments	—		95.0
Cash and short-term investments	1.5		5.0
Total	100.0	%	100.0	%

Schedule of expected future benefit payments

The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid:



Millions	Expected Benefit Payments
2017	$	2.1
2018	2.1
2019	2.0
2020	2.0
2021	1.9
2022 - 2026	8.4

Employee Share-Based Incentive Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2016

White Mountains

Share-based compensation arrangement by share-based payment award

Summary of performance share activity

The following summarizes performance share activity for the years ended December 31, 2016, 2015 and 2014 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	93,654		$	57.7		111,257		$	44.4		108,605		$	54.9
Shares paid or expired⁽¹⁾	(36,294	)	(41.0		)	(42,959	)	(30.8		)	(33,730	)	(24.5		)
New grants	22,615		—			29,195		—			39,590		—
Assumed forfeitures and cancellations⁽²⁾	378		.5			(3,839	)	(.3		)	(3,208	)	(.1		)
Expense recognized	—		25.2			—		44.4			—		14.1
End of period ⁽³⁾	80,353		$	42.4		93,654		$	57.7		111,257		$	44.4


⁽¹⁾	WTM performance share payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target. WTM performance shares payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance shares payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.


⁽²⁾	Amounts include changes in assumed forfeitures, as required under GAAP.


⁽³⁾	Outstanding performance share awards as of December 31, 2016, 2015 and 2014 exclude 7,315, 10,826 and 12,607 unvested performance shares awards for employees of Sirius Group.

Summary of performance shares outstanding and accrued expense for performance shares awarded under an Incentive Plan

The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2016 for each performance cycle:



$ in millions	Target WTM Performance Shares Outstanding		Accrued Expense
Performance cycle:
2016 – 2018	19,615		$	5.4
2015 – 2017	31,795		19.1
2014 – 2016	30,167		18.6
Sub-total	81,577		43.1
Assumed forfeitures	(1,224	)	(.7		)
Total at December 31, 2016	80,353		$	42.4

Schedule of Nonvested Restricted Stock Activity [Table Text Block]

The following outlines the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	70,675		$	15.7		83,314		$	14.3		94,130		$	17.0
Issued	25,365		20.2			23,640		15.7			23,440		13.1
Vested	(24,620	)	—			(36,279	)	—			(33,205	)	—
Forfeited	(800	)	(.3		)	—		—			(1,051	)	(.5		)
Expense recognized	—		(15.9		)	—		(14.3		)	—		(15.3		)
End of period ⁽¹⁾	70,620		$	19.7		70,675		$	15.7		83,314		$	14.3


⁽¹⁾	Outstanding restricted share awards as of December 31, 2016, 2015 and 2014 include 5,235, 9,205, and 12,477 unvested restricted shares for employees of Sirius Group.

OneBeacon

Share-based compensation arrangement by share-based payment award

Summary of performance share activity

The following summarizes activity for the years ended December 31, 2016, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:



	Year Ended December 31,
	2016					2015					2014
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	449,435		$	1.4		517,470		$	3.4		493,421		$	4.0
Payments and deferrals⁽¹⁾	(167,300	)	(.6		)	(181,290	)	(1.5		)	(142,138	)	(1.0		)
New awards	163,150		—			154,887		—			165,800		—
Forfeitures and cancellations⁽²⁾	7,234		—			(41,632	)	(.1		)	387		—
Expense recognized	—		.8			—		(.4		)	—		.4
End of period	452,519		$	1.6		449,435		$	1.4		517,470		$	3.4


⁽¹⁾	OneBeacon performance share payments in 2016 for the 2013-2015 performance cycle were at 24.3% of target. OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target.


⁽²⁾	Amounts include changes in assumed forfeitures, as required under GAAP.

Summary of performance shares outstanding and accrued expense for performance shares awarded under an Incentive Plan

The following summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan as of December 31, 2016 for each performance cycle:



$ in millions	Target OneBeacon Performance Shares Outstanding	Accrued Expense
Performance cycle:
2016 – 2018	163,150	$	.9
2015 – 2017	146,659	.7
2014 – 2016	142,710	—
Total at December 31, 2016	452,519	$	1.6

Schedule of Nonvested Restricted Stock Activity [Table Text Block]

The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the years ended December 31, 2016, 2015 and 2014.



	Year Ended December 31,
	2016					2015					2014
$ in millions	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Beginning of period	382,722		$	2.5		612,500		$	3.5		915,000		$	6.5
Issued	170,650		2.3			75,950		1.1			—		—
Vested	(157,500	)	—			(296,000	)	—			(300,000	)	—
Forfeited	—		—			(9,728	)	(.1		)	(2,500	)	—
Expense recognized	—		(2.7		)	—		(2.0		)	—		(3.0		)
End of period	395,872		$	2.1		382,722		$	2.5		612,500		$	3.5

Common Shareholders’ Equity and Non-controlling Interests Common Shareholders’ Equity and Non-controlling Interests (Tables)

12 Months Ended

Dec. 31, 2016

Noncontrolling Interest [Abstract]

Noncontrolling Interest

The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of December 31, 2016 and 2015:



	December 31, 2016					December 31, 2015
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon Ltd.	23.9	%	$	244.6		24.5	%	$	245.6
SIG Preference Shares	—		—			100.0		250.0

Other, excluding mutuals and reciprocals
HG Global	3.1		16.6			3.1		17.1
MediaAlpha	40.0		11.7			40.0		14.4
Dewar	18.8		3.9			19.0		3.7
Buzzmove	29.1		2.9			—		—
Wobi	5.0		.7			3.9		.6
Tranzact	—		—			36.8		79.4
Total other, excluding mutuals and reciprocals			35.8					115.2

Mutuals and reciprocals
BAM	100.0		(150.9		)	100.0		(140.0		)
SSIE	100.0		4.4			100.0		(16.0		)
Total mutuals and reciprocals			(146.5		)			(156.0		)
Total non-controlling interests			$	133.9				$	454.8

Segment Information (Tables)

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

Financial information for White Mountains' segments

Financial information for White Mountains’s segments follows:



			HG Global/BAM
Millions	OneBeacon		HG Global		BAM⁽¹⁾			Other Operations				Total
Year Ended December 31, 2016
Earned insurance and reinsurance premiums	$	1,100.6	$	4.4	$	1.5		$	7.5			$	1,114.0
Net investment income	50.6		2.2		6.8			27.2				86.8
Net investment income (loss) - BAM Surplus Note	—		17.8		(17.8		)	—				—
Net realized and unrealized investment gains (losses)	37.7		.1		.6			(28.1		)		10.3
Other revenue	5.5		—		1.1			143.0			⁽⁴⁾	149.6
Total revenues⁽³⁾	1,194.4		24.5		(7.8		)	149.6				1,360.7
Losses and LAE	656.0		—		—			8.0				664.0
Insurance and reinsurance acquisition expenses	206.0		.9		2.5			2.2				211.6
Other underwriting expenses	209.0		—		.4			.1				209.5
General and administrative expenses	13.0		1.4		38.2			234.8			⁽⁵⁾	287.4
Amortization of intangible assets	1.2		—		—			11.3				12.5
Interest expense	13.1		—		—			3.0				16.1
Total expenses	1,098.3		2.3		41.1			259.4				1,401.1
Pre-tax income (loss)	$	96.1	$	22.2	$	(48.9	)	$	(109.8	)		$	(40.4	)



				HG Global/BAM
Millions	OneBeacon			HG Global			BAM⁽¹⁾			Other Operations			Total
Year Ended December 31, 2015
Earned insurance and reinsurance premiums	$	1,176.2		$	2.5		$	.8		$	8.7		$	1,188.2
Net investment income	45.9			1.9			4.2			8.8			60.8
Net investment income (loss) - BAM Surplus Note	—			15.8			(15.8		)	—			—
Net realized and unrealized investment (losses) gains	(35.1		)	(.3		)	.9			259.9		⁽²⁾	225.4
Other (loss) revenue	(.6		)	—			.7			147.2		⁽⁴⁾	147.3
Total revenues⁽³⁾	1,186.4			19.9			(9.2		)	424.6			1,621.7
Losses and LAE	700.7			—			—			8.2			708.9
Insurance and reinsurance acquisition expenses	213.8			.6			2.3			3.4			220.1
Other underwriting expenses	218.2			—			.4			—			218.6
General and administrative expenses	14.1			1.4			35.4			240.7		⁽⁵⁾	291.6
Amortization of intangible assets	1.3			—			—			9.3			10.6
Interest expense	13.0			—			—			1.6			14.6
Total expenses	1,161.1			2.0			38.1			263.2			1,464.4
Pre-tax income (loss)	$	25.3		$	17.9		$	(47.3	)	$	161.4		$	157.3



			HG Global/BAM
Millions	OneBeacon		HG Global		BAM⁽¹⁾			Other Operations				Total
Year Ended December 31, 2014
Earned insurance and reinsurance premiums	$	1,177.1	$	1.4	$	.4		$	6.1			$	1,185.0
Net investment income	43.4		1.4		5.7			9.0				59.5
Net investment income (loss) - BAM Surplus Note	—		15.7		(15.7		)	—				—
Net realized and unrealized investment gains	40.4		1.7		6.6			29.8				78.5
Other revenue	5.8		—		.6			81.7			⁽⁴⁾	88.1
Total revenues⁽³⁾	1,266.7		20.2		(2.4		)	126.6				1,411.1
Losses and LAE	815.1		—		—			8.9				824.0
Insurance and reinsurance acquisition expenses	203.3		.3		1.8			0.8				206.2
Other underwriting expenses	179.2		—		.4			—				179.6
General and administrative expenses	12.4		1.6		35.9			158.3			⁽⁵⁾	208.2
Amortization of intangible assets	1.4		—		—			6.9				8.3
Interest expense	13.0		—		—			1.2				14.2
Total expenses	1,224.4		1.9		38.1			176.1				1,440.5
Pre-tax income (loss)	$	42.3	$	18.3	$	(40.5	)	$	(49.5	)		$	(29.4	)


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes BAM Surplus Notes and is not reduced by accruals of interest expense on the BAM Surplus Notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services.


⁽²⁾	Net realized and unrealized investment gains (losses) in the Other Operations segment includes the unrealized investment gain of $258.8 related to the investment in Symetra common shares, representing the difference between the carrying value under the equity method at November 5, 2015 and the fair value at December 31, 2015. See Note 17 — “Investments in Unconsolidated Affiliates”.


⁽³⁾	Total revenues includes both revenues from customers, as well as, investment performance.


⁽⁴⁾	Includes $116.5 from MediaAlpha for the year ended December 31, 2016; $105.5 from MediaAlpha for the year ended December 31, 2015; $65.3 from MediaAlpha for the year ended December 31, 2014.


⁽⁵⁾	Includes $109.6 from MediaAlpha for the year ended December 31, 2016; and $99.0 from MediaAlpha for the year ended December 31, 2015; $60.6 from MediaAlpha for the year ended December 31, 2014.



			HG Global/BAM
Selected Balance Sheet Data Millions	OneBeacon		HG Global		BAM				Other Operations		Eliminations			Held for Sale			Total
December 31, 2016:
Total investments	$	2,620.4	$	161.6	$	468.1			$	2,084.7	$	—		$	—		$	5,334.8
Reinsurance recoverable on paid and unpaid losses	179.5		—		—				—		—			—			179.5
Assets held for sale	—		—		—				—		—			10.1			10.1
Total assets	3,589.4		787.0		(78.9		)	⁽¹⁾	2,417.3		(180.2		)	10.1			6,544.7
Loss and LAE reserves	1,365.6		—		—				—		—			—			1,365.6
Liabilities held for sale	—		—		—				—		—			5.1			5.1
Total liabilities	2,564.2		247.6		72.0				98.8		(180.2		)	5.1			2,807.5
Total White Mountains’s common shareholders’ equity	780.6		522.8		—				2,294.9		—			5.0			3,603.3
Non-controlling interest	244.6		16.6		(150.9		)		23.6		—			—			133.9
December 31, 2015:
Total investments	$	2,591.4	$	136.2	$	460.3			$	1,083.2	$	—		$	—		$	4,271.1
Reinsurance recoverable on paid and unpaid losses	193.5		—		—				—		—			—			193.5
Assets held for sale	—		—		—				—		—			4,790.4			4,790.4
Total assets	3,600.8		739.0		(91.1		)	⁽¹⁾	1,378.8		(135.3		)	4,790.4			10,282.6
Loss and LAE reserves	1,389.8		—		—				—		—			—			1,389.8
Liabilities held for sale	—		—		—				—		—			3,047.4			3,047.4
Total liabilities	2,596.3		181.2		48.9				176.1		(135.3		)	3,047.4			5,914.6
Total White Mountains’s common shareholders’ equity	755.2		540.7		—				1,124.3		—			1,493.0			3,913.2
Non-controlling interest	249.3		17.1		(140.0		)		78.4		—			—		⁽²⁾	204.8

Schedule of selected balance sheet data by segment



			HG Global/BAM
Selected Balance Sheet Data Millions	OneBeacon		HG Global		BAM				Other Operations		Eliminations			Held for Sale			Total
December 31, 2016:
Total investments	$	2,620.4	$	161.6	$	468.1			$	2,084.7	$	—		$	—		$	5,334.8
Reinsurance recoverable on paid and unpaid losses	179.5		—		—				—		—			—			179.5
Assets held for sale	—		—		—				—		—			10.1			10.1
Total assets	3,589.4		787.0		(78.9		)	⁽¹⁾	2,417.3		(180.2		)	10.1			6,544.7
Loss and LAE reserves	1,365.6		—		—				—		—			—			1,365.6
Liabilities held for sale	—		—		—				—		—			5.1			5.1
Total liabilities	2,564.2		247.6		72.0				98.8		(180.2		)	5.1			2,807.5
Total White Mountains’s common shareholders’ equity	780.6		522.8		—				2,294.9		—			5.0			3,603.3
Non-controlling interest	244.6		16.6		(150.9		)		23.6		—			—			133.9
December 31, 2015:
Total investments	$	2,591.4	$	136.2	$	460.3			$	1,083.2	$	—		$	—		$	4,271.1
Reinsurance recoverable on paid and unpaid losses	193.5		—		—				—		—			—			193.5
Assets held for sale	—		—		—				—		—			4,790.4			4,790.4
Total assets	3,600.8		739.0		(91.1		)	⁽¹⁾	1,378.8		(135.3		)	4,790.4			10,282.6
Loss and LAE reserves	1,389.8		—		—				—		—			—			1,389.8
Liabilities held for sale	—		—		—				—		—			3,047.4			3,047.4
Total liabilities	2,596.3		181.2		48.9				176.1		(135.3		)	3,047.4			5,914.6
Total White Mountains’s common shareholders’ equity	755.2		540.7		—				1,124.3		—			1,493.0			3,913.2
Non-controlling interest	249.3		17.1		(140.0		)		78.4		—			—		⁽²⁾	204.8


⁽¹⁾	BAM total assets reflect the elimination of $503.0 in the BAM Surplus Notes issued to HG Global and its subsidiaries, and $108.0 and $90.2 in accrued interest related to the BAM Surplus Notes as of December 31, 2016 and 2015.

⁽²⁾ Excludes $250.0 related to SIG Preference Shares. See Note 14 — “Common Shareholders’ Equity and Non-controlling Interests”

Investments in Unconsolidated Affiliates (Tables)

12 Months Ended

Dec. 31, 2016

Investments in and Advances to Affiliates [Line Items]

Schedule of investments in unconsolidated affiliates

Schedule of changes in unconsolidated affiliates

The following table summarizes amounts recorded by White Mountains under the equity method relating to its investment in Symetra through November 5, 2015:



Millions
Equity method carrying value of investment in Symetra as of December 31, 2014 ⁽¹⁾	$	411.4
Equity in earnings ^(2)(3)(4)	25.1
Equity in net unrealized investment losses from Symetra’s fixed maturity portfolio ⁽⁵⁾⁽⁶⁾	(39.2		)
Distribution from Prospector Offshore Fund ⁽⁷⁾	12.4
Dividends received	(16.9		)
Equity method carrying value of investment in Symetra as of November 5, 2015⁽¹⁾⁽⁸⁾	$	392.8


⁽¹⁾	Includes White Mountains’s equity in net unrealized investment (losses) gains from Symetra’s fixed maturity portfolio of $(1.6) and $37.6, as of November 5, 2015 and December 31, 2014, which excludes tax benefit (expense) of $0.2 and $(2.7).


⁽²⁾	Equity in earnings for the period ended November 5, 2015 excludes tax expense of $1.6.


⁽³⁾	Equity in earnings for the period ended November 5, 2015 includes an increase of $2.3 relating to the pre-tax amortization of the Symetra common share basis difference.


⁽⁴⁾	Equity in earnings for the period ended November 5, 2015 includes losses of $(0.1) from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra


⁽⁵⁾	Net unrealized investment losses for the period ended November 5, 2015 includes an increase of $9.4 relating to the pre-tax amortization of the Symetra common share basis difference.


⁽⁶⁾	Net unrealized investment losses from Symetra’s fixed maturity portfolio excludes a tax benefit of $2.9 for the period ended November 5, 2015.


⁽⁷⁾	During 2015, pursuant to the redemption of White Mountains's investments in the Prospector Funds, 513,500 common shares of Symetra were distributed to White Mountains.


⁽⁸⁾	The aggregate value of White Mountains’s investment in common shares of Symetra was $651.2 based upon the quoted market price of $31.67 per share as of November 5, 2015.

Symetra

Investments in and Advances to Affiliates [Line Items]

Summarized statement of financial position of Symetra

The following table summarizes financial information for Symetra as of September 30, 2015:



	September 30,
Millions	2015
Symetra balance sheet data:
Total investments	$	32,409.2
Separate account assets	885.9
Total assets	34,962.8
Policyholder liabilities	29,492.0
Long-term debt	697.5
Separate account liabilities	885.9
Total liabilities	31,836.7
Common shareholders’ equity	3,126.1

The following table summarizes financial information for Symetra for the nine months ended September 30, 2015 and year ended December 31, 2014:



	Nine months ended September 30,			Year ended December 31,
Millions	2015			2014
Symetra income statement data:
Net premiums earned	$	539.3		$	629.1
Net investment income	994.3			1,320.5
Total revenues	1,605.9			2,182.4
Policy benefits	1,143.7			1,399.7
Total expenses	1,543.6			1,882.5
Net income	89.6			254.4
Comprehensive net (loss) income	(234.1		)	397.0

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Summary of the fair value and carrying value of financial instruments

The following table summarizes the fair values and book values as of December 31, 2016 and 2015:



	December 31, 2016				December 31, 2015
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
OBH Senior Notes	$	274.2	$	273.2	$	276.4	$	272.9
MediaAlpha Bank Facility	13.0		12.7		15.0		14.7

Discontinued Operations (Tables)

12 Months Ended

Dec. 31, 2016

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Summary of the fair value and carrying value of financial instruments

The following table summarizes the fair values and book values as of December 31, 2016 and 2015:



	December 31, 2016				December 31, 2015
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
OBH Senior Notes	$	274.2	$	273.2	$	276.4	$	272.9
MediaAlpha Bank Facility	13.0		12.7		15.0		14.7

Schedule of discontinued operations balance sheet, income statement, and cash flows

Net Assets Held for Sale



	December 31,
Millions	2016			2015
Assets held for sale
Fixed maturity investments, at fair value	$	6.6		$	2,383.5
Short-term investments, at amortized cost (which approximates fair value)	.2			352.1
Common equity securities, at fair value	—			174.4
Other long-term investments	—			72.2
Total Investments	6.8			2,982.2
Cash	.9			150.2
Reinsurance recoverable on unpaid losses	.3			283.6
Reinsurance recoverable on paid losses	—			10.2
Insurance and reinsurance premiums receivable	1.5			326.6
Deferred acquisition costs	—			74.6
Deferred tax asset	—			303.1
Ceded unearned insurance and reinsurance premiums	—			87.7
Accounts receivable on unsettled investment sales	—			29.0
Goodwill and other intangible assets	—			330.5
Other assets	.6			212.7
Total assets held for sale	$	10.1		$	4,790.4
Liabilities held for sale
Loss and loss adjustment expense reserves	$	5.0		$	1,650.4
Unearned insurance and reinsurance premiums	1.2			344.3
Debt	—			506.4
Deferred tax liability	—			270.6
Accrued incentive compensation	—			64.2
Funds held under reinsurance treaties	—			52.9
Other liabilities	(1.1		)	158.6
Total liabilities held for sale	5.1			3,047.4
Net assets held for sale	$	5.0		$	1,743.0

Net Income (Loss) from Discontinued Operations



	Year Ended December 31, 2016
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	240.1		$	—		$	240.1
Net investment income	14.4			—			14.4
Net realized and unrealized investment losses	(1.5		)	—			(1.5		)
Other revenue	.6			119.6			120.2
Total revenues	253.6			119.6			373.2
Expenses
Loss and loss adjustment expenses	154.9			—			154.9
Insurance and reinsurance acquisition expenses	59.0			—			59.0
Other underwriting expenses	30.9			—			30.9
Interest expense on debt	7.9			3.2			11.1
General and administrative expenses	8.3			118.8			127.1
Total expenses	261.0			122.0			383.0
Pre-tax loss	(7.4		)	(2.4		)	(9.8		)
Income tax benefit	3.1			6.4			9.5
Net (loss) income from discontinued operations	(4.3		)	4.0			(.3		)
Gain from sale of Sirius Group, net of tax	363.2			—			363.2
Gain from sale of Tranzact, net of tax	—			51.9			51.9
Total income from discontinued operations	358.9			55.9			414.8
Change in foreign currency translation and other from discontinued operations, net of tax	32.0			—			32.0
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	113.3			—			113.3
Comprehensive income from discontinued operations	$	504.2		$	55.9		$	560.1



	Year Ended December 31, 2015
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	847.0		$	—		$	847.0
Net investment income	40.7			—			40.7
Net realized and unrealized investment gains	15.1			—			15.1
Other (loss) revenue	(20.6		)	186.2			165.6
Total revenues	882.2			186.2			1,068.4
Expenses
Loss and loss adjustment expenses	422.7			—			422.7
Insurance and reinsurance acquisition expenses	189.8			—			189.8
Other underwriting expenses	107.9			—			107.9
Interest expense on debt	26.6			4.0			30.6
General and administrative expenses	27.0			185.3			212.3
Total expenses	774.0			189.3			963.3
Pre-tax income (loss)	108.2			(3.1		)	105.1
Income tax (expense) benefit	(27.1		)	.7			(26.4		)
Net income (loss) from discontinued operations	81.1			(2.4		)	78.7
Gain from sale of OneBeacon, net of tax	—			.3			.3
Gain from sale of Esurance and AFI, net of tax	—			17.9			17.9
Total income from discontinued operations	81.1			15.8			96.9
Change in foreign currency translation and other from discontinued operations, net of tax	(65.0		)	—			(65.0		)
Comprehensive income from discontinued operations	$	16.1		$	15.8		$	31.9



	Year Ended December 31, 2014
Millions	Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	873.9		$	.1		$	874.0
Net investment income	40.9			—			40.9
Net realized and unrealized investment gains	205.4			—			205.4
Other (loss) revenue	(62.4		)	43.0			(19.4		)
Total revenues	1,057.8			43.1			1,100.9
Expenses
Loss and loss adjustment expenses	345.3			(.7		)	344.6
Insurance and reinsurance acquisition expenses	193.6			.1			193.7
Other underwriting expenses	129.7			3.5			133.2
Interest expense on debt	26.3			1.4			27.7
General and administrative expenses	30.5			41.2			71.7
Total expenses	725.4			45.5			770.9
Pre-tax income (loss)	332.4			(2.4		)	330.0
Income tax (expense) benefit	(70.4		)	1.0			(69.4		)
Net income (loss) from discontinued operations	262.0			(1.4		)	260.6
Gain from sale of FFIC, net of tax	—			14.0			14.0
Loss from sale of OneBeacon, net of tax	—			(18.8		)	(18.8		)
Gain from sale of Esurance and AFI, net of tax	—			3.2			3.2
Total income (loss) from discontinued operations	262.0			(3.0		)	259.0
Change in foreign currency translation and other from discontinued operations, net of tax	(169.5		)	—			(169.5		)
Comprehensive income (loss) from discontinued operations	$	92.5		$	(3.0	)	$	89.5

Net Change in Cash from Discontinued Operations

The following summarizes the net change in cash associated with the businesses classified as discontinued operations:



	Year ended
	December 31,
Millions	2016			2015			2014
Net cash (used for) provided from operations	$	(24.7	)	$	56.0		$	48.9
Net cash provided from investing activities	221.7			(5.4		)	51.2
Net cash used for financing activities	(.2		)	(11.6		)	(64.5		)
Effect of exchange rate changes on cash	—			(4.5		)	(14.3		)
Net change in cash during the period	196.8			34.5			21.3
Cash balances at beginning of period	150.2			116.8			93.2
Net change in cash held for sale	(.3		)	(1.1		)	2.3
Cash sold as part of sale of consolidated subsidiaries	345.8			—			—
Cash balances at end of period	$	.9		$	150.2		$	116.8

Discontinued operations, computation of earnings per share

The following table outlines the computation of earnings per share for discontinued operations for the years ended December 31, 2016, 2015 and 2014:



	Year Ended December 31,
	2016			2015			2014
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	414.8		$	96.9		$	259.0
Allocation of income for participating unvested restricted common shares ⁽¹⁾	(5.4		)	(1.1		)	(3.3		)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts ⁽²⁾	$	409.4		$	95.8		$	255.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	5,014.9			5,879.2			6,104.9
Average unvested restricted common shares ⁽³⁾	(64.8		)	(68.0		)	(78.9		)
Basic earnings per share denominator	4,950.1			5,811.2			6,026.0
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	5,018.1			5,879.2			6,104.9
Average unvested restricted common shares⁽³⁾	(64.8		)	(68.0		)	(78.9		)
Diluted earnings per share denominator⁽⁴⁾	4,953.3			5,811.2			6,026.0
Basic earnings per share (in dollars):	$	82.71		$	16.48		$	42.43
Diluted earnings per share (in dollars):	$	82.66		$	16.48		$	42.43


⁽¹⁾	Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.


