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Document and Entity Information - $ / shares	3 Months Ended
Document and Entity Information - $ / shares	Mar. 31, 2017	Apr. 28, 2017	Dec. 31, 2016
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-Q
Document Period End Date	Mar. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		4,572,792
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q1
Common shares, par value per share (in dollars per share)	$ 1.00		$ 1.00

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets
Fixed maturity investments, at fair value	$ 4,175.4	$ 4,250.2
Short-term investments, at amortized cost (which approximates fair value)	230.9	287.0
Common equity securities, at fair value	601.0	474.3
Other long-term investments	331.7	323.3
Total investments	5,339.0	5,334.8
Cash	125.0	149.8
Reinsurance recoverable on unpaid losses	174.7	172.9
Reinsurance recoverable on paid losses	3.4	6.6
Insurance premiums receivable	227.5	229.9
Deferred acquisition costs	110.4	106.9
Deferred tax asset	125.5	126.7
Ceded unearned insurance premiums	52.4	44.2
Accrued investment income	27.2	26.1
Accounts receivable on unsettled investment sales	24.5	6.2
Goodwill and other intangible assets	53.0	55.9
Other assets	254.3	274.6
Assets held for sale	0.0	10.1
Total assets	6,516.9	6,544.7
Liabilities
Loss and loss adjustment expense reserves	1,368.8	1,365.6
Unearned insurance premiums	678.1	658.0
Debt	284.7	285.9
Accrued incentive compensation	83.2	140.0
Funds held under insurance contracts	148.7	153.0
Accounts payable on unsettled investment purchases	17.3	0.0
Other liabilities	178.5	199.9
Total liabilities held for sale	0.0	5.1
Total liabilities	2,759.3	2,807.5
White Mountains’s common shareholders’ equity
White Mountains’ common shares	4.6	4.6
Paid-in surplus	802.5	806.1
Retained earnings	2,821.8	2,797.2
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(0.6)	(1.4)
Pension liability	(3.1)	(3.2)
Total White Mountains’s common shareholders’ equity	3,625.2	3,603.3
Non-controlling interests
Non-controlling interests	132.4	133.9
Total equity	3,757.6	3,737.2
Total liabilities and equity	$ 6,516.9	$ 6,544.7

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Cash	$ 0.0	$ 0.0
Common shares, par value per share (in dollars per share)	$ 1.00	$ 1.00
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	4,572,792	4,563,814
Common shares, outstanding shares	4,572,792	4,563,814

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues:
Earned insurance premiums	$ 264.8	$ 282.1
Net investment income	26.1	17.9
Net realized and unrealized investment gains	51.3	29.5
Other revenue	43.7	39.8
Total revenues	385.9	369.3
Expenses:
Loss and loss adjustment expenses	151.7	161.1
Insurance Acquisition Expenses	46.6	52.7
Other Underwriting Expense	51.8	55.4
General and administrative expenses	94.2	87.1
Interest expense	3.7	4.5
Total expenses	348.0	360.8
Pre-tax income from continuing operations	37.9	8.5
Income tax (expense) benefit	(3.9)	9.7
Net income from continuing operations	34.0	18.2
Net (loss) gain on sale of discontinued operations, net of tax	(1.0)	0.0
Net income from discontinued operations, net of tax	0.0	1.1
Net income	33.0	19.3
Net loss (income) attributable to non-controlling interests	1.3	(6.3)
Net income attributable to White Mountains’s common shareholders	34.3	13.0
Other comprehensive income, net of tax:
Change in foreign currency translation, pension liability and other, net of tax	0.9	0.1
Comprehensive income attributable to White Mountains’s common shareholders	$ 35.2	$ 50.3
Basic income (loss) per share
Continuing operations	$ 7.72	$ 2.14
Discontinued operations	(0.22)	0.20
Total consolidated operations	7.50	2.34
Diluted income (loss) per share
Continuing operations	7.72	2.14
Discontinued operations	(0.22)	0.20
Total consolidated operations	7.50	2.34
Dividends declared per White Mountains’s common share	$ 1.00	$ 1.00
Sale of Tranzact [Member]
Expenses:
Net (loss) gain on sale of discontinued operations, net of tax	$ (1.0)	$ 0.0
Sirius Group
Revenues:
Net realized and unrealized investment gains		4.1
Other comprehensive income, net of tax:
Change in foreign currency translation, pension liability and other, net of tax	$ 0.0	$ 37.2

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	Foreign Currency Gain (Loss) AOCI, after tax	Foreign Currency Gain (Loss) Total	Foreign Currency Gain (Loss) Non-controlling interest	Foreign Currency Gain (Loss) Total Equity	Star & Shield LLC [Member] Common shares and paid-in surplus	Star & Shield LLC [Member] Retained earnings	Star & Shield LLC [Member] AOCI, after tax	Star & Shield LLC [Member] Total	Star & Shield LLC [Member] Non-controlling interest	Star & Shield LLC [Member] Total Equity
Balances at Dec. 31, 2015		$ 978.2	$ 3,084.9	$ (149.9)	$ 3,913.2	$ 454.8	$ 4,368.0
Increase (Decrease) in Shareholders' Equity
Net income	$ 19.3	0.0	13.0	0.0	13.0	6.3	19.3
Net change in foreign currency translation	0.1							$ 0.0	$ 0.0	$ 37.3	$ 37.3	$ 0.0	$ 37.3
Total comprehensive income		0.0	13.0	37.3	50.3	6.3	56.6
Dividends declared on common shares		0.0	(5.4)	0.0	(5.4)	0.0	(5.4)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(6.2)	(6.2)
Repurchases and retirements of common shares		(39.8)	(132.9)	0.0	(172.7)	0.0	(172.7)
Acquisition from non-controlling interests - OneBeacon		(2.7)	0.0	0.0	(2.7)	(8.8)	(11.5)
Net contributions from non-controlling interests		0.0	0.0	0.0	0.0	0.3	0.3
Noncontrolling Interest, Period Increase (Decrease)		0.0	0.0	0.0	0.0	4.8	4.8
Amortization of restricted share awards		3.2	0.0	0.0	3.2	0.2	3.4
Balances at Mar. 31, 2016		938.9	2,959.6	(112.6)	3,785.9	451.4	4,237.3
Balances at Dec. 31, 2016	3,737.2	810.7	2,797.2	(4.6)	3,603.3	133.9	3,737.2
Increase (Decrease) in Shareholders' Equity
Net income	33.0	0.0	34.3	0.0	34.3	(1.3)	33.0
Net change in foreign currency translation	0.9							$ 0.0	$ 0.0	$ 0.9	$ 0.9	$ 0.0	$ 0.9
Total comprehensive income		0.0	34.3	0.9	35.2	(1.3)	33.9
Dividends declared on common shares		0.0	(4.6)	0.0	(4.6)	0.0	(4.6)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(6.6)	(6.6)
Repurchases and retirements of common shares		(1.4)	(5.1)	0.0	(6.5)	(1.1)	(7.6)
Deconsolidation of non-controlling interests associated with the sale of Star & Shield														$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)
Net contributions from non-controlling interests		(4.8)	0.0	0.0	(4.8)	4.8	0.0
Noncontrolling Interest, Period Increase (Decrease)		0.0	0.0	0.0	0.0	6.9	6.9
Amortization of restricted share awards		2.6	0.0	0.0	2.6	0.2	2.8
Balances at Mar. 31, 2017	$ 3,757.6	$ 807.1	$ 2,821.8	$ (3.7)	$ 3,625.2	$ 132.4	$ 3,757.6

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Cash flows from operations:
Net income	$ 33.0	$ 19.3
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses	(51.3)	(29.5)
Deferred income tax (benefit) expense	(1.1)	5.0
Net (loss) income from discontinued operations	0.0	(1.1)
Net loss on sale of discontinued operations	1.0	0.0
Amortization and depreciation	14.6	11.4
Other operating items:
Net change in loss and loss adjustment expense reserves	2.9	(46.2)
Net change in reinsurance recoverable on paid and unpaid losses	1.4	26.5
Net change in unearned insurance premiums	20.7	13.9
Net change in deferred acquisition costs	(3.5)	(2.2)
Net change in ceded unearned premiums	(8.2)	(6.8)
Net change in funds held under insurance treaties	(4.3)	(2.8)
Net change in insurance premiums receivable	2.7	(11.0)
Net change in restricted cash	0.0	(2.6)
Net change in other assets and liabilities, net	(64.0)	(19.8)
Net cash used for operations - continuing operations	(56.1)	(45.9)
Net cash used for operations	(56.1)	(86.6)
Cash flows from investing activities:
Net change in short-term investments	55.6	(50.4)
Sales of fixed maturity and convertible investments	868.5	202.3
Maturities, calls and paydowns of fixed maturity and convertible investments	175.8	155.9
Sales of common equity securities	17.7	767.5
Distributions and redemptions of other long-term investments	16.5	3.0
Net settlement of investment cash flows and contributions with discontinued operations	0.0	(559.8)
Purchases of other long-term investments	(22.4)	(10.9)
Purchases of common equity securities	(113.4)	(86.1)
Purchases of fixed maturity and convertible investments	(957.6)	(315.2)
Purchases of unconsolidated affiliates and consolidated subsidiaries, net of cash acquired	0.0	(8.1)
Net change in unsettled investment purchases and sales	(1.0)	44.3
Net acquisitions of property and equipment	(0.4)	(1.3)
Net cash provided from investing activities - continuing operations	39.3	141.2
Net cash provided from investing activities	39.3	174.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit	0.0	102.5
Repayment of debt and revolving line of credit	(1.2)	0.0
Cash dividends paid to the Company’s common shareholders	(4.6)	(5.4)
Common shares repurchased	0.0	(166.8)
OneBeacon Ltd. common shares repurchased and retired	0.0	(10.6)
Distribution to non-controlling interest shareholders	(5.1)	(4.7)
Contributions to discontinued operations	0.0	(3.0)
Payments of contingent consideration related to purchases of consolidated subsidiaries	0.0	(7.8)
Capital contributions from BAM members	9.6	6.7
Other Financing Activities, Net	(7.6)	(7.2)
Net cash used for financing activities - continuing operations	(8.9)	(96.3)
Net cash used for financing activities	(8.9)	(104.6)
Effect of Exchange Rate on Cash and Cash Equivalents	0.0	0.0
Net change in cash during the period - continuing operations	(25.7)	(1.0)
Cash balances at beginning of period	149.8	167.2	$ 167.2
Add: cash held for sale at the beginning of period	0.9	1.2	1.2
Less: cash held for sale at the end of period	0.0	2.1	0.9
Cash balances at end of period	125.0	165.3	149.8
Supplemental cash flows information:
Interest paid	(0.2)	(0.2)
Net income tax refund from national governments	0.0	13.5
Discontinued Operations [Member]
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses	0.0	8.8
Net (loss) income from discontinued operations	0.0	(1.1)
Other operating items:
Net cash used for operations - discontinued operations	0.0	(40.7)
Cash flows from investing activities:
Net cash provided from investing activities - discontinued operations	0.0	33.6
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations	0.0	(8.3)
Cash balances at beginning of period	0.0	150.2	150.2
Cash balances at end of period	0.0	139.9	$ 0.0
Supplemental cash flows information:
Net income tax refund from national governments	$ 0.0	$ 36.4

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash and Cash Equivalents, at Carrying Value	$ 125.0	$ 165.3
Restricted cash balances	0.0	8.4
Effect of Exchange Rate on Cash and Cash Equivalents, Discontinued Operations	0.0	4.2
Discontinued Operations [Member]
Cash and Cash Equivalents, at Carrying Value	$ 0.0	$ 139.9

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation These interim 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. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, HG Global/BAM and Other Operations. As discussed further in the Company’s consolidated financial statements in Note 2 — “Significant Transactions”, on April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. Also, on July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. For the three months ended March 31, 2016, Sirius Group and Tranzact have been presented as discontinued operations in the statement of operations and comprehensive income. See Note 17 — “Held for Sale and Discontinued Operations”. The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of March 31, 2017 and December 31, 2016, White Mountains owned 75.7% and 76.1% of OneBeacon Ltd.’s outstanding common shares. The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is the first and only mutual bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM's insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). As of March 31, 2017 and December 31, 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, GAAP requires White Mountains to consolidate BAM’s results in its financial statements. BAM’s results are attributed to non-controlling interests. White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”) and certain consolidated and unconsolidated private capital investments. The consolidated private capital investments consist of QL Holdings LLC (“MediaAlpha”), Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). White Mountains’s Other Operations segment also includes its variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”) and Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. White Mountains was required to consolidate SSIE in its GAAP financial statements until White Mountains completed the sale of Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services, LLC on March 7, 2017. White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016. See Note 17 — “Held for Sale and Discontinued Operations”. All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2016 Annual Report on Form 10-K. 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. Refer to the Company’s 2016 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. 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 simplifies certain aspects of the accounting for share-based compensation. 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. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption White Mountains reported $6.5 million and $5.8 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2017 and 2016. Such payments were classified as operating cash flows prior to adoption. In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity will recognize excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position. Short-Duration Contracts Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnotes disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, 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 March 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. 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 variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718). The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of March 31, 2017, the unamortized debt issuance costs included in debt is $1.8 million. Recently Issued Accounting Pronouncements Cash Flow Statement In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230). Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented. The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. The types of assets included in the scope of the new guidance includes premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance. 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 new guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings accordingly, does not expect the adoption of ASU 2016-01 to have a significant impact on its financial statements. Revenue Recognition In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is to be recognized at an amount that reflects the consideration to which an entity expects to be entitled in exchange for goods or services transferred to customers. The new guidance sets forth the steps to be followed to recognize revenue: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. Subsequently, the FASB issued additional ASUs clarifying the guidance in and providing implementation guidance for ASU 2014-09. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, which delays the effective date of ASU 2014-09 and all related ASUs to annual and interim reporting periods beginning after December 15, 2017. Most of White Mountains’s revenue from customers relates to insurance contracts, which are excluded from the scope of ASU 2014-09, as are investment income and investments gains and losses. However, the new guidance is applicable to some of White Mountains’s revenue streams, including certain fee arrangements as well as commissions and other non-insurance revenues. White Mountains is evaluating the new guidance, but does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s non-insurance revenues from customers.

Significant Transactions

3 Months Ended

Mar. 31, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Significant Transactions

Sale of Star and 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 17 — “Held for Sale and Discontinued Operations”.

Buzzmove

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

Sale of Tranzact

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

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



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 book value from sale of Tranzact	$	82.1

In the first quarter of 2017, White Mountains recorded a $1.0 million reduction to the gain from sale of Tranzact in discontinued operations as a result of 2016 tax payments.

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 17 — “Held for Sale and Discontinued Operations”.

Sale of Sirius Group

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

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 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 within discontinued operations. See Note 17 — “Held for Sale and Discontinued Operations”.

Symetra

On February 1, 2016, Symetra closed its merger agreement with Sumitomo Life Insurance Company (“Sumitomo Life”) and White Mountains received proceeds of $658.0 million, or $32.00 per common share. White Mountains also received a special dividend of $0.50 per share as part of the transaction that was paid in the third quarter of 2015. See Note 14 — “Investments in Unconsolidated Affiliates”

OneBeacon Crop Business

On July 31, 2015, OneBeacon exited its multiple peril crop insurance (“MPCI”) and its related crop-hail business (collectively, “Crop Business”) as its exclusive managing general agency, Climate Crop Insurance Agency (“CCIA”), exited the business through a sale of the agency to an affiliate of AmTrust. As a result of the transaction, OneBeacon and CCIA agreed to an early termination of the existing five-year agreement. In connection with the termination of the agreement, OneBeacon received a payment of $3.0 million. Also related to the transaction, OneBeacon withdrew its 2016 Plan of Operations, which previously authorized it to write MPCI for the 2016 Reinsurance Year, and affiliates of AmTrust agreed to reinsure the Company’s remaining net Crop Business exposure for the 2015 Reinsurance Year under a related 100% quota share reinsurance agreement which, coupled with other transfer and assignment agreements as well as communications with policyholders and agents, had the effect of assumption reinsurance. As a result of this transaction, the Company has no material net exposure related to the Crop Business.

MediaAlpha

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

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

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

Wobi

On February 19, 2014, White Mountains acquired 54% of the outstanding common shares of Wobi for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition). During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $9.0 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. As of the acquisition date, White Mountains recognized total assets acquired related to Wobi of $13.4 million, including $5.5 million of goodwill and $2.9 million of other intangible assets; and total liabilities assumed of $0.7 million at their estimated acquisition date fair values.

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

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

During 2016, White Mountains purchased NIS 35.9 million (approximately $9.6 million based upon the foreign exchange spot rates at the dates of acquisitions) of convertible preferred shares of Wobi. As of both March 31, 2017 and December 31, 2016, White Mountains’s ownership share was 95.0% .

Loss and Loss Adjustment Expense Reserves

3 Months Ended

Mar. 31, 2017

Insurance Loss Reserves [Abstract]

Loss and Loss Adjustment Expense Reserves

The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three months ended March 31, 2017 and 2016:



	Three Months Ended
	March 31,
Millions	2017			2016
Gross beginning balance	$	1,365.6		$	1,389.8
Less beginning reinsurance recoverable on unpaid losses	(172.9		)	(186.0		)
Net loss and LAE reserves	1,192.7			1,203.8

Add: SSIE reserves held for sale at beginning of the period⁽¹⁾	4.7			5.5

Loss and LAE incurred relating to:
Current year losses	151.7			161.2
Prior year losses	—			(.1		)
Total incurred losses and LAE	151.7			161.1

Loss and LAE paid relating to:
Current year losses	(21.5		)	(23.1		)
Prior year losses	(129.1		)	(148.6		)
Total loss and LAE payments	(150.6		)	(171.7		)

Less: Deconsolidation of SSIE⁽¹⁾	4.4			—

Less: SSIE reserves held for sale at end of the period ⁽¹⁾	—			5.3

Net ending balance	1,194.1			1,193.4
Plus ending reinsurance recoverable on unpaid losses	174.7			150.4
Gross ending balance	$	1,368.8		$	1,343.8

⁽¹⁾ Resulting from the sale of Star & Shield in the first quarter of 2017, SSIE is no longer consolidated. See Note 17 — “Held for Sale and Discontinued Operations”.

Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2017

For the three months ended March 31, 2017, White Mountains did not experience any net loss reserve development on prior accident year reserves, as unfavorable reserve development at OneBeacon, primarily in Healthcare due to an adverse settlement on a single claim was offset by favorable reserve development driven by Technology, Accident & Health and

Entertainment resulting from favorable loss experience.

Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2016

For the three months ended March 31, 2016, White Mountains experienced net favorable loss reserve development of $0.1 million. For the three months ended March 31, 2016, OneBeacon did not experience any net loss reserve development on prior accident year reserves, as favorable development from several businesses, including Technology and Accident, was offset by unfavorable development primarily in Healthcare. For the three months ended March 31, 2016, SSIE had net favorable loss reserve development of $0.1 million.

Third Party Reinsurance	3 Months Ended
Third Party Reinsurance	Mar. 31, 2017
Reinsurance Disclosures [Abstract]
Third Party Reinsurance	Third Party Reinsurance In the normal course of business, White Mountains’s insurance subsidiaries may seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. OneBeacon At March 31, 2017, OneBeacon had $3.4 million and $174.7 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2016, OneBeacon had $6.6 million and $172.9 million of reinsurance recoverables on paid and unpaid losses. Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant. Effective May 1, 2017, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2018. The program provides coverage for OneBeacon's property business as well as certain acts of terrorism. Under the program, the first $20.0 million of losses resulting from any single catastrophe are retained, with 100.0% of the next $110.0 million of losses resulting from the catastrophe being reinsured. Any part of a catastrophe loss in excess of $130.0 million would be retained in full. In the event of a catastrophe, OneBeacon's property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium.

- Definition

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

+ References + Details

Investment Securities

3 Months Ended

Mar. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Investment Securities

Investments Securities

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

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

Realized investment gains (losses) resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of March 31, 2017 and December 31, 2016.

