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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 26, 2018	Jun. 30, 2017
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, 2017
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			$ 3,767,702,603
Entity Common Stock, Shares Outstanding		3,742,408
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed maturity investments, at fair value		$ 2,081.1
Common equity securities, at fair value		0.0
Total investments	$ 3,380.7	2,714.4
Goodwill and intangible assets	62.1	45.2
Total assets	3,659.2	6,520.2
Liabilities
Debt	23.8	12.7
Liabilities held for sale		2,569.3
Total liabilities	298.4	2,804.2
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 3,750,171 and 4,563,814 shares	3.8	4.6
Paid-in surplus	666.8	806.1
Retained earnings	2,823.2	2,776.6
Accumulated other comprehensive loss, after-tax:
Net unrealized foreign currency translation losses	(1.3)	(1.4)
Accumulated other comprehensive loss from net change in benefit plan assets and obligations	0.0	(3.2)
Total White Mountains’s common shareholders’ equity	3,492.5	3,582.7
Non-controlling interests
Non-controlling interests	(131.7)	133.3
Total equity	3,360.8	3,716.0
Total liabilities and equity	3,659.2	6,520.2
HG Global BAM
Assets
Fixed maturity investments, at fair value	623.6	585.2
Short-term investments, at amortized cost (which approximates fair value)	69.8	44.5
Total investments	693.4	629.7
Cash	25.6	27.0
Insurance premiums receivable	4.5	1.6
Deferred acquisition costs	14.8	10.6
Accrued investment income	3.4	2.9
Accounts receivable on unsettled investment sales	0.1	0.0
Other assets	5.6	5.8
Total assets	747.4	677.6
Liabilities
Unearned insurance premiums	136.8	82.9
Accrued incentive compensation	18.2	16.6
Accounts payable on unsettled investment purchases	0.6	0.0
Other liabilities	11.4	9.4
Total liabilities	167.0	108.9
MediaAlpha
Assets
Cash	9.1	1.7
Goodwill and intangible assets	53.7	36.6
Accounts receivable from publishers and advertisers	32.4	18.4
Other assets	1.3	0.9
Total assets	96.5	57.6
Liabilities
Debt	23.8	12.7
Amounts due to publishers and advertisers	31.6	13.0
Accrued incentive compensation	2.0	0.0
Other liabilities	2.4	2.6
Total liabilities	59.8	28.3
Other
Assets
Fixed maturity investments, at fair value	1,506.1	1,495.9
Short-term investments, at amortized cost (which approximates fair value)	106.3	130.4
Common equity securities, at fair value	866.1	285.6
Other long-term investments	208.8	172.8
Total investments	2,687.3	2,084.7
Cash	62.4	51.5
Accrued investment income	13.9	11.9
Accounts receivable on unsettled investment sales	20.9	4.8
Goodwill and intangible assets	8.4	8.6
Other assets	19.1	17.1
Assets held for sale	3.3	3,606.4
Total assets	2,815.3	5,785.0
Liabilities
Accrued incentive compensation	60.6	79.1
Other liabilities	11.0	18.6
Liabilities held for sale	0.0	2,569.3
Total liabilities	$ 71.6	$ 2,667.0

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
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	3,750,171	4,563,814
Common shares, outstanding shares	3,750,171	4,563,814

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Earned insurance premiums	$ 10.4	$ 13.4	$ 12.0
Net investment income	56.0	32.1	10.9
Net realized and unrealized investment gains	133.3	(27.4)	260.5
Advertising & commission revenues	167.0	118.3	107.4
Other revenues	7.1	21.3	49.2
Total revenues	373.8	157.7	440.0
Expenses:
Loss and loss adjustment expenses	1.1	8.0	8.2
Insurance acquisition expenses	4.1	5.6	6.3
Cost of Sales	139.4	102.0	93.6
Other underwriting expenses	0.4	0.4	0.4
General and administrative expenses	218.7	186.0	200.7
Amortization of other intangible assets	10.7	10.5	8.4
Interest expense	2.3	3.0	1.6
Total expenses	366.0	305.0	310.8
Pre-tax (loss) income from continuing operations	7.8	(147.3)	129.2
Gain (loss) from sale of other discontinued operations, net of tax	557.0	415.1	18.2
Income tax benefit (expense)	7.8	32.9	(12.7)
Net income (loss) from continuing operations	15.6	(114.4)	116.5
Net (loss) income from discontinued operations, net of tax	20.5	108.3	116.9
Income before equity in earnings of unconsolidated affiliates	593.1	409.0	251.6
Equity in earnings of unconsolidated affiliates	0.0	0.0	25.1
Net income	593.1	409.0	276.7
Net (income) loss attributable to non-controlling interests	34.1	(7.2)	18.5
Net income attributable to White Mountains’s common shareholders	627.2	401.8	295.2
OneBeacon
Expenses:
Gain (loss) from sale of other discontinued operations, net of tax	554.5	0.0	0.0
Net (loss) income from discontinued operations, net of tax	20.5	108.6	38.2
Sale of Sirius Group
Expenses:
Gain (loss) from sale of other discontinued operations, net of tax	(0.7)	363.2	0.0
Tranzact
Expenses:
Gain (loss) from sale of other discontinued operations, net of tax	3.2	51.9	0.0
Esurance
Expenses:
Gain (loss) from sale of other discontinued operations, net of tax	0.0	0.0	17.9
Other Discontinued Operations
Expenses:
Gain (loss) from sale of other discontinued operations, net of tax	0.0	0.0	0.3
HG Global BAM
Revenues:
Earned insurance premiums	9.4	5.9	3.3
Net investment income	12.3	9.0	6.1
Net realized and unrealized investment gains	0.6	0.7	0.6
Other revenues	1.0	1.1	0.7
Total revenues	23.3	16.7	10.7
Expenses:
Insurance acquisition expenses	4.0	3.4	2.9
Other underwriting expenses	0.4	0.4	0.4
General and administrative expenses	42.9	39.6	36.8
Total expenses	47.3	43.4	40.1
MediaAlpha
Revenues:
Advertising & commission revenues	163.2	116.5	105.5
Total revenues	163.2	116.5	105.5
Expenses:
Cost of Sales	135.9	97.8	90.7
General and administrative expenses	16.2	11.8	8.3
Amortization of other intangible assets	10.5	10.1	8.1
Interest expense	1.0	0.9	0.4
Total expenses	163.6	120.6	107.5
Other
Revenues:
Earned insurance premiums	1.0	7.5	8.7
Net investment income	43.7	23.1	4.8
Net realized and unrealized investment gains	132.7	(28.1)	259.9
Advertising & commission revenues	3.8	1.8	1.9
Other revenues	6.1	20.2	48.5
Total revenues	187.3	24.5	323.8
Expenses:
Loss and loss adjustment expenses	1.1	8.0	8.2
Insurance acquisition expenses	0.1	2.2	3.4
Cost of Sales	3.5	4.2	2.9
General and administrative expenses	148.9	124.1	147.0
Amortization of other intangible assets	0.2	0.4	0.5
Interest expense	1.3	2.1	1.2
Total expenses	$ 155.1	$ 141.0	$ 163.2

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income attributable to White Mountains’s common shareholders (3)	$ 627.2	$ 401.8	$ 295.2
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax	0.3	(0.7)	(42.8)
Comprehensive income (loss) from discontinued operations, net of tax	3.2	146.3	(65.0)
Comprehensive income	630.7	547.4	187.4
Comprehensive (income) loss attributable to non-controlling interests	(0.2)	(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders	$ 630.5	$ 547.1	$ 187.4
Basic earnings per share
Continuing operations	$ 11.56	$ (24.26)	$ 27.22
Discontinued operations	134.50	104.37	22.98
Total consolidated operations	146.06	80.11	50.20
Diluted earnings per share
Continuing operations	11.56	(24.26)	27.22
Discontinued operations	134.50	104.32	22.98
Total consolidated operations	146.06	80.06	50.20
Dividends declared and paid per White Mountains’s common share	$ 1	$ 1	$ 1

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	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	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 Total equity	Prospector Turtle Fund Non-controlling interests	Prospector Turtle Fund Total equity	Sale of Sirius Group	Sale of Sirius Group Non-controlling interests	Sale of Sirius Group Total equity	Tranzact Non-controlling interests	Tranzact Total equity	OneBeacon Non-controlling interests	OneBeacon Total equity	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total equity	Star & Shield LLC Non-controlling interests	Star & Shield LLC Total equity
Beginning balance at Dec. 31, 2014		$ 1,034.7	$ 3,003.0	$ (42.1)	$ 3,995.6	$ 542.6	$ 4,538.2
Increase (Decrease) in Shareholders' Equity
Net income	$ 276.7		295.2		295.2	(18.5)	276.7
Net change in pension liability and other accumulated comprehensive items								$ (73.2)	$ (73.2)		$ (73.2)
Other comprehensive income (loss), net of tax	(42.8)											$ (34.6)	$ (34.6)	$ (34.6)
Comprehensive income (loss)	187.4		295.2	(107.8)	187.4	(18.5)	168.9
Dividends declared on common shares			(6.0)		(6.0)		(6.0)
Dividends/distributions to non-controlling interests						(51.1)	(51.1)
Issuances of common shares		0.9			0.9		0.9
Repurchases and retirements of common shares	(284.2)	(67.0)	(217.2)		(284.2)		(284.2)
Net contributions from non-controlling interests						17.5	17.5
Acquisition of noncontrolling interest		(5.3)			(5.3)	(2.7)	(8.0)
Redemptions of the Prospector Turtle Fund															$ (31.5)	$ (31.5)
Amortization of restricted share and option awards		14.9			14.9	(2.0)	12.9
Ending balance at Dec. 31, 2015		978.2	3,075.0	(149.9)	3,903.3	454.3	4,357.6
Increase (Decrease) in Shareholders' Equity
Net income	409.0		401.8		401.8	7.2	409.0
Net change in pension liability and other accumulated comprehensive items								31.4	31.4	$ 0.3	31.7
Other comprehensive income (loss), net of tax	(0.7)											$ 113.9	$ 113.9	$ 113.9
Recognition of benefit plan assets and obligations from sale of OneBeacon																	$ 113.3
Comprehensive income (loss)	547.4		401.8	145.3	547.1	7.5	554.6
Dividends declared on common shares			(5.4)		(5.4)		(5.4)
Dividends/distributions to non-controlling interests						(22.7)	(22.7)
Issuances of common shares		9.1			9.1		9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)		(887.2)		(887.2)
Net contributions from non-controlling interests						27.3	27.3
Acquisition of noncontrolling interest		(2.7)			(2.7)	(8.8)	(11.5)
Acquisition of subsidiary						3.3	3.3
Deconsolidation of non-controlling interests associated with sale																		$ (250.0)	$ (250.0)	$ (78.4)	$ (78.4)
Amortization of restricted share and option awards		18.5			18.5	0.8	19.3
Ending balance at Dec. 31, 2016	3,582.7	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Increase (Decrease) in Shareholders' Equity
Net income	593.1		627.2		627.2	(34.1)	593.1
Net change in pension liability and other accumulated comprehensive items								$ 0.4	$ 0.4	$ 0.1	$ 0.5
Other comprehensive income (loss), net of tax	0.3
Recognition of benefit plan assets and obligations from sale of OneBeacon																								$ 2.9	$ 2.9	$ 2.9
Comprehensive income (loss)	630.7		627.2	3.3	630.5	(34.0)	596.5
Dividends declared on common shares			(4.6)		(4.6)		(4.6)
Dividends/distributions to non-controlling interests						(19.3)	(19.3)
Issuances of common shares		1.7			1.7		1.7
Repurchases and retirements of common shares	(723.9)	(147.9)	(576.0)		(723.9)	(5.2)	(729.1)
Net contributions from non-controlling interests		(4.6)			(4.6)	30.2	25.6
Issuance of shares of non-controlling interests		(4.1)			(4.1)	5.2	1.1
Deconsolidation of non-controlling interests associated with sale																						$ (238.3)	$ (238.3)				$ (4.4)	$ (4.4)
Amortization of restricted share and option awards		14.8			14.8	0.8	15.6
Ending balance at Dec. 31, 2017	$ 3,492.5	$ 670.6	$ 2,823.2	$ (1.3)	$ 3,492.5	$ (131.7)	$ 3,360.8

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operations:
Net income	$ 593.1	$ 409.0	$ 276.7
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses	(133.3)	27.4	(260.5)
Net gain on sale of unconsolidated affiliates	0.0	0.0	(19.8)
Amortization of restricted share and option awards	14.8	18.5	14.9
Amortization and depreciation	22.4	20.4	3.8
Deferred income tax (benefit) expense	(11.4)	(12.5)	11.3
Undistributed equity in earnings from unconsolidated affiliates, after-tax	0.0	0.0	(25.1)
Net income from discontinued operations	(20.5)	(108.3)	(116.9)
Net gain on sale of discontinued operations	(557.0)	(415.1)	(18.2)
Other operating items:
Net change in unearned insurance premiums	(54.5)	(31.7)	(23.9)
Net change in deferred acquisition costs	(4.2)	(3.7)	(3.0)
Net change in restricted cash	0.0	5.8	17.9
Net change in other assets and liabilities, net	(20.8)	(146.1)	29.0
Net cash used for continuing operations	(62.4)	(172.9)	(66.0)
Net cash provided from discontinued operations (Note 19)	157.0	23.6	248.4
Net cash provided from (used for) operations	94.6	(149.3)	182.4
Cash flows from investing activities:
Net change in short-term investments	(1.7)	(27.2)	6.9
Sales of fixed maturity and convertible investments	2,124.4	2,605.8	518.2
Maturities, calls and paydowns of fixed maturity and convertible investments	213.4	253.4	72.6
Sales of common equity securities	424.1	815.9	115.8
Distributions and redemptions of other long-term investments	29.4	17.3	44.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	1,131.0	2,646.2	24.0
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries	0.0	(141.6)	0.0
Purchases of other long-term investments	(84.1)	(38.5)	(73.9)
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries	167.7	(402.0)	18.0
Purchases of common equity securities	(881.2)	(278.3)	(86.4)
Purchases of fixed maturity and convertible investments	(2,365.2)	(4,407.0)	(533.7)
Purchases of consolidated affiliates and other assets (net of cash acquired)	(27.6)	(13.4)	(2.6)
Other investing activities, net	(14.7)	4.8	8.8
Net cash provided from investing activities — continuing operations	715.5	1,035.4	111.8
Net cash provided from (used for) investing activities — discontinued operations (Note 19)	3.0	241.4	(100.5)
Net cash provided from investing activities	718.5	1,276.8	11.3
Cash flows from financing activities:
Draw down of debt and revolving line of credit	376.0	352.5	171.5
Repayment of debt and revolving line of credit	(365.0)	(404.6)	(76.1)
Cash dividends paid to the Company’s common shareholders	(4.6)	(5.4)	(6.0)
Acquisitions of additional shares from non-controlling interest	(0.7)	0.0	(9.1)
Distributions from discontinued operations	45.2	57.2	52.7
Common shares repurchased	(714.6)	(881.3)	(268.6)
Proceeds from issuances of common shares	0.0	3.7	0.0
Capital contributions from non-controlling interest shareholders	0.5	0.0	0.0
Distributions to non-controlling interest shareholders	(2.0)	(1.1)	(7.5)
Payments to contingent considerations related to purchases of consolidated subsidiaries	0.0	(7.8)	0.0
Capital contributions from BAM members	37.4	38.0	29.2
Other financing activities, net	(9.3)	(5.8)	(6.7)
Net cash used for financing activities — continuing operations	(637.1)	(854.6)	(120.6)
Net cash used for financing activities — discontinued operations (Note 19)	(61.9)	(93.8)	(100.7)
Net cash used for financing activities	(699.0)	(948.4)	(221.3)
Net change in cash during the period - continuing operations	16.0	7.9	(74.8)
Cash balance at beginning of year	80.2	72.0	145.7
Add: cash held for sale at the beginning of period	0.0	0.9	1.2
Less: cash held for sale at the end of period	0.9	1.2	2.3
Cash balance at end of year	$ 97.1	$ 80.2	$ 72.0

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Cash Flows [Abstract]
Restricted cash balances, beginning of period	$ 0.0	$ 5.8	$ 23.7
Restricted cash balances, end of period	0.0	0.0	5.8
Discontinued operations, beginning of period	70.5	245.4	203.8
Discontinued operations, end of period	$ 0.0	$ 70.5	$ 245.4

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Significant Accounting Policies Basis of Presentation 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. 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 other entities required to be consolidated under GAAP. Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 15 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Use of Estimates 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. Reportable Segments White Mountains has determined its reportable segments based on the nature of the of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, MediaAlpha and Other Operations. See Note 13 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal 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 up to 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, 2017 and 2016, 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, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The MediaAlpha segment consists of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a leading marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha’s media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 400 advertisers and 300 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). During 2017, White Mountains revised certain of its previously issued financial statements for amounts relating to Wobi. See Note 20 — “Financial Statement Revisions”. White Mountains’s Other Operations segment also included its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts fully matured on June 30, 2016 and was liquidated in the third quarter of 2017, and its U.S.-based service provider, White Mountains Financial Services LLC, which was liquidated in the second quarter of 2017 (collectively, “WM Life Re”). Discontinued Operations and Assets and Liabilities Held for Sale On September 28, 2017, Intact Financial Corporation completed its previously announced acquisition of OneBeacon Insurance Group, Ltd. (“OneBeacon”) in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income and their assets and liabilities as held for sale in the balance sheet for all periods prior to the completion of each transaction. On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. White Mountains was required to consolidate SSIE in its GAAP financial statements until White Mountains completed the sale. White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016. See Note 19 — “Held for Sale and Discontinued Operations”. As of December 31, 2017, White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale. The property has been measured at its estimated fair value net of costs of disposal, of $3.3 million. The fair value reflects a write-down of $3.7 million, which has been recorded within other expenses. Significant Accounting Policies 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 7 — “Derivatives”. Short-Term Investments 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 or accreted cost, which approximated fair value as of December 31, 2017 and 2016. Investment Securities As of December 31, 2017, 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, including interests in hedge funds, private equity funds, non-controlling interests in private capital investments and foreign currency forward contracts. 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. 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, 2017 and 2016, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 94% of the 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 include fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, mortgage and asset-backed securities, municipal obligations, and foreign government, agency and provincial obligations. 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, which 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. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain 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 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 follows: 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. 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. Other Long-Term Investments Other long-term investments consist primarily of hedge funds, private equity funds, non-controlling interests in private capital investments and foreign currency forward contracts. See Note 3 — “Investment Securities”. White Mountains has taken the fair value option for most of its other long-term investments. 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. 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 as well as 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 methods and inputs for each underlying investment, White Mountains considers the inputs 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 the fair value of a hedge fund or private equity fund differs from the NAV 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 investment in the hedge fund or private equity fund. As of December 31, 2017 and 2016, White Mountains did not adjust the reported NAV of its investments in hedge funds and private equity funds. Derivatives Financial Instruments: White Mountains holds from time to time a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either other assets or other liabilities, aside from the foreign currency forward contracts which are recognized within other long-term investments, 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. From time to time, White Mountains holds warrants that it has received in the restructuring of certain of its common equity securities and fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives. Variable Annuity Reinsurance: In 2016, White Mountains completed the run-off of WM Life Re as all of its contracts matured as of June 30, 2016. WM Life Re entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. The liability for guaranteed minimum death benefits was classified as a derivative and measured at fair value. The liability for guaranteed minimum accumulation benefits was classified as an insurance liability, and was measured using 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. 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. All WM Life Re’s derivative financial 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. Receivables Receivables consist primarily of premiums receivable from BAM customers for municipal bond insurance policies and advertising fees receivable from publishers and advertisers of MediaAlpha. Incentive Compensation 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 of White Mountains. Non-share based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares or performance units, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 10 — “Employee Share-Based Incentive Compensation”. 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, 2017, 2016 and 2015. See Note 4 — “Goodwill and Other Intangible Assets”. Municipal Bond Guarantee 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. Installment premiums are measured at the present value of contractual premiums, discounted at the risk free rate, which is set at the inception of the insurance contract. 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. BAM’s obligation for outstanding contracts consists of the unearned premium reserve and any loss reserves. Loss reserves are recorded only to the extent that the present value of the expected amount of any losses to be paid, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2017 and 2016, BAM did not have any loss or LAE reserves. Revenue Recognition White Mountains recognizes advertising and publishing fee revenues based on the contractual amount of the fees, adjusted for any amounts expected to be refunded or uncollectible, when it has satisfied its contractual performance obligations, which is generally at the time each transaction is executed. For transactions where MediaAlpha acts as the principal, such as the Buyer and Open exchanges, revenue amounts are reported gross. For transactions where MediaAlpha acts as an agent facilitating transactions between third parties, revenue amounts are reported at the net fee billed. Agent and commission revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations adjustments, and are recognized when contractual performance obligations have been fulfilled. Cost of Sales White Mountains’s cost of sales consists primarily of revenue sharing payments to publisher partners and traffic acquisition costs to top tier search engines. Cost of sales are measured based on contract terms and recognized when the related revenue transactions are executed. Other segment cost of sales consist of salaries and related expenses, professional services and marketing and advertising expenses directly related to sales generation. These expenses are recognized as incurred. Federal and Foreign Income Taxes A number 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 functional currency for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using functional currencies other than the U.S. dollar. 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. 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. As of December 31, 2017 and 2016, White Mountains had unrealized foreign currency translation losses of $1.3 million and $1.4 million recorded in accumulated other comprehensive income on its consolidated balance sheet. Non-controlling Interests 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 11 — “Common Shareholders’ Equity and Non-controlling Interests”. Recently Adopted Changes in Accounting Principles Stock Compensation Effective January 1, 2017, White Mountains adopted 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 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 upon adoption changes were recognized in the statement of cash flows with no other significant effect on the rest of the financial statements. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, 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. 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 statements. Recently Issued Accounting Pronouncements Revenue Recognition Effective January 1, 2018, White Mountains plans to adopt ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under the new guidance, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The new guidance does not apply to revenue from insurance contracts, investment income or investments gains and losses. The new guidance is applicable to some of White Mountains’s revenue streams, including agent commissions and other non-insurance revenues. White Mountains does not expect adoption to have any significant effect on its financial statements. Stock Compensation In May 2017, the FASB issued ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. The new guidance becomes effective for fiscal years beginning after December 15, 2017. White Mountains does not expect adoption to have any effect on its financial statements. Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. The new guidance is effective for annual periods beginning after December 15, 2018. White Mountains does not expect adoption to have any effect on its financial statements. Definition of a Business In January 2017, the FASB issued ASU 2017-07, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or a group of similar identifiable assets, it is not a business. The new guidance is effective for annual and interim periods after December 15, 2017. White Mountains does not expect adoption to have any effect on its financial statements. Goodwill In January 2017, the FASB issue ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. 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 does not expect adoption to have any effect on its financial statements. 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 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. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include loans and trade receivables such as premium receivables and reinsurance recoverables on paid losses. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and accordingly does not expect adoption to have any effect on its financial statements. 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 expected impact of this 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 and accordingly, does not expect adoption to have any effect on its financial statements.

Significant Transactions

12 Months Ended

Dec. 31, 2017

Significant Transactions [Abstract]

Significant Transactions

Dispositions

OneBeacon

On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a net gain of $554.6 million, net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 19 — “Held for Sale and Discontinued Operations”.

Star & Shield

On March 7, 2017, White Mountains completed its sale of Star & Shield and its investment in SSIE surplus notes to K2 Insurances LLC. White Mountains did not recognize any gain or loss on the sale. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains’s GAAP financial statements. See Note 19 — “Held for Sale and Discontinued Operations”.

Tranzact

On July 21, 2016, White Mountains completed the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million. 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 6 — “Income Taxes”. The increase to White Mountains’s book value from the sale of Tranzact was $82.1 million.

The following table presents a reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value:



Millions	Year ended December 31, 2016
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
Increase to White Mountains’s book value 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 19 — “Held for Sale and Discontinued Operations”.

During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of a change in state tax expense.

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 the third quarter of 2016, there was a final true-up to Sirius Group’s tangible common shareholder’s equity that resulted in a $4.0 million reduction to the gain. 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 from Sirius Group.

During 2017, White Mountains recorded a $0.7 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees.

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.

The transactions to purchase the shares of 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 within discontinued operations. See Note 19 — “Held for Sale and Discontinued Operations”.

Acquisitions

The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The 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 British Pound Sterling (“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 other intangible assets, and total liabilities assumed of $0.1 million, reflecting acquisition date fair values.

On August 1, 2017, White Mountains acquired 37,409 newly-issued preferred shares of Buzzmove for GBP 4.0 million (approximately $5.0 million based upon the foreign exchange spot rate at the date of acquisition) and 5,808 common shares from the company founders for GBP 0.5 million (approximately $0.7 million based upon the spot rate at the date of acquisition). White Mountains’s ownership share in Buzzmove as of December 31, 2017 was 77.1%.

MediaAlpha

On January 15, 2016, MediaAlpha acquired certain assets from Oversee.net for an aggregate 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.

On October 5, 2017, MediaAlpha acquired certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0 million. The majority of assets acquired, which are included in other intangible assets, consists of customer relationships, a non-compete agreement from the seller and domain names. See Note 4 — “Goodwill and Other Intangibles Assets”.

On October 5, 2017, White Mountains acquired 131,579 newly-issued Class A common units of MediaAlpha for $12.5 million. As of December 31, 2017 and 2016, White Mountains’s ownership share in MediaAlpha was 64.4% and 60.0%.

Investment Securities

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Investment Securities

Investments Securities

White Mountains’s portfolio of investment securities held for general investment purposes consists of 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, 2017 and 2016.

Other long-term investments consist primarily of hedge funds, private equity funds, non-controlling interests in private capital investments and foreign currency forward contracts.

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.

The following table presents pre-tax net investment income for 2017, 2016 and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Investment income:
Fixed maturity investments	$	44.9		$	28.5		$	8.9
Short-term investments	1.8			.9			—
Common equity securities	10.6			4.0			4.0
Other long-term investments	1.2			1.1			2.4
Total investment income	58.5			34.5			15.3
Third-party investment expenses	(2.5		)	(2.4		)	(4.4		)
Net investment income, pre-tax	$	56.0		$	32.1		$	10.9

Net Realized and Unrealized Investment Gains (Losses)

The following table presents net realized and unrealized investment gains (losses) for 2017, 2016, and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Net realized investment gains, pre-tax	$	24.1		$	270.0		$	30.0
Net unrealized investment gains (losses), pre-tax	109.2			(297.4		)	230.5
Net realized and unrealized investment gains (losses), pre-tax	133.3			(27.4		)	260.5
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(12.9		)	2.7			(47.7		)
Net realized and unrealized investment gains (losses), after-tax	$	120.4		$	(24.7	)	$	212.8

Net Realized Investment Gains (Losses)

The following tables present net realized investment gains (losses) for 2017, 2016 and 2015:



	Year Ended December 31, 2017
Millions	Net realized (losses) gains			Net foreign exchange gains (losses)			Total net realized gains (losses) reflected in earnings
Fixed maturity investments	$	(1.6	)	$	4.1		$	2.5
Short-term investments	(.3		)	—			(.3		)
Common equity securities	18.1			6.0			24.1
Other long-term investments	19.1			(21.3		)	(2.2		)
Net realized investment gains (losses), pre-tax	35.3			(11.2		)	24.1
Income tax expense attributable to net realized investment gains (losses)	(8.9		)	—			(8.9		)
Net realized investment gains (losses), after-tax	$	26.4		$	(11.2	)	$	15.2



	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	$	(1.9	)	$	.3	$	(1.6	)
Short-term investments	.4			—		.4
Common equity securities	268.5			—		268.5
Other long-term investments	2.7			—		2.7
Net realized investment gains, pre-tax	269.7			.3		270.0
Income tax expense attributable to net realized investment gains	(45.6		)	—		(45.6		)
Net realized investment gains, after-tax	$	224.1		$	.3	$	224.4



	Year Ended December 31, 2015
Millions	Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(.2	)	$	—	$	(.2	)
Common equity securities	31.0			—		31.0
Other long-term investments	(.8		)	—		(.8		)
Net realized investment gains, pre-tax	30.0			—		30.0
Income tax expense attributable to net realized investment gains	(6.4		)	—		(6.4		)
Net realized investment gains, after-tax	$	23.6		$	—	$	23.6

Net Unrealized Investment Gains (Losses)

The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the years ended 2017, 2016 and 2015:



	Year Ended December 31, 2017
Millions	Net unrealized gains (losses)			Net foreign exchange gains (losses)			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	13.8		$	12.7		$	26.5
Common equity securities	99.3			—			99.3
Other long-term investments	(15.6		)	(1.0		)	(16.6		)
Net unrealized investment gains, pre-tax	97.5			11.7			109.2
Income tax expense attributable to net unrealized investment gains	(4.0		)	—			(4.0		)
Net unrealized investment gains, after-tax	$	93.5		$	11.7		$	105.2



	Year Ended December 31, 2016
Millions	Net unrealized losses			Net foreign exchange gains (losses)			Total net unrealized losses reflected in earnings
Fixed maturity investments	$	(14.6	)	$	2.1		$	(12.5	)
Common equity securities	(257.4		)	(3.3		)	(260.7		)
Other long-term investments	(22.7		)	(1.5		)	(24.2		)
Net unrealized investment losses, pre-tax	(294.7		)	(2.7		)	(297.4		)
Income tax benefit attributable to net unrealized investment losses	48.3			—			48.3
Net unrealized investment losses, after-tax	$	(246.4	)	$	(2.7	)	$	(249.1	)



	Year Ended December 31, 2015
Millions	Net unrealized gains (losses)			Net foreign exchange losses			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	.1		$	—		$	.1
Common equity securities	240.8			(3.9		)	236.9
Other long-term investments	(5.4		)	(1.1		)	(6.5		)
Net unrealized investment gains (losses), pre-tax	235.5			(5.0		)	230.5
Income tax expense attributable to net unrealized investment gains	(41.3		)	—			(41.3		)
Net unrealized investment gains (losses), after-tax	$	194.2		$	(5.0	)	$	189.2

White Mountains recognized gross realized investment gains of $61.5 million, $283.7 million and $48.7 million and gross realized investment losses of $37.4 million, $13.7 million and $18.6 million on sales of investment securities during 2017, 2016 and 2015.

The following table presents total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Fixed maturity investments	$	—		$	.1		$	—
Common equity securities	—			—			(9.0		)
Other long-term investments	(15.4		)	(14.3		)	0.9
Total net unrealized investment losses, pre-tax - Level 3 investments	$	(15.4	)	$	(14.2	)	$	(8.1	)

The following table presents 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:



	Year Ended December 31,
Millions	2017		2016			2015
Net change in pre-tax unrealized investment losses on investments in unconsolidated affiliates	$	—	$	—		$	(39.2	)
Income tax benefit	—		—			2.9
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	—		—			(36.3		)
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	—		—			1.4
Total investment losses through accumulated other comprehensive income	—		—			(34.9		)
Net realized and unrealized investment gains (losses), after-tax	120.4		(24.7		)	212.8
Total investment gains (losses) recorded during the period, after-tax	$	120.4	$	(24.7	)	$	177.9

Investment Holdings

The following tables present 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, 2017 and 2016.



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	297.8	$	—	$	(1.3	)	$	—	$	296.5
Debt securities issued by corporations	867.6		2.9		(4.3		)	14.7		880.9
Mortgage and asset-backed securities	697.2		1.6		(4.1		)	—		694.7
Municipal obligations	252.0		3.7		(.8		)	—		254.9
Foreign government, agency and provincial obligations	2.6		—		—			.1		2.7
Total fixed maturity investments	$	2,117.2	$	8.2	$	(10.5	)	$	14.8	$	2,129.7



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	112.1	$	—	$	(1.1	)	$	—	$	111.0
Debt securities issued by corporations	752.0		2.3		(10.1		)	2.1		746.3
Mortgage and asset-backed securities	986.9		.8		(7.9		)	—		979.8
Municipal obligations	238.7		1.1		(1.3		)	—		238.5
Foreign government, agency and provincial obligations	12.0		.1		—			—		12.1
Total fixed maturity investments	$	2,101.7	$	4.3	$	(20.4	)	$	2.1	$	2,087.7
Less: Fixed maturity investments reclassified to assets held for sale related to SSIE										6.6
Total fixed maturity investments										$	2,081.1

The weighted average duration of White Mountains’s fixed income portfolio was approximately 3.4 years when including short-term investments and approximately 3.7 years when excluding short-term investments as of December 31, 2017.

The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2017. 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, 2017
Millions	Cost or amortized cost		Carrying value
Due in one year or less	$	109.0	$	108.8
Due after one year through five years	663.0		660.9
Due after five years through ten years	464.9		472.0
Due after ten years	183.1		193.3
Mortgage and asset-backed securities	697.2		694.7
Total	$	2,117.2	$	2,129.7

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of December 31, 2017 and 2016:



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	739.7	$	129.4	$	(3.0	)	$	—		$	866.1
Other long-term investments	$	246.6	$	6.8	$	(39.7	)	$	(4.9	)	$	208.8



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	258.6	$	29.0	$	(2.0	)	$	—		$	285.6
Other long-term investments	$	194.0	$	7.9	$	(25.2	)	$	(3.9	)	$	172.8

Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $2.8 billion, $3.7 billion and $0.8 billion for the years ended December 31, 2017, 2016 and 2015.

Investments Held on Deposit or as Collateral

As of December 31, 2017 and 2016, investments of $204.6 million and $161.7 million, were held in trusts required to be maintained in relation to HG Global’s reinsurance agreements with BAM. 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 represent BAM’s state deposits and are included within the investment portfolio, totaled $6.0 million as of December 31, 2017 and 2016.

Fair Value Measurements as of December 31, 2017

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, 2017 and 2016, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 94% of the investment portfolio. See Note 1 — “Basis of Presentation and Significant Accounting Policies - Significant Accounting Policies - Investment Securities”.

Fair Value Measurements by Level

The following tables present White Mountains’s fair value measurements for investments as of December 31, 2017 and 2016 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 See Note 7 — “Derivatives — Variable Annuity Reinsurance”.



	December 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	296.5	$	296.5	$	—	$	—
Debt securities issued by corporations:
Consumer	185.1		—		185.1		—
Communications	127.8		—		127.8		—
Financials	114.8		—		114.8
Utilities	108.9		—		108.9		—
Materials	95.5		—		95.5		—
Health Care	94.3		—		94.3		—
Technology	80.5		—		80.5		—
Energy	48.1		—		48.1		—
Industrial	25.9		—		25.9		—
Total debt securities issued by corporations:	880.9		—		880.9		—

Mortgage and asset-backed securities	694.7		—		694.7		—
Municipal obligations	254.9		—		254.9		—
Foreign government, agency and provincial obligations	2.7		—		2.7		—
Total fixed maturity investments	2,129.7		296.5		1,833.2		—

Short-term investments ⁽¹⁾	176.1		151.0		25.1		—

Common equity securities:

Exchange traded funds ⁽²⁾	569.7		508.1		61.6		—
Health Care	17.1		17.1		—		—
Financials	16.3		16.3		—		—
Technology	15.1		15.1		—		—
Industrial	11.9		11.9		—		—
Communications	10.9		10.9		—		—
Consumer	10.7		10.7		—		—
Energy	3.8		3.8		—		—
Other ⁽³⁾	210.6		—		210.6		—
Total common equity securities	866.1		593.9		272.2		—
Other long-term investments ⁽⁴⁾⁽⁵⁾	87.2		—		—		87.2
Total investments	$	3,259.1	$	1,041.4	$	2,130.5	$	87.2

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ 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⁾Consists of two investments in unit trusts that primarily invest in international equities

⁽⁴⁾Excludes carrying value of $(3.7) related to foreign currency forward contracts.

⁽⁵⁾ Excludes carrying value of $125.3 associated with a hedge fund and private equity funds for which fair value is measured at NAV using the practical

expedient.



	December 31, 2016
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	111.0	$	101.5	$	9.5	$	—
Debt securities issued by corporations:
Consumer	190.8		—		190.8		—
Utilities	140.8		—		140.8		—
Health Care	114.9		—		114.9		—
Financials	79.7		—		79.7		—
Communications	72.0		—		72.0		—
Materials	65.0		—		65.0		—
Technology	48.8		—		48.8		—
Industrial	28.2		—		28.2		—
Energy	6.1		—		6.1		—
Total debt securities issued by corporations:	746.3		—		746.3		—

Mortgage and asset-backed securities	979.8		—		979.8		—
Municipal obligations	238.5		—		238.5		—
Foreign government, agency and provincial obligations	12.1		—		12.1		—
Total fixed maturity investments ⁽¹⁾	2,087.7		101.5		1,986.2		—

Short-term investments ⁽¹⁾⁽²⁾	175.0		162.3		12.7		—

Common equity securities:
Exchange traded funds ⁽³⁾	157.2		129.4		27.8		—
Health Care	13.9		13.9		—		—
Consumer	8.6		8.6		—		—
Financials	7.7		7.7		—		—
Technology	7.3		7.3		—		—
Communications	7.0		7.0		—		—
Energy	2.5		2.5		—		—
Industrial	1.5		1.5		—		—
Other ⁽⁴⁾	79.9		—		79.9		—
Total common equity securities	285.6		177.9		107.7		—
Other long-term investments ⁽⁵⁾⁽⁶⁾	91.4		—		—		91.4
Total investments ⁽¹⁾	$	2,639.7	$	441.7	$	2,106.6	$	91.4

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

⁽³⁾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.

⁽⁴⁾Consists of an investment in a unit trust that primarily invests in international equities.

⁽⁵⁾ Excludes carrying value of $(1.2) related to foreign currency forward contracts.

⁽⁶⁾Excludes carrying value of $82.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

Debt Securities Issued by Corporations

The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2017 and 2016:



	Fair Value at December 31,
Millions	2017		2016
AAA	$	1.6	$	—
AA	42.6		37.3
A	192.5		212.8
BBB	465.2		335.6
BB	161.7		143.2
B	17.3		17.4
Debt securities issued by corporations ⁽¹⁾⁽²⁾	$	880.9	$	746.3


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

⁽²⁾Includes carrying value of $4.2 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE.

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 shelves 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 did not hold any RMBS categorized as sub-prime as of December 31, 2017.

White Mountains considers 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. As of December 31, 2017, White Mountains did not hold any RMBS classified as non-prime.

The following table presents the carrying value of White Mountains’s mortgage and asset-backed securities as of December 31, 2017 and 2016:



	December 31, 2017						December 31, 2016
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	46.3	$	46.3	$	—	$	70.3	$	70.3	$	—
FNMA	84.5		84.5		—		235.5		235.5		—
FHLMC	62.0		62.0		—		59.5		59.5		—
Total Agency ⁽¹⁾	192.8		192.8		—		365.3		365.3		—
Non-agency:
Residential	—		—		—		70.3		70.3		—
Commercial	70.5		70.5		—		3.9		3.9		—
Total Non-agency	70.5		70.5		—		74.2		74.2		—

Total mortgage-backed securities	263.3		263.3		—		439.5		439.5		—
Other asset-backed securities:
Credit card receivables	206.0		206.0		—		214.2		214.2		—
Vehicle receivables	142.4		142.4		—		205.9		205.9		—
Other	83.0		83.0		—		120.2		120.2		—
Total other asset-backed securities	431.4		431.4		—		540.3		540.3		—
Total mortgage and asset-backed securities	$	694.7	$	694.7	$	—	$	979.8	$	979.8	$	—


⁽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 Commercial Mortgage-backed Securities

White Mountains’s non-agency CMBS portfolio is generally moderate-term and structurally senior and benefits from more than 35 points of subordination on average for both fixed rate and floating rate securities as of December 31, 2017. In general, subordination represents the percentage principal loss on the underlying collateral that would have to be absorbed by other securities lower in the capital structure before the more senior security incurs a loss. As of December 31, 2017, none of the underlying loans of the non-agency CMBS held by White Mountains were reported as non-performing.

The following table presents the amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities as of December 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Fixed rate CMBS	$	35.1	$	—	$	27.5	$	7.6
Floating rate CMBS	35.4		—		—		35.4
Total	$	70.5	$	—	$	27.5	$	43.0


⁽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.