⁽²⁾	Net income (loss) attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2016, 2015 and 2014.


⁽³⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Incentive Compensation Plans”.


⁽⁴⁾	The diluted earnings (loss) per share denominator for the years ended December 31, 2016, includes the impact of 40,000 common shares issuable upon exercise of the non-qualified options outstanding, which results in 3,217 incremental shares outstanding over the period.

Sirius Group

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Summary of the fair value and carrying value of financial instruments



	December 31, 2015
Millions	Fair Value		Carrying Value
SIG Senior Notes	$	410.0	$	399.8
SIG Preference Shares	255.0		250.0
Tranzact Bank Facility	102.8		102.9

- Definition

Tabular disclosure of information related to a disposal group. Includes, but is not limited to, a discontinued operation, disposal classified as held-for-sale or disposed of by means other than sale or disposal of an individually significant component.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721665-107760

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1474-107760

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721683-107760

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64862545&loc=SL51724579-110230

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5C
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721663-107760

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721659-107760

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721671-107760

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5D
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721677-107760

+ Details

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information



	2016 Three Months Ended											2015 Three Months Ended
Millions, except per share amounts	Dec. 31			Sept. 30		June 30			Mar. 31			Dec. 31			Sept. 30			June 30			Mar. 31
Revenues	$	268.2		$	365.0	$	358.2		$	369.3		$	602.6		$	296.1		$	368.7		$	354.3
Expenses	335.0			345.0		360.3			360.8			356.3			383.0			373.8			351.3
Pre-tax (loss) income	(66.8		)	20.0		(2.1		)	8.5			246.3			(86.9		)	(5.1		)	3.0
Tax benefit (expense)	23.0			6.7		6.0			9.7			1.0			1.6			2.2			(4.6		)
(Loss) income from continuing operations	(43.8		)	26.7		3.9			18.2			247.3			(85.3		)	(2.9		)	(1.6		)
(Loss) income from discontinued operations, net of tax	(6.8		)	61.9		358.6			1.1			19.6			6.5			(.5		)	71.3
Non-controlling interest in consolidated subsidiaries	17.3			3.1		(21.4		)	(6.3		)	(6.1		)	16.0			.9			7.3
Equity in earnings of unconsolidated affiliates	—			—		—			—			7.1			3.9			6.8			7.3
(Loss) income attributable to White Mountains’s common shareholders	$	(33.3	)	$	91.7	$	341.1		$	13.0		$	267.9		$	(58.9	)	$	4.3		$	84.3
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(5.81	)	$	6.12	$	(3.44	)	$	2.51		$	43.82		$	(11.10	)	$	0.80		$	1.76
Discontinued operations	(1.49		)	12.72		70.37			(.17		)	3.46			1.09			(.08		)	12.33
Total consolidated operations	$	(7.30	)	$	18.84	$	66.93		$	2.34		$	47.28		$	(10.01	)	$	0.72		$	14.09
Diluted
Continuing operations	$	(5.81	)	$	6.11	$	(3.43	)	$	2.51		$	43.79		$	(11.10	)	$	0.80		$	1.76
Discontinued operations	(1.49		)	12.69		70.22			(.17		)	3.46			1.09			(.08		)	12.33
Total consolidated operations	$	(7.30	)	$	18.80	$	66.79		$	2.34		$	47.25		$	(10.01	)	$	0.72		$	14.09

Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2012
Basis of Presentation
Debt issuance costs	$ 1.6
Assets held for sale	$ 10.1	$ 4,790.4
Percentage of par value of policy reinsured	15.00%
OneBeacon Ltd.
Basis of Presentation
Ownership interest (as a percent)	76.10%	75.50%
HG Global
Basis of Presentation
Surplus notes	$ 300.0			$ 503.0
Preferred stocks \| HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%
Common Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%
Wholly Owned Properties [Member] \| OneBeacon Ltd.
Basis of Presentation
Assets held for sale			$ 58.0

Summary of Significant Accounting Policies (Investment Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	94.00%	91.00%
Minimum percentage of variation expected	5.00%
Minimum variation expected \| $	$ 1.0
Securities outside variance threshold \| security	9

Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Workers compensation discount percent	2.50%	2.50%
Workers compensation discount amount	$ 1.6	$ 1.1

Summary of Significant Accounting Policies Summary of Significant Account Policies (Debt Issuance Costs) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Debt issuance costs	$ 1.6
OneBeacon
Debt Instrument [Line Items]
Debt issuance costs		$ 1.9

Summary of Significant Accounting Policies (Deferred Softward Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred software costs	$ 16.4	$ 15.8
Capitalized Computer Software, Amortization	$ 6.3	$ 4.4	$ 2.9
Minimum \| Software Development [Member]
Useful lives	18 months
Maximum \| Software Development [Member]
Useful lives	36 months
Maximum \| Software and Software Development Costs [Member]
Useful lives	5 years

Summary of Significant Accounting Policies (Foreign Currency Exchange) (Details) $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Significant Transactions
Net unrealized foreign currency translation losses	$ (1.4)	$ (145.6)
Israel, New Shekels
Significant Transactions
Foreign currency exchange rate	3.8476	3.9051	3.8865
SEK
Significant Transactions
Foreign currency exchange rate		8.4247	7.7737
GBP
Significant Transactions
Foreign currency exchange rate	0.8074	0.6757	0.6426
Euro Member Countries, Euro
Significant Transactions
Foreign currency exchange rate	0.9479	0.9189	0.8245

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Item Effected [Line Items]
Net investment income									$ 86.8	$ 60.8	$ 59.5
Income Tax Expense (Benefit)	$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)	$ 45.4	$ 0.2	14.8
OneBeacon Ltd.
Item Effected [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax											0.9	$ 0.4
Accounting Standards Update 2014-01 [Member] \| Restatement Adjustment [Member] \| OneBeacon Ltd.
Item Effected [Line Items]
Net investment income											1.7	1.9
Income Tax Expense (Benefit)											$ (2.3)	$ (2.2)

Significant Transactions Significant Transactions (Acquisitions) (Details) ₪ in Millions, $ in Millions		1 Months Ended	2 Months Ended				3 Months Ended									12 Months Ended
	Aug. 04, 2016 USD ($)	Jan. 31, 2014 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 ILS (₪)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 ILS (₪)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 14, 2014 USD ($)
Unusual or Infrequent Item [Line Items]
Assets							$ 6,544.7				$ 10,282.6						$ 6,544.7		$ 10,282.6
Liabilities							2,807.5				5,914.6						2,807.5		5,914.6
White Mountains’s common shareholders’ equity, including portion attributable to NCI							3,737.2				4,368.0						3,737.2		4,368.0
Other assets							274.6				273.2						274.6		273.2
Goodwill							31.7				24.1						31.7		24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)							24.2				31.3						24.2		31.3	38.7
Payments to Acquire Businesses and Interest in Affiliates																	13.4		2.6	205.0
Payments to Acquire Additional Interest in Subsidiaries																	0.0		9.1	0.0
Other revenue																	149.6		147.3	88.1
Non-controlling interest in consolidated subsidiaries							(17.3)	$ (3.1)	$ 21.4	$ 6.3	6.1	$ (16.0)	$ (0.9)	$ (7.3)			7.3		(18.1)	(22.2)
Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Assets	$ 11.5
Liabilities	$ 0.1
Ownership interest (as a percent)	70.90%
Goodwill	$ 7.6						7.6				0.0						7.6		0.0
Intangible Assets, Net (Excluding Goodwill)	1.1						1.0				0.0						1.0		0.0
Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Liabilities				$ 1.2
Ownership interest (as a percent)				56.20%
Goodwill				$ 0.3
MediaAlpha
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates															$ 28.1
Assets																					$ 70.1
Liabilities																					$ 10.0
Ownership interest (as a percent)																					60.00%
Goodwill							18.3				18.3						18.3		18.3		$ 18.3
Intangible Assets, Net (Excluding Goodwill)							18.3				$ 24.4						18.3		$ 24.4		$ 38.5
Percent of gross profit																		62.50%	62.50%
Additional cash that will be paid as part of the purchase price							$ 7.8										$ 7.8
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Liabilities						$ 0.7
Ownership interest (as a percent)							95.00%				96.10%						95.00%		96.10%
Goodwill						$ 5.5	$ 5.8				$ 5.8						$ 5.8		$ 5.8
Intangible Assets, Net (Excluding Goodwill)							$ 3.7				4.5						3.7		4.5
Star & Shield LLC [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		$ 1.8
Star & Shield Insurance Exchange [Member]
Unusual or Infrequent Item [Line Items]
Purchase of Surplus Notes											$ 4.0								4.0	$ 17.0
Other revenue																	21.0
LowFares.com [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates										$ 3.9
Common Stock \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)						54.00%
GBP \| Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	6.1
Israel, New Shekels \| Preferred stocks \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪					₪ 31.5													₪ 79.6
Israel, New Shekels \| Common Stock \| Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪			₪ 9.5															26.4
Israel, New Shekels \| Common Stock \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪					₪ 14.4													₪ 11.8
United States of America, Dollars \| Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 8.1
United States of America, Dollars \| Preferred stocks \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						$ 9.0													20.7
United States of America, Dollars \| Common Stock \| Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				2.4															6.5
United States of America, Dollars \| Common Stock \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						4.1													$ 3.1
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Assets				5.5		13.4
Intangible Assets, Net (Excluding Goodwill)				$ 2.8		$ 2.9
Common Stock \| Israel, New Shekels \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Additional Interest in Subsidiaries \| ₪																₪ 35.9
Common Stock \| United States of America, Dollars \| WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Additional Interest in Subsidiaries																	$ 9.6

Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended						6 Months Ended	12 Months Ended
	Apr. 18, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2014	Mar. 31, 2013	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 01, 2016	Dec. 23, 2014
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax									$ 415.1	$ 18.2	$ (1.6)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax										0.0	0.0
Total investments		$ 5,334.8							5,334.8	4,271.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold									2,646.2	24.0	0.0
Payments to Acquire Other Investments									39.5	78.0	114.7
Repayments of debt, principal									404.6	76.1	65.2
Sales of common equity securities									$ 1,200.8	461.4	$ 609.8
OneBeacon
Significant Transactions
Percentage of quota share reinsurance agreement									100.00%
Symetra
Significant Transactions
Sales of common equity securities					$ 658.0
Dividends Payable, Amount Per Share												$ 32.00
Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax				$ 363.2					$ 363.2
Total investments										686.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600.0			2,600.0
Payments to Acquire Other Investments	$ 161.8			$ 161.8
Sale of Sirius Group [Member]
Significant Transactions
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax								$ 113.3	113.3
Tranzact
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax			$ 51.9						$ 51.9
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	221.3
Repayments of debt, principal			56.3
Discontinued Operation, Tax Effect of Discontinued Operation			30.2
Increase of Book Value			$ 82.1
OneBeacon Runoff
Significant Transactions
Surplus notes													$ 101.0
Gain (loss) from sale of other discontinued operations, net of tax						$ 36.1
Essentia \| OneBeacon
Significant Transactions
Gain (Loss) on Disposition of Subsidiary							$ 23.0
Gain (Loss) on Disposition of Subsidiary, After Tax							$ 15.0
Gain (Loss) from Adjustments to Prior Period Extraordinary Items										(3.7)
Symetra
Significant Transactions
Sales of common equity securities					$ 658.0
Symetra \| Sirius Group
Significant Transactions
Total investments										528.6
Fair Value [Member] \| Other long-term investments \| OneBeacon Runoff
Significant Transactions
Surplus notes													$ 64.9
Crop Business
Significant Transactions
Proceeds related from termination of agreement										$ 3.0
Percentage of quota share reinsurance agreement										100.00%

Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss and Loss Adjustment Expense Reserve Activity
Workers compensation discount percent				2.50%	2.50%
Prior Accident Year				$ 14.8	$ (4.0)	$ 92.0
Increase (decrease) in reserves				664.0	708.9	824.0
Current Accident Year				649.2	712.9	732.0
Workers compensation discount amount				1.6	1.1
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	$ 75.5	$ 75.5	$ 14.3	15.4	(1.8)	89.8
Increase (decrease) in reserves		109.2	109.2			89.8
Current Accident Year		33.7	$ 33.7
Star & Shield Insurance Exchange [Member]
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				(0.6)	$ (2.2)	2.2
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Workers compensation discount amount				$ 1.6
Professional Insurance (1) \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		46.4
Increase (decrease) in reserves		69.3				59.1
Current Accident Year		22.9
Crop Business \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		0.0
Increase (decrease) in reserves		3.8				0.0
Current Accident Year		3.8
Specialty Products \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		51.7
Increase (decrease) in reserves		80.1				$ 61.8
Current Accident Year		28.4
Management's estimate [Member] \| Specialty Products \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Current Accident Year		$ 29.9
Minimum \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Years that Claims Close				5 years
Maximum \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Years that Claims Close				7 years
Healthcare Sector [Member] \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 40.7
Technology Sector [Member] \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				9.3
Programs [Member] \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				13.3
Accident \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				16.1
Entertainment \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				9.4
Financial Services Sector \| OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 8.2

Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 1,389.8	$ 1,342.2	$ 1,054.3
Less beginning reinsurance recoverable on unpaid losses	(186.0)	(161.6)	(80.2)
Net loss and LAE reserves	1,203.8	1,180.6	974.1
Current Accident Year	649.2	712.9	732.0
Prior Accident Year	14.8	(4.0)	92.0
Total incurred losses and LAE	664.0	708.9	824.0
Loss and LAE paid relating to current year losses	(192.8)	(208.8)	(202.6)
Loss and LAE paid relating to prior year losses	(483.1)	(479.1)	(420.8)
Total loss and LAE payments	(675.9)	(687.9)	(623.4)
Net loss and LAE reserves	1,192.7	1,203.8	1,180.6
Plus ending reinsurance recoverable on unpaid losses	172.9	186.0	161.6
Gross ending balance	1,365.6	1,389.8	1,342.2
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and LAE reserves	5.5	7.7
Net loss and LAE reserves	$ 4.7	$ 5.5	7.7
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and LAE reserves			$ 13.6

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year				$ 649.2	$ 712.9	$ 732.0
Prior Accident Year				14.8	(4.0)	92.0
Total incurred losses and LAE				664.0	708.9	824.0
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		$ 33.7	$ 33.7
Prior Accident Year	$ 75.5	75.5	14.3	15.4	$ (1.8)	89.8
Total incurred losses and LAE		109.2	$ 109.2			89.8
Professional Insurance (1) \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		22.9
Prior Accident Year		46.4
Total incurred losses and LAE		69.3				59.1
Specialty Property \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		(1.1)
Prior Accident Year		5.7
Total incurred losses and LAE		4.6				1.1
Crop Business \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		3.8
Prior Accident Year		0.0
Total incurred losses and LAE		3.8				0.0
Other \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.8
Prior Accident Year		(0.4)
Total incurred losses and LAE		2.4				1.6
Specialty Products \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		28.4
Prior Accident Year		51.7
Total incurred losses and LAE		80.1				61.8
Entertainment \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.5
Prior Accident Year		11.6
Total incurred losses and LAE		13.1				13.5
Government Risks \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.2
Prior Accident Year		7.1
Total incurred losses and LAE		8.3				8.5
Accident \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		0.0
Prior Accident Year		3.5
Total incurred losses and LAE		3.5				6.0
Other \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.6
Prior Accident Year		1.6
Total incurred losses and LAE		4.2				0.0
Specialty Industries \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		5.3
Prior Accident Year		23.8
Total incurred losses and LAE		$ 29.1				$ 28.0
Healthcare Sector [Member] \| OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Prior Accident Year				$ 40.7

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reconciliation of Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance [Abstract]
Statutory reserves (unaudited)	$ 1,192.7	$ 1,203.8	$ 1,180.6	$ 974.1
Reinsurance recoverable on unpaid losses and LAE	172.9	186.0
GAAP reserves	$ 1,365.6	$ 1,389.8	$ 1,342.2	$ 1,054.3

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Less: Discount on workers compensation reserves	$ (1.6)	$ (1.1)
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7	1,203.8	$ 1,180.6	$ 974.1
Plus: Total Reinsurance recoverables on unpaid losses	172.9	186.0
Total unpaid loss and LAE reserves	1,365.6	$ 1,389.8	$ 1,342.2	$ 1,054.3
OneBeacon
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	1,153.0
Less: Discount on workers compensation reserves	(1.6)
Total unpaid loss and allocated LAE reserves, net of reinsurance	1,151.4
Plus: Unallocated LAE	41.3
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7
Plus: Total Reinsurance recoverables on unpaid losses	$ 172.9
Percentage of quota share reinsurance agreement	100.00%
Total unpaid loss and LAE reserves	$ 1,365.6
OneBeacon \| Investing, Financing and Corporate
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Plus: Total Reinsurance recoverables on unpaid losses	18.3
Total unpaid loss and LAE reserves	18.3
Property \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
Property \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
Property \| OneBeacon \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
Plus: Total Reinsurance recoverables on unpaid losses	40.9
Property \| OneBeacon \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
Plus: Total Reinsurance recoverables on unpaid losses	13.2
Casualty \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
Casualty \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
Casualty \| OneBeacon \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
Plus: Total Reinsurance recoverables on unpaid losses	64.0
Casualty \| OneBeacon \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
Plus: Total Reinsurance recoverables on unpaid losses	19.9
Other \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	38.2
Other \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	47.4
Other \| OneBeacon \| Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	38.2
Plus: Total Reinsurance recoverables on unpaid losses	16.0
Other \| OneBeacon \| Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	47.4
Plus: Total Reinsurance recoverables on unpaid losses	$ 0.6

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Losses and Allocated LAE by Age, Net of Reinsurance) (Details)	Dec. 31, 2016
Specialty Products \| Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	70.10%
2	14.60%
3	5.90%
4	1.10%
5	0.30%
6	0.20%
7	0.40%
8	0.00%
9	0.00%
10	0.00%
Specialty Products \| Casualty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	11.30%
2	24.00%
3	17.80%
4	9.60%
5	4.70%
6	2.20%
7	0.90%
8	0.70%
9	0.20%
10	0.00%
Specialty Products \| Other
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	61.90%
2	29.50%
3	0.60%
4	0.00%
5	0.00%
6	0.00%
7	0.00%
8	0.00%
9	0.00%
10	0.00%
Specialty Industries \| Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	47.40%
2	27.30%
3	9.90%
4	4.10%
5	1.80%
6	0.70%
7	0.20%
8	0.10%
9	0.00%
10	0.00%
Specialty Industries \| Casualty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	11.40%
2	16.20%
3	12.90%
4	9.70%
5	6.80%
6	2.50%
7	0.80%
8	0.10%
9	0.20%
10	0.20%
Specialty Industries \| Other
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
1	30.40%
2	37.00%
3	10.80%
4	3.30%
5	1.10%
6	0.90%
7	0.20%
8	0.00%
9	0.10%
10	0.00%

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Property) (Details) - Property - Specialty Products $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 405.2
2007
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	8.6	$ 8.6	$ 8.6	$ 8.7	$ 7.7	$ 7.6	$ 7.6	$ 9.1	$ 11.1	$ 12.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	250
2008
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 26.8	26.8	26.8	26.8	26.6	25.6	25.4	29.2	$ 31.3
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	3,466
2009
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 51.6	52.5	52.6	52.9	53.3	52.9	52.1	$ 55.2
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	8,768
2010
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 55.8	55.8	55.9	56.7	55.6	49.5	$ 51.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	10,170
2011
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 60.7	60.8	60.6	60.2	59.8	$ 58.7
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims \| reported_claim	11,121
2012
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 92.2	92.2	92.9	86.7	$ 79.9
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims \| reported_claim	12,405
2013
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 27.7	27.6	27.6	$ 36.4
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims \| reported_claim	5,267
2014
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 31.6	32.1	$ 37.5
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims \| reported_claim	1,227
2015
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 25.7	$ 28.3
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims \| reported_claim	2,186
2016
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 24.5
Total IBNR plus expected development on reported claims	$ 5.2
Cumulative number of reported claims \| reported_claim	1,825

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Property) (Details) - Property - Specialty Products - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 375.0
All outstanding liabilities, net of reinsurance	0.0
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	8.6	$ 8.6	$ 8.6	$ 8.7	$ 7.0	$ 7.0	$ 7.0	$ 6.8	$ 3.7	$ 1.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	26.8	26.8	26.8	26.8	26.0	25.5	25.2	23.5	$ 15.5
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	51.6	51.6	51.6	51.9	51.9	51.2	47.3	$ 41.5
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	55.7	55.7	55.7	55.2	53.3	47.9	$ 43.1
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	60.5	60.4	60.1	59.7	59.0	$ 51.8
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	78.8	79.0	78.3	72.5	$ 58.8
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	27.4	27.1	26.9	$ 24.3
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	31.2	28.2	$ 19.8
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	22.1	$ 15.0
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 12.3

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Casualty) (Details) - Casualty - Specialty Products $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 1,976.1
2007
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	1,236
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 56.5	$ 57.1	$ 58.3	$ 61.8	$ 70.1	$ 70.5	$ 90.7	$ 97.3	$ 120.2	$ 138.3
Total IBNR plus expected development on reported claims	$ 2.5
2008
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	2,199
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 96.8	97.3	95.3	95.5	98.3	100.8	118.2	128.6	$ 145.6
Total IBNR plus expected development on reported claims	$ 3.3
2009
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	4,002
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 195.5	195.0	194.9	192.0	193.2	194.9	186.2	$ 169.5
Total IBNR plus expected development on reported claims	$ 4.9
2010
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	6,029
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 240.0	234.9	232.3	225.0	226.2	225.4	$ 211.5
Total IBNR plus expected development on reported claims	$ 7.8
2011
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	6,661
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 214.9	214.8	214.2	205.4	189.8	$ 194.9
Total IBNR plus expected development on reported claims	$ 9.4
2012
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	6,794
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 249.4	248.8	246.1	220.1	$ 213.5
Total IBNR plus expected development on reported claims	$ 14.4
2013
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	5,866
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 247.9	229.6	233.1	$ 205.2
Total IBNR plus expected development on reported claims	$ 20.3
2014
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	6,504
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 258.9	235.9	$ 240.5
Total IBNR plus expected development on reported claims	$ 44.4
2015
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	9,710
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 213.4	$ 206.9
Total IBNR plus expected development on reported claims	$ 78.5
2016
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	9,093
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 202.8
Total IBNR plus expected development on reported claims	$ 142.8

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Casualty) (Details) - Casualty - Specialty Products - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 1,411.7
All outstanding liabilities, net of reinsurance	4.3
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	54.0	$ 54.0	$ 53.6	$ 51.9	$ 50.7	$ 44.4	$ 41.7	$ 32.4	$ 24.8	$ 6.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	91.5	88.1	85.3	83.1	77.6	65.4	50.8	32.3	$ 8.8
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	187.8	178.3	169.9	161.4	147.8	124.0	81.8	$ 26.5
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	220.3	213.6	196.3	180.5	157.8	107.0	$ 32.0
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	192.5	186.2	163.7	131.3	88.9	$ 25.8
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	219.8	193.6	157.7	86.7	$ 25.8
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	194.2	143.1	86.3	$ 26.4
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	156.7	95.2	$ 34.3
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	78.4	$ 19.9
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 16.5

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Casualty) (Details) - Casualty - Specialty Industries $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 918.2
2007
Claims Development [Line Items]
Cumulative number of reported claims	1,770
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 36.9	$ 37.1	$ 36.0	$ 35.1	$ 35.1	$ 34.9	$ 34.0	$ 21.7	$ 24.3	$ 24.9
Total IBNR plus expected development on reported claims	$ 0.3
2008
Claims Development [Line Items]
Cumulative number of reported claims	1,835
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 22.1	22.2	23.7	25.2	25.8	26.4	30.4	26.2	$ 26.3
Total IBNR plus expected development on reported claims	$ 0.5
2009
Claims Development [Line Items]
Cumulative number of reported claims	2,982
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 51.0	51.6	53.6	54.8	49.6	51.5	51.1	$ 51.0
Total IBNR plus expected development on reported claims	$ 1.0
2010
Claims Development [Line Items]
Cumulative number of reported claims	4,791
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 77.2	75.5	73.3	69.4	69.3	72.4	$ 80.8
Total IBNR plus expected development on reported claims	$ 3.5
2011
Claims Development [Line Items]
Cumulative number of reported claims	5,254
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 112.2	111.8	100.5	90.2	87.5	$ 88.1
Total IBNR plus expected development on reported claims	$ 3.8
2012
Claims Development [Line Items]
Cumulative number of reported claims	6,298
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 98.8	101.8	102.0	103.4	$ 118.8
Total IBNR plus expected development on reported claims	$ 7.9
2013
Claims Development [Line Items]
Cumulative number of reported claims	7,107
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 138.2	133.6	131.5	$ 121.9
Total IBNR plus expected development on reported claims	$ 18.0
2014
Claims Development [Line Items]
Cumulative number of reported claims	7,633
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 124.2	127.4	$ 135.6
Total IBNR plus expected development on reported claims	$ 35.7
2015
Claims Development [Line Items]
Cumulative number of reported claims	7,315
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 127.7	$ 146.3
Total IBNR plus expected development on reported claims	$ 61.8
2016
Claims Development [Line Items]
Cumulative number of reported claims	5,701
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 129.9
Total IBNR plus expected development on reported claims	$ 98.4