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

Net Investment Income

White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments and dividend income from its common equity securities and other long- term investments. Pre-tax net investment income for the three months ended March 31, 2017 and 2016 consisted of the following:



	Three Months Ended
	March 31,
Millions	2017			2016
Investment income:
Fixed maturity investments	$	24.5		$	14.5
Short-term investments	.3			.2
Common equity securities	2.1			1.2
Other long-term investments	(.1		)	2.8
Total investment income	26.8			18.7
Third-party investment expenses	(.7		)	(.8		)
Net investment income, pre-tax	$	26.1		$	17.9

Net Realized and Unrealized Investment Gains (Losses)

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



	Three Months Ended
	March 31,
Millions	2017			2016
Net realized investment gains, pre-tax	$	1.1		$	256.8
Net unrealized investment gains (losses), pre-tax	50.2			(227.3		)
Net realized and unrealized investment gains, pre-tax	51.3			29.5
Income tax expense attributable to net realized and unrealized investment gains	(7.2		)	(8.5		)
Net realized and unrealized investment gains, after tax	$	44.1		$	21.0

Net realized investment gains (losses)

Net realized investment gains (losses) for the three months ended March 31, 2017 and 2016 consisted of the following:



	Three Months Ended								Three Months Ended
	March 31, 2017								March 31, 2016
Millions	Net realized (losses) gains			Net foreign currency gains		Total net realized (losses) gains reflected in earnings			Net realized (losses) gains			Net foreign currency gains (losses)		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(2.2	)	$	.1	$	(2.1	)	$	(1.1	)	$	—	$	(1.1	)
Common equity securities	1.2			.1		1.3			257.6			—		257.6
Other long-term investments	1.9			—		1.9			.3			—		.3
Net realized investment gains, pre-tax	.9			.2		1.1			256.8			—		256.8
Income tax expense attributable to net realized investment gains	(.7		)	—		(.7		)	(42.9		)	—		(42.9		)
Net realized investment gains, after tax	$	.2		$	.2	$	.4		$	213.9		$	—	$	213.9

Net unrealized investment gains (losses)

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



	Three Months Ended									Three Months Ended
	March 31, 2017									March 31, 2016
Millions	Net unrealized gains			Net foreign currency gains (losses)			Total net unrealized gains reflected in earnings			Net unrealized gains (losses)			Net foreign currency gains		Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	17.5		$	1.7		$	19.2		$	21.7		$	—	$	21.7
Common equity securities	29.1			.5			29.6			(249.8		)	2.4		(247.4		)
Other long-term investments	4.2			.2			4.4			(2.0		)	.4		(1.6		)
Forward contracts	—			(3.0		)	(3.0		)	—			—		—
Net unrealized investment gains (losses), pre-tax	50.8			(.6		)	50.2			(230.1		)	2.8		(227.3		)
Income tax (expense) benefit attributable to net unrealized investment gains (losses)	(6.5		)	—			(6.5		)	34.4			—		34.4
Net unrealized investment gains (losses), after tax	$	44.3		$	(.6	)	$	43.7		$	(195.7	)	$	2.8	$	(192.9	)

The following table summarizes the amount of total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the three months ended March 31, 2017 and 2016:



	Three Months Ended
	March 31,
Millions	2017			2016
Fixed maturity investments	$	.2		$	.5
Other long-term investments	(1.9		)	1.1
Total unrealized investment (losses) gains, pre-tax - Level 3 investments	$	(1.7	)	$	1.6

Investment Holdings

The cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of White Mountains’s fixed maturity investments as of March 31, 2017 and December 31, 2016 were as follows:



	March 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	$	110.7	$	—	$	(.4	)	$	—	$	110.3
Debt securities issued by corporations	1,680.2		10.5		(7.6		)	3.7		1,686.8
Municipal obligations	328.9		2.8		(1.4		)	—		330.3
Mortgage and asset-backed securities	2,020.0		5.1		(8.7		)	—		2,016.4
Foreign government, agency and provincial obligations	17.3		.4		—			.2		17.9
Preferred stocks	8.3		5.4		—			—		13.7
Total fixed maturity investments	$	4,165.4	$	24.2	$	(18.1	)	$	3.9	$	4,175.4



	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	$	281.7	$	.1	$	(3.5	)	$	—	$	278.3
Debt securities issued by corporations	1,512.6		8.4		(13.7		)	2.1		1,509.4
Municipal obligations	308.8		1.9		(1.7		)	—		309.0
Mortgage and asset-backed securities	2,141.7		2.6		(11.4		)	—		2,132.9
Foreign government, agency and provincial obligations	12.9		.3		—			—		13.2
Preferred stocks	8.3		5.7		—			—		14.0
Total fixed maturity investments	$	4,266.0	$	19.0	$	(30.3	)	$	2.1	$	4,256.8
Less: fixed maturity investments reclassified to assets held for sale related to SSIE										(6.6		)
Total fixed maturity investments										$	4,250.2

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



	March 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains (losses)			Carrying value
Common equity securities	$	537.9	$	65.0	$	(2.4	)	$	.5		$	601.0
Other long-term investments	$	321.9	$	39.7	$	(23.3	)	$	(6.6	)	$	331.7



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

Other Long-term Investments

Other long-term investments consist of the following as of March 31, 2017 and December 31, 2016:



	Carrying Value at
Millions	March 31, 2017			December 31, 2016
Hedge funds and private equity funds, at fair value	$	147.2		$	131.0
Private equity securities and limited liability companies, at fair value ⁽¹⁾⁽²⁾	72.4			72.0
OneBeacon Surplus Notes, at fair value ⁽¹⁾	69.6			71.9
Private convertible preferred securities, at fair value ⁽¹⁾	30.7			30.6
Tax advantaged federal affordable housing development fund ⁽³⁾	11.7			12.3
Partnership investments accounted for under the equity method	3.0			3.5
Forward Contracts	(4.2		)	(1.2		)
Other	1.3			3.2
Total other-long term investments	$	331.7		$	323.3

⁽¹⁾See Fair Value Measurements by Level table.

⁽²⁾White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OTH”) and has provided a $10.0 million surplus note facility under which OTH’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may, under certain circumstances, draw funds. At March 31, 2017, no funds had been drawn on the surplus note facility.

⁽³⁾Fund accounted for using the proportional amortization method.

Hedge Funds and Private Equity Funds

White Mountains holds investments in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments are generally estimated using the net asset value (“NAV”) of the funds. As of March 31, 2017, White Mountains held investments in 5 hedge funds and 20 private equity funds. The largest investment in a single fund was $54.2 million as of March 31, 2017 and $36.5 million as of December 31, 2016. The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of March 31, 2017 and December 31, 2016:



	March 31, 2017				December 31, 2016
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financials	$	54.2	$	—	$	36.5	$	—
Long/short equity REIT	19.7		—		19.9		—
Other	2.2		—		3.4		—
Total hedge funds	76.1		—		59.8		—

Private equity funds
Aerospace/Defense/Government	24.4		23.6		19.4		25.9
Manufacturing/Industrial	18.8		22.2		15.9		22.4
Multi-sector	11.0		2.0		11.4		2.0
Direct lending/Mezzanine debt	5.5		32.0		1.8		35.7
Healthcare	3.4		.4		3.5		.4
Energy infrastructure & services	2.8		3.0		14.1		3.2
Private equity secondaries	2.5		2.1		3.0		2.1
Financial Services	1.5		4.5		1.0		5.0
Insurance	.9		41.3		.8		41.3
Real estate	.3		.1		.3		.1
Total private equity funds	71.1		131.2		71.2		138.1
Total hedge funds and private equity funds included in other long-term investments	$	147.2	$	131.2	$	131.0	$	138.1

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 March 31, 2017, one hedge fund with a fair value of $38.3 million was subject to a lock-up period that expires on September 1, 2018.

The following summarizes the March 31, 2017 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:



	Notice Period
Millions Redemption frequency	30-59 days notice		60-89 days notice		90-119 days notice		Total
Monthly	$	—	$	—	$	—	$	—
Quarterly	15.9		—		—		15.9
Semi-annual	38.3		19.7		—		58.0
Annual	—		—		2.2		2.2
Total	$	54.2	$	19.7	$	2.2	$	76.1

Certain of White Mountains’s investments in hedge funds are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund’s underlying investments are liquidated. As of March 31, 2017, the hedge funds in liquidation had no value. The actual amount of any final distribution remittances remain subject to market fluctuations. The date at which such remittances, if any, will be received is not determinable as of March 31, 2017.

White Mountains has also submitted redemption requests for certain of its investments in active hedge funds. As of March 31, 2017, redemptions of $2.2 million are outstanding that would be subject to market fluctuations. The date at which such redemptions will be received is not determinable as of March 31, 2017. Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations.

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

As of March 31, 2017, investments in private equity funds were subject to lock-up periods as follows:



Millions	1-3 years	3 – 5 years	5 – 10 years	>10 years	Total
Private Equity Funds — expected lock-up period remaining	$18.4	$23.3	$22.2	$7.2	$71.1

OneBeacon Surplus Notes

In the fourth quarter of 2014, in conjunction with OneBeacon's sale of its runoff business to an affiliate of Armour Group Holdings Limited (the “OneBeacon Runoff Transaction”), OneBeacon provided financing in the form of the OneBeacon Surplus Notes with a par value of $101.0 million, which had a fair value of $69.6 million and $71.9 million as of March 31, 2017 and December 31, 2016. Subsequent to closing, the OneBeacon Surplus Notes are included in OneBeacon’s investment portfolio, classified within other long-term investments.

The internal valuation model used to estimate the fair value of the OneBeacon Surplus Notes is based on discounted expected cash flows using information as of the measurement date. The estimated fair value of the surplus notes is sensitive to changes in public debt credit spreads, as well as changes in estimates with respect to other variables including a discount to reflect the private nature of the notes (and the related lack of liquidity), the credit quality of the notes, based on the financial performance of the Issuer relative to expectations, and the timing, amount, and likelihood of interest and principal payments on the notes, which are subject to regulatory approval and therefore may vary from the contractual terms. For the purposes of estimating fair value, OneBeacon has assumed that all accrued but unpaid interest on the seller priority note since the date of issuance is paid in 2020, with regular annual interest payments on both the seller priority note and the pari passu note beginning in 2021, all accrued but unpaid interest on the pari passu note since the date of issuance is paid in 2025 and principal repayments begin on a graduated basis in 2030 for the seller priority note and 2035 for the pari passu note. Although these variables involve considerable judgment, OneBeacon does not currently expect any resulting changes in the estimated value of the surplus notes to be material to its financial position. An interest payment of $2.4 million was received in the three months ended March 31, 2016.

Below is a table illustrating the valuation adjustments taken to arrive at estimated fair value of the OneBeacon Surplus Notes as of March 31, 2017 and December 31, 2016:



	Type of Surplus Note						Total as of March 31, 2017			Total as of December 31, 2016
Millions	Seller Priority			Pari Passu			Total as of March 31, 2017			Total as of December 31, 2016
Par Value	$	57.9		$	43.1		$	101.0		$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments⁽¹⁾	7.2			1.8			9.0			5.1
Regulatory approval ⁽²⁾	2.9			(13.2		)	(10.3		)	(15.6		)
Liquidity adjustment ⁽³⁾	(19.9		)	(10.2		)	(30.1		)	(18.6		)
Total adjustments	(9.8		)	(21.6		)	(31.4		)	(29.1		)
Fair value⁽⁴⁾	$	48.1		$	21.5		$	69.6		$	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. The unfavorable year-to-date change in impact is due largely to an increase in the assumed liquidity spread to 400 basis points at March 31, 2017 from 250 basis points at December 31, 2016.

⁽⁴⁾ The decrease in the fair value of the surplus notes during the three months ended March 31, 2017 was driven primarily by an increase in the assumed liquidity spread, partially offset by the narrowing of non-investment grade credit spreads as well as the time value of money benefit generated by moving three months closer to modeled cash receipts.

Fair value measurements as of March 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 March 31, 2017 and December 31, 2016, White Mountains used quoted market prices or other observable inputs to determine fair value for approximately 93% and 94% of its investment portfolio. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, short-term investments, which include U.S. Treasury Bills and common equity securities. Investments valued using Level 2 inputs are primarily comprised of fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government, agency and provincial obligations and preferred stocks. Investments valued using Level 2 inputs also include certain passive exchange traded funds (“ETFs”) that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges and that management values using the fund manager’s published NAV to account for the difference in market close times. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include White Mountains’s investments in the OneBeacon Surplus Notes, as well as certain investments in fixed maturity investments, common equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Transfers between levels are based on investments held as of the beginning of the period.

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

White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources

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

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

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

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

White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for its other long-term investments, including obtaining and reviewing periodic and audited annual financial statements of hedge funds and private equity funds and discussing each fund’s pricing with the fund manager throughout the year. However, since the fund managers do not provide sufficient information to evaluate the pricing inputs and methods for each underlying investment, the inputs are considered to be unobservable. The fair value of White Mountains’s investments in hedge funds and private equity funds has generally been determined using the fund manager's NAV. In the event White Mountains believes that its estimate of NAV of a hedge fund or private equity fund differs from that reported by the fund manager due to illiquidity or other factors, White Mountains will adjust the reported NAV to more appropriately represent the fair value of its interest in the hedge fund or private equity fund investment. As of March 31, 2017 and December 31, 2016, White Mountains recorded negative adjustments of $1.0 million and $5.0 million to the reported NAV of certain investments in hedge funds and private equity funds.

Fair Value Measurements by Level

The following tables summarize White Mountains’s fair value measurements for investments as of March 31, 2017 and December 31, 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 Note 9.



	March 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	110.3	$	100.8	$	9.5	$	—

Debt securities issued by corporations:
Consumer	362.9		—		362.9		—
Financials	263.3		—		263.3		—
Health Care	248.4		—		248.4		—
Utilities	229.3		—		229.3		—
Industrial	174.5		—		174.5		—
Communications	135.9		—		135.9		—
Technology	118.1		—		118.1		—
Materials	101.9		—		101.9		—
Energy	52.5		—		52.5		—
Total debt securities issued by corporations	1,686.8		—		1,686.8		—

Mortgage and asset-backed securities	2,016.4		—		1,958.1		58.3
Municipal obligations	330.3		—		330.3		—
Foreign government, agency and provincial obligations	17.9		.6		17.3		—
Preferred stocks	13.7		—		13.7		—
Total fixed maturity investments	4,175.4		101.4		4,015.7		58.3

Short-term investments⁽⁴⁾	230.9		227.9		3.0		—

Common equity securities:
Exchange traded funds ⁽¹⁾	384.8		330.6		54.2		—
Consumer	32.2		32.2		—		—
Health Care	27.1		27.1		—		—
Financials	18.2		18.2		—		—
Technology	16.1		16.1		—		—
Communications	13.9		13.9		—		—
Industrial	9.0		9.0		—		—
Energy	7.7		7.7		—		—
Materials	5.1		5.1		—		—
Utilities	1.0		1.0		—		—
Other	85.9		—		85.9		—
Total common equity securities	601.0		460.9		140.1		—

Other long-term investments ⁽²⁾⁽³⁾	174.0		—		—		174.0
Total investments	$	5,181.3	$	790.2	$	4,158.8	$	232.3

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

⁽²⁾Excludes carrying value of $3.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(4.2) related to foreign currency forward contracts. Excludes carrying value of $11.7 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

⁽³⁾Excludes carrying value of $147.2 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Short-term investments are measured at amortized cost, which approximates fair value.



	December 31, 2016
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	278.3	$	268.8	$	9.5	$	—

Debt securities issued by corporations:
Consumer	385.6		—		385.6		—
Health Care	244.2		—		244.2		—
Utilities	180.3		—		180.3		—
Financials	176.0		—		176.0		—
Industrial	146.4		—		146.4		—
Communications	131.4		—		131.4		—
Materials	102.6		—		102.6		—
Technology	89.4		—		89.4		—
Energy	53.5		—		53.5		—
Total debt securities issued by corporations	1,509.4		—		1,509.4		—

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

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

Common equity securities:
Exchange traded funds⁽¹⁾	321.6		270.4		51.2		—
Health Care	20.9		20.9		—		—
Consumer	12.9		12.9		—		—
Financials	11.6		11.6		—		—
Technology	11.0		11.0		—		—
Communications	10.5		10.5		—		—
Energy	3.7		3.7		—		—
Industrial	2.2		2.2		—		—
Other	79.9		—		79.9		—
Total common equity securities	474.3		343.2		131.1		—

Other long-term investments⁽²⁾⁽³⁾	177.7		—		—		177.7
Total investments⁽⁴⁾	$	5,195.9	$	887.0	$	4,131.2	$	177.7

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

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

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

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

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

Debt securities issued by corporations

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



	Fair Value at
Millions	March 31, 2017		December 31, 2016
AA	93.1		100.9
A	484.7		381.9
BBB	846.5		786.5
BB	240.2		214.0
B	22.3		26.1
Debt securities issued by corporations⁽¹⁾	$	1,686.8	$	1,509.4

⁽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 Services ("Moody’s").

Mortgage and Asset-backed Securities

White Mountains purchases commercial mortgage-backed securities (“CMBS”) and residential mortgage-backed securities (“RMBS”) with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime 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 March 31, 2017.

White Mountains categorizes mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. As of March 31, 2017, White Mountains did not hold any RMBS classified as non-prime. White Mountains’s non-agency RMBS portfolio is generally moderate-term and structurally senior. White Mountains does not own any collateralized loan obligations. White Mountains does not own any collateralized debt obligations, with the exception of $33.9 million of non-agency RMBS resecuritization tranches, each a senior tranche in its own right and each collateralized by a single earlier vintage Super Senior or Senior non-agency RMBS.

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



	March 31, 2017						December 31, 2016
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	249.0	$	249.0	$	—	$	283.9	$	283.9	$	—
FNMA	270.1		270.1		—		278.3		278.3		—
FHLMC	86.1		86.1		—		89.8		89.8		—
Total Agency⁽¹⁾	605.2		605.2		—		652.0		652.0		—
Non-agency:
Residential	309.9		251.6		58.3		205.3		205.3		—
Commercial	126.9		126.9		—		127.5		127.5		—
Total Non-agency	436.8		378.5		58.3		332.8		332.8		—

Total mortgage-backed securities	1,042.0		983.7		58.3		984.8		984.8		—
Other asset-backed securities:
Credit card receivables	383.0		383.0		—		438.3		438.3		—
Vehicle receivables	353.0		353.0		—		479.5		479.5		—
Other	238.4		238.4		—		230.3		230.3		—
Total other asset-backed securities	974.4		974.4		—		1,148.1		1,148.1		—
Total mortgage and asset-backed securities	$	2,016.4	$	1,958.1	$	58.3	$	2,132.9	$	2,132.9	$	—

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

Non-agency Mortgage-backed Securities

The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of March 31, 2017 are as follows:



									Security Issuance Year
Millions	Fair Value		2004		2005		2006			2008		2010		2011		2012		2013		2014		2015		2016		2017
Non-agency RMBS	$	309.9	$	18.1	$	5.3	$	2.8		$	2.4	$	6.0	$	8.4	$	4.2	$	19.8	$	49.4	$	104.2	$	34.5	$	54.8
Non-agency CMBS	126.9		—		—		—			—		4.1		—		18.1		11.5		23.4		44.2		25.6		—
Total	$	436.8	$	18.1	$	5.3	$	2.8		$	2.4	$	10.1	$	8.4	$	22.3	$	31.3	$	72.8	$	148.4	$	60.1	$	54.8

Non-agency Residential Mortgage-backed Securities

The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of March 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Prime	$	309.9	$	104.9	$	205.0	$	—
Non-prime	—		—		—		—
Sub-prime	—		—		—		—
Total	$	309.9	$	104.9	$	205.0	$	—

⁽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 Mortgage-backed Securities

White Mountains’s non-agency CMBS portfolio is generally short-term and structurally senior, with more than 25 points of subordination on average for both fixed rate and floating rate as of March 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 March 31, 2017, none of the underlying loans of the non-agency CMBS held by White Mountains were reported as non-performing.

The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of March 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Fixed rate CMBS	$	115.2	$	1.6	$	70.1	$	43.5
Floating rate CMBS	11.7		—		—		11.7
Total	$	126.9	$	1.6	$	70.1	$	55.2

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

⁽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.

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 March 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 summarize the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2017 and 2016:



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Common equity securities		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2017	$	612.5		$	4,118.5		$	—	$	—	$	177.7		$	131.0		$	5,039.7		^(1)(2)(4)
Total realized and unrealized gains (losses)	22.4			25.5			.2		—		(1.9		)	8.2			54.4
Amortization/Accretion	—			(6.5		)	—		—		—			—			(6.5		)
Purchases	124.5			890.4			58.1		—		.2			22.1			1,095.3
Sales	(197.1		)	(866.9		)			—		(2.0		)	(14.1		)	(1,080.1		)
Deconsolidation of SSIE	—			(5.2		)	—		—		—						(5.2		)
Transfers in	—			—			—		—		—			—			—
Transfers out	—			—			—		—		—			—			—
Balance at March 31, 2017	$	562.3		$	4,155.8		$	58.3	$	—	$	174.0		$	147.2		$	5,097.6		⁽¹⁾⁽²⁾

⁽¹⁾Excludes carrying value of $3.5 and $3.0 at January 1, 2017 and March 31, 2017 associated with other long-term investments accounted for using the equity method and $(1.2) and $(4.2) related to foreign currency forward contracts. Excludes carrying value of $12.3 and $11.7 at January 1, 2017 and March 31, 2017 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

⁽²⁾Excludes carrying value of $287.1 and $230.9 at January 1, 2017 and March 31, 2017 associated with 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⁾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
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Common equity securities		Other long-term investments		Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2016	$	1,152.2		$	2,531.4		$	70.0	$	—	$	169.5	$	127.8		$	4,050.9		^(1)(2)(4)
Total realized and unrealized gains (losses)	11.3			19.1			.5		—		1.1		(2.4		)	29.6
Amortization/Accretion	—			(4.2		)	—		—		—		—			(4.2		)
Purchases	109.0			292.4			—		—		2.1		8.9			412.4
Sales	(852.9		)	(272.6		)	—		—		—		(3.0		)	(1,128.5		)
Transfers in	—			—			—		—		—		—			—
Transfers out	—			—			—		—		—		—			—
Balance at March 31, 2016	$	419.6		$	2,566.1		$	70.5	$	—	$	172.7	$	131.3		$	3,360.2		^(1)(2)(4)

⁽¹⁾Excludes carrying value of $3.8 and $3.7 at January 1, 2016 and March 31, 2016 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $14.7 and $14.1 at January 1, 2016 and March 31, 2016 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.

⁽²⁾Excludes carrying value of $211.3 million and $261.7 million at January 1, 2016 and March 31, 2016 associated with short-term investments of which $0.1 and $0.3 is classified as held for sale at January 1, 2016 and March 31, 2016.

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

⁽⁴⁾Includes carrying value of $9.5 and $9.4 of fixed maturity investments at January 1, 2016 and March 31, 2016 that is classified as assets held for sale related to SSIE.

Fair Value Measurements — transfers between levels - Three-month period ended March 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 the first three months of 2017 and the first three months of 2016, there were no fixed maturity investments classified as Level 3 measurements in the prior period that were transferred to Level 2 measurements.

Significant Unobservable Inputs

The following summarizes significant unobservable inputs used in estimating the fair value of investment securities, other than hedge funds and private equity funds, classified within Level 3 as of March 31, 2017 and December 31, 2016. The fair value of investments in hedge funds and private equity funds are generally estimated using the NAV of the funds.



Description	March 31, 2017
$ in millions, except share price	Rating⁽⁶⁾	Valuation Technique(s)	Fair Value⁽¹⁾	Unobservable Input
Non-agency residential mortgage-backed securities	AAA	Broker pricing	$58.3	Broker quote		102.56
Private equity security	NR	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	NR	Discounted cash flow	$22.1	Discount rate	-	25.0%
Private equity security	NR	Share price of most recent transaction	$3.4	Share price	-	$2.52
Private convertible preferred security	NR	Multiple of EBITDA	$3.7	EBITDA multiple	-	6.00
Private convertible preferred security	NR	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community development tax incentive investment	NR	Member share of GAAP net equity	$14.5	GAAP net equity		$14.5
Private equity security	NR	Discounted cash flow/ Option pricing method	$9.4	Discount rate	-	21.0%
				Time until expiration	-	4 years
			Volatility/Standard deviation		-	50.0%
			Risk free rate		-	1.00%
OneBeacon Surplus Notes:	NR
- Seller priority		Discounted cash flow	$48.1	Discount rate ⁽²⁾	-	10.6%
			Timing of interest payments ⁽⁴⁾		-	2020
			Timing of principal payments ⁽⁴⁾		-	2030
- Pari passu		Discounted cash flow	$21.5	Discount rate ⁽³⁾	-	15.0%
			Timing of interest payments ⁽⁵⁾		-	2021
			Timing of principal payments ⁽⁵⁾		-	2035

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

⁽²⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread, increased to 400 bps as of March 31, 2017 from 250 bps as of December 31, 2016 to reflect an increase in the assumed liquidity spread for the seller priority note.

⁽³⁾Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread, increased to 400 bps as of March 31, 2017 from 250 bps as of December 31, 2016 to reflect an increase in the assumed liquidity spread for the pari passu note.

⁽⁴⁾As of March 31, 2017, OneBeacon has assumed for the purpose of estimating fair value that all accrued but unpaid interest on the seller priority note since the date of issuance is paid in 2020, with regular annual interest payments beginning thereafter. Principal repayments are assumed to begin on a graduated basis in 2030.

⁽⁵⁾As of March 31, 2017, OneBeacon has assumed for the purpose of estimating fair value that regular annual interest payments on the pari passu note begin in 2021. All accrued but unpaid interest since the date of issuance is assumed to be paid in 2025. Principal repayments are assumed to begin on a graduated basis in 2035.