The following table presents the security issuance years of White Mountains’s investments in non-agency CMBS as of December 31, 2017:



			Security Issuance Year
Millions	Fair Value		2017		2016
Non-agency CMBS	$	70.5	$	67.0	$	3.5

Other Long-Term Investments

The following table presents the carrying values of White Mountains’s other long-term investments as of December 31, 2017 and 2016:



	Carrying Value at December 31,
Millions	2017			2016
Hedge funds and private equity funds, at fair value	$	125.3		$	82.6
Private equity securities, at fair value ^(1)(2)(3)	83.2			88.2
Foreign currency forward contracts	(3.7		)	(1.2		)
Other	4.0			3.2
Total other long-term investments	$	208.8		$	172.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾ Includes non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities.

⁽³⁾White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OneTitle”) and has provided a $10.0 million surplus note facility under which OneTitle’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may draw funds under certain circumstances. At December 31, 2017, no funds had been drawn on the surplus note facility.

Hedge Funds and Private Equity Funds

White Mountains invests in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments is generally estimated using the NAV of the funds. As of December 31, 2017, White Mountains held investments in one hedge fund and ten private equity funds. The largest investment in a single fund was $54.9 million as of December 31, 2017 and $21.5 million as of December 31, 2016.

The following table presents investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2017 and 2016:



	December 31, 2017				December 31, 2016
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	54.9	$	—	$	21.5	$	—
Long/short equity REIT	—		—		19.9		—
Total hedge funds	54.9		—		41.4		—

Private equity funds
Manufacturing/Industrial	43.3		10.4		19.4		22.9
Aerospace/Defense/Government	15.8		12.9		19.4		25.9
Direct lending	7.1		23.1		1.4		28.6
Financial Services	4.2		11.7		1.0		5.0
Insurance	—		41.2		—		41.2
Total private equity funds	70.4		99.3		41.2		123.6
Total hedge and private equity funds included in other long-term investments	$	125.3	$	99.3	$	82.6	$	123.6

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, 2017, White Mountains held one active hedge fund with a fair value of $54.9 million. The hedge fund is subject to a lock-up period that expires on September 1, 2018, with a semi-annual restriction on redemption frequency and an advance notice period requirement of not less than 45 days.

White Mountains redeemed its one investment in a long/short equity REIT hedge fund having a fair value of $20.8 million as of December 31, 2017. The bulk of the redemption proceeds were received early in the first quarter of 2018 with the balance expected in the second quarter of 2018.

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 its investors.

The following table presents investments in private equity funds that were subject to lock-up periods as of December 31, 2017:



Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private Equity Funds — expected lock-up period remaining	$	4.7	$	5.6	$	32.9	$	27.2	$	70.4

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, 2017 and 2016 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 present the changes in White Mountains’s fair value measurements by level for the years ended December 31, 2017 and 2016:



							Level 3 Investments						Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾			Total
Balance at January 1, 2017	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)
Net realized and unrealized gains (losses)	82.7			69.6			—			(15.3		)	20.4			157.4			⁽⁴⁾
Amortization/Accretion	—			(9.1		)	—			—			—			(9.1		)
Purchases	1,209.3			2,007.9			31.2			13.1			71.0			3,332.5
Sales	(681.1		)	(2,070.3		)	(12.5		)	(2.0		)	(48.7		)	(2,814.6		)
Deconsolidation of SSIE	—			(5.2		)	—			—			—			(5.2		)
Transfers in	—			18.7			—			—			—			18.7
Transfers out	—			—			(18.7		)	—			—			(18.7		)
Balance at December 31, 2017	$	890.4		$	2,105.4		$	—		$	87.2		$	125.3		$	3,208.3		⁽¹⁾⁽²⁾


⁽¹⁾	Excludes carrying value of $(3.7) and $(1.2) as of December 31, 2017 and January 1, 2017 associated with foreign currency forward contracts.


⁽²⁾	Excludes carrying value of $176.1 and $175.0 as of December 31, 2017 and January 1, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at January 1, 2017.


⁽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⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts and short-term investments of $23.8 and $0.3 for the year ended December 31, 2017.


⁽⁵⁾	Includes carrying value of $6.6 of fixed maturity investments at January 1, 2017 that is classified as assets held for sale related to SSIE.



							Level 3 Investments						Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾			Total
Balance at January 1, 2016	$	789.0		$	585.6		$	—		$	103.6		$	65.3		$	1,543.5		⁽¹⁾⁽²⁾
Net realized and unrealized gains (losses)	2.0			(8.0		)	.1			(14.3		)	(6.0		)	(26.2		)	⁽⁴⁾
Amortization/Accretion	.1			(6.2		)	—			—			—			(6.1		)
Purchases	1,746.9			2,868.8			69.9			2.2			40.5			4,728.3
Sales	(2,258.5		)	(1,416.4		)	—			(.1		)	(17.2		)	(3,692.2		)
Transfers in	—			70.0			—			—			—			70.0
Transfers out	—			—			(70.0		)	—			—			(70.0		)
Balance at December 31, 2016	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)


⁽¹⁾	Excludes carrying value of $175.0 and $142.0 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.

⁽²⁾ Includes carrying value of $9.5 and $6.6 of fixed maturity investments at January 1, 2016 and December 31, 2016 that is classified as assets held for sale related to SSIE.


⁽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⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts of $1.2 for the year ended December 31, 2016.


⁽⁵⁾	Excludes carrying value of $(1.2) as of December 31, 2016 associated with foreign currency forward contracts.

Fair Value Measurements — Transfers Between Levels - For Years Ended December 31, 2017 and 2016

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 2017, three fixed maturity investments classified as Level 3 measurements in the prior period 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, 2017. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $18.7 million for the period ended December 31, 2017.

During 2016, three fixed maturity investments classified as Level 3 measurements in the prior period were transferred to Level 2 measurements. These investments comprise the “Transfers out” of Level 3 and “Transfers in” to Level 2 of $70.0 million for the period ended December 31, 2016.

Significant Unobservable Inputs

The following tables present 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, 2017 and 2016. The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds.



$ in millions, except share price	December 31, 2017
Description	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	Implied share price	-	$.68
Private equity security	Share price of most recent transaction	$3.6	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.3	Discount rate		21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%

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



$ in millions, except share price	December 31, 2016
Description	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	Implied share price	-	$.68
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
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%

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

Goodwill and Other Intangible Assets

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Other Intangible Assets

White Mountains has recognized goodwill and other intangible assets at the acquisition date fair values in connection with its purchases of subsidiaries.

The following table presents the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company:



$ in millions	Weighted Average Economic life (in years)	December 31, 2017						December 31, 2016
$ 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
Buzzmove	N/A	7.6		—		7.6		7.6		—		7.6
Total goodwill		25.9		—		25.9		25.9		—		25.9

Other intangible assets:
MediaAlpha
Customer relationships	8	36.6		10.2		26.4		10.0		6.2		3.8
Information technology	5	33.3		24.3		9.0		32.4		17.9		14.5
Subtotal		69.9		34.5		35.4		42.4		24.1		18.3

Buzzmove
Trademark	7	.6		.1		.5		.6		.1		.5
Information technology	5	.5		.2		.3		.5		—		.5
Subtotal		1.1		.3		.8		1.1		.1		1.0

Total other intangible assets		71.0		34.8		36.2		43.5		24.2		19.3
Total goodwill and other intangible assets		$	96.9	$	34.8	$	62.1	$	69.4	$	24.2	$	45.2

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 other intangible asset over its remaining life. This approach was used to estimate the fair value of other 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 other 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 other intangible asset that provides an incremental benefit. Under this method, the fair value of the other intangible asset is calculated by comparing the value of the entity with and without the other intangible asset. This approach was used to estimate the fair value of favorable lease terms.

The following table presents goodwill and other intangible assets as of December 31, 2017 and December 31, 2016:



Millions	December 31, 2017			December 31, 2016
Goodwill:
MediaAlpha	$	18.3		$	18.3
Buzzmove	7.6			7.6
Total goodwill	25.9			25.9

Other intangible assets:
MediaAlpha	35.4			18.3
Buzzmove	.8			1.0
Total other intangible assets	36.2			19.3
Total goodwill and other intangible assets	62.1			45.2
Goodwill and other intangible assets held for sale	—			1.2
Goodwill and other intangible assets attributed to non-controlling interests	(21.1		)	(17.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders' equity	$	41.0		$	29.3

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



	December 31,
	2017					2016
Millions	Goodwill		Other intangible assets			Goodwill			Other intangible assets
Beginning balance	$	25.9	$	19.3		$	18.6		$	26.9
Add: Star & Shield amounts held for sale at beginning of the period ⁽¹⁾	—		—			—			.4
Acquisitions of businesses and asset groups ⁽²⁾			27.6			7.6			5.0
Wobi write-off	—		—			(.3		)	(2.5		)
Amortization, including foreign currency translation	—		(10.7		)	—			(10.5		)
Ending balance	$	25.9	$	36.2		$	25.9		$	19.3

⁽¹⁾See Note 19 — “Held for Sale and Discontinued Operations”.

⁽²⁾During 2017, amounts include certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0. See Note 2 — “Significant Transactions”.

Amortization expense was $10.7 million, $10.5 million and $8.4 million for the years ended December 31, 2017, 2016 and 2015.

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



Millions	Amortization expense
2018	$	10.2
2019	5.0
2020	3.2
2021	2.9
2022	2.8
Total	$	24.1

- 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

Debt

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Debt

The following table presents White Mountains’s debt outstanding as of December 31, 2017 and 2016:



	December 31,			Effective	December 31,			Effective
Millions	2017			Rate ⁽¹⁾	2016			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
Unamortized issue costs	—				—
WTM Bank Facility, carrying value	—				—
MediaAlpha Bank Facility	23.9			5.6%	—			N/A
Unamortized issuance cost	(.1		)		—
MediaAlpha Bank Facility, carrying value	23.8				—
Previous MediaAlpha Bank Facility	—			N/A	12.9			5.7%
Unamortized issuance cost	—				(.2		)
Previous MediaAlpha Bank Facility, carrying value	—				12.7
Total debt	$	23.8			$	12.7

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

The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2017:



Millions	December 31, 2017
Due in one year or less	$	3.1
Due in two to three years	12.1
Due in four to five years	8.7
Due after five years	—
Total	$	23.9

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”). On September 15, 2017, White Mountains borrowed a total of $350.0 million under the WTM Bank Facility, at a blended interest rate of 4.45%, to partially fund a self-tender offer and subsequently repaid the $350.0 million, after receiving the proceeds from the OneBeacon Transaction on September 28, 2017. 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%. As of December 31, 2017, the WTM Bank Facility was undrawn.

White Mountains recorded $0.6 million, $1.2 million, $0.1 million of interest expense on the WTM Bank Facility for the years ended December 31, 2017, 2016 and 2015.

The WTM 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.

MediaAlpha Bank Facility

On May 12, 2017, MediaAlpha entered into a secured credit facility (the “MediaAlpha Bank Facility”) with Western Alliance Bank, which had a total commitment of $20.0 million and had a maturity date of May 12, 2020. On October 5, 2017, MediaAlpha refinanced the MediaAlpha Bank Facility in order to fund the acquisition of certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com. The total commitment of the MediaAlpha Bank Facility was increased to $28.4 million and has a maturity date of October 6, 2020. The MediaAlpha Bank Facility consists of a $18.4 million term loan facility, which has an outstanding balance of $17.9 million as of December 31, 2017, and a revolving loan facility for $10.0 million, which has an outstanding balance of $6.0 million as of December 31, 2017.

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% on the term loan facility and 0.25% on the revolving credit facility as of December 31, 2017.

The MediaAlpha Bank Facility replaced MediaAlpha’s previous credit facility (the “Previous MediaAlpha Bank Facility”) with Opus Bank, which had a total commitment of $20.0 million.

In 2017, under the MediaAlpha Bank Facility, MediaAlpha borrowed $20.0 million and repaid $2.1 million on the term loan and borrowed $6.0 million on the revolving loan. In 2017, under the Previous MediaAlpha Bank Facility, MediaAlpha repaid $12.9 million.

White Mountains recorded $1.0 million, $0.9 million, $0.4 million of interest expense on the MediaAlpha Bank Facility and the Previous MediaAlpha Bank Facility for the years ended December 31, 2017, 2016 and 2015.

The MediaAlpha Bank Facility is secured by 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 fixed charge coverage ratio and an asset coverage ratio.

Debt Covenants

As of December 31, 2017, 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 $2.3 million, $3.0 million and $1.6 million in 2017, 2016 and 2015. Total interest paid by White Mountains for its indebtedness was $1.4 million, $2.1 million, and $0.1 million in 2017, 2016 and 2015.

Derivatives

12 Months Ended

Dec. 31, 2017

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. WM Life Re was liquidated in the third quarter of 2017.

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



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

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



	Gains (Losses)						Carrying Value as of
	Year Ended December 31,						December 31,
Millions	2016			2015			2016
Fixed income/interest rate	$	1.8		$	6.4		$	—
Foreign exchange	(4.8		)	(7.3		)	—
Equity	1.0			(7.9		)	—
Total	$	(2.0	)	$	(8.8	)	$	—

The following tables present the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the years ended December 31, 2016 and 2015:



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


⁽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.



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


⁽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 $5.8 as of December 31, 2015 posted as collateral to its reinsurance counterparties.

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. 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 policyholder behavior and risk margins, were generally the same as those used to estimate the fair value of variable annuity liabilities.

Forward Contracts

Derivative [Line Items]

Derivatives

Forward Contracts

White Mountains’s investment portfolio includes certain investment grade fixed maturity investments denominated in GBP and common equity securities denominated in Japanese Yen, Euros, GBP and other foreign currencies. White Mountains entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. The foreign currency forward contracts do not meet the criteria to be accounted for as a hedge. White Mountains actively manages its net foreign currency exposure and adjusts its foreign currency positions within ranges established by senior management. Mismatches between currency driven movements in foreign denominated investments and foreign currency forward contracts may result in net foreign currency positions being outside pre-defined ranges and/or may result in net foreign currency gains (losses). During the fourth quarter of 2017, White Mountains closed the foreign currency forward contracts associated with its common equity securities. As of December 31, 2017, White Mountains held $206.3 million (GBP 152.0 million) total gross notional value of a foreign currency forward contract.

White Mountains’s foreign currency forward contract is traded over-the-counter. The fair value of the foreign currency forward contract is estimated using OTC quotes for similar instruments and accordingly, the measurements are classified as Level 2 measurements as of December 31, 2017.

The derivative gains (losses) recognized in net realized and unrealized investment gains (losses) for the years ended December 31, 2017 and 2016 were $(23.8) million and $(1.2) million. White Mountains’s foreign currency forward contract is subject to a master netting agreement. As of December 31, 2017 and 2016, the gross liability amount offset under the master netting agreement and the net amount recognized in other long-term investments was $(3.7) million and $(1.2) million.

White Mountains does not hold or provide any collateral under its foreign currency forward contract.

The following table presents the gross notional amounts and carrying values associated with the foreign currency forward contracts as of December 31, 2017 and 2016:



	December 31, 2017						December 31, 2016
Millions	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾
Barclays Bank PLC	$	206.3	$	(3.7	)	A	$	184.6	$	(1.2	)	A-

⁽¹⁾Standard & Poor’s ratings “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings) and “A-” (which is the seventh highest of twenty-three creditworthiness ratings.

Income Taxes

12 Months Ended

Dec. 31, 2017

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, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States.

The following table presents the total income tax benefit for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Current tax (expense) benefit:
U.S. federal	$	(.3	)	$	21.4		$	—
State	(1.3		)	(.7		)	(.6		)
Non-U.S.	(2.0		)	(.3		)	(.8		)
Total current tax (expense) benefit	(3.6		)	20.4			(1.4		)
Deferred tax benefit (expense):
U.S. federal	11.4			12.5			(11.3		)
Total deferred tax benefit (expense)	11.4			12.5			(11.3		)
Total income tax benefit (expense)	$	7.8		$	32.9		$	(12.7	)

Effective Rate Reconciliation

The following table presents 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:



	Year Ended December 31,
Millions	2017			2016			2015
Tax (expense) benefit at the U.S. statutory rate	$	(2.7	)	$	51.6		$	(45.2	)
Differences in taxes resulting from:
Tax rate changes	(44.3		)	(3.9		)	(.5		)
Change in valuation allowance	42.6			6.9			(21.8		)
Non-U.S. earnings, net of foreign taxes	21.5			(19.2		)	58.5
Officer compensation	(4.1		)	—			—
Member surplus contributions	(3.0		)	(2.3		)	(1.5		)
Withholding tax	(2.0		)	(.2		)	(.5		)
Tax exempt interest and dividends	.5			.1			—
Tax reserve adjustments	(.3		)	—			—
Other, net	(.4		)	(.1		)	(1.7		)
Total income tax benefit (expense) on pre-tax income (loss)	$	7.8		$	32.9		$	(12.7	)

The non-U.S. component of pre-tax income (loss) was $71.3 million, $(66.3) million and $149.5 million for the years ended December 31, 2017, 2016 and 2015.

Tax Payments and Receipts

Net income tax payments to national governments (primarily the United States) totaled $2.0 million, $0.3 million, and $0.7 million for the years ended December 31, 2017, 2016 and 2015.

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.

The following table presents an outline of the significant components of White Mountains’s U.S. federal, state (net of federal benefit) and non-U.S. deferred tax assets and liabilities:



	December 31,
Millions	2017		2016
Deferred income tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	73.0	$	104.3
Non-U.S. net operating loss carryforwards	33.9		34.0
Incentive compensation	20.4		26.5
Investment basis difference	4.9		5.6
Other items	4.9		8.5
Total gross deferred income tax assets	137.1		178.9
Less: valuation allowances	109.6		146.2
Total net deferred income tax assets	27.5		32.7
Deferred income tax liabilities related to:
Member surplus contributions	24.1		30.0
Other items	2.1		2.7
Total deferred income tax liabilities	26.2		32.7
Net deferred tax asset	$	1.3	$	—

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.

Tax Cuts and Jobs Act of 2017

On December 22, 2017, the Tax Cuts and Jobs Act of 2017 (the “TCJA”) was enacted. Substantially all of the provisions of the TCJA are effective for taxable years beginning after December 31, 2017. The TCJA includes significant changes to the Internal Revenue Code of 1986 (the “Code”), including amendments which significantly change the taxation of individuals and business entities. The more significant changes in the TCJA that impact White Mountains are reductions in the corporate federal income tax rate from 35% to 21% and several technical provisions including, among others, limiting the utilization of net operating losses arising after December 31, 2017 to 80% of taxable income with an indefinite carryforward.

The TCJA did not have a material impact on White Mountains’s financial statements in 2017 due to a full valuation allowance previously having been recorded against its U.S. deferred tax assets. Under U.S. GAAP, specifically ASC Topic 740, Income Taxes, the tax effects of changes in tax laws must be recognized in the period in which the law is enacted, or December 22, 2017 for the TCJA. ASC 740 also requires deferred tax assets and liabilities to be measured at the enacted tax rate expected to apply when temporary differences are to be realized or settled. Thus, at the date of enactment, White Mountains’s deferred taxes were re-measured based upon the new tax rate. For White Mountains, a change in deferred taxes was recorded as an adjustment to our deferred tax provision for $43.1 million of federal tax expense, which was offset by a change in the valuation allowance.

The staff of the U.S. Securities and Exchange Commission has recognized the complexity of reflecting the impacts of the TCJA and on December 22, 2017 issued guidance in Staff Accounting Bulletin 118 (“SAB 118”) which clarifies accounting for income taxes under ASC 740 if information is not yet available or complete and provides for up to a one-year period in which to complete the required analyses and accounting (the measurement period). SAB 118 describes three scenarios associated with a company’s status of accounting for income tax reform: (1) a company is complete with its accounting for certain effects of tax reform, (2) a company is able to determine a reasonable estimate for certain effects of tax reform and records that estimate as a provisional amount, or (3) a company is not able to determine a reasonable estimate and therefore continues to apply ASC 740, based on the provisions of the tax laws that were in effect immediately prior to the TCJA being enacted. White Mountains completed its accounting for the effects of the TCJA, which have been reflected in the December 31, 2017 financial statements.

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 $109.6 million valuation allowance as of December 31, 2017, $74.8 million relates to deferred tax assets on net operating losses in the United States and other federal and state deferred tax benefits, $20.3 million relates to deferred tax assets on net operating losses and net investment unrealized gains and losses in Luxembourg subsidiaries, $13.5 million relates to net operating losses and other deferred tax benefits in Israeli subsidiaries and $1.0 million relates to net operating losses in a U.K. subsidiary. Of the $146.2 million valuation allowance as of December 31, 2016, $112.5 million relates to deferred tax assets on net operating losses in the United States and other federal and state deferred tax benefits, $24.0 million relates to deferred tax assets on net operating losses in Luxembourg subsidiaries, $9.4 million relates to net operating losses in Israeli subsidiaries and $0.3 million relates to net operating losses in a U.K. subsidiary.

United States

During 2017 and 2016, White Mountains recorded tax (benefit) expense of $(21.5) million and $17.1 million to release or establish a valuation allowance against deferred tax assets of Guilford Holdings, Inc. and subsidiaries (“Guilford”). Guilford consists of MediaAlpha, various service companies and certain other investments that are included in the Other Operations segment. The TCJA reduced the corporate income tax from 35% to 21%, which reduced the deferred tax assets of Guilford by $20.4 million. White Mountains recorded a $20.4 million tax expense for the reduction, which was offset with a tax benefit due to the release in the valuation allowance against the deferred tax assets. During 2016, Guilford had income in discontinued operations that was 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 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 losses within continuing operations in 2016, White Mountains recorded a tax benefit of $21.4 million in continuing operations, with an offsetting tax expense in discontinued operations. During 2017 and 2016, Guilford continued to have a full valuation allowance recorded against its deferred tax assets as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

During 2017 and 2016, White Mountains recorded tax (benefit) expense of $(18.4) million and $3.6 million to release or

establish valuation allowances against deferred tax assets of BAM. The reduction in the corporate tax rate under the TCJA reduced the deferred tax assets of BAM by $22.7 million. White Mountains recorded a $22.7 million tax expense for the reduction, which was offset with a tax benefit due to the release in the valuation allowance against the deferred tax assets. Also during 2017 and 2016, BAM had income in other comprehensive income that was available to offset its loss from continuing operations. As a result, BAM recorded a tax benefit of $10.1 million and $11.0 million, in continuing operations, with an offsetting tax expense in paid-in surplus. During 2017 and 2016, BAM continued to have a full valuation allowance recorded against its deferred tax assets as White Mountains management is unsure it will generate sufficient taxable income to utilize the deferred tax assets.

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.

Non-U.S. Jurisdictions

During 2017, White Mountains recorded tax expense of $6.4 million for a reduction to deferred tax assets, and a corresponding reduction to the valuation allowance, which primarily related to the recapture of previously deducted losses due to the intra-group distribution of Guilford offset by the tax deduction for the write down on the investment in Wobi in Luxembourg-domiciled subsidiaries. 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 2017 and 2016, White Mountains recorded tax expense of $3.0 million and $3.4 million to establish a valuation allowance against deferred tax assets at certain Israel-domiciled subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets.

During 2017 and 2016, White Mountains recorded tax expense of $0.7 million and $0.1 million to establish a valuation allowance against deferred tax assets at its U.K. subsidiaries, as White Mountains management does not currently anticipate sufficient taxable income to utilize the deferred tax assets.

Net Operating Loss and Capital Loss Carryforwards

The following table presents net operating loss and capital loss carryforwards as of December 31, 2017, the expiration dates and the deferred tax assets thereon:



	December 31, 2017
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2018-2022	$	.3		$	—		$	—		$	—		$	.3
2023-2027	—			—			—			—			—
2028-2037	332.0			46.4			—			—			378.4
No expiration date	—			31.3			6.7			54.7			92.7
Total	$	332.3		$	77.7		$	6.7		$	54.7		$	471.4
Gross deferred tax asset	73.0			20.2			1.1			12.6			106.9
Valuation allowance	(73.0		)	(20.2		)	(1.1		)	(12.6		)	(106.9		)
Net deferred tax asset	$	—		$	—		$	—		$	—		$	—

Included in the U.S. net operating loss carryforwards are losses of $3.1 million subject to an annual limitation on utilization under Internal Revenue Code Section 382. These loss carryforwards will begin to expire in 2032. Also included in the U.S. net operating loss carryforwards are losses of $6.7 million due to additional deductions related to equity compensation. These loss carryforwards will begin to expire in 2032. As of December 31, 2017, there are U.S. alternative minimum tax credit carryforwards of $1.3 million, which are refundable under provisions of the TCJA.

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. There were no uncertain tax positions for the years ended December 31, 2016 and 2015.

The following table presents a reconciliation of the beginning and ending amount of unrecognized tax benefits for 2017:



Millions	Permanent Differences ⁽¹⁾		Temporary Differences ⁽²⁾		Interest and Penalties ⁽³⁾		Total
Balance at January 1, 2017	$	—	$	—	$	—	$	—
Changes in prior year tax positions	.1		—		—		.1
Tax positions taken during the current year	.2		—		—		.2
Balance at December 31, 2017	$	.3	$	—	$	—	$	.3


⁽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.

White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2017, 2016 and 2015, White Mountains did not recognize any net interest (income) expense. There was no accrued interest as of December 31, 2017 and December 31, 2016.

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 2013.

In the second quarter of 2016, White Mountains recorded an increase in deferred tax assets of $0.6 million and a corresponding increase in valuation allowance of $0.6 million related to the settlement of the IRS audit of Guilford for tax year 2012.

In the first quarter of 2018, the Israeli Tax Authority commenced an examination of the 2013 to 2016 income tax returns for Wobi. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.

Municipal Bond Guarantee Insurance

12 Months Ended

Dec. 31, 2017

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 the initial capitalization of BAM, a newly formed mutual municipal bond insurer. As of December 31, 2017, 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. At inception, BAM and HG Re also entered into a first loss reinsurance treaty (“FLRT”). HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. During 2017, HG Global and BAM made certain changes to the ceding commission arrangements under the FLRT. These changes serve to accelerate growth in BAM’s statutory capital but do not impact the net risk premium ceded from BAM to HG Re. 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.

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%, 3.78% and 4.60% for 2016, 2017 and 2018. 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%. No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the New York State Department of Financial Services (“NYDFS”). BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes only to the extent that its remaining qualified statutory capital and other capital resources continue to support its outstanding obligations, business plan and its AA stable rating from S&P. BAM repaid $4.0 million on the BAM Surplus Notes and $1.0 million on the accrued interest during the year ended December 31, 2017.

In order to further support BAM’s long-term capital position and business prospects, in 2017 HG Global agreed to contribute the $203.0 million of Series A BAM Surplus Notes (“Series A Notes”) into the supplemental collateral trust (the “Supplemental Trust”) at HG Re. The Supplemental Trust already held the $300.0 million of Series B BAM Surplus Notes (“Series B Notes”). Assets held in the Supplemental Trust serve to collateralize HG Re’s obligations to BAM under the FLRT. HG Global and BAM also changed the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The Supplemental Trust target balance is equal to approximately $603.0 million. As the BAM Surplus Notes are repaid over time, the BAM Surplus Notes will be replaced in the Supplemental Trust by cash and fixed income securities. The collateral trust balances must be at target levels before capital can be distributed out of the Supplemental Trust. In connection with the contribution, Series A Notes were merged with the Series B Notes.

Under GAAP, if the terms of a debt instrument are amended, unless there is a greater than 10% change in the expected discounted future cash flows of such instrument, a change in the instrument’s carrying value is not permitted. White Mountains has determined that the impact of the changes made during 2017 to the terms of the BAM Surplus Notes on the expected discounted future cash flows was not greater than 10%.

As of December 31, 2017 and 2016, the collateral trusts held assets of $715.1 million and $465.4 million, which included $499.0 million and $300.0 million of BAM Surplus Notes. As of December 31, 2017, HG Global has accrued $126.0 million of interest receivable on the BAM Surplus Notes.

The following table presents a schedule of BAM’s insured obligations as of December 31, 2017 and 2016:



	December 31, 2017		December 31, 2016
Contracts outstanding	6,371		4,807
Remaining weighted average contract period (in years)	10.9		10.8
Contractual debt service outstanding (in millions):
Principal	$	42,090.6	$	33,057.3
Interest	21,057.1		16,396.6
Total debt service outstanding	$	63,147.7	$	49,453.9

Gross unearned insurance premiums	$	136.8	$	82.9

The following table presents a schedule of BAM’s future premium revenues as of December 31, 2017:



Millions	December 31, 2017
January 1, 2018 - March 31, 2018	$	2.9
April 1, 2018 - June 30, 2018	2.9
July 1, 2018 - September 30, 2018	2.9
October 1, 2018 - December 31, 2018	2.8
	11.5

2019	11.1
2020	10.7
2021	10.2
2022	9.7
2023 and thereafter	83.6
Total gross unearned insurance premiums	$	136.8

The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the years ended December 31, 2017, 2016 and 2015:



Millions	December 31, 2017		December 31, 2016		December 31, 2015
Gross written premiums	$	63.2	$	38.6	$	25.9
Assumed (ceded) written premiums	—		—		—
Net written premiums	$	63.2	$	38.6	$	25.9

Earnings Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Earnings Per Share

White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding.

The following table presents the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2017, 2016 and 2015. See Note 19 — “Held for Sale and Discontinued Operations”.



	Year Ended December 31,
	2017			2016			2015
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	627.2		$	401.8		$	295.2
Less: total income from discontinued operations, net of tax	577.5			523.4			135.1
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	49.7			(121.6		)	160.1
Allocation of earnings to participating restricted common shares ⁽¹⁾	(.7		)	1.5			(1.9		)
Basic and diluted earnings per share numerators	$	49.0		$	(120.1	)	$	158.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,293.8			5,014.9			5,879.2
Average unvested restricted common shares ⁽²⁾	(54.3		)	(64.8		)	(68.0		)
Basic earnings per share denominator	4,239.5			4,950.1			5,811.2
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	4,293.8			5,018.1			5,879.2
Average unvested restricted common shares ⁽²⁾	(54.3		)	(64.8		)	(68.0		)
Diluted earnings per share denominator ⁽³⁾	4,239.5			4,953.3			5,811.2
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00		$	1.00
Undistributed earnings (losses)	$	10.56		$	(25.26	)	$	26.22
Basic and diluted earnings per share	$	11.56		$	(24.26	)	$	27.22


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


⁽²⁾	Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “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 resulted 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.

The following table presents the undistributed net earnings (losses) from continuing operations for the years ended December 31, 2017, 2016 and 2015. See Note 19 — “Held for Sale and Discontinued Operations”.



	Year Ended December 31,
Millions		2017			2016			2015
Undistributed net earnings - continuing operations:
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	49.0		$	(120.1	)	$	158.2
Dividends declared, net of restricted common share amounts ⁽¹⁾		(4.5		)	(5.4		)	(5.9		)
Total undistributed net earnings (losses), net of restricted common share amounts		$	44.5		$	(125.5	)	$	152.3

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

Employee Share-Based Incentive Compensation Plans

12 Months Ended

Dec. 31, 2017

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 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 provides for grants of various types of share-based and non-share-based incentive awards to key employees and directors 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

Performance shares are designed to reward employees for meeting company-wide performance targets. 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 service 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 for the vested portion of the awards over the related service periods. 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 measures used for determining performance share payouts is growth in White Mountains’s adjusted book value per share and intrinsic value per share. Intrinsic value per share is generally calculated by adjusting adjusted book value per share for differences between the adjusted book value of certain assets and liabilities and White Mountains’s estimate of their underlying intrinsic values.

The following table presents performance share activity for the years ended December 31, 2017, 2016 and 2015 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2017					2016					2015
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	80,353		$	42.4		93,654		$	57.7		111,257		$	44.4
Shares paid or expired ⁽¹⁾	(30,838	)	(21.9		)	(36,294	)	(41.0		)	(42,959	)	(30.8		)
New grants	17,710		—			22,615		—			29,195		—
Forfeitures ⁽²⁾	(16,710	)	(9.3		)	378		.5			(3,839	)	(.3		)
Expense recognized	—		34.6			—		25.2			—		44.4
End of period ⁽³⁾	50,515		$	45.8		80,353		$	42.4		93,654		$	57.7


⁽¹⁾	WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017 ranged from 34% to 76% of target. WTM performance shares 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.


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


⁽³⁾	Outstanding performance share awards as of December 31, 2017, 2016 and 2015 exclude 2,195, 7,315 and 10,826 unvested performance shares awards for employees of Sirius Group.

For the 2014-2016 performance cycle, all performance shares earned were settled in cash. For the performance shares earned in the 2013-2015 performance cycle, the Company issued 5,000 common shares and settled the remainder in cash. For the 2012-2014 performance cycle, all performance shares earned were settled in cash. If the outstanding WTM performance shares had vested on December 31, 2017, the total additional compensation cost to be recognized would have been $20.2 million, based on accrual factors as of December 31, 2017 (common share price and payout assumptions).

The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2017 for each performance cycle:



$ in millions	Target WTM Performance Shares Outstanding		Accrued Expense
Performance cycle:
2017 – 2019	16,680		$	8.3
2016 – 2018	16,235		14.7
2015 – 2017	18,370		23.6
Sub-total	51,285		46.6
Assumed forfeitures	(770	)	(.8		)
Total	50,515		$	45.8

For the 2017-2019 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 5% average growth in adjusted book value per share and intrinsic value per share. Average growth of 1% or less would result in no payout and average growth of 9% or more would result in a payout of 200%.

For the 2016-2018 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 4% average growth in adjusted book value per share and intrinsic value per share. Average growth of 0% or less would result in no payout and average growth of 8% or more would result in a payout of 200%.

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% average growth in adjusted book value per share and intrinsic value per share. Average growth of 2% or less would result in no payout and average growth of 14% or more would result in a payout of 200%.

For investment personnel for the 2016-2018 and 2017-2019 performance cycles, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is based one-third on average growth in adjusted book value per share and intrinsic value per share (as described above) and two-thirds on achieving a total return on invested assets as measured against metrics based on U.S. Treasury Note.

For investment personnel for the 2015-2017 performance cycle, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is based one-third on average growth in adjusted book value per share and intrinsic value per share, 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 fixed income assets as measured against metrics based on the Bloomberg Barclays U.S. Intermediate Aggregate Index returns.

Restricted Shares

The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
	2017					2016					2015
$ 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	70,620		$	19.7		70,675		$	15.7		83,314		$	14.3
Issued	17,985		16.3			25,365		20.2			23,640		15.7
Vested	(28,846	)	—			(24,620	)	—			(36,279	)	—
Forfeited	(6,004	)	(3.5		)	(800	)	(.3		)	—		—
Expense recognized	—		(18.2		)	—		(15.9		)	—		(14.3		)
End of period ⁽¹⁾	53,755		$	14.3		70,620		$	19.7		70,675		$	15.7


⁽¹⁾	Outstanding restricted share awards as of December 31, 2017, 2016 and 2015 include 2,195, 5,235, and 9,205 unvested restricted shares for employees of Sirius Group.

During 2017, White Mountains issued 17,485 restricted shares that vest on January 1, 2020, 250 restricted shares that vest on January 1, 2019 and 250 restricted shares that vest on January 1, 2018. 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. The unrecognized compensation cost as of December 31, 2017 is expected to be recognized ratably over the remaining vesting periods.

Non-Qualified Options

As of January 20, 2017, the 125,000 Non-Qualified Options issued to the Company’s former Chairman and CEO had been exercised. During the first quarter of 2017, 40,000 Non-Qualified Options, with an intrinsic value of $4.4 million, were exercised in exchange for 5,142 common shares with an equal total market value. 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 and the fixed strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011.

Common Shareholders’ Equity and Non-controlling Interests

12 Months Ended

Dec. 31, 2017

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, 2017, White Mountains may repurchase an additional 643,130 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 2017, the Company repurchased 832,725 common shares for $723.9 million at an average share price of $869, which were comprised of 821,732 common shares repurchased under the board authorization for $713.1 million at an average share price of $870 and 10,993 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 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.

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.

Common Shares Issued

During 2017, the Company issued a total of 25,086 common shares, which consisted of 17,985 restricted shares to key personnel, 5,142 shares issued to the Company’s retired CEO as a result of exercised options, and 1,959 shares issued to directors of the Company.

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 retired 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 personnel and 1,376 shares issued to directors of the Company.

Dividends on Common Shares

For the years ended December 31, 2017, 2016 and 2015, the Company declared and paid cash dividends totaling $4.6 million, $5.4 million and $6.0 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 presents 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, 2017 and 2016:



	December 31, 2017					December 31, 2016
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon	—	%	$	—		23.9	%	$	244.6

Other, excluding mutuals and reciprocals
HG Global	3.1		15.9			3.1		16.6
MediaAlpha	35.7		13.1			40.0		11.7
Dewar⁽¹⁾	—		—			18.8		3.9
Buzzmove	22.9		2.5			29.1		3.0
Total other, excluding mutuals and reciprocals			31.5					35.2

Mutuals and reciprocals
BAM	100.0		(163.2		)	100.0		(150.9		)
SSIE	—		—			100.0		4.4
Total mutuals and reciprocals			(163.2		)			(146.5		)
Total non-controlling interests			$	(131.7	)			$	133.3

⁽¹⁾ Dewar is a subsidiary of OneBeacon.

Statutory Capital and Surplus	12 Months Ended
Statutory Capital and Surplus	Dec. 31, 2017
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, 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 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. 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, 2017, HG Re had statutory capital and surplus of $677.0 million. As a Special Purpose Insurer, HG Re has a nominal minimum regulatory capital requirement of $1. BAM is domiciled in New York and is subject to regulation by the 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, 2017, 2016 and 2015 was $25.4 million, $32.7 million and $32.0 million. BAM’s members’ surplus, as reported to regulatory authorities as of December 31, 2017, was $427.3 million, which exceeds the minimum members’ surplus necessary for BAM to maintain its New York State financial guarantee insurance license of $66.0 million. 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. Following is a description of the dividend capacity of White Mountains’s reinsurance and other operating subsidiaries: HG Global/BAM As of December 31, 2017, 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 2017. As of December 31, 2017, HG Global has accrued $235.6 million of dividends payable to holders of its preferred shares, $227.9 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 FLRT with BAM. As of December 31, 2017, HG Re had statutory capital and surplus of $677.0 million, $715.1 million of assets held in the collateral trusts pursuant to the FLRT with BAM and less than $1.0 million of cash and investments outside the collateral trusts. 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 3.78% for 2017 and is 4.60% for 2018. 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 to the extent that its capital resources continues to support its outstanding obligations, business plan and ratings. No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the NYDFS. BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes only to the extent that its remaining qualified statutory capital other capital resources continue to support its outstanding obligations, business plan and its AA stable rating from S&P. During 2017, HG Global and BAM agreed to change the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The NYDFS approved the change during the third quarter of 2017. During 2017, BAM repaid $4.0 million on the BAM Surplus Notes and $1.0 million on the accrued interest. BAM did not make any payments on the BAM Surplus Notes in 2016 or 2015. MediaAlpha During 2017, MediaAlpha paid $5.2 million of dividends, $3.3 million of which was paid to White Mountains. As of December 31, 2017, MediaAlpha had $9.1 million of net unrestricted cash. Other Operations During 2017, White Mountains paid a $4.6 million common share dividend. As of December 31, 2017, the Company and its intermediate holding companies held $1,682.6 million of net unrestricted cash, short-term investments and fixed maturity investments, $866.1 million of common equity securities and $62.0 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 HG Global/BAM, MediaAlpha and Other Operations. As a result of the Sirius Group and Tranzact sales and the OneBeacon Transaction, the results of operations for Sirius Group and OneBeacon, previously reported in their own respective segments, and Tranzact, previously reported in the Other Operations segment, have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of operations and comprehensive income. See Note 19 — “Held for Sale and Discontinued Operations”.

Beginning in the second quarter of 2017, MediaAlpha’s results have been presented as a separate segment within White Mountains’s consolidated financial statements. Prior year amounts have been reclassified to conform to the current period’s presentation.

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.

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 municipal 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 up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. BAM's results are attributed to non-controlling interests.

MediaAlpha is a marketing technology company that develops transparent and efficient platforms for the buying and selling of insurance and other vertical-specific performance media (i.e., clicks, calls and leads). MediaAlpha’s exchange technology, machine learning and analytical tools facilitate transparent, real-time transactions between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory).

White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, WM Capital, its wholly-owned investment management subsidiary, WM Advisors, and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments consist of Wobi and Buzzmove. White Mountains’s Other Operations segment also includes its variable annuity reinsurance business, WM Life Re.