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Casualty) (Details) - Casualty - Specialty Industries - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 558.3
All outstanding liabilities, net of reinsurance	0.1
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	35.9	$ 34.3	$ 32.9	$ 32.4	$ 29.9	$ 27.9	$ 13.8	$ 7.9	$ 5.5	$ 2.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	20.9	20.8	21.2	19.9	17.3	14.2	11.5	6.9	$ 3.2
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	47.4	46.3	46.3	42.9	34.5	25.0	16.6	$ 5.1
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	68.9	65.3	57.5	51.4	37.3	21.1	$ 8.7
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	96.4	88.9	69.8	53.7	34.2	$ 11.1
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	79.7	67.8	50.6	31.9	$ 12.9
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	96.6	72.7	45.5	$ 18.6
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	61.4	40.2	$ 16.6
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	39.3	$ 13.6
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 11.8

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Other) (Details) - Other - Specialty Products $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 478.0
2007
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,073
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 26.8	$ 26.7	$ 26.8
2008
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,192
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 31.2	31.2	31.2	31.2	31.2	31.2	31.2	31.5	$ 31.7
2009
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,138
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 30.9	30.9	30.9	30.9	30.8	31.3	31.4	$ 33.8
2010
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,182
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 32.5	32.5	32.5	32.6	32.5	34.3	$ 34.7
2011
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,306
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 35.4	35.4	35.4	35.4	35.5	$ 33.2
2012
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,324
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 37.0	36.9	36.9	36.9	$ 36.6
2013
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	5,510
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 39.2	39.2	40.6	$ 39.6
2014
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims	9,971
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 92.9	90.8	$ 96.6
2015
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims	10,704
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 73.2	$ 75.5
2016
Claims Development [Line Items]
Total IBNR plus expected development on reported claims	$ 33.8
Cumulative number of reported claims	4,306
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 77.1

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Other) (Details) - Other - Specialty Products - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 439.9
All outstanding liabilities, net of reinsurance	0.1
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	38.2
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.4	$ 19.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	31.2	31.2	31.2	31.2	31.2	31.2	31.2	31.1	$ 21.3
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	30.9	30.9	30.9	30.9	30.9	30.8	30.8	$ 21.6
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	32.5	32.5	32.5	32.5	32.5	32.4	$ 22.4
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	35.4	35.4	35.4	35.4	35.2	$ 25.2
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	37.0	36.9	36.9	36.8	$ 26.1
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	39.2	39.2	39.0	$ 27.4
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	90.3	88.5	$ 46.2
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	70.6	$ 41.7
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 44.2

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Other) (Details) - Other - Specialty Industries $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 294.8
2007
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	291
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.5	$ 4.6	$ 4.3
Total IBNR plus expected development on reported claims	$ 0.0
2008
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	975
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 12.3	12.1	12.1	11.8	11.9	11.9	12.2	10.8	$ 10.4
Total IBNR plus expected development on reported claims	$ 0.0
2009
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	1,763
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 21.7	21.5	21.4	20.9	19.3	19.0	19.2	$ 19.1
Total IBNR plus expected development on reported claims	$ 0.2
2010
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	2,449
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 28.4	28.4	28.2	27.9	27.1	26.6	$ 25.6
Total IBNR plus expected development on reported claims	$ 0.0
2011
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	3,638
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 38.2	38.2	38.5	37.7	36.0	$ 35.1
Total IBNR plus expected development on reported claims	$ 0.2
2012
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	3,861
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 41.6	41.1	40.1	39.6	$ 42.0
Total IBNR plus expected development on reported claims	$ 0.3
2013
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	4,245
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 42.5	42.2	41.2	$ 41.1
Total IBNR plus expected development on reported claims	$ 0.7
2014
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	3,497
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 40.6	40.4	$ 42.6
Total IBNR plus expected development on reported claims	$ 1.9
2015
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	3,301
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 31.9	$ 46.6
Total IBNR plus expected development on reported claims	$ 4.9
2016
Claims Development [Line Items]
Cumulative number of reported claims \| reported_claim	2,222
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 33.2
Total IBNR plus expected development on reported claims	$ 19.4

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Other) (Details) - Other - Specialty Industries - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 247.1
All outstanding liabilities, net of reinsurance	(0.3)
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	47.4
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	4.4	$ 4.4	$ 4.4	$ 4.3	$ 4.3	$ 4.3	$ 4.3	$ 4.3	$ 4.0	$ 1.2
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	12.2	12.0	12.0	11.7	11.8	11.7	11.1	9.0	$ 3.8
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	21.4	21.4	21.2	19.2	18.3	17.7	14.6	$ 6.3
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	28.1	28.0	27.6	26.9	25.1	22.9	$ 8.3
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	37.3	37.2	36.5	34.6	29.4	$ 13.5
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	39.1	38.3	35.8	30.7	$ 13.2
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	39.6	37.2	29.3	$ 13.0
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	34.8	28.9	$ 12.6
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	22.2	$ 9.8
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 8.0

Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Property) (Details) - Property - Specialty Industries $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 1,279.9
2007
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	94.9	$ 95.1	$ 95.1	$ 95.1	$ 95.3	$ 95.1	$ 94.9	$ 101.3	$ 104.8	$ 110.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	4,828
2008
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 129.7	129.8	130.0	132.7	132.0	129.3	129.6	137.6	$ 147.3
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| reported_claim	6,603
2009
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 118.6	118.7	118.6	118.8	117.9	118.2	118.6	$ 124.5
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims \| reported_claim	6,718
2010
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 131.2	131.4	129.7	130.0	127.2	131.5	$ 131.4
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims \| reported_claim	8,617
2011
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 142.5	143.2	143.5	141.1	141.5	$ 138.5
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims \| reported_claim	9,704
2012
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 130.1	130.4	126.7	123.8	$ 126.9
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims \| reported_claim	9,697
2013
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 143.6	142.9	127.0	$ 125.1
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims \| reported_claim	8,085
2014
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 130.1	123.5	$ 123.9
Total IBNR plus expected development on reported claims	$ 3.9
Cumulative number of reported claims \| reported_claim	8,352
2015
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 138.9	$ 142.3
Total IBNR plus expected development on reported claims	$ 10.2
Cumulative number of reported claims \| reported_claim	8,607
2016
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 120.3
Total IBNR plus expected development on reported claims	$ 32.8
Cumulative number of reported claims \| reported_claim	7,429

Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Property) (Details) - Property - Specialty Industries - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 1,171.4
All outstanding liabilities, net of reinsurance	0.0
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	94.6	$ 94.5	$ 94.5	$ 94.2	$ 93.4	$ 91.7	$ 87.6	$ 82.7	$ 70.9	$ 42.2
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	129.3	129.3	129.2	129.1	129.5	123.4	116.8	101.4	$ 62.5
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	118.4	118.0	117.2	115.5	111.0	105.4	85.9	$ 50.4
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	130.8	129.3	125.9	122.8	118.2	106.2	$ 69.0
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	141.4	139.5	138.0	130.7	117.4	$ 72.5
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	131.9	128.5	116.4	103.1	$ 64.2
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	137.7	126.1	101.5	$ 59.8
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	111.6	94.6	$ 56.5
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	114.4	$ 68.6
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 61.3

Third Party Reinsurance (Written and Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Direct	$ 1,245.2	$ 1,325.7	$ 1,296.3
Assumed	28.0	36.0	65.9
Gross written premiums	1,273.2	1,361.7	1,362.2
Ceded	(127.4)	(189.1)	(123.2)
Net written premiums	1,145.8	1,172.6	1,239.0
Earned premiums:
Direct Premiums amount	1,198.1	1,322.0	1,233.5
Assumed	29.4	45.9	70.9
Gross earned premiums	1,227.5	1,367.9	1,304.4
Ceded	(113.5)	(179.7)	(119.4)
Net earned premiums	1,114.0	1,188.2	1,185.0
Losses and LAE:
Direct	693.8	802.5	802.8
Assumed	21.2	55.7	115.7
Gross losses and LAE	715.0	858.2	918.5
Ceded	(51.0)	(149.3)	(94.5)
Total incurred losses and LAE	664.0	708.9	824.0
OneBeacon
Written premiums:
Direct	1,193.3	1,279.9	1,257.5
Assumed	28.0	36.0	65.9
Gross written premiums	1,221.3	1,315.9	1,323.4
Ceded	(120.6)	(179.3)	(106.5)
Net written premiums	1,100.7	1,136.6	1,216.9
Earned premiums:
Direct Premiums amount	1,177.0	1,298.0	1,209.1
Assumed	29.4	45.9	70.9
Gross earned premiums	1,206.4	1,343.9	1,280.0
Ceded	(105.8)	(167.7)	(102.9)
Net earned premiums	1,100.6	1,176.2	1,177.1
Losses and LAE:
Direct	679.5	783.0	778.7
Assumed	21.2	55.7	115.7
Gross losses and LAE	700.7	838.7	894.4
Ceded	(44.7)	(138.0)	(79.3)
Total incurred losses and LAE	656.0	700.7	815.1
HG/BAM
Written premiums:
Direct	38.6	25.9	16.2
Assumed	0.0	0.0	0.0
Gross written premiums	38.6	25.9	16.2
Ceded	0.0	0.0	0.0
Net written premiums	38.6	25.9	16.2
Earned premiums:
Direct Premiums amount	5.9	3.3	1.8
Assumed	0.0	0.0	0.0
Gross earned premiums	5.9	3.3	1.8
Ceded	0.0	0.0	0.0
Net earned premiums	5.9	3.3	1.8
Losses and LAE:
Direct	0.0	0.0	0.0
Assumed	0.0	0.0	0.0
Gross losses and LAE	0.0	0.0	0.0
Ceded	0.0	0.0	0.0
Total incurred losses and LAE	0.0	0.0	0.0
HG/BAM [Member]
Written premiums:
Ceded	27.2	19.3	12.3
Earned premiums:
Ceded	21.0	16.0	1.4
Other Segments
Written premiums:
Direct	13.3	19.9	22.6
Assumed	0.0	0.0	0.0
Gross written premiums	13.3	19.9	22.6
Ceded	(6.8)	(9.8)	(16.7)
Net written premiums	6.5	10.1	5.9
Earned premiums:
Direct Premiums amount	15.2	20.7	22.6
Assumed	0.0	0.0	0.0
Gross earned premiums	15.2	20.7	22.6
Ceded	(7.7)	(12.0)	(16.5)
Net earned premiums	7.5	8.7	6.1
Losses and LAE:
Direct	14.3	19.5	24.1
Assumed	0.0	0.0	0.0
Gross losses and LAE	14.3	19.5	24.1
Ceded	(6.3)	(11.3)	(15.2)
Total incurred losses and LAE	$ 8.0	8.2	8.9
AMTrust [Member]
Written premiums:
Ceded		33.3
Earned premiums:
Ceded		33.3
Losses and LAE:
Ceded		$ 33.4
Percentage of quota share reinsurance agreement		100.00%
Star & Shield Insurance Exchange [Member]
Written premiums:
Ceded			16.0
Earned premiums:
Ceded			15.7
Losses and LAE:
Ceded			$ 16.9

Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended			60 Months Ended
	Mar. 31, 2017	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2020	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses				$ 6.6		$ 7.5
Reinsurance recoverable on unpaid losses				172.9		$ 186.0	$ 161.6	$ 80.2
OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses				6.6
Reinsurance recoverable on unpaid losses				172.9
Property Per Risk Reinsurance Program \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				100.0
Property Per Risk Reinsurance Program \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Workers compensation \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				40.0
Reinsurance retention policy, excess retention, amount reinsured				20.0
Workers compensation \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				$ 6.0
Property Catastrophe Program \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage				100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation				$ 110.0
Property Catastrophe Program \| Maximum \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				130.0
Property Catastrophe Program \| Minimum \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				20.0
Property Catastrophe Program \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				20.0
Reinsurance retention policy, excess retention, amount reinsured				110.0
Property Catastrophe Program \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				20.0
Specialty Property \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				6.0
Reinsurance retention policy, excess retention, amount reinsured				34.0
Specialty Property \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				6.0
Film Completion Bond Business [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				2.0
Reinsurance retention policy, excess retention, amount reinsured				38.0
Film Completion Bond Business [Member] \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				2.0
Combined Healthcare [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				10.0
Inland and Ocean Marine \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				2.5
Reinsurance retention policy, excess retention, amount reinsured				57.5
Inland and Ocean Marine \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				7.0
Combined Healthcare/ Casualty \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				10.0
Combined Healthcare/ Casualty \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Property Per-Risk, Acts of Terrorism \| Maximum \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				100,000.0
Surety Reinsurance Program [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				5.0
Reinsurance retention policy, excess retention, amount reinsured				45.0
Surety Reinsurance Program [Member] \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				5.0
Other Casualty Business [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				8.0
Other Casualty Business [Member] \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
HMO Provider Excess Reinsurance [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				5.0
HMO Provider Excess Reinsurance [Member] \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				5.0
Healthcare Professional Liability [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				7.0
Healthcare Professional Liability [Member] \| OneBeacon \| Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Casualty 2nd layer \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				11.0
Scenario, Forecast [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act		80.00%	83.00%
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act					1.00%
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act		20.00%	17.00%
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act	1.00%
Scenario, Forecast [Member] \| Property Per-Risk, Acts of Terrorism \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			$ 140.0
Reinsurance Retention Policy, Reinsured Risk, Percentage			83.00%
Scenario, Forecast [Member] \| Property Per-Risk, Acts of Terrorism \| Maximum \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			$ 100,000.0
Department of Treasury [Member] \| Scenario, Forecast [Member] \| Property and Casualty, Commercial Insurance Product Line [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured		$ 200.0	$ 140.0		$ 20.0
Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member] \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 179.5
Reinsurance recoverables, percentage of total				100.00%
Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member] \| AM Best A plus \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 73.9
Reinsurance recoverables, percentage of total				41.00%
Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member] \| AM Best A minus \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 79.7
Reinsurance recoverables, percentage of total				44.00%
Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member] \| AM Best B \| OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 25.9
Reinsurance recoverables, percentage of total				15.00%

Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	$ 172.9	$ 186.0	$ 161.6	$ 80.2
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid and unpaid losses	179.5	$ 193.5
OneBeacon \| Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 179.5
Reinsurance recoverables, percentage of total	100.00%
Minimum \| OneBeacon \| Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	$ 20.0
Maximum \| OneBeacon \| Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	130.0
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	172.9
OneBeacon \| Specialty Property
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	34.0
Reinsurance retention policy, amount retained	6.0
OneBeacon \| Combined Healthcare [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	10.0
OneBeacon \| Surety Reinsurance Program [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	45.0
Reinsurance retention policy, amount retained	5.0
OneBeacon \| Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	110.0
Reinsurance retention policy, amount retained	20.0
OneBeacon \| Property Per Risk Reinsurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	100.0
Reinsurance retention policy, amount retained	3.0
OneBeacon \| HMO Provider Excess Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	5.0
OneBeacon \| Inland and Ocean Marine
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	57.5
Reinsurance retention policy, amount retained	2.5
OneBeacon \| Film Completion Bond Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	38.0
Reinsurance retention policy, amount retained	2.0
OneBeacon \| Casualty Clash/ Workers Compensation Catastrophe [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	34.0
Reinsurance retention policy, amount retained	6.0
OneBeacon \| Workers compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	20.0
Reinsurance retention policy, amount retained	40.0
OneBeacon \| Financial Institutions - Professional Liability Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	10.0
OneBeacon \| Casualty 2nd layer
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	11.0
OneBeacon \| Other Casualty Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	8.0
Reinsurance retention policy, amount retained	3.0
OneBeacon \| Workers compensation per occurrence [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	8.0
Reinsurance retention policy, amount retained	2.0
OneBeacon \| Healthcare Professional Liability [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	7.0
Reinsurance retention policy, amount retained	3.0
OneBeacon \| Maximum \| Specialty Property
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon \| Maximum \| Surety Reinsurance Program [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon \| Maximum \| Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	20.0
OneBeacon \| Maximum \| Property Per Risk Reinsurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
OneBeacon \| Maximum \| HMO Provider Excess Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon \| Maximum \| Inland and Ocean Marine
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	7.0
OneBeacon \| Maximum \| Film Completion Bond Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	2.0
OneBeacon \| Maximum \| Casualty Clash/ Workers Compensation Catastrophe [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon \| Maximum \| Workers compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon \| Maximum \| Financial Institutions - Professional Liability Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon \| Maximum \| Other Casualty Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
OneBeacon \| Maximum \| Workers compensation per occurrence [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	2.0
OneBeacon \| Maximum \| Healthcare Professional Liability [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
One Beacon Insurance Company
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	18.3
AM Best A plus \| OneBeacon \| Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 73.9
Reinsurance recoverables, percentage of total	41.00%
AM Best A minus \| OneBeacon \| Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 79.7
Reinsurance recoverables, percentage of total	44.00%
AM Best B \| OneBeacon \| Reinsurer Concentration Risk [Member] \| Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 25.9
Reinsurance recoverables, percentage of total	15.00%

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment income:
Total investment income	$ 89.9	$ 66.2	$ 72.4
Third-party investment expenses	(3.1)	(5.4)	(12.9)
Net investment income, pre-tax	86.8	60.8	59.5
Fixed maturity investments
Investment income:
Total investment income	77.3	52.6	51.3
Short-term investments
Investment income:
Total investment income	1.1	0.2	0.1
Common Stock
Investment income:
Total investment income	7.5	10.1	16.6
Other long-term investments
Investment income:
Total investment income	$ 4.0	$ 3.3	$ 4.4

Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 280.3	$ 77.3	$ 166.8
Net unrealized investment (losses) gains, pre-tax	(270.0)	148.1	(88.3)
Net realized and unrealized investment gains, pre-tax	10.3	225.4	78.5
Income tax expense attributable to net realized and unrealized investment gains	(8.8)	(35.2)	(18.0)
Net realized and unrealized investment gains, after tax	$ 1.5	$ 190.2	$ 60.5

Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Realized Gain (Loss)	$ 279.9	$ 76.9	$ 166.8
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.4	0.0
Net realized investment gains, pre-tax	280.3	77.3	166.8
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(49.5)	(22.8)	(28.4)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	49.5	(22.8)	(28.4)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	230.4	54.1	138.4
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	0.4	0.4	0.0
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	230.8	54.5	138.4
Fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	(2.1)	1.9	5.8
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.0	0.0
Net realized investment gains, pre-tax	(1.7)	1.9	5.8
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0.4
Foreign Currency Transaction Gain (Loss), Realized	0.0
Net realized investment gains, pre-tax	0.4
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	280.7	64.4	138.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.4	0.0
Net realized investment gains, pre-tax	280.7	64.8	138.0
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0.9	10.6	23.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0
Net realized investment gains, pre-tax	$ 0.9	$ 10.6	$ 23.0

Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ (267.3)	$ 152.9	$ (79.3)
Foreign Currency Transaction Gain (Loss), Unrealized	(2.7)	(4.8)	(9.0)
Net unrealized investment (losses) gains, pre-tax	(270.0)	148.1	(88.3)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	40.7	(12.3)	9.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	(0.1)	0.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(40.7)	(12.4)	10.4
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(226.6)	140.6	(69.4)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(2.7)	(4.9)	(8.5)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(229.3)	135.7	(77.9)
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Foreign Currency Transaction Gain (Loss), Unrealized	2.1	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	(10.0)	(15.6)	11.3
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Foreign Currency Transaction Gain (Loss), Unrealized	(3.3)	(3.7)	(7.7)
Net unrealized investment (losses) gains, pre-tax	(257.9)	203.9	(90.7)
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	(1.1)	(1.3)
Net unrealized investment (losses) gains, pre-tax	(0.9)	$ (40.2)	$ (8.9)
Forward Contracts [Member]
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	(1.2)
Net unrealized investment (losses) gains, pre-tax	$ (1.2)

Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ (267.3)	$ 152.9	$ (79.3)
Realized gains	315.5	112.9	194.0
Realized losses	35.2	35.6	27.2
Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.2	(23.1)	7.7
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Fixed maturity investments \| Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.1	(1.1)	1.9
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Common equity securities \| Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0	(9.0)	5.8
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Other long-term investments \| Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 6.1	$ (13.0)	$ 0.0

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$ 0.0	$ (39.2)	$ 81.2
Income tax benefit (expense)	0.0	2.9	(5.9)
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(36.3)	75.3
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(34.9)	75.3
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	1.5	190.2	60.5
Total investment gains recorded during the period, after-tax	1.5	155.3	135.8
Symetra
Investments in and Advances to Affiliates [Line Items]
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(34.9)	75.3
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	$ 0.0	$ 1.4	$ 0.0

Investment Securities (Investment Holdings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Including Short Term Fixed
Investment securities
Duration of Fixed Maturities	2 years 9 months 18 days
US Government and agency obligations
Investment securities
Trading Securities, Cost	$ 281.7	160.4
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	278.3	160.0
Trading Securities, Unrealized Holding Gain	0.1	0.0
Trading Securities, Unrealized Holding Loss	(3.5)	(0.4)
Debt securities issued by corporations
Investment securities
Trading Securities, Cost	1,512.6	1,001.0
Net foreign currency gains	2.1	0.0
Fixed maturity investments, at fair value	1,509.4	1,000.0
Trading Securities, Unrealized Holding Gain	8.4	4.3
Trading Securities, Unrealized Holding Loss	(13.7)	(5.3)
Municipal obligations
Investment securities
Trading Securities, Cost	308.8	227.8
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	309.0	228.8
Trading Securities, Unrealized Holding Gain	1.9	2.2
Trading Securities, Unrealized Holding Loss	(1.7)	(1.2)
Mortgage and asset-backed securities
Investment securities
Trading Securities, Cost	2,141.7	1,170.6
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	2,132.9	1,167.0
Trading Securities, Unrealized Holding Gain	2.6	2.0
Trading Securities, Unrealized Holding Loss	(11.4)	(5.6)
Foreign government, agency and provincial obligations
Investment securities
Trading Securities, Cost	12.9	1.0
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	13.2	1.2
Trading Securities, Unrealized Holding Gain	0.3	0.2
Trading Securities, Unrealized Holding Loss	0.0	0.0
Preferred stocks
Investment securities
Trading Securities, Cost	8.3	78.3
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	14.0	82.7
Trading Securities, Unrealized Holding Gain	5.7	4.4
Trading Securities, Unrealized Holding Loss	0.0	0.0
Fixed Income Investments [Member]
Investment securities
Trading Securities, Cost	4,266.0	2,639.1
Net foreign currency gains	2.1	0.0
Total fixed maturity investments	4,256.8	2,639.7
Trading Securities, Unrealized Holding Gain	19.0	13.1
Trading Securities, Unrealized Holding Loss	$ (30.3)	(12.5)
Excluding Short Term Investments
Investment securities
Duration of Fixed Maturities	3 years
Star & Shield Insurance Exchange [Member]
Investment securities
Fixed maturity investments reclassified to assets held for sale (1)	$ (6.6)	(9.5)
Fixed maturity investments \| Star & Shield Insurance Exchange [Member]
Investment securities
Fixed maturity investments, at fair value	$ 6.6	$ 9.5	$ 10.1

Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	4,266.0
Trading Securities by Maturity, Fair Value
Trading Securities	4,256.8
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	212.8
Due after one year through five years	1,272.6
Due after five years through ten years	462.8
Due after ten years	167.8
Trading Securities by Maturity, Fair Value
Due in one year or less	213.3
Due after one year through five years	1,270.2
Due after five years through ten years	458.4
Due after ten years	168.0
Mortgage and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	2,132.9	1,167.0
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	2,141.7
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	2,132.9
Preferred stocks
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	14.0	82.7
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	8.3
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	14.0
Fixed maturity investments \| Star & Shield Insurance Exchange [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 6.6	$ 9.5	$ 10.1

Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Sales and maturities of investment securities	5,385.3	2,162.8	$ 3,337.5
Common Stock
Investment securities
Trading Securities, Cost	440.8	822.5
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	474.3	1,113.9
Trading Securities, Unrealized Holding Gain	35.9	302.8
Trading Securities, Unrealized Holding Loss	(2.4)	(11.4)
Other long-term investments
Investment securities
Trading Securities, Cost	314.9	304.5
Net foreign currency gains	(3.9)	(2.3)
Fixed maturity investments, at fair value	323.3	315.8
Trading Securities, Unrealized Holding Gain	40.3	32.0
Trading Securities, Unrealized Holding Loss	$ (28.0)	$ (18.4)

Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Investments held in trusts	$ 165,900,000	$ 143,600,000
Assets held by insurance regulators	$ 93,500,000	$ 88,000,000
Percentage of investments recorded at fair value	94.00%	91.00%
Surety Product Line [Member] \| OneBeacon
Investment securities
Assets Held in Collateral	$ 153,000,000	$ 137,700,000
Private equity funds
Investment securities
Other than Temporary Impairment Losses, Investments	5,000,000	$ 2,400,000.0
Fixed maturity investments \| White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	$ 0

Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Debt securities issued by corporations
Investment securities
Fair value investments	1,509.4	1,000.0
BBB \| Debt securities issued by corporations
Investment securities
Fair value investments	786.5	507.1
A \| Debt securities issued by corporations
Investment securities
Fair value investments	381.9	397.7
AA \| Debt securities issued by corporations
Investment securities
Fair value investments	100.9	95.2
Standard & Poor's, BB Rating [Member] \| Debt securities issued by corporations
Investment securities
Fair value investments	214.0	0.0
Standard & Poor's, B Rating [Member] \| Debt securities issued by corporations
Investment securities
Fair value investments	$ 26.1	$ 0.0