⁽⁶⁾ Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.



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

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

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

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

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

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

- Definition

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

+ References + Details

Goodwill and Other Intangible Assets

3 Months Ended

Mar. 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 shows the change in goodwill and other intangible assets:



	Three Months Ended March 31,
Millions	2017								2016
	Goodwill		Other intangible assets			Total			Goodwill		Other intangible assets			Total
Beginning balance	$	31.7	$	24.2		$	55.9		$	24.1	$	31.3		$	55.4
Add: Amounts held for sale at beginning of the period⁽¹⁾	—		—			—			—		.4			.4
Acquisition of businesses	—		—			—			—		3.9			3.9
Amortization, including foreign currency translation	—		(2.9		)	(2.9		)	—		(3.1		)	(3.1		)
Less: Amounts held for sale at end of the period⁽¹⁾	—		—			—			—		.3			.3
Ending balance	$	31.7	$	21.3		$	53.0		$	24.1	$	32.2		$	56.3

⁽¹⁾See Note 17 — “Held for Sale and Discontinued Operations”.

- 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

3 Months Ended

Mar. 31, 2017

Debt Disclosure [Abstract]

Debt

White Mountains’s debt outstanding as of March 31, 2017 and December 31, 2016 consisted of the following:



Millions	March 31, 2017			Effective Rate ⁽¹⁾	December 31, 2016			Effective Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
OBH Senior Notes, at face value	275.0			4.7%	275.0			4.7%
Unamortized original issue discount and debt issuance costs	(1.8		)		(1.8		)
OBH Senior Notes, carrying value	273.2				273.2
OneBeacon Bank Facility	—			N/A	—			N/A
MediaAlpha Bank Facility	11.7			6.1%	12.9			5.7%
Unamortized issuance cost	(.2		)		(.2		)
MediaAlpha Bank Facility, carrying value	11.5				12.7
Total debt	$	284.7			$	285.9

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

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”). As of March 31, 2017, the WTM Bank Facility was undrawn.

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.

OBH Senior Notes

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

The OBH Senior Notes were issued under indentures that contain restrictive covenants which, among other things, limit the ability of OneBeacon Ltd., OBH, and their respective subsidiaries to create liens and enter into sale and leaseback transactions and limits the ability of OneBeacon Ltd. and OBH to consolidate, merge or transfer its properties and assets. The indentures do not contain any financial ratios or specified levels of net worth or liquidity to which the OneBeacon Ltd. or OBH must adhere. In addition, a failure by OneBeacon Ltd. or OBH or their respective subsidiaries to pay principal and interest on covered debt, where such failure results in the acceleration of at least $75.0 million of the principal amount of covered debt, could trigger the acceleration of the OBH Senior Notes.

OneBeacon Bank Facility

On September 29, 2015, OneBeacon Ltd. and OneBeacon U.S. Holdings, Inc. (“OBH”), as co-borrowers and co-guarantors, entered into a revolving credit facility administered by U.S. Bank N.A. and also including BMO Harris Bank N.A., which has a total commitment of $65.0 million and has a maturity date of September 29, 2019 (the “OneBeacon Bank Facility”). As of March 31, 2017, the OneBeacon Bank Facility was undrawn.

The OneBeacon Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards.

MediaAlpha Bank Facility

On July 23, 2015, MediaAlpha entered into a secured credit facility with Opus Bank, which has a total commitment of $20.0 million and has a maturity date of July 23, 2019 (the “MediaAlpha Bank Facility”). The MediaAlpha Bank Facility consists of a $15.0 million term loan facility, which has an outstanding balance of $11.7 million as of March 31, 2017, and a revolving loan facility for $5.0 million, which is undrawn as of March 31, 2017. During the three months ended March 31, 2017, MediaAlpha repaid $1.2 million under the term loan facility. The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.5% as of March 31, 2017.

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 maximum leverage ratio.

Compliance

At March 31, 2017, White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Gibraltar, Ireland, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’ income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2017, represented a net effective tax rate of 10.3%. The effective tax rate for the three months ended March 31, 2017 was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended March 31, 2016, represented a net effective tax rate of (114.1)%. As noted below, the effective tax rate for the three months ended March 31, 2016 was impacted by a $12.8 million tax benefit on the settlement of the 2007-2009 IRS exam. Without the tax benefit on the settlement of the 2007-2009 IRS exam there would have been tax expense, which would have resulted in a net effective tax rate that is approximately the same as the U.S. statutory rate of 35%. In arriving at the effective tax rate for the three months ended March 31, 2017 and 2016, White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2017 and 2016. 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. In the first and second quarters of 2016 White Mountains recorded tax benefits of $12.8 million and $3.5 million respectively related to the settlement of IRS audits of certain subsidiaries of OneBeacon for tax years 2007-2009 and 2010-2012. 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 Holdings, Inc. and subsidiaries for tax year 2012. 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.

Derivatives

3 Months Ended

Mar. 31, 2017

Derivatives

Variable Annuity Reinsurance

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

The following table summarizes the pre-tax operating results of WM Life Re for the three months ended March 31, 2016.



	Three Months Ended
	March 31,
Millions		2016
Fees, included in other revenue		$	1.0
Change in fair value of variable annuity liability, included in other revenue		(.4		)
Change in fair value of derivatives, included in other revenue		(1.7		)
Foreign exchange, included in other revenue		.9
Other investment income and losses		—
Total revenue		(.2		)
Death benefit claims paid, included in general and administrative expenses		(.1		)
General and administrative expenses		(.8		)
Pre-tax loss		$	(1.1	)

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



	Gains (losses)
	Three Months Ended			Carrying Value
	March 31,			As of
Millions	2016			December 31, 2016
Fixed income/interest rate	$	1.8		$	—
Foreign exchange	(4.2		)	—
Equity	.7			—
Total	$	(1.7	)	$	—

The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2016.



	Three Months Ended March 31, 2016
	Variable Annuity Liabilities			Derivative Instruments
Millions	Level 3			Level 3 ⁽¹⁾			Level 2 ⁽¹⁾⁽²⁾			Level 1 ⁽³⁾			Total
Beginning of period	$	.3		$	2.7		$	16.5		$	.9		$	20.1
Purchases	—			—			—			—			—
Realized and unrealized (losses) gains	(.4		)	1.2			1.1			(4.0		)	(1.7		)
Transfers in	—			—			—			—			—
Sales/settlements	—			(1.3		)	(7.3		)	3.3			(5.3		)
End of period	$	(.1	)	$	2.6		$	10.3		$	.2		$	13.1

⁽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.

All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements. The fair value of White Mountains’s variable annuity reinsurance liabilities were estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, were generally unobservable inputs and significantly impacted the fair value estimates. Generally, the liabilities associated with these guarantees increased with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains used derivative instruments to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policyholder behavior and risk margins, were generally the same as those used to estimate the fair value of variable annuity liabilities.

WM Life Re entered into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee. In the case of OTC derivatives, WM Life Re had exposure to credit risk for amounts that were uncollateralized by counterparties. WM Life Re’s internal risk management guidelines established net counterparty exposure thresholds that took into account OTC counterparties’ credit ratings. The OTC derivative contracts were subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements.

In the case of exchange traded instruments, WM Life Re had exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re had master netting agreements with certain of its counterparties whereby the collateral provided (held) was calculated on a net basis. The following summarizes amounts offset under master netting agreements:



	March 31, 2016
Millions	Gross asset amounts before offsets ⁽¹⁾		Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other Assets
Interest rate contracts
OTC	$	2.2	$	(2.5	)	$	(.3	)
Foreign exchange contracts
OTC	12.7		—			12.7
Exchange traded	—		(.1		)	(.1		)
Equity contracts
OTC	1.5		(.9		)	.6
Exchange traded	.3		(.1		)	.2
Total⁽²⁾	$	16.7	$	(3.6	)	$	13.1

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

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

There were no open derivatives instruments and no exposure to credit losses on OTC and exchanged traded derivatives subsequent to June 30, 2016.

The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets as of March 31, 2016:



	March 31, 2016
Millions	Net amount of assets reflected in Balance Sheet		Collateral provided to counter-party - Cash		Collateral provided to counter-party - Financial Instruments		Net amount of exposure after effect of collateral provided		Excess collateral provided to counter-party- Cash		Excess collateral provided - Financial Instruments		Counter-party collateral held by WM Life Re- Cash		Net amount of exposure to counter-party		Standard & Poor's Rating⁽¹⁾
JP Morgan	$	6.5	$	—	$	—	$	6.5	$	—	$	—	$	5.4	$	1.1	A	+
Bank of America	1.4		—		—		1.4		—		—		—		1.4		A
Citigroup - OTC	5.0		—		—		5.0		—		—		.5		4.5		A
Citigroup - Exchange Traded	.2		—		—		.2		7.9		—		—		8.1		A
Total	$	13.1	$	—	$	—	$	13.1	$	7.9	$	—	$	5.9	$	15.1

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

“BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).

Forward Contracts

Derivatives

Forward Contracts

White Mountains investment portfolio contains investment grade fixed maturity investments denominated in British Pound Sterling (GBP) and common equity securities denominated in Euro (EUR). White Mountains entered into foreign currency forward contracts to manage its GBP and EUR foreign currency exposure. The contracts do not meet the criteria to be accounted for as a hedge. White Mountains monitors its exposure to foreign currency and adjusts its foreign currency positions within the risk guidelines and ranges established by senior management. While White Mountains actively manages its foreign currency positions, mismatches between movements in foreign currency rates and its foreign currency forward contract may result in foreign currency positions being outside pre-defined ranges and/or foreign currency losses. At March 31, 2017, White Mountains held $299.9 million (GBP 200.0 million and EUR 50.0 million) total gross notional value of foreign currency forward contracts.

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

The net realized and unrealized derivative loss recognized in net realized and unrealized investment gains (losses) for the period ended March 31, 2017 was $3.0 million. White Mountains’s forward contracts are subject to master netting agreements. As of March 31, 2017 and December 31, 2016, the gross liability amount offset under the master netting agreement and the net amount recognized in other long-term investments was $(4.2) million and $(1.2) million.

White Mountains does not hold or provide any collateral under its forward contracts. The following table summarizes the notional amount and the uncollateralized balance associated with the forward currency contracts:



	March 31, 2017
Millions	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾
Barclays Bank PLC	$	247.2	$	(3.2	)	A-
JP Morgan	52.7		(1.0		)	A-
Total	$	299.9	$	(4.2	)

⁽¹⁾Standard & Poor’s ratings “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6441202&loc=d3e80720-113993

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41620-113959

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579240-113959

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624177-113959

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80784-113994

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41641-113959

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

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ Details

Municipal Bond Guarantee

3 Months Ended

Mar. 31, 2017

Guarantees [Abstract]

Municipal Bond Guarantee Insurance

In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of March 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. Through HG Re, which had statutory capital and surplus of $467.4 million at December 31, 2016, HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. 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.

For the three months ended March 31, 2017, HG Global had pre-tax income of $6.6 million, which included $4.8 million of interest income on the BAM Surplus Notes. For the three months ended March 31, 2016, HG Global had pre-tax income of $7.3 million, which included $4.5 million of interest income on the BAM Surplus Notes.

For the three months ended March 31, 2017, White Mountains reported pre-tax losses of $12.2 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.8 million of interest expense on the BAM Surplus Notes. For the three months ended March 31, 2016, White Mountains reported pre-tax losses of $7.6 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.5 million of interest expense on the BAM Surplus Notes.

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% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which is 3.54% and 3.78% for 2016 and 2017. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8%. BAM is required to seek regulatory approval to pay interest and principal on its surplus notes only when adequate capital resources have accumulated beyond BAM’s initial capitalization and a level that continues to support its outstanding obligations, business plan and ratings.

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

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



	March 31, 2017		December 31, 2016
Contracts outstanding	5,191		4,807
Remaining weighted average contract period outstanding (in years)	10.8		10.8
Contractual debt service outstanding (in millions):
Principal	$	35,222.1	$	33,057.3
Interest	17,589.9		16,396.6
Total debt service outstanding	$	52,812.0	$	49,453.9

Gross unearned insurance premiums	$	99.8	$	83.0

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



Millions	March 31, 2017
April 1, 2017 - December 31, 2017	$	6.3

January 1, 2018 - March 31, 2018	2.1
April 1, 2018 - June 30, 2018	2.0
July 1, 2018 - September 30, 2018	2.0
October 1, 2018 - December 31, 2018	2.0
	8.1

2019	7.8
2020	7.4
2021	7.0
2022 and thereafter	63.2
Total gross unearned insurance premiums	$	99.8

Earnings Per Share

3 Months Ended

Mar. 31, 2017

Earnings Per Share [Abstract]

Earnings Per Share

Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share from continuing operations for the three months ended March 31, 2017 and 2016. See Note 17 — “Held for Sale and Discontinued Operations”.



	Three Months Ended
	March 31,
	2017			2016
Basic and diluted earnings per share numerators (in millions):
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	$	35.3		$	11.9
Allocation of income for unvested restricted common shares	(.4		)	(.1		)
Dividends declared on participating restricted common shares⁽¹⁾	(.1		)	(.1		)
Total allocation to restricted common shares	(.5		)	(.2		)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	34.8		$	11.7
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	34.8		$	11.7
Dividends declared net of restricted common share amounts⁽¹⁾	(4.5		)	(5.9		)
Total undistributed net earnings, net of restricted common share amounts	$	30.3		$	5.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,564.6			5,539.6
Average unvested restricted shares⁽²⁾	(52.5		)	(54.0		)
Basic earnings per share denominator	4,512.1			5,485.6
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period⁽³⁾	4,564.6			5,540.5
Average unvested restricted common shares⁽²⁾	(52.5		)	(54.0		)
Diluted earnings per share denominator⁽³⁾	4,512.1			5,486.5
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains’s common shareholders	$	7.72		$	2.14
Dividends declared and paid	(1.00		)	(1.00		)
Undistributed earnings	$	6.72		$	1.14
Diluted earnings per share (in dollars):
Net income (loss) attributable to White Mountains’s common shareholders	$	7.72		$	2.14
Dividends declared and paid	(1.00		)	(1.00		)
Undistributed earnings	$	6.72		$	1.14

⁽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 15 — “Employee Share-Based Compensation Plans”.

⁽³⁾The diluted earnings per share denominator for the three months ended March 31, 2016 includes the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 882 incremental shares outstanding over the period. The incremental shares had an effect on diluted earning per share of less than $0.01.

Non-controlling Interest

3 Months Ended

Mar. 31, 2017

Noncontrolling Interest [Abstract]

Non-controlling Interests

The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of March 31, 2017 and December 31, 2016:



	March 31, 2017					December 31, 2016
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon Ltd.	24.3	%	$	250.7		23.9	%	$	244.6

Other, excluding mutuals and reciprocals
HG Global	3.1		16.5			3.1		16.6
MediaAlpha	40.0		11.0			40.0		11.7
Wobi	5.0		1.0			5.0		.7
Dewar	18.8		3.4			18.8		3.9
Buzzmove	29.1		2.8			29.1		2.9
Total other, excluding mutuals and reciprocals			34.7					35.8
Mutuals and reciprocals
BAM	100.0		(153.0		)	100.0		(150.9		)
SSIE	—		—			100.0		4.4
Total mutuals and reciprocals			(153.0		)			(146.5		)
Total non-controlling interests			$	132.4				$	133.9

Segment Information

3 Months Ended

Mar. 31, 2017

Segment Reporting [Abstract]

Segment Informartion

Segment Information

White Mountains has determined that its reportable segments are OneBeacon, HG Global/BAM and Other Operations. As a result of the Sirius Group and Tranzact sales, the results of operations for Sirius Group, previously reported in its own segment, 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. Prior year amounts have been reclassified to conform to the current period’s presentation. See Note 17 — “Held for Sale and Discontinued Operations”.

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

Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows:



			HG Global/BAM
Millions	OneBeacon		HG		BAM⁽¹⁾			Other Operations			Total
Three Months Ended March 31, 2017
Earned insurance premiums	$	261.8	$	1.5	$	.5		$	1.0		$	264.8
Net investment income	12.2		.6		2.0			11.3			26.1
Net investment income (loss) - surplus note interest	—		4.8		(4.8		)	—			—
Net realized and unrealized investment gains	15.0		.3		1.0			35.0			51.3
Other revenue	3.4		—		.4			39.9		⁽²⁾	43.7
Total revenues	292.4		7.2		(.9		)	87.2			385.9
Losses and LAE	150.6		—		—			1.1			151.7
Insurance acquisition expenses	45.3		.3		.9			.1			46.6
Other underwriting expenses	51.7		—		.1			—			51.8
General and administrative expenses	4.7		.3		10.3			76.0		⁽³⁾	91.3
Amortization of other intangible assets	.3		—		—			2.6			2.9
Interest expense	3.3		—		—			.4			3.7
Total expenses	255.9		.6		11.3			80.2			348.0
Pre-tax income (loss)	$	36.5	$	6.6	$	(12.2	)	$	7.0		$	37.9



			HG Global/BAM
Millions	OneBeacon		HG		BAM⁽¹⁾			Other Operations				Total
Three Months Ended March 31, 2016
Earned insurance premiums	$	278.6	$	.9	$	.3		$	2.3			$	282.1
Net investment income	14.4		.5		1.6			1.4				17.9
Net investment income (loss) - surplus note interest	—		4.5		(4.5		)	—				—
Net realized and unrealized investment gains	16.6		2.1		4.9			5.9				29.5
Other revenue	.9		—		.1			38.8			⁽²⁾	39.8
Total revenues	310.5		8.0		2.4			48.4				369.3
Losses and LAE	158.8		—		—			2.3				161.1
Insurance acquisition expenses	51.0		.2		.7			.8				52.7
Other underwriting expenses	55.3		—		.1			—				55.4
General and administrative expenses	3.6		.5		9.2			70.7			⁽³⁾	84.0
Amortization of other intangible assets	.3		—		—			2.8				3.1
Interest expense	3.3		—		—			1.2				4.5
Total expenses	272.3		.7		10.0			77.8				360.8
Pre-tax income (loss)	$	38.2	$	7.3	$	(7.6	)	$	(29.4	)		$	8.5

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

⁽²⁾Includes $32.5 from MediaAlpha for the three months ended March 31, 2017, and $32.7 from MediaAlpha for the three months ended March 31, 2016.

⁽³⁾Includes $30.9 from MediaAlpha for the three months ended March 31, 2017, and $30.5 from MediaAlpha for the three months ended March 31, 2016.

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8981-108599

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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+ Details

Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates	3 Months Ended
	Mar. 31, 2017
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in Unconsolidated Affiliates	Investments in Unconsolidated Affiliates White Mountains’s investments in unconsolidated affiliates represent investments in other companies in which White Mountains has a significant voting and economic interest but does not control the entity. Symetra In August 2015, Symetra announced it had entered into a definitive merger agreement with Sumitomo Life Insurance Company (“Sumitomo Life”) pursuant to which Sumitomo Life would acquire all of the outstanding shares of Symetra. Following the announcement and Symetra shareholders’ November 5, 2015 meeting to approve the transaction, White Mountains relinquished its representation on Symetra’s board of directors. As a result, White Mountains changed its accounting for Symetra common shares from the equity method to fair value as of December 31, 2015. During the fourth quarter of 2015, White Mountains recognized $258.8 million ($241.1 million after tax) of unrealized investment gains through net income, representing the difference between the carrying value of Symetra common shares under the equity method at the date of change and fair value at December 31, 2015. White Mountains also received a special dividend of $0.50 per share as part of the transaction that was paid in the third quarter of 2015. On February 1, 2016, Symetra closed its definitive merger agreement with Sumitomo Life and White Mountains received proceeds of $658.0 million, or $32.00 per common share. White Mountains recognized $4.7 million in pre-tax net investment gains associated with Symetra in the first quarter of 2016.

Employee Share-Based Incentive Compensation Plans

3 Months Ended

Mar. 31, 2017

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

Employee Share-Based Incentive Compensation Plans

White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non share-based incentive awards to key employees and service providers of White Mountains. White Mountains’s share-based compensation incentive awards consist of performance shares, restricted shares and stock options.

Share-Based Compensation Based on White Mountains Common Shares

WTM Performance Shares

Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. The following table summarizes performance share activity for the three months ended March 31, 2017 and 2016 for performance shares granted under the WTM Incentive Plan:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	80,353		$	42.4		93,654		$	57.7
Shares paid or expired⁽¹⁾	(30,167	)	(20.8		)	—		—
New grants	16,460		—			16,215		—
Forfeitures and cancellations⁽²⁾	(9,841	)	(5.7		)	(1,186	)	(.6		)
Expense recognized	—		8.1			—		14.2
End of period⁽³⁾	56,805		$	24.0		108,683		$	71.3

⁽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 share payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target.

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

⁽³⁾Outstanding performance share awards as of March 31, 2017 and 2016 exclude 2,195 and 10,826 performance share awards granted to employees of Sirius Group.

For performance shares earned in 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. If all the outstanding WTM performance shares had vested on March 31, 2017, the total additional compensation cost to be recognized would have been $30.5 million, based on accrual factors at March 31, 2017 (common share price and payout assumptions).

Performance Shares granted under the WTM Incentive Plan

The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at March 31, 2017 for each performance cycle:



Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2017 – 2019	16,460		$	1.2
2016 – 2018	19,415		7.3
2015 – 2017	21,795		15.8
Sub-total	57,670		24.3
Assumed forfeitures	(865	)	(.3		)
March 31, 2017	56,805		$	24.0

Restricted Shares

The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2017 and 2016:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	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
Issued	16,735		15.8			21,215		16.8
Vested	(22,015	)	—			(24,620	)	—
Forfeited	(5,200	)	(2.8		)	(800	)	(.3		)
Expense recognized	—		(3.6		)	—		(3.0		)
End of period⁽¹⁾	60,140		$	29.1		66,470		$	29.2

⁽¹⁾Restricted share awards outstanding as of March 31, 2017 and 2016 include 5,235 and 2,195 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.

During the three months ended March 31, 2017, White Mountains issued 16,735 restricted shares that vest on January 1, 2020. During the first three months of 2016, White Mountains issued 21,215 restricted shares that vest on January 1, 2019. The unrecognized compensation cost at March 31, 2017 is expected to be recognized ratably over the remaining vesting periods.

Stock Options

Non-Qualified Options

As March 31, 2017, the 125,000 Non-Qualified options issued to the Company’s former Chairman and CEO have 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 less the fixed strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011.

Share-Based Compensation Based on OneBeacon Ltd. Common Shares

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

OneBeacon Performance Shares

OneBeacon performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. OneBeacon performance share awards generally vest at the end of a three-year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of OneBeacon Ltd. common shares at the time awards are paid. No OneBeacon performance shares were issued for the 2017-2019 cycle.