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

The following tables present the financial information for White Mountains’s segments:



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations		Total
Year Ended December 31, 2017
Earned insurance premiums	$	9.4		$	—		$	1.0	$	10.4
Net investment income	12.3			—			43.7		56.0
Net realized and unrealized investment gains	.6			—			132.7		133.3
Advertising and commission revenues ⁽²⁾	—			163.2			3.8		167.0
Other revenues	1.0			—			6.1		7.1
Total revenues	23.3			163.2			187.3		373.8
Losses and LAE	—			—			1.1		1.1
Insurance acquisition expenses	4.0			—			.1		4.1
Other underwriting expenses	.4			—			—		.4
Cost of sales	—			135.9			3.5		139.4
General and administrative expenses	42.9			26.7			149.1		218.7
Interest expense	—			1.0			1.3		2.3
Total expenses	47.3			163.6			155.1		366.0
Pre-tax (loss) income	$	(24.0	)	$	(.4	)	$	32.2	$	7.8


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Approximately 27% of MediaAlpha’s advertising revenue was associated with one customer for the years ended December 31, 2017.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2016
Earned insurance premiums	$	5.9		$	—		$	7.5		$	13.4
Net investment income	9.0			—			23.1			32.1
Net realized and unrealized investment gains (losses)	.7			—			(28.1		)	(27.4		)
Advertising and commission revenues⁽²⁾	—			116.5			1.8			118.3
Other revenues	1.1			—			20.2			21.3
Total revenues	16.7			116.5			24.5			157.7
Losses and LAE	—			—			8.0			8.0
Insurance acquisition expenses	3.4			—			2.2			5.6
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			97.8			4.2			102.0
General and administrative expenses	39.6			21.9			124.5			186.0
Interest expense	—			.9			2.1			3.0
Total expenses	43.4			120.6			141.0			305.0
Pre-tax loss	$	(26.7	)	$	(4.1	)	$	(116.5	)	$	(147.3	)


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Approximately 24% of MediaAlpha’s advertising revenue was associated with one customer for the years ended December 31, 2016.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2015
Earned insurance premiums	$	3.3		$	—		$	8.7		$	12.0
Net investment income	6.1			—			4.8			10.9
Net realized and unrealized investment gains	.6			—			259.9		⁽²⁾	260.5
Advertising and commission revenues ⁽³⁾	—			105.5			1.9			107.4
Other revenues	.7			—			48.5			49.2
Total revenues	10.7			105.5			323.8			440.0
Losses and LAE	—			—			8.2			8.2
Insurance acquisition expenses	2.9			—			3.4			6.3
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			90.7			2.9			93.6
General and administrative expenses	36.8			16.4			147.5			200.7
Interest expense	—			.4			1.2			1.6
Total expenses	40.1			107.5			163.2			310.8
Pre-tax (loss) income	$	(29.4	)	$	(2.0	)	$	160.6		$	129.2


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Net realized and unrealized investment gains 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 14 — “Investments in Unconsolidated Affiliates”.


⁽³⁾	Approximately 52% of MediaAlpha’s advertising revenue was associated with three customers for the year ended December 31, 2015.




Selected Balance Sheet Data Millions	HG Global/BAM				MediaAlpha		Other Operations		Eliminations			Held for Sale		Total
December 31, 2017:
Total investments	$	693.4			$	—	$	2,687.3	$	—		$	—	$	3,380.7
Total assets	747.4			⁽¹⁾	96.5		3,039.9		(227.9		)	3.3		3,659.2
Total liabilities	394.9				59.8		71.6		(227.9		)	—		298.4
Total White Mountains’s common shareholders’ equity	499.8				23.6		2,965.8		—			3.3		3,492.5
Non-controlling interest	(147.3		)		13.1		2.5		—			—		(131.7		)
December 31, 2016:
Total investments	$	629.7			$	—	$	2,084.7	$	—		$	—	$	2,714.4
Assets held for sale	—				—		—		—			3,606.4		3,606.4
Total assets	677.6			⁽¹⁾	57.6		2,358.8		(180.2		)	3,606.4		6,520.2
Liabilities held for sale	—				—		—		—			2,569.3		2,569.3
Total liabilities	289.1				28.3		97.7		(180.2		)	2,569.3		2,804.2
Total White Mountains’s common shareholders’ equity	522.8				17.6		2,258.1		—			784.2		3,582.7
Non-controlling interest	(134.3		)		11.7		3.0		—			252.9		133.3


⁽¹⁾	As of December 2017 and 2016,BAM’s total assets reflect the elimination of $499.0 and $503.0 of BAM Surplus Notes issued to HG Global and its subsidiaries, and $126.0 and $108.0 in accrued interest related to the BAM Surplus Notes.

As of December 31, 2017 and 2016, HG Global’s total liabilities includes $227.9 million and $180.2 million of accrued preferred dividends payable to the Company. Other Operations total assets includes the preferred dividends receivable and its eliminated in consolidation. As of December 31, 2017 and 2016, 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 the Company, is $727.7 million and $703.0 million.

Investments in Unconsolidated Affiliates

12 Months Ended

Dec. 31, 2017

Equity Method Investments and Joint Ventures [Abstract]

Investments in Unconsolidated Affiliates

Investments in Unconsolidated Entities

White Mountains’s investments in unconsolidated entities are included within other long-term investments and consist of investments in common equity securities or similar instruments, which give White Mountains the ability to exert significant influence over the investee’s operating and financial policies (“equity method eligible unconsolidated entities”). Such investments may be accounted for under either the equity method or alternatively, White Mountains may elect to account for them under the fair value option.

The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments:



	December 31
Millions	2017		2016
Equity method eligible private equity securities, at fair value	$	58.0	$	55.6
Investments, accounted for under the equity method	4.6		3.5
Total investments in equity method eligible unconsolidated entities	62.6		59.1
Other unconsolidated investments ⁽¹⁾	146.2		113.7
Total other long-term investments	$	208.8	$	172.8

⁽¹⁾Consists of other long-term investments that are not equity method eligible.

The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of December 31, 2017 and 2016:



Investee	Ownership Interest	Instrument Held
Compare.com	22%	Common shares
durchblicker	45%	Common shares
OneTitle	20%	Common shares
PassportCard	50%	Common shares
Tuckerman Capital Fund III	21%	Units

The following tables presents aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities:



	December 31,
Millions	2017		2016
Balance sheet data ⁽¹⁾:
Total assets	$	75.4	$	79.4
Total liabilities	24.2		18.8

⁽¹⁾Financial data submitted by investees is on a one-quarter lag.



	Year Ended December 31,
Millions	2017			2016			2015
Income statement data ⁽¹⁾:
Revenues	$	60.0		$	32.9		$	12.9
Expenses	(66.8		)	(76.4		)	(69.9		)

⁽¹⁾Financial data submitted by investees is on a one-quarter lag.

Symetra

White Mountains accounted for its investment in Symetra common shares under the equity method until November 5, 2015 when, upon relinquishing its representation on Symetra’s board of directors, it no longer had the ability to exert significant influence. On November 5, 2015, White Mountains began accounting for its investment in Symetra at 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 November 5, 2015 and fair value at December 31, 2015.

The following tables presents 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 tables presents financial information for Symetra for the nine months ended September 30, 2015:



	Nine months ended September 30, 2015
Millions	Nine months ended September 30, 2015
Symetra income statement data:
Net premiums earned	$	539.3
Net investment income	994.3
Total revenues	1,605.9
Policy benefits	1,143.7
Total expenses	1,543.6
Net income	89.6
Comprehensive net loss	(234.1		)

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.

Variable Interest Entities	12 Months Ended
Variable Interest Entities	Dec. 31, 2017
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 writes. A portion of the premium is a member’s surplus contribution (“MSC”) and the remainder is a risk premium. In the event of a municipal bond refunding, the MSC from the original issuance can be reutilized, in effect serving as a credit against the total insurance premium on the refunding of the municipal bond. Issuers of debt insured by BAM are members of BAM so long as any of their BAM-insured debt is outstanding, and as members they have certain interests in BAM, including the right to vote for BAM’s directors and to receive dividends in the future, if declared. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE. At inception, BAM and HG Re also entered into the FLRT. HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. HG Re’s obligations under the FLRT are satisfied by the assets in the Regulation 114 Trust and the Supplemental Trust. Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, under the FLRT, HG Holdings Ltd, a subsidiary of HG Global, has the right to designate two directors for election to BAM’s board of directors. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. Prospector Offshore Fund In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. (the “Prospector Offshore Fund”) 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, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

White Mountains accounts for its financial instruments at fair value with the exception of the WTM Bank Facility, which was undrawn at December 31, 2017 and December 31, 2016, the MediaAlpha Bank Facility and the Previous MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount.

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2017 and December 31, 2016:



	December 31, 2017				December 31, 2016
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
MediaAlpha Bank Facility	$	23.9	$	23.8	$	—	$	—
Previous MediaAlpha Bank Facility	—		—		13.0		12.7

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, 2017
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie, the founder and Managing Member of Prospector Partners, LLC (“Prospector”), retired from the White Mountains 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 the Consulting Agreement described below were terminated. Pursuant to an investment management agreements with WM Advisors and OneBeacon, Prospector charged WM Advisors and OneBeacon fees based on the amount of assets under management. During 2015, Prospector earned $2.1 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. Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector. For the year ended December 31, 2015, White Mountains and OneBeacon incurred $0.4 million in management fees and $0.1 million in incentive fees. Other Relationships and Transactions During 2017, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company. On July 13, 2017, the Company repurchased 235,000 White Mountains common shares for $850.00 per share, the market price at the time the agreement was reached. During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 White Mountains 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 White Mountains 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 in two transactions. On June 1, 2015, the Company repurchased 19,688 White Mountains 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 White Mountains common shares for $761.50, the market price at the time the agreement was reached. 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, WM Advisors earned $8.0 million in fees from Symetra.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2017
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 $3.5 million, $3.4 million and $2.0 million for the years ended December 31, 2017, 2016 and 2015. 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 $2.7 million, $2.4 million, $2.2 million, and $9.1 million for the years 2018, 2019, 2020 and 2021 and thereafter. White Mountains also has future binding commitments to fund certain other long-term investments. These commitments, which total $109.3 million, do not have fixed funding dates. Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to 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 if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following description presents significant legal contingencies, ongoing non-claims related litigation or arbitration as of December 31, 2017: Esurance On October 7, 2011, the Company completed the sale of its Esurance Holdings, Inc. and its subsidiaries and Answer Financial Inc. and its 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. Sirius Tax Contingencies A subsidiary of Sirius Group, which was sold by White Mountains in 2016, has been denied interest deductions by the Swedish Tax Authority (“STA”) for tax years 2013-2016. The Swedish subsidiary has filed an appeal in the Swedish Administrative Court for tax year 2013 and will preserve its right to contest the STA’s decision for later years if needed. Sirius Group believes it is more likely than not that it will prevail in the court proceedings. However, if the ultimate decision in the Swedish courts is unfavorable, White Mountains would be required to indemnify Sirius Group for the tax effect of the interest deductions of approximately $19 million.

Discontinued Operations

12 Months Ended

Dec. 31, 2017

Discontinued Operations and Disposal Groups [Abstract]

Discontinued Operations

Held for Sale and Discontinued Operations

OneBeacon

On September 28, 2017, Intact Financial Corporation completed its previously announced acquisition of OneBeacon in an all-cash transaction for $18.10 per share. White Mountains received total proceeds of $1.3 billion and recorded a gain of $554.6 million, net of transaction costs. For 2017 through the closing date of the transaction, net income from discontinued operations related to OneBeacon was $20.5 million. Net income from discontinued operations related to OneBeacon was $108.6 million and $38.2 million for the years ended December 31, 2016 and 2015.

Star & Shield

On March 7, 2017, White Mountains completed the sale of Star & Shield and its investment in SSIE surplus notes to K2 Insurances LLC. White Mountains did not recognize any gain or loss on the sale. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains's GAAP financial statements.

Tranzact

On July 21, 2016, White Mountains completed the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million at closing. On October 5, 2016, 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.

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 and 2015, White Mountains recorded net income (loss) from discontinued operations of $6.1 million and $(1.9) 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, including $30.2 million of tax expense recorded to gain from sale of Tranzact in discontinued operations and $8.8 million of tax benefit recorded to net income from discontinued operations.

During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of state tax expense.

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. During 2017, White Mountains recorded a $0.7 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees.

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 and 2015, $3.7 million and $205.0 million, of which $200.8 million is related to Symetra, 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 and 2015, White Mountains recorded $(4.3) million and $81.1 million of net (loss) income from discontinued operations and $32.0 million and $(65.0) million of other comprehensive income (loss) from Sirius Group.

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.

Esurance

For the year ended December 31, 2015, White Mountains recorded a gain from sale of Esurance Holdings, Inc. and its subsidiaries and Answer Financial Inc. and its subsidiaries (collectively, “Esurance”) in discontinued operations of $17.9 million, which primarily related to a payment 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 18 — “Commitments and Contingencies”.

Net Assets Held for Sale

The following table presents the assets and liabilities associated with business classified as held for sale as of December 31, 2016. Amounts presented relate to OneBeacon, Star & Shield and SSIE and the Company’s Guilford, Connecticut property. Assets held for sale as of December 31, 2017 consist solely of the Company’s Guilford, Connecticut property, which is recorded at the amount of its estimated fair value, net of estimated costs of disposal, of $3.3 million. The related writedown of $3.7 million has been recorded within other expenses.



Millions	December 31, 2016
Assets held for sale
Fixed maturity investments, at fair value	$	2,175.7
Short-term investments, at amortized cost (which approximates fair value)	112.3
Common equity securities, at fair value	188.7
Other long-term investments	150.5
Total investments	2,627.2
Cash	70.5
Reinsurance recoverable on unpaid losses	179.8
Insurance and reinsurance premiums receivable	229.8
Deferred acquisition costs	96.3
Deferred tax asset	126.7
Ceded unearned insurance and reinsurance premiums	44.2
Accounts receivable on unsettled investment sales	1.4
Goodwill and other intangible assets	1.2
Accrued investment income	11.3
Other assets	218.0
Total assets held for sale	$	3,606.4
Liabilities held for sale
Loss and loss adjustment expense reserves	$	1,370.6
Unearned insurance and reinsurance premiums	576.3
Debt	273.2
Accrued incentive compensation	44.3
Funds held under reinsurance treaties	153.0
Other liabilities	151.9
Total liabilities held for sale	2,569.3
Net assets held for sale	$	1,037.1

Net Income (Loss) from Discontinued Operations

The following tables present the results of operations, including related income taxes associated with the business classified as discontinued operations. For the year ended December 31, 2017 and 2016, the amounts presented relate to OneBeacon, Sirius Group and Tranzact. For the year ended December 31, 2015, the amounts presented relate to OneBeacon, Sirius Group, Tranzact and Esurance. 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, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7



	Year Ended December 31, 2015
Millions	OneBeacon			Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	1,176.2		$	847.0		$	—		$	2,023.2
Net investment income	45.9			40.7			—			86.6
Net realized and unrealized investment (losses) gains	(35.1		)	15.1			—			(20.0		)
Other (loss) revenues	(.6		)	(20.6		)	186.2			165.0
Total revenues	1,186.4			882.2			186.2			2,254.8
Expenses
Loss and loss adjustment expenses	700.7			422.7			—			1,123.4
Insurance and reinsurance acquisition expenses	213.8			189.8			—			403.6
Other underwriting expenses	218.2			107.9			—			326.1
Interest expense	15.4			26.6			4.0			46.0
General and administrative expenses	13.0			27.0			185.3			225.3
Total expenses	1,161.1			774.0			189.3			2,124.4
Pre-tax income (loss)	25.3			108.2			(3.1		)	130.4
Income tax benefit (expense)	12.9			(27.1		)	.7			(13.5		)
Net income (loss) from discontinued operations	38.2			81.1			(2.4		)	116.9
Gain from sale of OneBeacon runoff, net of tax	.3			—			—			.3
Gain from sale of Esurance, net of tax	—			—			17.9			17.9
Total income from discontinued operations	38.5			81.1			15.5			135.1
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	—			(65.0		)	—			(65.0		)
Comprehensive income from discontinued operations	$	38.5		$	16.1		$	15.5		$	70.1

Net Change in Cash from Discontinued Operations

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



	Year ended December 31,
Millions	2017			2016			2015
Net cash provided from operations	$	157.0		$	23.6		$	248.4
Net cash provided from (used for) investing activities	3.0			241.4			(100.5		)
Net cash used for financing activities	(61.9		)	(93.8		)	(100.7		)
Effect of exchange rate changes on cash	—			—			(4.5		)
Net change in cash during the period	98.1			171.2			42.7
Cash balances at beginning of period	70.5			245.4			203.8
Net change in cash held for sale	(.9		)	(.3		)	(1.1		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)	—
Cash balances at end of period	$	—		$	70.5		$	245.4

Earnings Per Share from Discontinued Operations

White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in dividends and earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding.

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
	2017			2016			2015
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	627.2		$	401.8		$	295.2
Less: total income (loss) from continuing operations, net of tax	49.7			(121.6		)	160.1
Net income from discontinued operations attributable to White Mountains’s common shareholders	577.5			523.4			135.1
Allocation of earnings to participating restricted common shares ⁽¹⁾	(7.3		)	(6.8		)	(1.6		)
Basic and diluted earnings per share numerators	$	570.2		$	516.6		$	133.5
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,293.8			5,014.9			5,879.2
Average unvested restricted common shares ⁽³⁾	(54.3		)	(64.8		)	(68.0		)
Basic earnings per share denominator	4,239.5			4,950.1			5,811.2
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	4,293.8			5,018.1			5,879.2
Average unvested restricted common shares ⁽³⁾	(54.3		)	(64.8		)	(68.0		)
Diluted earnings per share denominator ⁽⁴⁾	4,239.5			4,953.3			5,811.2
Basic earnings per share (in dollars) - discontinued operations:	$	134.50		$	104.37		$	22.98
Diluted earnings per share (in dollars) - discontinued operations:	$	134.50		$	104.32		$	22.98


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


⁽²⁾	Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2017, 2016 and 2015.


⁽³⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Compensation Plans”.

⁽⁴⁾

The diluted earnings 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 resulted 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.

Fair Value of Financial Instrument in Liabilities Held for Sale

The OBH Senior Notes are recorded as debt at face value less unamortized original issue discount. The following table presents the fair value and carrying value of this financial instrument as of December 31, 2016:



	December 31, 2016
Millions	Fair Value		Carrying Value
OBH Senior Notes	$	274.2	$	273.2

The fair value estimates for the OBH Senior Notes has been determined using quoted market prices. The OBH Senior Notes are considered a Level 2 measurement.

OneBeacon Surplus Notes in Assets Held for Sale

In the fourth quarter of 2014, in conjunction with OneBeacon’s sale of its runoff business to an affiliate of Armour Group Holdings Limited, OneBeacon provided financing in the form of surplus notes (the “OneBeacon Surplus Notes”) with a par value of $101.0 million, which had a fair value of $71.9 million as of December 31, 2016. The OneBeacon Surplus Notes, issued by one of the transferred entities, Bedivere Insurance Company (the “Issuer”) were in the form of both seller priority and pari passu notes.

Subsequent to the OneBeacon Transaction, 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 of the OneBeacon Surplus Notes is based on discounted expected cash flows using information as of the measurement date.

The following table presents the valuation adjustments taken to arrive at estimated fair value of the OneBeacon Surplus Notes as of December 31, 2016:



Millions	December 31, 2016
Par Value	$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments ⁽¹⁾	5.1
Regulatory approval ⁽²⁾	(15.6		)
Liquidity adjustment ⁽³⁾	(18.6		)
Total adjustments	(29.1		)
Fair value	$	71.9

⁽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 National Association of Insurance Commissioners’ risk-based capital standards for property and casualty companies. The favorable year-to-date 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 three months 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-to-date change in impact is driven primarily by the narrowing of non-investment grade credit spreads, which causes negative valuation impact from the anticipated delay in securing regulatory approval to be lower.

⁽³⁾ Represents impact of liquidity spread to account for OneBeacon's sole ownership of the notes, lack of a trading market, and unique nature of the ongoing regulatory approval process.

Financial Statement Revisions

12 Months Ended

Dec. 31, 2017

Accounting Changes and Error Corrections [Abstract]

Financial Statement Revisions

In October 2017, White Mountains discovered that the former CEO of Wobi, one of its overseas portfolio companies, had been reporting overstated commission revenues and related receivables to White Mountains. Upon discovery of the overstatements, White Mountains initiated an investigation, conducted by outside counsel, of the reporting of these overstatements by Wobi to White Mountains. As a result of this investigation, White Mountains has revised certain of its previously issued financial statements. The revisions resulted in reductions to commission revenues (included in advertising and commission revenues) and commissions receivable (included in other assets). In addition, the overstatements led White Mountains to write down the goodwill and other intangible assets related to Wobi to zero. This write down is recorded in general and administrative expenses for the years ended December 31, 2016 and 2015. Wobi also conducted a separate investigation with the assistance of Israeli counsel to support the preparation of Wobi’s standalone financial statements. That separate investigation has been completed and its results did not materially impact White Mountains’s financial statements.

White Mountains evaluated the impact of the misstatements resulting from the overstatements at Wobi on prior periods’ financial statements in accordance with SEC Staff Accounting Bulletin (“SAB”) Topic 1.M, Materiality, and concluded the misstatements were not material to any previously reported financial statements. However, while not material to any previously reported annual or quarterly period, the aggregate amount of prior period misstatements could be material to White Mountains's results for the full fiscal year ended December 31, 2017. White Mountains has therefore revised all periods impacted including its consolidated balance sheets as of December 31, 2016 and 2015; and its consolidated statements of operations and comprehensive income, consolidated statements of changes in equity, cash flows and earnings per share for the years ended December 31, 2016 and 2015. The revisions also reflect a previously recorded out of period adjustment in 2015 and 2014.

The following tables presents the impact of these revisions to each of the previously reported consolidated statements. Amounts previously reported reflect the reclassification of OneBeacon, Sirius Group and Tranzact to discontinued operations for all applicable periods presented.

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME



	As previously reported						Adjustments										As revised
	Years ended December 31,
Millions, except for per share amounts	2016			2015			2016					2015					2016			2015
Revenues:
Financial Guarantee (HG Global/BAM)
Financial Guarantee revenues	$	16.7		$	10.7		$	—				$	—				$	16.7		$	10.7

Marketing Technology (MediaAlpha)
Marketing Technology revenues	116.5			105.5			—					—					116.5			105.5

Other
Advertising & commission revenues	10.4			4.6			(8.6		)			(2.7		)			1.8			1.9
Other revenues ⁽¹⁾⁽²⁾	22.7			314.5			—					7.4					22.7			321.9
Other revenues	33.1			319.1			(8.6		)			4.7					24.5			323.8
Total revenues	166.3			435.3			(8.6		)			4.7					157.7			440.0
Expenses:
Financial Guarantee (HG Global/BAM)
Financial Guarantee expenses	43.4			40.1			—					—					43.4			40.1

Marketing Technology (MediaAlpha)
Marketing Technology expenses	120.6			107.5			—					—					120.6			107.5

Other
Other expenses ⁽³⁾	137.6			154.5			3.0			⁽⁵	⁾	8.2			⁽⁵	⁾	140.6			162.7
Amortization of other intangible assets	1.2			1.2			(.8		)			(.7		)			.4			.5
Other expenses	138.8			155.7			2.2					7.5					141.0			163.2
Total expenses	302.8			303.3			2.2					7.5					305.0			310.8
Pre-tax (loss) income	(136.5		)	132.0			(10.8		)			(2.8		)			(147.3		)	129.2
Income benefit (expense)	32.9			(12.7		)	—					—					32.9			(12.7		)
Net (loss) income from continuing operations	(103.6		)	119.3			(10.8		)			(2.8		)			(114.4		)	116.5
Gain on sale of discontinued operations, net of tax	415.1			18.2			—					—					415.1			18.2
Net income from discontinued operations	108.3			116.9			—					—					108.3			116.9
Income before equity in earnings of unconsolidated affiliates	419.8			254.4			(10.8		)			(2.8		)			409.0			251.6
Equity in earnings of unconsolidated affiliates	—			25.1			—					—					—			25.1
Net income ⁽⁴⁾	419.8			279.5			(10.8		)			(2.8		)			409.0			276.7
Net (income) loss attributable to non-controlling interests	(7.3		)	18.1			.1					.4					(7.2		)	18.5
Net income attributable to White Mountains's common shareholders	412.5			297.6			(10.7		)			(2.4		)			401.8			295.2
Other comprehensive income (loss), net of tax ⁽¹⁾	145.6			(100.4		)	—					(7.4		)			145.6			(107.8		)
Comprehensive income	558.1			197.2			(10.7		)			(9.8		)			547.4			187.4
Comprehensive income attributable to non-controlling interests	(.3		)	—			—					—					(.3		)	—
Comprehensive income attributable to White Mountains's common shareholders	$	557.8		$	197.2		$	(10.7	)			$	(9.8	)			$	547.1		$	187.4
Basic and diluted earnings per share - continuing operations	$	(22.13	)	$	27.63		$	(2.13	)			$	(.41	)			$	(24.26	)	$	27.22

⁽¹⁾In 2015, White Mountains recorded a foreign currency translation gain related to its investment in Symetra in net income when it should have been recorded through other comprehensive income. The correction to properly reflect the translation amount through other comprehensive income did not have any impact on comprehensive income attributable to White Mountains's common shareholders or to book value per share.

⁽²⁾Total other revenues include earned insurance premiums, net investment income, net realized and unrealized losses, and other revenues.

⁽³⁾Total other expenses include loss and loss adjustment expenses, insurance acquisition expenses, other underwriting expenses, cost of sales, general and administrative expenses, and interest expense.

⁽⁴⁾The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities.

⁽⁵⁾The adjustments to other expenses is primarily related to the write-off of goodwill and intangible assets for Wobi.

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY



	As previously reported									Adjustments									As revised
	Years ended December 31,
Millions	2016			2015			2014			2016			2015			2014			2016			2015			2014

Common shares and paid-in surplus	$	810.7		$	978.2		$	1,034.7		$	—		$	—		$	—		$	810.7		$	978.2		$	1,034.7

Retained earnings, beginning of year	3,084.9			3,010.5			2,801.9			(9.9		)	(7.5		)	(.5		)	3,075.0			3,003.0			2,801.4
Share repurchases	(694.8		)	(217.2		)	(97.4		)	—			—			—			(694.8		)	(217.2		)	(97.4		)
Net income	412.5			297.6			312.2			(10.7		)	(2.4		)	(7.0		)	401.8			295.2			305.2
Dividends	(5.4		)	(6.0		)	(6.2		)	—			—			—			(5.4		)	(6.0		)	(6.2		)
Retained earnings, end of year	2,797.2			3,084.9			3,010.5			(20.6		)	(9.9		)	(7.5		)	2,776.6			3,075.0			3,003.0

Accumulated other comprehensive (loss) income, after-tax, beginning of year	(149.9		)	(49.5		)	52.1			—			7.4			.5			(149.9		)	(42.1		)	52.6
Net change in foreign currency translation	31.4			(65.8		)	(168.2		)	—			(7.4		)	6.9			31.4			(73.2		)	(161.3		)
Net other changes in AOCI	113.9			(34.6		)	66.6			—			—			—			113.9			(34.6		)	66.6
Accumulated other comprehensive loss, after-tax, end of year	(4.6		)	(149.9		)	(49.5		)	—			—			7.4			(4.6		)	(149.9		)	(42.1		)

Total White Mountains Common Shareholders' Equity	3,603.3			3,913.2			3,995.7			(20.6		)	(9.9		)	(.1		)	3,582.7			3,903.3			3,995.6

Non-controlling interests, beginning of year	454.8			542.7			491.7			(.5		)	(.1		)	—			454.3			542.6			491.7
Net income (loss)	7.3			(18.1		)	(22.2		)	(.1		)	(.4		)	(.1		)	7.2			(18.5		)	(22.3		)
Other changes in NCI	(328.2		)	(69.8		)	73.2			—			—			—			(328.2		)	(69.8		)	73.2
Non-controlling interests, end of year	133.9			454.8			542.7			(.6		)	(.5		)	(.1		)	133.3			454.3			542.6

Total equity	$	3,737.2		$	4,368.0		$	4,538.4		$	(21.2	)	$	(10.4	)	$	(.2	)	$	3,716.0		$	4,357.6		$	4,538.2

CONSOLIDATED BALANCE SHEETS



	December 31, 2016
Millions	As previously reported			Adjustments			As revised
Assets
Financial Guarantee (HG Global/BAM)
Financial Guarantee assets	$	677.6		$	—		$	677.6

Marketing Technology (MediaAlpha)
Marketing Technology assets	57.6			—			57.6

Other
Investments	2,084.7			—			2,084.7
Goodwill	13.4			(5.8		)	7.6
Other intangible assets	4.7			(3.7		)	1.0
Other assets — commissions receivable	14.8			(14.8		)	—
Other assets ⁽¹⁾	85.5			(.2		)	85.3
Assets held for sale	3,606.4			—			3,606.4
Other segment assets	5,809.5			(24.5		)	5,785.0

Total assets	$	6,544.7		$	(24.5	)	$	6,520.2
Liabilities
Financial Guarantee (HG Global/BAM)
Total Financial Guarantee liabilities	$	108.9		$	—		$	108.9

Marketing Technology (MediaAlpha)
Total Marketing Technology liabilities	28.3			—			28.3

Other
Accrued incentive compensation	79.1			—			79.1
Other liabilities ⁽²⁾	21.9			(3.3		)	18.6
Liabilities held for sale	2,569.3			—			2,569.3
Total Other segment liabilities	2,670.3			(3.3		)	2,667.0

Total liabilities	2,807.5			(3.3		)	2,804.2

Equity
White Mountains's common shares	4.6			—			4.6
Paid in surplus	806.1			—			806.1
Retained earnings	2,797.2			(20.6		)	2,776.6
Accumulated other comprehensive income, net of tax	(4.6		)	—			(4.6		)
Total White Mountains's common shareholders' equity	3,603.3			(20.6		)	3,582.7
Non-controlling interests	133.9			(.6		)	133.3
Total equity	3,737.2			(21.2		)	3,716.0
Total liabilities and equity	$	6,544.7		$	(24.5	)	$	6,520.2

Subsequent Event	12 Months Ended
Subsequent Event	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events DavidShield On January 24, 2018, White Mountains acquired a 50% equity stake in DavidShield Life Insurance Agency (2000) Ltd. for approximately $28.0 million, its joint venture partner in PassportCard and a provider of expatriate medical and other accident and health insurance coverages. CrossHarbor On January 26, 2018, White Mountains entered into an agreement to invest up to $50.0 million in CrossHarbor Institutional Partners 2018, L.P. CrossHarbor Capital is an alternative investment management firm that invests in core-plus, value-add, and opportunistic commercial real estate transactions. Kudu On February 5, 2018, White Mountains entered into an agreement to invest up to $125.0 million in Kudu Investment Management, LLC (“Kudu”), a capital provider to asset management and wealth management firms. Through February of 2018, White Mountains has invested $1.8 million in Kudu.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quartely Financial Data (Unaudited)

SELECTED QUARTERLY FINANCIAL DATA

(Unaudited)

The following table presents selected quarterly financial data for 2017 and 2016. 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 OneBeacon, Tranzact, Sirius Group, and Esurance, the results of operations for OneBeacon, Tranzact, Sirius Group and Esurance have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of comprehensive income. See Note 19 — “Held for Sale and Discontinued Operations”.

Prior year amounts have been reclassified to conform to the current period’s presentation. Prior year amounts have also been adjusted for the impact of White Mountains’s financial statement revisions.



	2017 Three Months Ended										2016 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	$	114.0	$	87.5	$	83.5		$	88.8		$	(5.5	)	$	56.8		$	48.4		$	58.0
Expenses	108.8		79.1		85.7			92.4			69.0			70.6			74.1			91.3
Pre-tax income (loss)	5.2		8.4		(2.2		)	(3.6		)	(74.5		)	(13.8		)	(25.7		)	(33.3		)
Tax benefit	2.5		4.0		1.0			.3			10.2			17.1			4.0			1.6
Income (loss) from continuing operations	7.7		12.4		(1.2		)	(3.3		)	(64.3		)	3.3			(21.7		)	(31.7		)
Income (loss) from discontinued operations, net of tax	4.3		539.1		2.8			31.3			8.0			84.4			383.6			47.4
Non-controlling interest in consolidated subsidiaries	10.5		10.6		12.0			1.0			17.4			3.1			(21.5		)	(6.2		)
Income (loss) attributable to White Mountains’s common shareholders	$	22.5	$	562.1	$	13.6		$	29.0		$	(38.9	)	$	90.8		$	340.4		$	9.5
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	4.85	$	5.36	$	2.36		$	(0.50	)	$	(10.27	)	$	1.31		$	(8.47	)	$	(6.85	)
Discontinued operations	1.15		125.45		.61			6.86			1.75			17.34			75.27			8.55
Total consolidated operations	$	6.00	$	130.81	$	2.97		$	6.36		$	(8.52	)	$	18.65		$	66.80		$	1.70
Diluted
Continuing operations	$	4.85	$	5.36	$	2.36		$	(0.50	)	$	(10.27	)	$	1.31		$	(8.47	)	$	(6.85	)
Discontinued operations	1.15		125.45		.61			6.86			1.75			17.30			75.11			8.55
Total consolidated operations	$	6.00	$	130.81	$	2.97		$	6.36		$	(8.52	)	$	18.61		$	66.64		$	1.70

The following tables present the impact of the financial statement revisions to previously reported selected quarterly financial data. See Note 20 — “Financial Statement Revisions”. The impact of the financial statement revisions for the three months ended September 30, 2016 was included in the period ended September 30, 2017 Form 10-Q.



	Three Months Ended June 30, 2017									Three Months Ended March 31, 2017
	As previously reported ⁽¹⁾									As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised			As previously reported ⁽¹⁾		Adjustments			As revised
Revenues	$	85.4		$	(1.9	)	$	83.5		$	93.5	$	(4.7	)	$	88.8
Expenses	85.8			(.1		)	85.7			92.2		.2			92.4
Pre-tax (loss) income	(.4		)	(1.8		)	(2.2		)	1.3		(4.9		)	(3.6		)
Tax benefit	1.0						1.0			.3		—			.3
Income (loss) from continuing operations	.6			(1.8		)	(1.2		)	1.6		(4.9		)	(3.3		)
Income from discontinued operations, net of tax	2.8			—			2.8			31.3		—			31.3
Non-controlling interest in consolidated subsidiaries	12.1			(.1		)	12.0			1.3		(.3		)	1.0
Income attributable to White Mountains’s common shareholders	$	15.5		$	(1.9	)	$	13.6		$	34.2	$	(5.2	)	$	29.0
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	2.78		$	(.42	)	$	2.36		$	.65	$	(1.15	)	$	(0.50	)
Discontinued operations	.61			—			.61			6.86		—			6.86
Total consolidated operations	$	3.39		$	(.42	)	$	2.97		$	7.51	$	(1.15	)	$	6.36
Diluted
Continuing operations	$	2.78		$	(.42	)	$	2.36		$	.65	$	(1.15	)	$	(0.50	)
Discontinued operations	.61			—			.61			6.86		—			6.86
Total consolidated operations	$	3.39		$	(.42	)	$	2.97		$	7.51	$	(1.15	)	$	6.36

⁽¹⁾ Amounts previously reported reflect the effect of reclassifying OneBeacon as discontinued operations. See Note 19 — “Held for Sale and Discontinued Operations”.



	Three Months Ended December 31, 2016
	As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised
Revenues	$	.3		$	(5.8	)	$	(5.5	)
Expenses	69.1			(.1		)	69.0
Pre-tax loss	(68.8		)	(5.7		)	(74.5		)
Tax benefit	10.2			—			10.2
Loss from continuing operations	(58.6		)	(5.7		)	(64.3		)
Income from discontinued operations, net of tax	8.0			—			8.0
Non-controlling interest in consolidated subsidiaries	17.3			.1			17.4
Loss attributable to White Mountains’s common shareholders	$	(33.3	)	$	(5.6	)	$	(38.9	)
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(9.04	)	$	(1.23	)	$	(10.27	)
Discontinued operations	1.75			—			1.75
Total consolidated operations	$	(7.29	)	$	(1.23	)	$	(8.52	)
Diluted
Continuing operations	$	(9.04	)	$	(1.23	)	$	(10.27	)
Discontinued operations	1.75			—			1.75
Total consolidated operations	$	(7.29	)	$	(1.23	)	$	(8.52	)

⁽¹⁾ Amounts previously reported reflect the effect of reclassifying OneBeacon, Tranzact, Sirius, and Esurance as discontinued operations. See Note 19 — “Held for Sale and Discontinued Operations”.



	Three Months Ended June 30, 2016									Three Months Ended March 31, 2016
	As previously reported ⁽¹⁾									As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised			As previously reported ⁽¹⁾			Adjustments			As revised
Revenues	$	49.2		$	(.8	)	$	48.4		$	58.8		$	(.8	)	$	58.0
Expenses	74.3			(.2		)	74.1			88.5			2.8			91.3
Pre-tax loss	(25.1		)	(.6		)	(25.7		)	(29.7		)	(3.6		)	(33.3		)
Tax benefit	4.0			—			4.0			1.6			—			1.6
Loss from continuing operations	(21.1		)	(.6		)	(21.7		)	(28.1		)	(3.6		)	(31.7		)
Income from discontinued operations, net of tax	383.6			—			383.6			47.4			—			47.4
Non-controlling interest in consolidated subsidiaries	(21.4		)	(.1		)	(21.5		)	(6.3		)	.1			(6.2		)
Income attributable to White Mountains’s common shareholders	$	341.1		$	(.7	)	$	340.4		$	13.0		$	(3.5	)	$	9.5
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(8.34	)	$	(.13	)	$	(8.47	)	$	(6.22	)	$	(.63	)	$	(6.85	)
Discontinued operations	75.27			—			75.27			8.55			—			8.55
Total consolidated operations	$	66.93		$	(.13	)	$	66.80		$	2.33		$	(.63	)	$	1.70
Diluted
Continuing operations	$	(8.34	)	$	(.13	)	$	(8.47	)	$	(6.22	)	$	(.63	)	$	(6.85	)
Discontinued operations	75.11			—			75.11			8.55			—			8.55
Total consolidated operations	$	66.77		$	(.13	)	$	66.64		$	2.33		$	(.63	)	$	1.70

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

12 Months Ended

Dec. 31, 2017

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, 2017



Millions	Cost		Carrying Value		Fair Value
Fixed maturity investments:
U.S. Government and government agencies and authorities	$	297.8	$	296.5	$	296.5
Debt securities issued by corporations	867.6		880.9		880.9
Mortgage and asset-backed securities	697.2		694.7		694.7
States, municipalities and political subdivisions	252.0		254.9		254.9
Foreign governments	2.6		2.7		2.7
Total fixed maturity investments	2,117.2		2,129.7		2,129.7
Short-term investments	176.1		176.1		176.1
Common equity securities:
Exchange traded funds	512.5		569.7		569.7
Banks, trust and insurance companies	12.8		16.3		16.3
Industrial, miscellaneous and other	214.4		280.1		280.1
Total common equity securities	739.7		866.1		866.1
Other long-term investments ⁽¹⁾	246.6		208.8		208.8
Total investments	$	3,279.6	$	3,380.7	$	3,380.7


⁽¹⁾	Includes carrying value of $(3.7) related to foreign currency forward contracts.

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, 2017

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⁽¹⁾



	December 31,
Millions	2017			2016
Assets:
Cash	$	14.9		$	3.1
Fixed maturity investments, at fair value	869.6			80.0
Common equity securities, at fair value	641.8			—
Other long-term investments ⁽²⁾	(3.7		)	(1.2		)
Short-term investments, at amortized cost	57.2			12.5
Other assets	30.9			21.8
Investments in consolidated subsidiaries ⁽³⁾	1,914.8			4,877.6
Total assets	$	3,525.5		$	4,993.8
Liabilities:
Payable to subsidiary ⁽⁴⁾	$	11.8		$	1,387.2
Other liabilities	21.2			23.9
Total liabilities	33.0			1,411.1
White Mountains’s common shareholders’ equity ⁽³⁾	3,492.5			3,582.7
Total liabilities and equity	$	3,525.5		$	4,993.8

⁽¹⁾ These condensed unconsolidated financial statements reflect the results of operations, financial position and cash flows for the Company. Investments in wholly-owned and majority-owned subsidiaries are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net basis as equity in earnings from consolidated and unconsolidated subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾As of December 31, 2017 and 2016, other investments includes $(3.7) and $(1.2) related to foreign currency forward contracts. See Note 7 — “Derivatives”.