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
GNMA
Investment securities
Fair value investments	283.9	265.5
FNMA
Investment securities
Fair value investments	278.3	42.2
FHLMC
Investment securities
Fair value investments	89.8	22.8
Total Agency
Investment securities
Fair value investments	652.0	330.5
Residential
Investment securities
Debt Instrument, Collateral Amount	30.2
Fair value investments	205.3	133.2
Commercial
Investment securities
Fair value investments	127.5	140.4
Total Non-agency
Investment securities
Fair value investments	332.8	273.6
Total mortgage-backed securities
Investment securities
Fair value investments	984.8	604.1
Credit card receivables
Investment securities
Fair value investments	438.3	217.7
Vehicle receivables
Investment securities
Fair value investments	479.5	269.7
Other
Investment securities
Fair value investments	230.3	75.5
Total other asset-backed securities
Investment securities
Fair value investments	1,148.1	562.9
Total mortgage and asset- backed securities
Investment securities
Fair value investments	2,132.9	1,167.0
Level 2 Inputs
Investment securities
Fair value investments	4,118.5	2,531.4	$ 2,372.9
Level 2 Inputs \| GNMA
Investment securities
Fair value investments	283.9	265.5
Level 2 Inputs \| FNMA
Investment securities
Fair value investments	278.3	42.2
Level 2 Inputs \| FHLMC
Investment securities
Fair value investments	89.8	22.8
Level 2 Inputs \| Total Agency
Investment securities
Fair value investments	652.0	330.5
Level 2 Inputs \| Residential
Investment securities
Fair value investments	205.3	133.2
Level 2 Inputs \| Commercial
Investment securities
Fair value investments	127.5	140.4
Level 2 Inputs \| Total Non-agency
Investment securities
Fair value investments	332.8	273.6
Level 2 Inputs \| Total mortgage-backed securities
Investment securities
Fair value investments	984.8	604.1
Level 2 Inputs \| Credit card receivables
Investment securities
Fair value investments	438.3	217.7
Level 2 Inputs \| Vehicle receivables
Investment securities
Fair value investments	479.5	269.7
Level 2 Inputs \| Other
Investment securities
Fair value investments	230.3	75.5
Level 2 Inputs \| Total other asset-backed securities
Investment securities
Fair value investments	1,148.1	562.9
Level 2 Inputs \| Total mortgage and asset- backed securities
Investment securities
Fair value investments	2,132.9	1,167.0
Level 3 Inputs \| GNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| FNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| FHLMC
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total Agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Residential
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Commercial
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total Non-agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total mortgage-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Credit card receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Vehicle receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Other
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total other asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs \| Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0.0	$ 0.0

Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Residential
Investment securities
Fair value investments	205.3	133.2
Commercial
Investment securities
Fair value investments	127.5	140.4
Total Non-agency
Investment securities
Fair value investments	332.8	$ 273.6
Securities Issued in 2004 \| Residential
Investment securities
Fair value investments	19.4
Securities Issued in 2004 \| Commercial
Investment securities
Fair value investments	0.0
Securities Issued in 2004 \| Total Non-agency
Investment securities
Fair value investments	19.4
2005 \| Residential
Investment securities
Fair value investments	5.7
2005 \| Commercial
Investment securities
Fair value investments	0.0
2005 \| Total Non-agency
Investment securities
Fair value investments	5.7
2006 \| Residential
Investment securities
Fair value investments	3.0
2006 \| Commercial
Investment securities
Fair value investments	0.0
2006 \| Total Non-agency
Investment securities
Fair value investments	3.0
2007 \| Residential
Investment securities
Fair value investments	0.0
2007 \| Commercial
Investment securities
Fair value investments	0.0
2007 \| Total Non-agency
Investment securities
Fair value investments	0.0
2008 \| Residential
Investment securities
Fair value investments	2.7
2008 \| Commercial
Investment securities
Fair value investments	0.0
2008 \| Total Non-agency
Investment securities
Fair value investments	2.7
2009 \| Residential
Investment securities
Fair value investments	0.0
2009 \| Commercial
Investment securities
Fair value investments	0.0
2009 \| Total Non-agency
Investment securities
Fair value investments	0.0
2010 \| Residential
Investment securities
Fair value investments	7.4
2010 \| Commercial
Investment securities
Fair value investments	4.3
2010 \| Total Non-agency
Investment securities
Fair value investments	11.7
2011 \| Residential
Investment securities
Fair value investments	9.9
2011 \| Commercial
Investment securities
Fair value investments	0.0
2011 \| Total Non-agency
Investment securities
Fair value investments	9.9
2012 \| Residential
Investment securities
Fair value investments	5.0
2012 \| Commercial
Investment securities
Fair value investments	18.1
2012 \| Total Non-agency
Investment securities
Fair value investments	23.1
2013 \| Residential
Investment securities
Fair value investments	15.3
2013 \| Commercial
Investment securities
Fair value investments	11.5
2013 \| Total Non-agency
Investment securities
Fair value investments	26.8
2014 \| Residential
Investment securities
Fair value investments	53.3
2014 \| Commercial
Investment securities
Fair value investments	23.4
2014 \| Total Non-agency
Investment securities
Fair value investments	76.7
Securities Issued in 2015 \| Residential
Investment securities
Fair value investments	48.3
Securities Issued in 2015 \| Commercial
Investment securities
Fair value investments	44.4
Securities Issued in 2015 \| Total Non-agency
Investment securities
Fair value investments	92.7
Securities Issued in 2016 \| Residential
Investment securities
Fair value investments	35.3
Securities Issued in 2016 \| Commercial
Investment securities
Fair value investments	25.8
Securities Issued in 2016 \| Total Non-agency
Investment securities
Fair value investments	$ 61.1

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Residential
Investment securities
Fair value investments	205.3	$ 133.2
Residential \| Prime [Member]
Investment securities
Fair value investments	205.3
Residential \| Non-prime
Investment securities
Fair value investments	0.0
Residential \| Subprime [Member]
Investment securities
Fair value investments	0.0
Residential \| Super Senior
Investment securities
Fair value investments	150.6
Residential \| Super Senior \| Prime [Member]
Investment securities
Fair value investments	150.6
Residential \| Super Senior \| Non-prime
Investment securities
Fair value investments	0.0
Residential \| Super Senior \| Subprime [Member]
Investment securities
Fair value investments	0.0
Residential \| Senior
Investment securities
Fair value investments	54.7
Residential \| Senior \| Prime [Member]
Investment securities
Fair value investments	54.7
Residential \| Senior \| Non-prime
Investment securities
Fair value investments	0.0
Residential \| Senior \| Subprime [Member]
Investment securities
Fair value investments	0.0
Residential \| Subordinate
Investment securities
Fair value investments	0.0
Residential \| Subordinate \| Prime [Member]
Investment securities
Fair value investments	0.0
Residential \| Subordinate \| Non-prime
Investment securities
Fair value investments	0.0
Residential \| Subordinate \| Subprime [Member]
Investment securities
Fair value investments	$ 0.0

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Cost	$ 5,308.8
Other Investments	323.3	$ 315.8
Other long-term investments
Investment securities
Other Investments	2.0	3.2
Limited liability companies and private equity securities [Member]
Investment securities
Other Investments	72.0	82.1
Surplus Note
Investment securities
Other Investments	71.9	51.5
Convertible Preferred Stock
Investment securities
Other Investments	30.6	32.7
Affordable Housing Development Fund [Member]
Investment securities
Other Investments	12.3	14.7
Partnership investments [Member]
Investment securities
Other Investments	3.5	3.8
Total hedge and private equity funds included in other long-term investments
Investment securities
Other Investments	$ 131.0	$ 127.8

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Fixed rate CMBS
Investment securities
Average Basis Points of Subordination	25.00
Fair value investments	$ 116.0
Fixed rate CMBS \| Super Senior
Investment securities
Fair value investments	8.7
Fixed rate CMBS \| Senior
Investment securities
Fair value investments	59.1
Fixed rate CMBS \| Subordinate
Investment securities
Fair value investments	48.2
Floating rate CMBS
Investment securities
Fair value investments	11.5
Floating rate CMBS \| Super Senior
Investment securities
Fair value investments	0.0
Floating rate CMBS \| Senior
Investment securities
Fair value investments	0.0
Floating rate CMBS \| Subordinate
Investment securities
Fair value investments	11.5
Commercial
Investment securities
Fair value investments	127.5	$ 140.4
Commercial \| Super Senior
Investment securities
Fair value investments	8.7
Commercial \| Senior
Investment securities
Fair value investments	59.1
Commercial \| Subordinate
Investment securities
Fair value investments	$ 59.7

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2016 USD ($) fund	Dec. 31, 2015 USD ($)
Private equity funds
Investment securities
Number of Investments Held \| fund	23
Trading Securities, Other	$ 71.2
Hedge funds
Investment securities
Number of Investments Held \| fund	5
Trading Securities, Other	$ 59.8
Total hedge and private equity funds included in other long-term investments
Investment securities
Trading Securities, Other	131.0	$ 127.8
Fair value unfunded commitments	138.1	82.2
Other Long-term Investments Largest Single Investment	36.5	22.7
Private equity funds \| Private equity funds
Investment securities
Trading Securities, Other	71.2	90.8
Fair value unfunded commitments	138.1	82.2
Private equity funds \| Multi-sector
Investment securities
Trading Securities, Other	11.4	14.8
Fair value unfunded commitments	2.0	2.1
Private equity funds \| Energy infrastructure & services
Investment securities
Trading Securities, Other	14.1	20.7
Fair value unfunded commitments	3.2	3.4
Private equity funds \| Real estate
Investment securities
Trading Securities, Other	0.3	0.4
Fair value unfunded commitments	0.1	0.1
Private equity funds \| Private equity secondaries
Investment securities
Trading Securities, Other	3.0	4.4
Fair value unfunded commitments	2.1	2.1
Private equity funds \| Direct Lending [Member]
Investment securities
Trading Securities, Other	1.8
Fair value unfunded commitments	35.7
Private equity funds \| Financial Services Sector
Investment securities
Trading Securities, Other	1.0
Fair value unfunded commitments	5.0
Private equity funds \| Healthcare
Investment securities
Trading Securities, Other	3.5	3.8
Fair value unfunded commitments	0.4	0.4
Private equity funds \| Insurance
Investment securities
Trading Securities, Other	0.8	2.0
Fair value unfunded commitments	41.3	41.3
Private equity funds \| Aerospace/Defense/Government
Investment securities
Trading Securities, Other	19.4	19.8
Fair value unfunded commitments	25.9	30.3
Private equity funds \| Industrial
Investment securities
Trading Securities, Other	15.9	24.9
Fair value unfunded commitments	22.4	2.5
Hedge funds \| Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Investment securities
Trading Securities, Other	36.5	12.8
Fair value unfunded commitments	0.0	0.0
Hedge funds \| Hedge funds
Investment securities
Trading Securities, Other	59.8	37.0
Fair value unfunded commitments	0.0	0.0
Hedge funds \| Long/short equity REIT
Investment securities
Trading Securities, Other	19.9	20.6
Fair value unfunded commitments	0.0	0.0
Hedge funds \| Other hedge funds
Investment securities
Trading Securities, Other	3.4	3.6
Fair value unfunded commitments	$ 0.0	$ 0.0

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Dec. 31, 2016 USD ($) fund
Investment securities
Number of Investments Held \| fund	5
Trading Securities, Other	$ 59.8
Distributions from inactive hedge funds	1.0
Outstanding redemptions	2.4
30-59 days notice
Investment securities
Trading Securities, Other	37.5
60-89 days notice
Investment securities
Trading Securities, Other	19.9
90-119 days notice
Investment securities
Trading Securities, Other	2.4
Monthly
Investment securities
Trading Securities, Other	0.0
Monthly \| 30-59 days notice
Investment securities
Trading Securities, Other	0.0
Monthly \| 60-89 days notice
Investment securities
Trading Securities, Other	0.0
Monthly \| 90-119 days notice
Investment securities
Trading Securities, Other	0.0
Quarterly
Investment securities
Trading Securities, Other	15.9
Quarterly \| 30-59 days notice
Investment securities
Trading Securities, Other	15.9
Quarterly \| 60-89 days notice
Investment securities
Trading Securities, Other	0.0
Quarterly \| 90-119 days notice
Investment securities
Trading Securities, Other	0.0
Semi-annual
Investment securities
Trading Securities, Other	41.5
Semi-annual \| 30-59 days notice
Investment securities
Trading Securities, Other	21.6
Semi-annual \| 60-89 days notice
Investment securities
Trading Securities, Other	19.9
Semi-annual \| 90-119 days notice
Investment securities
Trading Securities, Other	$ 0.0
Lock-up period expiring 2018
Investment securities
Number of Investments Held \| fund	1
Trading Securities, Other	$ 21.5
Annual
Investment securities
Trading Securities, Other	2.4
Annual \| 30-59 days notice
Investment securities
Trading Securities, Other	0.0
Annual \| 60-89 days notice
Investment securities
Trading Securities, Other	0.0
Annual \| 90-119 days notice
Investment securities
Trading Securities, Other	$ 2.4

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity funds $ in Millions	Dec. 31, 2016 USD ($)
Investment securities
Trading Securities, Other	$ 71.2
1-3 years
Investment securities
Trading Securities, Other	14.8
3 – 5 years
Investment securities
Trading Securities, Other	3.5
5 – 10 years
Investment securities
Trading Securities, Other	50.0
10 years
Investment securities
Trading Securities, Other	$ 2.9

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Investment securities
Short-term investments, at amortized cost (which approximates fair value)	$ 287.0	$ 211.2
Fixed maturity investments, at fair value	4,250.2	2,630.2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,050.9	3,343.6
Total realized and unrealized gains (losses)	(11.6)	226.8
Amortization/Accretion	(19.3)	(19.6)
Purchases	6,381.7	2,377.1
Sales	(5,385.2)	(2,210.1)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(61.4)
Transfers in	118.0	41.8
Transfers out	(118.0)	(41.8)
Fair value investments	5,039.7	4,050.9	$ 3,343.6
Net realized and unrealized investment gains	10.3	225.4	78.5
Trading Securities, Change in Unrealized Holding Gain (Loss)	(267.3)	152.9	(79.3)
Discontinued Operations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains	(1.5)	15.1	205.4
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,134.4
Fair value investments	5,195.9	4,134.4
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,152.2	550.6
Total realized and unrealized gains (losses)	11.3	263.0
Amortization/Accretion	0.1	0.0
Purchases	2,243.7	814.5
Sales	(2,794.8)	(825.7)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(43.5)
Transfers in	0.0	0.0
Transfers out	0.0	(1.2)
Fair value investments	612.5	1,152.2	550.6
Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,363.5
Fair value investments	887.0	1,363.5
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,531.4	2,372.9
Total realized and unrealized gains (losses)	0.2	(13.7)
Amortization/Accretion	(19.4)	(19.6)
Purchases	3,979.8	1,436.0
Sales	(2,491.5)	(1,286.0)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0.0
Transfers in	118.0	41.8
Transfers out	0.0	0.0
Fair value investments	4,118.5	2,531.4	2,372.9
Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,531.4
Fair value investments	4,131.2	2,531.4
Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.2	(23.1)	7.7
Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	239.5
Fair value investments	177.7	239.5
Prospector
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains		0.8
Symetra
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases		394.5
Symetra \| Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases		394.5
Trading Securities, Change in Unrealized Holding Gain (Loss)		258.8
Star & Shield Insurance Exchange [Member] \| Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	6.6	9.5	10.1
Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Common equity securities \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0	39.5
Total realized and unrealized gains (losses)	0.0	7.8
Amortization/Accretion	0.0	0.0
Sales	0.0	(43.7)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(3.6)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Fair value investments	0.0	0.0	39.5
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0	(9.0)	5.8
Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,639.7
Fair value investments	4,256.8	2,639.7
Fixed maturity investments \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	134.0
Fair value investments	269.4	134.0
Fixed maturity investments \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,435.7
Fair value investments	3,987.4	2,435.7
Fixed maturity investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	70.0	76.4
Total realized and unrealized gains (losses)	0.1	(1.1)
Amortization/Accretion	0.0	0.0
Purchases	120.8	35.3
Sales	(72.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0.0
Transfers in	0.0	0.0
Transfers out	(118.0)	(40.6)
Fair value investments	$ 0.0	$ 70.0	76.4
Number of investments \| Investment	3	6
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 0.1	$ (1.1)	1.9
Fixed maturity investments \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	70.0
Fair value investments	0.0	70.0
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5
Fair value investments	177.7	169.5
Other long-term investments \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Other long-term investments \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Other long-term investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5	125.9
Total realized and unrealized gains (losses)	6.1	(20.0)
Amortization/Accretion	0.0	0.0
Purchases	2.2	76.5
Sales	(0.1)	(12.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Fair value investments	177.7	169.5	125.9
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.1	(13.0)	0.0
Other long-term investments \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5
Fair value investments	177.7	169.5
Carrying value of investment accounted for using the equity method	3.5	3.8	5.2
Other long-term investments \| Affordable Housing Development Fund [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	14.7	16.8
Fair value investments	12.3	14.7	16.8
Short-term Investments [Member] \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	211.3	376.8
Fair value investments	287.1	211.3	376.8
Short-term Investments [Member] \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	211.3
Fair value investments	274.4	211.3
Short-term Investments [Member] \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	12.7	0.0
Short-term Investments [Member] \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Short-term Investments [Member] \| Star & Shield Insurance Exchange [Member] \| Level 1 Inputs \| Short-term Investments [Member]
Investment securities
Short-term investments, at amortized cost (which approximates fair value)	0.1	0.1
Forward Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0
Forward Contracts [Member] \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	(1.2)
Total hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	127.8	178.3
Total realized and unrealized gains (losses)	(6.1)	(9.2)
Amortization/Accretion	0.0	0.0
Purchases	35.2	14.8
Sales	(25.9)	(41.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(14.3)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Fair value investments	$ 131.0	$ 127.8	$ 178.3

Investment Securities (Significant Unobservable Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	$ 118.0	$ 41.8
Standard Poors NR Rating [Member] \| Preferred stocks \| Par Value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 70.0
Standard Poors NR Rating [Member] \| Preferred stocks \| Discount Yield
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		70.0
Standard Poors NR Rating [Member] \| Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Average Share Price Used As Unobservable Input	4 years	4 years
Volatility Rate	50.00%	60.00%
Risk Free Rate	1.00%	1.15%
Standard Poors NR Rating [Member] \| Private equity funds \| Option pricing method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	0.21
Assets, Fair Value Disclosure	$ 9.3	$ 9.6
Standard Poors NR Rating [Member] \| Private equity funds \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	0.25
Assets, Fair Value Disclosure	$ 22.1
Standard Poors NR Rating [Member] \| Convertible Preferred Stock \| Multiple of EBITDA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	6.0	6.0
Assets, Fair Value Disclosure	$ 3.6	$ 5.7
Standard Poors NR Rating [Member] \| Community Reinvestment Vehicle [Member] \| GAAP net equity [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	14.3	14.3
Assets, Fair Value Disclosure	$ 14.3	$ 14.3
Standard Poors NR Rating [Member] \| Seller Priority Surplus Note [Member] \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	0.096	0.13
Assets, Fair Value Disclosure	$ 51.1	$ 38.0
Standard Poors NR Rating [Member] \| Pari Passu Surplus Note [Member] \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	0.15	0.224
Assets, Fair Value Disclosure	$ 20.8	$ 13.5
Level 2
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	0.0	0.0
Level 3 Inputs \| Fixed maturity investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	$ 118.0	$ 40.6
Investment A \| Standard Poors NR Rating [Member] \| Private equity funds \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	1.00	1.00
Assets, Fair Value Disclosure	$ 21.0	$ 21.0
Investment B [Member] \| Standard Poors NR Rating [Member] \| Private equity funds \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	2.52	1.03
Assets, Fair Value Disclosure	$ 3.2	$ 33.8
Investment C [Member] \| Standard Poors NR Rating [Member] \| Private equity funds \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		2.52
Assets, Fair Value Disclosure		$ 3.0
Investment E \| Standard Poors NR Rating [Member] \| Convertible Preferred Stock \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	3.83	3.83
Assets, Fair Value Disclosure	$ 27.0	$ 27.0
Pari Passu Surplus Note [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
basis points	2.50%	2.50%
Seller Priority Surplus Note [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
basis points	2.50%	2.50%

Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
RBC Score	250.00%
Total realized and unrealized gains (losses)	$ 11.6	$ (226.8)
OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(29.1)	(49.5)
Current market rates on repayments \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	5.1	(15.1)
Regulatory Approval [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(15.6)	(24.2)
Liquidity Adjustment [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(18.6)	(10.2)
Fair Value [Member] \| OneBeacon
Surplus notes	71.9	51.5
Par Value [Member] \| OneBeacon
Surplus notes	101.0	$ 101.0
Seller Priority Surplus Note [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(6.8)
Seller Priority Surplus Note [Member] \| Current market rates on repayments \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	6.2
Seller Priority Surplus Note [Member] \| Regulatory Approval [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(0.2)
Seller Priority Surplus Note [Member] \| Liquidity Adjustment [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(12.8)
Seller Priority Surplus Note [Member] \| Fair Value [Member] \| OneBeacon
Surplus notes	51.1
Seller Priority Surplus Note [Member] \| Par Value [Member] \| OneBeacon
Surplus notes	57.9
Pari Passu Surplus Note [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(22.3)
Pari Passu Surplus Note [Member] \| Current market rates on repayments \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(1.1)
Pari Passu Surplus Note [Member] \| Regulatory Approval [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(15.4)
Pari Passu Surplus Note [Member] \| Liquidity Adjustment [Member] \| OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(5.8)
Pari Passu Surplus Note [Member] \| Fair Value [Member] \| OneBeacon
Surplus notes	20.8
Pari Passu Surplus Note [Member] \| Par Value [Member] \| OneBeacon
Surplus notes	$ 43.1

Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 14, 2013
Debt Instrument [Line Items]
Debt	$ 285.9	$ 337.6
Interest expense on debt	16.1	14.6	$ 14.2
Draw down of debt and revolving line of credit	352.5	171.5	133.6
WTM Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility				$ 425.0
Interest expense on debt	1.2	0.1	$ 0.3
Prior W T M Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility				$ 375.0
OneBeacon \| 2012 OBH Senior Notes
Debt Instrument [Line Items]
Debt instrument at face value	275.0	275.0
Unamortized original issue discount	(1.8)	(2.1)
Debt	$ 273.2	$ 272.9
Effective yield (as a percent)	4.70%	4.70%
White Mountains \| WTM Bank Facility
Debt Instrument [Line Items]
Debt	$ 0.0	$ 50.0
Effective yield (as a percent)		3.90%
MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility		$ 20.0
Debt instrument at face value	12.9	15.0
Unamortized Debt Issuance Expense	$ (0.2)	$ (0.3)
Effective yield (as a percent)	5.70%	5.50%
Carrying Value \| OneBeacon \| 2012 OBH Senior Notes
Debt Instrument [Line Items]
Debt	$ 273.2	$ 272.9
Carrying Value \| MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Debt	$ 12.7	$ 14.7

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Aug. 04, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 14, 2014	Feb. 19, 2014
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 58.6		$ 53.6
Amortization of Intangible Assets	34.4		22.3
Intangible Assets, Net (Excluding Goodwill)	24.2		31.3	$ 38.7
Goodwill	31.7		24.1	$ 23.8
Intangible Assets, Gross (Including Goodwill)	90.3		77.7
Goodwill and intangible assets	55.9		55.4
MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	42.4		38.5
Amortization of Intangible Assets	24.1		14.1
Intangible Assets, Net (Excluding Goodwill)	18.3		24.4		$ 38.5
Goodwill	18.3		18.3		$ 18.3
WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	5.7		5.7
Amortization of Intangible Assets	2.0		1.2
Intangible Assets, Net (Excluding Goodwill)	3.7		4.5
Goodwill	5.8		5.8			$ 5.5
Buzzmove [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	1.1		0.0
Amortization of Intangible Assets	0.1		0.0
Intangible Assets, Net (Excluding Goodwill)	1.0	$ 1.1	0.0
Goodwill	7.6	$ 7.6	0.0
Developed Technology Rights [Member] \| MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	32.4		32.0
Amortization of Intangible Assets	17.9		11.2
Intangible Assets, Net (Excluding Goodwill)	$ 14.5		20.8
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Developed Technology Rights [Member] \| WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 3.6		3.6
Amortization of Intangible Assets	1.2		0.7
Intangible Assets, Net (Excluding Goodwill)	$ 2.4		2.9
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Developed Technology Rights [Member] \| Buzzmove [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 0.5		0.0
Amortization of Intangible Assets	0.0		0.0
Intangible Assets, Net (Excluding Goodwill)	$ 0.5		0.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Customer Relationships [Member] \| MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 10.0		6.5
Amortization of Intangible Assets	6.2		2.9
Intangible Assets, Net (Excluding Goodwill)	$ 3.8		3.6
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Trademarks [Member] \| WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 2.1		2.1
Amortization of Intangible Assets	0.8		0.5
Intangible Assets, Net (Excluding Goodwill)	$ 1.3		1.6
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Trademarks [Member] \| Buzzmove [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 0.6		0.0
Amortization of Intangible Assets	0.1		0.0
Intangible Assets, Net (Excluding Goodwill)	$ 0.5		0.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Other Intangible Assets [Member] \| OneBeacon
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 9.4		9.4
Amortization of Intangible Assets	8.2		7.0
Intangible Assets, Net (Excluding Goodwill)	$ 1.2		$ 2.4
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years

Debt and Standby Letter of Credit Facilities (Schedule of Contractual Repayments) (Details) $ in Millions	Dec. 31, 2016 USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 5.0
Due in two to three years	7.9
Due in four to five years	0.0
Due after five years	275.0
Total	$ 287.9

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Beginning balance	$ 24.1	$ 23.8
Acquisitions of businesses	7.6	0.3
Ending balance	31.7	24.1	$ 23.8
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	31.3	38.7
Finite-Lived and Indefinite-Lived Intangibles Acquired	1.1	2.8
Amortization, including foreign currency translation	(12.5)	(10.6)	(8.3)
Ending balance	24.2	31.3	38.7
LowFares.com [Member]
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived and Indefinite-Lived Intangibles Acquired	3.9
Star & Shield LLC [Member]
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	$ 0.4	0.8
Ending balance		$ 0.4	$ 0.8