The following table summarizes performance share activity for the three months ended March 31, 2017 and 2016 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:



	Three Months Ended March 31,
	2017				2016
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense		Target Performance Shares Outstanding		Accrued Expense
Beginning of period	452,519		$	1.6	449,435		$	1.4
Shares paid or expired⁽¹⁾	(142,710	)	—		(167,300	)	(.7		)
New grants	—		—		163,150		—
Assumed forfeitures and cancellations⁽²⁾	(13	)	—		(4,079	)	—
Expense recognized	—		.3		—		.4
End of period	309,796		$	1.9	441,206		$	1.1

⁽¹⁾ There were no payments made in 2017 for the 2014-2016 performance cycle; those performance shares did not meet the threshold performance goals and expired. OneBeacon performance share payments in 2016 for the 2013-2015 performance cycle were at 24.3% of target.

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

If the outstanding OneBeacon performance shares had been vested on March 31, 2017, the total additional compensation cost to be recognized would have been $1.8 million, based on accrual factors at March 31, 2017 (common share price, accumulated dividends and payout assumptions).

The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at March 31, 2017 for each performance cycle:



Millions, except share amounts	Target Performance Shares Outstanding	Accrued Expense
Performance cycle:
2016 – 2018	163,150	$	1.1
2015 – 2017	146,646	.8
Sub-total	309,796	1.9
Assumed forfeitures	—	—
March 31, 2017	309,796	$	1.9

OneBeacon Restricted Shares

The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three months ended March 31, 2017 and 2016:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	395,872		$	2.1		382,722		$	2.5
Issued	461,160		7.4			170,650		2.3
Vested	(157,500	)	—			(157,500	)	—
Forfeited	—		—			—		—
Expense recognized	—		(.9		)	—		(.5		)
End of period	699,532		$	8.6		395,872		$	4.3

On February 28, 2017, OneBeacon issued 461,160 restricted shares, of which 235,000 are scheduled to cliff vest on August 28, 2018, 115,450 are scheduled to cliff vest on January 1, 2020, and 110,710 are scheduled to vest in two equal installments on February 24, 2018 and February 24, 2019.

On February 24, 2016, OneBeacon issued 170,650 shares of restricted shares, of which 92,500 restricted shares vest on February 24, 2018 and 78,150 vest on January 1, 2019.

On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments. The first installment vested on February 22, 2014. Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent five years were reduced by 35,000 shares.

As of March 31, 2017, unrecognized compensation expense of $8.6 million related to restricted stock awards is expected to be recognized over the remaining vesting periods.

OneBeacon Restricted Stock Units

During the first three months of 2017, the OneBeacon Compensation Committee awarded to certain employees 240,160 restricted stock units (“RSUs”) that are schedule to vest on December 31, 2019, contingent upon the approval of the OneBeacon 2017 Long-Term Incentive Plan at the 2017 Annual General Meeting to be held May 24, 2017, all of which were outstanding as of March 31, 2017.

During the first three months of 2016, the OneBeacon Compensation Committee awarded to certain employees 222,449 RSUs that are scheduled to vest on December 31, 2018, of which, net of forfeiture assumptions, 208,499 were outstanding as of March 31, 2017.

At vesting the RSUs will be paid out in cash or shares at the discretion of the OneBeacon Compensation Committee. For the three months ended March 31, 2017 and 2016, the expense associated with the RSUs was $0.6 million and $0.4 million. If all of the outstanding RSUs had been vested on March 31, 2017, the total additional compensation cost to be recognized would have been $7.0 million, based on current accrual factors (common share price and accumulated dividends) as of March 31, 2017.

- Definition

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

+ References + Details

Fair Value of Financial Instruments

3 Months Ended

Mar. 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 OBH Senior Notes and MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount.

The following table summarizes the fair value and carrying value of these financial instruments as of March 31, 2017 and December 31, 2016:



	March 31, 2017				December 31, 2016
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
OBH Senior Notes	$	279.4	$	273.2	$	274.2	$	273.2
MediaAlpha Bank Facility	11.7		11.5		13.0		12.7

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

The fair value estimate for the MediaAlpha Bank Facility has been determined based on a discounted cash flows approach and is considered to be a 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

Discontinued Operations

3 Months Ended

Mar. 31, 2017

Discontinued Operations

Held for Sale and Discontinued Operations

Sirius Group

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

Through April 18, 2016, Sirius Group’s results are reported as discontinued operations and assets and liabilities held for sale within White Mountains’s GAAP financial statements. Assets held for sale did not include White Mountains’s investment in OneBeacon and certain other investments that are 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 loss from discontinued operations does not include White Mountains’s net investment income and realized and unrealized investment gains and losses associated with these investments. For the three months ended March 31, 2016, $4.1 million of realized and unrealized investment gains and losses, net of related tax effects, that are included in the Sirius Group legal entities have been excluded from net loss from discontinued operations. For the three months ended March 31, 2016, White Mountains recorded $0.9 million of net loss from discontinued operations and $37.2 million of other comprehensive income from Sirius Group.

Tranzact

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

During 2016, 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. In the first quarter of 2017, White Mountains recorded a $1.0 million reduction to the gain from sale of Tranzact in discontinued operations as a result of 2016 tax payments.

During 2016, White Mountains recognized a $21.4 million tax benefit in continuing operations related to the reversal of a valuation allowance that resulted from the gain on the sale of Tranzact recognized within discontinued operations. This tax benefit was recorded in continuing operations with an offsetting amount of net tax expense recorded in discontinued operations, $30.2 million of tax expense was recorded to gain from sale of Tranzact in discontinued operations and a $8.8 million tax benefit was recorded to net income from discontinued operations.

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 three months ended March 31 2016, White Mountains recorded a net loss from discontinued operations of $2.0 million from Tranzact.

Star & Shield

On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in SSIE Surplus Notes which resulted in a $21.0 million total decrease to net income attributable to White Mountains's common shareholders and a corresponding increase to net income attributable to non-controlling interests. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services, LLC and the sale was completed on March 7, 2017. 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.

Discontinued Operations [Member]

Discontinued Operations

Summary of Reclassified Balances and Related Items

Net Assets Held for Sale

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



Millions	December 31, 2016
Assets held for sale
Fixed maturity investments, at fair value	$	6.6
Short-term investments, at amortized cost (which approximates fair value)	.2
Total investments	6.8
Cash	.9
Reinsurance recoverable on unpaid losses	.3
Insurance and reinsurance premiums receivable	1.5
Other assets	.6
Total assets held for sale	$	10.1
Liabilities held for sale
Loss and loss adjustment expense reserves	$	5.0
Unearned insurance and reinsurance premiums	1.2
Other liabilities	(1.1		)
Total liabilities held for sale	5.1
Net assets held for sale	$	5.0

Net Income (Loss) from Discontinued Operations

The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations. For the three months ended March 31, 2016, the amounts presented relate to Sirius Group and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains.



	Three Months Ended								Three Months Ended
	March 31, 2017								March 31, 2016
Millions	Sirius Group		Other Disc Ops			Total			Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	—	$	—		$	—		$	202.4		$	—		$	202.4
Net investment income	—		—			—			12.2			—			12.2
Net realized and unrealized losses	—		—			—			(8.8		)	—			(8.8		)
Other revenue	—		—			—			(4.1		)	57.8			53.7
Total revenues	—		—			—			201.7			57.8			259.5
Expenses
Loss and loss adjustment expenses	—		—			—			113.7			—			113.7
Insurance and reinsurance acquisition expenses	—		—			—			48.4			—			48.4
Other underwriting expenses	—		—			—			26.7			—			26.7
General and administrative expenses	—		—			—			8.1			53.4			61.5
Interest expense	—		—			—			6.6			1.4			8.0
Total expenses	—		—			—			203.5			54.8			258.3
Pre-tax (loss) income	—		—			—			(1.8		)	3.0			1.2
Income tax expense (benefit)	—		—			—			.9			(1.0		)	(.1		)
Net (loss) income from discontinued operations	—		—			—			(.9		)	2.0			1.1
Net loss from sale of discontinued operations	—		(1.0		)	(1.0		)	—			—			—
Total (loss) income from discontinued operations	$	—	$	(1.0	)	$	(1.0	)	$	(.9	)	$	2.0		$	1.1
Change in foreign currency translation and other from discontinued operations	—		—			—			37.2			—			37.2
Comprehensive (loss) income from discontinued operations	$	—	$	(1.0	)	$	(1.0	)	$	36.3		$	2.0		$	38.3

Net Change in Cash from Discontinued Operations

The following summarizes the net change in cash, including income tax (payment to) refund from national governments and interest paid associated with the business classified as discontinued operations:



	Three Months Ended
	March 31,
(Millions)	2017			2016
Net cash (used for) provided from operations	$	—		$	(40.7	)
Net cash provided from investing activities	—			33.6
Net cash (used for) provided from financing activities	—			(8.3		)
Effect of exchange rate changes on cash	—			4.2
Net change in cash during the period	—			(11.2		)
Cash balances at beginning of period	.9			150.2
Net change in cash held for sale	(.9		)	.9
Cash balances at end of period	$	—		$	139.9
Supplemental cash flows information:
Interest paid	$	—		$	(1.4	)
Net income tax payment to national governments	$	—		$	(36.4	)

Earnings Per Share

Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three months ended March 31, 2017 and 2016:



	Three Months Ended
	March 31,
	2017			2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	(1.0	)	$	1.1
Allocation of income for participating unvested restricted common shares⁽¹⁾	—			—
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts⁽²⁾	$	(1.0	)	$	1.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,564.6			5,539.6
Average unvested restricted common shares⁽³⁾	(52.5		)	(54.0		)
Basic earnings per share denominator	4,512.1			5,485.6
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period⁽⁴⁾	4,564.6			5,540.5
Average unvested restricted common shares⁽³⁾	(52.5		)	(54.0		)
Diluted earnings per share denominator⁽⁴⁾	4,512.1			5,486.5
Basic earnings per share (in dollars):	$	(.22	)	$	.20
Diluted earnings per share (in dollars):	$	(.22	)	$	.20

⁽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 three months ended March 31, 2017 and 2016.

⁽³⁾ Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 15 - “Employee Share-Based Compensation Plans”.

⁽⁴⁾The diluted earnings per share denominator for the three months ended March 31, 2016 includes the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding, which results in 882 incremental shares outstanding over the period. The incremental shares had an effect on diluted earning per share of less than $0.01 per share.

- Definition

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

+ References

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+ Details

Contingencies	3 Months Ended
Contingencies	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3 — “Loss and Loss Adjustment Expense Reserves”. White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of March 31, 2017: Tribune Company In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the U.S. District Court for the Southern District of New York (the “SDNY”). Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon and OneBeacon-sponsored benefit plans received approximately $32.0 million, for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs’ appealed. On March 29, 2016, a three judge panel of the U.S Second Circuit Court of Appeals affirmed the dismissal of the Noteholder Actions. On July 22, 2016, the Plaintiff's petition to the Second Circuit for reconsideration or for a rehearing en banc was denied in full. On September 9, 2016 the Plaintiffs filed for a writ of certiorari, seeking review in the U. S. Supreme Court. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the SDNY and was stayed pending the motion to dismiss in the Noteholder Actions. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014 and the motion was granted on January 6, 2017. The plaintiff has requested permission to move the SDNY to certify the decision as a final judgment capable of immediate appeal. No amount has been accrued in connection with this matter as of March 31, 2017, as the amount of loss, if any, cannot be reasonably estimated.

- Definition

The entire disclosure for loss and gain contingencies. Describes any existing condition, situation, or set of circumstances involving uncertainty as of the balance sheet date (or prior to issuance of the financial statements) as to a probable or reasonably possible loss incurred by an entity that will ultimately be resolved when one or more future events occur or fail to occur, and typically discloses the amount of loss recorded or a range of possible loss, or an assertion that no reasonable estimate can be made.

+ References

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-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14615-108349

+ Details

OneBeacon Pension Plan	3 Months Ended
OneBeacon Pension Plan	Mar. 31, 2017
Compensation Related Costs [Abstract]
OneBeacon Pension Plan	Note 19. OneBeacon Pension Plan OneBeacon previously sponsored the OneBeacon qualified pension plan (the “Qualified Plan”). During the three months ended March 31, 2016, the Qualified Plan finalized its termination by purchasing group annuity contracts from the Principal Financial Group (“Principal Financial”), and making lump sum distributions to Qualified Plan participants electing such payments, which eliminated the remaining Qualified Plan liability, and also ceased administratively paying benefits. As a result of these transactions, the Company recognized a pre-tax pension settlement charge of $0.3 million during the three months ended March 31, 2016, and no longer has a projected benefit obligation with respect to the Qualified Plan. OneBeacon transferred $47.1 million of excess invested assets from the Qualified Plan into the trust supporting the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (“KSOP”), which OneBeacon determined to be the Qualified Replacement Plan (“QRP”) with $13.8 million of excess invested assets remaining in the Qualified Plan trust as of March 31, 2017 in order to wind-down potential post-termination obligations of that plan, as approved by way of a March 2016 private letter ruling from the IRS. The invested assets related to both the legacy Qualified Plan trust and the QRP are included in other assets and are accounted for at fair value with related income recognized in net other revenues. OneBeacon continues to sponsor a non-qualified, non-contributory, defined benefit pension plan (“Non-qualified Plan”) covering certain employees who were employed as of December 31, 2001 and former employees who had met the eligibility requirements, as well as retirees. The Non-qualified Plan was frozen and curtailed in 2002, resulting in the pension benefit obligation being equal to the accumulated benefit obligation. The benefits are based primarily on years of service and employees’ compensation through December 31, 2002. OneBeacon’s funding policy is generally to contribute amounts to satisfy actual disbursements for the calendar year.

Subsequent Event Subsequent Event	3 Months Ended
Subsequent Event Subsequent Event	Mar. 31, 2017
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Note 20. Subsequent Event On May 2, 2017, OneBeacon announced it had entered into a definitive agreement to be acquired by Intact Financial Corporation in an all-cash transaction for $18.10 per share, or roughly 1.65x tangible book value (the “Transaction”). In addition, White Mountains entered into a definitive agreement to vote its shares of OneBeacon Ltd. in favor of the “Transaction”. White Mountains owns 75.7% of OneBeacon’s outstanding common shares, representing 96.9% of the voting power. White Mountains expects to receive gross proceeds of $1.3 billion from the Transaction, which is expected to close in the fourth quarter of 2017. It is subject to regulatory approval and other customary closing conditions.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2017
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued 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 simplifies certain aspects of the accounting for share-based compensation. 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. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption White Mountains reported $6.5 million and $5.8 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2017 and 2016. Such payments were classified as operating cash flows prior to adoption. In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity will recognize excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position. Short-Duration Contracts Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnotes disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, 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 March 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. 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 variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718). The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of March 31, 2017, the unamortized debt issuance costs included in debt is $1.8 million. Recently Issued Accounting Pronouncements Cash Flow Statement In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230). Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented. The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. The types of assets included in the scope of the new guidance includes premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance. 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 new guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings accordingly, does not expect the adoption of ASU 2016-01 to have a significant impact on its financial statements. Revenue Recognition In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is to be recognized at an amount that reflects the consideration to which an entity expects to be entitled in exchange for goods or services transferred to customers. The new guidance sets forth the steps to be followed to recognize revenue: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. Subsequently, the FASB issued additional ASUs clarifying the guidance in and providing implementation guidance for ASU 2014-09. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, which delays the effective date of ASU 2014-09 and all related ASUs to annual and interim reporting periods beginning after December 15, 2017. Most of White Mountains’s revenue from customers relates to insurance contracts, which are excluded from the scope of ASU 2014-09, as are investment income and investments gains and losses. However, the new guidance is applicable to some of White Mountains’s revenue streams, including certain fee arrangements as well as commissions and other non-insurance revenues. White Mountains is evaluating the new guidance, but does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s non-insurance revenues from customers.

Significant Transactions (Tables)

3 Months Ended

Mar. 31, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Reconciliation to the increase in book value

A reconciliation of the gain reported in discontinued operations to the impact to White Mountains's book value is as follows:



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 book value from sale of Tranzact	$	82.1

Loss and Loss Adjustment Expense Reserves (Tables)

3 Months Ended

Mar. 31, 2017

Insurance Loss Reserves [Abstract]

Loss and loss adjustment expense ("LAE") reserve activities



	Three Months Ended
	March 31,
Millions	2017			2016
Gross beginning balance	$	1,365.6		$	1,389.8
Less beginning reinsurance recoverable on unpaid losses	(172.9		)	(186.0		)
Net loss and LAE reserves	1,192.7			1,203.8

Add: SSIE reserves held for sale at beginning of the period⁽¹⁾	4.7			5.5

Loss and LAE incurred relating to:
Current year losses	151.7			161.2
Prior year losses	—			(.1		)
Total incurred losses and LAE	151.7			161.1

Loss and LAE paid relating to:
Current year losses	(21.5		)	(23.1		)
Prior year losses	(129.1		)	(148.6		)
Total loss and LAE payments	(150.6		)	(171.7		)

Less: Deconsolidation of SSIE⁽¹⁾	4.4			—

Less: SSIE reserves held for sale at end of the period ⁽¹⁾	—			5.3

Net ending balance	1,194.1			1,193.4
Plus ending reinsurance recoverable on unpaid losses	174.7			150.4
Gross ending balance	$	1,368.8		$	1,343.8

⁽¹⁾ Resulting from the sale of Star & Shield in the first quarter of 2017, SSIE is no longer consolidated. See Note 17 — “Held for Sale and Discontinued Operations”.

Investment Securities (Tables)

3 Months Ended

Mar. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Pre-tax net investment income

Pre-tax net investment income for the three months ended March 31, 2017 and 2016 consisted of the following:



	Three Months Ended
	March 31,
Millions	2017			2016
Investment income:
Fixed maturity investments	$	24.5		$	14.5
Short-term investments	.3			.2
Common equity securities	2.1			1.2
Other long-term investments	(.1		)	2.8
Total investment income	26.8			18.7
Third-party investment expenses	(.7		)	(.8		)
Net investment income, pre-tax	$	26.1		$	17.9

Schedule of Realized and Unrealized Gain (Loss) on Investments

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



	Three Months Ended
	March 31,
Millions	2017			2016
Net realized investment gains, pre-tax	$	1.1		$	256.8
Net unrealized investment gains (losses), pre-tax	50.2			(227.3		)
Net realized and unrealized investment gains, pre-tax	51.3			29.5
Income tax expense attributable to net realized and unrealized investment gains	(7.2		)	(8.5		)
Net realized and unrealized investment gains, after tax	$	44.1		$	21.0

Realized Gain (Loss) on Investments

Net realized investment gains (losses) for the three months ended March 31, 2017 and 2016 consisted of the following:



	Three Months Ended								Three Months Ended
	March 31, 2017								March 31, 2016
Millions	Net realized (losses) gains			Net foreign currency gains		Total net realized (losses) gains reflected in earnings			Net realized (losses) gains			Net foreign currency gains (losses)		Total net realized (losses) gains reflected in earnings
Fixed maturity investments	$	(2.2	)	$	.1	$	(2.1	)	$	(1.1	)	$	—	$	(1.1	)
Common equity securities	1.2			.1		1.3			257.6			—		257.6
Other long-term investments	1.9			—		1.9			.3			—		.3
Net realized investment gains, pre-tax	.9			.2		1.1			256.8			—		256.8
Income tax expense attributable to net realized investment gains	(.7		)	—		(.7		)	(42.9		)	—		(42.9		)
Net realized investment gains, after tax	$	.2		$	.2	$	.4		$	213.9		$	—	$	213.9

Schedule of unrealized gain (loss) on investments

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



	Three Months Ended									Three Months Ended
	March 31, 2017									March 31, 2016
Millions	Net unrealized gains			Net foreign currency gains (losses)			Total net unrealized gains reflected in earnings			Net unrealized gains (losses)			Net foreign currency gains		Total net unrealized gains (losses) reflected in earnings
Fixed maturity investments	$	17.5		$	1.7		$	19.2		$	21.7		$	—	$	21.7
Common equity securities	29.1			.5			29.6			(249.8		)	2.4		(247.4		)
Other long-term investments	4.2			.2			4.4			(2.0		)	.4		(1.6		)
Forward contracts	—			(3.0		)	(3.0		)	—			—		—
Net unrealized investment gains (losses), pre-tax	50.8			(.6		)	50.2			(230.1		)	2.8		(227.3		)
Income tax (expense) benefit attributable to net unrealized investment gains (losses)	(6.5		)	—			(6.5		)	34.4			—		34.4
Net unrealized investment gains (losses), after tax	$	44.3		$	(.6	)	$	43.7		$	(195.7	)	$	2.8	$	(192.9	)

Net unrealized investment gains (losses) for Level 3 investments



	Three Months Ended
	March 31,
Millions	2017			2016
Fixed maturity investments	$	.2		$	.5
Other long-term investments	(1.9		)	1.1
Total unrealized investment (losses) gains, pre-tax - Level 3 investments	$	(1.7	)	$	1.6

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



	March 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	$	110.7	$	—	$	(.4	)	$	—	$	110.3
Debt securities issued by corporations	1,680.2		10.5		(7.6		)	3.7		1,686.8
Municipal obligations	328.9		2.8		(1.4		)	—		330.3
Mortgage and asset-backed securities	2,020.0		5.1		(8.7		)	—		2,016.4
Foreign government, agency and provincial obligations	17.3		.4		—			.2		17.9
Preferred stocks	8.3		5.4		—			—		13.7
Total fixed maturity investments	$	4,165.4	$	24.2	$	(18.1	)	$	3.9	$	4,175.4



	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	$	281.7	$	.1	$	(3.5	)	$	—	$	278.3
Debt securities issued by corporations	1,512.6		8.4		(13.7		)	2.1		1,509.4
Municipal obligations	308.8		1.9		(1.7		)	—		309.0
Mortgage and asset-backed securities	2,141.7		2.6		(11.4		)	—		2,132.9
Foreign government, agency and provincial obligations	12.9		.3		—			—		13.2
Preferred stocks	8.3		5.7		—			—		14.0
Total fixed maturity investments	$	4,266.0	$	19.0	$	(30.3	)	$	2.1	$	4,256.8
Less: fixed maturity investments reclassified to assets held for sale related to SSIE										(6.6		)
Total fixed maturity investments										$	4,250.2



	March 31, 2017
Millions	Cost or amortized cost		Gross unrealized gains		Gross unrealized losses			Net foreign currency gains (losses)			Carrying value
Common equity securities	$	537.9	$	65.0	$	(2.4	)	$	.5		$	601.0
Other long-term investments	$	321.9	$	39.7	$	(23.3	)	$	(6.6	)	$	331.7



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

Other Long-term Investments

Other long-term investments consist of the following as of March 31, 2017 and December 31, 2016:



	Carrying Value at
Millions	March 31, 2017			December 31, 2016
Hedge funds and private equity funds, at fair value	$	147.2		$	131.0
Private equity securities and limited liability companies, at fair value ⁽¹⁾⁽²⁾	72.4			72.0
OneBeacon Surplus Notes, at fair value ⁽¹⁾	69.6			71.9
Private convertible preferred securities, at fair value ⁽¹⁾	30.7			30.6
Tax advantaged federal affordable housing development fund ⁽³⁾	11.7			12.3
Partnership investments accounted for under the equity method	3.0			3.5
Forward Contracts	(4.2		)	(1.2		)
Other	1.3			3.2
Total other-long term investments	$	331.7		$	323.3

⁽¹⁾See Fair Value Measurements by Level table.