⁽³⁾As of December 31, 2016, investments in consolidated affiliates and the Company’s common shareholders’ equity were revised to include the impact of the Wobi adjustments of $(20.6). See Note 20 — “Financial Statement Revisions”.

⁽⁴⁾ During 2016, the Company used cash proceeds received from the issuance of debt to its direct wholly-owned subsidiary, Lone Tree Holdings, Ltd.(“LTH”), primarily to fund repurchases of its common shares.

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME⁽¹⁾



	Year Ended December 31,
Millions	2017			2016			2015
Revenues (loss) (including realized gains and losses)	$	27.3		$	(1.0	)	$	5.0
Expenses	99.7			68.2			59.9
Pre-tax loss	(72.4		)	(69.2		)	(54.9		)
Income tax expense	(1.4		)	(.5		)	—
Net loss	(73.8		)	(69.7		)	(54.9		)
Equity in earnings from consolidated and unconsolidated subsidiaries ⁽²⁾	701.0			471.5			350.1
Net income attributable to White Mountains’s common shareholders	627.2			401.8			295.2
Other comprehensive income (loss) items, after-tax	3.3			145.3			(100.4		)
Comprehensive income attributable to White Mountains’s common shareholders	$	630.5		$	547.1		$	194.8
Computation of net income available to common shareholders:
Net income available to common shareholders	$	627.2		$	401.8		$	295.2

⁽¹⁾ These condensed unconsolidated financial statements reflect the results of operations, financial position and cash flows for the Company. Investments in wholly-owned and majority-owned subsidiaries are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net basis as equity in earnings of subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾ During 2016 and 2015, equity in earnings from consolidated and unconsolidated affiliates was revised to include the impact of the Wobi adjustments of $(10.7) and $(2.4). See Note 20 — “Financial Statement Revisions”.

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

SCHEDULE II

(continued)

CONDENSED STATEMENTS OF CASH FLOWS ⁽¹⁾⁽²⁾



	Year Ended December 31,
Millions	2017			2016			2015
Net income attributable to White Mountains’s common shareholders ⁽³⁾	$	627.2		$	401.8		$	295.2
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses on sales of investments	(18.5		)	1.1			—
Undistributed earnings from subsidiaries ⁽³⁾	(701.0		)	(471.5		)	(350.1		)
Other non-cash reconciling items, primarily amortization of restricted share and option awards ⁽⁴⁾	31.1			17.9			(.4		)
Accumulated earnings distributed from subsidiary in cash ⁽⁵⁾	1,256.7			—			—
Net change in other assets and liabilities ⁽⁶⁾	(4.9		)	(5.6		)	28.1
Net cash provided from (used for) operations	1,190.6			(56.3		)	(27.2		)
Cash flows from investing activities:
Net change in short-term investments	(24.7		)	10.9			7.6
Purchases of investment securities	(474.7		)	—			—
Sales and maturities of investment securities	367.1			—			—
Issuance of debt from subsidiaries ⁽⁷⁾	382.0			992.0			271.0
Repayment of debt to subsidiaries	—			(5.0		)	(35.0		)
Contributions to subsidiaries ⁽⁸⁾	(700.0		)	—			—
Distributions from subsidiaries⁽⁹⁾	—			—			15.0
Net cash provided from investing activities	(450.3		)	997.9			258.6
Cash flows from financing activities:
Draw down of revolving line of credit ⁽¹⁰⁾	350.0			350.0			125.0
Repayment of revolving line of credit ⁽¹⁰⁾	(350.0		)	(400.0		)	(75.0		)
Proceeds from issuances of common shares	—			3.7			—
Repurchases and retirement of common shares ⁽⁷⁾	(714.6		)	(881.3		)	(268.6		)
Dividends paid on common shares	(4.6		)	(5.4		)	(6.0		)
Payments of restricted shares withholding taxes	(9.3		)	(5.8		)	(6.7		)
Net cash used for financing activities	(728.5		)	(938.8		)	(231.3		)
Net increase in cash during the year	11.8			2.8			.1
Cash balance at beginning of year	3.1			.3			.2
Cash balance at end of year	$	14.9		$	3.1		$	.3
Supplemental cash flow information:
Interest paid	$	(.6	)	$	(1.2	)	$	—

⁽¹⁾ These condensed unconsolidated financial statements reflect the results of operations, financial position and cash flows for the Company. Investments in wholly-owned and majority-owned subsidiaries are accounted for using the equity method. Under the equity method, investments in subsidiaries are recorded on the condensed balance sheets at the amount of the Company’s ownership percentage of the subsidiary’s GAAP book value. The income from subsidiaries is reported on a net basis as equity in earnings of subsidiaries on the condensed statements of operations and comprehensive income. Capital contributions to and distributions from subsidiaries are presented within investing activities on the condensed statements of cash flows.

⁽²⁾ During 2017, LTH, a direct wholly-owned subsidiary of the Company, merged into the Company. The merger was treated as a liquidation for financial statement purposes. As part of the liquidation, significant non-cash balances that were transferred from LTH to the Company included ending net equity of $2,810.4, intercompany balances of $1,863.1, investments in its subsidiaries of $964.4, short-term investments of $13.0 and other liabilities of $14.1.

⁽³⁾ During 2016 and 2015, net income attributable to White Mountains’s common shareholders and undistributed earnings from subsidiaries was revised to include the impact of the Wobi adjustments of $(10.7) and $(2.4). See Note 20 — “Financial Statement Revisions”.

⁽⁴⁾ For the years ended December 31, 2017, 2016, and 2015, amortization of restricted share and option awards was $14.8, $18.5 and $14.9.

⁽⁵⁾ During 2017, as part of its liquidation into the Company, LTH transferred $1,256.7 of cash, which included $1,037.6 of the proceeds from the sale of OneBeacon, to the Company.

⁽⁶⁾ For 2017, 2016 and 2015, net change in other assets and liabilities also included a $11.6, $0.2, and $2.4 net change in payables to the Company’s subsidiaries.


⁽⁷⁾	During 2017, the Company had non-cash issuance of debt from LTH of $94.2. During 2017, 2016 and 2015, the Company used cash proceeds received from the issuance of debt from LTH, primarily to fund repurchases of its common shares.

⁽⁸⁾During 2017, the Company contributed $700.0 to its direct wholly-owned subsidiary, Guilford Holdings, Inc.

⁽⁹⁾

During 2017, the Company received non-cash distributions of $1,238.9 from LTH, prior to its liquidation. The distribution was completed through the transfer of fixed maturity investments and common equity securities. During 2016, the Company received a non-cash distribution of $80.0 from LTH. The distribution was completed through the transfer of fixed maturity investments. During 2015, the Company received cash distributions of $15.0 from LTH.


⁽¹⁰⁾	The WTM Bank Facility presented in Note 5 — “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, 2017

Supplementary Insurance Information [Abstract]

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUPPLEMENTARY INSURANCE INFORMATION ⁽³⁾



Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H		Column I		Column J		Column K
Millions Segment	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, 2017
HG Global/BAM	$	14.8	$	—	$	136.8	$	—	$	9.4	$	12.3	$	—	$	4.0	$	.4	$	63.2
Other operations ⁽²⁾	—		—		—		—		1.0		—		1.1		.1		—		0.9
December 31, 2016
HG Global/BAM	10.6		—		82.9		—		5.9		9.0		—		3.4		.4		38.6
Other operations ⁽²⁾	—		—		—		—		7.5		.2		8.0		2.2		.1		6.5
December 31, 2015
HG Global/BAM	6.9		—		50.2		—		3.3		6.1		—		2.9		.4		25.9
Other operations ⁽²⁾	—		—		—		—		8.7		.2		8.2		3.4		—		10.1

⁽¹⁾ The amounts shown exclude net investment income relating to non-insurance operations of $43.7, $22.9 and $4.6 for the twelve months ended December 31, 2017, 2016 and 2015, respectively.

⁽²⁾The Other operations amounts shown relate to SSIE. White Mountains completed the sale of SSIE on March 7, 2017. See Note 19 — “Held for Sale and Discontinued Operations”.

⁽³⁾Schedule excludes activity related to OneBeacon and Sirius Group for all periods presented. See Note 19 — “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, 2017

Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]

SCHEDULE IV REINSURANCE

WHITE MOUNTAINS INSURANCE GROUP, LTD.

REINSURANCE ⁽²⁾



Column A	Column B		Column C			Column D		Column E		Column F
$ in millions Premiums earned	Gross amount		Ceded to other companies			Assumed from other companies		Net amount		Percentage of amount assumed to net
Years ended:
December 31, 2017
HG Global/BAM	$	9.4	$	—		$	—	$	9.4	—	%
Other operations ⁽¹⁾	1.0		—			—		1.0		—
December 31, 2016
HG Global/BAM	5.9		—			—		5.9		—	%
Other operations ⁽¹⁾	15.2		(7.7		)	—		7.5		—
December 31, 2015
HG Global/BAM	3.3		—			—		3.3		—	%
Other operations ⁽¹⁾	20.7		(12.0		)	$	—	8.7		—	%


⁽¹⁾	The Other operations amounts shown relate to SSIE. White Mountains completed the sale of SSIE on March 7, 2017. See Note 19 — “Held for Sale and Discontinued Operations”.


⁽²⁾	Schedule excludes activity related to OneBeacon and Sirius Group for all periods presented. See Note 19 — “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, 2017

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 ⁽²⁾



Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H					Column I		Column J		Column K
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		Claims and Claims Adjustment Expenses Incurred Related to					Amortization of deferred policy acquisition costs		Paid Claims and Claims Adjustment Expenses		Premiums written
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		Current Year		Prior Year			Amortization of deferred policy acquisition costs		Paid Claims and Claims Adjustment Expenses		Premiums written
Other operations ⁽¹⁾:
2017	$	—	$	—	$	—	$	—	$	1.0	$	—	$	1.1	$	—		$	0.1	$	—	$	0.9
2016	—		—		—		—		7.5		.2		8.6		(0.6		)	2.2		8.8		6.5
2015	—		—		—		—		8.7		.2		10.4		(2.2		)	3.4		10.4		10.1


⁽¹⁾	The Other operations amounts shown relate to SSIE. White Mountains completed the sale of SSIE on March 7, 2017. See Note 19 — “Held for Sale and Discontinued Operations”.


⁽²⁾	Schedule excludes amounts related to OneBeacon and Sirius Group for all periods presented. See Note 19 — “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, 2017
Accounting Policies [Abstract]
Nature of Operations and Basis of Presentation	Basis of Presentation 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. 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 other entities required to be consolidated under GAAP. Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. See Note 15 — “Variable Interest Entities”. Intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.
Use of Estimates	Use of Estimates 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.
Reportable Segments	Reportable Segments White Mountains has determined its reportable segments based on the nature of the of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. White Mountains’s reportable segments are HG Global/BAM, MediaAlpha and Other Operations. See Note 13 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal 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 up to 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, 2017 and 2016, 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, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The MediaAlpha segment consists of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a leading marketing technology company that develops technology that enables the programmatic buying and selling of vertical-specific, performance-based media between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory) through cost-per-click, cost-per-call and cost-per-lead pricing models. MediaAlpha’s media buying platform enables advertisers to create and automate data-driven bidding strategies designed to improve the efficiency and enhance overall performance of their marketing campaigns that target high-intent consumers at the time and place they are ready to purchase. MediaAlpha’s publisher platform is used by publishers to sell their vertical-specific, performance-based media to advertisers through transparent, programmatic, auction-based marketplaces. MediaAlpha works with 400 advertisers and 300 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company, its wholly-owned subsidiary, White Mountains Capital, Inc. (“WM Capital”), its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), and its other intermediate holding companies, as well as certain consolidated and unconsolidated private capital and other investments. The consolidated private capital investments include Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). During 2017, White Mountains revised certain of its previously issued financial statements for amounts relating to Wobi. See Note 20 — “Financial Statement Revisions”. White Mountains’s Other Operations segment also included its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts fully matured on June 30, 2016 and was liquidated in the third quarter of 2017, and its U.S.-based service provider, White Mountains Financial Services LLC, which was liquidated in the second quarter of 2017 (collectively, “WM Life Re”).
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 7 — “Derivatives”.
Short-term Investments	Short-Term Investments 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 or accreted cost, which approximated fair value as of December 31, 2017 and 2016.
Investment securities	Investment Securities As of December 31, 2017, 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, including interests in hedge funds, private equity funds, non-controlling interests in private capital investments and foreign currency forward contracts. 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. 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, 2017 and 2016, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 94% of the 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 include fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, mortgage and asset-backed securities, municipal obligations, and foreign government, agency and provincial obligations. 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, which 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. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain 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 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 follows: 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. 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.
Other Long-term investments	Other Long-Term Investments Other long-term investments consist primarily of hedge funds, private equity funds, non-controlling interests in private capital investments and foreign currency forward contracts. See Note 3 — “Investment Securities”. White Mountains has taken the fair value option for most of its other long-term investments. 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. 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 as well as 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 methods and inputs for each underlying investment, White Mountains considers the inputs 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 the fair value of a hedge fund or private equity fund differs from the NAV 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 investment in the hedge fund or private equity fund. As of December 31, 2017 and 2016, White Mountains did not adjust the reported NAV of its investments in hedge funds and private equity funds.
Derivative Financial Instruments	Derivatives Financial Instruments: White Mountains holds from time to time a variety of derivative financial instruments for risk management purposes. White Mountains recognizes all derivatives as either other assets or other liabilities, aside from the foreign currency forward contracts which are recognized within other long-term investments, 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 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	Variable Annuity Reinsurance: In 2016, White Mountains completed the run-off of WM Life Re as all of its contracts matured as of June 30, 2016. WM Life Re entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. The liability for guaranteed minimum death benefits was classified as a derivative and measured at fair value. The liability for guaranteed minimum accumulation benefits was classified as an insurance liability, and was measured using 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. 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. All WM Life Re’s derivative financial 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.
Receivables	Receivables Receivables consist primarily of premiums receivable from BAM customers for municipal bond insurance policies and advertising fees receivable from publishers and advertisers of MediaAlpha.
Incentive Compensation	Incentive Compensation 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 of White Mountains. Non-share based awards are recognized over the related service periods based on management’s best estimate of the amounts at which the awards are expected to be paid. Share-based compensation which is typically settled in cash, such as performance shares or performance units, is classified as a liability-type award. The compensation cost for liability-classified awards is measured initially at the grant date fair value and remeasured each reporting period until settlement. The compensation cost for equity-classified awards expected to be settled in shares, such as options and restricted shares, is measured at the original grant date fair value of the award. The compensation cost for all awards is recognized for the vested portion of the awards over the related service periods. See Note 10 — “Employee Share-Based Incentive Compensation”.
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, 2017, 2016 and 2015. See Note 4 — “Goodwill and Other Intangible Assets”.
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee 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. Installment premiums are measured at the present value of contractual premiums, discounted at the risk free rate, which is set at the inception of the insurance contract. 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. BAM’s obligation for outstanding contracts consists of the unearned premium reserve and any loss reserves. Loss reserves are recorded only to the extent that the present value of the expected amount of any losses to be paid, net of any expected recoveries, exceeds the associated unearned premium reserve. As of December 31, 2017 and 2016, BAM did not have any loss or LAE reserves.
Revenue Recognition	Revenue Recognition White Mountains recognizes advertising and publishing fee revenues based on the contractual amount of the fees, adjusted for any amounts expected to be refunded or uncollectible, when it has satisfied its contractual performance obligations, which is generally at the time each transaction is executed. For transactions where MediaAlpha acts as the principal, such as the Buyer and Open exchanges, revenue amounts are reported gross. For transactions where MediaAlpha acts as an agent facilitating transactions between third parties, revenue amounts are reported at the net fee billed. Agent and commission revenues are measured based on the contractual rates with insurance carriers, net of any amounts expected to be uncollectible and any amounts associated with expected policy cancellations adjustments, and are recognized when contractual performance obligations have been fulfilled.
Federal and foreign income taxes	Federal and Foreign Income Taxes A number 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 functional currency for White Mountains’s non-U.S. based subsidiaries are measured, in most instances, using functional currencies other than the U.S. dollar. 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. 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.
Noncontrolling Interest	Non-controlling Interests 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 11 — “Common Shareholders’ Equity and Non-controlling Interests”.
Recent Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Stock Compensation Effective January 1, 2017, White Mountains adopted 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 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 upon adoption changes were recognized in the statement of cash flows with no other significant effect on the rest of the financial statements. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, 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. 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 statements. Recently Issued Accounting Pronouncements Revenue Recognition Effective January 1, 2018, White Mountains plans to adopt ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under the new guidance, revenue is recognized at an amount that reflects the consideration to which an entity expects to be entitled once it fulfills its performance obligations under the terms of its contract with the customer. The new guidance does not apply to revenue from insurance contracts, investment income or investments gains and losses. The new guidance is applicable to some of White Mountains’s revenue streams, including agent commissions and other non-insurance revenues. White Mountains does not expect adoption to have any significant effect on its financial statements. Stock Compensation In May 2017, the FASB issued ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. The new guidance becomes effective for fiscal years beginning after December 15, 2017. White Mountains does not expect adoption to have any effect on its financial statements. Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium will be amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. The new guidance is effective for annual periods beginning after December 15, 2018. White Mountains does not expect adoption to have any effect on its financial statements. Definition of a Business In January 2017, the FASB issued ASU 2017-07, Business Combinations: Clarifying the Definition of a Business (ASC 805), which clarifies the definition of a business and affects the determination of whether acquisitions or disposals are accounted for as assets or as a business. Under the new guidance, when substantially all of the fair value of the assets is concentrated in a single identifiable asset or a group of similar identifiable assets, it is not a business. The new guidance is effective for annual and interim periods after December 15, 2017. White Mountains does not expect adoption to have any effect on its financial statements. Goodwill In January 2017, the FASB issue ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changes the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same. However, the second step required under the existing guidance has been eliminated. Goodwill is considered impaired if the carrying value exceeds the estimated fair value. The new guidance is effective for fiscal years beginning after December 15, 2019. 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 does not expect adoption to have any effect on its financial statements. 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 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. This differs from current GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include loans and trade receivables such as premium receivables and reinsurance recoverables on paid losses. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and accordingly does not expect adoption to have any effect on its financial statements. 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 expected impact of this 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 and accordingly, does not expect adoption to have any effect on its financial statements.

Significant Transactions (Tables)

12 Months Ended

Dec. 31, 2017

Significant Transactions [Abstract]

Reconciliation to the increase in book value

The following table presents a reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value:



Millions	Year ended December 31, 2016
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
Increase to White Mountains’s book value from sale of Tranzact	$	82.1

Investment Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Pre-tax net investment income

The following table presents pre-tax net investment income for 2017, 2016 and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Investment income:
Fixed maturity investments	$	44.9		$	28.5		$	8.9
Short-term investments	1.8			.9			—
Common equity securities	10.6			4.0			4.0
Other long-term investments	1.2			1.1			2.4
Total investment income	58.5			34.5			15.3
Third-party investment expenses	(2.5		)	(2.4		)	(4.4		)
Net investment income, pre-tax	$	56.0		$	32.1		$	10.9

Net Realized and Unrealized Investment Gains and Losses

The following table presents net realized and unrealized investment gains (losses) for 2017, 2016, and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Net realized investment gains, pre-tax	$	24.1		$	270.0		$	30.0
Net unrealized investment gains (losses), pre-tax	109.2			(297.4		)	230.5
Net realized and unrealized investment gains (losses), pre-tax	133.3			(27.4		)	260.5
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(12.9		)	2.7			(47.7		)
Net realized and unrealized investment gains (losses), after-tax	$	120.4		$	(24.7	)	$	212.8

Realized Gain (Loss) on Investments

The following tables present net realized investment gains (losses) for 2017, 2016 and 2015:



	Year Ended December 31, 2017
Millions	Net realized (losses) gains			Net foreign exchange gains (losses)			Total net realized gains (losses) reflected in earnings
Fixed maturity investments	$	(1.6	)	$	4.1		$	2.5
Short-term investments	(.3		)	—			(.3		)
Common equity securities	18.1			6.0			24.1
Other long-term investments	19.1			(21.3		)	(2.2		)
Net realized investment gains (losses), pre-tax	35.3			(11.2		)	24.1
Income tax expense attributable to net realized investment gains (losses)	(8.9		)	—			(8.9		)
Net realized investment gains (losses), after-tax	$	26.4		$	(11.2	)	$	15.2



	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	$	(1.9	)	$	.3	$	(1.6	)
Short-term investments	.4			—		.4
Common equity securities	268.5			—		268.5
Other long-term investments	2.7			—		2.7
Net realized investment gains, pre-tax	269.7			.3		270.0
Income tax expense attributable to net realized investment gains	(45.6		)	—		(45.6		)
Net realized investment gains, after-tax	$	224.1		$	.3	$	224.4



	Year Ended December 31, 2015
Millions	Net realized (losses) gains			Net foreign exchange gains		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(.2	)	$	—	$	(.2	)
Common equity securities	31.0			—		31.0
Other long-term investments	(.8		)	—		(.8		)
Net realized investment gains, pre-tax	30.0			—		30.0
Income tax expense attributable to net realized investment gains	(6.4		)	—		(6.4		)
Net realized investment gains, after-tax	$	23.6		$	—	$	23.6

Unrealized Gain (Loss) on Investments

The following tables present net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the years ended 2017, 2016 and 2015:



	Year Ended December 31, 2017
Millions	Net unrealized gains (losses)			Net foreign exchange gains (losses)			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	13.8		$	12.7		$	26.5
Common equity securities	99.3			—			99.3
Other long-term investments	(15.6		)	(1.0		)	(16.6		)
Net unrealized investment gains, pre-tax	97.5			11.7			109.2
Income tax expense attributable to net unrealized investment gains	(4.0		)	—			(4.0		)
Net unrealized investment gains, after-tax	$	93.5		$	11.7		$	105.2



	Year Ended December 31, 2016
Millions	Net unrealized losses			Net foreign exchange gains (losses)			Total net unrealized losses reflected in earnings
Fixed maturity investments	$	(14.6	)	$	2.1		$	(12.5	)
Common equity securities	(257.4		)	(3.3		)	(260.7		)
Other long-term investments	(22.7		)	(1.5		)	(24.2		)
Net unrealized investment losses, pre-tax	(294.7		)	(2.7		)	(297.4		)
Income tax benefit attributable to net unrealized investment losses	48.3			—			48.3
Net unrealized investment losses, after-tax	$	(246.4	)	$	(2.7	)	$	(249.1	)



	Year Ended December 31, 2015
Millions	Net unrealized gains (losses)			Net foreign exchange losses			Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	.1		$	—		$	.1
Common equity securities	240.8			(3.9		)	236.9
Other long-term investments	(5.4		)	(1.1		)	(6.5		)
Net unrealized investment gains (losses), pre-tax	235.5			(5.0		)	230.5
Income tax expense attributable to net unrealized investment gains	(41.3		)	—			(41.3		)
Net unrealized investment gains (losses), after-tax	$	194.2		$	(5.0	)	$	189.2

Net unrealized investment gains (losses) for Level 3 investments



	Year Ended December 31,
Millions	2017			2016			2015
Fixed maturity investments	$	—		$	.1		$	—
Common equity securities	—			—			(9.0		)
Other long-term investments	(15.4		)	(14.3		)	0.9
Total net unrealized investment losses, pre-tax - Level 3 investments	$	(15.4	)	$	(14.2	)	$	(8.1	)

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



	Year Ended December 31,
Millions	2017		2016			2015
Net change in pre-tax unrealized investment losses on investments in unconsolidated affiliates	$	—	$	—		$	(39.2	)
Income tax benefit	—		—			2.9
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	—		—			(36.3		)
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	—		—			1.4
Total investment losses through accumulated other comprehensive income	—		—			(34.9		)
Net realized and unrealized investment gains (losses), after-tax	120.4		(24.7		)	212.8
Total investment gains (losses) recorded during the period, after-tax	$	120.4	$	(24.7	)	$	177.9

Investment holdings, fixed maturity investments



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	297.8	$	—	$	(1.3	)	$	—	$	296.5
Debt securities issued by corporations	867.6		2.9		(4.3		)	14.7		880.9
Mortgage and asset-backed securities	697.2		1.6		(4.1		)	—		694.7
Municipal obligations	252.0		3.7		(.8		)	—		254.9
Foreign government, agency and provincial obligations	2.6		—		—			.1		2.7
Total fixed maturity investments	$	2,117.2	$	8.2	$	(10.5	)	$	14.8	$	2,129.7



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains		Carrying value
U.S. Government and agency obligations	$	112.1	$	—	$	(1.1	)	$	—	$	111.0
Debt securities issued by corporations	752.0		2.3		(10.1		)	2.1		746.3
Mortgage and asset-backed securities	986.9		.8		(7.9		)	—		979.8
Municipal obligations	238.7		1.1		(1.3		)	—		238.5
Foreign government, agency and provincial obligations	12.0		.1		—			—		12.1
Total fixed maturity investments	$	2,101.7	$	4.3	$	(20.4	)	$	2.1	$	2,087.7
Less: Fixed maturity investments reclassified to assets held for sale related to SSIE										6.6
Total fixed maturity investments										$	2,081.1

Schedule of contractual maturities of fixed maturities



	December 31, 2017
Millions	Cost or amortized cost		Carrying value
Due in one year or less	$	109.0	$	108.8
Due after one year through five years	663.0		660.9
Due after five years through ten years	464.9		472.0
Due after ten years	183.1		193.3
Mortgage and asset-backed securities	697.2		694.7
Total	$	2,117.2	$	2,129.7

Investment holdings, equity securities, convertible fixed maturities and other long-term investments



	December 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	739.7	$	129.4	$	(3.0	)	$	—		$	866.1
Other long-term investments	$	246.6	$	6.8	$	(39.7	)	$	(4.9	)	$	208.8



	December 31, 2016
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency losses			Carrying value
Common equity securities	$	258.6	$	29.0	$	(2.0	)	$	—		$	285.6
Other long-term investments	$	194.0	$	7.9	$	(25.2	)	$	(3.9	)	$	172.8

Fair value measurements by level, investment securities



	December 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	296.5	$	296.5	$	—	$	—
Debt securities issued by corporations:
Consumer	185.1		—		185.1		—
Communications	127.8		—		127.8		—
Financials	114.8		—		114.8
Utilities	108.9		—		108.9		—
Materials	95.5		—		95.5		—
Health Care	94.3		—		94.3		—
Technology	80.5		—		80.5		—
Energy	48.1		—		48.1		—
Industrial	25.9		—		25.9		—
Total debt securities issued by corporations:	880.9		—		880.9		—

Mortgage and asset-backed securities	694.7		—		694.7		—
Municipal obligations	254.9		—		254.9		—
Foreign government, agency and provincial obligations	2.7		—		2.7		—
Total fixed maturity investments	2,129.7		296.5		1,833.2		—

Short-term investments ⁽¹⁾	176.1		151.0		25.1		—

Common equity securities:

Exchange traded funds ⁽²⁾	569.7		508.1		61.6		—
Health Care	17.1		17.1		—		—
Financials	16.3		16.3		—		—
Technology	15.1		15.1		—		—
Industrial	11.9		11.9		—		—
Communications	10.9		10.9		—		—
Consumer	10.7		10.7		—		—
Energy	3.8		3.8		—		—
Other ⁽³⁾	210.6		—		210.6		—
Total common equity securities	866.1		593.9		272.2		—
Other long-term investments ⁽⁴⁾⁽⁵⁾	87.2		—		—		87.2
Total investments	$	3,259.1	$	1,041.4	$	2,130.5	$	87.2

⁽¹⁾Short-term investments are measured at amortized cost, which approximates fair value.

⁽²⁾ 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⁾Consists of two investments in unit trusts that primarily invest in international equities

⁽⁴⁾Excludes carrying value of $(3.7) related to foreign currency forward contracts.

⁽⁵⁾ Excludes carrying value of $125.3 associated with a hedge fund and private equity funds for which fair value is measured at NAV using the practical

expedient.



	December 31, 2016
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	111.0	$	101.5	$	9.5	$	—
Debt securities issued by corporations:
Consumer	190.8		—		190.8		—
Utilities	140.8		—		140.8		—
Health Care	114.9		—		114.9		—
Financials	79.7		—		79.7		—
Communications	72.0		—		72.0		—
Materials	65.0		—		65.0		—
Technology	48.8		—		48.8		—
Industrial	28.2		—		28.2		—
Energy	6.1		—		6.1		—
Total debt securities issued by corporations:	746.3		—		746.3		—

Mortgage and asset-backed securities	979.8		—		979.8		—
Municipal obligations	238.5		—		238.5		—
Foreign government, agency and provincial obligations	12.1		—		12.1		—
Total fixed maturity investments ⁽¹⁾	2,087.7		101.5		1,986.2		—

Short-term investments ⁽¹⁾⁽²⁾	175.0		162.3		12.7		—

Common equity securities:
Exchange traded funds ⁽³⁾	157.2		129.4		27.8		—
Health Care	13.9		13.9		—		—
Consumer	8.6		8.6		—		—
Financials	7.7		7.7		—		—
Technology	7.3		7.3		—		—
Communications	7.0		7.0		—		—
Energy	2.5		2.5		—		—
Industrial	1.5		1.5		—		—
Other ⁽⁴⁾	79.9		—		79.9		—
Total common equity securities	285.6		177.9		107.7		—
Other long-term investments ⁽⁵⁾⁽⁶⁾	91.4		—		—		91.4
Total investments ⁽¹⁾	$	2,639.7	$	441.7	$	2,106.6	$	91.4

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

⁽³⁾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.

⁽⁴⁾Consists of an investment in a unit trust that primarily invests in international equities.

⁽⁵⁾ Excludes carrying value of $(1.2) related to foreign currency forward contracts.

⁽⁶⁾Excludes carrying value of $82.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

Debt securities issued by corporations, credit ratings

The following table presents the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2017 and 2016:



	Fair Value at December 31,
Millions	2017		2016
AAA	$	1.6	$	—
AA	42.6		37.3
A	192.5		212.8
BBB	465.2		335.6
BB	161.7		143.2
B	17.3		17.4
Debt securities issued by corporations ⁽¹⁾⁽²⁾	$	880.9	$	746.3


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

⁽²⁾Includes carrying value of $4.2 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE.

Mortgage-backed, asset-backed securities

The following table presents the carrying value of White Mountains’s mortgage and asset-backed securities as of December 31, 2017 and 2016:



	December 31, 2017						December 31, 2016
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	46.3	$	46.3	$	—	$	70.3	$	70.3	$	—
FNMA	84.5		84.5		—		235.5		235.5		—
FHLMC	62.0		62.0		—		59.5		59.5		—
Total Agency ⁽¹⁾	192.8		192.8		—		365.3		365.3		—
Non-agency:
Residential	—		—		—		70.3		70.3		—
Commercial	70.5		70.5		—		3.9		3.9		—
Total Non-agency	70.5		70.5		—		74.2		74.2		—

Total mortgage-backed securities	263.3		263.3		—		439.5		439.5		—
Other asset-backed securities:
Credit card receivables	206.0		206.0		—		214.2		214.2		—
Vehicle receivables	142.4		142.4		—		205.9		205.9		—
Other	83.0		83.0		—		120.2		120.2		—
Total other asset-backed securities	431.4		431.4		—		540.3		540.3		—
Total mortgage and asset-backed securities	$	694.7	$	694.7	$	—	$	979.8	$	979.8	$	—


⁽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 commercial mortgage securities, type of interest rate and tranche levels

The following table presents the amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities as of December 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Fixed rate CMBS	$	35.1	$	—	$	27.5	$	7.6
Floating rate CMBS	35.4		—		—		35.4
Total	$	70.5	$	—	$	27.5	$	43.0


⁽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

Non-agency commercial backed mortgage securities

The following table presents the security issuance years of White Mountains’s investments in non-agency CMBS as of December 31, 2017:



			Security Issuance Year
Millions	Fair Value		2017		2016
Non-agency CMBS	$	70.5	$	67.0	$	3.5

Other Investments Not Readily Marketable

The following table presents the carrying values of White Mountains’s other long-term investments as of December 31, 2017 and 2016:



	Carrying Value at December 31,
Millions	2017			2016
Hedge funds and private equity funds, at fair value	$	125.3		$	82.6
Private equity securities, at fair value ^(1)(2)(3)	83.2			88.2
Foreign currency forward contracts	(3.7		)	(1.2		)
Other	4.0			3.2
Total other long-term investments	$	208.8		$	172.8

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾ Includes non-controlling interests in common equity securities, limited liability companies and private convertible preferred securities.

Other long-term investments

The following table presents investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2017 and 2016:



	December 31, 2017				December 31, 2016
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financial	$	54.9	$	—	$	21.5	$	—
Long/short equity REIT	—		—		19.9		—
Total hedge funds	54.9		—		41.4		—

Private equity funds
Manufacturing/Industrial	43.3		10.4		19.4		22.9
Aerospace/Defense/Government	15.8		12.9		19.4		25.9
Direct lending	7.1		23.1		1.4		28.6
Financial Services	4.2		11.7		1.0		5.0
Insurance	—		41.2		—		41.2
Total private equity funds	70.4		99.3		41.2		123.6
Total hedge and private equity funds included in other long-term investments	$	125.3	$	99.3	$	82.6	$	123.6

Fair Value of private equity funds subject to lock-up periods

The following table presents investments in private equity funds that were subject to lock-up periods as of December 31, 2017:



Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private Equity Funds — expected lock-up period remaining	$	4.7	$	5.6	$	32.9	$	27.2	$	70.4

Rollforward of fair value investments by level

The following tables present the changes in White Mountains’s fair value measurements by level for the years ended December 31, 2017 and 2016:



							Level 3 Investments						Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾			Total
Balance at January 1, 2017	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)
Net realized and unrealized gains (losses)	82.7			69.6			—			(15.3		)	20.4			157.4			⁽⁴⁾
Amortization/Accretion	—			(9.1		)	—			—			—			(9.1		)
Purchases	1,209.3			2,007.9			31.2			13.1			71.0			3,332.5
Sales	(681.1		)	(2,070.3		)	(12.5		)	(2.0		)	(48.7		)	(2,814.6		)
Deconsolidation of SSIE	—			(5.2		)	—			—			—			(5.2		)
Transfers in	—			18.7			—			—			—			18.7
Transfers out	—			—			(18.7		)	—			—			(18.7		)
Balance at December 31, 2017	$	890.4		$	2,105.4		$	—		$	87.2		$	125.3		$	3,208.3		⁽¹⁾⁽²⁾


⁽¹⁾	Excludes carrying value of $(3.7) and $(1.2) as of December 31, 2017 and January 1, 2017 associated with foreign currency forward contracts.


⁽²⁾	Excludes carrying value of $176.1 and $175.0 as of December 31, 2017 and January 1, 2017 classified as short-term investments, of which $0.1 is classified as held for sale at January 1, 2017.


⁽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⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts and short-term investments of $23.8 and $0.3 for the year ended December 31, 2017.


⁽⁵⁾	Includes carrying value of $6.6 of fixed maturity investments at January 1, 2017 that is classified as assets held for sale related to SSIE.



							Level 3 Investments						Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾
Millions	Level 1 Investments			Level 2 Investments			Fixed maturity investments			Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV ⁽³⁾			Total
Balance at January 1, 2016	$	789.0		$	585.6		$	—		$	103.6		$	65.3		$	1,543.5		⁽¹⁾⁽²⁾
Net realized and unrealized gains (losses)	2.0			(8.0		)	.1			(14.3		)	(6.0		)	(26.2		)	⁽⁴⁾
Amortization/Accretion	.1			(6.2		)	—			—			—			(6.1		)
Purchases	1,746.9			2,868.8			69.9			2.2			40.5			4,728.3
Sales	(2,258.5		)	(1,416.4		)	—			(.1		)	(17.2		)	(3,692.2		)
Transfers in	—			70.0			—			—			—			70.0
Transfers out	—			—			(70.0		)	—			—			(70.0		)
Balance at December 31, 2016	$	279.5		$	2,093.8		$	—		$	91.4		$	82.6		$	2,547.3		^(1)(2)(5)


⁽¹⁾	Excludes carrying value of $175.0 and $142.0 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.

⁽²⁾ Includes carrying value of $9.5 and $6.6 of fixed maturity investments at January 1, 2016 and December 31, 2016 that is classified as assets held for sale related to SSIE.


⁽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⁾ Excludes realized and unrealized losses associated with foreign currency forward contracts of $1.2 for the year ended December 31, 2016.


⁽⁵⁾	Excludes carrying value of $(1.2) as of December 31, 2016 associated with foreign currency forward contracts.

Schedule of significant unobservable inputs used in estimating the fair value of investment securities



$ in millions, except share price	December 31, 2017
Description	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	Implied share price	-	$.68
Private equity security	Share price of most recent transaction	$3.6	Share price	-	$2.52
Private convertible preferred security	Multiple of EBITDA	$0.6	EBITDA multiple	-	6.00
Private convertible preferred security	Discounted cash flow	$14.5	Implied share price	-	$2.06
Private equity security	Discounted cash flow/ Option pricing method	$11.3	Discount rate		21.0%
			Time until expiration	-	4 years
		Volatility/Standard deviation		-	50.0%
		Risk free rate		-	1.77%

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



$ in millions, except share price	December 31, 2016
Description	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	Implied share price	-	$.68
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
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%

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

Goodwill and Other Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2017

Acquired Finite-Lived Intangible Assets [Line Items]

Schedule of Acquired Finite-Lived Intangible Assets by Major Class [Table Text Block]

The following table presents the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, by company:



$ in millions	Weighted Average Economic life (in years)	December 31, 2017						December 31, 2016
$ 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
Buzzmove	N/A	7.6		—		7.6		7.6		—		7.6
Total goodwill		25.9		—		25.9		25.9		—		25.9

Other intangible assets:
MediaAlpha
Customer relationships	8	36.6		10.2		26.4		10.0		6.2		3.8
Information technology	5	33.3		24.3		9.0		32.4		17.9		14.5
Subtotal		69.9		34.5		35.4		42.4		24.1		18.3

Buzzmove
Trademark	7	.6		.1		.5		.6		.1		.5
Information technology	5	.5		.2		.3		.5		—		.5
Subtotal		1.1		.3		.8		1.1		.1		1.0

Total other intangible assets		71.0		34.8		36.2		43.5		24.2		19.3
Total goodwill and other intangible assets		$	96.9	$	34.8	$	62.1	$	69.4	$	24.2	$	45.2

Schedule of Intangible Assets and Goodwill

The following table presents goodwill and other intangible assets as of December 31, 2017 and December 31, 2016:



Millions	December 31, 2017			December 31, 2016
Goodwill:
MediaAlpha	$	18.3		$	18.3
Buzzmove	7.6			7.6
Total goodwill	25.9			25.9

Other intangible assets:
MediaAlpha	35.4			18.3
Buzzmove	.8			1.0
Total other intangible assets	36.2			19.3
Total goodwill and other intangible assets	62.1			45.2
Goodwill and other intangible assets held for sale	—			1.2
Goodwill and other intangible assets attributed to non-controlling interests	(21.1		)	(17.1		)
Goodwill and other intangible assets included in White Mountains’s common shareholders' equity	$	41.0		$	29.3

Schedule of Goodwill and Intangible Assets Rollforward

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



	December 31,
	2017					2016
Millions	Goodwill		Other intangible assets			Goodwill			Other intangible assets
Beginning balance	$	25.9	$	19.3		$	18.6		$	26.9
Add: Star & Shield amounts held for sale at beginning of the period ⁽¹⁾	—		—			—			.4
Acquisitions of businesses and asset groups ⁽²⁾			27.6			7.6			5.0
Wobi write-off	—		—			(.3		)	(2.5		)
Amortization, including foreign currency translation	—		(10.7		)	—			(10.5		)
Ending balance	$	25.9	$	36.2		$	25.9		$	19.3

Schedule of Finite-Lived Intangible Assets, Future Amortization Expense

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



Millions	Amortization expense
2018	$	10.2
2019	5.0
2020	3.2
2021	2.9
2022	2.8
Total	$	24.1

Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of debt outstanding

The following table presents White Mountains’s debt outstanding as of December 31, 2017 and 2016:



	December 31,			Effective	December 31,			Effective
Millions	2017			Rate ⁽¹⁾	2016			Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
Unamortized issue costs	—				—
WTM Bank Facility, carrying value	—				—
MediaAlpha Bank Facility	23.9			5.6%	—			N/A
Unamortized issuance cost	(.1		)		—
MediaAlpha Bank Facility, carrying value	23.8				—
Previous MediaAlpha Bank Facility	—			N/A	12.9			5.7%
Unamortized issuance cost	—				(.2		)
Previous MediaAlpha Bank Facility, carrying value	—				12.7
Total debt	$	23.8			$	12.7

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

Schedule of contractual repayments of debt

The following table presents a schedule of contractual repayments of White Mountains’s debt as of December 31, 2017:



Millions	December 31, 2017
Due in one year or less	$	3.1
Due in two to three years	12.1
Due in four to five years	8.7
Due after five years	—
Total	$	23.9

- 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=84159872&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=75038535&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=84159872&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=84159872&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=84159872&loc=d3e21521-112644

+ Details

Derivatives (Tables)

12 Months Ended

Dec. 31, 2017

Derivative [Line Items]

Schedule of collateral

the gross notional amounts and carrying values associated with the foreign currency forward contracts as of December 31, 2017 and 2016:



	December 31, 2017						December 31, 2016
Millions	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾
Barclays Bank PLC	$	206.3	$	(3.7	)	A	$	184.6	$	(1.2	)	A-

Variable Annuity Reinsurance

Derivative [Line Items]

Pre-tax operating results

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



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

Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument



	Gains (Losses)						Carrying Value as of
	Year Ended December 31,						December 31,
Millions	2016			2015			2016
Fixed income/interest rate	$	1.8		$	6.4		$	—
Foreign exchange	(4.8		)	(7.3		)	—
Equity	1.0			(7.9		)	—
Total	$	(2.0	)	$	(8.8	)	$	—

Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table

The following tables present the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the years ended December 31, 2016 and 2015:



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


⁽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.