Debt and Standby Letter of Credit Facilities (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 30, 2012	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 29, 2015	Aug. 14, 2013
Debt Instrument [Line Items]
Debt issuance costs			$ 1.6
Interest expense on debt			16.1	$ 14.6	$ 14.2
Repayments of debt, principal			404.6	76.1	65.2
Draw down of debt and revolving line of credit			352.5	171.5	133.6
Interest Paid			14.8	12.8	13.0
Expenses related to issuance, including underwriting fees			1.6
Debt			285.9	337.6
OneBeacon Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility						$ 65.0
WTM Bank Facility
Debt Instrument [Line Items]
Issuance of debt, net of debt issuance costs			350.0	125.0
Interest expense on debt			1.2	0.1	0.3
Total commitment under revolving credit facility							$ 425.0
Repayments of debt, principal			$ 400.0	$ 75.0
Debt, Weighted Average Interest Rate			3.85%	3.74%
Prior W T M Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility							$ 375.0
One Beacon US Holdings Inc. \| 2012 OBH Senior Notes
Debt Instrument [Line Items]
Debt instrument at face value	$ 275.0
Percentage of par value at which debt was issued	99.90%
Issuance of debt, net of debt issuance costs	$ 272.9
Interest rate (as a percent)	4.60%
Effective yield (as a percent)	4.70%
Interest Expense			$ 13.1	$ 13.0	$ 13.0
One Beacon US Holdings Inc. \| Senior Notes \| 2003 OBH Senior Notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.875%
Debt Instrument, Repurchase Amount		$ 269.8
Aggregate purchase price of senior notes purchased and retired		275.9
Loss on retirement of debt		(6.3)
Write off of deferred issuance costs		$ 0.2
MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.50%
Debt instrument at face value			$ 12.9	$ 15.0
Effective yield (as a percent)			5.70%	5.50%
Total commitment under revolving credit facility				$ 20.0
Term Loan [Member] \| MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility			$ 15.0
Repayments of debt, principal			2.1
Revolving Credit Facility [Member] \| MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility			5.0
Repayments of debt, principal			$ 2.5

Goodwill and Other Intangible Assets (Amortization Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of Intangible Assets	$ 12.5	$ 10.6	$ 8.3
Finite-lived intangible assets, next twelve months	10.9
Finite-lived intangible assets, year two	8.3
Finite-lived intangible assets, year three	2.3
Finite-lived intangible assets, year four	0.7
Finite-lived intangible assets, year five	0.9
Total	$ 23.1

Income Taxes (Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current tax expense (benefit):
U.S. federal									$ 50.7	$ 9.1	$ (4.3)
State									(1.6)	(1.8)	(2.0)
Non-U.S.									(0.8)	(1.5)	(1.4)
Total current tax expense									48.3	5.8	(7.7)
Deferred tax expense (benefit):
U.S. federal									(2.3)	(6.7)	22.8
State									(0.6)	1.1	(0.3)
Total deferred tax (expense) benefit									(2.9)	(5.6)	22.5
Income Tax Expense (Benefit)	$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)	$ 45.4	$ 0.2	$ 14.8

Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)									35.00%
Tax expense (benefit) at the U.S. statutory rate									$ 14.1	$ (55.1)	$ 10.3
Differences in taxes resulting from:
Tax reserve adjustments									13.7	(1.7)	5.2
Change in valuation allowance									9.1	(19.4)	(35.7)
Non-U.S. earnings, net of foreign taxes									6.9	76.7	37.8
Tax rate changes									(3.5)	0.0	0.0
Tax exempt interest and dividends									2.5	2.6	2.5
Other, net									(0.5)	(1.2)	(2.4)
Other, net									3.1	(1.7)	(2.9)
Income Tax Expense (Benefit)	$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)	45.4	0.2	14.8
Pre-tax (loss) income from continuing operations	$ (66.8)	$ 20.0	$ (2.1)	$ 8.5	$ 246.3	$ (86.9)	$ (5.1)	$ 3.0	(40.4)	157.3	(29.4)
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total									(12.8)	(6.5)	2.7
Non-US
Differences in taxes resulting from:
Pre-tax (loss) income from continuing operations									$ 16.3	$ 208.5	$ 60.4

Income Taxes (Deferred Tax Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$ 189.3	$ 191.0
Unearned premiums	37.8	37.5
Non-U.S. net operating loss carryforwards	29.5	33.8
Loss reserve discount	18.1	26.8
Tax credit carryforwards	20.7	16.9
Accrued interest	2.2	1.3
Other items	4.0	8.6
Total gross deferred income tax assets	362.4	383.7
Less: valuation allowances	(146.6)	(165.4)
Total net deferred income tax assets	215.8	218.3
Deferred income tax liabilities related to:
Deferred acquisition costs	33.6	34.9
Net unrealized investment gains	2.9	28.4
Deferred Tax Liabilities, Deferred Expense, Reserves and Accruals	6.4	3.5
Deferred Tax Liabilities, Deferred Expense, Capitalized Software	4.7	4.5
Investment basis difference	4.1	15.2
Other items	0.3	0.0
Total deferred income tax liabilities	89.1	105.5
Net deferred tax asset	126.7	112.8
OneBeacon Runoff
Deferred income tax assets related to:
Runoff Transaction	12.6	12.6
Members Surplus Contributions [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	30.0	19.0
Surplus Note
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	7.1	0.0
Incentive Compensation [Member]
Deferred income tax assets related to:
Compensation	42.5	49.0
Deferred Compensation, Share-based Payments [Member]
Deferred income tax assets related to:
Compensation	5.7	$ 6.2
UNITED STATES
Deferred income tax assets related to:
Tax credit carryforwards	$ 18.0

Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation Allowance [Line Items]
Valuation allowance	$ 146.6				$ 165.4				$ 146.6	$ 165.4
Change in valuation allowance									(9.1)	19.4	$ 35.7
Income Tax Expense (Benefit)	(23.0)	$ (6.7)	$ (6.0)	$ (9.7)	(1.0)	$ (1.6)	$ (2.2)	$ 4.6	(45.4)	(0.2)	$ (14.8)
Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	24.4								24.4
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance	117.3				131.5				117.3	131.5
Net Operating Losses in Israel Subsidiaries [Member]
Valuation Allowance [Line Items]
Valuation allowance	4.6				5.5				4.6	5.5
Net Operating Losses in UK Subsidiaries [Member] [Member]
Valuation Allowance [Line Items]
Valuation allowance	0.3								0.3
Non-US \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance										0.3
Non-US \| Net Operating Losses in Israel Subsidiaries [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									1.1	3.0
UNITED KINGDOM \| UNITED STATES
Valuation Allowance [Line Items]
Change in valuation allowance									0.1
Subsidiaries \| Luxembourg \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					28.3					28.3
Subsidiaries \| NETHERLANDS \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					$ 0.1					0.1
Non-US \| UNITED STATES
Valuation Allowance [Line Items]
Change in valuation allowance									2.8
Guilford Holdings, Inc. \| UNITED STATES
Valuation Allowance [Line Items]
Change in valuation allowance									17.1	9.1
BAM
Valuation Allowance [Line Items]
Change in valuation allowance									3.6	6.3
Income Tax Expense (Benefit)									(11.0)	(8.7)
Houston General Insurance [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									0.1	(0.5)
Star & Shield Insurance Exchange [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									(6.9)	$ 1.2
Continuing Operations [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									(21.4)
Stock Compensation Plan [Member]
Valuation Allowance [Line Items]
Operating Loss Carryforwards	$ 9.9								$ 9.9

Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net deferred tax assets
Total net deferred income tax assets	$ 215.8	$ 218.3
Change in valuation allowance	(9.1)	19.4	$ 35.7
Alternative minimum tax credit carryforwards, not subject to expiration	2.3
Tax credit carryforwards	20.7	16.9
UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	(4.2)
Net deferred tax assets
Foreign tax credit carryforwards, subject to expiration	0.4
Tax credit carryforwards	18.0
Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	675.5
Net deferred tax assets
Gross deferred tax asset	218.8
Valuation allowance	(144.9)
Total net deferred income tax assets	73.9
Operating and Capital Loss Carryforward \| UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	558.7
Net deferred tax assets
Gross deferred tax asset	189.3
Valuation allowance	(115.4)
Total net deferred income tax assets	73.9
Operating and Capital Loss Carryforward \| Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	93.8
Net deferred tax assets
Gross deferred tax asset	24.4
Valuation allowance	(24.4)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward \| Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	19.8
Net deferred tax assets
Gross deferred tax asset	4.6
Valuation allowance	(4.6)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward \| UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	3.2
Net deferred tax assets
Gross deferred tax asset	0.5
Valuation allowance	(0.5)
Total net deferred income tax assets	0.0
No expiration date [Member] \| Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	116.8
No expiration date [Member] \| Operating and Capital Loss Carryforward \| UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
No expiration date [Member] \| Operating and Capital Loss Carryforward \| Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	93.8
No expiration date [Member] \| Operating and Capital Loss Carryforward \| Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	19.8
No expiration date [Member] \| Operating and Capital Loss Carryforward \| UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	3.2
2027-2036 \| Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	556.3
2027-2036 \| Operating and Capital Loss Carryforward \| UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	556.3
2027-2036 \| Operating and Capital Loss Carryforward \| Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2027-2036 \| Operating and Capital Loss Carryforward \| Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2024-2033 [Member] \| Operating and Capital Loss Carryforward \| UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2022-2026 \| Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	1.6
2022-2026 \| Operating and Capital Loss Carryforward \| UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	1.6
2022-2026 \| Operating and Capital Loss Carryforward \| Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2022-2026 \| Operating and Capital Loss Carryforward \| Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2019-2013 [Member] \| Operating and Capital Loss Carryforward \| UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2017-2021 \| Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2017-2021 \| Operating and Capital Loss Carryforward \| UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2017-2021 \| Operating and Capital Loss Carryforward \| Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2017-2021 \| Operating and Capital Loss Carryforward \| Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
2014-2018 [Member] \| Operating and Capital Loss Carryforward \| UNITED KINGDOM
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.0
Star & Shield Insurance Exchange [Member]
Net deferred tax assets
Change in valuation allowance	$ (6.9)	$ 1.2

Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	12 Months Ended
Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Percentage of likelihood for realization of benefit upon ultimate settlement	50.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	$ 24.3	$ 35.0	$ 33.7
Changes in prior year tax positions	(5.4)	(10.7)	(4.9)
Lapse in statute of limitations			(1.1)
Settlements with tax authorities	(13.7)
Tax positions taken during the current year			7.3
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense	5.2
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	(8.2)	1.7	(2.2)
Ending balance	5.2	24.3	35.0
Permanent Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	5.4	5.4	8.4
Changes in prior year tax positions	0.0	0.0	(2.2)
Lapse in statute of limitations			(0.8)
Settlements with tax authorities	(5.4)
Tax positions taken during the current year			0.0
Ending balance	0.0	5.4	5.4
Temporary Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	10.7	23.1	16.6
Changes in prior year tax positions	(5.5)	(12.4)	(0.8)
Lapse in statute of limitations			0.0
Settlements with tax authorities	0.0
Tax positions taken during the current year			7.3
Ending balance	5.2	10.7	23.1
Interest and Penalties Net of Tax Benefit [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		8.2
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance		6.5	8.7
Changes in prior year tax positions	0.1	$ 1.7	(1.9)
Lapse in statute of limitations			(0.3)
Settlements with tax authorities	(8.3)
Tax positions taken during the current year			0.0
Ending balance	$ 0.0		$ 6.5

Income Taxes (Tax Examinations) (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Income Tax Expense (Benefit)	$ (23.0)	$ (6.7)	$ (6.0)	$ (9.7)	$ (1.0)	$ (1.6)	$ (2.2)	$ 4.6		$ (45.4)	$ (0.2)	$ (14.8)
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense	$ 5.2									$ 5.2
Tax Assessment Year 2010 to 2012 [Member]
Income Tax Examination [Line Items]
Income Tax Expense (Benefit)			(3.5)
Tax Assessment Year 2005 to 2006 [Member]
Income Tax Examination [Line Items]
Income Tax Expense (Benefit)									$ (5.0)
Tax Assessment Year 2007 to 2009 [Member]
Income Tax Examination [Line Items]
Income Tax Expense (Benefit)				$ (12.8)
Tax Year 2012 [Member]
Income Tax Examination [Line Items]
Operating Loss Carryforwards			$ 0.6

Derivatives (Details) $ in Millions	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Notional amount	$ 184.6

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pre-tax operating results of WM Life Re
Total revenues	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3	$ 1,360.7	$ 1,621.7	$ 1,411.1
General and administrative expenses									(299.9)	(302.2)	(216.5)
Pre-tax (loss) income from continuing operations	$ (66.8)	$ 20.0	$ (2.1)	$ 8.5	$ 246.3	$ (86.9)	$ (5.1)	$ 3.0	(40.4)	157.3	(29.4)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue									1.2	9.3	18.6
Change in Fair Value of Variable Annuity Liability, included in other revenues									(0.3)	(0.4)	52.9
Change in fair value of derivatives, included in other revenue									(2.0)	(8.8)	(72.4)
Foreign exchange, included in other revenue									1.3	(1.3)	(3.2)
Other investment income and losses									0.0	(0.4)	(1.4)
Total revenues									0.2	(1.6)	(5.5)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses									0.0	0.0	0.6
Death benefit claims paid, included in general and administrative expenses									(0.3)	(0.1)	(0.1)
General and administrative expenses									(2.6)	(4.0)	(4.3)
Pre-tax (loss) income from continuing operations									$ (2.7)	$ (5.7)	(9.3)
Level 3 Inputs \| Projected Surrender Assumption [Member] \| White Mountains Life Re
Pre-tax operating results of WM Life Re
Change in fair value of derivatives, included in other revenue											$ 0.2

Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (2.0)	$ (8.8)	$ (72.4)
Derivative, Fair Value, Net	0.0	20.1	56.4	$ 69.2
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	1.8	6.4	(33.7)
Derivative, Fair Value, Net	0.0	0.5
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(4.8)	(7.3)	(1.3)
Derivative, Fair Value, Net	0.0	14.8
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	1.0	(7.9)	(37.4)
Derivative, Fair Value, Net	$ 0.0	$ 4.8
Level 3 \| Projected Surrender Assumption [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue			$ 0.2

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in derivative instruments
Purchases	$ 6,381.7	$ 2,377.1
Level 2
Changes in derivative instruments
Purchases	3,979.8	1,436.0
Level 1
Changes in derivative instruments
Purchases	2,243.7	814.5
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	20.1	56.4	$ 69.2
Purchases			0.0
Realized and unrealized gains (losses)	2.0	8.8	72.4
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(18.1)	(27.5)	59.6
Ending balance	0.0	20.1	56.4
Collateral Cash and Investments	0.0	5.8	33.2
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	0.0	0.0	0.6
White Mountains Life Re \| Level 3
Changes in variable annuity (liabilities)
Purchases			0.0
White Mountains Life Re \| Level 2
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	16.5	33.8	4.7
Purchases			0.0
Realized and unrealized gains (losses)	0.7	7.5	71.0
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(15.8)	(9.8)	100.1
Ending balance	0.0	16.5	33.8
White Mountains Life Re \| Level 1
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	0.9	3.7	1.1
Purchases			0.0
Realized and unrealized gains (losses)	4.2	(8.4)	(37.2)
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	3.3	(11.2)	(34.6)
Ending balance	0.0	0.9	3.7
Variable Annuity [Member] \| White Mountains Life Re \| Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in variable annuity (liabilities)
Beginning balance	0.3	0.7	52.8
Net realized and unrealized (losses) gains	0.3	0.4	(53.5)
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0
Ending balance	0.0	0.3	0.7
Derivative [Member] \| White Mountains Life Re \| Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	2.7	18.9	63.4
Purchases			0.0
Realized and unrealized gains (losses)	(2.9)	9.7	38.6
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(5.6)	(6.5)	(5.9)
Ending balance	$ 0.0	$ 2.7	$ 18.9

Derivatives (Additional Collateral) (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Credit risk on derivative instruments
Collateral Cash and Investments	$ 0.0	$ 5.8	$ 33.2
Cash
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	0.0	$ 5.8
Fixed maturity investments
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 0.0

Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details) - White Mountains Life Re $ in Millions	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Derivative Asset	$ 23.2
Derivative Liability	(3.1)
Net amounts recognized in Other Assets	20.1
Interest Rate Contract \| Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	2.4
Derivative Liability	(2.1)
Net amounts recognized in Other Assets	0.3
Interest Rate Contract \| Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	0.1
Derivative Liability	(0.1)
Net amounts recognized in Other Assets	0.0
Foreign Exchange Contract \| Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	15.0
Derivative Liability	0.0
Net amounts recognized in Other Assets	15.0
Foreign Exchange Contract \| Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	0.1
Derivative Liability	(0.3)
Net amounts recognized in Other Assets	(0.2)
Equity Contract \| Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	4.4
Derivative Liability	(0.6)
Net amounts recognized in Other Assets	3.8
Equity Contract \| Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	1.2
Derivative Liability	0.0
Net amounts recognized in Other Assets	$ 1.2

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Dec. 31, 2015 USD ($) Rating
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 20,100
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 19,000
S&P Credit Ratings \| Rating	23
Derivative, Collateral, Right to Reclaim Cash	$ 0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	20,100
Excess collateral provided to counterparty in cash	5,800
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	6,900
A \| Citigroup OTC [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	9,900
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	8,500
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	9,900
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	1,400
A \| Bank of America [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	700
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	700
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	700
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	0
A \| Citigroup Exchange Traded [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1,000
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	6,800
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1,000
Excess collateral provided to counterparty in cash	5,800
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	0
Standard & Poor's, A+ Rating [Member] \| J P Morgan Chase [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	8,500
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	3,000
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	8,500
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	$ 5,500

Derivatives (Forward Contracts) (Details) $ in Millions	12 Months Ended
Derivatives (Forward Contracts) (Details) $ in Millions	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Notional amount	$ 184.6
Forward Contracts [Member]
Derivative [Line Items]
Realized and unrealized gains (losses)	1.2
Other Investments [Member]
Derivative [Line Items]
Derivative Liability	(1.2)
GBP
Derivative [Line Items]
Notional amount	150.0
Barclays [Member] \| Standard & Poor's, A- Rating [Member] \| Forward Contracts [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(1.2)
Notional amount	$ 184.6

Municipal Bond Guarantee Insurance (Details) $ in Millions	3 Months Ended								12 Months Ended
Municipal Bond Guarantee Insurance (Details) $ in Millions	Dec. 31, 2016 USD ($) Contract	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Contract	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax (loss) income from continuing operations	$ (66.8)	$ 20.0	$ (2.1)	$ 8.5	$ 246.3	$ (86.9)	$ (5.1)	$ 3.0	$ (40.4)	$ 157.3	$ (29.4)
Net investment income									86.8	$ 60.8	59.5
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Quarter	1.9								1.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months	7.3								7.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	7.0								7.0
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three	6.6								6.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four	6.3								6.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five	5.9								5.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten	49.9								49.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Second Quarter	1.8								1.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Third Quarter	1.8								1.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Fourth Quarter	$ 1.8								$ 1.8
Municipal bond contract \| Contract	4,807				3,103				4,807	3,103
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									10 years 10 months	12 years 10 months
Municipal bond outstanding principal	$ 33,057.3				$ 22,556.0				$ 33,057.3	$ 22,556.0
Municipal bond outstanding interest	16,396.6				11,984.4				16,396.6	11,984.4
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	83.0				50.2				83.0	50.2
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 300.0								$ 300.0			503.0
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 14.5
Preferred stocks \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	96.90%								96.90%
Common Stock \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.40%								88.40%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 503.0								$ 503.0
Pre-tax (loss) income from continuing operations									(48.9)	(47.3)	(40.5)
Net investment income									6.8	4.2	5.7
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	467.2				452.9				467.2	452.9
Collateral Held in Supplement Trust	$ 465.4				$ 441.0				$ 465.4	441.0
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax (loss) income from continuing operations									$ 22.2	17.9	18.3
Net investment income									2.2	1.9	1.4
Surplus Note
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									0.0	0.0	0.0
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									(17.8)	(15.8)	(15.7)
Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									17.8	$ 15.8	$ 15.7
Interest Income [Member] \| Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Accrued Liabilities	$ 108.0								$ 108.0

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to White Mountains’ common shareholders									$ (2.3)	$ 200.7	$ 53.2
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares(1)									0.0	(2.3)	(0.7)
Dividends declared on participating restricted common shares(1)									0.0	(0.1)	(0.1)
Total allocation to restricted common shares									0.0	(2.4)	(0.8)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									(2.3)	198.3	52.4
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									(2.3)	198.3	52.4
Dividends declared net of restricted common share amounts(1)									(5.4)	(5.9)	(6.1)
Total undistributed net earnings, net of restricted common share amounts									$ (7.7)	$ 192.4	$ 46.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,014,900	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Basic earnings per share denominator									4,950,100	5,811,200	6,026,000
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									5,018,100	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Diluted earnings (loss) per share denominator									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$ (5.81)	$ 6.12	$ (3.44)	$ 2.51	$ 43.82	$ (11.10)	$ 0.80	$ 1.76	$ (0.47)	$ 34.12	$ 8.70
Dividends declared and paid									(1.00)	(1.00)	(1.00)
Undistributed (loss) earnings									(1.47)	33.12	7.70
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$ (5.81)	$ 6.11	$ (3.43)	$ 2.51	$ 43.79	$ (11.10)	$ 0.80	$ 1.76	(0.47)	34.12	8.70
Dividends declared and paid									(1.00)	(1.00)	(1.00)
Undistributed (loss) earnings									$ (1.47)	$ 33.12	$ 7.70

Earnings Per Share (Footnotes) (Details) - shares	12 Months Ended
Earnings Per Share (Footnotes) (Details) - shares	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Incremental common shares attributable to share-based payment arrangements	3,217
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Incremental common shares attributable to share-based payment arrangements	40,000	125,000

Retirement and Postretirement Plans (Benefit Obligations) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in projected benefit obligation:
Projected benefit obligation at beginning of year		$ 109.5	$ 119.7
Service cost		0.2	0.9	$ 0.6
Interest cost		1.5	4.6	4.7
Settlement loss (gain) on projected benefit obligation	$ (1.8)	1.2	(1.8)
Assumption changes		(0.9)	(3.5)
Actuarial loss		0.4	0.9
Annuity contract purchases related to termination of Qualified Plan (3)		0.0	0.0	(0.3)
Benefits paid directly by OneBeacon		(2.1)	(2.2)
Projected benefit obligation at end of year	23.8	23.8	109.5	119.7
Change in plan assets:
Fair value of plan assets at beginning of year		139.8	146.0
Actual return on plan assets		6.3	7.5
(Unfunded) over funded status at end of year	(10.8)	(10.8)	30.3
Fair value of plan assets at end of year	13.0	13.0	139.8	146.0
Amounts recognized in the financial statements
Net balance sheet asset recorded in other assets	13.0	13.0	55.8
Net balance sheet liability recorded in other liabilities	(23.8)	(23.8)	(25.5)
Defined Benefit Plan, Amounts Recognized in Balance Sheet	(10.8)	(10.8)	30.3
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)	0.0	$ 0.0
Projected Benefit Obligations [Member]
Amounts recognized in the financial statements
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(1.2)
Benefit and Expenses Paid with Plan Assets [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		17.0	13.7
Contract Termination [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		0.0	4.6
Annuity Contract Purchases [Member]
Change in projected benefit obligation:
Annuity contract purchases related to termination of Qualified Plan (3)		(69.0)	0.0
Defined Benefit Plan, Other Costs		69.0	0.0
Qualified Plan [Member]
Change in plan assets:
Fair value of plan assets at beginning of year		139.4
Fair value of plan assets at end of year	12.8	12.8	139.4
Other Postretirement Benefit Plan, Defined Benefit [Member]
Amounts recognized in the financial statements
Net balance sheet liability recorded in other liabilities	23.8	23.8
Other Postretirement Benefit Plan, Defined Benefit [Member] \| OneBeacon Ltd.
Defined Benefit Plan Disclosure [Line Items]
Amount set aside in irrevocable rabbi trust	20.6	20.6
Transfer of Assets [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		47.1	0.0
Other Expense [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		17.0	$ 13.7
Principal Financial Group [Member]
Amounts recognized in the financial statements
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Group Annuity Contracts	69.0	69.0
Change in plan assets:
Fair value of plan assets at end of year	13.0	13.0
Amounts recognized in the financial statements
Net balance sheet asset recorded in other assets	$ 13.0	13.0
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)
Defined Benefit Plan, Assets Transferred to (from) Plan		47.1
Qualified Plan [Member] \| Other Expense [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		$ (17.0)

Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		$ 3.0	$ 0.0	$ 0.0
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest
Accumulated other comprehensive loss beginning balance	$ (6.5)	(8.0)	(8.0)
Increase (decrease) in accumulated other comprehensive loss:
Amortization of net actuarial gains recognized during the year		1.3	1.3
Net actuarial (losses) gains occurring during the year		3.2	(1.3)
Other Comprehensive Income Defined Benefit Plans Other Adjustments before Tax		(3.0)	0.0
Accumulated other comprehensive loss ending balance		(6.5)	(8.0)	$ (8.0)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets
Projected benefit obligation		23.8	25.5
Accumulated benefit obligation		23.8	25.5
Fair value of plan assets		0.0	0.0
Qualified Plan [Member]
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets
Projected benefit obligation		0.0	84.0
Accumulated benefit obligation		0.0	84.0
Fair value of plan net assets		13.0	$ 139.8
Scenario, Forecast [Member] \| OneBeacon Ltd.
Increase (decrease) in accumulated other comprehensive loss:
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012	$ 0.9
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		$ 3.0