⁽³⁾Fund accounted for using the proportional amortization method.

Other long-term investments

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



	March 31, 2017				December 31, 2016
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Hedge funds
Long/short banks and financials	$	54.2	$	—	$	36.5	$	—
Long/short equity REIT	19.7		—		19.9		—
Other	2.2		—		3.4		—
Total hedge funds	76.1		—		59.8		—

Private equity funds
Aerospace/Defense/Government	24.4		23.6		19.4		25.9
Manufacturing/Industrial	18.8		22.2		15.9		22.4
Multi-sector	11.0		2.0		11.4		2.0
Direct lending/Mezzanine debt	5.5		32.0		1.8		35.7
Healthcare	3.4		.4		3.5		.4
Energy infrastructure & services	2.8		3.0		14.1		3.2
Private equity secondaries	2.5		2.1		3.0		2.1
Financial Services	1.5		4.5		1.0		5.0
Insurance	.9		41.3		.8		41.3
Real estate	.3		.1		.3		.1
Total private equity funds	71.1		131.2		71.2		138.1
Total hedge funds and private equity funds included in other long-term investments	$	147.2	$	131.2	$	131.0	$	138.1

Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds

The following summarizes the March 31, 2017 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:



	Notice Period
Millions Redemption frequency	30-59 days notice		60-89 days notice		90-119 days notice		Total
Monthly	$	—	$	—	$	—	$	—
Quarterly	15.9		—		—		15.9
Semi-annual	38.3		19.7		—		58.0
Annual	—		—		2.2		2.2
Total	$	54.2	$	19.7	$	2.2	$	76.1

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

As of March 31, 2017, investments in private equity funds were subject to lock-up periods as follows:



Millions	1-3 years	3 – 5 years	5 – 10 years	>10 years	Total
Private Equity Funds — expected lock-up period remaining	$18.4	$23.3	$22.2	$7.2	$71.1

Par Value to Fair Value Reconciliation of Surplus Notes

Below is a table illustrating the valuation adjustments taken to arrive at estimated fair value of the OneBeacon Surplus Notes as of March 31, 2017 and December 31, 2016:



	Type of Surplus Note						Total as of March 31, 2017			Total as of December 31, 2016
Millions	Seller Priority			Pari Passu			Total as of March 31, 2017			Total as of December 31, 2016
Par Value	$	57.9		$	43.1		$	101.0		$	101.0
Fair value adjustments to reflect:
Current market rates on public debt and contract-based repayments⁽¹⁾	7.2			1.8			9.0			5.1
Regulatory approval ⁽²⁾	2.9			(13.2		)	(10.3		)	(15.6		)
Liquidity adjustment ⁽³⁾	(19.9		)	(10.2		)	(30.1		)	(18.6		)
Total adjustments	(9.8		)	(21.6		)	(31.4		)	(29.1		)
Fair value⁽⁴⁾	$	48.1		$	21.5		$	69.6		$	71.9

⁽¹⁾

⁽²⁾

Fair value measurements by level, investment securities



	March 31, 2017
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	110.3	$	100.8	$	9.5	$	—

Debt securities issued by corporations:
Consumer	362.9		—		362.9		—
Financials	263.3		—		263.3		—
Health Care	248.4		—		248.4		—
Utilities	229.3		—		229.3		—
Industrial	174.5		—		174.5		—
Communications	135.9		—		135.9		—
Technology	118.1		—		118.1		—
Materials	101.9		—		101.9		—
Energy	52.5		—		52.5		—
Total debt securities issued by corporations	1,686.8		—		1,686.8		—

Mortgage and asset-backed securities	2,016.4		—		1,958.1		58.3
Municipal obligations	330.3		—		330.3		—
Foreign government, agency and provincial obligations	17.9		.6		17.3		—
Preferred stocks	13.7		—		13.7		—
Total fixed maturity investments	4,175.4		101.4		4,015.7		58.3

Short-term investments⁽⁴⁾	230.9		227.9		3.0		—

Common equity securities:
Exchange traded funds ⁽¹⁾	384.8		330.6		54.2		—
Consumer	32.2		32.2		—		—
Health Care	27.1		27.1		—		—
Financials	18.2		18.2		—		—
Technology	16.1		16.1		—		—
Communications	13.9		13.9		—		—
Industrial	9.0		9.0		—		—
Energy	7.7		7.7		—		—
Materials	5.1		5.1		—		—
Utilities	1.0		1.0		—		—
Other	85.9		—		85.9		—
Total common equity securities	601.0		460.9		140.1		—

Other long-term investments ⁽²⁾⁽³⁾	174.0		—		—		174.0
Total investments	$	5,181.3	$	790.2	$	4,158.8	$	232.3

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

⁽³⁾Excludes carrying value of $147.2 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.

⁽⁴⁾Short-term investments are measured at amortized cost, which approximates fair value.



	December 31, 2016
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	278.3	$	268.8	$	9.5	$	—

Debt securities issued by corporations:
Consumer	385.6		—		385.6		—
Health Care	244.2		—		244.2		—
Utilities	180.3		—		180.3		—
Financials	176.0		—		176.0		—
Industrial	146.4		—		146.4		—
Communications	131.4		—		131.4		—
Materials	102.6		—		102.6		—
Technology	89.4		—		89.4		—
Energy	53.5		—		53.5		—
Total debt securities issued by corporations	1,509.4		—		1,509.4		—

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

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

Common equity securities:
Exchange traded funds⁽¹⁾	321.6		270.4		51.2		—
Health Care	20.9		20.9		—		—
Consumer	12.9		12.9		—		—
Financials	11.6		11.6		—		—
Technology	11.0		11.0		—		—
Communications	10.5		10.5		—		—
Energy	3.7		3.7		—		—
Industrial	2.2		2.2		—		—
Other	79.9		—		79.9		—
Total common equity securities	474.3		343.2		131.1		—

Other long-term investments⁽²⁾⁽³⁾	177.7		—		—		177.7
Total investments⁽⁴⁾	$	5,195.9	$	887.0	$	4,131.2	$	177.7

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

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

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

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

Debt securities issued by corporations, credit ratings

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



	Fair Value at
Millions	March 31, 2017		December 31, 2016
AA	93.1		100.9
A	484.7		381.9
BBB	846.5		786.5
BB	240.2		214.0
B	22.3		26.1
Debt securities issued by corporations⁽¹⁾	$	1,686.8	$	1,509.4

⁽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 Services ("Moody’s").

Mortgage-backed, asset-backed securities

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



	March 31, 2017						December 31, 2016
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
GNMA	$	249.0	$	249.0	$	—	$	283.9	$	283.9	$	—
FNMA	270.1		270.1		—		278.3		278.3		—
FHLMC	86.1		86.1		—		89.8		89.8		—
Total Agency⁽¹⁾	605.2		605.2		—		652.0		652.0		—
Non-agency:
Residential	309.9		251.6		58.3		205.3		205.3		—
Commercial	126.9		126.9		—		127.5		127.5		—
Total Non-agency	436.8		378.5		58.3		332.8		332.8		—

Total mortgage-backed securities	1,042.0		983.7		58.3		984.8		984.8		—
Other asset-backed securities:
Credit card receivables	383.0		383.0		—		438.3		438.3		—
Vehicle receivables	353.0		353.0		—		479.5		479.5		—
Other	238.4		238.4		—		230.3		230.3		—
Total other asset-backed securities	974.4		974.4		—		1,148.1		1,148.1		—
Total mortgage and asset-backed securities	$	2,016.4	$	1,958.1	$	58.3	$	2,132.9	$	2,132.9	$	—

Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities

The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of March 31, 2017 are as follows:



									Security Issuance Year
Millions	Fair Value		2004		2005		2006			2008		2010		2011		2012		2013		2014		2015		2016		2017
Non-agency RMBS	$	309.9	$	18.1	$	5.3	$	2.8		$	2.4	$	6.0	$	8.4	$	4.2	$	19.8	$	49.4	$	104.2	$	34.5	$	54.8
Non-agency CMBS	126.9		—		—		—			—		4.1		—		18.1		11.5		23.4		44.2		25.6		—
Total	$	436.8	$	18.1	$	5.3	$	2.8		$	2.4	$	10.1	$	8.4	$	22.3	$	31.3	$	72.8	$	148.4	$	60.1	$	54.8

Non-agency residential mortgage securities, collateral quality and tranche levels

The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of March 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Prime	$	309.9	$	104.9	$	205.0	$	—
Non-prime	—		—		—		—
Sub-prime	—		—		—		—
Total	$	309.9	$	104.9	$	205.0	$	—

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

Non-agency commercial mortgage securities, type of interest rate and tranche levels

The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of March 31, 2017:



Millions	Fair Value		Super Senior ⁽¹⁾		Senior ⁽²⁾		Subordinate ⁽³⁾
Fixed rate CMBS	$	115.2	$	1.6	$	70.1	$	43.5
Floating rate CMBS	11.7		—		—		11.7
Total	$	126.9	$	1.6	$	70.1	$	55.2

⁽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.

Rollforward of fair value investments by level

The following tables summarize the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2017 and 2016:



							Level 3 Investments
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Common equity securities		Other long-term investments			Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2017	$	612.5		$	4,118.5		$	—	$	—	$	177.7		$	131.0		$	5,039.7		^(1)(2)(4)
Total realized and unrealized gains (losses)	22.4			25.5			.2		—		(1.9		)	8.2			54.4
Amortization/Accretion	—			(6.5		)	—		—		—			—			(6.5		)
Purchases	124.5			890.4			58.1		—		.2			22.1			1,095.3
Sales	(197.1		)	(866.9		)			—		(2.0		)	(14.1		)	(1,080.1		)
Deconsolidation of SSIE	—			(5.2		)	—		—		—						(5.2		)
Transfers in	—			—			—		—		—			—			—
Transfers out	—			—			—		—		—			—			—
Balance at March 31, 2017	$	562.3		$	4,155.8		$	58.3	$	—	$	174.0		$	147.2		$	5,097.6		⁽¹⁾⁽²⁾

⁽²⁾Excludes carrying value of $287.1 and $230.9 at January 1, 2017 and March 31, 2017 associated with short-term investments, of which $0.1 is classified as held for sale at January 1, 2017.

⁽⁴⁾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
Millions	Level 1 investments			Level 2 investments			Fixed maturity investments		Common equity securities		Other long-term investments		Hedge Funds and Private Equity Funds measured at NAV⁽³⁾			Total
Balance at January 1, 2016	$	1,152.2		$	2,531.4		$	70.0	$	—	$	169.5	$	127.8		$	4,050.9		^(1)(2)(4)
Total realized and unrealized gains (losses)	11.3			19.1			.5		—		1.1		(2.4		)	29.6
Amortization/Accretion	—			(4.2		)	—		—		—		—			(4.2		)
Purchases	109.0			292.4			—		—		2.1		8.9			412.4
Sales	(852.9		)	(272.6		)	—		—		—		(3.0		)	(1,128.5		)
Transfers in	—			—			—		—		—		—			—
Transfers out	—			—			—		—		—		—			—
Balance at March 31, 2016	$	419.6		$	2,566.1		$	70.5	$	—	$	172.7	$	131.3		$	3,360.2		^(1)(2)(4)

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

⁽⁴⁾Includes carrying value of $9.5 and $9.4 of fixed maturity investments at January 1, 2016 and March 31, 2016 that is classified as assets held for sale related to SSIE.

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



Description	March 31, 2017
$ in millions, except share price	Rating⁽⁶⁾	Valuation Technique(s)	Fair Value⁽¹⁾	Unobservable Input
Non-agency residential mortgage-backed securities	AAA	Broker pricing	$58.3	Broker quote		102.56
Private equity security	NR	Share price of most recent transaction	$21.0	Share price	-	$1.00
Private equity security	NR	Discounted cash flow	$22.1	Discount rate	-	25.0%
Private equity security	NR	Share price of most recent transaction	$3.4	Share price	-	$2.52
Private convertible preferred security	NR	Multiple of EBITDA	$3.7	EBITDA multiple	-	6.00
Private convertible preferred security	NR	Share price of most recent transaction	$27.0	Share price	-	$3.83
Community development tax incentive investment	NR	Member share of GAAP net equity	$14.5	GAAP net equity		$14.5
Private equity security	NR	Discounted cash flow/ Option pricing method	$9.4	Discount rate	-	21.0%
				Time until expiration	-	4 years
			Volatility/Standard deviation		-	50.0%
			Risk free rate		-	1.00%
OneBeacon Surplus Notes:	NR
- Seller priority		Discounted cash flow	$48.1	Discount rate ⁽²⁾	-	10.6%
			Timing of interest payments ⁽⁴⁾		-	2020
			Timing of principal payments ⁽⁴⁾		-	2030
- Pari passu		Discounted cash flow	$21.5	Discount rate ⁽³⁾	-	15.0%
			Timing of interest payments ⁽⁵⁾		-	2021
			Timing of principal payments ⁽⁵⁾		-	2035

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

⁽⁶⁾ Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.



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

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

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

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

Goodwill and Other Intangible Assets (Tables)

3 Months Ended

Mar. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Intangible Assets and Goodwill

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



	Three Months Ended March 31,
Millions	2017								2016
	Goodwill		Other intangible assets			Total			Goodwill		Other intangible assets			Total
Beginning balance	$	31.7	$	24.2		$	55.9		$	24.1	$	31.3		$	55.4
Add: Amounts held for sale at beginning of the period⁽¹⁾	—		—			—			—		.4			.4
Acquisition of businesses	—		—			—			—		3.9			3.9
Amortization, including foreign currency translation	—		(2.9		)	(2.9		)	—		(3.1		)	(3.1		)
Less: Amounts held for sale at end of the period⁽¹⁾	—		—			—			—		.3			.3
Ending balance	$	31.7	$	21.3		$	53.0		$	24.1	$	32.2		$	56.3

⁽¹⁾See Note 17 — “Held for Sale and Discontinued Operations”.

Debt (Tables)

3 Months Ended

Mar. 31, 2017

Debt Disclosure [Abstract]

Schedule of debt outstanding

White Mountains’s debt outstanding as of March 31, 2017 and December 31, 2016 consisted of the following:



Millions	March 31, 2017			Effective Rate ⁽¹⁾	December 31, 2016			Effective Rate ⁽¹⁾
WTM Bank Facility	$	—		N/A	$	—		N/A
OBH Senior Notes, at face value	275.0			4.7%	275.0			4.7%
Unamortized original issue discount and debt issuance costs	(1.8		)		(1.8		)
OBH Senior Notes, carrying value	273.2				273.2
OneBeacon Bank Facility	—			N/A	—			N/A
MediaAlpha Bank Facility	11.7			6.1%	12.9			5.7%
Unamortized issuance cost	(.2		)		(.2		)
MediaAlpha Bank Facility, carrying value	11.5				12.7
Total debt	$	284.7			$	285.9

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

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

+ Details

Derivatives (Tables)

3 Months Ended

Mar. 31, 2017

Derivative Instruments and Hedging Activities Disclosures [Line Items]

Schedule of Collateral



	March 31, 2017
Millions	Notional Amount		Carrying Value			Standard & Poor's Rating ⁽¹⁾
Barclays Bank PLC	$	247.2	$	(3.2	)	A-
JP Morgan	52.7		(1.0		)	A-
Total	$	299.9	$	(4.2	)

⁽¹⁾Standard & Poor’s ratings “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).

Variable Annuity

Derivative Instruments and Hedging Activities Disclosures [Line Items]

Pre-tax operating results of WM Life Re

The following table summarizes the pre-tax operating results of WM Life Re for the three months ended March 31, 2016.



	Three Months Ended
	March 31,
Millions		2016
Fees, included in other revenue		$	1.0
Change in fair value of variable annuity liability, included in other revenue		(.4		)
Change in fair value of derivatives, included in other revenue		(1.7		)
Foreign exchange, included in other revenue		.9
Other investment income and losses		—
Total revenue		(.2		)
Death benefit claims paid, included in general and administrative expenses		(.1		)
General and administrative expenses		(.8		)
Pre-tax loss		$	(1.1	)

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



	Gains (losses)
	Three Months Ended			Carrying Value
	March 31,			As of
Millions	2016			December 31, 2016
Fixed income/interest rate	$	1.8		$	—
Foreign exchange	(4.2		)	—
Equity	.7			—
Total	$	(1.7	)	$	—

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

The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2016.



	Three Months Ended March 31, 2016
	Variable Annuity Liabilities			Derivative Instruments
Millions	Level 3			Level 3 ⁽¹⁾			Level 2 ⁽¹⁾⁽²⁾			Level 1 ⁽³⁾			Total
Beginning of period	$	.3		$	2.7		$	16.5		$	.9		$	20.1
Purchases	—			—			—			—			—
Realized and unrealized (losses) gains	(.4		)	1.2			1.1			(4.0		)	(1.7		)
Transfers in	—			—			—			—			—
Sales/settlements	—			(1.3		)	(7.3		)	3.3			(5.3		)
End of period	$	(.1	)	$	2.6		$	10.3		$	.2		$	13.1

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

⁽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.

Exchange traded and OTC derivative instruments

The following summarizes amounts offset under master netting agreements:



	March 31, 2016
Millions	Gross asset amounts before offsets ⁽¹⁾		Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other Assets
Interest rate contracts
OTC	$	2.2	$	(2.5	)	$	(.3	)
Foreign exchange contracts
OTC	12.7		—			12.7
Exchange traded	—		(.1		)	(.1		)
Equity contracts
OTC	1.5		(.9		)	.6
Exchange traded	.3		(.1		)	.2
Total⁽²⁾	$	16.7	$	(3.6	)	$	13.1

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

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

Net exposure to collateral



	March 31, 2016
Millions	Net amount of assets reflected in Balance Sheet		Collateral provided to counter-party - Cash		Collateral provided to counter-party - Financial Instruments		Net amount of exposure after effect of collateral provided		Excess collateral provided to counter-party- Cash		Excess collateral provided - Financial Instruments		Counter-party collateral held by WM Life Re- Cash		Net amount of exposure to counter-party		Standard & Poor's Rating⁽¹⁾
JP Morgan	$	6.5	$	—	$	—	$	6.5	$	—	$	—	$	5.4	$	1.1	A	+
Bank of America	1.4		—		—		1.4		—		—		—		1.4		A
Citigroup - OTC	5.0		—		—		5.0		—		—		.5		4.5		A
Citigroup - Exchange Traded	.2		—		—		.2		7.9		—		—		8.1		A
Total	$	13.1	$	—	$	—	$	13.1	$	7.9	$	—	$	5.9	$	15.1

“BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).

Municipal Bond Guarantee (Tables)

3 Months Ended

Mar. 31, 2017

Guarantees [Abstract]

Schedule of Municipal Bond Guarantee Insured Obligations

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



	March 31, 2017		December 31, 2016
Contracts outstanding	5,191		4,807
Remaining weighted average contract period outstanding (in years)	10.8		10.8
Contractual debt service outstanding (in millions):
Principal	$	35,222.1	$	33,057.3
Interest	17,589.9		16,396.6
Total debt service outstanding	$	52,812.0	$	49,453.9

Gross unearned insurance premiums	$	99.8	$	83.0

Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Table Text Block]

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



Millions	March 31, 2017
April 1, 2017 - December 31, 2017	$	6.3

January 1, 2018 - March 31, 2018	2.1
April 1, 2018 - June 30, 2018	2.0
July 1, 2018 - September 30, 2018	2.0
October 1, 2018 - December 31, 2018	2.0
	8.1

2019	7.8
2020	7.4
2021	7.0
2022 and thereafter	63.2
Total gross unearned insurance premiums	$	99.8

Earnings Per Share (Tables)

3 Months Ended

Mar. 31, 2017

Earnings Per Share [Abstract]

Computation of earnings per share

The following table outlines the Company’s computation of earnings per share from continuing operations for the three months ended March 31, 2017 and 2016. See Note 17 — “Held for Sale and Discontinued Operations”.



	Three Months Ended
	March 31,
	2017			2016
Basic and diluted earnings per share numerators (in millions):
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	$	35.3		$	11.9
Allocation of income for unvested restricted common shares	(.4		)	(.1		)
Dividends declared on participating restricted common shares⁽¹⁾	(.1		)	(.1		)
Total allocation to restricted common shares	(.5		)	(.2		)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	34.8		$	11.7
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$	34.8		$	11.7
Dividends declared net of restricted common share amounts⁽¹⁾	(4.5		)	(5.9		)
Total undistributed net earnings, net of restricted common share amounts	$	30.3		$	5.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,564.6			5,539.6
Average unvested restricted shares⁽²⁾	(52.5		)	(54.0		)
Basic earnings per share denominator	4,512.1			5,485.6
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period⁽³⁾	4,564.6			5,540.5
Average unvested restricted common shares⁽²⁾	(52.5		)	(54.0		)
Diluted earnings per share denominator⁽³⁾	4,512.1			5,486.5
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains’s common shareholders	$	7.72		$	2.14
Dividends declared and paid	(1.00		)	(1.00		)
Undistributed earnings	$	6.72		$	1.14
Diluted earnings per share (in dollars):
Net income (loss) attributable to White Mountains’s common shareholders	$	7.72		$	2.14
Dividends declared and paid	(1.00		)	(1.00		)
Undistributed earnings	$	6.72		$	1.14

⁽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 15 — “Employee Share-Based Compensation Plans”.