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


⁽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 $5.8 as of December 31, 2015 posted as collateral to its reinsurance counterparties.

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of total income tax benefit (expense)

The following table presents the total income tax benefit for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
Millions	2017			2016			2015
Current tax (expense) benefit:
U.S. federal	$	(.3	)	$	21.4		$	—
State	(1.3		)	(.7		)	(.6		)
Non-U.S.	(2.0		)	(.3		)	(.8		)
Total current tax (expense) benefit	(3.6		)	20.4			(1.4		)
Deferred tax benefit (expense):
U.S. federal	11.4			12.5			(11.3		)
Total deferred tax benefit (expense)	11.4			12.5			(11.3		)
Total income tax benefit (expense)	$	7.8		$	32.9		$	(12.7	)

Reconciliation of the U.S. federal statutory income tax rate and actual effective tax rate on pre-tax income



	Year Ended December 31,
Millions	2017			2016			2015
Tax (expense) benefit at the U.S. statutory rate	$	(2.7	)	$	51.6		$	(45.2	)
Differences in taxes resulting from:
Tax rate changes	(44.3		)	(3.9		)	(.5		)
Change in valuation allowance	42.6			6.9			(21.8		)
Non-U.S. earnings, net of foreign taxes	21.5			(19.2		)	58.5
Officer compensation	(4.1		)	—			—
Member surplus contributions	(3.0		)	(2.3		)	(1.5		)
Withholding tax	(2.0		)	(.2		)	(.5		)
Tax exempt interest and dividends	.5			.1			—
Tax reserve adjustments	(.3		)	—			—
Other, net	(.4		)	(.1		)	(1.7		)
Total income tax benefit (expense) on pre-tax income (loss)	$	7.8		$	32.9		$	(12.7	)

Schedule of components of deferred income tax assets and liabilities

The following table presents an outline of the significant components of White Mountains’s U.S. federal, state (net of federal benefit) and non-U.S. deferred tax assets and liabilities:



	December 31,
Millions	2017		2016
Deferred income tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$	73.0	$	104.3
Non-U.S. net operating loss carryforwards	33.9		34.0
Incentive compensation	20.4		26.5
Investment basis difference	4.9		5.6
Other items	4.9		8.5
Total gross deferred income tax assets	137.1		178.9
Less: valuation allowances	109.6		146.2
Total net deferred income tax assets	27.5		32.7
Deferred income tax liabilities related to:
Member surplus contributions	24.1		30.0
Other items	2.1		2.7
Total deferred income tax liabilities	26.2		32.7
Net deferred tax asset	$	1.3	$	—

Schedule of net operating and capital loss carryforwards by expiration dates and the deferred tax assets thereon

The following table presents net operating loss and capital loss carryforwards as of December 31, 2017, the expiration dates and the deferred tax assets thereon:



	December 31, 2017
Millions	United States			Luxembourg			United Kingdom			Israel			Total
2018-2022	$	.3		$	—		$	—		$	—		$	.3
2023-2027	—			—			—			—			—
2028-2037	332.0			46.4			—			—			378.4
No expiration date	—			31.3			6.7			54.7			92.7
Total	$	332.3		$	77.7		$	6.7		$	54.7		$	471.4
Gross deferred tax asset	73.0			20.2			1.1			12.6			106.9
Valuation allowance	(73.0		)	(20.2		)	(1.1		)	(12.6		)	(106.9		)
Net deferred tax asset	$	—		$	—		$	—		$	—		$	—

Reconciliation of changes in the amount of unrecognized tax benefits

The following table presents a reconciliation of the beginning and ending amount of unrecognized tax benefits for 2017:



Millions	Permanent Differences ⁽¹⁾		Temporary Differences ⁽²⁾		Interest and Penalties ⁽³⁾		Total
Balance at January 1, 2017	$	—	$	—	$	—	$	—
Changes in prior year tax positions	.1		—		—		.1
Tax positions taken during the current year	.2		—		—		.2
Balance at December 31, 2017	$	.3	$	—	$	—	$	.3


⁽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.

Municipal Bond Guarantee Insurance (Tables)

12 Months Ended

Dec. 31, 2017

Insurance [Abstract]

Financial Guarantee Insurance Contracts, Premium Received over Contract Period

The following table presents a schedule of BAM’s future premium revenues as of December 31, 2017:



Millions	December 31, 2017
January 1, 2018 - March 31, 2018	$	2.9
April 1, 2018 - June 30, 2018	2.9
July 1, 2018 - September 30, 2018	2.9
October 1, 2018 - December 31, 2018	2.8
	11.5

2019	11.1
2020	10.7
2021	10.2
2022	9.7
2023 and thereafter	83.6
Total gross unearned insurance premiums	$	136.8

Schedule of Insured Obligations

The following table presents a schedule of BAM’s insured obligations as of December 31, 2017 and 2016:



	December 31, 2017		December 31, 2016
Contracts outstanding	6,371		4,807
Remaining weighted average contract period (in years)	10.9		10.8
Contractual debt service outstanding (in millions):
Principal	$	42,090.6	$	33,057.3
Interest	21,057.1		16,396.6
Total debt service outstanding	$	63,147.7	$	49,453.9

Gross unearned insurance premiums	$	136.8	$	82.9

Schedule of Net Written Premiums

The following table presents a schedule of net written premiums included in White Mountains’s HG Global/BAM segment for the years ended December 31, 2017, 2016 and 2015:



Millions	December 31, 2017		December 31, 2016		December 31, 2015
Gross written premiums	$	63.2	$	38.6	$	25.9
Assumed (ceded) written premiums	—		—		—
Net written premiums	$	63.2	$	38.6	$	25.9

Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Computation of earnings per share



	Year Ended December 31,
	2017			2016			2015
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	627.2		$	401.8		$	295.2
Less: total income from discontinued operations, net of tax	577.5			523.4			135.1
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	49.7			(121.6		)	160.1
Allocation of earnings to participating restricted common shares ⁽¹⁾	(.7		)	1.5			(1.9		)
Basic and diluted earnings per share numerators	$	49.0		$	(120.1	)	$	158.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,293.8			5,014.9			5,879.2
Average unvested restricted common shares ⁽²⁾	(54.3		)	(64.8		)	(68.0		)
Basic earnings per share denominator	4,239.5			4,950.1			5,811.2
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	4,293.8			5,018.1			5,879.2
Average unvested restricted common shares ⁽²⁾	(54.3		)	(64.8		)	(68.0		)
Diluted earnings per share denominator ⁽³⁾	4,239.5			4,953.3			5,811.2
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$	1.00		$	1.00		$	1.00
Undistributed earnings (losses)	$	10.56		$	(25.26	)	$	26.22
Basic and diluted earnings per share	$	11.56		$	(24.26	)	$	27.22


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


⁽²⁾	Restricted shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Incentive Compensation Plans”.

⁽³⁾

Schedule of Undistributed Net Earnings



	Year Ended December 31,
Millions		2017			2016			2015
Undistributed net earnings - continuing operations:
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	49.0		$	(120.1	)	$	158.2
Dividends declared, net of restricted common share amounts ⁽¹⁾		(4.5		)	(5.4		)	(5.9		)
Total undistributed net earnings (losses), net of restricted common share amounts		$	44.5		$	(125.5	)	$	152.3

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

Net Assets Held for Sale



Millions	December 31, 2016
Assets held for sale
Fixed maturity investments, at fair value	$	2,175.7
Short-term investments, at amortized cost (which approximates fair value)	112.3
Common equity securities, at fair value	188.7
Other long-term investments	150.5
Total investments	2,627.2
Cash	70.5
Reinsurance recoverable on unpaid losses	179.8
Insurance and reinsurance premiums receivable	229.8
Deferred acquisition costs	96.3
Deferred tax asset	126.7
Ceded unearned insurance and reinsurance premiums	44.2
Accounts receivable on unsettled investment sales	1.4
Goodwill and other intangible assets	1.2
Accrued investment income	11.3
Other assets	218.0
Total assets held for sale	$	3,606.4
Liabilities held for sale
Loss and loss adjustment expense reserves	$	1,370.6
Unearned insurance and reinsurance premiums	576.3
Debt	273.2
Accrued incentive compensation	44.3
Funds held under reinsurance treaties	153.0
Other liabilities	151.9
Total liabilities held for sale	2,569.3
Net assets held for sale	$	1,037.1

Net Income (Loss) from Discontinued Operations



	Year Ended December 31, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7



	Year Ended December 31, 2015
Millions	OneBeacon			Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	1,176.2		$	847.0		$	—		$	2,023.2
Net investment income	45.9			40.7			—			86.6
Net realized and unrealized investment (losses) gains	(35.1		)	15.1			—			(20.0		)
Other (loss) revenues	(.6		)	(20.6		)	186.2			165.0
Total revenues	1,186.4			882.2			186.2			2,254.8
Expenses
Loss and loss adjustment expenses	700.7			422.7			—			1,123.4
Insurance and reinsurance acquisition expenses	213.8			189.8			—			403.6
Other underwriting expenses	218.2			107.9			—			326.1
Interest expense	15.4			26.6			4.0			46.0
General and administrative expenses	13.0			27.0			185.3			225.3
Total expenses	1,161.1			774.0			189.3			2,124.4
Pre-tax income (loss)	25.3			108.2			(3.1		)	130.4
Income tax benefit (expense)	12.9			(27.1		)	.7			(13.5		)
Net income (loss) from discontinued operations	38.2			81.1			(2.4		)	116.9
Gain from sale of OneBeacon runoff, net of tax	.3			—			—			.3
Gain from sale of Esurance, net of tax	—			—			17.9			17.9
Total income from discontinued operations	38.5			81.1			15.5			135.1
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	—			(65.0		)	—			(65.0		)
Comprehensive income from discontinued operations	$	38.5		$	16.1		$	15.5		$	70.1

Net Change in Cash from Discontinued Operations

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



	Year ended December 31,
Millions	2017			2016			2015
Net cash provided from operations	$	157.0		$	23.6		$	248.4
Net cash provided from (used for) investing activities	3.0			241.4			(100.5		)
Net cash used for financing activities	(61.9		)	(93.8		)	(100.7		)
Effect of exchange rate changes on cash	—			—			(4.5		)
Net change in cash during the period	98.1			171.2			42.7
Cash balances at beginning of period	70.5			245.4			203.8
Net change in cash held for sale	(.9		)	(.3		)	(1.1		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)	—
Cash balances at end of period	$	—		$	70.5		$	245.4

- 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=84176398&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=84176398&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

Employee Share-Based Incentive Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2017

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

Summary of performance share activity

The following table presents performance share activity for the years ended December 31, 2017, 2016 and 2015 for performance shares granted under the WTM Incentive Plan:



	Year Ended December 31,
	2017					2016					2015
$ in millions	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	80,353		$	42.4		93,654		$	57.7		111,257		$	44.4
Shares paid or expired ⁽¹⁾	(30,838	)	(21.9		)	(36,294	)	(41.0		)	(42,959	)	(30.8		)
New grants	17,710		—			22,615		—			29,195		—
Forfeitures ⁽²⁾	(16,710	)	(9.3		)	378		.5			(3,839	)	(.3		)
Expense recognized	—		34.6			—		25.2			—		44.4
End of period ⁽³⁾	50,515		$	45.8		80,353		$	42.4		93,654		$	57.7


⁽¹⁾	WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017 ranged from 34% to 76% of target. WTM performance shares 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.


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


⁽³⁾	Outstanding performance share awards as of December 31, 2017, 2016 and 2015 exclude 2,195, 7,315 and 10,826 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

following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2017 for each performance cycle:



$ in millions	Target WTM Performance Shares Outstanding		Accrued Expense
Performance cycle:
2017 – 2019	16,680		$	8.3
2016 – 2018	16,235		14.7
2015 – 2017	18,370		23.6
Sub-total	51,285		46.6
Assumed forfeitures	(770	)	(.8		)
Total	50,515		$	45.8

Schedule of Nonvested Restricted Stock Activity



	Year Ended December 31,
	2017					2016					2015
$ 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	70,620		$	19.7		70,675		$	15.7		83,314		$	14.3
Issued	17,985		16.3			25,365		20.2			23,640		15.7
Vested	(28,846	)	—			(24,620	)	—			(36,279	)	—
Forfeited	(6,004	)	(3.5		)	(800	)	(.3		)	—		—
Expense recognized	—		(18.2		)	—		(15.9		)	—		(14.3		)
End of period ⁽¹⁾	53,755		$	14.3		70,620		$	19.7		70,675		$	15.7


⁽¹⁾	Outstanding restricted share awards as of December 31, 2017, 2016 and 2015 include 2,195, 5,235, and 9,205 unvested restricted shares for employees of Sirius Group.

Common Shareholders’ Equity and Non-controlling Interests Common Shareholders’ Equity and Non-controlling Interests (Tables)

12 Months Ended

Dec. 31, 2017

Noncontrolling Interest [Abstract]

Noncontrolling Interest



	December 31, 2017					December 31, 2016
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon	—	%	$	—		23.9	%	$	244.6

Other, excluding mutuals and reciprocals
HG Global	3.1		15.9			3.1		16.6
MediaAlpha	35.7		13.1			40.0		11.7
Dewar⁽¹⁾	—		—			18.8		3.9
Buzzmove	22.9		2.5			29.1		3.0
Total other, excluding mutuals and reciprocals			31.5					35.2

Mutuals and reciprocals
BAM	100.0		(163.2		)	100.0		(150.9		)
SSIE	—		—			100.0		4.4
Total mutuals and reciprocals			(163.2		)			(146.5		)
Total non-controlling interests			$	(131.7	)			$	133.3

⁽¹⁾ Dewar is a subsidiary of OneBeacon.

Segment Information (Tables)

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Segment Reporting [Abstract]

Financial information for White Mountains' segments

he following tables present the financial information for White Mountains’s segments:



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations		Total
Year Ended December 31, 2017
Earned insurance premiums	$	9.4		$	—		$	1.0	$	10.4
Net investment income	12.3			—			43.7		56.0
Net realized and unrealized investment gains	.6			—			132.7		133.3
Advertising and commission revenues ⁽²⁾	—			163.2			3.8		167.0
Other revenues	1.0			—			6.1		7.1
Total revenues	23.3			163.2			187.3		373.8
Losses and LAE	—			—			1.1		1.1
Insurance acquisition expenses	4.0			—			.1		4.1
Other underwriting expenses	.4			—			—		.4
Cost of sales	—			135.9			3.5		139.4
General and administrative expenses	42.9			26.7			149.1		218.7
Interest expense	—			1.0			1.3		2.3
Total expenses	47.3			163.6			155.1		366.0
Pre-tax (loss) income	$	(24.0	)	$	(.4	)	$	32.2	$	7.8


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Approximately 27% of MediaAlpha’s advertising revenue was associated with one customer for the years ended December 31, 2017.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2016
Earned insurance premiums	$	5.9		$	—		$	7.5		$	13.4
Net investment income	9.0			—			23.1			32.1
Net realized and unrealized investment gains (losses)	.7			—			(28.1		)	(27.4		)
Advertising and commission revenues⁽²⁾	—			116.5			1.8			118.3
Other revenues	1.1			—			20.2			21.3
Total revenues	16.7			116.5			24.5			157.7
Losses and LAE	—			—			8.0			8.0
Insurance acquisition expenses	3.4			—			2.2			5.6
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			97.8			4.2			102.0
General and administrative expenses	39.6			21.9			124.5			186.0
Interest expense	—			.9			2.1			3.0
Total expenses	43.4			120.6			141.0			305.0
Pre-tax loss	$	(26.7	)	$	(4.1	)	$	(116.5	)	$	(147.3	)


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Approximately 24% of MediaAlpha’s advertising revenue was associated with one customer for the years ended December 31, 2016.



Millions	HG Global/BAM ⁽¹⁾			MediaAlpha			Other Operations			Total
Year Ended December 31, 2015
Earned insurance premiums	$	3.3		$	—		$	8.7		$	12.0
Net investment income	6.1			—			4.8			10.9
Net realized and unrealized investment gains	.6			—			259.9		⁽²⁾	260.5
Advertising and commission revenues ⁽³⁾	—			105.5			1.9			107.4
Other revenues	.7			—			48.5			49.2
Total revenues	10.7			105.5			323.8			440.0
Losses and LAE	—			—			8.2			8.2
Insurance acquisition expenses	2.9			—			3.4			6.3
Other underwriting expenses	.4			—			—			.4
Cost of sales	—			90.7			2.9			93.6
General and administrative expenses	36.8			16.4			147.5			200.7
Interest expense	—			.4			1.2			1.6
Total expenses	40.1			107.5			163.2			310.8
Pre-tax (loss) income	$	(29.4	)	$	(2.0	)	$	160.6		$	129.2


⁽¹⁾	BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes the 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 NYDFS.


⁽²⁾	Net realized and unrealized investment gains 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 14 — “Investments in Unconsolidated Affiliates”.


⁽³⁾	Approximately 52% of MediaAlpha’s advertising revenue was associated with three customers for the year ended December 31, 2015.




Selected Balance Sheet Data Millions	HG Global/BAM				MediaAlpha		Other Operations		Eliminations			Held for Sale		Total
December 31, 2017:
Total investments	$	693.4			$	—	$	2,687.3	$	—		$	—	$	3,380.7
Total assets	747.4			⁽¹⁾	96.5		3,039.9		(227.9		)	3.3		3,659.2
Total liabilities	394.9				59.8		71.6		(227.9		)	—		298.4
Total White Mountains’s common shareholders’ equity	499.8				23.6		2,965.8		—			3.3		3,492.5
Non-controlling interest	(147.3		)		13.1		2.5		—			—		(131.7		)
December 31, 2016:
Total investments	$	629.7			$	—	$	2,084.7	$	—		$	—	$	2,714.4
Assets held for sale	—				—		—		—			3,606.4		3,606.4
Total assets	677.6			⁽¹⁾	57.6		2,358.8		(180.2		)	3,606.4		6,520.2
Liabilities held for sale	—				—		—		—			2,569.3		2,569.3
Total liabilities	289.1				28.3		97.7		(180.2		)	2,569.3		2,804.2
Total White Mountains’s common shareholders’ equity	522.8				17.6		2,258.1		—			784.2		3,582.7
Non-controlling interest	(134.3		)		11.7		3.0		—			252.9		133.3

Schedule of selected balance sheet data by segment




Selected Balance Sheet Data Millions	HG Global/BAM				MediaAlpha		Other Operations		Eliminations			Held for Sale		Total
December 31, 2017:
Total investments	$	693.4			$	—	$	2,687.3	$	—		$	—	$	3,380.7
Total assets	747.4			⁽¹⁾	96.5		3,039.9		(227.9		)	3.3		3,659.2
Total liabilities	394.9				59.8		71.6		(227.9		)	—		298.4
Total White Mountains’s common shareholders’ equity	499.8				23.6		2,965.8		—			3.3		3,492.5
Non-controlling interest	(147.3		)		13.1		2.5		—			—		(131.7		)
December 31, 2016:
Total investments	$	629.7			$	—	$	2,084.7	$	—		$	—	$	2,714.4
Assets held for sale	—				—		—		—			3,606.4		3,606.4
Total assets	677.6			⁽¹⁾	57.6		2,358.8		(180.2		)	3,606.4		6,520.2
Liabilities held for sale	—				—		—		—			2,569.3		2,569.3
Total liabilities	289.1				28.3		97.7		(180.2		)	2,569.3		2,804.2
Total White Mountains’s common shareholders’ equity	522.8				17.6		2,258.1		—			784.2		3,582.7
Non-controlling interest	(134.3		)		11.7		3.0		—			252.9		133.3


⁽¹⁾	As of December 2017 and 2016,BAM’s total assets reflect the elimination of $499.0 and $503.0 of BAM Surplus Notes issued to HG Global and its subsidiaries, and $126.0 and $108.0 in accrued interest related to the BAM Surplus Notes.

Investments in Unconsolidated Affiliates (Tables)

12 Months Ended

Dec. 31, 2017

Investments in and Advances to Affiliates [Table Text Block]

The following table presents the carrying values of investments in equity method eligible unconsolidated entities recorded within other long-term investments:



	December 31
Millions	2017		2016
Equity method eligible private equity securities, at fair value	$	58.0	$	55.6
Investments, accounted for under the equity method	4.6		3.5
Total investments in equity method eligible unconsolidated entities	62.6		59.1
Other unconsolidated investments ⁽¹⁾	146.2		113.7
Total other long-term investments	$	208.8	$	172.8

⁽¹⁾Consists of other long-term investments that are not equity method eligible.

Affiliated Entity [Member]

Equity Method Investments [Table Text Block]

The following table presents White Mountains’s investments in equity method eligible unconsolidated entities as of December 31, 2017 and 2016:



Investee	Ownership Interest	Instrument Held
Compare.com	22%	Common shares
durchblicker	45%	Common shares
OneTitle	20%	Common shares
PassportCard	50%	Common shares
Tuckerman Capital Fund III	21%	Units

Total investments in equity method eligible unconsolidated entities

Equity Method Investments [Table Text Block]

The following tables presents aggregated summarized financial information for White Mountains’s investments in equity method eligible unconsolidated entities:



	December 31,
Millions	2017		2016
Balance sheet data ⁽¹⁾:
Total assets	$	75.4	$	79.4
Total liabilities	24.2		18.8

⁽¹⁾Financial data submitted by investees is on a one-quarter lag.



	Year Ended December 31,
Millions	2017			2016			2015
Income statement data ⁽¹⁾:
Revenues	$	60.0		$	32.9		$	12.9
Expenses	(66.8		)	(76.4		)	(69.9		)

⁽¹⁾Financial data submitted by investees is on a one-quarter lag.

Symetra

Equity Method Investments [Table Text Block]

The following tables presents 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 tables presents financial information for Symetra for the nine months ended September 30, 2015:



	Nine months ended September 30, 2015
Millions	Nine months ended September 30, 2015
Symetra income statement data:
Net premiums earned	$	539.3
Net investment income	994.3
Total revenues	1,605.9
Policy benefits	1,143.7
Total expenses	1,543.6
Net income	89.6
Comprehensive net loss	(234.1		)

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Summary of the fair value and carrying value of financial instruments

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2017 and December 31, 2016:



	December 31, 2017				December 31, 2016
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
MediaAlpha Bank Facility	$	23.9	$	23.8	$	—	$	—
Previous MediaAlpha Bank Facility	—		—		13.0		12.7

Discontinued Operations (Tables)

12 Months Ended

Dec. 31, 2017

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

Schedule of discontinued operations balance sheet, income statement, and cash flows



	Year Ended December 31,
Millions		2017			2016			2015
Undistributed net earnings - continuing operations:
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts		$	49.0		$	(120.1	)	$	158.2
Dividends declared, net of restricted common share amounts ⁽¹⁾		(4.5		)	(5.4		)	(5.9		)
Total undistributed net earnings (losses), net of restricted common share amounts		$	44.5		$	(125.5	)	$	152.3

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

Net Assets Held for Sale



Millions	December 31, 2016
Assets held for sale
Fixed maturity investments, at fair value	$	2,175.7
Short-term investments, at amortized cost (which approximates fair value)	112.3
Common equity securities, at fair value	188.7
Other long-term investments	150.5
Total investments	2,627.2
Cash	70.5
Reinsurance recoverable on unpaid losses	179.8
Insurance and reinsurance premiums receivable	229.8
Deferred acquisition costs	96.3
Deferred tax asset	126.7
Ceded unearned insurance and reinsurance premiums	44.2
Accounts receivable on unsettled investment sales	1.4
Goodwill and other intangible assets	1.2
Accrued investment income	11.3
Other assets	218.0
Total assets held for sale	$	3,606.4
Liabilities held for sale
Loss and loss adjustment expense reserves	$	1,370.6
Unearned insurance and reinsurance premiums	576.3
Debt	273.2
Accrued incentive compensation	44.3
Funds held under reinsurance treaties	153.0
Other liabilities	151.9
Total liabilities held for sale	2,569.3
Net assets held for sale	$	1,037.1

Net Income (Loss) from Discontinued Operations



	Year Ended December 31, 2017
Millions	OneBeacon		Sirius Group			Tranzact		Total
Revenues
Earned insurance premiums	$	807.6	$	—		$	—	$	807.6
Net investment income	39.7		—			—		39.7
Net realized and unrealized investment gains	38.8		—			—		38.8
Other revenues	7.7		—			—		7.7
Total revenues	893.8		—			—		893.8
Expenses
Loss and loss adjustment expenses	546.0		—			—		546.0
Insurance and reinsurance acquisition expenses	145.6		—			—		145.6
Other underwriting expenses	156.2		—			—		156.2
General and administrative expenses	21.2		—			—		21.2
Interest expense	10.0							10.0
Total expenses	879.0		—			—		879.0
Pre-tax income	14.8		—			—		14.8
Income tax benefit	5.7		—			—		5.7
Net income from discontinued operations	20.5		—			—		20.5
Gain (loss) from sale of discontinued operations, net of tax	554.5		(.7		)	3.2		557.0
Total income (loss) from discontinued operations	575.0		(.7		)	3.2		577.5
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	.3		—			—		.3
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax	2.9		—			—		2.9
Comprehensive income (loss) from discontinued operations	$	578.2	$	(.7	)	$	3.2	$	580.7



	Year Ended December 31, 2016
Millions	OneBeacon		Sirius Group			Tranzact			Total
Revenues
Earned insurance premiums	$	1,100.6	$	240.1		$	—		$	1,340.7
Net investment income	50.6		14.4			—			65.0
Net realized and unrealized investment gains (losses)	37.7		(1.5		)	—			36.2
Other revenues	5.5		.6			119.6			125.7
Total revenues	1,194.4		253.6			119.6			1,567.6
Expenses
Loss and loss adjustment expenses	656.0		154.9			—			810.9
Insurance and reinsurance acquisition expenses	206.0		59.0			—			265.0
Other underwriting expenses	209.0		30.9			—			239.9
General and administrative expenses	14.2		10.4			116.7			141.3
Interest expense	13.1		7.9			3.2			24.2
Total expenses	1,098.3		263.1			119.9			1,481.3
Pre-tax income (loss)	96.1		(9.5		)	(.3		)	86.3
Income tax benefit	12.5		3.1			6.4			22.0
Net income (loss) from discontinued operations	108.6		(6.4		)	6.1			108.3
Gain from sale of discontinued operations, net of tax	—		363.2			51.9			415.1
Total income from discontinued operations	108.6		356.8			58.0			523.4
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	1.0		32.0			—			33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax	—		113.3			—			113.3
Comprehensive income from discontinued operations	$	109.6	$	502.1		$	58.0		$	669.7



	Year Ended December 31, 2015
Millions	OneBeacon			Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	1,176.2		$	847.0		$	—		$	2,023.2
Net investment income	45.9			40.7			—			86.6
Net realized and unrealized investment (losses) gains	(35.1		)	15.1			—			(20.0		)
Other (loss) revenues	(.6		)	(20.6		)	186.2			165.0
Total revenues	1,186.4			882.2			186.2			2,254.8
Expenses
Loss and loss adjustment expenses	700.7			422.7			—			1,123.4
Insurance and reinsurance acquisition expenses	213.8			189.8			—			403.6
Other underwriting expenses	218.2			107.9			—			326.1
Interest expense	15.4			26.6			4.0			46.0
General and administrative expenses	13.0			27.0			185.3			225.3
Total expenses	1,161.1			774.0			189.3			2,124.4
Pre-tax income (loss)	25.3			108.2			(3.1		)	130.4
Income tax benefit (expense)	12.9			(27.1		)	.7			(13.5		)
Net income (loss) from discontinued operations	38.2			81.1			(2.4		)	116.9
Gain from sale of OneBeacon runoff, net of tax	.3			—			—			.3
Gain from sale of Esurance, net of tax	—			—			17.9			17.9
Total income from discontinued operations	38.5			81.1			15.5			135.1
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax	—			(65.0		)	—			(65.0		)
Comprehensive income from discontinued operations	$	38.5		$	16.1		$	15.5		$	70.1

Net Change in Cash from Discontinued Operations

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



	Year ended December 31,
Millions	2017			2016			2015
Net cash provided from operations	$	157.0		$	23.6		$	248.4
Net cash provided from (used for) investing activities	3.0			241.4			(100.5		)
Net cash used for financing activities	(61.9		)	(93.8		)	(100.7		)
Effect of exchange rate changes on cash	—			—			(4.5		)
Net change in cash during the period	98.1			171.2			42.7
Cash balances at beginning of period	70.5			245.4			203.8
Net change in cash held for sale	(.9		)	(.3		)	(1.1		)
Cash sold as part of sale of consolidated subsidiaries	(167.7		)	(345.8		)	—
Cash balances at end of period	$	—		$	70.5		$	245.4

Discontinued operations, computation of earnings per share

The following table presents the Company’s computation of earnings per share for discontinued operations for the years ended December 31, 2017, 2016 and 2015:



	Year Ended December 31,
	2017			2016			2015
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	627.2		$	401.8		$	295.2
Less: total income (loss) from continuing operations, net of tax	49.7			(121.6		)	160.1
Net income from discontinued operations attributable to White Mountains’s common shareholders	577.5			523.4			135.1
Allocation of earnings to participating restricted common shares ⁽¹⁾	(7.3		)	(6.8		)	(1.6		)
Basic and diluted earnings per share numerators	$	570.2		$	516.6		$	133.5
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,293.8			5,014.9			5,879.2
Average unvested restricted common shares ⁽³⁾	(54.3		)	(64.8		)	(68.0		)
Basic earnings per share denominator	4,239.5			4,950.1			5,811.2
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	4,293.8			5,018.1			5,879.2
Average unvested restricted common shares ⁽³⁾	(54.3		)	(64.8		)	(68.0		)
Diluted earnings per share denominator ⁽⁴⁾	4,239.5			4,953.3			5,811.2
Basic earnings per share (in dollars) - discontinued operations:	$	134.50		$	104.37		$	22.98
Diluted earnings per share (in dollars) - discontinued operations:	$	134.50		$	104.32		$	22.98


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


⁽²⁾	Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2017, 2016 and 2015.


⁽³⁾	Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 10 — “Employee Share-Based Compensation Plans”.

⁽⁴⁾

Summary of the fair value and carrying value of financial instruments

The following tables presents the fair value and carrying value of these financial instruments as of December 31, 2017 and December 31, 2016:



	December 31, 2017				December 31, 2016
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
MediaAlpha Bank Facility	$	23.9	$	23.8	$	—	$	—
Previous MediaAlpha Bank Facility	—		—		13.0		12.7

Par Value to Fair Value Reconciliation of Surplus Notes

presents the valuation adjustments taken to arrive at estimated fair value of the OneBeacon Surplus Notes as of December 31, 2016:



Millions	December 31, 2016
Par Value	$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments ⁽¹⁾	5.1
Regulatory approval ⁽²⁾	(15.6		)
Liquidity adjustment ⁽³⁾	(18.6		)
Total adjustments	(29.1		)
Fair value	$	71.9

⁽¹⁾

⁽²⁾

⁽³⁾ Represents impact of liquidity spread to account for OneBeacon's sole ownership of the notes, lack of a trading market, and unique nature of the ongoing regulatory approval process.

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

The following table presents the fair value and carrying value of this financial instrument as of December 31, 2016:



	December 31, 2016
Millions	Fair Value		Carrying Value
OBH Senior Notes	$	274.2	$	273.2

- Definition

+ References

+ Details

Financial Statement Revisions (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Changes and Error Corrections [Abstract]

Schedule of Error Corrections and Prior Period Adjustments

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME



	As previously reported						Adjustments										As revised
	Years ended December 31,
Millions, except for per share amounts	2016			2015			2016					2015					2016			2015
Revenues:
Financial Guarantee (HG Global/BAM)
Financial Guarantee revenues	$	16.7		$	10.7		$	—				$	—				$	16.7		$	10.7

Marketing Technology (MediaAlpha)
Marketing Technology revenues	116.5			105.5			—					—					116.5			105.5

Other
Advertising & commission revenues	10.4			4.6			(8.6		)			(2.7		)			1.8			1.9
Other revenues ⁽¹⁾⁽²⁾	22.7			314.5			—					7.4					22.7			321.9
Other revenues	33.1			319.1			(8.6		)			4.7					24.5			323.8
Total revenues	166.3			435.3			(8.6		)			4.7					157.7			440.0
Expenses:
Financial Guarantee (HG Global/BAM)
Financial Guarantee expenses	43.4			40.1			—					—					43.4			40.1

Marketing Technology (MediaAlpha)
Marketing Technology expenses	120.6			107.5			—					—					120.6			107.5

Other
Other expenses ⁽³⁾	137.6			154.5			3.0			⁽⁵	⁾	8.2			⁽⁵	⁾	140.6			162.7
Amortization of other intangible assets	1.2			1.2			(.8		)			(.7		)			.4			.5
Other expenses	138.8			155.7			2.2					7.5					141.0			163.2
Total expenses	302.8			303.3			2.2					7.5					305.0			310.8
Pre-tax (loss) income	(136.5		)	132.0			(10.8		)			(2.8		)			(147.3		)	129.2
Income benefit (expense)	32.9			(12.7		)	—					—					32.9			(12.7		)
Net (loss) income from continuing operations	(103.6		)	119.3			(10.8		)			(2.8		)			(114.4		)	116.5
Gain on sale of discontinued operations, net of tax	415.1			18.2			—					—					415.1			18.2
Net income from discontinued operations	108.3			116.9			—					—					108.3			116.9
Income before equity in earnings of unconsolidated affiliates	419.8			254.4			(10.8		)			(2.8		)			409.0			251.6
Equity in earnings of unconsolidated affiliates	—			25.1			—					—					—			25.1
Net income ⁽⁴⁾	419.8			279.5			(10.8		)			(2.8		)			409.0			276.7
Net (income) loss attributable to non-controlling interests	(7.3		)	18.1			.1					.4					(7.2		)	18.5
Net income attributable to White Mountains's common shareholders	412.5			297.6			(10.7		)			(2.4		)			401.8			295.2
Other comprehensive income (loss), net of tax ⁽¹⁾	145.6			(100.4		)	—					(7.4		)			145.6			(107.8		)
Comprehensive income	558.1			197.2			(10.7		)			(9.8		)			547.4			187.4
Comprehensive income attributable to non-controlling interests	(.3		)	—			—					—					(.3		)	—
Comprehensive income attributable to White Mountains's common shareholders	$	557.8		$	197.2		$	(10.7	)			$	(9.8	)			$	547.1		$	187.4
Basic and diluted earnings per share - continuing operations	$	(22.13	)	$	27.63		$	(2.13	)			$	(.41	)			$	(24.26	)	$	27.22

⁽²⁾Total other revenues include earned insurance premiums, net investment income, net realized and unrealized losses, and other revenues.

⁽⁵⁾The adjustments to other expenses is primarily related to the write-off of goodwill and intangible assets for Wobi.