Retirement and Postretirement Plans (Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic benefit cost (income)
Service cost	$ 0.2	$ 0.9	$ 0.6
Interest cost	1.5	4.6	4.7
Expected return on plan assets	(1.0)	(8.7)	(8.5)
Amortization of net actuarial losses	1.3	1.3	0.3
Net periodic pension (cost) income before special termination benefits	2.0	(1.9)	(2.9)
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	(3.0)	0.0	0.0
Special termination benefits expense	0.0	0.0	0.3
Total net periodic benefit income	$ (1.0)	$ (1.9)	$ (2.6)
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount Yield (as a percent)	3.78%	3.22%
Discount rate (as a percent)	3.88%
Pension Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (as a percent)		3.91%
Long-Duration Contracts, Assumptions by Product and Guarantee, Estimated Average Investment Yield	3.00%	6.00%

Retirement and Postretirement Plans (Qualified Plan's Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 13.0	$ 139.8	$ 146.0
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 139.4
Asset allocations (as a percent)	100.00%	100.00%
Qualified Plan [Member] \| Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 139.4
Qualified Plan [Member] \| Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 0.0
Qualified Plan [Member] \| Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Asset allocations (as a percent)	1.50%	5.00%
Qualified Plan [Member] \| Fixed maturity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 132.4
Asset allocations (as a percent)	0.00%	95.00%
Qualified Plan [Member] \| Fixed maturity investments \| Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 132.4
Qualified Plan [Member] \| Fixed maturity investments \| Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Fixed maturity investments \| Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Common equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 0.0
Asset allocations (as a percent)	98.50%	0.00%
Qualified Plan [Member] \| Common equity securities \| Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 0.0
Qualified Plan [Member] \| Common equity securities \| Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Common equity securities \| Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	7.0
Qualified Plan [Member] \| Short-term investments \| Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	7.0
Qualified Plan [Member] \| Short-term investments \| Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] \| Short-term investments \| Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 0.0

Retirement and Postretirement Plans Retirement and Postretirment Plans (Assumptions) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Qualified Plan [Member]
Discount Yield (as a percent)	3.78%	3.22%
Discount rate (as a percent)	3.88%
Pension Costs [Member]
Discount rate (as a percent)		3.91%
Long-Duration Contracts, Assumptions by Product and Guarantee, Estimated Average Investment Yield	3.00%	6.00%

Retirement and Postretirement Plans (Cash Flow) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee savings plan
Defined Benefit Plan, Benefit Obligation	$ 23,800,000	$ 109,500,000	$ 119,700,000
(Unfunded) over funded status at end of year	(10,800,000)	30,300,000
OneBeacon Ltd.
Expected Benefit Payments
Expected future benefit payments, next twelve months	2,100,000
Expected future benefit payments, year two	2,100,000
Expected future benefit payments, year three	2,000,000
Expected future benefit payments, year four	2,000,000
Expected future benefit payments, year five	1,900,000
Expected future benefit payments, five fiscal years thereafter	8,400,000
Employee savings plan
Anticipated contribution to plan		2,100,000
OneBeacon
Employee savings plan
Assets Held-in-trust, Noncurrent	29,300,000	33,900,000
Withdraw from rabbi trust	5,500,000
Disability and Health Benefits [Member] \| OneBeacon
Employee savings plan
Post-employment benefit liability	3,100,000	4,100,000
Death benefits [Member] \| OneBeacon
Employee savings plan
Postemployment Benefits Liability	12,800,000	12,500,000
One Beacon Employee Stock Ownership Plan
Employee savings plan
Defined Contribution Plan, Cost Recognized	$ 5,500,000	5,700,000	4,400,000
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 118,500
Variable contribution provided to participants (as percentage)	6.00%
One Beacon Employee Stock Ownership Plan \| OneBeacon
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Cost Recognized	$ 3,200,000	$ 3,000,000	$ 2,700,000
IRS \| OneBeacon
Employee savings plan
Defined Benefit Plan Compensation LImits	$ 265,000

Employee Share-Based Incentive Compensation Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2014
Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 118,500
Variable contribution provided to participants (as percentage)	6.00%
OneBeacon \| IRS
Share-based compensation arrangement by share-based payment award
Defined Benefit Plan Compensation LImits	$ 265,000
Range of Performance Cycle from 2011 to 2013 [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Vested (in shares)		(3,570)

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) multiplier shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares
Unamortized grant date fair value
Beginning of period	$ 140.7
End of period (3)	$ 140.0	$ 140.7
Performance Shares \| Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Vested (in shares) \| shares			3,570
Performance Shares \| Range Of Performance Cycle From 2013 to 2015 [Member]
Unamortized grant date fair value
Vested (in shares) \| shares	5,000
Performance Shares \| White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range \| multiplier	0
Multiplier for determining payout, high end of range \| multiplier	2
Performance cycle period	3 years
Performance Shares \| W T M Incentive and Phantom Plan [Member] \| Performance Cycle From 2012 to 2014
Unamortized grant date fair value
Performance goal percentage for minimum payout		91.00%
Payout for maximum growth target percentage		145.50%
Performance Shares \| W T M Incentive and Phantom Plan [Member] \| Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout			88.00%
Payout for maximum growth target percentage			131.50%
Performance Shares \| W T M Incentive and Phantom Plan [Member] \| Range Of Performance Cycle From 2013 to 2015 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout	140.00%
Payout for maximum growth target percentage	142.00%
Performance Shares \| W T M Incentive Plan \| Range Of Performance Cycle From 2014-2016 [Member]
Unamortized grant date fair value
Payout for maximum growth target percentage	200.00%
Performance Shares \| W T M Incentive Plan \| White Mountains
Target Performance Share activity
Beginning of period \| shares	93,654	111,257	108,605
Shares paid or expired(1) \| shares	(36,294)	(42,959)	(33,730)
New grants \| shares	22,615	29,195	39,590
Assumed forfeitures and cancellations(2) \| shares	378	(3,839)	(3,208)
End of period (3) \| shares	80,353	93,654	111,257
Unamortized grant date fair value
Beginning of period	$ 57.7	$ 44.4	$ 54.9
Shares paid or expired(1)	(41.0)	(30.8)	(24.5)
Assumed forfeitures and cancellations(2)	0.5	(0.3)	(0.1)
Expense recognized	25.2	44.4	14.1
End of period (3)	42.4	$ 57.7	$ 44.4
Unamortized grant date fair value	$ 23.1

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 140.0	$ 140.7
Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	9.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 \| Performance Shares
Share-based compensation arrangement by share-based payment award
Vested (in shares)	(5,000)
Performance Cycle From 2013 to 2015 \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	140.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	142.00%
Performance Cycle From 2012 to 2014 \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout		91.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage		145.50%
OneBeacon \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	452,519	449,435	517,470	493,421
Accrued incentive compensation	$ 1.6	$ 1.4	$ 3.4	$ 4.0
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	12.00%	13.00%	14.00%
OneBeacon \| Range Of Performance Cycle From 2016-2018 [Member] \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	163,150
Incentive compensation payable before forfeitures	$ 0.9
OneBeacon \| Range Of Performance Cycle From 2015 to 2017 [Member] \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	146,659
Incentive compensation payable before forfeitures	$ 0.7
OneBeacon \| Range Of Performance Cycle From 2014-2016 [Member] \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	142,710
Incentive compensation payable before forfeitures	$ 0.0
White Mountains \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	81,577
Incentive compensation payable before forfeitures	$ 43.1
forfeitures	(1,224)
Assumed forfeitures in period amount	(0.7)
Shares outstanding (in shares)	80,353	93,654	111,257	108,605
Accrued incentive compensation	$ 42.4	$ 57.7	$ 44.4	$ 54.9
White Mountains \| Range Of Performance Cycle From 2016-2018 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	19,615
Incentive compensation payable before forfeitures	$ 5.4
White Mountains \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	31,795
Incentive compensation payable before forfeitures	$ 19.1
White Mountains \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	30,167
Incentive compensation payable before forfeitures	$ 18.6
Sirius Group \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	7,315	10,826	12,607
Minimum \| Range Of Performance Cycle From 2016-2018 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	0.00%
Minimum \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Minimum \| OneBeacon \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	5.00%	6.00%	7.00%
Maximum \| Range Of Performance Cycle From 2016-2018 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Maximum \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Maximum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	15.00%
Maximum \| OneBeacon \| Long Term Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	19.00%	20.00%	21.00%

Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Share Cliff Vesting in January 2017 [Member]
Target Performance Share activity
Issued (in shares)			23,440
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2019 [Member]
Target Performance Share activity
Issued (in shares)	24,615
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2018 [Member]
Target Performance Share activity
Issued (in shares)	750	23,640
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	70,675	83,314	94,130
Issued (in shares)	25,365	23,640	23,440
Vested (in shares)	(24,620)	(36,279)	(33,205)
Assumed forfeitures and cancellations (in shares)	(800)	0	(1,051)
End of period (3)	70,620	70,675	83,314
Unamortized grant date fair value
Beginning balance	$ 15.7	$ 14.3	$ 17.0
Issued	20.2	15.7	13.1
Assumed forfeitures and cancellations	(0.3)	0.0	(0.5)
Expense recognized	(15.9)	(14.3)	(15.3)
Ending balance	$ 19.7	$ 15.7	$ 14.3
White Mountains \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	93,654	111,257	108,605
Assumed forfeitures and cancellations (in shares)	378	(3,839)	(3,208)
End of period (3)	80,353	93,654	111,257
Unamortized grant date fair value
Assumed forfeitures and cancellations	$ 0.5	$ (0.3)	$ (0.1)
Expense recognized	(25.2)	$ (44.4)	$ (14.1)
Ending balance	$ 23.1
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	9,205	12,477
End of period (3)	5,235	9,205	12,477
Sirius Group \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	10,826	12,607
End of period (3)	7,315	10,826	12,607
Range of Performance Cycle from 2011 to 2013 [Member] \| Performance Shares
Target Performance Share activity
Vested (in shares)			(3,570)
Range of Performance Cycle from 2011 to 2013 [Member] \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage			131.50%
Range of Performance Cycle from 2011 to 2013 [Member] \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage			37.10%
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%

Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	3,217
Shares Issued, Price Per Share	$ 742
Qualified Plan [Member]
Share-based compensation arrangement by share-based payment award
Percentage of common shares outstanding	3.00%	3.00%
Eligible contribution percentage	3.00%	3.00%	2.00%
Chief Executive Officer
Share-based compensation arrangement by share-based payment award
Shares Issued, Price Per Share	$ 742
Employee Stock Option [Member]
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	40,000	125,000
Employee Stock Option [Member] \| Chief Executive Officer
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	40,000	125,000
Employee Stock Option [Member] \| Combination One
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	5,000
Shares Issued, Price Per Share	$ 742
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0.4
Employee Stock Option [Member] \| Combination Two [Member]
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	80,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 8.4
Common Stock
Share-based compensation arrangement by share-based payment award
Incremental common shares attributable to share-based payment arrangements	9,930
Long Term Incentive Plan \| Range Of Performance Cycle From 2013 to 2015 [Member]
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage	24.30%

Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Expense Amortized
Beginning of period	$ 140.7
End of period (3)	$ 140.0	$ 140.7
OneBeacon \| Performance Shares \| Long Term Incentive Plan
Target options outstanding
Beginning of period	449,435	517,470	493,421
Shares paid or expired (in shares)	(167,300)	(181,290)	(142,138)
New grants	163,150	154,887	165,800
Assumed forfeitures and cancellations (in shares)	7,234	(41,632)	387
End of period (3)	452,519	449,435	517,470
Expense Amortized
Beginning of period	$ 1.4	$ 3.4	$ 4.0
Payments and deferrals	(0.6)	(1.5)	(1.0)
Assumed forfeitures and cancellations	0.0	(0.1)	0.0
Expense recognized	0.8	(0.4)	0.4
End of period (3)	$ 1.6	$ 1.4	$ 3.4
Range Of Performance Cycle From 2013 to 2015 [Member] \| Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	24.30%
Performance Cycle From 2012 to 2014 \| Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage		45.70%
Range of Performance Cycle from 2011 to 2013 [Member] \| Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage			37.10%

Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 140.0	$ 140.7
OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	452,519	449,435	517,470	493,421
Accrued incentive compensation	$ 1.6	$ 1.4	$ 3.4	$ 4.0
Unamortized grant date fair value	$ 2.1
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	12.00%	13.00%	14.00%
Range Of Performance Cycle From 2016-2018 [Member] \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	163,150
Incentive compensation payable before forfeitures	$ 0.9
Range Of Performance Cycle From 2015 to 2017 [Member] \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	146,659
Incentive compensation payable before forfeitures	$ 0.7
Range Of Performance Cycle From 2014-2016 [Member] \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	142,710
Incentive compensation payable before forfeitures	$ 0.0
Maximum \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	19.00%	20.00%	21.00%
Percentage of performance shares earned	200.00%	200.00%	200.00%
Minimum \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	5.00%	6.00%	7.00%

Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions		1 Months Ended	12 Months Ended
	Feb. 24, 2016 shares	May 31, 2011 shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	May 25, 2011 Installment
Restricted Stock Units (RSUs) [Member] \| OneBeacon \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Employee Service Share-based Compensation, Total Compensation Cost Not Yet Recognized \| $			$ 3.9
Defined Contribution Plan, Cost Recognized \| $			$ 2.1	$ 0.8
Percentage of common shares outstanding			100.00%
Target Performance Share activity
Issued (in shares)			233,461	226,778
Restricted Share Cliff Vesting in January 2018 [Member] \| One Beacon Insurance Company \| Restricted Stock
Target Performance Share activity
Issued (in shares)				75,950
Restricted Stock \| Sirius Group \| W T M Incentive Plan
Target Performance Share activity
Beginning of period			9,205	12,477
End of period (3)			5,235	9,205	12,477
Restricted Stock \| OneBeacon \| Long Term Incentive Plan
Target Performance Share activity
Beginning of period			382,722	612,500	915,000
Issued (in shares)			170,650	75,950	0
Vested (in shares)			(157,500)	(296,000)	(300,000)
Assumed forfeitures and cancellations (in shares)			0	(9,728)	(2,500)
Employee Service Share-based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not yet Recognized \| $			$ 0.0	$ (0.1)
End of period (3)			395,872	382,722	612,500
Unamortized grant date fair value
Beginning balance \| $			$ 2.5	$ 3.5	$ 6.5
Issued \| $			2.3	1.1	0.0
Vested \| $			0.0		0.0
Expense recognized \| $			(2.7)	(2.0)	(3.0)
Ending balance \| $			$ 2.1	$ 2.5	$ 3.5
Restricted Stock \| One Beacon Insurance Company \| Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	170,650
Unamortized grant date fair value
Forfeited shares		35,000
Restricted Stock \| One Beacon Insurance Company \| Long Term Incentive Plan \| Chief Executive Officer
Target Performance Share activity
Issued (in shares)		630,000
Unamortized grant date fair value
Number of installments in which the shares vest \| Installment						4
Forfeited shares		35,000
Years performance shares will be forfeited		5 years
Vesting in January 2019 [Member] \| Restricted Stock \| One Beacon Insurance Company \| Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	78,150
Vesting in February 2018 [Member] \| Restricted Stock \| One Beacon Insurance Company \| Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	92,500
Restricted Share Cliff Vesting in January 2017 [Member] \| Restricted Stock Units (RSUs) [Member] \| OneBeacon \| Long Term Incentive Plan
Target Performance Share activity
End of period (3)			173,042
Restricted Share Cliff Vesting in January 2018 [Member] \| Restricted Stock Units (RSUs) [Member] \| OneBeacon \| Long Term Incentive Plan
Target Performance Share activity
End of period (3)			214,506

Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Defined Contribution Plan, Cost Recognized	$ 5.5	$ 5.7	$ 4.4
Qualified Plan [Member]
Share-based compensation arrangement by share-based payment award
Eligible contribution percentage	3.00%	3.00%	2.00%
Percentage of common shares outstanding	3.00%	3.00%

Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2007	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		$ 887.2	$ 284.2	$ 134.5
Value of each common share repurchased and retired during the period (in dollars per share)		$ 802	$ 733	$ 617
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		47,030	25,016	28,405
Shares issued to directors (in shares)		1,735	1,376	1,395
Dividends, Common Stock [Abstract]
Cash dividends declared and paid		$ 5.4	$ 6.0	$ 6.2
Dividends delared and paid (in dollars per share)		$ 1.00	$ 1.00	$ 1.00
Non-controlling interests		$ 133.9	$ 454.8
Non-controlling Interests
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		0.0	0.0	$ 0.0
Total Non-Controlling Excluding Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 35.8	$ 115.2
Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		25,365	23,640	23,440
Performance Shares
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		5,000		3,570
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		1,106,145	387,495	217,879
General Board Authorization [Member]
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares		878,130
Common shares repurchased and retired during the period		1,098,123	361,839	207,404
Value of common shares repurchased and retired during the period		$ 881.4	$ 267.4	$ 128.2
Value of each common share repurchased and retired during the period (in dollars per share)		$ 803	$ 739	$ 618
Chief Executive Officer
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		14,930
Repurchased for Employee Benefit Plans [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		8,022	10,802	10,475
White Mountains
Dividends, Common Stock [Abstract]
Capital contributions					$ 594.5
Sirius Internation Group Ltd \| Preferred stocks
Dividends, Common Stock [Abstract]
Preference shares floating rate of dividend amount in excess of libor	3.20%
Preferred Stock Redemption Basis Spread on Treasury Issue Rate	45.00%
OneBeacon Ltd.
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		23.90%	24.50%
Non-controlling interests		$ 244.6	$ 245.6
HG/BAM [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (150.9)	$ (140.0)
Star & Shield Insurance Exchange [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ 4.4	$ (16.0)
Mutual and Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests		$ (146.5)	$ (156.0)
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.10%	3.10%
Non-controlling interests		$ 16.6	$ 17.1
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		40.00%	40.00%
Non-controlling interests		$ 11.7	$ 14.4
Tranzact
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners			36.80%
Non-controlling interests			$ 79.4
WOBI [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		5.00%	3.90%
Non-controlling interests		$ 0.7	$ 0.6
Buzzmove [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		29.10%	0.00%
Non-controlling interests		$ 2.9	$ 0.0
A W G Dewar Inc [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		18.80%	19.00%
Non-controlling interests		$ 3.9	$ 3.7
SIG Preference Shares
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		0.00%	100.00%
Non-controlling interests		$ 0.0	$ 250.0
BAM Management
Dividends, Common Stock [Abstract]
Capital contributions					$ 14.5
OneBeacon
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 244.6	$ 249.3

Statutory Capital and Surplus (Details) $ in Millions	3 Months Ended	12 Months Ended
Statutory Capital and Surplus (Details) $ in Millions	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) yr	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017	Dec. 31, 2013 USD ($)
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		$ 3,603.3	$ 3,913.2
Amount available for dividend distributions without prior approval from regulatory authorities		60.0
Amount not available for dividend distribution		502.5
Distributions made		5.4
Fair value investments	$ 3,343.6	5,039.7	4,050.9	$ 3,343.6
Trading Securities		4,256.8
Other long-term investments		323.3	315.8
OneBeacon
Statutory Accounting Practices [Line Items]
Statutory net income (loss)		36.7	44.5	(14.2)
Stockholders' Equity Attributable to Parent		780.6	755.2
Distributions made		79.2	80.0
Unrestricted cash		64.5
Other Operations
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		2,294.9	1,124.3
Unrestricted cash		1,655.0
Total Capital Contributed to Subsidiary		703.0	676.0
Other long-term investments		66.9
Unregulated Operation
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities		2,300.0
Unregulated Operation \| OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		5.7	5.3
Unrestricted cash		56.4
White Mountains Advisors LLC \| Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash		17.8
Star & Shield Insurance Exchange [Member] \| Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		4.4	5.0
Statutory net income (loss)		20.4	(3.5)
Surplus required		0.4
OneBeacon
Statutory Accounting Practices [Line Items]
Dividends available, net of noncontrolling interest		177.1
HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value		$ 619.0
Ownership interest (as a percent)		96.90%
Preferred stock dividend rate		6.00%
HG Re \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 467.2
Collateral Held in Supplement Trust		465.4
BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		431.5
Statutory net income (loss)		32.7	32.0	31.8
Surplus required		66.0
Subsidiaries
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		562.5
One Beacon Insurance Company \| OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made	700.5
Noncash and Part Noncash Distribution Received	151.2
Split Rock [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		270.2	249.5
Statutory net income (loss)		$ 42.4	45.5	46.2
Percent Available for Dividend Distribution without Prior Approval from Regulatory Agency in Regulated Entities		15.00%
Split Rock [Member] \| Regulated Operation \| OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 270.2
Amount available for dividend distributions without prior approval from regulatory authorities		60.1
Distributions made		25.0
Capital		210.1
Statutory accounting practices earned surplus		60.1
Total Capital Contributed to Subsidiary			85.0
Statutory Accounting Practices, Statutory Amount Available for Contributions without Regulatory Approval		31.5
White Mountains \| OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		60.3
Atlantic Speciality Insurance Company (ASIC) [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Surplus required		107.5
Atlantic Speciality Insurance Company (ASIC) [Member] \| Regulated Operation \| OneBeacon
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities		11.4
Distributions made		26.5
Capital		624.8
Statutory accounting practices earned surplus		$ 69.0
Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
basis points		3.00%
Preferred stocks \| HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends Payable		$ 186.2
Preferred stocks \| White Mountains \| HG Global \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends Payable		180.2
Common Equities [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Trading Securities		11.9
Common Equities [Member] \| Other Operations
Statutory Accounting Practices [Line Items]
Trading Securities		285.6
Retained earnings
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	$ 3,010.5	$ 2,797.2	3,084.9	$ 3,010.5		$ 2,801.9
Retained earnings \| Split Rock [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		25.00%
Retained earnings \| Atlantic Speciality Insurance Company (ASIC) [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		10.00%
U.S and Bermuda [Member] \| OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 624.8	$ 622.3
Surplus Note \| Unregulated Operation \| OneBeacon
Statutory Accounting Practices [Line Items]
Fair value investments		$ 71.9
Combination One \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate		5
Combination Two [Member] \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr		3
US Treasury Rate [Member] \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		3.54%
Fixed interest rate [Member] \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
Scenario, Forecast [Member] \| US Treasury Rate [Member] \| Surplus Note \| BAM \| HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)					3.78%

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 1,114.0	$ 1,188.2	$ 1,185.0
Net investment income									86.8	60.8	59.5
Net realized and unrealized investment gains									10.3	225.4	78.5
Other revenue									149.6	147.3	88.1
Total revenues	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3	1,360.7	1,621.7	1,411.1
Loss and loss adjustment expenses									664.0	708.9	824.0
Insurance acquisition expenses									211.6	220.1	206.2
Other underwriting expenses									209.5	218.6	179.6
General and administrative expenses									299.9	302.2	216.5
Amortization of Intangible Assets									12.5	10.6	8.3
Interest expense on debt									16.1	14.6	14.2
Total expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3	1,401.1	1,464.4	1,440.5
Pre-tax (loss) income from continuing operations	$ (66.8)	$ 20.0	$ (2.1)	$ 8.5	$ 246.3	$ (86.9)	$ (5.1)	$ 3.0	(40.4)	157.3	(29.4)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									287.4	291.6	208.2
OneBeacon
Segment reporting information
Earned insurance premiums									1,100.6	1,176.2	1,177.1
Net investment income									50.6	45.9	43.4
Net realized and unrealized investment gains									37.7	(35.1)	40.4
Other revenue									5.5	(0.6)	5.8
Total revenues									1,194.4	1,186.4	1,266.7
Loss and loss adjustment expenses									656.0	700.7	815.1
Insurance acquisition expenses									206.0	213.8	203.3
Other underwriting expenses									209.0	218.2	179.2
Interest expense on debt									13.1	13.0	13.0
Total expenses									1,098.3	1,161.1	1,224.4
Pre-tax (loss) income from continuing operations									96.1	25.3	42.3
OneBeacon \| Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									13.0	14.1	12.4
BAM
Segment reporting information
Earned insurance premiums									1.5	0.8	0.4
Net investment income									6.8	4.2	5.7
Net realized and unrealized investment gains									0.6	0.9	6.6
Other revenue									1.1	0.7	0.6
Total revenues									(7.8)	(9.2)	(2.4)
Loss and loss adjustment expenses									0.0	0.0	0.0
Insurance acquisition expenses									2.5	2.3	1.8
Other underwriting expenses									0.4	0.4	0.4
Interest expense on debt									0.0	0.0	0.0
Total expenses									41.1	38.1	38.1
Pre-tax (loss) income from continuing operations									(48.9)	(47.3)	(40.5)
BAM \| Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									$ 38.2	35.4	35.9
HG Global
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 4.4	2.5	1.4
Net investment income									2.2	1.9	1.4
Net realized and unrealized investment gains									0.1	(0.3)	1.7
Other revenue									0.0	0.0	0.0
Total revenues									24.5	19.9	20.2
Loss and loss adjustment expenses									0.0	0.0	0.0
Insurance acquisition expenses									0.9	0.6	0.3
Other underwriting expenses									0.0	0.0	0.0
Interest expense on debt									0.0	0.0	0.0
Total expenses									2.3	2.0	1.9
Pre-tax (loss) income from continuing operations									22.2	17.9	18.3
HG Global \| Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									1.4	1.4	1.6
Other Segments
Segment reporting information
Earned insurance premiums									7.5	8.7	6.1
Net investment income									27.2	8.8	9.0
Net realized and unrealized investment gains									(28.1)	259.9	29.8
Other revenue									143.0	147.2	81.7
Total revenues									149.6	424.6	126.6
Loss and loss adjustment expenses									8.0	8.2	8.9
Insurance acquisition expenses									2.2	3.4	0.8
Other underwriting expenses									0.1	0.0	0.0
Interest expense on debt									3.0	1.6	1.2
Total expenses									259.4	263.2	176.1
Pre-tax (loss) income from continuing operations									(109.8)	161.4	(49.5)
Other Segments \| Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									234.8	240.7	158.3
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets									12.5	10.6	8.3
General and Administrative Expense [Member] \| OneBeacon
Segment reporting information
Amortization of Intangible Assets									1.2	1.3	1.4
General and Administrative Expense [Member] \| BAM
Segment reporting information
Amortization of Intangible Assets									0.0	0.0	0.0
General and Administrative Expense [Member] \| HG Global
Segment reporting information
Amortization of Intangible Assets									0.0	0.0	0.0
General and Administrative Expense [Member] \| Other Segments
Segment reporting information
Amortization of Intangible Assets									11.3	9.3	6.9
Symetra \| Other Segments
Segment reporting information
Net realized and unrealized investment gains										258.8
MediaAlpha [Member] \| Other Segments
Segment reporting information
Other revenue									116.5	105.5	65.3
General and administrative expenses									109.6	99.0	60.6
Surplus Note
Segment reporting information
Net investment income									0.0	0.0	0.0
Surplus Note \| OneBeacon
Segment reporting information
Net investment income									0.0	0.0	0.0
Surplus Note \| BAM
Segment reporting information
Net investment income									(17.8)	(15.8)	(15.7)
Surplus Note \| HG Global
Segment reporting information
Net investment income									17.8	15.8	15.7
Surplus Note \| Other Segments
Segment reporting information
Net investment income									$ 0.0	$ 0.0	$ 0.0