Non-controlling Interest (Tables)

3 Months Ended

Mar. 31, 2017

Noncontrolling Interest [Abstract]

Noncontrolling Interest Disclosure



	March 31, 2017					December 31, 2016
$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
OneBeacon Ltd.	24.3	%	$	250.7		23.9	%	$	244.6

Other, excluding mutuals and reciprocals
HG Global	3.1		16.5			3.1		16.6
MediaAlpha	40.0		11.0			40.0		11.7
Wobi	5.0		1.0			5.0		.7
Dewar	18.8		3.4			18.8		3.9
Buzzmove	29.1		2.8			29.1		2.9
Total other, excluding mutuals and reciprocals			34.7					35.8
Mutuals and reciprocals
BAM	100.0		(153.0		)	100.0		(150.9		)
SSIE	—		—			100.0		4.4
Total mutuals and reciprocals			(153.0		)			(146.5		)
Total non-controlling interests			$	132.4				$	133.9

Segment Information (Tables)

3 Months Ended

Mar. 31, 2017

Segment Reporting [Abstract]

Financial information for White Mountains' segments

Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows:



			HG Global/BAM
Millions	OneBeacon		HG		BAM⁽¹⁾			Other Operations			Total
Three Months Ended March 31, 2017
Earned insurance premiums	$	261.8	$	1.5	$	.5		$	1.0		$	264.8
Net investment income	12.2		.6		2.0			11.3			26.1
Net investment income (loss) - surplus note interest	—		4.8		(4.8		)	—			—
Net realized and unrealized investment gains	15.0		.3		1.0			35.0			51.3
Other revenue	3.4		—		.4			39.9		⁽²⁾	43.7
Total revenues	292.4		7.2		(.9		)	87.2			385.9
Losses and LAE	150.6		—		—			1.1			151.7
Insurance acquisition expenses	45.3		.3		.9			.1			46.6
Other underwriting expenses	51.7		—		.1			—			51.8
General and administrative expenses	4.7		.3		10.3			76.0		⁽³⁾	91.3
Amortization of other intangible assets	.3		—		—			2.6			2.9
Interest expense	3.3		—		—			.4			3.7
Total expenses	255.9		.6		11.3			80.2			348.0
Pre-tax income (loss)	$	36.5	$	6.6	$	(12.2	)	$	7.0		$	37.9



			HG Global/BAM
Millions	OneBeacon		HG		BAM⁽¹⁾			Other Operations				Total
Three Months Ended March 31, 2016
Earned insurance premiums	$	278.6	$	.9	$	.3		$	2.3			$	282.1
Net investment income	14.4		.5		1.6			1.4				17.9
Net investment income (loss) - surplus note interest	—		4.5		(4.5		)	—				—
Net realized and unrealized investment gains	16.6		2.1		4.9			5.9				29.5
Other revenue	.9		—		.1			38.8			⁽²⁾	39.8
Total revenues	310.5		8.0		2.4			48.4				369.3
Losses and LAE	158.8		—		—			2.3				161.1
Insurance acquisition expenses	51.0		.2		.7			.8				52.7
Other underwriting expenses	55.3		—		.1			—				55.4
General and administrative expenses	3.6		.5		9.2			70.7			⁽³⁾	84.0
Amortization of other intangible assets	.3		—		—			2.8				3.1
Interest expense	3.3		—		—			1.2				4.5
Total expenses	272.3		.7		10.0			77.8				360.8
Pre-tax income (loss)	$	38.2	$	7.3	$	(7.6	)	$	(29.4	)		$	8.5

⁽²⁾Includes $32.5 from MediaAlpha for the three months ended March 31, 2017, and $32.7 from MediaAlpha for the three months ended March 31, 2016.

⁽³⁾Includes $30.9 from MediaAlpha for the three months ended March 31, 2017, and $30.5 from MediaAlpha for the three months ended March 31, 2016.

Employee Share-Based Incentive Compensation Plans (Tables)

3 Months Ended

Mar. 31, 2017

White Mountains

Share-based compensation arrangement by share-based payment award

Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan

The following table summarizes performance share activity for the three months ended March 31, 2017 and 2016 for performance shares granted under the WTM Incentive Plan:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense			Target Performance Shares Outstanding		Accrued Expense
Beginning of period	80,353		$	42.4		93,654		$	57.7
Shares paid or expired⁽¹⁾	(30,167	)	(20.8		)	—		—
New grants	16,460		—			16,215		—
Forfeitures and cancellations⁽²⁾	(9,841	)	(5.7		)	(1,186	)	(.6		)
Expense recognized	—		8.1			—		14.2
End of period⁽³⁾	56,805		$	24.0		108,683		$	71.3

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

⁽³⁾Outstanding performance share awards as of March 31, 2017 and 2016 exclude 2,195 and 10,826 performance share awards granted to employees of Sirius Group.

Share-based Compensation, Performance Shares Award Outstanding Activity [Table Text Block]

The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at March 31, 2017 for each performance cycle:



Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense
Performance cycle:
2017 – 2019	16,460		$	1.2
2016 – 2018	19,415		7.3
2015 – 2017	21,795		15.8
Sub-total	57,670		24.3
Assumed forfeitures	(865	)	(.3		)
March 31, 2017	56,805		$	24.0

Summary of restricted shares activity

The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2017 and 2016:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	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
Issued	16,735		15.8			21,215		16.8
Vested	(22,015	)	—			(24,620	)	—
Forfeited	(5,200	)	(2.8		)	(800	)	(.3		)
Expense recognized	—		(3.6		)	—		(3.0		)
End of period⁽¹⁾	60,140		$	29.1		66,470		$	29.2

OneBeacon [Member]

Share-based compensation arrangement by share-based payment award

Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan

The following table summarizes performance share activity for the three months ended March 31, 2017 and 2016 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:



	Three Months Ended March 31,
	2017				2016
Millions, except share amounts	Target Performance Shares Outstanding		Accrued Expense		Target Performance Shares Outstanding		Accrued Expense
Beginning of period	452,519		$	1.6	449,435		$	1.4
Shares paid or expired⁽¹⁾	(142,710	)	—		(167,300	)	(.7		)
New grants	—		—		163,150		—
Assumed forfeitures and cancellations⁽²⁾	(13	)	—		(4,079	)	—
Expense recognized	—		.3		—		.4
End of period	309,796		$	1.9	441,206		$	1.1

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

Share-based Compensation, Performance Shares Award Outstanding Activity [Table Text Block]

The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at March 31, 2017 for each performance cycle:



Millions, except share amounts	Target Performance Shares Outstanding	Accrued Expense
Performance cycle:
2016 – 2018	163,150	$	1.1
2015 – 2017	146,646	.8
Sub-total	309,796	1.9
Assumed forfeitures	—	—
March 31, 2017	309,796	$	1.9

Summary of restricted shares activity

The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three months ended March 31, 2017 and 2016:



	Three Months Ended March 31,
	2017					2016
Millions, except share amounts	Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
Non-vested,
Beginning of period	395,872		$	2.1		382,722		$	2.5
Issued	461,160		7.4			170,650		2.3
Vested	(157,500	)	—			(157,500	)	—
Forfeited	—		—			—		—
Expense recognized	—		(.9		)	—		(.5		)
End of period	699,532		$	8.6		395,872		$	4.3

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2017

Fair Value Disclosures [Abstract]

Summary of the fair value and carrying value of financial instruments

The following table summarizes the fair value and carrying value of these financial instruments as of March 31, 2017 and December 31, 2016:



	March 31, 2017				December 31, 2016
Millions	Fair Value		Carrying Value		Fair Value		Carrying Value
OBH Senior Notes	$	279.4	$	273.2	$	274.2	$	273.2
MediaAlpha Bank Facility	11.7		11.5		13.0		12.7

Discontinued Operations (Tables)

3 Months Ended

Mar. 31, 2017

Disposal Groups, Including Discontinued Operations [Table Text Block]

Summary of Reclassified Balances and Related Items

Net Assets Held for Sale

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



Millions	December 31, 2016
Assets held for sale
Fixed maturity investments, at fair value	$	6.6
Short-term investments, at amortized cost (which approximates fair value)	.2
Total investments	6.8
Cash	.9
Reinsurance recoverable on unpaid losses	.3
Insurance and reinsurance premiums receivable	1.5
Other assets	.6
Total assets held for sale	$	10.1
Liabilities held for sale
Loss and loss adjustment expense reserves	$	5.0
Unearned insurance and reinsurance premiums	1.2
Other liabilities	(1.1		)
Total liabilities held for sale	5.1
Net assets held for sale	$	5.0

Net Income (Loss) from Discontinued Operations



	Three Months Ended								Three Months Ended
	March 31, 2017								March 31, 2016
Millions	Sirius Group		Other Disc Ops			Total			Sirius Group			Other Disc Ops			Total
Revenues
Earned insurance premiums	$	—	$	—		$	—		$	202.4		$	—		$	202.4
Net investment income	—		—			—			12.2			—			12.2
Net realized and unrealized losses	—		—			—			(8.8		)	—			(8.8		)
Other revenue	—		—			—			(4.1		)	57.8			53.7
Total revenues	—		—			—			201.7			57.8			259.5
Expenses
Loss and loss adjustment expenses	—		—			—			113.7			—			113.7
Insurance and reinsurance acquisition expenses	—		—			—			48.4			—			48.4
Other underwriting expenses	—		—			—			26.7			—			26.7
General and administrative expenses	—		—			—			8.1			53.4			61.5
Interest expense	—		—			—			6.6			1.4			8.0
Total expenses	—		—			—			203.5			54.8			258.3
Pre-tax (loss) income	—		—			—			(1.8		)	3.0			1.2
Income tax expense (benefit)	—		—			—			.9			(1.0		)	(.1		)
Net (loss) income from discontinued operations	—		—			—			(.9		)	2.0			1.1
Net loss from sale of discontinued operations	—		(1.0		)	(1.0		)	—			—			—
Total (loss) income from discontinued operations	$	—	$	(1.0	)	$	(1.0	)	$	(.9	)	$	2.0		$	1.1
Change in foreign currency translation and other from discontinued operations	—		—			—			37.2			—			37.2
Comprehensive (loss) income from discontinued operations	$	—	$	(1.0	)	$	(1.0	)	$	36.3		$	2.0		$	38.3

Net Change in Cash from Discontinued Operations



	Three Months Ended
	March 31,
(Millions)	2017			2016
Net cash (used for) provided from operations	$	—		$	(40.7	)
Net cash provided from investing activities	—			33.6
Net cash (used for) provided from financing activities	—			(8.3		)
Effect of exchange rate changes on cash	—			4.2
Net change in cash during the period	—			(11.2		)
Cash balances at beginning of period	.9			150.2
Net change in cash held for sale	(.9		)	.9
Cash balances at end of period	$	—		$	139.9
Supplemental cash flows information:
Interest paid	$	—		$	(1.4	)
Net income tax payment to national governments	$	—		$	(36.4	)

Earnings Per Share

Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three months ended March 31, 2017 and 2016:



	Three Months Ended
	March 31,
	2017			2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$	(1.0	)	$	1.1
Allocation of income for participating unvested restricted common shares⁽¹⁾	—			—
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts⁽²⁾	$	(1.0	)	$	1.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	4,564.6			5,539.6
Average unvested restricted common shares⁽³⁾	(52.5		)	(54.0		)
Basic earnings per share denominator	4,512.1			5,485.6
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period⁽⁴⁾	4,564.6			5,540.5
Average unvested restricted common shares⁽³⁾	(52.5		)	(54.0		)
Diluted earnings per share denominator⁽⁴⁾	4,512.1			5,486.5
Basic earnings per share (in dollars):	$	(.22	)	$	.20
Diluted earnings per share (in dollars):	$	(.22	)	$	.20

⁽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 three months ended March 31, 2017 and 2016.

⁽³⁾ Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 15 - “Employee Share-Based Compensation Plans”.

- Definition

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

+ References

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

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

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

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721683-107760

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64862545&loc=SL51724579-110230

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5C
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721663-107760

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721659-107760

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5A
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721671-107760

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5D
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721677-107760

+ Details

Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 31, 2012
Basis of Presentation
Debt Issuance Cost	$ 1.8
OneBeacon Ltd.
Basis of Presentation
Ownership interest (as a percent)	75.70%	76.10%
HG Global
Basis of Presentation
Noncontrolling equity percentage	3.10%	3.10%
Surplus Notes				$ 503.0
Percentage of par value of policy reinsured	15.00%
OneBeacon [Member]
Basis of Presentation
Noncontrolling equity percentage	24.30%	23.90%
Debt Issuance Cost			$ 1.9
Preferred Stock \| HG Global
Basis of Presentation
Noncontrolling equity percentage		96.90%
Ownership interest (as a percent)	96.90%
Common Stock \| HG Global
Basis of Presentation
Noncontrolling equity percentage		88.40%
Ownership interest (as a percent)	88.40%

Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions				1 Months Ended	2 Months Ended				3 Months Ended							12 Months Ended
	Aug. 04, 2016 USD ($)	Apr. 18, 2016 USD ($)	Feb. 01, 2016 USD ($) $ / shares	Feb. 01, 2016 $ / shares	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 ILS (₪)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 ILS (₪)	Dec. 31, 2015 USD ($)	Mar. 14, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt									$ 1.2					$ 0.0
Payments to Acquire Other Investments									22.4					10.9
Investments									5,339.0	$ 5,334.8							$ 5,334.8
Sales of common equity securities									17.7					767.5
Payments to Acquire Businesses and Interest in Affiliates									0.0					8.1
Other Income									43.7					39.8
Assets									6,516.9	6,544.7							6,544.7
Goodwill									31.7	31.7			$ 24.1	24.1			31.7		$ 24.1
Intangible Assets, Net (Excluding Goodwill)									21.3	24.2			32.2	32.2			24.2		$ 31.3
Liabilities									2,759.3	2,807.5							2,807.5
Net Income (Loss) Attributable to Noncontrolling Interest									1.3					(6.3)
Net (loss) gain on sale of discontinued operations, net of tax									$ (1.0)					0.0
Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)	70.90%
Assets	$ 11.5
Goodwill	7.6
Intangible Assets, Net (Excluding Goodwill)	1.1
Liabilities	0.1
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						56.20%
Goodwill						$ 0.3
Liabilities						1.2
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)									95.00%
Goodwill								$ 5.5
Liabilities								$ 0.7
QuoteLab [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																				60.00%
Payments to Acquire Interest in Subsidiaries and Affiliates															$ 28.1
Assets																				$ 70.1
Goodwill																				18.3
Intangible Assets, Net (Excluding Goodwill)																				38.5
Liabilities																				$ 10.0
Percent of gross profit																		62.50%	62.50%
Additional cash that will be paid as part of the purchase price													7.8	7.8
LowFares.com
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates													$ 3.9	3.9
Common Stock \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)								54.00%
United Kingdom, Pounds \| Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	6.1
Israel, New Shekels \| Common Stock \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries \| ₪																₪ 35.9
Israel, New Shekels \| Preferred Stock [Member] \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪							₪ 31.5											₪ 79.6
Israel, New Shekels \| Common Stock \| Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪					₪ 9.5													26.4
Israel, New Shekels \| Common Stock \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates \| ₪							₪ 14.4											₪ 11.8
United States of America, Dollars \| Buzzmove [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 8.1
United States of America, Dollars \| Common Stock \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries																	9.6
United States of America, Dollars \| Preferred Stock [Member] \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								$ 9.0											$ 20.7
United States of America, Dollars \| Common Stock \| Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						2.4													6.5
United States of America, Dollars \| Common Stock \| WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								4.1											3.1
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets						5.5		13.4
Intangible Assets, Net (Excluding Goodwill)						$ 2.8		$ 2.9
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Other Investments		$ 161.8										$ 161.8
Investments																			686.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 2,600.0										$ 2,600.0
Net (loss) gain on sale of discontinued operations, net of tax											$ 4.0						363.2
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in foreign currency translation and other, net of tax																	113.3
Tranzact [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt											56.3
Increase of Book Value											82.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold										$ 1.2	221.3
Net (loss) gain on sale of discontinued operations, net of tax									$ (1.0)		51.9						51.9
Discontinued Operation, Tax Effect of Discontinued Operation											$ 30.2						$ 30.2
Crop insurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related from termination of agreement																			$ 3.0
Percentage of Quota Share Reinsurance Agreement																		100.00%	100.00%
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities			$ 658.0
Dividends Payable, Amount Per Share \| $ / shares			$ 32.00	$ 32.00
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities														$ 658.0
Common Stock, Dividends, Per Share, Cash Paid \| $ / shares				$ 0.50
Symetra \| Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments																			$ 528.6

Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Gross beginning balance	$ 1,365.6	$ 1,389.8
Less beginning reinsurance recoverable on unpaid losses	(172.9)	(186.0)
Net loss and LAE reserves	1,192.7	1,203.8
Loss and LAE incurred relating to current year losses	151.7	161.2
Net (favorable) and unfavorable loss reserve development	0.0	0.1
Total incurred losses and LAE	151.7	161.1
Current year losses	(21.5)	(23.1)
Prior year losses	(129.1)	(148.6)
Total loss and LAE payments	(150.6)	(171.7)
Net ending balance	1,194.1	1,193.4
Plus ending reinsurance recoverable on unpaid losses	174.7	150.4
Gross ending balance	1,368.8	1,343.8
Star & Shield Insurance Exchange
Net ending balance	0.0
Star & Shield Insurance Exchange
Net (favorable) and unfavorable loss reserve development		0.1
Star & Shield Insurance Exchange
Net loss and LAE reserves	4.7	5.5
Net ending balance	$ 4.4	$ 5.3

Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Jun. 30, 2016
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ 0.0	$ (0.1)
Loss and LAE incurred relating to current year losses	$ 151.7	161.2
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development		$ (0.1)
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development			$ 0.0

Third Party Reinsurance (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 3.4	$ 6.6
Reinsurance recoverable on unpaid losses	174.7	172.9
OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	3.4	6.6
Reinsurance recoverable on unpaid losses	$ 174.7	$ 172.9
Property Catastrophe Program [Member] \| OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation	$ 110.0
Property Catastrophe Program [Member] \| Minimum \| OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	20.0
Property Catastrophe Program [Member] \| Maximum [Member] \| OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 130.0

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Investment income
Total investment income	$ 26.8	$ 18.7
Third-party investment expenses	(0.7)	(0.8)
Net investment income, pre-tax	26.1	17.9
Fixed maturity investments.
Investment income
Investment Income, Net, Amortization of Discount and Premium	24.5	14.5
Short-term investments
Investment income
Investment Income, Net, Amortization of Discount and Premium	0.3	0.2
Common Stock
Investment income
Investment Income, Net, Amortization of Discount and Premium	2.1	1.2
Other long-term investments
Investment income
Investment Income, Net, Amortization of Discount and Premium	$ (0.1)	$ 2.8

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	$ 1.1	$ 256.8
Net unrealized investment gains (losses), pre-tax	50.2	(227.3)
Net realized and unrealized investment gains, pre-tax	51.3	29.5
Income tax expense attributable to net realized and unrealized investment gains	(7.2)	(8.5)
Net realized and unrealized investment gains, after tax	44.1	21.0
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	(2.1)	(1.1)
Net unrealized investment gains (losses), pre-tax	19.2	21.7
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	1.3	257.6
Net unrealized investment gains (losses), pre-tax	29.6	(247.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	1.9	0.3
Net unrealized investment gains (losses), pre-tax	$ 4.4	$ (1.6)

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 0.9	$ 256.8
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.7)	(42.9)
Trading securities realized holding gain (loss) on investments after tax	0.2	213.9
Foreign Currency Transaction Gain (Loss), Realized	0.2	0.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	0.2	0.0
Net realized investment gains, pre-tax	1.1	256.8
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.7)	(42.9)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	0.4	213.9
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(2.2)	(1.1)
Foreign Currency Transaction Gain (Loss), Realized	0.1	0.0
Net realized investment gains, pre-tax	(2.1)	(1.1)
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	1.2	257.6
Foreign Currency Transaction Gain (Loss), Realized	0.1	0.0
Net realized investment gains, pre-tax	1.3	257.6
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	1.9	0.3
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment gains, pre-tax	$ 1.9	$ 0.3

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 50.8	$ (230.1)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(6.5)	34.4
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	44.3	(195.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.6)	2.8
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.6)	2.8
Net unrealized investment gains (losses), pre-tax	50.2	(227.3)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(6.5)	34.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	43.7	(192.9)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	17.5	21.7
Foreign Currency Transaction Gain (Loss), Unrealized	1.7	0.0
Net unrealized investment gains (losses), pre-tax	19.2	21.7
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	29.1	(249.8)
Foreign Currency Transaction Gain (Loss), Unrealized	0.5	2.4
Net unrealized investment gains (losses), pre-tax	29.6	(247.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	4.2	(2.0)
Foreign Currency Transaction Gain (Loss), Unrealized	0.2	0.4
Net unrealized investment gains (losses), pre-tax	4.4	(1.6)
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	(3.0)	0.0
Net unrealized investment gains (losses), pre-tax	$ (3.0)	$ 0.0

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 50.8	$ (230.1)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	17.5	21.7
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	29.1	(249.8)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	4.2	(2.0)
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.7)	1.6
Level 3 Inputs \| Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.2	0.5
Level 3 Inputs \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (1.9)	$ 1.1

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,175.4	$ 4,250.2
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	110.7	281.7
Trading Securities, Unrealized Holding Gain	0.0	0.1
Trading Securities, Unrealized Holding Loss	(0.4)	(3.5)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	110.3	278.3
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,680.2	1,512.6
Trading Securities, Unrealized Holding Gain	10.5	8.4
Trading Securities, Unrealized Holding Loss	(7.6)	(13.7)
Net foreign currency gains (losses)	3.7	2.1
Fixed maturity investments, at fair value	1,686.8	1,509.4
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	328.9	308.8
Trading Securities, Unrealized Holding Gain	2.8	1.9
Trading Securities, Unrealized Holding Loss	(1.4)	(1.7)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	330.3	309.0
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,020.0	2,141.7
Trading Securities, Unrealized Holding Gain	5.1	2.6
Trading Securities, Unrealized Holding Loss	(8.7)	(11.4)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	2,016.4	2,132.9
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	17.3	12.9
Trading Securities, Unrealized Holding Gain	0.4	0.3
Trading Securities, Unrealized Holding Loss	0.0	0.0
Net foreign currency gains (losses)	0.2	0.0
Fixed maturity investments, at fair value	17.9	13.2
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	8.3	8.3
Trading Securities, Unrealized Holding Gain	5.4	5.7
Trading Securities, Unrealized Holding Loss	0.0	0.0
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	13.7	14.0
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,165.4	4,266.0
Trading Securities, Unrealized Holding Gain	24.2	19.0
Trading Securities, Unrealized Holding Loss	(18.1)	(30.3)
Net foreign currency gains (losses)	3.9	2.1
Total fixed maturity investments including assets held for sale	$ 4,175.4	4,256.8
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale		$ (6.6)	$ (9.4)	$ (9.5)

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,175.4	$ 4,250.2
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	537.9	440.8
Trading Securities, Unrealized Holding Gain	65.0	35.9
Trading Securities, Unrealized Holding Loss	(2.4)	(2.4)
Net foreign currency gains (losses)	0.5	0.0
Fixed maturity investments, at fair value	601.0	474.3
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	321.9	314.9
Trading Securities, Unrealized Holding Gain	39.7	40.3
Trading Securities, Unrealized Holding Loss	(23.3)	(28.0)
Net foreign currency gains (losses)	(6.6)	(3.9)
Fixed maturity investments, at fair value	$ 331.7	$ 323.3

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments	$ 22.4	$ 10.9
Other Investments	(331.7)		$ (323.3)
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(4.2)		(1.2)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(1.3)		(3.2)
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(147.2)		(131.0)
Limited liability companies and private equity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(72.4)		(72.0)
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(69.6)		(71.9)
Convertible Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(30.7)		(30.6)
Affordable Housing Development Fund
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(11.7)	$ (14.1)	(12.3)	$ (14.7)
Partnership investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(3.0)		(3.5)
Hedge Funds Equity R E I T [Member] \| Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.7		19.9
Unfunded Commitments	$ 0.0		$ 0.0
OneTitle [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Ownership interest (as a percent)	20.00%
Surplus Notes	$ 10.0