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY



	As previously reported									Adjustments									As revised
	Years ended December 31,
Millions	2016			2015			2014			2016			2015			2014			2016			2015			2014

Common shares and paid-in surplus	$	810.7		$	978.2		$	1,034.7		$	—		$	—		$	—		$	810.7		$	978.2		$	1,034.7

Retained earnings, beginning of year	3,084.9			3,010.5			2,801.9			(9.9		)	(7.5		)	(.5		)	3,075.0			3,003.0			2,801.4
Share repurchases	(694.8		)	(217.2		)	(97.4		)	—			—			—			(694.8		)	(217.2		)	(97.4		)
Net income	412.5			297.6			312.2			(10.7		)	(2.4		)	(7.0		)	401.8			295.2			305.2
Dividends	(5.4		)	(6.0		)	(6.2		)	—			—			—			(5.4		)	(6.0		)	(6.2		)
Retained earnings, end of year	2,797.2			3,084.9			3,010.5			(20.6		)	(9.9		)	(7.5		)	2,776.6			3,075.0			3,003.0

Accumulated other comprehensive (loss) income, after-tax, beginning of year	(149.9		)	(49.5		)	52.1			—			7.4			.5			(149.9		)	(42.1		)	52.6
Net change in foreign currency translation	31.4			(65.8		)	(168.2		)	—			(7.4		)	6.9			31.4			(73.2		)	(161.3		)
Net other changes in AOCI	113.9			(34.6		)	66.6			—			—			—			113.9			(34.6		)	66.6
Accumulated other comprehensive loss, after-tax, end of year	(4.6		)	(149.9		)	(49.5		)	—			—			7.4			(4.6		)	(149.9		)	(42.1		)

Total White Mountains Common Shareholders' Equity	3,603.3			3,913.2			3,995.7			(20.6		)	(9.9		)	(.1		)	3,582.7			3,903.3			3,995.6

Non-controlling interests, beginning of year	454.8			542.7			491.7			(.5		)	(.1		)	—			454.3			542.6			491.7
Net income (loss)	7.3			(18.1		)	(22.2		)	(.1		)	(.4		)	(.1		)	7.2			(18.5		)	(22.3		)
Other changes in NCI	(328.2		)	(69.8		)	73.2			—			—			—			(328.2		)	(69.8		)	73.2
Non-controlling interests, end of year	133.9			454.8			542.7			(.6		)	(.5		)	(.1		)	133.3			454.3			542.6

Total equity	$	3,737.2		$	4,368.0		$	4,538.4		$	(21.2	)	$	(10.4	)	$	(.2	)	$	3,716.0		$	4,357.6		$	4,538.2

CONSOLIDATED BALANCE SHEETS



	December 31, 2016
Millions	As previously reported			Adjustments			As revised
Assets
Financial Guarantee (HG Global/BAM)
Financial Guarantee assets	$	677.6		$	—		$	677.6

Marketing Technology (MediaAlpha)
Marketing Technology assets	57.6			—			57.6

Other
Investments	2,084.7			—			2,084.7
Goodwill	13.4			(5.8		)	7.6
Other intangible assets	4.7			(3.7		)	1.0
Other assets — commissions receivable	14.8			(14.8		)	—
Other assets ⁽¹⁾	85.5			(.2		)	85.3
Assets held for sale	3,606.4			—			3,606.4
Other segment assets	5,809.5			(24.5		)	5,785.0

Total assets	$	6,544.7		$	(24.5	)	$	6,520.2
Liabilities
Financial Guarantee (HG Global/BAM)
Total Financial Guarantee liabilities	$	108.9		$	—		$	108.9

Marketing Technology (MediaAlpha)
Total Marketing Technology liabilities	28.3			—			28.3

Other
Accrued incentive compensation	79.1			—			79.1
Other liabilities ⁽²⁾	21.9			(3.3		)	18.6
Liabilities held for sale	2,569.3			—			2,569.3
Total Other segment liabilities	2,670.3			(3.3		)	2,667.0

Total liabilities	2,807.5			(3.3		)	2,804.2

Equity
White Mountains's common shares	4.6			—			4.6
Paid in surplus	806.1			—			806.1
Retained earnings	2,797.2			(20.6		)	2,776.6
Accumulated other comprehensive income, net of tax	(4.6		)	—			(4.6		)
Total White Mountains's common shareholders' equity	3,603.3			(20.6		)	3,582.7
Non-controlling interests	133.9			(.6		)	133.3
Total equity	3,737.2			(21.2		)	3,716.0
Total liabilities and equity	$	6,544.7		$	(24.5	)	$	6,520.2

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information



	2017 Three Months Ended										2016 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	$	114.0	$	87.5	$	83.5		$	88.8		$	(5.5	)	$	56.8		$	48.4		$	58.0
Expenses	108.8		79.1		85.7			92.4			69.0			70.6			74.1			91.3
Pre-tax income (loss)	5.2		8.4		(2.2		)	(3.6		)	(74.5		)	(13.8		)	(25.7		)	(33.3		)
Tax benefit	2.5		4.0		1.0			.3			10.2			17.1			4.0			1.6
Income (loss) from continuing operations	7.7		12.4		(1.2		)	(3.3		)	(64.3		)	3.3			(21.7		)	(31.7		)
Income (loss) from discontinued operations, net of tax	4.3		539.1		2.8			31.3			8.0			84.4			383.6			47.4
Non-controlling interest in consolidated subsidiaries	10.5		10.6		12.0			1.0			17.4			3.1			(21.5		)	(6.2		)
Income (loss) attributable to White Mountains’s common shareholders	$	22.5	$	562.1	$	13.6		$	29.0		$	(38.9	)	$	90.8		$	340.4		$	9.5
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	4.85	$	5.36	$	2.36		$	(0.50	)	$	(10.27	)	$	1.31		$	(8.47	)	$	(6.85	)
Discontinued operations	1.15		125.45		.61			6.86			1.75			17.34			75.27			8.55
Total consolidated operations	$	6.00	$	130.81	$	2.97		$	6.36		$	(8.52	)	$	18.65		$	66.80		$	1.70
Diluted
Continuing operations	$	4.85	$	5.36	$	2.36		$	(0.50	)	$	(10.27	)	$	1.31		$	(8.47	)	$	(6.85	)
Discontinued operations	1.15		125.45		.61			6.86			1.75			17.30			75.11			8.55
Total consolidated operations	$	6.00	$	130.81	$	2.97		$	6.36		$	(8.52	)	$	18.61		$	66.64		$	1.70



	Three Months Ended June 30, 2017									Three Months Ended March 31, 2017
	As previously reported ⁽¹⁾									As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised			As previously reported ⁽¹⁾		Adjustments			As revised
Revenues	$	85.4		$	(1.9	)	$	83.5		$	93.5	$	(4.7	)	$	88.8
Expenses	85.8			(.1		)	85.7			92.2		.2			92.4
Pre-tax (loss) income	(.4		)	(1.8		)	(2.2		)	1.3		(4.9		)	(3.6		)
Tax benefit	1.0						1.0			.3		—			.3
Income (loss) from continuing operations	.6			(1.8		)	(1.2		)	1.6		(4.9		)	(3.3		)
Income from discontinued operations, net of tax	2.8			—			2.8			31.3		—			31.3
Non-controlling interest in consolidated subsidiaries	12.1			(.1		)	12.0			1.3		(.3		)	1.0
Income attributable to White Mountains’s common shareholders	$	15.5		$	(1.9	)	$	13.6		$	34.2	$	(5.2	)	$	29.0
Income (loss) attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	2.78		$	(.42	)	$	2.36		$	.65	$	(1.15	)	$	(0.50	)
Discontinued operations	.61			—			.61			6.86		—			6.86
Total consolidated operations	$	3.39		$	(.42	)	$	2.97		$	7.51	$	(1.15	)	$	6.36
Diluted
Continuing operations	$	2.78		$	(.42	)	$	2.36		$	.65	$	(1.15	)	$	(0.50	)
Discontinued operations	.61			—			.61			6.86		—			6.86
Total consolidated operations	$	3.39		$	(.42	)	$	2.97		$	7.51	$	(1.15	)	$	6.36

⁽¹⁾ Amounts previously reported reflect the effect of reclassifying OneBeacon as discontinued operations. See Note 19 — “Held for Sale and Discontinued Operations”.



	Three Months Ended December 31, 2016
	As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised
Revenues	$	.3		$	(5.8	)	$	(5.5	)
Expenses	69.1			(.1		)	69.0
Pre-tax loss	(68.8		)	(5.7		)	(74.5		)
Tax benefit	10.2			—			10.2
Loss from continuing operations	(58.6		)	(5.7		)	(64.3		)
Income from discontinued operations, net of tax	8.0			—			8.0
Non-controlling interest in consolidated subsidiaries	17.3			.1			17.4
Loss attributable to White Mountains’s common shareholders	$	(33.3	)	$	(5.6	)	$	(38.9	)
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(9.04	)	$	(1.23	)	$	(10.27	)
Discontinued operations	1.75			—			1.75
Total consolidated operations	$	(7.29	)	$	(1.23	)	$	(8.52	)
Diluted
Continuing operations	$	(9.04	)	$	(1.23	)	$	(10.27	)
Discontinued operations	1.75			—			1.75
Total consolidated operations	$	(7.29	)	$	(1.23	)	$	(8.52	)



	Three Months Ended June 30, 2016									Three Months Ended March 31, 2016
	As previously reported ⁽¹⁾									As previously reported ⁽¹⁾
Millions, except per share amounts	As previously reported ⁽¹⁾			Adjustments			As revised			As previously reported ⁽¹⁾			Adjustments			As revised
Revenues	$	49.2		$	(.8	)	$	48.4		$	58.8		$	(.8	)	$	58.0
Expenses	74.3			(.2		)	74.1			88.5			2.8			91.3
Pre-tax loss	(25.1		)	(.6		)	(25.7		)	(29.7		)	(3.6		)	(33.3		)
Tax benefit	4.0			—			4.0			1.6			—			1.6
Loss from continuing operations	(21.1		)	(.6		)	(21.7		)	(28.1		)	(3.6		)	(31.7		)
Income from discontinued operations, net of tax	383.6			—			383.6			47.4			—			47.4
Non-controlling interest in consolidated subsidiaries	(21.4		)	(.1		)	(21.5		)	(6.3		)	.1			(6.2		)
Income attributable to White Mountains’s common shareholders	$	341.1		$	(.7	)	$	340.4		$	13.0		$	(3.5	)	$	9.5
(Loss) income attributable to White Mountains’s common shareholders per share:
Basic
Continuing operations	$	(8.34	)	$	(.13	)	$	(8.47	)	$	(6.22	)	$	(.63	)	$	(6.85	)
Discontinued operations	75.27			—			75.27			8.55			—			8.55
Total consolidated operations	$	66.93		$	(.13	)	$	66.80		$	2.33		$	(.63	)	$	1.70
Diluted
Continuing operations	$	(8.34	)	$	(.13	)	$	(8.47	)	$	(6.22	)	$	(.63	)	$	(6.85	)
Discontinued operations	75.11			—			75.11			8.55			—			8.55
Total consolidated operations	$	66.77		$	(.13	)	$	66.64		$	2.33		$	(.63	)	$	1.70

Summary of Significant Accounting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) publisher advertiser	Dec. 31, 2016 USD ($)	Sep. 28, 2017 $ / shares	Dec. 31, 2012 USD ($)
Basis of Presentation
Net unrealized foreign currency translation losses	$ (1.3)	$ (1.4)
Percentage of par value of policy reinsured	15.00%
Percentage of investments recorded at fair value	94.00%	94.00%
HG Global
Basis of Presentation
Surplus notes				$ 503.0
MediaAlpha
Basis of Presentation
Ownership interest (as a percent)	64.40%	60.00%
Number of advertisers \| advertiser	400
Number of publishers \| publisher	300
Preferred stocks \| HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%	96.90%
Common Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%	88.40%
Intact Financial Corporation \| OneBeacon
Basis of Presentation
Sale of stock, price per share \| $ / shares			$ 18.10

Summary of Significant Accounting Policies (Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	94.00%	94.00%
Minimum Percentage of Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	5.00%
Minimum Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	$ 1.0

Significant Transactions (Dispositions) (Details) - USD ($) $ in Millions					3 Months Ended	12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold						$ 1,131.0	$ 2,646.2	$ 24.0
Gain (loss) from sale of other discontinued operations, net of tax						557.0	415.1	18.2
Repayments of debt, principal						365.0	404.6	76.1
Payments to acquire other investments						84.1	38.5	73.9
OneBeacon
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300.0
Gain (loss) from sale of other discontinued operations, net of tax	$ 554.6					554.5	0.0	0.0
Tranzact
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Gain (loss) from sale of other discontinued operations, net of tax		51.9				3.2	51.9	0.0
Repayments of debt, principal			56.3
Discontinued operation, tax effect of discontinued operation		30.2
Increase of book value		$ 82.1	$ 82.1
Sirius Group
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Gain (loss) from sale of other discontinued operations, net of tax					$ (4.0)		363.2
Payments to acquire other investments				$ 161.8
Sale of Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						(0.7)	363.2	0.0
Recognition of benefit plan assets and obligations from sale of OneBeacon							113.3
Discontinued Operations \| OneBeacon
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						554.5	0.0
Discontinued operation, tax effect of discontinued operation						(5.7)	(12.5)	(12.9)
Recognition of benefit plan assets and obligations from sale of OneBeacon						2.9	0.0
Discontinued Operations \| Tranzact
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						3.2	51.9
Discontinued operation, tax effect of discontinued operation						0.0	(6.4)
Recognition of benefit plan assets and obligations from sale of OneBeacon						0.0	0.0
Discontinued Operations \| Sirius Group
Significant Transactions
Gain (loss) from sale of other discontinued operations, net of tax						(0.7)	363.2
Discontinued operation, tax effect of discontinued operation						0.0	(3.1)	$ 27.1
Recognition of benefit plan assets and obligations from sale of OneBeacon						$ 0.0	$ 113.3

Significant Transactions Significant Transactions (Acquisitions) (Details) - USD ($) $ in Millions					12 Months Ended
	Oct. 05, 2017	Aug. 01, 2017	Aug. 04, 2016	Jan. 15, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unusual or Infrequent Item [Line Items]
Assets					$ 3,659.2	$ 6,520.2
Goodwill					25.9	25.9	$ 18.6
Intangible assets, net (excluding goodwill)					36.2	19.3
Liabilities					298.4	2,804.2
Payments to acquire businesses and interest in affiliates					$ 27.6	13.4	$ 2.6
Buzzmove
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)			70.90%		77.10%
Assets			$ 11.5
Goodwill			7.6		$ 7.6	7.6
Intangible assets, net (excluding goodwill)			1.1		$ 0.8	$ 1.0
Liabilities			0.1
LowFares.com
Unusual or Infrequent Item [Line Items]
Payments to acquire businesses and interest in affiliates				$ 3.9
Healthplans.com
Unusual or Infrequent Item [Line Items]
Payments to acquire businesses and interest in affiliates	$ 28.0
MediaAlpha
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)					64.40%	60.00%
Assets					$ 96.5	$ 57.6
Liabilities					$ 59.8	$ 28.3
Preferred stocks \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 5.0
Stock issued during period, shares, acquisitions		37,409
Common Stock \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 0.7
Stock issued during period, shares, acquisitions		5,808
GBP \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates			6.1
GBP \| Preferred stocks \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 4.0
GBP \| Common Stock \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 0.5
United States of America, Dollars \| Buzzmove
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates			$ 8.1
Class A unit \| MediaAlpha
Unusual or Infrequent Item [Line Items]
Capital units, purchased	131,579
Capital units, value	$ 12.5

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment income:
Total investment income	$ 58.5	$ 34.5	$ 15.3
Third-party investment expenses	(2.5)	(2.4)	(4.4)
Net investment income, pre-tax	56.0	32.1	10.9
Fixed maturity investments
Investment income:
Total investment income	44.9	28.5	8.9
Short-term investments
Investment income:
Total investment income	1.8	0.9	0.0
Common Stock
Investment income:
Total investment income	10.6	4.0	4.0
Other long-term investments
Investment income:
Total investment income	$ 1.2	$ 1.1	$ 2.4

Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 24.1	$ 270.0	$ 30.0
Net unrealized investment gains (losses), pre-tax	109.2	(297.4)	230.5
Net realized and unrealized investment gains (losses), pre-tax	133.3	(27.4)	260.5
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(12.9)	2.7	(47.7)
Net realized and unrealized investment gains (losses), after-tax	$ 120.4	$ (24.7)	$ 212.8

Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Net realized investment gains (losses), pre-tax	$ 35.3	$ 269.7	$ 30.0
Net realized investment gains (losses), foreign, pre-tax	(11.2)	0.3	0.0
Net realized investment gains, pre-tax, total	24.1	270.0	30.0
Income tax expense attributable to net realized investment gains (losses)	(8.9)	(45.6)	(6.4)
Income tax expense attributable to net realized investment gains (losses), foreign	0.0	0.0	0.0
Income tax expense attributable to net realized investment gains, total	(8.9)	(45.6)	(6.4)
Net realized investment gains (losses), after-tax	26.4	224.1	23.6
Net realized investment gains (losses), foreign, after-tax	(11.2)	0.3	0.0
Net realized investment gains (losses), total, after-tax	15.2	224.4	23.6
Fixed maturity investments
Investment securities
Net realized investment gains (losses), pre-tax	(1.6)	(1.9)	(0.2)
Net realized investment gains (losses), foreign, pre-tax	4.1	0.3	0.0
Net realized investment gains, pre-tax, total	2.5	(1.6)	(0.2)
Short-term investments
Investment securities
Net realized investment gains (losses), pre-tax	(0.3)	0.4
Net realized investment gains (losses), foreign, pre-tax	0.0	0.0
Net realized investment gains, pre-tax, total	(0.3)	0.4
Common equity securities
Investment securities
Net realized investment gains (losses), pre-tax	18.1	268.5	31.0
Net realized investment gains (losses), foreign, pre-tax	6.0	0.0	0.0
Net realized investment gains, pre-tax, total	24.1	268.5	31.0
Other long-term investments
Investment securities
Net realized investment gains (losses), pre-tax	19.1	2.7	(0.8)
Net realized investment gains (losses), foreign, pre-tax	(21.3)	0.0	0.0
Net realized investment gains, pre-tax, total	$ (2.2)	$ 2.7	$ (0.8)

Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Unrealized investment gains	$ 97.5	$ (294.7)	$ 235.5
Foreign Currency Transaction Gain (Loss), Unrealized	11.7	(2.7)	(5.0)
Net unrealized investment gains (losses), pre-tax	109.2	(297.4)	230.5
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(4.0)	48.3	(41.3)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(4.0)	48.3	(41.3)
Unrealized investment gains after tax	93.5	(246.4)	194.2
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	11.7	(2.7)	(5.0)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	105.2	(249.1)	189.2
Fixed maturity investments
Investment securities
Unrealized investment gains	13.8	(14.6)	0.1
Foreign Currency Transaction Gain (Loss), Unrealized	12.7	2.1	0.0
Net unrealized investment gains (losses), pre-tax	26.5	(12.5)	0.1
Common equity securities
Investment securities
Unrealized investment gains	99.3	(257.4)	240.8
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	(3.3)	(3.9)
Net unrealized investment gains (losses), pre-tax	99.3	(260.7)	236.9
Other long-term investments
Investment securities
Unrealized investment gains	(15.6)	(22.7)	(5.4)
Foreign Currency Transaction Gain (Loss), Unrealized	(1.0)	(1.5)	(1.1)
Net unrealized investment gains (losses), pre-tax	$ (16.6)	$ (24.2)	$ (6.5)

Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Realized gains	$ 61.5	$ 283.7	$ 48.7
Realized losses	37.4	13.7	18.6
Unrealized investment gains	97.5	(294.7)	235.5
Level 3 Inputs
Investment securities
Unrealized investment gains	(15.4)	(14.2)	(8.1)
Fixed maturity investments
Investment securities
Unrealized investment gains	13.8	(14.6)	0.1
Fixed maturity investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.1	0.0
Common equity securities
Investment securities
Unrealized investment gains	99.3	(257.4)	240.8
Common equity securities \| Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.0	(9.0)
Other long-term investments
Investment securities
Unrealized investment gains	(15.6)	(22.7)	(5.4)
Other long-term investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	$ (15.4)	$ (14.3)	$ 0.9

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized investment losses on investments in unconsolidated affiliates	$ 0.0	$ 0.0	$ (39.2)
Income tax benefit	0.0	0.0	2.9
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	0.0	0.0	(36.3)
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	120.4	(24.7)	212.8
Total investment gains (losses) recorded during the period, after-tax	120.4	(24.7)	177.9
Symetra
Investments in and Advances to Affiliates [Line Items]
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	0.0	0.0	1.4
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	$ 0.0	$ 0.0	$ (34.9)

Investment Securities (Investment Holdings) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fixed maturity investments, at fair value		$ 2,081,100,000
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	$ 297,800,000	112,100,000
Gross unrealized gains	0	0
Gross unrealized losses	(1,300,000)	(1,100,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	296,500,000	111,000,000
Debt securities issued by corporations
Investment securities
Cost or amortized cost	867,600,000	752,000,000
Gross unrealized gains	2,900,000	2,300,000
Gross unrealized losses	(4,300,000)	(10,100,000)
Net foreign currency gains	14,700,000	2,100,000
Fixed maturity investments, at fair value	880,900,000	746,300,000
Mortgage and asset-backed securities
Investment securities
Cost or amortized cost	697,200,000	986,900,000
Gross unrealized gains	1,600,000	800,000
Gross unrealized losses	(4,100,000)	(7,900,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	694,700,000	979,800,000
Municipal obligations
Investment securities
Cost or amortized cost	252,000,000	238,700,000
Gross unrealized gains	3,700,000	1,100,000
Gross unrealized losses	(800,000)	(1,300,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	254,900,000	238,500,000
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	2,600,000	12,000,000
Gross unrealized gains	0	100,000
Gross unrealized losses	0	0
Net foreign currency gains	100,000	0
Fixed maturity investments, at fair value	2,700,000	12,100,000
Fixed income investments
Investment securities
Cost or amortized cost	2,117,200,000	2,101,700,000
Gross unrealized gains	8,200,000	4,300,000
Gross unrealized losses	(10,500,000)	(20,400,000)
Net foreign currency gains	14,800,000	2,100,000
Total fixed maturity investments	$ 2,129,700,000	2,087,700,000
Including Short Term Fixed
Investment securities
Duration of fixed maturities (years)	3 years 4 months 26 days
Excluding Short Term Investments
Investment securities
Duration of fixed maturities (years)	3 years 8 months 15 days
Star & Shield Insurance Exchange
Investment securities
Fixed maturity investments, at fair value		6,600,000.0
Less: Fixed maturity investments reclassified to assets held for sale		$ 6,600,000

Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	$ 2,117.2
Trading Securities by Maturity, Fair Value
Trading Securities	2,129.7
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	109.0
Due after one year through five years	663.0
Due after five years through ten years	464.9
Due after ten years	183.1
Trading Securities by Maturity, Fair Value
Due in one year or less	108.8
Due after one year through five years	660.9
Due after five years through ten years	472.0
Due after ten years	193.3
Mortgage and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Mortgage and asset-backed securities	697.2
Trading Securities by Maturity, Fair Value
Mortgage and asset-backed securities	$ 694.7

Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fixed maturity investments, at fair value		$ 2,081.1
Sales and maturities of investment securities	$ 2,791.3	3,692.4	$ 750.7
Common Stock
Investment securities
Cost or amortized cost	739.7	258.6
Gross unrealized gains	129.4	29.0
Gross unrealized losses	(3.0)	(2.0)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	866.1	285.6
Other long-term investments
Investment securities
Cost or amortized cost	246.6	194.0
Gross unrealized gains	6.8	7.9
Gross unrealized losses	(39.7)	(25.2)
Net foreign currency gains	(4.9)	(3.9)
Fixed maturity investments, at fair value	$ 208.8	$ 172.8

Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Investments held in trusts	$ 204.6	$ 161.7
Assets held by insurance regulators	$ 6.0	$ 6.0
Percentage of investments recorded at fair value	94.00%	94.00%

Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Debt securities issued by corporations
Investment securities
Fair value investments	880.9	746.3
AAA \| Debt securities issued by corporations
Investment securities
Fair value investments	1.6	0.0
AA \| Debt securities issued by corporations
Investment securities
Fair value investments	42.6	37.3
A \| Debt securities issued by corporations
Investment securities
Fair value investments	192.5	212.8
BBB \| Debt securities issued by corporations
Investment securities
Fair value investments	465.2	335.6
BB \| Debt securities issued by corporations
Investment securities
Fair value investments	161.7	143.2
B \| Debt securities issued by corporations
Investment securities
Fair value investments	$ 17.3	17.4
Star & Shield Insurance Exchange \| Fixed maturity investments
Investment securities
Fair value investments		6.6
Discontinued Operations, Held-for-sale \| Star & Shield Insurance Exchange \| Fixed maturity investments
Investment securities
Fair value investments		$ 4.2

Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
GNMA
Investment securities
Fair value investments	46.3	70.3
FNMA
Investment securities
Fair value investments	84.5	235.5
FHLMC
Investment securities
Fair value investments	62.0	59.5
Total Agency
Investment securities
Fair value investments	192.8	365.3
Residential
Investment securities
Fair value investments	0.0	70.3
Commercial
Investment securities
Fair value investments	70.5	3.9
Total Non-agency
Investment securities
Fair value investments	70.5	74.2
Total mortgage-backed securities
Investment securities
Fair value investments	263.3	439.5
Credit card receivables
Investment securities
Fair value investments	206.0	214.2
Vehicle receivables
Investment securities
Fair value investments	142.4	205.9
Other
Investment securities
Fair value investments	83.0	120.2
Total other asset-backed securities
Investment securities
Fair value investments	431.4	540.3
Total mortgage and asset- backed securities
Investment securities
Fair value investments	694.7	979.8
Level 2 Inputs
Investment securities
Fair value investments	2,105.4	2,093.8	$ 585.6
Level 2 Inputs \| GNMA
Investment securities
Fair value investments	46.3	70.3
Level 2 Inputs \| FNMA
Investment securities
Fair value investments	84.5	235.5
Level 2 Inputs \| FHLMC
Investment securities
Fair value investments	62.0	59.5
Level 2 Inputs \| Total Agency
Investment securities
Fair value investments	192.8	365.3
Level 2 Inputs \| Residential
Investment securities
Fair value investments	0.0	70.3
Level 2 Inputs \| Commercial
Investment securities
Fair value investments	70.5	3.9
Level 2 Inputs \| Total Non-agency
Investment securities
Fair value investments	70.5	74.2
Level 2 Inputs \| Total mortgage-backed securities
Investment securities
Fair value investments	263.3	439.5
Level 2 Inputs \| Credit card receivables
Investment securities
Fair value investments	206.0	214.2
Level 2 Inputs \| Vehicle receivables
Investment securities
Fair value investments	142.4	205.9
Level 2 Inputs \| Other
Investment securities
Fair value investments	83.0	120.2
Level 2 Inputs \| Total other asset-backed securities
Investment securities
Fair value investments	431.4	540.3
Level 2 Inputs \| Total mortgage and asset- backed securities
Investment securities
Fair value investments	694.7	979.8
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 Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Fixed rate CMBS
Investment securities
Average basis points of subordination	35.00
Fair value investments	$ 35.1
Fixed rate CMBS \| Super Senior
Investment securities
Fair value investments	0.0
Fixed rate CMBS \| Senior
Investment securities
Fair value investments	27.5
Fixed rate CMBS \| Subordinate
Investment securities
Fair value investments	7.6
Floating rate CMBS
Investment securities
Fair value investments	35.4
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	35.4
Commercial
Investment securities
Fair value investments	70.5	$ 3.9
Commercial \| Super Senior
Investment securities
Fair value investments	0.0
Commercial \| Senior
Investment securities
Fair value investments	27.5
Commercial \| Subordinate
Investment securities
Fair value investments	$ 43.0

- 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 + Details

Investment Securities (Summary of Non-agency Commercial Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Commercial
Investment securities
Fair value investments	70.5	3.9
Total Non-agency
Investment securities
Fair value investments	70.5	$ 74.2
2017 \| Commercial
Investment securities
Fair value investments	67.0
2016 \| Commercial
Investment securities
Fair value investments	$ 3.5

- 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 + Details

Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Hedge and private equity funds included in other long-term investments
Investment securities
Other investments	$ 125.3	$ 82.6
Private equity securities and limited liability companies, at fair value
Investment securities
Other investments	83.2	88.2
Forward Contracts
Investment securities
Other investments	(3.7)	(1.2)
Other long-term investments
Investment securities
Other investments	$ 4.0	$ 3.2
OneTitle
Investment securities
Ownership interest (as a percent)	20.00%
Surplus notes	$ 10.0

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2017 USD ($) fund	Dec. 31, 2016 USD ($)
Long/short equity REIT
Investment securities
Carrying value	$ 20.8
Hedge funds
Investment securities
Number of investments held \| fund	1
Private equity funds
Investment securities
Number of investments held \| fund	10
Carrying value	$ 70.4
Total hedge and private equity funds included in other long-term investments
Investment securities
Other long-term investments largest single investment	54.9	$ 21.5
Carrying value	125.3	82.6
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 99.3	123.6
Hedge funds \| Long/short banks and financial
Investment securities
Number of investments held \| fund	1
Carrying value	$ 54.9	21.5
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Hedge funds \| Long/short equity REIT
Investment securities
Carrying value	0.0	19.9
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Hedge funds \| Hedge funds
Investment securities
Carrying value	54.9	41.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Private equity funds \| Private equity funds
Investment securities
Carrying value	70.4	41.2
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	99.3	123.6
Private equity funds \| Manufacturing/Industrial
Investment securities
Carrying value	43.3	19.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	10.4	22.9
Private equity funds \| Aerospace/Defense/Government
Investment securities
Carrying value	15.8	19.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	12.9	25.9
Private equity funds \| Direct lending
Investment securities
Carrying value	7.1	1.4
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	23.1	28.6
Private equity funds \| Financial Services
Investment securities
Carrying value	4.2	1.0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	11.7	5.0
Private equity funds \| Insurance
Investment securities
Carrying value	0.0	0.0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 41.2	$ 41.2
Lock-up period expiring 2018 \| Hedge funds
Investment securities
Number of investments held \| fund	1
Carrying value	$ 54.9

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity funds $ in Millions	Dec. 31, 2017 USD ($)
Investment securities
Carrying value	$ 70.4
1 – 3 years
Investment securities
Carrying value	4.7
3 – 5 years
Investment securities
Carrying value	5.6
5 – 10 years
Investment securities
Carrying value	32.9
10 years
Investment securities
Carrying value	$ 27.2

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)	Jan. 01, 2017 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 2,547,300,000	$ 1,543,500,000
Total realized and unrealized gains (losses)	(157,400,000)	(26,200,000)
Amortization/Accretion	(9,100,000)	(6,100,000)
Purchases	3,332,500,000	4,728,300,000
Sales	(2,814,600,000)	(3,692,200,000)
Transfers in	18,700,000	70,000,000
Transfers out	(18,700,000)	(70,000,000)
Fair value investments	3,208,300,000	2,547,300,000	$ 1,543,500,000
Net realized and unrealized investment gains (losses)	133,300,000	(27,400,000)	260,500,000
Fixed maturity investments, at fair value		2,081,100,000
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,639,700,000
Fair value investments	3,259,100,000	2,639,700,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	279,500,000	789,000,000
Total realized and unrealized gains (losses)	82,700,000	2,000,000
Amortization/Accretion	0	100,000
Purchases	1,209,300,000	1,746,900,000
Sales	(681,100,000)	(2,258,500,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	890,400,000	279,500,000	789,000,000
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	441,700,000
Fair value investments	1,041,400,000	441,700,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,093,800,000	585,600,000
Total realized and unrealized gains (losses)	69,600,000	(8,000,000)
Amortization/Accretion	(9,100,000)	(6,200,000)
Purchases	2,007,900,000	2,868,800,000
Sales	(2,070,300,000)	(1,416,400,000)
Transfers in	18,700,000	70,000,000
Transfers out	0	0
Fair value investments	2,105,400,000	2,093,800,000	585,600,000
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	2,106,600,000
Fair value investments	2,130,500,000	2,106,600,000
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	91,400,000
Fair value investments	87,200,000	91,400,000
Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(5,200,000)
Fixed maturity investments, at fair value		6,600,000.0
Star & Shield Insurance Exchange \| Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Star & Shield Insurance Exchange \| Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(5,200,000)
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	2,087,700,000
Fair value investments	2,129,700,000	2,087,700,000
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	101,500,000
Fair value investments	296,500,000	101,500,000
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	1,986,200,000
Fair value investments	1,833,200,000	1,986,200,000
Fixed maturity investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	0
Total realized and unrealized gains (losses)	0	100,000
Amortization/Accretion	0	0
Purchases	31,200,000	69,900,000
Sales	(12,500,000)
Transfers in	0	0
Transfers out	18,700,000	(70,000,000)
Fair value investments	$ 0	$ 0	0
Number of investments \| Investment	3	3
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	$ 0
Fair value investments	0	$ 0
Fixed maturity investments \| Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	6,600,000
Fair value investments		6,600,000
Fixed maturity investments, at fair value		6,600,000	9,500,000
Fixed maturity investments \| Star & Shield Insurance Exchange \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
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	91,400,000
Fair value investments	87,200,000	91,400,000
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
Fair value investments	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
Fair value investments	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	91,400,000	103,600,000
Total realized and unrealized gains (losses)	(15,300,000)	(14,300,000)
Amortization/Accretion	0	0
Purchases	13,100,000	2,200,000
Sales	(2,000,000)	(100,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	87,200,000	91,400,000	103,600,000
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	91,400,000
Fair value investments	87,200,000	91,400,000
Other long-term investments \| Star & Shield Insurance Exchange \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
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	82,600,000	65,300,000
Total realized and unrealized gains (losses)	20,400,000	(6,000,000)
Amortization/Accretion	0	0
Purchases	71,000,000	40,500,000
Sales	(48,700,000)	(17,200,000)
Transfers in	0	0
Transfers out	0	0
Fair value investments	125,300,000	82,600,000	65,300,000
Total hedge and private equity funds included in other long-term investments \| Star & Shield Insurance Exchange \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses)	(300,000.0)
Short-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	175,000,000	142,000,000
Fair value investments	176,100,000	175,000,000	142,000,000
Short-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	162,300,000
Fair value investments	151,000,000	162,300,000
Short-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	12,700,000
Fair value investments	25,100,000	12,700,000
Short-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	0
Fair value investments	0	0
Short-term investments \| Star & Shield Insurance Exchange
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	100,000
Fair value investments		100,000
Short-term investments \| Star & Shield Insurance Exchange \| Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Short-term investments, at amortized cost (which approximates fair value)		100,000	$ 0	$ 100,000
Foreign Exchange Forward [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains (losses)	(23,800,000.0)	(1,200,000.0)
Foreign Exchange Forward [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	(1,200,000)
Fair value investments	$ (3,700,000)	$ (1,200,000)

Investment Securities (Significant Unobservable Inputs) (Details) - Standard Poors NR Rating - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		0.21
Time until expiration of security	4 years	4 years
Volatility Rate	50.00%	50.00%
Risk Free Rate	1.77%	1.00%
Private equity funds \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 21.0
Unobservable Inputs	1
Private equity funds \| Discounted cash flow/ Option pricing method
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 11.3	$ 9.3
Unobservable Inputs	0.21
Convertible Preferred Stock \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 3.6
Unobservable Inputs	2.521
Convertible Preferred Stock \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 14.5
Unobservable Inputs	2.06
Convertible Preferred Stock \| Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 3.6
Unobservable Inputs		6.0
Investment A \| Private equity funds \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 21.0
Unobservable Inputs		1.00
Investment B \| Private equity funds \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 22.1
Unobservable Inputs		.68
Investment B \| Private equity funds \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 22.1
Unobservable Inputs	.68
Investment C \| Private equity funds \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 3.2
Unobservable Inputs		2.52
Investment E \| Convertible Preferred Stock \| Share price of most recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 27.0
Unobservable Inputs		3.83
Investment E \| Convertible Preferred Stock \| Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 0.6
Unobservable Inputs	6

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Aug. 04, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 25.9	$ 25.9		$ 18.6
Intangible assets, gross (excluding goodwill)	71.0	43.5
Amortization of Intangible Assets	34.8	24.2
Intangible assets, net (excluding goodwill)	36.2	19.3
Intangible assets, gross (including goodwill)	96.9	69.4
Goodwill and intangible assets	62.1	45.2
MediaAlpha
Goodwill [Line Items]
Goodwill	18.3	18.3
Intangible assets, gross (excluding goodwill)	69.9	42.4
Amortization of Intangible Assets	34.5	24.1
Intangible assets, net (excluding goodwill)	35.4	18.3
Buzzmove
Goodwill [Line Items]
Goodwill	7.6	7.6	$ 7.6
Intangible assets, gross (excluding goodwill)	1.1	1.1
Amortization of Intangible Assets	0.3	0.1
Intangible assets, net (excluding goodwill)	$ 0.8	1.0	$ 1.1
Customer relationships \| MediaAlpha
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Intangible assets, gross (excluding goodwill)	$ 36.6	10.0
Amortization of Intangible Assets	10.2	6.2
Intangible assets, net (excluding goodwill)	$ 26.4	3.8
Information technology \| MediaAlpha
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Intangible assets, gross (excluding goodwill)	$ 33.3	32.4
Amortization of Intangible Assets	24.3	17.9
Intangible assets, net (excluding goodwill)	$ 9.0	14.5
Information technology \| Buzzmove
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Intangible assets, gross (excluding goodwill)	$ 0.5	0.5
Amortization of Intangible Assets	0.2	0.0
Intangible assets, net (excluding goodwill)	$ 0.3	0.5
Trademark \| Buzzmove
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Intangible assets, gross (excluding goodwill)	$ 0.6	0.6
Amortization of Intangible Assets	0.1	0.1
Intangible assets, net (excluding goodwill)	0.5	0.5
Other intangible assets
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	$ 36.2	$ 19.3		$ 26.9

Goodwill and Other Intangible Assets (Summary of Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 25.9	$ 25.9	$ 18.6
Intangible assets, net (excluding goodwill)	36.2	19.3
Goodwill and intangible assets	62.1	45.2
MediaAlpha
Goodwill [Line Items]
Goodwill	18.3	18.3
Buzzmove
Goodwill [Line Items]
Goodwill	7.6	7.6
Other intangible assets
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	36.2	19.3	$ 26.9
Other intangible assets \| MediaAlpha
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	35.4	18.3
Other intangible assets \| Buzzmove
Goodwill [Line Items]
Intangible assets, net (excluding goodwill)	0.8	1.0
Discontinued Operations, Held-for-sale
Goodwill [Line Items]
Goodwill and intangible assets	0.0	1.2
Non-controlling interests
Goodwill [Line Items]
Goodwill and intangible assets	(21.1)	(17.1)
Parent
Goodwill [Line Items]
Goodwill and intangible assets	$ 41.0	$ 29.3

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 05, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance		$ 25.9	$ 18.6
Acquisitions of businesses and asset groups (2)			7.6
Wobi write-off			(0.3)
Ending balance		25.9	25.9	$ 18.6
Finite-lived Intangible Assets [Roll Forward]
Beginning balance		19.3
Amortization, including foreign currency translation		(10.7)	(10.5)	(8.4)
Ending balance		36.2	19.3
Payments to acquire businesses and interest in affiliates		27.6	13.4	2.6
Other intangible assets
Finite-lived Intangible Assets [Roll Forward]
Beginning balance		19.3	26.9
Acquisitions of businesses and asset groups (2)		27.6	5.0
Wobi write-off			(2.5)
Amortization, including foreign currency translation		(10.7)	(10.5)
Ending balance		$ 36.2	19.3	26.9
Star & Shield LLC \| Other intangible assets
Finite-lived Intangible Assets [Roll Forward]
Add: Star & Shield amounts held for sale at beginning of the period			$ 0.4
Less: Star & Shield amounts held for sale at end of the period				$ 0.4
Healthplans.com
Finite-lived Intangible Assets [Roll Forward]
Payments to acquire businesses and interest in affiliates	$ 28.0

Goodwill and Other Intangible Assets (Amortization Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of other intangible assets	$ 10.7	$ 10.5	$ 8.4
2018	10.2
2019	5.0
2020	3.2
2021	2.9
2022	2.8
Total	$ 24.1

Debt (Debt Outstanding) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt	$ 23.8	$ 12.7
White Mountains \| WTM Bank Facility
Debt Instrument [Line Items]
Debt	0.0	0.0
MediaAlpha
Debt Instrument [Line Items]
Debt	23.8	12.7
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt instrument at face value	23.9	0.0
Unamortized Debt Issuance Expense	$ (0.1)	0.0
Effective yield (as a percent)	5.60%
MediaAlpha \| Previous MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt instrument at face value	$ 0.0	12.9
Unamortized Debt Issuance Expense	0.0	$ (0.2)
Effective yield (as a percent)		5.70%
Debt	0.0	$ 12.7
Carrying Value \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt	23.8	0.0
Carrying Value \| MediaAlpha \| Previous MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt	$ 0.0	$ 12.7

Debt (Schedule of Contractual Repayments) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 3.1
Due in two to three years	12.1
Due in four to five years	8.7
Due after five years	0.0
Total	$ 23.9

Debt (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Debt (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 05, 2017	May 12, 2017	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument [Line Items]
Repayments of debt, principal	$ 365.0	$ 404.6	$ 76.1
Interest expense	2.3	3.0	1.6
Draw down of debt and revolving line of credit	376.0	352.5	171.5
Interest paid	1.4	2.1	0.1
WTM Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility							$ 425.0
Issuance of debt, net of debt issuance costs	$ 350.0	$ 350.0
Debt, weighted average interest rate	4.45%	3.85%
Repayments of debt, principal	$ 350.0	$ 400.0
Interest expense	0.6	1.2	0.1
MediaAlpha Bank Facility
Debt Instrument [Line Items]
Interest expense	1.0	0.9	0.4
MediaAlpha
Debt Instrument [Line Items]
Interest expense	1.0	$ 0.9	$ 0.4
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility				$ 28.4	$ 20.0
Draw down of debt and revolving line of credit	6.0
MediaAlpha \| Previous MediaAlpha Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility						$ 20.0
Repayments of debt, principal	$ 12.9
Term Loan \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.50%
Term Loan \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility	$ 18.4
Issuance of debt, net of debt issuance costs	20.0
Repayments of debt, principal	2.1
Long-term line of credit	17.9
Revolving Credit Facility \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility	10.0
Long-term line of credit	$ 6.0
Line of Credit \| MediaAlpha Bank Facility
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.25%

Derivatives (Details) $ in Millions	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Notional amount	$ 206.3

Income Taxes (Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax expense (benefit):
U.S. federal									$ (0.3)	$ 21.4	$ 0.0
State									(1.3)	(0.7)	(0.6)
Non-U.S.									(2.0)	(0.3)	(0.8)
Total current tax expense									(3.6)	20.4	(1.4)
Deferred tax expense (benefit):
U.S. federal									11.4	12.5	(11.3)
Total deferred tax benefit (expense)									11.4	12.5	(11.3)
Income Tax Expense (Benefit)	$ 2.5	$ 4.0	$ 1.0	$ 0.3	$ 10.2	$ 17.1	$ 4.0	$ 1.6	$ 7.8	$ 32.9	$ (12.7)

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pre-tax operating results of WM Life Re
Total revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
General and administrative expenses									(218.7)	(186.0)	(200.7)
Pre-tax (loss) income from continuing operations	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (74.5)	$ (13.8)	$ (25.7)	$ (33.3)	$ 7.8	(147.3)	129.2
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue										1.2	9.3
Change in fair value of variable annuity liability, included in other revenue										(0.3)	(0.4)
Change in fair value of derivatives, included in other revenue										(2.0)	(8.8)
Foreign exchange, included in other revenue										1.3	(1.3)
Other investment loss										0.0	(0.4)
Total revenues										0.2	(1.6)
Death benefit claims paid, included in general and administrative expenses										(0.3)	(0.1)
General and administrative expenses										(2.6)	(4.0)
Pre-tax (loss) income from continuing operations										$ (2.7)	$ (5.7)

Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)	21.00%	35.00%									35.00%
Tax expense (benefit) at the U.S. statutory rate											$ (2.7)	$ 51.6	$ (45.2)
Differences in taxes resulting from:
Tax rate changes											(44.3)	(3.9)	(0.5)
Change in valuation allowance											42.6	6.9	(21.8)
Non-U.S. earnings, net of foreign taxes											21.5	(19.2)	58.5
Officer compensation											(4.1)	0.0	0.0
Member surplus contributions											(3.0)	(2.3)	(1.5)
Withholding tax											(2.0)	(0.2)	(0.5)
Tax exempt interest and dividends											0.5	0.1	0.0
Tax reserve adjustments											(0.3)	0.0	0.0
Other, net											(0.4)	(0.1)	(1.7)
Income Tax Expense (Benefit)			$ 2.5	$ 4.0	$ 1.0	$ 0.3	$ 10.2	$ 17.1	$ 4.0	$ 1.6	7.8	32.9	(12.7)
Pre-tax (loss) income from continuing operations			$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (74.5)	$ (13.8)	$ (25.7)	$ (33.3)	7.8	(147.3)	129.2
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total											2.0	0.3	0.7
Non-US
Differences in taxes resulting from:
Pre-tax (loss) income from continuing operations											$ 71.3	$ (66.3)	$ 149.5

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
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)
Carrying value	0.0	20.1	$ 56.4
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
Carrying value	0.0
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)
Carrying value	0.0
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)
Carrying value	$ 0.0

Income Taxes (Deferred Tax Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 73.0	$ 104.3
Non-U.S. net operating loss carryforwards	33.9	34.0
Compensation	20.4	26.5
Investment basis difference	4.9	5.6
Other items	4.9	8.5
Total gross deferred income tax assets	137.1	178.9
Less: valuation allowances	109.6	146.2
Total net deferred income tax assets	27.5	32.7
Deferred income tax liabilities related to:
Other items	2.1	2.7
Total deferred income tax liabilities	26.2	32.7
Net deferred tax asset	1.3	0.0
Members Surplus Contributions
Deferred income tax liabilities related to:
Deferred tax liabilities, members surplus contributions	$ 24.1	$ 30.0

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in derivative instruments
Purchases	$ 3,332.5	$ 4,728.3
Level 2
Changes in derivative instruments
Purchases	2,007.9	2,868.8
Level 1
Changes in derivative instruments
Purchases	1,209.3	1,746.9
White Mountains Life Re
Changes in derivative instruments
Beginning of period	0.0	20.1	$ 56.4
Purchases		0.0	0.0
Realized and unrealized gains (losses)		2.0	8.8
Transfers in (out)		0.0	0.0
Sales/settlements		(18.1)	(27.5)
Ending balance		0.0	20.1
Collateral Cash and Investments			5.8
White Mountains Life Re \| Level 3
Changes in variable annuity (liabilities)
Purchases			0.0
White Mountains Life Re \| Level 2
Changes in derivative instruments
Beginning of period	0.0	16.5	33.8
Purchases		0.0	0.0
Realized and unrealized gains (losses)		0.7	7.5
Transfers in (out)		0.0	0.0
Sales/settlements		(15.8)	(9.8)
Ending balance		0.0	16.5
White Mountains Life Re \| Level 1
Changes in derivative instruments
Beginning of period	0.0	0.9	3.7
Purchases		0.0	0.0
Realized and unrealized gains (losses)		4.2	(8.4)
Transfers in (out)		0.0	0.0
Sales/settlements		3.3	(11.2)
Ending balance		0.0	0.9
Variable Annuity \| White Mountains Life Re \| Level 3
Changes in variable annuity (liabilities)
Beginning balance	0.0	0.3	(0.7)
Purchases		0.0
Net realized and unrealized (losses) gains		0.3	0.4
Transfers in (out)		0.0	0.0
Sales/settlements		0.0	0.0
Ending balance		0.0	0.3
Derivative \| White Mountains Life Re \| Level 3
Changes in derivative instruments
Beginning of period	$ 0.0	2.7	18.9
Purchases		0.0	0.0
Realized and unrealized gains (losses)		(2.9)	9.7
Transfers in (out)		0.0	0.0
Sales/settlements		(5.6)	(6.5)
Ending balance		$ 0.0	$ 2.7

Income Taxes (Tax Cuts and Jobs Act of 2017) (Details) - USD ($) $ in Millions			12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Dec. 31, 2017
Tax Cuts and Jobs of Act 2017 [Line Items]
U.S. statutory rate (as a percent)	21.00%	35.00%		35.00%
Unites States
Tax Cuts and Jobs of Act 2017 [Line Items]
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense				$ 43.1
Forecast
Tax Cuts and Jobs of Act 2017 [Line Items]
Percentage of utilization of net operating losses			80.00%

Derivatives (Forward Contracts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional amount	$ 206.3
Fair value investments	3,208.3	$ 2,547.3	$ 1,543.5
Forward Contracts
Derivative [Line Items]
Realized and unrealized gains (losses)	(23.8)	(1.2)
Other Investments
Derivative [Line Items]
Net amount recognized in other log term investments	(3.7)	(1.2)
GBP
Derivative [Line Items]
Notional amount	152.0
Barclays Bank PLC \| A \| Forward Contracts
Derivative [Line Items]
Notional amount	206.3
Fair value investments	$ (3.7)	(1.2)
Barclays Bank PLC \| A- \| Forward Contracts
Derivative [Line Items]
Notional amount		184.6
Carrying value		$ (1.2)

Income Taxes (Valuation Allowance) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Valuation allowance			$ 109.6				$ 146.2				$ 109.6	$ 146.2
Change in valuation allowance											$ (42.6)	(6.9)	$ 21.8
U.S. statutory rate (as a percent)	21.00%	35.00%									35.00%
Income Tax Expense (Benefit)			(2.5)	$ (4.0)	$ (1.0)	$ (0.3)	(10.2)	$ (17.1)	$ (4.0)	$ (1.6)	$ (7.8)	(32.9)	$ 12.7
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance			74.8				112.5				74.8	112.5
United Kingdom \| Net operating losses in UK Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance											0.7	0.1
Unites States
Valuation Allowance [Line Items]
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense											43.1
Unites States \| U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Change in valuation allowance											6.4
Non-US \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance												2.8
Israel \| Net operating losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance											3.0	3.4
Subsidiaries \| Net operating losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			13.5				9.4				13.5	9.4
Subsidiaries \| Luxembourg \| Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			20.3				24.0				20.3	24.0
Subsidiaries \| United Kingdom \| Net operating losses in UK Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance			$ 1.0				$ 0.3				1.0	0.3
Guilford Holdings, Inc. \| Unites States
Valuation Allowance [Line Items]
Change in valuation allowance											(21.5)	17.1
U.S. statutory rate (as a percent)	21.00%	35.00%
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense											20.4
BAM
Valuation Allowance [Line Items]
Change in valuation allowance											(18.4)	3.6
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense											22.7
Income Tax Expense (Benefit)											(10.1)	(11.0)
Star & Shield Insurance Exchange
Valuation Allowance [Line Items]
Decrease in valuation allowance												$ (6.9)
Continuing Operations \| Guilford Holdings, Inc. \| Unites States
Valuation Allowance [Line Items]
Change in valuation allowance											$ (21.4)

Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax assets
Total net deferred income tax assets	$ 27.5	$ 32.7
Alternative minimum tax credit carryforwards, not subject to expiration	1.3
Unites States
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	3.1
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	471.4
Net deferred tax assets
Gross deferred tax asset	106.9
Valuation allowance	(106.9)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward \| Unites States
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	332.3
Net deferred tax assets
Gross deferred tax asset	73.0
Valuation allowance	(73.0)
Total net deferred income tax assets	0.0
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	77.7
Net deferred tax assets
Gross deferred tax asset	20.2
Valuation allowance	(20.2)
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	6.7
Net deferred tax assets
Gross deferred tax asset	1.1
Valuation allowance	(1.1)
Total net deferred income tax assets	0.0
Operating and Capital Loss Carryforward \| Israel
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	54.7
Net deferred tax assets
Gross deferred tax asset	12.6
Valuation allowance	(12.6)
Total net deferred income tax assets	0.0
2018-2022 \| 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	0.3
2018-2022 \| Operating and Capital Loss Carryforward \| Unites States
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	0.3
2018-2022 \| 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
2018-2022 \| 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
2018-2022 \| Operating and Capital Loss Carryforward \| Israel
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	0.0
2023-2027 \| 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	0.0
2023-2027 \| Operating and Capital Loss Carryforward \| Unites States
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	0.0
2023-2027 \| 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
2023-2027 \| 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
2023-2027 \| Operating and Capital Loss Carryforward \| Israel
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	0.0
2028-2037 \| 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	378.4
2028-2037 \| Operating and Capital Loss Carryforward \| Unites States
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	332.0
2028-2037 \| 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	46.4
2028-2037 \| 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
2028-2037 \| Operating and Capital Loss Carryforward \| Israel
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 \| 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	92.7
No expiration date \| Operating and Capital Loss Carryforward \| Unites 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 \| 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	31.3
No expiration date \| 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	6.7
No expiration date \| Operating and Capital Loss Carryforward \| Israel
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating loss carryforwards	54.7
2030 \| 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	$ (6.7)

Income Taxes (Uncertain Tax Positions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 0.0
Changes in prior year tax positions	0.1
Tax positions taken during the current year	0.2
Ending balance	0.3	$ 0.0
Permanent Differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	0.0
Changes in prior year tax positions	0.1
Tax positions taken during the current year	0.2
Ending balance	0.3	0.0
Temporary Differences
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	0.0
Changes in prior year tax positions	0.0
Tax positions taken during the current year	0.0
Ending balance	0.0	0.0
Interest and Penalties Net of Tax Benefit
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	0.0
Changes in prior year tax positions	0.0
Tax positions taken during the current year	0.0
Ending balance	$ 0.0	$ 0.0

Income Taxes (Tax Examinations) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Income Tax Examination [Line Items]
Increase in deferred tax assets	$ 137.1	$ 178.9
Tax Year 2012
Income Tax Examination [Line Items]
Increase in deferred tax assets			$ 0.6
Increase in valuation allowance			$ 0.6

Municipal Bond Guarantee Insurance - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of discounted future cash flows	10.00%
Percentage of par value of policy reinsured	15.00%
Percentage of risk premium ceded	60.00%
Repayments of debt, principal	$ 365.0	$ 404.6	$ 76.1
Interest paid	1.4	$ 2.1	$ 0.1
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				503.0
Assets Held-in-trust	$ 603.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Year of U.S. Treasury Rate \| yr	1
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%
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%
Collateral held in supplement trust	$ 715.1	$ 465.4
Interest receivable	126.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	499.0	300.0
BAM \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 499.0	$ 503.0
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)	3.78%	3.54%
Debt instrument, basis spread on variable rate	30000.00%
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Repayments of debt, principal	$ 4.0
Interest paid	$ 1.0
Combination One \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate \| yr	5
Combination Two \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate \| yr	3
Fixed interest rate \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)	8.00%
Series A BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 203.0
Series B BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 300.0
Forecast \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)					4.60%

Municipal Bond Guarantee Insurance - Schedule of BAM's Insured Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross unearned insurance premiums	$ 136.8	$ 82.9
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Contracts outstanding \| Contract	6,371	4,807
Remaining weighted average contract period (in years)	10 years 11 months	10 years 10 months
Principal	$ 42,090.6	$ 33,057.3
Interest	21,057.1	16,396.6
Total debt service outstanding	63,147.7	$ 49,453.9
Gross unearned insurance premiums	$ 136.8

Municipal Bond Guarantee Insurance - Schedule of BAM's Future Expected Premium Revenues (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Total gross unearned insurance premiums	$ 136.8	$ 82.9
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
January 1, 2018 - March 31, 2018	2.9
April 1, 2018 - June 30, 2018	2.9
July 1, 2018 - September 30, 2018	2.9
October 1, 2018 - December 31, 2018	2.8
Total	11.5
2019	11.1
2020	10.7
2021	10.2
2022	9.7
2023 and thereafter	83.6
Total gross unearned insurance premiums	$ 136.8

Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross written premiums	$ 63.2	$ 38.6	$ 25.9
Assumed (ceded) written premiums	0.0	0.0	0.0
Net written premiums	$ 63.2	$ 38.6	$ 25.9

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Less: total income from discontinued operations, net of tax									577.5	523.4	135.1
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders									49.7	(121.6)	160.1
Allocation of earnings to participating restricted common shares									0.7	(1.5)	1.9
Basic and diluted earnings per share numerators									49.0	(120.1)	158.2
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									$ 49.0	$ (120.1)	$ 158.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									4,293,800	5,014,900	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Basic earnings per share denominator									4,239,500	4,950,100	5,811,200
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									4,293,800	5,018,100	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Diluted earnings (loss) per share denominator									4,239,500	4,953,300	5,811,200
Basic and diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid per White Mountains’s common share									$ 1	$ 1	$ 1
Undistributed earnings (loss)									10.56	(25.26)	26.22
Basic and diluted earnings per share (in usd per share)									$ 11.56	$ (24.26)	$ 27.22

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
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Incremental common shares attributable to share-based payment arrangements	40,000
Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		125,000

Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 49.0	$ (120.1)	$ 158.2
Dividends declared net of restricted common share amounts	4.5	5.4	5.9
Total undistributed net earnings (losses), net of restricted common share amounts	$ 44.5	$ (125.5)	$ 152.3

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) multiplier shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Vested (in shares)		5,000
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
W T M Incentive Plan \| White Mountains
Target Performance Share activity
Beginning of period	80,353	93,654	111,257
Shares paid or expired	(30,838)	(36,294)	(42,959)
New grants	17,710	22,615	29,195
Assumed forfeitures and cancellations	(16,710)	378	(3,839)
End of period	50,515	80,353	93,654
Unamortized grant date fair value
Beginning of period \| $	$ 42.4	$ 57.7	$ 44.4
Shares paid or expired \| $	(21.9)	(41.0)	(30.8)
Assumed forfeitures and cancellations \| $	(9.3)	0.5	(0.3)
Expense recognized \| $	34.6	25.2	44.4
End of period \| $	45.8	$ 42.4	$ 57.7
Unamortized grant date fair value \| $	$ 20.2
W T M Incentive Plan \| Sirius Group
Target Performance Share activity
Beginning of period	7,315	10,826
End of period	2,195	7,315	10,826
W T M Incentive and Phantom Plan \| Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Performance goal percentage for minimum payout	34.00%
Payout for maximum growth target percentage	76.00%
W T M Incentive and Phantom Plan \| Performance Cycle From 2012 to 2014
Unamortized grant date fair value
Performance goal percentage for minimum payout		140.00%
Payout for maximum growth target percentage		142.00%
W T M Incentive and Phantom Plan \| Performance Cycle from 2011 to 2013
Unamortized grant date fair value
Performance goal percentage for minimum payout			91.00%
Payout for maximum growth target percentage			145.50%

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Range Of Performance Cycle From 2017-2019
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	5.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2016-2018
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	4.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2015-2017
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%
White Mountains
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	51,285
Incentive compensation payable before forfeitures	$ 46.6
Assumed forfeitures	(770)
Assumed forfeitures in period amount	(0.8)
Shares outstanding (in shares)	50,515	80,353	93,654	111,257
Accrued incentive compensation	$ 45.8	$ 42.4	$ 57.7	$ 44.4
White Mountains \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	16,680
Incentive compensation payable before forfeitures	$ 8.3
White Mountains \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	16,235
Incentive compensation payable before forfeitures	$ 14.7
White Mountains \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	18,370
Incentive compensation payable before forfeitures	$ 23.6
Minimum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	1.00%
Minimum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	0.00%
Minimum \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Maximum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	9.00%
Maximum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	8.00%
Maximum \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%

Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2020
Target Performance Share activity
Issued (in shares)	17,485
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2019
Target Performance Share activity
Issued (in shares)	250	24,615
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2018
Target Performance Share activity
Issued (in shares)	250	750	23,640
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	70,620	70,675	83,314
Issued (in shares)	17,985	25,365	23,640
Vested (in shares)	(28,846)	(24,620)	(36,279)
Assumed forfeitures and cancellations (in shares)	(6,004)	(800)	0
End of period	53,755	70,620	70,675
Unamortized grant date fair value
Beginning balance	$ 19.7	$ 15.7	$ 14.3
Issued	16.3	20.2	15.7
Assumed forfeitures and cancellations	(3.5)	(0.3)	0.0
Expense recognized	(18.2)	(15.9)	(14.3)
Ending balance	$ 14.3	$ 19.7	$ 15.7
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	5,235	9,205
End of period	2,195	5,235	9,205

Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended	12 Months Ended
	Jan. 20, 2017	Mar. 31, 2017	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Shares issued, price per share			$ 742
Employee Stock Option \| Combination One
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised			5,000
Options, exercises in period, intrinsic value			$ 0.4
Shares issued, price per share			$ 742
Employee Stock Option \| Combination Two
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised			80,000
Options, exercises in period, intrinsic value			$ 8.4
Conversion of stock, shares converted			9,930
Employee Stock Option \| Chief Executive Officer
Share-based compensation arrangement by share-based payment award
Non-option equity instruments, exercised	125,000	40,000
Options, exercises in period, intrinsic value		$ 4.4
Common Stock
Share-based compensation arrangement by share-based payment award
Conversion of stock, shares converted		5,142

Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 723.9	$ 887.2	$ 284.2
Value of each common share repurchased and retired during the period (in dollars per share)	$ 869	$ 802	$ 733
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	25,086	47,030	25,016
Shares issued to directors (in shares)	1,959	1,735	1,376
Stock issued, in satisfaction of performance shares		5,000
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	$ 4.6	$ 5.4	$ 6.0
Dividends declared and paid (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Non-controlling Equity	$ (131.7)	$ 133.3
Total Non-Controlling Excluding Reciprocals
Dividends, Common Stock [Abstract]
Non-controlling Equity	$ 31.5	$ 35.2
Restricted Stock
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	17,985	25,365	23,640
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	832,725	1,106,145	387,495
General Board Authorization
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares	643,130
Common shares repurchased and retired during the period	821,732	1,098,123	361,839
Value of common shares repurchased and retired during the period	$ 713.1	$ 881.4	$ 267.4
Value of each common share repurchased and retired during the period (in dollars per share)	$ 870	$ 803	$ 739
Chief Executive Officer
Common Shares Issued [Abstract]
Stock issued during period, shares, new issues	5,142	14,930
Repurchased for Employee Benefit Plans
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	10,993	8,022	10,802
OneBeacon Ltd.
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	23.90%
Non-controlling Equity	$ 0.0	$ 244.6
HG Global
Dividends, Common Stock [Abstract]
Non-controlling Percentage	3.10%	3.10%
Non-controlling Equity	$ 15.9	$ 16.6
MediaAlpha
Dividends, Common Stock [Abstract]
Non-controlling Percentage	35.70%	40.00%
Non-controlling Equity	$ 13.1	$ 11.7
Dewar
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	18.80%
Non-controlling Equity	$ 0.0	$ 3.9
Buzzmove
Dividends, Common Stock [Abstract]
Non-controlling Percentage	22.90%	29.10%
Non-controlling Equity	$ 2.5	$ 3.0
BAM
Dividends, Common Stock [Abstract]
Non-controlling Percentage	100.00%	100.00%
Non-controlling Equity	$ (163.2)	$ (150.9)
Star & Shield Insurance Exchange
Dividends, Common Stock [Abstract]
Non-controlling Percentage	0.00%	100.00%
Non-controlling Equity	$ 0.0	$ 4.4
Total mutuals and reciprocals
Dividends, Common Stock [Abstract]
Non-controlling Equity	$ (163.2)	$ (146.5)

Statutory Capital and Surplus (Details)	12 Months Ended
Statutory Capital and Surplus (Details)	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	$ 365,000,000	$ 404,600,000	$ 76,100,000
Interest paid	1,400,000	2,100,000	100,000
Distributions made	4,600,000
Common equity securities	2,129,700,000
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash	1,682,600,000
Other long-term investments	62,000,000
HG Re \| HG Global BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	677,000,000
Surplus required	1
Collateral held in supplement trust	715,100,000
Cash and investments outside of collateral trusts	1,000,000
BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	427,300,000
Statutory net income (loss)	25,400,000	$ 32,700,000	$ 32,000,000
Surplus required	66,000,000
HG Global \| HG Global BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value	$ 619,000,000
Ownership interest (as a percent)	96.90%
Preferred stock dividend rate	6.00%
White Mountains \| MediaAlpha
Statutory Accounting Practices [Line Items]
Distributions made	$ 3,300,000
MediaAlpha
Statutory Accounting Practices [Line Items]
Ownership interest (as a percent)	64.40%	60.00%
Distributions made	$ 5,200,000
Unrestricted cash	9,100,000
Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	4,000,000
Interest paid	$ 1,000,000
Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	3.78%	3.54%
Debt instrument, basis spread on variable rate	30000.00%
Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Debt instrument, basis spread on variable rate		3.00%
Preferred stocks \| HG Global \| HG Global BAM
Statutory Accounting Practices [Line Items]
Dividends payable	$ 235,600,000
Preferred stocks \| White Mountains \| HG Global \| HG Global BAM
Statutory Accounting Practices [Line Items]
Dividends payable	227,900,000
Common Equities \| Other Operations
Statutory Accounting Practices [Line Items]
Common equity securities	$ 866,100,000
Combination One \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr	5
Combination One \| Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate	5
Combination Two \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr	3
Combination Two \| Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate \| yr	3
US Treasury Rate \| Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	3.78%
Fixed interest rate \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Fixed interest rate \| Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Forecast \| Surplus Note \| BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				4.60%
Forecast \| US Treasury Rate \| Surplus Note \| BAM \| HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				4.60%

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 10.4	$ 13.4	$ 12.0
Net investment income									56.0	32.1	10.9
Net realized and unrealized investment gains									133.3	(27.4)	260.5
Advertising & commission revenues									167.0	118.3	107.4
Other revenues									7.1	21.3	49.2
Total revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	373.8	157.7	440.0
Loss and loss adjustment expenses									1.1	8.0	8.2
Insurance acquisition expenses									4.1	5.6	6.3
Other underwriting expenses									0.4	0.4	0.4
Cost of sales									139.4	102.0	93.6
General and administrative expenses									218.7	186.0	200.7
Interest expense									2.3	3.0	1.6
Total expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Pre-tax (loss) income from continuing operations	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	$ (74.5)	$ (13.8)	$ (25.7)	$ (33.3)	7.8	(147.3)	129.2
HG Global BAM
Segment reporting information
Advertising & commission revenues									0.0	0.0	0.0
Total revenues									23.3		10.7
Loss and loss adjustment expenses									0.0	0.0	0.0
Cost of sales									0.0	0.0	0.0
Interest expense									0.0	0.0	0.0
Total expenses									47.3	43.4	40.1
Pre-tax (loss) income from continuing operations									(24.0)	(26.7)	(29.4)
MediaAlpha
Segment reporting information
Earned insurance premiums									0.0	0.0	0.0
Net investment income									0.0	0.0	0.0
Net realized and unrealized investment gains									0.0	0.0	0.0
Advertising & commission revenues									163.2	116.5	105.5
Other revenues									0.0	0.0	0.0
Total revenues									163.2	116.5	105.5
Loss and loss adjustment expenses									0.0	0.0	0.0
Insurance acquisition expenses									0.0	0.0	0.0
Other underwriting expenses									0.0	0.0	0.0
Cost of sales									135.9	97.8	90.7
General and administrative expenses									26.7	21.9	16.4
Interest expense									1.0	0.9	0.4
Total expenses									163.6	120.6	107.5
Pre-tax (loss) income from continuing operations									$ (0.4)	$ (4.1)	$ (2.0)
Percentage of Advertising Revenue									27.00%	24.00%	52.00%
Other Segments
Segment reporting information
Earned insurance premiums									$ 1.0	$ 7.5	$ 8.7
Net investment income									43.7	23.1	4.8
Net realized and unrealized investment gains									132.7	(28.1)	259.9
Advertising & commission revenues									3.8	1.8	1.9
Other revenues									6.1	20.2	48.5
Total revenues									187.3	24.5	323.8
Loss and loss adjustment expenses									1.1	8.0	8.2
Insurance acquisition expenses									0.1	2.2	3.4
Other underwriting expenses									0.0	0.0	0.0
Cost of sales									3.5	4.2	2.9
General and administrative expenses									149.1	124.5	147.5
Interest expense									1.3	2.1	1.2
Total expenses									155.1	141.0	163.2
Pre-tax (loss) income from continuing operations									32.2	(116.5)	160.6
HG Global BAM
Segment reporting information
Earned insurance premiums									9.4	5.9	3.3
Net investment income									12.3	9.0	6.1
Net realized and unrealized investment gains									0.6	0.7	0.6
Other revenues									1.0	1.1	0.7
Total revenues									23.3	16.7	10.7
Insurance acquisition expenses									4.0	3.4	2.9
Other underwriting expenses									0.4	0.4	0.4
General and administrative expenses									42.9	39.6	36.8
Total expenses									$ 47.3	$ 43.4	40.1
Symetra \| Other Segments
Segment reporting information
Net realized and unrealized investment gains											$ 258.8

Segment Information (Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2012
Segment reporting information
Total investments	$ 3,380.7	$ 2,714.4
Assets held for sale		3,606.4
Assets	3,659.2	6,520.2
Liabilities held for sale		2,569.3
Liabilities	298.4	2,804.2
White Mountains’ common shareholders’ equity	3,492.5	3,582.7
Non-controlling interests	(131.7)	133.3
HG Global BAM
Segment reporting information
Total investments	693.4	629.7
Assets held for sale		0.0
Assets	747.4	677.6
Liabilities held for sale		0.0
Liabilities	394.9	289.1
White Mountains’ common shareholders’ equity	499.8	522.8
Non-controlling interests	(147.3)	(134.3)
BAM
Segment reporting information
Surplus notes	499.0	300.0
Accrued interest on surplus notes	126.0	108.0
MediaAlpha
Segment reporting information
Total investments	0.0	0.0
Assets held for sale		0.0
Assets	96.5	57.6
Liabilities held for sale		0.0
Liabilities	59.8	28.3
White Mountains’ common shareholders’ equity	23.6	17.6
Non-controlling interests	13.1	11.7
Other Operations
Segment reporting information
Total investments	2,687.3	2,084.7
Assets held for sale		0.0
Assets	3,039.9	2,358.8
Liabilities held for sale		0.0
Liabilities	71.6	97.7
White Mountains’ common shareholders’ equity	2,965.8	2,258.1
Non-controlling interests	2.5	3.0
Total Capital Contributed to Subsidiary	727.7	703.0
Eliminations
Segment reporting information
Assets	(227.9)	(180.2)
Liabilities	(227.9)	(180.2)
Discontinued Operations, Held-for-sale
Segment reporting information
Total investments	0.0	0.0
Assets held for sale		3,606.4
Assets	3.3	3,606.4
Liabilities held for sale		2,569.3
Liabilities	0.0	2,569.3
White Mountains’ common shareholders’ equity	3.3	784.2
Non-controlling interests	0.0	252.9
Continuing Operations
Segment reporting information
Non-controlling interests	(131.7)	133.3
HG Global
Segment reporting information
Non-controlling interests	15.9	16.6
Surplus notes			$ 503.0
HG Global \| BAM
Segment reporting information
Surplus notes	499.0	503.0
Preferred Dividends Payable \| HG Global \| Other Operations
Segment reporting information
Accounts Payable and Accrued Liabilities	$ 227.9	$ 180.2

Investments in Unconsolidated Affiliates (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Equity method eligible private equity securities, at fair value
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	$ 83.2	$ 88.2
White Mountains
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	208.8	172.8
White Mountains \| Equity method eligible private equity securities, at fair value
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	58.0	55.6
White Mountains \| Investments, accounted for under the equity method
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	4.6	3.5
White Mountains \| Total investments in equity method eligible unconsolidated entities
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	62.6	59.1
White Mountains \| Other Investments
Investments in and Advances to Affiliates [Line Items]
Total other long-term investments	$ 146.2	$ 113.7

Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (White Mountain's Investments) (Details)	Dec. 31, 2017
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	22.00%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%
OneTitle
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	20.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%
Tuckerman Capital Fund III
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	21.00%

Investments in Unconsolidated Affiliates (Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended							12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in and Advances to Affiliates
Total assets				$ 6,520.2				$ 3,659.2	$ 6,520.2
Total liabilities				2,804.2				298.4	2,804.2
Revenues	$ 87.5	$ 83.5	$ 88.8	(5.5)	$ 56.8	$ 48.4	$ 58.0	373.8	157.7	$ 440.0
White Mountains
Investments in and Advances to Affiliates
Total assets				79.4				75.4	79.4
Total liabilities				$ 18.8				24.2	18.8
Revenues								60.0	32.9	12.9
Expenses								$ (66.8)	$ (76.4)	$ (69.9)

Investments in Unconsolidated Affiliates (Symetra & Hamer Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in unconsolidated affiliates
Unrealized investment gains			$ 97.5	$ (294.7)	$ 235.5
Unrealized investment gains after tax			93.5	(246.4)	194.2
Equity in earnings of unconsolidated affiliates			$ 0.0	$ 0.0	$ 25.1
Symetra
Investments in unconsolidated affiliates
Unrealized investment gains	$ 258.8
Unrealized investment gains after tax	$ 241.1
Hamer
Investments in unconsolidated affiliates
Gain on disposal of equity method investment		$ 20.0
Equity in earnings of unconsolidated affiliates		$ 1.6

Investments in Unconsolidated Affiliates (Symetra Summary of Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended							9 Months Ended	12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items]
Total investments				$ 2,714.4					$ 3,380.7	$ 2,714.4
Assets				6,520.2					3,659.2	6,520.2
Long-term debt				12.7					23.8	12.7
Liabilities				2,804.2					298.4	2,804.2
Common shareholders’ equity				3,716.0					3,360.8	3,716.0
Earned insurance premiums									10.4	13.4	$ 12.0
Net investment income									56.0	32.1	10.9
Revenues	$ 87.5	$ 83.5	$ 88.8	(5.5)	$ 56.8	$ 48.4	$ 58.0		373.8	157.7	440.0
Expenses	$ 79.1	$ 85.7	$ 92.4	$ 69.0	$ 70.6	$ 74.1	$ 91.3		366.0	305.0	310.8
Net income									593.1	409.0	276.7
Comprehensive income (loss)									$ 630.7	$ 547.4	$ 187.4
Symetra
Investments in and Advances to Affiliates [Line Items]
Total investments								$ 32,409.2
Separate account assets								885.9
Assets								34,962.8
Policyholder liabilities								29,492.0
Long-term debt								697.5
Separate account liabilities								885.9
Liabilities								31,836.7
Common shareholders’ equity								3,126.1
Earned insurance premiums								539.3
Net investment income								994.3
Revenues								1,605.9
Policy benefits								1,143.7
Expenses								1,543.6
Net income								89.6
Comprehensive income (loss)								$ (234.1)

Variable Interest Entities (Details) - director	12 Months Ended
Variable Interest Entities (Details) - director	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2015
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured		15.00%
BAM
Variable Interest Entity [Line Items]
Right to elect directors on board number	2
Prospector Offshore Fund
Variable Interest Entity [Line Items]
Ownership of variable interest entity			67.60%
First Loss Reinsurance Treaty \| HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%
First Loss Reinsurance Treaty \| BAM
Variable Interest Entity [Line Items]
Percentage of premiums	60.00%

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 23.8	$ 12.7
MediaAlpha
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	23.8	12.7
MediaAlpha \| Previous MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	0.0	12.7
MediaAlpha \| Fair value \| MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	23.9	0.0
MediaAlpha \| Fair value \| Previous MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	0.0	13.0
MediaAlpha \| Carrying Value \| MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	23.8	0.0
MediaAlpha \| Carrying Value \| Previous MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 0.0	$ 12.7

Transactions with Related Persons (Details) - USD ($) $ / shares in Units, $ in Millions						12 Months Ended
	Jul. 13, 2017	Sep. 15, 2016	Apr. 19, 2016	Sep. 17, 2015	Jun. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Value of each common share repurchased and retired during the period (in dollars per share)						$ 869	$ 802	$ 733
Prospector
Related Party Transaction [Line Items]
Asset management fees								$ 2.1
Shares paid or expired								5.5
Management fees, base revenue								0.4
Incentive fees incurred								0.1
Franklin Mutual Advisers
Related Party Transaction [Line Items]
Value of each common share repurchased and retired during the period (in dollars per share)	$ 850.00	$ 820.00	$ 807.00	$ 761.50	$ 650.03
White Mountains Advisors LLC \| Symetra
Related Party Transaction [Line Items]
Asset management fees								$ 8.0
W T M Incentive Plan \| Performance Shares \| White Mountains
Related Party Transaction [Line Items]
Number of performance shares granted						17,710	22,615	29,195
Shares paid or expired						$ 21.9	$ 41.0	$ 30.8
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						832,725	1,106,145	387,495
Common Stock \| Franklin Mutual Advisers
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period	235,000	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Rent expense	$ 3.5	$ 3.4	$ 2.0
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	2.7
Capital lease payments due in two years	2.4
Capital lease payments due in three years	2.2
Capital Lease payments due in four years and thereafter	9.1
Commitments to fund other-long term investments	109.3
Swedish Tax Authority \| Sirius Group
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Estimate of possible loss (up to)	$ 19.0

Discontinued Operations (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended								12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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													$ 1,131.0	$ 2,646.2	$ 24.0
Gain (loss) from sale of other discontinued operations, net of tax													557.0	415.1	18.2
Net (loss) income from discontinued operations, net of tax					$ 4.3	$ 539.1	$ 2.8	$ 31.3	$ 8.0	$ 84.4	$ 383.6	$ 47.4	20.5	108.3	116.9
Income Tax (expense) benefit					2.5	$ 4.0	$ 1.0	$ 0.3	10.2	17.1	$ 4.0	$ 1.6	7.8	32.9	(12.7)
Payments to acquire other investments													84.1	38.5	73.9
Total investments					$ 3,380.7				$ 2,714.4				3,380.7	2,714.4
Net realized and unrealized investment gains													133.3	(27.4)	260.5
OneBeacon
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	$ 1,300.0
Gain (loss) from sale of other discontinued operations, net of tax	$ 554.6												554.5	0.0	0.0
Net (loss) income from discontinued operations, net of tax													20.5	108.6	38.2
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
Gain (loss) from sale of other discontinued operations, net of tax		51.9											3.2	51.9	0.0
Discontinued operation, tax effect of discontinued operation		$ 30.2
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
Gain (loss) from sale of other discontinued operations, net of tax										$ (4.0)				363.2
Payments to acquire other investments				$ 161.8
Total investments															686.2
Net realized and unrealized investment gains														3.7	205.0
Sirius Group \| Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total investments															528.6
Net realized and unrealized investment gains															200.8
Sale of Sirius Group
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													(0.7)	363.2	0.0
Recognition of other comprehensive income from sale of subsidiary														113.3
Esurance
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	0.0	17.9
Discontinued Operations \| OneBeacon
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													554.5	0.0
Net (loss) income from discontinued operations, net of tax													20.5	108.6	38.2
Discontinued operation, tax effect of discontinued operation													(5.7)	(12.5)	(12.9)
Recognition of other comprehensive income from sale of subsidiary													2.9	0.0
Net realized and unrealized investment gains													38.8	37.7	(35.1)
Net change in pension liability and other accumulated comprehensive items													0.3	1.0	0.0
Discontinued Operations \| Tranzact
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													3.2	51.9
Net (loss) income from discontinued operations, net of tax													0.0	6.1	(1.9)
Income Tax (expense) benefit														21.4
Tax effect of discontinued operations														30.2
Discontinued operation, tax effect of discontinued operation													0.0	(6.4)
Recognition of other comprehensive income from sale of subsidiary													0.0	0.0
Net realized and unrealized investment gains													0.0	0.0
Net change in pension liability and other accumulated comprehensive items													0.0	0.0
Discontinued Operations \| Sirius Group
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													(0.7)	363.2
Net (loss) income from discontinued operations, net of tax													0.0	(6.4)
Discontinued operation, tax effect of discontinued operation													0.0	(3.1)	27.1
Recognition of other comprehensive income from sale of subsidiary													0.0	113.3
Net realized and unrealized investment gains													0.0	(1.5)	15.1
Net change in pension liability and other accumulated comprehensive items													$ 0.0	32.0	(65.0)
Discontinued Operations \| Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net (loss) income from discontinued operations, net of tax														(4.3)	81.1
Net change in pension liability and other accumulated comprehensive items														32.0
Discontinued Operations \| Esurance
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
OneBeacon \| Intact Financial Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of stock, price per share	$ 18.10
Net gain (loss) from sale of discontinued operations \| Discontinued Operations \| Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, tax effect of discontinued operation														$ 8.8

Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets held for sale
Fixed maturity investments, at fair value		$ 2,081.1
Common equity securities, at fair value		0.0
Total investments	$ 3,380.7	2,714.4
Goodwill and intangible assets	62.1	45.2
Liabilities held for sale
Debt	23.8	12.7
Liabilities held for sale		2,569.3
Sirius Group, Tranzact, and Star and Shield \| Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale		3,606.4
Assets held for sale
Fixed maturity investments, at fair value		2,175.7
Short-term investments, at amortized cost (which approximates fair value)		112.3
Common equity securities, at fair value		188.7
Other long-term investments		150.5
Total investments		2,627.2
Cash		70.5
Reinsurance recoverable on unpaid losses		179.8
Insurance premiums receivable		229.8
Deferred acquisition costs		96.3
Deferred tax asset		126.7
Ceded unearned insurance and reinsurance premiums		44.2
Accounts receivable on unsettled investment sales		1.4
Goodwill and intangible assets		1.2
Accrued investment income		11.3
Other assets		218.0
Assets held for sale		3,606.4
Liabilities held for sale
Loss and loss adjustment expense reserves		1,370.6
Unearned insurance and reinsurance premiums		576.3
Debt		273.2
Accrued incentive compensation		44.3
Funds held under insurance contracts		153.0
Other liabilities		151.9
Liabilities held for sale		2,569.3
Net assets held for sale		$ 1,037.1
Connecticut \| Sirius Group, Tranzact, and Star and Shield \| Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	3.3
Other expense	3.7
Assets held for sale
Assets held for sale	$ 3.3