Segment Information (Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Total investments	$ 5,334.8	$ 4,271.1
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
Assets held for sale	10.1	4,790.4
Assets	6,544.7	10,282.6
Loss and loss adjustment expense reserves	1,365.6	1,389.8	$ 1,342.2	$ 1,054.3
Liabilities held for sale	5.1	3,047.4
Liabilities	2,807.5	5,914.6
Non-controlling interests	133.9	454.8
White Mountains’ common shareholders’ equity	3,603.3	3,913.2
OneBeacon
Segment reporting information
Total investments	2,620.4	2,591.4
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
Assets held for sale	0.0	0.0
Assets	3,589.4	3,600.8
Loss and loss adjustment expense reserves	1,365.6	1,389.8
Liabilities held for sale	0.0	0.0
Liabilities	2,564.2	2,596.3
Non-controlling interests	244.6	249.3
White Mountains’ common shareholders’ equity	780.6	755.2
HG Global
Segment reporting information
Total investments	161.6	136.2
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	787.0	739.0
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	247.6	181.2
Non-controlling interests	16.6	17.1
White Mountains’ common shareholders’ equity	522.8	540.7
BAM
Segment reporting information
Total investments	468.1	460.3
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	(78.9)	(91.1)
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	72.0	48.9
Non-controlling interests	(150.9)	(140.0)
White Mountains’ common shareholders’ equity	0.0	0.0
Surplus notes	503.0
Accrued interest on surplus notes	108.0	90.2
Other Operations
Segment reporting information
Total investments	2,084.7	1,083.2
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	2,417.3	1,378.8
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	98.8	176.1
Non-controlling interests	23.6	78.4
White Mountains’ common shareholders’ equity	2,294.9	1,124.3
Total Capital Contributed to Subsidiary	703.0	676.0
Consolidation, Eliminations [Member]
Segment reporting information
Assets	(180.2)	(135.3)
Liabilities	(180.2)	(135.3)
Discontinued Operations, Held-for-sale [Member]
Segment reporting information
Total investments	0.0	0.0
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	10.1	4,790.4
Assets	10.1	4,790.4
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	5.1	3,047.4
Liabilities	5.1	3,047.4
Non-controlling interests	0.0	0.0
White Mountains’ common shareholders’ equity	5.0	1,493.0
Continuing Operations [Member]
Segment reporting information
Non-controlling interests	133.9	204.8
SIG Preference Shares
Segment reporting information
Non-controlling interests		250.0
HG Global
Segment reporting information
Non-controlling interests	16.6	17.1
Surplus notes	300.0				$ 503.0
Preferred Dividends Payable [Member] \| HG Global \| Other Operations
Segment reporting information
Accounts Payable and Accrued Liabilities	$ 180.2	$ 135.3

Investments in Unconsolidated Affiliates (Details) - USD ($)	12 Months Ended
	Dec. 31, 2014	Nov. 05, 2015
Safeco Corporation [Member]
Investments in and Advances to Affiliates [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 1,350,000,000
Symetra
Investments in and Advances to Affiliates [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	195,000,000
Symetra \| Common Stock
Investments in and Advances to Affiliates [Line Items]
Investments in unconsolidated affiliates	$ 411,400,000	$ 392,800,000

Investments in Unconsolidated Affiliates (Symetra Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended		10 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Nov. 05, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 01, 2016	Dec. 31, 2011
Investments in unconsolidated affiliates
Trading Securities, Change in Unrealized Holding Gain (Loss)						$ (267,300,000)	$ 152,900,000	$ (79,300,000)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax						(226,600,000)	140,600,000	(69,400,000)
Realized gains						315,500,000	112,900,000	194,000,000
After tax amortization through equity in net unrealized losses						0	(36,300,000)	75,300,000
Sales of common equity securities						$ 1,200,800,000	$ 461,400,000	609,800,000
Common shares, par value per share (in dollars per share)		$ 1.00				$ 1.00	$ 1.00
Symetra
Investments in unconsolidated affiliates
Payments to Acquire Interest in Subsidiaries and Affiliates								195,000,000
Trading Securities, Change in Unrealized Holding Gain (Loss)		$ 258,800,000
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax		$ 241,100,000
Realized gains	$ 4,700,000
Shares of symetra financial corporation owned (in shares)					20,562,379
Ownership interest (as a percent)					17.70%
Fair value of investment										$ 261,000,000
Estimated fair value of investment per share										$ 15
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					$ 159,900,000
Term of amortization (in years)			30 years
Dividends Payable, Amount Per Share									$ 32.00
Common Stock \| Symetra
Investments in unconsolidated affiliates
Dividends received					$ (16,900,000)
Per share price, class of warrant (in dollars per share)					$ 31.67
equity in earnings of unconsolidated affiliates [Member] \| Symetra
Investments in unconsolidated affiliates
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					$ 32,600,000
After tax amortization through equity in earnings				$ 2,200,000				2,800,000
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Symetra
Investments in unconsolidated affiliates
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					$ 127,300,000
After tax amortization through equity in net unrealized losses				$ 8,900,000				$ 11,700,000
Symetra
Investments in unconsolidated affiliates
Sales of common equity securities	$ 658,000,000

Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended								10 Months Ended	12 Months Ended
	Nov. 05, 2015	Dec. 31, 2014	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Nov. 05, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax			$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 7.1	$ 3.9	$ 6.8	$ 7.3		$ 0.0	$ 25.1	$ 45.6
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates												0.0	(39.2)	81.2
Income Tax Expense (Benefit)			$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)		$ 45.4	$ 0.2	14.8
Common shares, issued shares			4,563,814				5,623,735					4,563,814	5,623,735
Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax											$ 25.1
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates											(39.2)
Carrying value of a distribution											12.4
Dividends received											(16.9)
Carrying value at the end of the period	$ 392.8	$ 411.4									392.8			411.4
Loss from dilutive effect resulting from dividend and issuance of restricted shares											(0.1)
Aggregate value of White Mountains common shares of Symetra at quoted market price	$ 651.2										$ 651.2
Share Price	$ 31.67										$ 31.67
Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$ (1.6)	$ 37.6
equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											$ (1.6)
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											2.9
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											0.2			$ (2.7)
impairment of net unrealized gains (losses) [Member] \| Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates											9.4
Amortization of common share basis difference [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax											$ 2.3
Prospector
Changes in carrying value of investment in Symetra
Common shares, issued shares							513,500						513,500

Investments in Unconsolidated Affiliates (Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments in and Advances to Affiliates
Total investments	$ 5,334.8				$ 4,271.1					$ 5,334.8	$ 4,271.1
Total assets	6,544.7				10,282.6					6,544.7	10,282.6
Long-term debt	285.9				337.6					285.9	337.6
Total liabilities	2,807.5				5,914.6					2,807.5	5,914.6
Common shareholders’ equity	3,737.2				4,368.0					3,737.2	4,368.0
Earned insurance premiums										1,114.0	1,188.2	$ 1,185.0
Net investment income										86.8	60.8	59.5
(Loss) revenues (including realized gains and losses)	268.2	$ 365.0	$ 358.2	$ 369.3	602.6	$ 296.1	$ 368.7	$ 354.3		1,360.7	1,621.7	1,411.1
Expenses	$ 335.0	$ 345.0	$ 360.3	$ 360.8	$ 356.3	383.0	$ 373.8	$ 351.3		1,401.1	1,464.4	1,440.5
Net income										419.8	279.5	290.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										$ 558.1	$ 197.2	207.3
Symetra
Investments in and Advances to Affiliates
Total investments						32,409.2			$ 32,409.2
Separate account assets						885.9			885.9
Total assets						34,962.8			34,962.8
Policyholder liabilities						29,492.0			29,492.0
Long-term debt						697.5			697.5
Separate account liabilities						885.9			885.9
Total liabilities						31,836.7			31,836.7
Common shareholders’ equity						$ 3,126.1			3,126.1
Earned insurance premiums									539.3			629.1
Net investment income									994.3			1,320.5
(Loss) revenues (including realized gains and losses)									1,605.9			2,182.4
Liability for Future Policy Benefits, Period Expense (Income)									1,143.7			1,399.7
Expenses									1,543.6			1,882.5
Net income									89.6			254.4
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ (234.1)			$ 397.0

Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (Hamer and Bri-Mar) (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 7.1	$ 3.9	$ 6.8	$ 7.3		$ 0.0	$ 25.1	$ 45.6
Hamer, LLC
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Realized Gain (Loss) on Disposal									$ 20.0
Equity in earnings of unconsolidated affiliates, net of tax									$ 1.6			$ 1.9

Variable Interest Entities (Details) $ in Millions	3 Months Ended								12 Months Ended
Variable Interest Entities (Details) $ in Millions	Dec. 31, 2016 USD ($) trust	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) trust director	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Other revenue									$ 149.6	$ 147.3	$ 88.1
Accumulated losses	$ 2,797.2				$ 3,084.9				2,797.2	3,084.9
Noncontrolling interest in VIE	133.9				454.8				133.9	454.8
Non-controlling interest in consolidated subsidiaries	$ (17.3)	$ (3.1)	$ 21.4	$ 6.3	6.1	$ (16.0)	$ (0.9)	$ (7.3)	7.3	(18.1)	(22.2)
Payments to Acquire Other Investments									$ 39.5	78.0	114.7
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Other revenue									$ 0.0	0.0	$ 0.0
Noncontrolling interest in VIE	$ 16.6				17.1				16.6	17.1
Star & Shield Insurance Exchange [Member]
Variable Interest Entity [Line Items]
Surplus notes					21.0					21.0
Total assets related to consolidated VIE's	9.2				14.2				9.2	14.2
Total liabilities related to consolidated VIE's	4.8				30.3				4.8	$ 30.3
Prospector
Variable Interest Entity [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage										67.60%
Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	5.0								5.0
Total assets related to consolidated VIE's	5.1								5.1
Total liabilities related to consolidated VIE's	5.0								5.0
Accumulated losses	0.5								0.5
Accrued interest on surplus notes	0.6								0.6
Common Stock \| BAM
Variable Interest Entity [Line Items]
Issuance of notes												$ 503.0
Star & Shield Insurance Exchange [Member]
Variable Interest Entity [Line Items]
Other revenue									21.0
Noncontrolling interest in VIE	4.4				(16.0)				$ 4.4	$ (16.0)
BAM
Variable Interest Entity [Line Items]
Right to elect directors on board number \| director									2
HG Global
Variable Interest Entity [Line Items]
Surplus notes	300.0								$ 300.0			$ 503.0
Noncontrolling interest in VIE	$ 16.6				$ 17.1				$ 16.6	$ 17.1
First Loss Reinsurance Treaty [Member] \| HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
First Loss Reinsurance Treaty [Member] \| BAM
Variable Interest Entity [Line Items]
Percentage of Premiums	60.00%								60.00%
First Loss Reinsurance Treaty [Member] \| HG Global
Variable Interest Entity [Line Items]
Number of Trust \| trust	2								2
Amendment [Member] \| Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	$ 5.0								$ 5.0

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 285.9	$ 337.6
OneBeacon \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	273.2	272.9
OneBeacon \| Fair value \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	274.2	276.4
OneBeacon \| Carrying Value \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	273.2	272.9
MediaAlpha [Member] \| Fair value \| MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	13.0	15.0
MediaAlpha [Member] \| Carrying Value \| MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 12.7	$ 14.7

Transactions with Related Persons (Details) - USD ($)	3 Months Ended				12 Months Ended
Transactions with Related Persons (Details) - USD ($)	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Assets					$ 6,544,700,000	$ 10,282,600,000
Sales of common equity securities					$ 1,200,800,000	$ 461,400,000	$ 609,800,000
Treasury Stock Acquired, Average Cost Per Share					$ 802	$ 733	$ 617
OneBeacon
Related Party Transaction [Line Items]
Assets					$ 3,589,400,000	$ 3,600,800,000
Prospector
Related Party Transaction [Line Items]
Asset Management Fees						2,100,000	$ 6,500,000
Shares paid or expired(1)						$ 5,500,000
Percentage of fees in excess of $500,000 revenue chargeable						6.00%
Amount of revenue over which 6% of fees chargeable						$ 500,000
Management Fees, Base Revenue						400,000	1,100,000
Incentive fees incurred						100,000	700,000
Franklin Mutual Advisers [Member]
Related Party Transaction [Line Items]
Treasury Stock Acquired, Average Cost Per Share	$ 820.00	$ 807.00	$ 761.50	$ 650.03
White Mountains Advisors LLC \| Symetra
Related Party Transaction [Line Items]
Asset Management Fees						$ 8,000,000	18,400,000
Combination One \| Prospector
Related Party Transaction [Line Items]
Assets					200,000,000
Combination Two [Member] \| Prospector
Related Party Transaction [Line Items]
Assets					200,000,000
Combination Three [Member] \| Prospector
Related Party Transaction [Line Items]
Assets					$ 400,000,000
revenue sharing agreement \| Prospector
Related Party Transaction [Line Items]
Fees from Revenue Sharing Agreement with Related Party							$ 400,000
First Threshold Limit \| White Mountains Advisors LLC \| Prospector
Related Party Transaction [Line Items]
basis points						1.00%
Second Threshold Limit \| White Mountains Advisors LLC \| Prospector
Related Party Transaction [Line Items]
basis points						0.50%
Third Threshold Limit \| White Mountains Advisors LLC \| Prospector
Related Party Transaction [Line Items]
basis points						0.25%
W T M Incentive Plan \| Performance Shares \| White Mountains
Related Party Transaction [Line Items]
Number of performance shares granted					22,615	29,195	39,590
Shares paid or expired(1)					$ 41,000,000	$ 30,800,000	$ 24,500,000
W T M Incentive Plan \| Performance Shares \| White Mountains \| Prospector
Related Party Transaction [Line Items]
Number of performance shares granted						7,000
Common Stock
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period					1,106,145	387,495	217,879
Common Stock \| Franklin Mutual Advisers [Member]
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period	305,000	325,000	26,300	19,688
2014-2016 Cycle \| W T M Incentive Plan \| Performance Shares \| White Mountains \| Prospector
Related Party Transaction [Line Items]
Number of performance shares granted						6,250

Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Rent expense	$ 14.1	$ 14.0	$ 12.9
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	11.4
Capital lease payments due in two years	9.5
Capital lease payments due in three years	8.0
Capital Lease payments due in four years and thereafter	32.6
Commitments to fund other-long term investments	138.1
Assets	6,544.7	10,282.6
Reserve for probable liability from mandatory shared market mechanisms	8.0
OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease obligation		1.7
Capital lease asset		$ 2.4
Assets Held under Capital Leases [Member] \| OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Assets	$ 1.3

Commitments and Contingencies (Legal Contingencies) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2007	Dec. 31, 2016
OneBeacon \| Pending Litigation [Member] \| Tribune Company Litigation [Member]
Loss Contingencies [Line Items]
Proceeds From Tribune Common Stock	$ 32.0
OneTitle [Member]
Loss Contingencies [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		20.00%
Surplus notes		$ 10.0
Sirius Group \| Pending Litigation [Member] \| Tribune Company Litigation [Member]
Loss Contingencies [Line Items]
Proceeds From Tribune Common Stock	$ 6.1

Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended									6 Months Ended	12 Months Ended
Discontinued Operations (Details) - USD ($) $ in Millions	Apr. 18, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold												$ 2,646.2	$ 24.0	$ 0.0
Payments to Acquire Other Investments												39.5	78.0	114.7
Income Tax Expense (Benefit)		$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)			45.4	0.2	14.8
Net realized and unrealized investment gains												10.3	225.4	78.5
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax													0.0	0.0
Total investments		5,334.8				4,271.1						5,334.8	4,271.1
OCI, Change in foreign currency transaction and translation adjustment, net of tax												0.3	(0.5)	(10.7)
Gain (loss) from sale of other discontinued operations, net of tax												415.1	18.2	(1.6)
Loss and loss adjustment expense reserves		1,365.6				1,389.8				$ 1,342.2		1,365.6	1,389.8	1,342.2	$ 1,054.3
Net (loss) income from discontinued operations, net of tax		(6.8)	61.9	358.6	$ 1.1	19.6	$ 6.5	$ (0.5)	$ 71.3			(0.3)	78.7	260.6
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600.0			2,600.0
Payments to Acquire Other Investments	$ 161.8			161.8
Net realized and unrealized investment gains												3.7	205.0	2.7
Total investments						686.2							686.2
Gain (loss) from sale of other discontinued operations, net of tax				$ 363.2								363.2
Sirius Group \| Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains													200.8
Total investments						528.6							528.6
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax											$ 113.3	113.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax												32.0	(65.0)	(169.5)
Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		1.2	221.3
Discontinued Operation, Tax Effect of Discontinued Operation			30.2
Gain (loss) from sale of other discontinued operations, net of tax			$ 51.9									51.9
OneBeacon Runoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax										$ 36.1
Esurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax												0.0	17.9	3.2
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains												(1.5)	15.1	205.4
OCI, Change in foreign currency transaction and translation adjustment, net of tax													(65.0)	(169.5)
Discontinued Operation, Tax Effect of Discontinued Operation												(9.5)	26.4	69.4
Total income from discontinued operations												414.8	96.9	259.0
Net (loss) income from discontinued operations, net of tax												(0.3)	78.7	260.6
Discontinued Operations \| Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net realized and unrealized investment gains												(1.5)	15.1	205.4
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax												113.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax												32.0	(65.0)	(169.5)
Discontinued Operation, Tax Effect of Discontinued Operation												(3.1)	27.1	70.4
Gain (loss) from sale of other discontinued operations, net of tax												363.2		0.0
Total income from discontinued operations												358.9	81.1	262.0
Net (loss) income from discontinued operations, net of tax												(4.3)	81.1	262.0
Discontinued Operations \| Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income Tax Expense (Benefit)												21.4
Tax Effect of Discontinued Operations												30.2
Discontinued Operation, Tax Effect of Discontinued Operation												8.8
Gain (loss) from sale of other discontinued operations, net of tax												51.9
Net (loss) income from discontinued operations, net of tax												6.1	(1.9)	0.4
Discontinued Operations \| OneBeacon Runoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax													0.3	$ (18.8)
Loss and loss adjustment expense reserves		$ 18.3				$ 20.4						18.3	20.4
Percentage of quota share reinsurance agreement														100.00%
Total income from discontinued operations													(0.5)
Discontinued Operations \| Esurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax												28.3	$ 17.9	$ 3.2
Discontinued Operations \| Fireman's Fund Insurance Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) from sale of other discontinued operations, net of tax														$ 14.0
Discontinued Operations, Held-for-sale [Member] \| Star & Shield LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other Expenses												$ 21.0

Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets held for sale
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	287.0	211.2
Common equity securities, at fair value	474.3	1,113.9
Other long-term investments	323.3	315.8
Total investments	5,334.8	4,271.1
Cash Including Restricted Cash	149.8	173.0
Reinsurance recoverable on unpaid losses	172.9	186.0
Reinsurance recoverable on paid losses	6.6	7.5
Insurance premiums receivable	229.9	220.3
Deferred acquisition costs	106.9	107.6
Deferred Income Tax Assets	126.7	112.8
Ceded unearned insurance premiums	44.2	29.5
Accounts receivable on unsettled investment sales	6.2	41.9
Goodwill and intangible assets	55.9	55.4
Other assets	274.6	273.2
Assets held for sale	10.1	4,790.4
Liabilities held for sale
Loss and loss adjustment expense reserves	1,365.6	1,389.8	$ 1,342.2	$ 1,054.3
Unearned insurance and reinsurance premiums	658.0	610.5
Debt	285.9	337.6
Accrued incentive compensation	140.0	140.7
Funds held under insurance contracts	153.0	137.8
Other liabilities	199.9	250.8
Liabilities held for sale	5.1	3,047.4
Sirius Group, Tranzact, and Star and Shield [Member] \| Discontinued Operations, Held-for-sale [Member]
Assets held for sale
Fixed maturity investments, at fair value		2,383.5
Short-term investments, at amortized cost (which approximates fair value)		352.1
Common equity securities, at fair value		174.4
Other long-term investments		72.2
Total investments		2,982.2
Cash Including Restricted Cash		150.2
Reinsurance recoverable on unpaid losses		283.6
Reinsurance recoverable on paid losses		10.2
Insurance premiums receivable		326.6
Deferred acquisition costs		74.6
Deferred Income Tax Assets		303.1
Ceded unearned insurance premiums		87.7
Accounts receivable on unsettled investment sales		29.0
Goodwill and intangible assets		330.5
Other assets		212.7
Assets held for sale		4,790.4
Liabilities held for sale
Loss and loss adjustment expense reserves		1,650.4
Unearned insurance and reinsurance premiums		344.3
Debt		506.4
Deferred tax liability		270.6
Accrued incentive compensation		64.2
Funds held under insurance contracts		52.9
Other liabilities		158.6
Liabilities held for sale		3,047.4
Net assets held for sale		$ 1,743.0
Star & Shield Insurance Exchange [Member] \| Discontinued Operations, Held-for-sale [Member]
Assets held for sale
Fixed maturity investments, at fair value	6.6
Short-term investments, at amortized cost (which approximates fair value)	0.2
Common equity securities, at fair value	0.0
Other long-term investments	0.0
Total investments	6.8
Cash Including Restricted Cash	0.9
Reinsurance recoverable on unpaid losses	0.3
Reinsurance recoverable on paid losses	0.0
Insurance premiums receivable	1.5
Deferred acquisition costs	0.0
Deferred Income Tax Assets	0.0
Ceded unearned insurance premiums	0.0
Accounts receivable on unsettled investment sales	0.0
Goodwill and intangible assets	0.0
Other assets	0.6
Assets held for sale	10.1
Liabilities held for sale
Loss and loss adjustment expense reserves	5.0
Unearned insurance and reinsurance premiums	1.2
Debt	0.0
Deferred tax liability	0.0
Accrued incentive compensation	0.0
Funds held under insurance contracts	0.0
Other liabilities	(1.1)
Liabilities held for sale	5.1
Net assets held for sale	$ 5.0

Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Earned insurance premiums										$ 1,114.0	$ 1,188.2	$ 1,185.0
Net investment income										86.8	60.8	59.5
Net realized and unrealized investment gains										10.3	225.4	78.5
Other revenue										149.6	147.3	88.1
(Loss) revenues (including realized gains and losses)	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3		1,360.7	1,621.7	1,411.1
Expenses
Loss and loss adjustment expenses										664.0	708.9	824.0
Insurance acquisition expenses										211.6	220.1	206.2
Other underwriting expenses										209.5	218.6	179.6
Interest expense on debt										16.1	14.6	14.2
General and administrative expenses										299.9	302.2	216.5
Total expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3		1,401.1	1,464.4	1,440.5
Net (loss) income from discontinued operations, net of tax	$ (6.8)	61.9	358.6	$ 1.1	$ 19.6	$ 6.5	$ (0.5)	$ 71.3		(0.3)	78.7	260.6
Gain (loss) from sale of other discontinued operations, net of tax										415.1	18.2	(1.6)
OCI, Change in foreign currency transaction and translation adjustment, net of tax										0.3	(0.5)	(10.7)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax											0.0	0.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										558.1	197.2	207.3
Discontinued Operations
Revenues
Earned insurance premiums										240.1	847.0	874.0
Net investment income										14.4	40.7	40.9
Net realized and unrealized investment gains										(1.5)	15.1	205.4
Other revenue										120.2	165.6	(19.4)
(Loss) revenues (including realized gains and losses)										373.2	1,068.4	1,100.9
Expenses
Loss and loss adjustment expenses										154.9	422.7	344.6
Insurance acquisition expenses										59.0	189.8	193.7
Other underwriting expenses										30.9	107.9	133.2
Interest expense on debt										11.1	30.6	27.7
General and administrative expenses										127.1	212.3	71.7
Total expenses										383.0	963.3	770.9
Net (loss) income from discontinued operations										(9.8)	105.1	330.0
Income tax benefit										9.5	(26.4)	(69.4)
Net (loss) income from discontinued operations, net of tax										(0.3)	78.7	260.6
Total income from discontinued operations										414.8	96.9	259.0
OCI, Change in foreign currency transaction and translation adjustment, net of tax											(65.0)	(169.5)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										560.1	31.9	89.5
Sirius Group
Revenues
Net realized and unrealized investment gains										3.7	205.0	2.7
Expenses
Gain (loss) from sale of other discontinued operations, net of tax			$ 363.2							363.2
Sirius Group \| Discontinued Operations
Revenues
Earned insurance premiums										240.1	847.0	873.9
Net investment income										14.4	40.7	40.9
Net realized and unrealized investment gains										(1.5)	15.1	205.4
Other revenue										0.6	(20.6)	(62.4)
(Loss) revenues (including realized gains and losses)										253.6	882.2	1,057.8
Expenses
Loss and loss adjustment expenses										154.9	422.7	345.3
Insurance acquisition expenses										59.0	189.8	193.6
Other underwriting expenses										30.9	107.9	129.7
Interest expense on debt										7.9	26.6	26.3
General and administrative expenses										8.3	27.0	30.5
Total expenses										261.0	774.0	725.4
Net (loss) income from discontinued operations										(7.4)	108.2	332.4
Income tax benefit										3.1	(27.1)	(70.4)
Net (loss) income from discontinued operations, net of tax										(4.3)	81.1	262.0
Gain (loss) from sale of other discontinued operations, net of tax										363.2		0.0
Total income from discontinued operations										358.9	81.1	262.0
OCI, Change in foreign currency transaction and translation adjustment, net of tax										32.0	(65.0)	(169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax										113.3
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										504.2	16.1	92.5
Tranzact
Expenses
Income tax benefit		(30.2)
Gain (loss) from sale of other discontinued operations, net of tax		$ 51.9								51.9
Tranzact \| Discontinued Operations
Expenses
Income tax benefit										(8.8)
Net (loss) income from discontinued operations, net of tax										6.1	(1.9)	0.4
Gain (loss) from sale of other discontinued operations, net of tax										51.9
Other Operations \| Discontinued Operations
Revenues
Earned insurance premiums										0.0	0.0	0.1
Net investment income										0.0	0.0	0.0
Net realized and unrealized investment gains										0.0	0.0	0.0
Other revenue										119.6	186.2	43.0
(Loss) revenues (including realized gains and losses)										119.6	186.2	43.1
Expenses
Loss and loss adjustment expenses										0.0	0.0	(0.7)
Insurance acquisition expenses										0.0	0.0	0.1
Other underwriting expenses										0.0	0.0	3.5
Interest expense on debt										3.2	4.0	1.4
General and administrative expenses										118.8	185.3	41.2
Total expenses										122.0	189.3	45.5
Net (loss) income from discontinued operations										(2.4)	(3.1)	(2.4)
Income tax benefit										6.4	0.7	1.0
Net (loss) income from discontinued operations, net of tax										4.0	(2.4)	(1.4)
Total income from discontinued operations										55.9	15.8	(3.0)
OCI, Change in foreign currency transaction and translation adjustment, net of tax											0.0	0.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										55.9	15.8	(3.0)
Fireman's Fund Insurance Company [Member] \| Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax												14.0
OneBeacon Runoff
Expenses
Gain (loss) from sale of other discontinued operations, net of tax									$ 36.1
OneBeacon Runoff \| Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax											0.3	(18.8)
Total income from discontinued operations											(0.5)
Esurance [Member]
Expenses
Gain (loss) from sale of other discontinued operations, net of tax										0.0	17.9	3.2
Esurance [Member] \| Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax										28.3	17.9	3.2
Other Segments
Revenues
Earned insurance premiums										7.5	8.7	6.1
Net investment income										27.2	8.8	9.0
Net realized and unrealized investment gains										(28.1)	259.9	29.8
Other revenue										143.0	147.2	81.7
(Loss) revenues (including realized gains and losses)										149.6	424.6	126.6
Expenses
Loss and loss adjustment expenses										8.0	8.2	8.9
Insurance acquisition expenses										2.2	3.4	0.8
Other underwriting expenses										0.1	0.0	0.0
Interest expense on debt										3.0	1.6	1.2
Total expenses										259.4	263.2	176.1
Symetra \| Other Segments
Revenues
Net realized and unrealized investment gains											258.8
MediaAlpha [Member] \| Other Segments
Revenues
Other revenue										116.5	105.5	65.3
Expenses
General and administrative expenses										$ 109.6	$ 99.0	$ 60.6

Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									$ 0.0	$ (2.3)	$ (0.7)
Net income attributable to White Mountains’s common shareholders	$ (33.3)	$ 91.7	$ 341.1	$ 13.0	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 412.5	$ 297.6	$ 312.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,014,900	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Basic earnings per share denominator									4,950,100	5,811,200	6,026,000
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									5,018,100	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Diluted earnings per share denominator(4)									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):	$ (7.30)	$ 18.84	$ 66.93	$ 2.34	$ 47.28	$ (10.01)	$ 0.72	$ 14.09	$ 82.24	$ 50.60	$ 51.13
Earnings Per Share, Diluted	$ (7.30)	$ 18.80	$ 66.79	$ 2.34	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 82.19	$ 50.60	$ 51.13
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total income from discontinued operations									$ 414.8	$ 96.9	$ 259.0
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									(5.4)	1.1	(3.3)
Net income attributable to White Mountains’s common shareholders									$ 409.4	$ 95.8	$ 255.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,014,900	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Basic earnings per share denominator									4,950,100	5,811,200	6,026,000
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									5,018,100	5,879,200	6,104,900
Average unvested restricted common shares									(64,800)	(68,000)	(78,900)
Diluted earnings per share denominator(4)									4,953,300	5,811,200	6,026,000
Basic earnings per share (in dollars):									$ 82.71	$ 16.48	$ 42.43
Earnings Per Share, Diluted									$ 82.66	$ 16.48	$ 42.43

Discontinued Operations Discontinued Operations (Earnings Per Share) (Footnotes) (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental common shares attributable to share-based payment arrangements	3,217
Employee Stock Option [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental common shares attributable to share-based payment arrangements	40,000	125,000

Discontinued Operations Discontinued Operations (Cash Flows Table) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Provided by (Used in) Operating Activities, Discontinued Operations	$ (24.7)	$ 56.0	$ 48.9
Cash Provided by (Used in) Investing Activities, Discontinued Operations	221.7	(5.4)	51.2
Cash Provided by (Used in) Financing Activities, Discontinued Operations	(0.2)	(11.6)	(64.5)
Net Cash Provided by (Used in) Discontinued Operations	196.8	34.5	21.3
Cash balance at beginning of year	167.2	232.7	233.5
Cash and Cash Equivalents, Period Increase (Decrease)	(17.7)	(66.6)	1.5
Cash balance at end of year	149.8	167.2	232.7
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Effect of Exchange Rate on Cash	0.0	(4.5)	(14.3)
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash balance at beginning of year	150.2	116.8	93.2
Cash and Cash Equivalents, Period Increase (Decrease)	(0.3)	(1.1)	2.3
Cash Divested from Deconsolidation	345.8
Cash balance at end of year	$ 0.9	$ 150.2	$ 116.8

Discontinued Operations Discontinued Operations (Fair Value) (Details) $ in Millions	Dec. 31, 2015 USD ($)
Sirius Group \| SIG Senior Notes \| Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 410.0
Sirius Group \| SIG Senior Notes \| Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	399.8
Sirius Group \| SIG Preference Shares \| Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	255.0
Sirius Group \| SIG Preference Shares \| Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	250.0
Tranzact \| Tranzact Bank Facility [Member] \| Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	102.8
Tranzact \| Tranzact Bank Facility [Member] \| Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 102.9

Subsequent Event (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Subsequent Event [Line Items]
Sales of common equity securities	$ 1,200.8	$ 461.4	$ 609.8
Common shares, par value per share (in dollars per share)	$ 1.00	$ 1.00

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
(Loss) revenues (including realized gains and losses)	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3	$ 1,360.7	$ 1,621.7	$ 1,411.1
Total expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3	1,401.1	1,464.4	1,440.5
Pre-tax (loss) income from continuing operations	(66.8)	20.0	(2.1)	8.5	246.3	(86.9)	(5.1)	3.0	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	23.0	6.7	6.0	9.7	1.0	1.6	2.2	(4.6)	45.4	0.2	14.8
Net income (loss) from continuing operations	(43.8)	26.7	3.9	18.2	247.3	(85.3)	(2.9)	(1.6)	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(6.8)	61.9	358.6	1.1	19.6	6.5	(0.5)	71.3	(0.3)	78.7	260.6
Non-controlling interest in consolidated subsidiaries	17.3	3.1	(21.4)	(6.3)	(6.1)	16.0	0.9	7.3	(7.3)	18.1	22.2
Equity in earnings of unconsolidated affiliates, net of tax	0.0	0.0	0.0	0.0	7.1	3.9	6.8	7.3	0.0	25.1	45.6
Net income attributable to White Mountains’s common shareholders	$ (33.3)	$ 91.7	$ 341.1	$ 13.0	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 412.5	$ 297.6	$ 312.2
Basic earnings per share
Continuing operations	$ (5.81)	$ 6.12	$ (3.44)	$ 2.51	$ 43.82	$ (11.10)	$ 0.80	$ 1.76	$ (0.47)	$ 34.12	$ 8.70
Discontinued operations	(1.49)	12.72	70.37	(0.17)	3.46	1.09	(0.08)	12.33	82.71	16.48	42.43
Total consolidated operations	(7.30)	18.84	66.93	2.34	47.28	(10.01)	0.72	14.09	82.24	50.60	51.13
Diluted earnings per share
Continuing operations	(5.81)	6.11	(3.43)	2.51	43.79	(11.10)	0.80	1.76	(0.47)	34.12	8.70
Discontinued operations	(1.49)	12.69	70.22	(0.17)	3.46	1.09	(0.08)	12.33	82.66	16.48	42.43
Total consolidated operations	$ (7.30)	$ 18.80	$ 66.79	$ 2.34	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 82.19	$ 50.60	$ 51.13

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Cost	5,308.8
Carrying Value	5,341.5
Fair Value	5,341.5
Fair value investments	5,039.7	4,050.9	$ 3,343.6
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	4,266.0
Carrying Value	4,256.8
Fair Value	4,256.8
US Government and agency obligations
Summary of investments other than investments in related parties
Cost	281.7
Carrying Value	278.3
Fair Value	278.3
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,512.6
Carrying Value	1,509.4
Fair Value	1,509.4
Municipal obligations
Summary of investments other than investments in related parties
Cost	308.8
Carrying Value	309.0
Fair Value	309.0
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	2,141.7
Carrying Value	2,132.9
Fair Value	2,132.9
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	12.9
Carrying Value	13.2
Fair Value	13.2
Preferred stocks
Summary of investments other than investments in related parties
Cost	8.3
Carrying Value	14.0
Fair Value	14.0
Short-term investments
Summary of investments other than investments in related parties
Cost	287.1
Carrying Value	287.1
Fair Value	287.1
Exchange Traded Funds [Member]
Summary of investments other than investments in related parties
Cost	316.4
Carrying Value	321.6
Fair Value	321.6
Common equity securities
Summary of investments other than investments in related parties
Cost	440.8
Carrying Value	474.3
Fair Value	474.3
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	11.2
Carrying Value	11.6
Fair Value	11.6
Public utilities
Summary of investments other than investments in related parties
Cost	0.0
Carrying Value	0.0
Fair Value	0.0
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	113.2
Carrying Value	141.1
Fair Value	141.1
Other long-term investments
Summary of investments other than investments in related parties
Cost	314.9
Carrying Value	323.3
Fair Value	323.3
Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	5,195.9	4,134.4
Fair value measured on a recurring basis \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	177.7	169.5
Level 3 \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	177.7	169.5	125.9
Level 3 \| Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	177.7	239.5
Level 3 \| Fair value measured on a recurring basis \| Forward Contracts [Member]
Summary of investments other than investments in related parties
Fair value investments	(1.2)
Level 3 \| Fair value measured on a recurring basis \| Other long-term investments
Summary of investments other than investments in related parties
Carrying value of investment accounted for using the equity method	3.5	3.8	5.2
Fair value investments	177.7	169.5
Affordable Housing Development Fund [Member] \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	12.3	14.7	16.8
Star & Shield Insurance Exchange [Member] \| Fixed maturity investments
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	6.6	9.5	$ 10.1
Star & Shield Insurance Exchange [Member] \| Short-term investments
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	$ 0.1	$ 0.1

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Other long-term investments	$ 323.3	$ 315.8
Assets
Cash Including Restricted Cash	149.8	173.0
Fixed maturity investments, at fair value	4,250.2	2,630.2
Short-term investments, at amortized cost	287.0	211.2
Other assets	274.6	273.2
Total assets	6,544.7	10,282.6
Liabilities
Debt	285.9	337.6
Other liabilities	199.9	250.8
Total liabilities	2,807.5	5,914.6
White Mountains’s common shareholders’ equity	3,737.2	4,368.0
Total liabilities and equity	6,544.7	10,282.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Other long-term investments	(1.2)
Assets
Cash Including Restricted Cash	3.1	0.3
Fixed maturity investments, at fair value	80.0	0.0
Short-term investments, at amortized cost	12.5	23.4
Other assets	1.4	6.3
Investments in consolidated and unconsolidated affiliates	4,898.2	4,350.9
Total assets	4,994.0	4,380.9
Liabilities
Debt	0.0	50.0
Payable to subsidiary (2)	1,387.2	400.0
Other liabilities	3.5	17.7
Total liabilities	1,390.7	467.7
White Mountains’s common shareholders’ equity	3,603.3	3,913.2
Total liabilities and equity	$ 4,994.0	$ 4,380.9

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	$ 5,039.7				$ 4,050.9				$ 5,039.7	$ 4,050.9	$ 3,343.6
(Loss) revenues (including realized gains and losses)	268.2	$ 365.0	$ 358.2	$ 369.3	602.6	$ 296.1	$ 368.7	$ 354.3	1,360.7	1,621.7	1,411.1
Expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3	1,401.1	1,464.4	1,440.5
Pre-tax loss	(66.8)	20.0	(2.1)	8.5	246.3	(86.9)	(5.1)	3.0	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	23.0	6.7	6.0	9.7	1.0	1.6	2.2	(4.6)	45.4	0.2	14.8
Net income (loss) from continuing operations	(43.8)	26.7	3.9	18.2	247.3	(85.3)	(2.9)	(1.6)	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(6.8)	61.9	358.6	1.1	19.6	6.5	(0.5)	71.3	(0.3)	78.7	260.6
Net income attributable to White Mountains’s common shareholders	(33.3)	91.7	341.1	13.0	267.9	(58.9)	4.3	84.3	412.5	297.6	312.2
Comprehensive income attributable to White Mountains’ common shareholders									557.8	197.2	210.6
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders	(33.3)	$ 91.7	$ 341.1	$ 13.0	267.9	$ (58.9)	$ 4.3	$ 84.3	412.5	297.6	312.2
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
(Loss) revenues (including realized gains and losses)									(1.0)	5.0	0.3
Expenses									68.2	59.9	32.3
Pre-tax loss									(69.2)	(54.9)	(32.0)
Income Tax Expense (Benefit)									(0.5)	0.0	0.9
Net income (loss) from continuing operations									(69.7)	(54.9)	(31.1)
Net (loss) income from discontinued operations, net of tax									0.0	0.0	13.9
Equity in earnings (losses) from consolidated and unconsolidated affiliates									482.2	352.5	329.4
Net income attributable to White Mountains’s common shareholders									412.5	297.6	312.2
Other comprehensive income (loss), after-tax									145.3	(100.4)	(101.6)
Comprehensive income attributable to White Mountains’ common shareholders									557.8	197.2	210.6
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									412.5	297.6	$ 312.2
Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	5,195.9				4,134.4				5,195.9	4,134.4
Level 3 \| Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	177.7				$ 239.5				177.7	$ 239.5
Level 3 \| Forward Contracts [Member] \| Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	$ (1.2)								$ (1.2)

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ (33,300,000)	$ 91,700,000	$ 341,100,000	$ 13,000,000	$ 267,900,000	$ (58,900,000)	$ 4,300,000	$ 84,300,000	$ 412,500,000	$ 297,600,000	$ 312,200,000
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(10,300,000)	(225,400,000)	(78,500,000)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	6,800,000	$ (61,900,000)	$ (358,600,000)	(1,100,000)	(19,600,000)	$ (6,500,000)	$ 500,000	(71,300,000)	300,000	(78,700,000)	(260,600,000)
Net change in other assets and liabilities, net									(226,700,000)	49,700,000	(142,400,000)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(130,600,000)	119,700,000	69,900,000
Net cash (used for) provided from discontinued operations									(24,700,000)	56,000,000	48,900,000
Net cash (used for) provided from operations									(155,300,000)	175,700,000	118,800,000
Cash flows from investing activities:
Net decrease in short-term investments									(70,100,000)	140,000,000	(138,000,000)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									1,055,100,000	16,700,000	(3,300,000)
Net cash provided from investing activities — discontinued operations									221,700,000	(5,400,000)	51,200,000
Net cash provided from investing activities									1,276,800,000	11,300,000	47,900,000
Cash flows from financing activities:
Draw down of debt and revolving line of credit									352,500,000	171,500,000	133,600,000
Repayment of debt and revolving line of credit									(404,600,000)	(76,100,000)	(65,200,000)
Proceeds from issuances of common shares									3,700,000	0	0
Repurchases and retirement of common shares									(881,300,000)	(268,600,000)	(128,200,000)
Dividends paid on common shares									(5,400,000)	(6,000,000)	(6,200,000)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(942,200,000)	(203,000,000)	(65,100,000)
Net cash used for financing activities — discontinued operations									(200,000)	(11,600,000)	(64,500,000)
Net cash used for financing activities									(942,400,000)	(214,600,000)	(129,600,000)
Net change in cash during the period - continuing operations									(17,700,000)	(66,600,000)	1,500,000
Cash balance at beginning of year				167,200,000				232,700,000	167,200,000	232,700,000	233,500,000
Cash balance at end of year	149,800,000				167,200,000				149,800,000	167,200,000	232,700,000
Interest Paid									(14,800,000)	(12,800,000)	(13,000,000)
OneBeacon Ltd. common shares repurchased and retired									(11,500,000)	(3,700,000)	(1,700,000)
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payable to subsidiary (2)									200,000.0	2,400,000.0	700,000.0
Transfer from Investments									80,000,000.0
Net income attributable to White Mountains’s common shareholders									412,500,000	297,600,000	312,200,000
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									1,100,000	0	(200,000)
Undistributed current earnings from subsidiaries									(482,200,000)	(352,500,000)	(329,400,000)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0	0	(13,900,000)
Other non-cash reconciling items including restricted share and option amortization									17,900,000	(400,000)	12,900,000
Net change in other assets and liabilities, net									(11,400,000)	21,400,000	(7,600,000)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(62,100,000)	(33,900,000)	(26,000,000)
Net cash (used for) provided from discontinued operations									0	0	13,900,000
Net cash (used for) provided from operations									(62,100,000)	(33,900,000)	(12,100,000)
Cash flows from investing activities:
Net decrease in short-term investments									10,900,000	7,600,000	(29,600,000)
Purchases of investment securities									0	0	(7,600,000)
Sales and maturities of investment securities									0	0	39,400,000
Issuance of debt (to) from subsidiaries									992,000,000	271,000,000	144,600,000
Repayment of debt (to) from subsidiaries									(5,000,000)	(35,000,000)	(30,000,000)
Receipt of cash flow from discontinued operations									0	0	13,900,000
Distributions from subsidiaries									0	15,000,000	29,700,000
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									997,900,000	258,600,000	160,400,000
Net cash provided from investing activities — discontinued operations									0	0	(13,900,000)
Net cash provided from investing activities									997,900,000	258,600,000	146,500,000
Cash flows from financing activities:
Draw down of debt and revolving line of credit									350,000,000	125,000,000	65,000,000
Repayment of debt and revolving line of credit									(400,000,000)	(75,000,000)	(65,000,000)
Proceeds from issuances of common shares									3,700,000	0	0
Repurchases and retirement of common shares									(881,300,000)	(268,600,000)	(128,200,000)
Dividends paid on common shares									(5,400,000)	(6,000,000)	(6,200,000)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(933,000,000)	(224,600,000)	(134,400,000)
Net cash used for financing activities — discontinued operations									0	0	0
Net cash used for financing activities									(933,000,000)	(225,000,000)	(134,400,000)
Net change in cash during the period - continuing operations									2,800,000	100,000	0
Cash balance at beginning of year				$ 300,000				$ 200,000	300,000	200,000	200,000
Cash balance at end of year	$ 3,100,000				$ 300,000				3,100,000	300,000	200,000
Proceeds from Dividends										15,000,000	29,700,000
Interest Paid									$ (1,200,000)	$ 0	$ (300,000)

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 86.8	$ 60.8	$ 59.5
OneBeacon
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	96.3	100.7	103.2
Future policy benefits, losses, claims and loss expenses	1,365.6	1,389.8	1,342.2
Unearned premiums	575.1	560.3	588.3
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	1,100.6	1,176.2	1,177.1
Net investment income	50.6	45.9	43.4
Benefits, claims, losses, and settlement expenses	656.0	700.7	815.1
Amortization of deferred policy acquisition costs	206.0	213.8	203.3
Other operating expenses	209.0	218.2	179.2
Premiums written	1,100.7	1,136.6	1,216.9
Net investment income	50.6	45.9	43.4
HG Global-BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	10.6	6.9	4.0
Future policy benefits, losses, claims and loss expenses	0.0	0.0	0.0
Unearned premiums	82.9	50.2	27.6
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	5.9	3.3	1.8
Net investment income	9.0	6.1	7.1
Benefits, claims, losses, and settlement expenses	0.0	0.0	0.0
Amortization of deferred policy acquisition costs	3.4	2.9	2.1
Other operating expenses	0.4	0.4	0.4
Premiums written	38.6	25.9	16.2
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0.0	0.0	0.0
Future policy benefits, losses, claims and loss expenses	5.0	6.0	7.8
Unearned premiums	1.2	2.1	0.8
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	7.5	8.7	6.1
Net investment income	0.2	0.2	0.2
Benefits, claims, losses, and settlement expenses	8.0	8.2	8.9
Amortization of deferred policy acquisition costs	2.2	3.4	0.8
Other operating expenses	0.1	0.0	0.0
Premiums written	6.5	10.1	5.9
Non-insurance [Member] \| Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 27.0	$ 8.6	$ 8.8

SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Premiums earned
Gross amount	$ 1,198.1	$ 1,322.0	$ 1,233.5
Ceded to other companies	(113.5)	(179.7)	(119.4)
Assumed from other companies	29.4	45.9	70.9
Net earned premiums	1,114.0	1,188.2	1,185.0
OneBeacon
Premiums earned
Gross amount	1,177.0	1,298.0	1,209.1
Ceded to other companies	(105.8)	(167.7)	(102.9)
Assumed from other companies	29.4	45.9	70.9
Net earned premiums	$ 1,100.6	$ 1,176.2	$ 1,177.1
Percentage of amount assumed to net	2.70%	3.90%	6.00%
HG/BAM
Premiums earned
Gross amount	$ 5.9	$ 3.3	$ 1.8
Ceded to other companies	0.0	0.0	0.0
Assumed from other companies	0.0	0.0	0.0
Net earned premiums	$ 5.9	$ 3.3	$ 1.8
Percentage of amount assumed to net	0.00%	0.00%	0.00%
Other Segments
Premiums earned
Gross amount	$ 15.2	$ 20.7	$ 22.6
Ceded to other companies	(7.7)	(12.0)	(16.5)
Assumed from other companies	0.0	0.0	0.0
Net earned premiums	$ 7.5	$ 8.7	$ 6.1
Percentage of amount assumed to net	0.00%	0.00%	0.00%

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance recoverable on paid losses: Allowance for reinsurance balances
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	$ 0.7	$ 2.2	$ 14.3
Charged to costs and expenses	(0.2)	0.1	0.5
Charged to other accounts (1)	0.0	0.0	0.0
Deductions described	0.0	(1.4)	(11.6)
Balance at end of period	0.9	0.7	2.2
Property and casualty insurance and reinsurance premiums receivable: Allowance for uncollectible accounts
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	2.0	1.9	3.1
Charged to costs and expenses	0.3	(0.8)	(0.2)
Charged to other accounts (1)	(0.4)	(0.7)	(1.0)
Deductions described	0.0	0.0	0.0
Balance at end of period	$ 1.3	$ 2.0	$ 1.9

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Accident Year	$ 649.2	$ 712.9	$ 732.0
Prior Accident Year	$ 14.8	$ (4.0)	$ 92.0
Rate at which reserves are discounted (as a percent)	2.50%	2.50%	2.50%
OneBeacon
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 96.3	$ 100.7	$ 103.2
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,365.6	1,389.8	1,342.2
Discount, if any, deducted in Column C	1.6	1.1	1.0
Unearned Premiums	575.1	560.3	588.3
Earned Premiums	1,100.6	1,176.2	1,177.1
Net investment income	50.6	45.9	43.4
Current Accident Year	640.6	702.5	725.3
Prior Accident Year	15.4	(1.8)	89.8
Amortization of deferred policy acquisition	(206.0)	(213.8)	(203.3)
Paid Claims and Claims Adjustment Expenses	667.1	677.5	608.6
Premiums written	1,100.7	1,136.6	1,216.9
Other Operations
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	0.0	0.0	0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses	5.0	6.0	7.8
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	1.2	2.1	0.8
Earned Premiums	7.5	8.7	6.1
Net investment income	0.2	0.2	0.2
Current Accident Year	8.6	10.4	6.7
Prior Accident Year	(0.6)	(2.2)	2.2
Amortization of deferred policy acquisition	0.0	0.0	0.0
Paid Claims and Claims Adjustment Expenses	8.8	10.4	14.8
Premiums written	$ 6.5	$ 10.1	$ 5.9