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Mar. 31, 2017 USD ($) fund	Dec. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments \| $	$ 331.7	$ 323.3
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments \| fund	5
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments \| fund	20
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Largest single fund investment \| $	$ 54.2	$ 36.5

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 76.1
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	147.2	$ 131.0
Unfunded Commitments	131.2	138.1
Private equity funds \| Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.8	14.1
Unfunded Commitments	3.0	3.2
Private equity funds \| Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	11.0	11.4
Unfunded Commitments	2.0	2.0
Private equity funds \| Direct Lending [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.5	1.8
Unfunded Commitments	32.0	35.7
Private equity funds \| Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.5	3.0
Unfunded Commitments	2.1	2.1
Private equity funds \| Financial Services Sector [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.5	1.0
Unfunded Commitments	4.5	5.0
Private equity funds \| Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.4	3.5
Unfunded Commitments	0.4	0.4
Private equity funds \| Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.9	0.8
Unfunded Commitments	41.3	41.3
Private equity funds \| Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.4	19.4
Unfunded Commitments	23.6	25.9
Private equity funds \| Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.8	15.9
Unfunded Commitments	22.2	22.4
Private equity funds \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1	71.2
Unfunded Commitments	131.2	138.1
Private equity funds \| Real Estate Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	0.3
Unfunded Commitments	0.1	0.1
Hedge funds \| Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.2	36.5
Unfunded Commitments	0.0	0.0
Hedge funds \| Hedge Funds Equity R E I T [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.7	19.9
Unfunded Commitments	0.0	0.0
Hedge funds \| Hedge Funds, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.2	3.4
Unfunded Commitments	0.0	0.0
Hedge funds \| Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	76.1	59.8
Unfunded Commitments	$ 0.0	$ 0.0

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 76.1
Redemption requests	2.2
30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.2
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.7
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.2
Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Monthly \| 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Monthly \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Monthly \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	15.9
Quarterly \| 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	15.9
Quarterly \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Quarterly \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Semi-annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.0
Semi-annual \| 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	38.3
Semi-annual \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.7
Semi-annual \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.2
Annual \| 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Annual \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0
Annual \| Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 2.2
September 2018 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of Investments	1
Other long-term investments	$ 38.3

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	$ (1.3)	$ 6.3
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1
1-3 years \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.4
3-5 years \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	23.3
5-10 years \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.2
Greater than 10 years \| Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 7.2

Investment Securities OneBeacon Surplus Notes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 23, 2014
Interest Paid	$ 0.2	$ 0.2
OneBeacon [Member]
Assets, Fair Value Adjustment	(31.4)		$ (29.1)
Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	$ (9.8)
Surplus Note
Interest Paid		$ 2.4
Surplus Note \| OneBeacon [Member]
Basis points	4.00%		2.50%
Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	$ (21.6)
Surplus Note
RBC Score	250.00%		250.00%
Par Value [Member] \| OneBeacon [Member]
Surplus Notes	$ 101.0		$ 101.0	$ 101.0
Par Value [Member] \| Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Surplus Notes	57.9
Par Value [Member] \| Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Surplus Notes	43.1
Fair Value [Member] \| OneBeacon [Member]
Surplus Notes	69.6		71.9
Fair Value [Member] \| Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Surplus Notes	48.1
Fair Value [Member] \| Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Surplus Notes	21.5
Current market rates on repayments [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	9.0		5.1
Current market rates on repayments [Member] \| Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	7.2
Current market rates on repayments [Member] \| Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	1.8
Regulatory Approval [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	(10.3)		(15.6)
Regulatory Approval [Member] \| Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	2.9
Regulatory Approval [Member] \| Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	(13.2)
Liquidity Adjustment [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	(30.1)		$ (18.6)
Liquidity Adjustment [Member] \| Seller Priority Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	(19.9)
Liquidity Adjustment [Member] \| Pari Passu Surplus Note [Member] \| OneBeacon [Member]
Assets, Fair Value Adjustment	$ (10.2)

Investment Securities (Fair Value Measurements) (Details) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1.0
Hedge Funds and Private Equity Funds [Member]
Other than Temporary Impairment Losses, Investments	$ 1.0	$ 5.0

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,686.8	1,509.4
AA \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	93.1	100.9
A \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	484.7	381.9
BBB \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	846.5	786.5
BB \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	240.2	214.0
Standard & Poor's, B Rating [Member] \| Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 22.3	$ 26.1

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Non-Agency Residential Mortgage [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.9
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 126.9	$ 127.5

- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	115.2
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	249.0	283.9
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	270.1	278.3
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	86.1	89.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	605.2	652.0
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	309.9	205.3
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.9	127.5
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	436.8	332.8
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,042.0	984.8
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	383.0	438.3
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	353.0	479.5
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	238.4	230.3
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	974.4	1,148.1
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,016.4	2,132.9
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,155.8	4,118.5	$ 2,566.1	$ 2,531.4
Level 2 Inputs \| Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	249.0	283.9
Level 2 Inputs \| Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	270.1	278.3
Level 2 Inputs \| Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	86.1	89.8
Level 2 Inputs \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	605.2	652.0
Level 2 Inputs \| Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	251.6	205.3
Level 2 Inputs \| Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.9	127.5
Level 2 Inputs \| Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	378.5	332.8
Level 2 Inputs \| Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	983.7	984.8
Level 2 Inputs \| Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	383.0	438.3
Level 2 Inputs \| Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	353.0	479.5
Level 2 Inputs \| Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	238.4	230.3
Level 2 Inputs \| Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	974.4	1,148.1
Level 2 Inputs \| Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,958.1	2,132.9
Level 3 Inputs \| Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.3	0.0
Level 3 Inputs \| Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.3	0.0
Level 3 Inputs \| Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.3	0.0
Level 3 Inputs \| Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
Level 3 Inputs \| Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 58.3	$ 0.0

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	309.9	$ 205.3
Residential Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	104.9
Residential Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	205.0
Residential Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Prime \| Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	309.9
Prime \| Residential Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	104.9
Prime \| Residential Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	205.0
Prime \| Residential Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime \| Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime \| Residential Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime \| Residential Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Non-prime \| Residential Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime \| Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime \| Residential Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime \| Residential Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Sub-prime \| Residential Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0.0

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,097.6	$ 5,039.7	$ 3,360.2	$ 4,050.9
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	0.0025
Fair value investments	$ 115.2
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.6
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1
Commercial Mortgage Backed Securities Fixed Rate [Member] \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	43.5
Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Commercial Mortgage Backed Securities Floating Rate [Member] \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate [Member] \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0
Commercial Mortgage Backed Securities Floating Rate [Member] \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.9	$ 127.5
Commercial Mortgage Backed Securities \| Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.6
Commercial Mortgage Backed Securities \| Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1
Commercial Mortgage Backed Securities \| Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 55.2

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Rollforward of Fair Value Measurements by Level (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 1,080,100,000	$ 1,128,500,000
Balance at January 1, 2017	5,039,700,000	4,050,900,000
Total realized and unrealized gains (losses)	(54,400,000)	29,600,000
Amortization/Accretion	(6,500,000)	(4,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	1,095,300,000	412,400,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	5,097,600,000	3,360,200,000
Other Investments	331,700,000		$ 323,300,000
Net realized and unrealized investment gains, after tax	44,100,000	21,000,000
Net realized and unrealized investment gains	51,300,000	29,500,000
Short-term investments, at amortized cost (which approximates fair value)	$ 230,900,000		287,000,000
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers	(5.00%)
Maximum price difference provided by pricing services	$ 1,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	197,100,000	852,900,000
Balance at January 1, 2017	612,500,000	1,152,200,000
Total realized and unrealized gains (losses)	22,400,000	11,300,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	124,500,000	109,000,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	562,300,000	419,600,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	866,900,000	272,600,000
Balance at January 1, 2017	4,118,500,000	2,531,400,000
Total realized and unrealized gains (losses)	25,500,000	19,100,000
Amortization/Accretion	(6,500,000)	(4,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	890,400,000	292,400,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	4,155,800,000	2,566,100,000
Fixed maturity investments. \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Balance at January 1, 2017	0	70,000,000
Total realized and unrealized gains (losses)	200,000	500,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,100,000	0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	58,300,000	70,500,000
Common equity securities \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Balance at January 1, 2017	0	0
Total realized and unrealized gains (losses)	0	0
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	0	0
Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	2,000,000
Balance at January 1, 2017	177,700,000	169,500,000
Total realized and unrealized gains (losses)	(1,900,000)	1,100,000
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	200,000	2,100,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	174,000,000	172,700,000
Hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	14,100,000	3,000,000
Balance at January 1, 2017	131,000,000	127,800,000
Total realized and unrealized gains (losses)	8,200,000	(2,400,000)
Amortization/Accretion	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	22,100,000	8,900,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2017	147,200,000	131,300,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	5,195,900,000
Balance at March 31, 2017	5,181,300,000
Fair value measured on a recurring basis \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	887,000,000
Balance at March 31, 2017	790,200,000
Fair value measured on a recurring basis \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	4,131,200,000
Balance at March 31, 2017	4,158,800,000
Fair value measured on a recurring basis \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	232,300,000
Fair value measured on a recurring basis \| Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	4,256,800,000
Balance at March 31, 2017	4,175,400,000
Fair value measured on a recurring basis \| Fixed maturity investments. \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	269,400,000
Balance at March 31, 2017	101,400,000
Fair value measured on a recurring basis \| Fixed maturity investments. \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	3,987,400,000
Balance at March 31, 2017	4,015,700,000
Fair value measured on a recurring basis \| Fixed maturity investments. \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	58,300,000
Fair value measured on a recurring basis \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	174,000,000
Fair value measured on a recurring basis \| Other long-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Fair value measured on a recurring basis \| Other long-term investments \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	177,700,000
Balance at March 31, 2017	174,000,000
Carrying value of investment accounted for using the equity method	3,000,000	3,700,000	3,500,000	$ 3,800,000
Fair value measured on a recurring basis \| Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	287,100,000
Balance at March 31, 2017	230,900,000
Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	274,400,000
Balance at March 31, 2017	227,900,000
Short-term investments, at amortized cost (which approximates fair value)		211,300,000	287,100,000	261,700,000
Fair value measured on a recurring basis \| Short-term investments \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	12,700,000
Balance at March 31, 2017	3,000,000
Fair value measured on a recurring basis \| Short-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2017	0
Balance at March 31, 2017	0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	0	(8,800,000)
Star & Shield Insurance Exchange \| Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments, at amortized cost (which approximates fair value)		300,000	100,000	100,000
Affordable Housing Development Fund
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	11,700,000	14,100,000	12,300,000	14,700,000
Affordable Housing Development Fund \| Fair value measured on a recurring basis \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	11,700,000		12,300,000
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale		$ 9,400,000	6,600,000	$ 9,500,000
Forward Contracts \| Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, fair value, net	(4,200,000.0)		$ (1,200,000.0)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	5,200,000
Star & Shield Insurance Exchange \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	5,200,000
Star & Shield Insurance Exchange \| Fixed maturity investments. \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange \| Common equity securities \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Star & Shield Insurance Exchange \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 0

Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 input
Standard & Poor's, AAA Rating \| Non-Agency Residential Mortgage [Member] \| Broker Pricing [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 58.3
Fair Value Measurements, Valuation Techniques	102.56
NR \| Private equity funds \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 22.1	$ 22.1
Fair Value Measurements, Valuation Techniques	0.25	0.25
NR \| Private equity funds \| Option pricing method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 9.4	$ 9.3
Fair Value Measurements, Valuation Techniques	0.21	0.21
NR \| Convertible Preferred Stock \| Multiple of EBITA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 3.7	$ 3.6
Unobservable Inputs \| input			0
Fair Value Measurements, Valuation Techniques	6	6.0
NR \| Community Reinvestment Vehicle [Member] \| GAAP net equity [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 14.5	$ 14.3
Fair Value Measurements, Valuation Techniques	14.5	14.3
NR \| Seller Priority Surplus Note [Member] \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 48.1	$ 51.1
Fair Value Measurements, Valuation Techniques	0.106	0.096
NR \| Pari Passu Surplus Note [Member] \| Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21.5	$ 20.8
Fair Value Measurements, Valuation Techniques	0.150	0.15
Investment A [Member] \| NR \| Private equity funds \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21.0	$ 21.0
Fair Value Measurements, Valuation Techniques	1	1.00
Investment B [Member] \| NR \| Private equity funds \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 3.4	$ 3.2
Fair Value Measurements, Valuation Techniques	2.521	2.52
Investment E [Member] \| NR \| Convertible Preferred Stock \| Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 27.0	$ 27.0
Fair Value Measurements, Valuation Techniques	3.83	3.83
Investment C \| NR \| Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Time Until Expiration of Security	4 years	4 years
Volatility Rate	50.00%	50.00%
Risk Free Rate	1.00%	1.00%
Standard & Poor's, B Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	4.00%	2.50%
Standard & Poor's, CCC Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	4.00%	2.50%

Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates	$ 0.0		$ 8.1
Goodwill	31.7	$ 24.1	24.1	$ 31.7	$ 24.1
Intangible Assets, Net (Excluding Goodwill)	$ 21.3	32.2	32.2	$ 24.2	$ 31.3
LowFares.com
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		$ 3.9	$ 3.9

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 31.7	$ 24.1	$ 31.7	$ 24.1
Intangible Assets, Net (Excluding Goodwill)	21.3	32.2	24.2	31.3
Goodwill and other intangible assets	53.0	56.3	55.9	55.4
Acquisition of businesses	0.0	3.9
Disposal Group, Including Discontinued Operation, Intangible Assets	0.0	0.3	0.0	0.4
Amortization of Intangible Assets	(2.9)	(3.1)
Intangible Asset and Goodwill [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of businesses		3.9
Disposal Group, Including Discontinued Operation, Intangible Assets	0.0	0.3	$ 0.0	$ 0.4
Amortization of Intangible Assets	$ (2.9)	$ (3.1)

Debt (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
Debt (Details) - USD ($) $ in Millions	Nov. 30, 2012	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2012	Dec. 31, 2016	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument
Total debt		$ 284.7			$ 285.9
Notional amount		299.9
Repayments of Debt		1.2	$ 0.0
Draw down of debt and revolving line of credit		$ 0.0	$ 102.5
MediaAlpha Bank Facility [Member]
Debt Instrument
Basis points		1.50%
2012 OBH Senior Notes
Debt Instrument
Proceeds from Issuance of Debt				$ 275.0
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 425.0
OB Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility		$ 65.0
One Beacon Holdings Inc [Member] \| 2012 OBH Senior Notes
Debt Instrument
Debt Instrument, Interest Rate, Effective Percentage	4.70%
Proceeds from Issuance of Debt	$ 272.9
Debt Instrument, Interest Rate, Stated Percentage	4.60%
Debt Instrument, Issue Price as Percentage of Par Value	99.90%
OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value		275.0			275.0
Unamortized original issue discount		(1.8)			(1.8)
Total debt		$ 273.2			$ 273.2
Debt Instrument, Interest Rate, Effective Percentage		4.70%			4.70%
White Mountains \| WTM Bank Facility
Debt Instrument
Total debt		$ 0.0			$ 0.0
MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument
Debt instrument at face value		$ 11.7			$ 12.9
Debt Instrument, Interest Rate, Effective Percentage		6.10%			5.70%
Unamortized Debt Issuance Expense		$ (0.2)			$ (0.2)
Line of Credit Facility, Fair Value of Amount Outstanding		11.5			$ 12.7
Total commitment under revolving credit facility						$ 20.0
Notional amount		15.0
Line of Credit Facility, Remaining Borrowing Capacity		5.0
MediaAlpha [Member] \| Term Loan [Member]
Debt Instrument
Repayments of Debt		1.2
Debt Instrument, Redemption, Period One [Member] \| OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Total debt		$ 75.0

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
Income Taxes (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Percent	10.30%			(114.10%)
U.S. statutory rate (as a percent)	35.00%	35.00%		35.00%
Income Tax Expense (Benefit)	$ 3.9			$ (9.7)
Tax Year 2012 [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets, Gross			$ 0.6
Operating Loss Carryforwards, Valuation Allowance			0.6
Tax Assessment Year 2010 to 2012 [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)			$ (3.5)
Tax Assessment Year 2007 to 2009 [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)				$ (12.8)
Discontinued Operations [Member] \| Tranzact [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)					$ (21.4)

Derivatives (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2015
Notional amount	$ 299.9
Collective account values of underlying variable annuities as a percentage of the guarantee value		113.00%

Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 385.9	$ 369.3
General and administrative expenses	(94.2)	(87.1)
Pre-tax income from continuing operations	$ 37.9	8.5
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue		1.0
Change in fair value of variable annuity liability, included in other revenue		(0.4)
Realized and unrealized gains (losses)		(1.7)
Foreign exchange, included in other revenue		0.9
Other investment income and losses		0.0
Total revenues		(0.2)
Liability for Future Policy Benefits, Other Increase (Decrease)		(0.1)
General and administrative expenses		(0.8)
Pre-tax income from continuing operations		$ (1.1)

Derivatives Realized and Unrealized Gains (Losses) (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Variable Annuity Reinsurance [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	$ 13.1
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(1.7)
Derivative, fair value, net	13.1	$ 0.0	$ 20.1
Derivative Transfers In (Out), Net	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	5.3
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	1.8
Derivative, fair value, net		0.0
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(4.2)
Derivative, fair value, net		0.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.7
Derivative, fair value, net		$ 0.0
Level 2 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	1.1
Derivative, fair value, net	10.3		16.5
Derivative Transfers In (Out), Net	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	7.3
Level 1 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(4.0)
Derivative, fair value, net	0.2		0.9
Derivative Transfers In (Out), Net	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	(3.3)
Fair Value, Inputs, Level 3 [Member] \| Variable Annuity
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0.1		(0.3)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(0.4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers	0.0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements	0.0
Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	1.2
Derivative, fair value, net	2.6		$ 2.7
Derivative Transfers In (Out), Net	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	$ 1.3

Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015
Variable Annuity Reinsurance [Line Items]
Notional amount	$ 299.9
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value			113.00%
White Mountains Life Re
Changes in derivative instruments
Beginning of period	$ 0.0	$ 20.1
Purchases		0.0
Realized and unrealized gains (losses)		(1.7)
Transfers in		0.0
Sales/settlements		(5.3)
End of period		13.1
White Mountains Life Re \| Level 2 Inputs
Changes in derivative instruments
Beginning of period		16.5
Purchases		0.0
Realized and unrealized gains (losses)		1.1
Transfers in		0.0
Sales/settlements		(7.3)
End of period		10.3
White Mountains Life Re \| Level 1 Inputs
Changes in derivative instruments
Beginning of period		0.9
Purchases		0.0
Realized and unrealized gains (losses)		(4.0)
Transfers in		0.0
Sales/settlements		3.3
End of period		0.2
Variable Annuity \| White Mountains Life Re \| Level 3 Inputs
Changes in derivative instruments
Beginning balance		(0.3)
Realized and unrealized gains (losses)		(0.4)
Transfers in		0.0
Sales/settlements		0.0
Ending balance		0.1
Purchases		0.0
Derivative [Member] \| White Mountains Life Re \| Level 3 Inputs
Changes in derivative instruments
Beginning of period		2.7
Purchases		0.0
Realized and unrealized gains (losses)		1.2
Transfers in		0.0
Sales/settlements		(1.3)
End of period		$ 2.6

Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) - White Mountains Life Re $ in Millions	Mar. 31, 2016 USD ($)
Offsetting Liabilities [Line Items]
Derivative Assets	$ 16.7
Derivative Liabilities	(3.6)
Net amounts recognized in Other assets	13.1
Over The Counter \| Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.2
Derivative Liabilities	(2.5)
Net amounts recognized in Other assets	(0.3)
Over The Counter \| Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	12.7
Derivative Liabilities	0.0
Net amounts recognized in Other assets	12.7
Over The Counter \| Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.5
Derivative Liabilities	(0.9)
Net amounts recognized in Other assets	0.6
Exchange Traded \| Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.0
Derivative Liabilities	(0.1)
Net amounts recognized in Other assets	(0.1)
Exchange Traded \| Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.3
Derivative Liabilities	(0.1)
Net amounts recognized in Other assets	$ 0.2

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Mar. 31, 2016 USD ($) Rating
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 13,100
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	13,100
Excess collateral provided to counterparty in cash	7,900
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	5,900
Net amount of exposure to counter-party	$ 15,100
S and P Credit Ratings \| Rating	23
Standard & Poor's, A Rating [Member] \| Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 1,400
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,400
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	1,400
Standard & Poor's, A Rating [Member] \| Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	5,000
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	5,000
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	500
Net amount of exposure to counter-party	4,500
Standard & Poor's, A Rating [Member] \| Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	200
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	200
Excess collateral provided to counterparty in cash	7,900
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	8,100
Standard & Poor's, A plus \| JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	6,500
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	6,500
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	5,400
Net amount of exposure to counter-party	$ 1,100

Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount	$ 299.9
Forward Contracts
Derivative [Line Items]
Notional amount	299.9
Derivative, fair value, net	(4.2)
Realized and unrealized gains (losses)	(3.0)
Other Investments [Member]
Derivative [Line Items]
Derivative Liability	(4.2)	$ (1.2)
United Kingdom, Pounds
Derivative [Line Items]
Notional amount	200.0
Euro Member Countries, Euro
Derivative [Line Items]
Notional amount	50.0
Barclays [Member] \| Standard & Poor's, A- Rating [Member] \| Forward Contracts
Derivative [Line Items]
Notional amount	247.2
Derivative, fair value, net	(3.2)
JP Morgan \| Standard & Poor's, A- Rating [Member] \| Forward Contracts
Derivative [Line Items]
Notional amount	52.7
Derivative, fair value, net	$ (1.0)

Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	3 Months Ended
Municipal Bond Guarantee (Details) $ in Millions	Jul. 31, 2012 USD ($)	Mar. 31, 2017 USD ($) yr Contract	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Contract
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ 37.9	$ 8.5
Net investment income (loss) - surplus note interest		$ 0.0	$ 0.0
BAM
Guarantor Obligations [Line Items]
Municipal bond contract \| Contract		5,191		4,807
Insured Financial Obligations, Remaining Weighted Average Contract Period		10 years 10 months	10 years 10 months
Municipal bond outstanding principal		$ 35,222.1		$ 33,057.3
Municipal bond outstanding interest		17,589.9		16,396.6
Municipal bond debt service outstanding		$ 52,812.0		49,453.9
Year of U.S. Treasury Rate \| yr		1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months		$ 6.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		8.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three		7.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five		7.0
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four		7.4
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten		63.2
Financial Guarantee Insurance Contracts, Unearned Premium Revenue		99.8		83.0
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 594.5
Surplus Notes	503.0
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ 6.6
HG Global \| Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		96.90%
HG Global \| Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		88.40%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ (12.2)
BAM
Guarantor Obligations [Line Items]
Pre-tax (loss) income		(12.2)	$ (7.6)
Net investment income (loss) - surplus note interest		$ (4.8)	(4.5)
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance				$ 467.4
Percentage of par value of policy reinsured		15.00%
HG Global
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ 6.6	7.3
Net investment income (loss) - surplus note interest		4.8	$ 4.5
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 14.5
Surplus Note \| BAM
Guarantor Obligations [Line Items]
Interest Income, Other		$ 4.8
Derivative, Fixed Interest Rate		3.78%		3.54%
Surplus Note \| Build America Mutual
Guarantor Obligations [Line Items]
Interest expense		$ 4.8
Combination One \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate \| yr		5
Combination Two \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate \| yr		3
US Treasury Rate [Member] \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Basis points		3.00%
Fixed interest rate [Member] \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate		8.00%
Fourth Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		$ 2.0
Third Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		2.0
Second Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		2.0
First Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		$ 2.1