Discontinued Operations (Net Income (Loss) from Discontinued Operations) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Earned insurance premiums											$ 10.4	$ 13.4	$ 12.0
Net investment income											56.0	32.1	10.9
Net realized and unrealized investment gains											133.3	(27.4)	260.5
Other revenues											7.1	21.3	49.2
Revenues				$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	373.8	157.7	440.0
Expenses
Loss and loss adjustment expenses											1.1	8.0	8.2
Insurance acquisition expenses											4.1	5.6	6.3
Other underwriting expenses											0.4	0.4	0.4
General and administrative expenses											218.7	186.0	200.7
Interest expense											2.3	3.0	1.6
Total expenses				79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Net (loss) income from discontinued operations, net of tax			$ 4.3	$ 539.1	$ 2.8	$ 31.3	$ 8.0	84.4	$ 383.6	$ 47.4	20.5	108.3	116.9
Gain (loss) from sale of other discontinued operations, net of tax											557.0	415.1	18.2
Total income (loss) from discontinued operations											577.5	523.4	135.1
Comprehensive income (loss) from discontinued operations											630.7	547.4	187.4
Discontinued Operations
Expenses
Total income (loss) from discontinued operations											577.5	523.4	135.1
OneBeacon
Expenses
Net (loss) income from discontinued operations, net of tax											20.5	108.6	38.2
Gain (loss) from sale of other discontinued operations, net of tax	$ 554.6										554.5	0.0	0.0
OneBeacon \| Discontinued Operations
Revenues
Earned insurance premiums											807.6	1,100.6	1,176.2
Net investment income											39.7	50.6	45.9
Net realized and unrealized investment gains											38.8	37.7	(35.1)
Other revenues											7.7	5.5	(0.6)
Revenues											893.8	1,194.4	1,186.4
Expenses
Loss and loss adjustment expenses											546.0	656.0	700.7
Insurance acquisition expenses											145.6	206.0	213.8
Other underwriting expenses											156.2	209.0	218.2
General and administrative expenses											21.2	14.2	13.0
Interest expense											10.0	13.1	15.4
Total expenses											879.0	1,098.3	1,161.1
Pre-tax income											14.8	96.1	25.3
Income tax benefit											5.7	12.5	12.9
Net (loss) income from discontinued operations, net of tax											20.5	108.6	38.2
Gain (loss) from sale of other discontinued operations, net of tax											554.5	0.0
Total income (loss) from discontinued operations											575.0	108.6	38.5
Net change in pension liability and other accumulated comprehensive items											0.3	1.0	0.0
Recognition of other comprehensive income from sale of subsidiary											2.9	0.0
Comprehensive income (loss) from discontinued operations											578.2	109.6	38.5
Sale of Sirius Group
Expenses
Gain (loss) from sale of other discontinued operations, net of tax											(0.7)	363.2	0.0
Recognition of other comprehensive income from sale of subsidiary												113.3
Sale of Sirius Group \| Discontinued Operations
Expenses
Net (loss) income from discontinued operations, net of tax												(4.3)	81.1
Net change in pension liability and other accumulated comprehensive items												32.0
Sirius Group
Revenues
Net realized and unrealized investment gains												3.7	205.0
Expenses
Gain (loss) from sale of other discontinued operations, net of tax								$ (4.0)				363.2
Sirius Group \| Discontinued Operations
Revenues
Earned insurance premiums											0.0	240.1	847.0
Net investment income											0.0	14.4	40.7
Net realized and unrealized investment gains											0.0	(1.5)	15.1
Other revenues											0.0	0.6	(20.6)
Revenues											0.0	253.6	882.2
Expenses
Loss and loss adjustment expenses											0.0	154.9	422.7
Insurance acquisition expenses											0.0	59.0	189.8
Other underwriting expenses											0.0	30.9	107.9
General and administrative expenses											0.0	10.4	27.0
Interest expense												7.9	26.6
Total expenses											0.0	263.1	774.0
Pre-tax income											0.0	(9.5)	108.2
Income tax benefit											0.0	3.1	(27.1)
Net (loss) income from discontinued operations, net of tax											0.0	(6.4)
Gain (loss) from sale of other discontinued operations, net of tax											(0.7)	363.2
Total income (loss) from discontinued operations											(0.7)	356.8	81.1
Net change in pension liability and other accumulated comprehensive items											0.0	32.0	(65.0)
Recognition of other comprehensive income from sale of subsidiary											0.0	113.3
Comprehensive income (loss) from discontinued operations											(0.7)	502.1	16.1
Tranzact
Expenses
Income tax benefit		$ (30.2)
Gain (loss) from sale of other discontinued operations, net of tax		$ 51.9									3.2	51.9	0.0
Tranzact \| Discontinued Operations
Revenues
Earned insurance premiums											0.0	0.0
Net investment income											0.0	0.0
Net realized and unrealized investment gains											0.0	0.0
Other revenues											0.0	119.6
Revenues											0.0	119.6
Expenses
Loss and loss adjustment expenses											0.0	0.0
Insurance acquisition expenses											0.0	0.0
Other underwriting expenses											0.0	0.0
General and administrative expenses											0.0	116.7
Interest expense												3.2
Total expenses											0.0	119.9
Pre-tax income											0.0	(0.3)
Income tax benefit											0.0	6.4
Net (loss) income from discontinued operations, net of tax											0.0	6.1	(1.9)
Gain (loss) from sale of other discontinued operations, net of tax											3.2	51.9
Total income (loss) from discontinued operations											3.2	58.0
Net change in pension liability and other accumulated comprehensive items											0.0	0.0
Recognition of other comprehensive income from sale of subsidiary											0.0	0.0
Comprehensive income (loss) from discontinued operations											3.2	58.0
Other Operations \| Discontinued Operations
Revenues
Earned insurance premiums													0.0
Net investment income													0.0
Net realized and unrealized investment gains													0.0
Other revenues													186.2
Revenues													186.2
Expenses
Loss and loss adjustment expenses													0.0
Insurance acquisition expenses													0.0
Other underwriting expenses													0.0
General and administrative expenses													185.3
Interest expense													4.0
Total expenses													189.3
Pre-tax income													(3.1)
Income tax benefit													0.7
Net (loss) income from discontinued operations, net of tax													(2.4)
Total income (loss) from discontinued operations													15.5
Net change in pension liability and other accumulated comprehensive items													0.0
Comprehensive income (loss) from discontinued operations													15.5
Total \| Discontinued Operations
Revenues
Earned insurance premiums											807.6	1,340.7	2,023.2
Net investment income											39.7	65.0	86.6
Net realized and unrealized investment gains											38.8	36.2	(20.0)
Other revenues											7.7	125.7	165.0
Revenues											893.8	1,567.6	2,254.8
Expenses
Loss and loss adjustment expenses											546.0	810.9	1,123.4
Insurance acquisition expenses											145.6	265.0	403.6
Other underwriting expenses											156.2	239.9	326.1
General and administrative expenses											21.2	141.3	225.3
Interest expense											10.0	24.2	46.0
Total expenses											879.0	1,481.3	2,124.4
Pre-tax income											14.8	86.3	130.4
Income tax benefit											5.7	22.0	(13.5)
Net (loss) income from discontinued operations, net of tax											20.5	108.3	116.9
Gain (loss) from sale of other discontinued operations, net of tax											557.0	415.1
Total income (loss) from discontinued operations											577.5	523.4	135.1
Net change in pension liability and other accumulated comprehensive items											0.3	33.0	(65.0)
Recognition of other comprehensive income from sale of subsidiary											2.9	113.3
Comprehensive income (loss) from discontinued operations											$ 580.7	$ 669.7	70.1
OneBeacon Runoff \| Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax													0.3
Esurance & AFI \| Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax													$ 17.9

Discontinued Operations Discontinued Operations (Cash Flows Table) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from discontinued operations (Note 19)	$ 157.0	$ 23.6	$ 248.4
Net cash provided from (used for) investing activities — discontinued operations (Note 19)	3.0	241.4	(100.5)
Net cash used for financing activities — discontinued operations (Note 19)	(61.9)	(93.8)	(100.7)
Effect of exchange rate changes on cash	0.0	0.0	(4.5)
Net cash provided from operations	98.1	171.2	42.7
Cash balance at beginning of year	80.2	72.0	145.7
Net change in cash during the period - continuing operations	16.0	7.9	(74.8)
Cash balance at end of year	97.1	80.2	72.0
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash balance at beginning of year	70.5	245.4	203.8
Net change in cash during the period - continuing operations	(0.9)	(0.3)	(1.1)
Cash sold as part of sale of consolidated subsidiaries	(167.7)	(345.8)	0.0
Cash balance at end of year	$ 0.0	$ 70.5	$ 245.4

Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Less: total income (loss) from continuing operations attributable to White Mountains’s common shareholders									49.7	(121.6)	160.1
Total income (loss) from discontinued operations									577.5	523.4	135.1
Allocation of earnings to participating restricted common shares									(0.7)	1.5	(1.9)
Basic and diluted earnings per share numerators									$ 49.0	$ (120.1)	$ 158.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									4,293,800	5,014,900	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Basic earnings per share denominator									4,239,500	4,950,100	5,811,200
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									4,293,800	5,018,100	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Diluted earnings per share denominator									4,239,500	4,953,300	5,811,200
Basic earnings per share (in dollars) - discontinued operations:	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.65	$ 66.80	$ 1.70	$ 146.06	$ 80.11	$ 50.20
Diluted earnings per share (in dollars) - discontinued operations:	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.61	$ 66.64	$ 1.70	$ 146.06	$ 80.06	$ 50.20
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders									$ 627.2	$ 401.8	$ 295.2
Less: total income (loss) from continuing operations attributable to White Mountains’s common shareholders									49.7	(121.6)	160.1
Total income (loss) from discontinued operations									577.5	523.4	135.1
Allocation of earnings to participating restricted common shares									(7.3)	(6.8)	(1.6)
Basic and diluted earnings per share numerators									$ 570.2	$ 516.6	$ 133.5
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									4,293,800	5,014,900	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Basic earnings per share denominator									4,239,500	4,950,100	5,811,200
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive shares outstanding									4,293,800	5,018,100	5,879,200
Average unvested restricted common shares									(54,300)	(64,800)	(68,000)
Diluted earnings per share denominator									4,239,500	4,953,300	5,811,200
Basic earnings per share (in dollars) - discontinued operations:									$ 134.50	$ 104.37	$ 22.98
Diluted earnings per share (in dollars) - discontinued operations:									$ 134.50	$ 104.32	$ 22.98

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
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
Common Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		125,000

Discontinued Operations Discontinued Operations (Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 23.8	$ 12.7
OneBeacon \| OBH Senior Notes \| Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt		274.2
OneBeacon \| OBH Senior Notes \| Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt		$ 273.2

Discontinued Operations Discontinued Operations (Surplus Notes) (Details) - OneBeacon - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of discontinued operations, net of tax		$ (29.1)
Par Value
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Surplus notes			$ 101.0
Fair Value
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Surplus notes		$ 71.9
Seller Priority Surplus Note \| Current market rates on repayments
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
RBC Score	250.00%

Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Advertising & commission revenues									167.0	118.3	107.4
Total expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Amortization of other intangible assets									10.7	10.5	8.4
Pre-tax (loss) income from continuing operations	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income tax benefit (expense)	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Net income (loss) from continuing operations	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Gain (loss) from sale of other discontinued operations, net of tax									557.0	415.1	18.2
Net (loss) income from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Income before equity in earnings of unconsolidated affiliates									593.1	409.0	251.6
Equity in earnings of unconsolidated affiliates									0.0	0.0	25.1
Net income									593.1	409.0	276.7
Net (income) loss attributable to non-controlling interests	10.5	10.6	12.0	1.0	17.4	3.1	(21.5)	(6.2)	34.1	(7.2)	18.5
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	13.6	29.0	(38.9)	$ 90.8	340.4	9.5	627.2	401.8	295.2
Other comprehensive income (loss), net of tax									0.3	(0.7)	(42.8)
Comprehensive income									630.7	547.4	187.4
Comprehensive (income) loss attributable to non-controlling interests									(0.2)	(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders									630.5	$ 547.1	$ 187.4
Basic and diluted earnings per share - continuing operations										$ (24.26)	$ 27.22
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues			85.4	93.5	0.3		49.2	58.8		$ 166.3	$ 435.3
Total expenses			85.8	92.2	69.1		74.3	88.5		302.8	303.3
Pre-tax (loss) income from continuing operations			(0.4)	1.3	(68.8)		(25.1)	(29.7)		(136.5)	132.0
Income tax benefit (expense)			1.0	0.3	10.2		4.0	1.6		32.9	(12.7)
Net income (loss) from continuing operations			0.6	1.6	(58.6)		(21.1)	(28.1)		(103.6)	119.3
Gain (loss) from sale of other discontinued operations, net of tax										415.1	18.2
Net (loss) income from discontinued operations, net of tax			2.8	31.3	8.0		383.6	47.4		108.3	116.9
Income before equity in earnings of unconsolidated affiliates										419.8	254.4
Equity in earnings of unconsolidated affiliates										0.0	25.1
Net income										419.8	279.5
Net (income) loss attributable to non-controlling interests			12.1	1.3	17.3		(21.4)	(6.3)		(7.3)	18.1
Net income attributable to White Mountains’s common shareholders (3)			15.5	34.2	(33.3)		341.1	13.0		412.5	297.6
Comprehensive income										558.1	197.2
Comprehensive (income) loss attributable to non-controlling interests										(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders										$ 557.8	$ 197.2
Basic and diluted earnings per share - continuing operations										$ (22.13)	$ 27.63
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues			(1.9)	(4.7)	(5.8)		(0.8)	(0.8)		$ (8.6)	$ 4.7
Total expenses			(0.1)	0.2	(0.1)		(0.2)	2.8		2.2	7.5
Pre-tax (loss) income from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Income tax benefit (expense)				0.0	0.0		0.0	0.0		0.0	0.0
Net income (loss) from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Gain (loss) from sale of other discontinued operations, net of tax										0.0	0.0
Net (loss) income from discontinued operations, net of tax			0.0	0.0	0.0		0.0	0.0		0.0	0.0
Income before equity in earnings of unconsolidated affiliates										(10.8)	(2.8)
Equity in earnings of unconsolidated affiliates										0.0	0.0
Net income										(10.8)	(2.8)
Net (income) loss attributable to non-controlling interests			(0.1)	(0.3)	0.1		(0.1)	0.1		0.1	0.4
Net income attributable to White Mountains’s common shareholders (3)			$ (1.9)	$ (5.2)	$ (5.6)		$ (0.7)	$ (3.5)		(10.7)	(2.4)
Comprehensive income										(10.7)	(9.8)
Comprehensive (income) loss attributable to non-controlling interests										0.0	0.0
Comprehensive income attributable to White Mountains’s common shareholders										$ (10.7)	$ (9.8)
Basic and diluted earnings per share - continuing operations										$ (2.13)	$ (0.41)
HG Global BAM
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									23.3	$ 16.7	$ 10.7
Total expenses									47.3	43.4	40.1
HG Global BAM \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										16.7	10.7
Total expenses										43.4	40.1
HG Global BAM \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										0.0	0.0
Total expenses										0.0	0.0
MediaAlpha
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									163.2	116.5	105.5
Advertising & commission revenues									163.2	116.5	105.5
Total expenses									163.6	120.6	107.5
Amortization of other intangible assets									10.5	10.1	8.1
MediaAlpha \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										116.5	105.5
Total expenses										120.6	107.5
MediaAlpha \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										0.0	0.0
Total expenses										0.0	0.0
Other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									187.3	24.5	323.8
Advertising & commission revenues									3.8	1.8	1.9
Other revenues										22.7	321.9
Total expenses									155.1	141.0	163.2
Other expense										140.6	162.7
Amortization of other intangible assets									0.2	0.4	0.5
Other \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										33.1	319.1
Advertising & commission revenues										10.4	4.6
Other revenues										22.7	314.5
Total expenses										138.8	155.7
Other expense										137.6	154.5
Amortization of other intangible assets										1.2	1.2
Other \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										(8.6)	4.7
Advertising & commission revenues										(8.6)	(2.7)
Other revenues										0.0	7.4
Total expenses										2.2	7.5
Other expense										3.0	8.2
Amortization of other intangible assets										(0.8)	(0.7)
Discontinued Operations
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income attributable to White Mountains’s common shareholders (3)									$ 627.2	401.8	295.2
Other comprehensive income (loss), net of tax										145.6	(107.8)
Discontinued Operations \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other comprehensive income (loss), net of tax										145.6	(100.4)
Discontinued Operations \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other comprehensive income (loss), net of tax										$ 0.0	$ (7.4)

Financial Statement Revisions - Statement of Shareholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	$ 3,716.0
Repurchases and retirements of common shares	(723.9)	$ (887.2)	$ (284.2)
Net income	593.1	409.0	276.7
Ending balance	3,360.8	3,716.0
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	3,737.2
Net income		419.8	279.5
Ending balance		3,737.2
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(21.2)
Net income		(10.8)	(2.8)
Ending balance		(21.2)
Common shares and paid-in surplus
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	810.7	978.2	1,034.7
Repurchases and retirements of common shares	(147.9)	(192.4)	(67.0)
Ending balance		810.7	978.2	$ 1,034.7
Common shares and paid-in surplus \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	810.7	978.2	1,034.7
Ending balance		810.7	978.2	1,034.7
Common shares and paid-in surplus \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	0.0	0.0	0.0
Ending balance		0.0	0.0	0.0
Retained earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	2,776.6	3,075.0	3,003.0	2,801.4
Repurchases and retirements of common shares	(576.0)	(694.8)	(217.2)	(97.4)
Net income	627.2	401.8	295.2	305.2
Dividends declared on common shares	(4.6)	(5.4)	(6.0)	(6.2)
Net other changes in AOCI		113.9	(34.6)	66.6
Ending balance		2,776.6	3,075.0	3,003.0
Retained earnings \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	2,797.2	3,084.9	3,010.5	2,801.9
Repurchases and retirements of common shares		(694.8)	(217.2)	(97.4)
Net income		412.5	297.6	312.2
Dividends declared on common shares		(5.4)	(6.0)	(6.2)
Net other changes in AOCI		113.9	(34.6)	66.6
Ending balance		2,797.2	3,084.9	3,010.5
Retained earnings \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(20.6)	(9.9)	(7.5)	(0.5)
Repurchases and retirements of common shares		0.0	0.0	0.0
Net income		(10.7)	(2.4)	(7.0)
Dividends declared on common shares		0.0	0.0	0.0
Net other changes in AOCI		0.0	0.0	0.0
Ending balance		(20.6)	(9.9)	(7.5)
AOCL, after-tax
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(4.6)	(149.9)	(42.1)	52.6
Ending balance		(4.6)	(149.9)	(42.1)
AOCL, after-tax \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(4.6)	(149.9)	(49.5)	52.1
Net change in pension liability and other accumulated comprehensive items		31.4	(65.8)	(168.2)
Ending balance		(4.6)	(149.9)	(49.5)
AOCL, after-tax \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	0.0	0.0	7.4	0.5
Ending balance		0.0	0.0	7.4
Parent
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	3,582.7	3,903.3	3,995.6
Repurchases and retirements of common shares	(723.9)	(887.2)	(284.2)
Net income	627.2	401.8	295.2
Dividends declared on common shares	(4.6)	(5.4)	(6.0)
Ending balance		3,582.7	3,903.3	3,995.6
Parent \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	3,603.3	3,913.2	3,995.7
Ending balance		3,603.3	3,913.2	3,995.7
Parent \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(20.6)	(9.9)	(0.1)
Ending balance		(20.6)	(9.9)	(0.1)
Non-controlling interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	133.3	454.3	542.6	491.7
Repurchases and retirements of common shares	(5.2)
Net income	(34.1)	7.2	(18.5)	(22.3)
Other changes in NCI		(328.2)	(69.8)	73.2
Ending balance		133.3	454.3	542.6
Non-controlling interests \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	133.9	454.8	542.7	491.7
Net income		7.3	(18.1)	(22.2)
Other changes in NCI		(328.2)	(69.8)	73.2
Ending balance		133.9	454.8	542.7
Non-controlling interests \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(0.6)	(0.5)	(0.1)	0.0
Net income		(0.1)	(0.4)	(0.1)
Other changes in NCI		0.0	0.0	0.0
Ending balance		(0.6)	(0.5)	(0.1)
Total equity
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	3,716.0	4,357.6	4,538.2
Repurchases and retirements of common shares	(729.1)	(887.2)	(284.2)
Net income	593.1	409.0	276.7
Dividends declared on common shares	(4.6)	(5.4)	(6.0)
Ending balance		3,716.0	4,357.6	4,538.2
Total equity \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	3,737.2	4,368.0	4,538.4
Ending balance		3,737.2	4,368.0	4,538.4
Total equity \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Beginning balance	(21.2)	(10.4)	(0.2)
Ending balance		(21.2)	(10.4)	(0.2)
Foreign Currency Gain (Loss) \| AOCL, after-tax
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net change in pension liability and other accumulated comprehensive items	0.4	31.4	(73.2)	(161.3)
Foreign Currency Gain (Loss) \| AOCL, after-tax \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net change in pension liability and other accumulated comprehensive items		0.0	(7.4)	$ 6.9
Foreign Currency Gain (Loss) \| Parent
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net change in pension liability and other accumulated comprehensive items	0.4	31.4	(73.2)
Foreign Currency Gain (Loss) \| Non-controlling interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net change in pension liability and other accumulated comprehensive items	0.1	0.3
Foreign Currency Gain (Loss) \| Total equity
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net change in pension liability and other accumulated comprehensive items	$ 0.5	$ 31.7	$ (73.2)

Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	$ 3,659.2	$ 6,520.2
Investments	3,380.7	2,714.4
Goodwill	25.9	25.9	$ 18.6
Intangible assets	71.0	43.5
Total liabilities	298.4	2,804.2
Liabilities held for sale		2,569.3
White Mountains’s common shares	3.8	4.6
Paid-in surplus	666.8	806.1
Retained earnings	2,823.2	2,776.6
Accumulated other comprehensive income, net of tax		(4.6)
Total White Mountains’s common shareholders’ equity	3,492.5	3,582.7
Non-controlling interests	(131.7)	133.3
Total equity	3,360.8	3,716.0
Total liabilities and equity	3,659.2	6,520.2
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		6,544.7
Total liabilities		2,807.5
White Mountains’s common shares		4.6
Paid-in surplus		806.1
Retained earnings		2,797.2
Accumulated other comprehensive income, net of tax		(4.6)
Total White Mountains’s common shareholders’ equity		3,603.3
Non-controlling interests		133.9
Total equity		3,737.2
Total liabilities and equity		6,544.7
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		(24.5)
Total liabilities		(3.3)
White Mountains’s common shares		0.0
Paid-in surplus		0.0
Retained earnings		(20.6)
Accumulated other comprehensive income, net of tax		0.0
Total White Mountains’s common shareholders’ equity		(20.6)
Non-controlling interests		(0.6)
Total equity		(21.2)
Total liabilities and equity		(24.5)
HG Global BAM
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	747.4	677.6
Investments	693.4	629.7
Total liabilities	167.0	108.9
Accrued incentive compensation	18.2	16.6
Other liabilities	11.4	9.4
HG Global BAM \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		677.6
Total liabilities		108.9
HG Global BAM \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		0.0
Total liabilities		0.0
MediaAlpha
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	96.5	57.6
Total liabilities	59.8	28.3
Accrued incentive compensation	2.0	0.0
Other liabilities	2.4	2.6
MediaAlpha \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		57.6
Total liabilities		28.3
MediaAlpha \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		0.0
Total liabilities		0.0
Other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets	2,815.3	5,785.0
Investments	2,687.3	2,084.7
Goodwill		7.6
Intangible assets		1.0
Other intangible assets		0.0
Other assets		85.3
Assets held for sale	3.3	3,606.4
Total liabilities	71.6	2,667.0
Accrued incentive compensation	60.6	79.1
Other liabilities	11.0	18.6
Liabilities held for sale	$ 0.0	2,569.3
Other \| Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		5,809.5
Investments		2,084.7
Goodwill		13.4
Intangible assets		4.7
Other intangible assets		14.8
Other assets		85.5
Assets held for sale		3,606.4
Total liabilities		2,670.3
Accrued incentive compensation		79.1
Other liabilities		21.9
Liabilities held for sale		2,569.3
Other \| Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		(24.5)
Investments		0.0
Goodwill		(5.8)
Intangible assets		(3.7)
Other intangible assets		(14.8)
Other assets		(0.2)
Assets held for sale		0.0
Total liabilities		(3.3)
Accrued incentive compensation		0.0
Other liabilities		(3.3)
Liabilities held for sale		$ 0.0

Subsequent Event (Details) - Subsequent Event - USD ($) $ in Millions	Feb. 28, 2018	Feb. 05, 2018	Jan. 26, 2018	Jan. 24, 2018
DavidShield & PassportCard
Subsequent Event [Line Items]
Ownership interest (as a percent)				50.00%
Investment (up to)				$ 28
CrossHarbor
Subsequent Event [Line Items]
Investment (up to)			$ 50
Kudu Investment Management, LLC
Subsequent Event [Line Items]
Investment (up to)	$ 2	$ 125

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Fourth Quarter Events [Line Items]
Revenues (loss) (including realized gains and losses)		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Total expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Pre-tax (loss) income from continuing operations	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income tax benefit (expense)	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Net income (loss) from continuing operations	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Net (loss) income from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Non-controlling interest in consolidated subsidiaries	10.5	10.6	12.0	1.0	17.4	3.1	(21.5)	(6.2)	34.1	(7.2)	18.5
Equity in earnings of unconsolidated affiliates									0.0	0.0	25.1
Net income attributable to White Mountains’s common shareholders	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Basic earnings per share
Continuing operations	$ 4.85	$ 5.36	$ 2.36	$ (0.50)	$ (10.27)	$ 1.31	$ (8.47)	$ (6.85)	$ 11.56	$ (24.26)	$ 27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.34	75.27	8.55	134.50	104.37	22.98
Total consolidated operations	6.00	130.81	2.97	6.36	(8.52)	18.65	66.80	1.70	146.06	80.11	50.20
Diluted earnings per share
Continuing operations	4.85	5.36	2.36	(0.50)	(10.27)	1.31	(8.47)	(6.85)	11.56	(24.26)	27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.30	75.11	8.55	134.50	104.32	22.98
Total consolidated operations	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.61	$ 66.64	$ 1.70	$ 146.06	$ 80.06	$ 50.20

Selected Quarterly Financial Data (Unaudited) Selected Quarterly Financial Data Revised (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Pre-tax (loss) income from continuing operations	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income Tax (expense) benefit	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Net (loss) income from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Non-controlling interest in consolidated subsidiaries	10.5	10.6	12.0	1.0	17.4	3.1	(21.5)	(6.2)	34.1	(7.2)	18.5
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Continuing operations	$ 4.85	$ 5.36	$ 2.36	$ (0.50)	$ (10.27)	$ 1.31	$ (8.47)	$ (6.85)	$ 11.56	$ (24.26)	$ 27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.34	75.27	8.55	134.50	104.37	22.98
Earnings Per Share, Basic	6.00	130.81	2.97	6.36	(8.52)	18.65	66.80	1.70	146.06	80.11	50.20
Continuing operations	4.85	5.36	2.36	(0.50)	(10.27)	1.31	(8.47)	(6.85)	11.56	(24.26)	27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.30	75.11	8.55	134.50	104.32	22.98
Diluted earnings per share (in dollars) - discontinued operations:	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.61	$ 66.64	$ 1.70	$ 146.06	$ 80.06	$ 50.20
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues			$ 85.4	$ 93.5	$ 0.3		$ 49.2	$ 58.8		$ 166.3	$ 435.3
Expenses			85.8	92.2	69.1		74.3	88.5		302.8	303.3
Pre-tax (loss) income from continuing operations			(0.4)	1.3	(68.8)		(25.1)	(29.7)		(136.5)	132.0
Income Tax (expense) benefit			1.0	0.3	10.2		4.0	1.6		32.9	(12.7)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest			0.6	1.6	(58.6)		(21.1)	(28.1)		(103.6)	119.3
Net (loss) income from discontinued operations, net of tax			2.8	31.3	8.0		383.6	47.4		108.3	116.9
Non-controlling interest in consolidated subsidiaries			12.1	1.3	17.3		(21.4)	(6.3)		(7.3)	18.1
Net income attributable to White Mountains’s common shareholders (3)			$ 15.5	$ 34.2	$ (33.3)		$ 341.1	$ 13.0		412.5	297.6
Continuing operations			$ 2.78	$ 0.65	$ (9.04)		$ (8.34)	$ (6.22)
Discontinued operations			0.61	6.86	1.75		75.27	8.55
Earnings Per Share, Basic			3.39	7.51	(7.29)		66.93	2.33
Continuing operations			2.78	0.65	(9.04)		(8.34)	(6.22)
Discontinued operations			0.61	6.86	1.75		75.11	8.55
Diluted earnings per share (in dollars) - discontinued operations:			$ 3.39	$ 7.51	$ (7.29)		$ 66.77	$ 2.33
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues			$ (1.9)	$ (4.7)	$ (5.8)		$ (0.8)	$ (0.8)		(8.6)	4.7
Expenses			(0.1)	0.2	(0.1)		(0.2)	2.8		2.2	7.5
Pre-tax (loss) income from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Income Tax (expense) benefit				0.0	0.0		0.0	0.0		0.0	0.0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Net (loss) income from discontinued operations, net of tax			0.0	0.0	0.0		0.0	0.0		0.0	0.0
Non-controlling interest in consolidated subsidiaries			(0.1)	(0.3)	0.1		(0.1)	0.1		0.1	0.4
Net income attributable to White Mountains’s common shareholders (3)			$ (1.9)	$ (5.2)	$ (5.6)		$ (0.7)	$ (3.5)		$ (10.7)	$ (2.4)
Continuing operations			$ (0.42)	$ (1.15)	$ (1.23)		$ (0.13)	$ (0.63)
Discontinued operations			0.00	0.00	0.00		0.00	0.00
Earnings Per Share, Basic			(0.42)	(1.15)	(1.23)		(0.13)	(0.63)
Continuing operations			(0.42)	(1.15)	(1.23)		(0.13)	(0.63)
Discontinued operations			0.00	0.00	0.00		0.00	0.00
Diluted earnings per share (in dollars) - discontinued operations:			$ (0.42)	$ (1.15)	$ (1.23)		$ (0.13)	$ (0.63)

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of investments other than investments in related parties
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Cost	3,279.6
Carrying Value	3,380.7
Fair Value	3,380.7
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,117.2
Carrying Value	2,129.7
Fair Value	2,129.7
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	297.8
Carrying Value	296.5
Fair Value	296.5
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	867.6
Carrying Value	880.9
Fair Value	880.9
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	697.2
Carrying Value	694.7
Fair Value	694.7
Municipal obligations
Summary of investments other than investments in related parties
Cost	252.0
Carrying Value	254.9
Fair Value	254.9
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	2.6
Carrying Value	2.7
Fair Value	2.7
Short-term investments
Summary of investments other than investments in related parties
Cost	176.1
Carrying Value	176.1
Fair Value	176.1
Exchange Traded Funds
Summary of investments other than investments in related parties
Cost	512.5
Carrying Value	569.7
Fair Value	569.7
Common equity securities
Summary of investments other than investments in related parties
Cost	739.7
Carrying Value	866.1
Fair Value	866.1
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	12.8
Carrying Value	16.3
Fair Value	16.3
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	214.4
Carrying Value	280.1
Fair Value	280.1
Other long-term investments
Summary of investments other than investments in related parties
Cost	246.6
Carrying Value	208.8
Fair Value	208.8
Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	3,259.1	2,639.7
Fair value measured on a recurring basis \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	87.2	91.4
Level 3 \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	87.2	91.4	$ 103.6
Level 3 \| Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	87.2	91.4
Level 3 \| Fair value measured on a recurring basis \| Foreign Exchange Forward [Member]
Summary of investments other than investments in related parties
Fair value investments	(3.7)	(1.2)
Level 3 \| Fair value measured on a recurring basis \| Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	$ 87.2	$ 91.4

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Balance Sheet) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturity investments, at fair value		$ 2,081.1
Common equity securities, at fair value		0.0
Total assets	$ 3,659.2	6,520.2
Liabilities
Debt	23.8	12.7
Total liabilities	298.4	2,804.2
White Mountains’s common shareholders’ equity (3)	3,360.8	3,716.0
Total liabilities and equity	3,659.2	6,520.2
Fair value investments	3,208.3	2,547.3	$ 1,543.5
White Mountains Insurance Group Ltd.
Assets
Cash	14.9	3.1
Fixed maturity investments, at fair value	869.6	80.0
Common equity securities, at fair value			$ 641.8
Other long-term investments	(3.7)	(1.2)
Short-term investments, at amortized cost	57.2	12.5
Other assets	30.9	21.8
Investments in consolidated subsidiaries (3)	1,914.8	4,877.6
Total assets	3,525.5	4,993.8
Liabilities
Payable to subsidiary	11.8	1,387.2
Other liabilities	21.2	23.9
Total liabilities	33.0	1,411.1
White Mountains’s common shareholders’ equity (3)	3,492.5	3,582.7
Total liabilities and equity	3,525.5	4,993.8
WOBI
Assets
Investments in consolidated subsidiaries (3)		(20.6)
Barclays Bank PLC \| A \| Forward Contracts
Liabilities
Fair value investments	$ (3.7)	$ (1.2)

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Statement of Operations and Comprehensive Income) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized gains and losses)		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Pre-tax loss	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income Tax (expense) benefit	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Net income (loss) from continuing operations	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Net gain from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Net income attributable to White Mountains’s common shareholders	22.5	562.1	13.6	29.0	(38.9)	90.8	340.4	9.5	627.2	401.8	295.2
Other comprehensive income (loss), net of tax									0.3	(0.7)	(42.8)
Comprehensive income attributable to White Mountains’ common shareholders									630.5	547.1	187.4
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	627.2	401.8	295.2
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized gains and losses)									27.3	(1.0)	5.0
Expenses									99.7	68.2	59.9
Pre-tax loss									(72.4)	(69.2)	(54.9)
Income Tax (expense) benefit									(1.4)	(0.5)	0.0
Net income (loss) from continuing operations									(73.8)	(69.7)	(54.9)
Equity in earnings from consolidated and unconsolidated subsidiaries (2)									701.0	471.5	350.1
Net income attributable to White Mountains’s common shareholders									627.2	401.8	295.2
Other comprehensive income (loss), net of tax									3.3	145.3	(100.4)
Comprehensive income attributable to White Mountains’ common shareholders									630.5	547.1	194.8
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									$ 627.2	401.8	295.2
WOBI
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Equity in earnings from consolidated and unconsolidated subsidiaries (2)										$ (10.7)	$ (2.4)

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Cash Flows) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	$ 3,360.8				$ 3,716.0				$ 3,360.8	$ 3,716.0
Net income attributable to White Mountains’s common shareholders (3)	22.5	$ 562.1	$ 13.6	$ 29.0	(38.9)	$ 90.8	$ 340.4	$ 9.5	627.2	401.8	$ 295.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses									(133.3)	27.4	(260.5)
Net gain from discontinued operations, net of tax	4.3	$ 539.1	$ 2.8	31.3	8.0	$ 84.4	$ 383.6	47.4	20.5	108.3	116.9
Net change in other assets and liabilities, net									(20.8)	(146.1)	29.0
Net cash provided from (used for) operations									94.6	(149.3)	182.4
Cash flows from investing activities:
Net change in short-term investments									(1.7)	(27.2)	6.9
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									715.5	1,035.4	111.8
Cash Provided by (Used in) Investing Activities, Discontinued Operations									3.0	241.4	(100.5)
Net cash provided from investing activities									718.5	1,276.8	11.3
Payments for (Proceeds from) Other Investing Activities									14.7	(4.8)	(8.8)
Cash flows from financing activities:
Draw down of debt and revolving line of credit									376.0	352.5	171.5
Repayment of revolving line of credit									(365.0)	(404.6)	(76.1)
Proceeds from issuances of common shares									0.0	3.7	0.0
Repurchases and retirement of common shares									(714.6)	(881.3)	(268.6)
Dividends paid on common shares									(4.6)	(5.4)	(6.0)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(637.1)	(854.6)	(120.6)
Net cash used for financing activities — discontinued operations (Note 19)									(61.9)	(93.8)	(100.7)
Net cash used for financing activities									(699.0)	(948.4)	(221.3)
Net increase in cash during the year									16.0	7.9	(74.8)
Cash balance at beginning of year				80.2				72.0	80.2	72.0	145.7
Cash balance at end of year	97.1				80.2				97.1	80.2	72.0	$ 145.7
Interest paid									(1.4)	(2.1)	(0.1)
Amortization of restricted share and option awards									14.8	18.5	14.9
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(62.4)	(172.9)	(66.0)
Cash Provided by (Used in) Operating Activities, Discontinued Operations									157.0	23.6	248.4
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	3,492.5				3,582.7				3,492.5	3,582.7
Net income attributable to White Mountains’s common shareholders (3)									627.2	401.8	295.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses									(18.5)	1.1	0.0
Undistributed earnings from subsidiaries (3)									(701.0)	(471.5)	(350.1)
Other non-cash reconciling items primarily amortization of restricted share and option awards (4)									31.1	17.9	(0.4)
Net cash transferred from subsidiary									1,256.7	0.0	0.0
Net change in other assets and liabilities, net									(4.9)	(5.6)	28.1
Net cash provided from (used for) operations									1,190.6	(56.3)	(27.2)
Cash flows from investing activities:
Net change in short-term investments									(24.7)	10.9	7.6
Purchases of investment securities									(474.7)	0.0	0.0
Sales and maturities of investment securities									367.1	0.0	0.0
Issuance of debt from subsidiaries									382.0	992.0	271.0
Repayment of debt to subsidiaries									0.0	(5.0)	(35.0)
Distributions from subsidiaries									0.0	0.0	15.0
Net cash provided from investing activities									(450.3)	997.9	258.6
Cash flows from financing activities:
Draw down of debt and revolving line of credit									350.0	350.0	125.0
Repayment of revolving line of credit									(350.0)	(400.0)	(75.0)
Proceeds from issuances of common shares									0.0	3.7	0.0
Repurchases and retirement of common shares									(714.6)	(881.3)	(268.6)
Dividends paid on common shares									(4.6)	(5.4)	(6.0)
Payments of restricted shares withholding taxes									(9.3)	(5.8)	(6.7)
Net cash used for financing activities									(728.5)	(939.0)	(231.3)
Net increase in cash during the year									11.8	2.8	0.1
Cash balance at beginning of year				$ 3.1				$ 0.3	3.1	0.3	0.2
Cash balance at end of year	14.9				3.1				14.9	3.1	0.3	0.2
Interest paid									(0.6)	(1.2)	0.0
Equity in earnings from consolidated and unconsolidated subsidiaries (2)									701.0	471.5	350.1
Payable to subsidiary									11.6	0.2	2.4
Payments to Acquire Interest in Subsidiaries and Affiliates										700.0	0.0	$ 0.0
Transfer from Investments									1,238.9	80.0
Proceeds from Dividends											15.0
Other liabilities	21.2				23.9				21.2	23.9
Investments in consolidated subsidiaries (3)	1,914.8				4,877.6				1,914.8	4,877.6
Short-term investments, at amortized cost (which approximates fair value)	57.2				12.5				57.2	12.5
WOBI
Cash flows from financing activities:
Equity in earnings from consolidated and unconsolidated subsidiaries (2)										(10.7)	$ (2.4)
Investments in consolidated subsidiaries (3)					$ (20.6)					$ (20.6)
Lone Tree Holdings Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	2,810.4								2,810.4
Cash flows from financing activities:
Other liabilities	14.1								14.1
Investments in consolidated subsidiaries (3)	964.4								964.4
Short-term investments, at amortized cost (which approximates fair value)	13.0								13.0
Lone Tree Holdings Ltd. \| White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Payable to subsidiary									1,256.7
Proceeds from divestiture of business									1,037.6
Non-cash reconciling item [Member] \| White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Issuance of debt from subsidiaries									94.2
intercompany balances [Member] \| Lone Tree Holdings Ltd.
Cash flows from financing activities:
Other liabilities	$ 1,863.1								$ 1,863.1

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 56.0	$ 32.1	$ 10.9
HG Global BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	14.8	10.6	6.9
Future policy benefits, losses, claims and loss expenses	0.0	0.0	0.0
Unearned premiums	136.8	82.9	50.2
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	9.4	5.9	3.3
Net investment income	12.3	9.0	6.1
Benefits, claims, losses, and settlement expenses	0.0	0.0	0.0
Amortization of deferred policy acquisition costs	4.0	3.4	2.9
Other operating expenses	0.4	0.4	0.4
Premiums written	63.2	38.6	25.9
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0.0	0.0	0.0
Future policy benefits, losses, claims and loss expenses	0.0	0.0	0.0
Unearned premiums	0.0	0.0	0.0
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	1.0	7.5	8.7
Net investment income	0.0	0.2	0.2
Benefits, claims, losses, and settlement expenses	1.1	8.0	8.2
Amortization of deferred policy acquisition costs	0.1	2.2	3.4
Other operating expenses	0.0	0.1	0.0
Premiums written	0.9	6.5	10.1
Non-insurance \| Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 43.7	$ 22.9	$ 4.6

SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums earned
Net earned premiums	$ 10.4	$ 13.4	$ 12.0
HG Global/BAM
Premiums earned
Gross amount	9.4	5.9	3.3
Ceded to other companies	0.0	0.0	0.0
Assumed from other companies	0.0	0.0	0.0
Net earned premiums	$ 9.4	$ 5.9	$ 3.3
Percentage of amount assumed to net	0.00%	0.00%	0.00%
Other Segments
Premiums earned
Gross amount	$ 1.0	$ 15.2	$ 20.7
Ceded to other companies	0.0	(7.7)	(12.0)
Assumed from other companies	0.0	0.0	0.0
Net earned premiums	$ 1.0	$ 7.5	$ 8.7
Percentage of amount assumed to net	0.00%	0.00%	0.00%

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - Other Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	0.0	0.0	0.0
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	0.0	0.0	0.0
Earned Premiums	1.0	7.5	8.7
Net investment income	0.0	0.2	0.2
Current Accident Year	1.1	8.6	10.4
Prior Accident Year	0.0	(0.6)	(2.2)
Amortization of deferred policy acquisition	0.1	2.2	3.4
Paid Claims and Claims Adjustment Expenses	0.0	8.8	10.4
Premiums written	$ 0.9	$ 6.5	$ 10.1