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 35.3		$ 11.9
Allocation of income for unvested restricted common shares	(0.4)		(0.1)
Dividends declared on participating restricted common shares	(0.1)		(0.1)
Total allocation to restricted common shares	(0.5)		(0.2)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	34.8		11.7
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	34.8		11.7
Dividends declared net of restricted common share amounts	(4.5)		(5.9)
Total undistributed net earnings, net of restricted common share amounts	$ 30.3		$ 5.8
Basic earnings per share denominators
Total average common shares outstanding during the period	4,564,600		5,539,600
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,564,600		5,539,600
Average unvested restricted common shares	(52,500)		(54,000)
Average outstanding dilutive options to acquire common shares	4,564,600		5,540,500
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Basic earnings per share (in dollars):
Continuing operations	$ 7.72		$ 2.14
Dividends declared	(1.00)		(1.00)
Undistributed earnings (loss) (in dollars per share)	6.72		1.14
Earnings Per Share, Diluted [Abstract]
Continuing operations	7.72		2.14
Dividends declared	(1.00)		(1.00)
Undistributed earnings (loss) (in dollars per share)	6.72		$ 1.14
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			882
Earnings Per Share, Diluted	$ 7.50		$ 2.34
Discontinued Operations [Member]
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	$ 0.0		$ 0.0
Basic earnings per share denominators
Total average common shares outstanding during the period	4,564,600		5,540,500
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,564,600		5,540,500
Average unvested restricted common shares	(52,500)		(54,000)
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Earnings Per Share, Diluted [Abstract]
Earnings Per Share, Diluted	$ (0.22)		$ 0.20
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		120,000
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			125,000
Earnings Per Share, Diluted			$ 0.01

Non-controlling Interest (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 132.4	$ 133.9
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 34.7	$ 35.8
OneBeacon [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.30%	23.90%
Total non-controlling interests	$ 250.7	$ 244.6
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 16.5	$ 16.6
MediaAlpha [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 11.0	$ 11.7
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	5.00%	5.00%
Total non-controlling interests	$ 1.0	$ 0.7
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	18.80%	18.80%
Total non-controlling interests	$ 3.4	$ 3.9
Buzzmove [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	29.10%	29.10%
Total non-controlling interests	$ 2.8	$ 2.9
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (153.0)	$ (150.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	100.00%
Total non-controlling interests	$ 0.0	$ 4.4
Star & Shield Insurance Exchange \| Total mutual and reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (153.0)	$ (146.5)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Segment reporting information
Premiums Earned, Net	$ 264.8	$ 282.1
Net investment income	26.1	17.9
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	51.3	29.5
Other Income	43.7	39.8
Total revenues	385.9	369.3
Loss and loss adjustment expenses	151.7	161.1
Insurance Acquisition Expenses	46.6	52.7
Other Underwriting Expense	51.8	55.4
General and administrative expenses	94.2	87.1
Amortization of Intangible Assets	2.9	3.1
Interest expense	3.7	4.5
Total expenses	348.0	360.8
Pre-tax (loss) income	37.9	8.5
OneBeacon [Member]
Segment reporting information
Premiums Earned, Net	261.8	278.6
Net investment income	12.2	14.4
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	15.0	16.6
Other Income	3.4	0.9
Total revenues	292.4	310.5
Loss and loss adjustment expenses	150.6	158.8
Insurance Acquisition Expenses	45.3	51.0
Other Underwriting Expense	51.7	55.3
Interest expense	3.3	3.3
Total expenses	255.9	272.3
Pre-tax (loss) income	36.5	38.2
HG Global
Segment reporting information
Premiums Earned, Net	1.5	0.9
Net investment income	0.6	0.5
Net investment income (loss) - surplus note interest	4.8	4.5
Net realized and unrealized investment gains	0.3	2.1
Other Income	0.0	0.0
Total revenues	7.2	8.0
Loss and loss adjustment expenses	0.0	0.0
Insurance Acquisition Expenses	0.3	0.2
Other Underwriting Expense	0.0	0.0
Interest expense	0.0	0.0
Total expenses	0.6	0.7
Pre-tax (loss) income	6.6	7.3
BAM
Segment reporting information
Premiums Earned, Net	0.5	0.3
Net investment income	2.0	1.6
Net investment income (loss) - surplus note interest	(4.8)	(4.5)
Net realized and unrealized investment gains	1.0	4.9
Other Income	0.4	0.1
Total revenues	(0.9)	2.4
Loss and loss adjustment expenses	0.0	0.0
Insurance Acquisition Expenses	0.9	0.7
Other Underwriting Expense	0.1	0.1
Interest expense	0.0	0.0
Total expenses	11.3	10.0
Pre-tax (loss) income	(12.2)	(7.6)
Other Operations
Segment reporting information
Premiums Earned, Net	1.0	2.3
Net investment income	11.3	1.4
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	35.0	5.9
Other Income	39.9	38.8
Total revenues	87.2	48.4
Loss and loss adjustment expenses	1.1	2.3
Insurance Acquisition Expenses	0.1	0.8
Other Underwriting Expense	0.0	0.0
Interest expense	0.4	1.2
Total expenses	80.2	77.8
Pre-tax (loss) income	7.0	(29.4)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	91.3	84.0
Segment Reconciling Items [Member] \| OneBeacon [Member]
Segment reporting information
General and administrative expenses	4.7	3.6
Segment Reconciling Items [Member] \| HG Global
Segment reporting information
General and administrative expenses	0.3	0.5
Segment Reconciling Items [Member] \| BAM
Segment reporting information
General and administrative expenses	10.3	9.2
Segment Reconciling Items [Member] \| Other Operations
Segment reporting information
General and administrative expenses	76.0	70.7
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	2.9	3.1
General and Administrative Expense [Member] \| OneBeacon [Member]
Segment reporting information
Amortization of Intangible Assets	0.3	0.3
General and Administrative Expense [Member] \| HG Global
Segment reporting information
Amortization of Intangible Assets	0.0	0.0
General and Administrative Expense [Member] \| BAM
Segment reporting information
Amortization of Intangible Assets	0.0	0.0
General and Administrative Expense [Member] \| Other Operations
Segment reporting information
Amortization of Intangible Assets	2.6	2.8
MediaAlpha [Member] \| Other Operations
Segment reporting information
Other Income	32.5	32.7
General and administrative expenses	$ 30.9	$ 30.5

Investments in Unconsolidated Affiliates (Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended
	Feb. 01, 2016	Feb. 01, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments			$ 50.8	$ (230.1)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax			44.3	(195.7)
Sales of common equity securities			$ 17.7	767.5
Common shares, par value per share (in dollars per share)			$ 1.00			$ 1.00
Net realized and unrealized investment gains			$ 51.3	29.5
Symetra
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments					$ 258.8
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax					$ 241.1
Sales of common equity securities	$ 658.0
Common shares, par value per share (in dollars per share)	$ 32.00	$ 32.00
Net realized and unrealized investment gains				4.7
Symetra
Investments in unconsolidated affiliates
Common Stock, Dividends, Per Share, Cash Paid		$ 0.50
Sales of common equity securities				$ 658.0

- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References + Details

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	4,572,792		4,563,814
Accrued Expense
Accrued expense at the beginning of period	$ 140.0
Accrued expense at the end of the period	$ 83.2		$ 140.0
Performance Share \| W T M Incentive Plan \| Range Of Performance Cycle From 2014-2016 [Member]
Accrued Expense
Percentage of target payout - minimum actual	34.00%
Percentage of target payout - maximum actual	76.00%
Performance Share \| W T M Incentive Plan \| Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares			5,000
Accrued Expense
Percentage of target payout - minimum actual			140.00%
Percentage of target payout - maximum actual			142.00%
White Mountains \| Restricted Share Cliff Vesting in January 2020 [Member] \| W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	16,735
White Mountains \| Performance Share \| W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	80,353	93,654	93,654
Shares paid or expired (in shares)	(30,167)	0
New grants (in shares)	16,460	16,215
Assumed forfeitures and cancellations	(9,841)	(1,186)
Ending of period (in shares)	56,805	108,683	80,353
Accrued Expense
Accrued expense at the beginning of period	$ 42.4	$ 57.7	$ 57.7
Payments and deferrals	(20.8)	0.0
Assumed forfeitures and cancellations	(5.7)	(0.6)
Expense recognized	8.1	14.2
Accrued expense at the end of the period	$ 24.0	$ 71.3	$ 42.4
Vesting Period	3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 30.5
Sirius Group \| Performance Share \| W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	2,195	10,826

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 83.2	$ 140.0
White Mountains \| Performance Share \| W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	57,670
Accrued Expense	$ 24.3
Target Performance Shares Outstanding, Assumed forfeitures	(865)
Accrued Expense, Assumed forfeitures	(0.3)
Total Target Performance Shares Outstanding	56,805	80,353	108,683	93,654
Total Accrued Expense	$ 24.0	$ 42.4	$ 71.3	$ 57.7
White Mountains \| Performance Share \| W T M Incentive Plan \| Performance Cycle 2017 to 2019
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	16,460
Accrued Expense	$ 1.2
White Mountains \| Performance Share \| W T M Incentive Plan \| Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	19,415
Accrued Expense	$ 7.3
White Mountains \| Performance Share \| W T M Incentive Plan \| Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	21,795
Accrued Expense	$ 15.8

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
White Mountains \| Restricted Share Cliff Vesting in January 2020 [Member]
Target Restricted Share Activity
Issued (in shares)	16,735
White Mountains \| Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)		21,215
White Mountains \| Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	70,620	70,675
Issued (in shares)	16,735	21,215
Vested (in shares)	(22,015)	(24,620)
Assumed forfeitures and cancellations	(5,200)	(800)
Ending of period (in shares)	60,140	66,470
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 19.7	$ 15.7
Issued	15.8	16.8
Assumed forfeitures and cancellations	(2.8)	(0.3)
Expense recognized	(3.6)	(3.0)
Unamortized grant date fair value, ending balance	$ 29.1	$ 29.2
Sirius Group \| Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	5,235	2,195

Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Expense Amortized
Accrued expense at the beginning of period	$ 140.0
Accrued expense at the end of the period	$ 83.2		$ 140.0
OneBeacon [Member] \| Performance Share \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
OneBeacon [Member] \| One Beacon Long Term Incentive Plan \| Performance Cycle 2013 to 2015
Expense Amortized
Percentage of target payout - minimum actual			24.30%
OneBeacon [Member] \| One Beacon Long Term Incentive Plan \| Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	452,519	449,435	449,435
Shares paid or expired (in shares)	(142,710)	(167,300)
New grants (in shares)	0	163,150
Assumed forfeitures and cancellations	(13)	(4,079)
Ending of period (in shares)	309,796	441,206	452,519
Expense Amortized
Accrued expense at the beginning of period	$ 1.6	$ 1.4	$ 1.4
Payments and deferrals	0.0	(0.7)
Assumed forfeitures and cancellations	0.0	0.0
Expense recognized	0.3	0.4
Accrued expense at the end of the period	1.9	$ 1.1	$ 1.6
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 1.8

Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 83.2	$ 140.0
OneBeacon [Member] \| Performance Share \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	309,796
Incentive Compensation Payable before Forfeitures	$ 1.9
Target Performance Shares Outstanding, Assumed forfeitures	0
Accrued Expense, Assumed forfeitures	0.0
Total Target Performance Shares Outstanding	309,796	452,519	441,206	449,435
Accrued incentive compensation	$ 1.9	$ 1.6	$ 1.1	$ 1.4
OneBeacon [Member] \| Performance Share \| One Beacon Long Term Incentive Plan \| Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	163,150
Incentive Compensation Payable before Forfeitures	$ 1.1
OneBeacon [Member] \| Performance Share \| One Beacon Long Term Incentive Plan \| Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	146,646
Incentive Compensation Payable before Forfeitures	$ 0.8

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon [Member] - One Beacon Long Term Incentive Plan $ in Millions			1 Months Ended	3 Months Ended
	Feb. 28, 2017 shares	Feb. 24, 2016 shares	May 31, 2011 shares	Mar. 31, 2017 USD ($) shares	Mar. 31, 2016 USD ($) shares	May 25, 2011 Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				395,872	382,722
Issued (in shares)	461,160	170,650		461,160	170,650
Vested (in shares)				(157,500)	(157,500)
Assumed forfeitures and cancellations				0	0
Ending of period (in shares)				699,532	395,872
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance \| $				$ 2.1	$ 2.5
Issued \| $				7.4	2.3
Assumed forfeitures and cancellations \| $				0.0	0.0
Expense recognized \| $				(0.9)	(0.5)
Unamortized grant date fair value, ending balance \| $				$ 8.6	$ 4.3
Restricted Stock \| Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)			630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest \| Installment						4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years			35,000
Performance Share
Target Restricted Share Activity
Beginning of period (in shares)				452,519	449,435
Assumed forfeitures and cancellations				(13)	(4,079)
Ending of period (in shares)				309,796	441,206
Unamortized grant date fair value
Assumed forfeitures and cancellations \| $				$ 0.0	$ 0.0
Expense recognized \| $				(0.3)	$ (0.4)
Unamortized grant date fair value, ending balance \| $				$ 1.8
Performance Share \| Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares			35,000
Years Performance Shares will be Forfeited			5 years
Restricted Stock Units (RSUs) [Member]
Target Restricted Share Activity
Issued (in shares)				240,160	222,449
Ending of period (in shares)				208,499
Unamortized grant date fair value
Expense recognized \| $				$ (0.6)	$ (0.4)
Restricted Share Cliff Vesting in August 2018 [Member] \| Restricted Stock
Target Restricted Share Activity
Issued (in shares)	235,000
Restricted Share Cliff Vesting in January 2020 [Member] \| Restricted Stock
Target Restricted Share Activity
Issued (in shares)	115,450
Restricted Share Vesting in February 2018 and February 2019 [Member] \| Restricted Stock
Target Restricted Share Activity
Issued (in shares)	110,710
Restricted Share Vesting in February 2018 [Member] \| Restricted Stock
Target Restricted Share Activity
Issued (in shares)		92,500
Restricted Share Vesting in February 2019 [Member] \| Restricted Stock
Target Restricted Share Activity
Issued (in shares)		78,150

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - OneBeacon [Member] - USD ($) $ in Millions			3 Months Ended
	Feb. 28, 2017	Feb. 24, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units (RSUs) [Member] \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)			240,160	222,449
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number			208,499
Expense recognized			$ 0.6	$ 0.4
Restricted Stock \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	461,160	170,650	461,160	170,650
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number			699,532	395,872	395,872	382,722
Expense recognized			$ 0.9	$ 0.5
Additional compensation cost that would be recognized if all outstanding performance shares vested			8.6	$ 4.3	$ 2.1	$ 2.5
Scenario, Plan \| Restricted Stock Units (RSUs) [Member]
Share-based compensation arrangement by share-based payment award
Additional compensation cost that would be recognized if all outstanding performance shares vested			$ 7.0

Employee Share-Based Incentive Compensation Plans WTM Stock options (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Shares Issued, Price Per Share			$ 742
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		882
Employee Stock Option [Member]
Share-based compensation arrangement by share-based payment award
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		125,000
Common Stock [Member]
Share-based compensation arrangement by share-based payment award
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	5,142
Chief Executive Officer \| Employee Stock Option [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 4,400,000
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Exercised	125,000
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	40,000
Combination Two \| Employee Stock Option [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value			$ 8,400,000
Conversion of Stock, Shares Converted			9,930
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			80,000
Combination One \| Employee Stock Option [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value			$ 400,000
Shares Issued, Price Per Share			$ 742
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			5,000

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 284.7	$ 285.9
OneBeacon [Member] \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	273.2	273.2
OneBeacon [Member] \| Book Value \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	273.2	273.2
OneBeacon [Member] \| Estimate of Fair Value Measurement [Member] \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt Instrument, Fair Value Disclosure	279.4	274.2
MediaAlpha [Member] \| Book Value \| MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	11.5	12.7
MediaAlpha [Member] \| Estimate of Fair Value Measurement [Member] \| MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt Instrument, Fair Value Disclosure	$ 11.7	$ 13.0

Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended					12 Months Ended
Discontinued Operations (Details) - USD ($) $ in Millions	Apr. 18, 2016	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Payments to Acquire Other Investments		$ 22.4				$ 10.9
Investments		5,339.0	$ 5,334.8				$ 5,334.8
Net realized and unrealized investment gains		51.3				29.5
Net (loss) gain on sale of discontinued operations, net of tax		(1.0)				0.0
Net (loss) income from discontinued operations		0.0				1.1
Net change in foreign currency translation		0.9				0.1
Other liabilities		178.5	199.9				199.9
Income Tax Expense (Benefit)		3.9				(9.7)
Net Realized and Unrealized Gain (Loss) on Trading Securities		51.3				29.5
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600.0				$ 2,600.0
Payments to Acquire Other Investments	$ 161.8				$ 161.8
Investments								$ 686.2
Net realized and unrealized investment gains						4.1
Net (loss) gain on sale of discontinued operations, net of tax				$ 4.0			363.2
Net change in foreign currency translation		0.0				37.2
Sale of Sirius Group [Member]
Discontinued Operations [Line Items}
Change in foreign currency translation and other, net of tax							113.3
Tranzact [Member]
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 1.2	221.3
Net (loss) gain on sale of discontinued operations, net of tax		(1.0)		51.9			51.9
Discontinued Operation, Tax Effect of Discontinued Operation				$ 30.2			30.2
Discontinued Operations [Member]
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		0.0				(8.8)
Net (loss) income from discontinued operations		0.0				1.1
Net change in foreign currency translation		0.0				37.2
Discontinued Operation, Tax Effect of Discontinued Operation		0.0				(0.1)
Discontinued Operations [Member] \| Sirius Group
Discontinued Operations [Line Items}
Net realized and unrealized investment gains		0.0				(8.8)
Net (loss) income from discontinued operations		0.0				(0.9)
Net change in foreign currency translation		0.0				37.2
Discontinued Operation, Tax Effect of Discontinued Operation		$ 0.0				0.9
Discontinued Operations [Member] \| Tranzact [Member]
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations						$ (2.0)
Income Tax Expense (Benefit)							(21.4)
Tax Effect of Extraordinary Item, Gain (Loss)							30.2
Discontinued Operation, Tax Effect of Discontinued Operation							(8.8)
Discontinued Operations, Held-for-sale \| Star & Shield LLC [Member]
Discontinued Operations [Line Items}
Other Expenses							$ 21.0

Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	$ 4,175.4				$ 4,250.2
Short-term investments, at amortized cost (which approximates fair value)	230.9				287.0
Common equity securities, at fair value	601.0				474.3
Other long-term investments	331.7				323.3
Total investments	5,339.0				5,334.8
Cash	125.0				149.8
Reinsurance recoverable on unpaid losses	174.7				172.9
Reinsurance recoverable on paid losses	3.4				6.6
Insurance premiums receivable	227.5				229.9
Deferred acquisition costs	110.4				106.9
Deferred tax asset	125.5				126.7
Ceded unearned insurance premiums	52.4				44.2
Accounts receivable on unsettled investment sales	24.5				6.2
Goodwill and other intangible assets	53.0		$ 56.3		55.9	$ 55.4
Other assets	(254.3)				(274.6)
Assets held for sale	0.0				10.1
Loss and loss adjustment expense reserves	1,368.8		1,343.8		1,365.6	1,389.8
Unearned insurance premiums	678.1				658.0
Debt	284.7				285.9
Accrued incentive compensation	83.2				140.0
Funds held under insurance contracts	148.7				153.0
Accounts payable on unsettled investment purchases	17.3				0.0
Other liabilities	178.5				199.9
Total liabilities held for sale	0.0				5.1
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	264.8		282.1
Net investment income	26.1		17.9
Net realized and unrealized investment gains	51.3		29.5
Total revenues	385.9		369.3
Loss and loss adjustment expenses	151.7		161.1
Insurance Acquisition Expenses	46.6		52.7
Other Underwriting Expense	51.8		55.4
General and administrative expenses	94.2		87.1
Interest expense	3.7		4.5
Total expenses	348.0		360.8
Net (loss) income from discontinued operations	0.0		(1.1)
Net (loss) gain on sale of discontinued operations, net of tax	(1.0)		0.0
Net change in foreign currency translation	0.9		0.1
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	149.8		167.2		167.2
Cash balances at end of period	125.0		165.3		149.8
Net income tax payment (refund) from national governments	0.0		(13.5)
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments						$ 686.2
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains			4.1
Net (loss) gain on sale of discontinued operations, net of tax		$ 4.0			363.2
Net change in foreign currency translation	0.0		37.2
Tranzact [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation		30.2			30.2
Net (loss) gain on sale of discontinued operations, net of tax	(1.0)	$ 51.9			51.9
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net Change in Cash Reclassified to Assets Held-for-sale	(0.9)		0.9
Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0.0		202.4
Net investment income	0.0		12.2
Net realized and unrealized investment gains	0.0		(8.8)
Other revenue	0.0		53.7
Total revenues	0.0		259.5
Loss and loss adjustment expenses	0.0		113.7
Insurance Acquisition Expenses	0.0		48.4
Other Underwriting Expense	0.0		26.7
General and administrative expenses	0.0		61.5
Interest expense	0.0		8.0
Total expenses	0.0		258.3
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0		1.2
Discontinued Operation, Tax Effect of Discontinued Operation	0.0		(0.1)
Net (loss) income from discontinued operations	0.0		(1.1)
Net loss from discontinued operations, net of tax	(1.0)		1.1
Net change in foreign currency translation	0.0		37.2
Total comprehensive income	(1.0)		38.3
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash used for operations - discontinued operations	0.0		(40.7)
Net cash provided from investing activities - discontinued operations	0.0		33.6
Net cash used for financing activities - discontinued operations	0.0		(8.3)
Effect of exchange rate changes on cash	0.0		4.2
Net change in cash during the period	0.0		(11.2)
Cash balances at beginning of period	0.0		150.2		150.2
Cash balances at end of period	0.0		139.9		0.0
Interest paid	0.0		1.4
Net income tax payment (refund) from national governments	0.0		(36.4)
Discontinued Operations [Member] \| Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value					6.6
Short-term investments, at amortized cost (which approximates fair value)					0.2
Total investments					6.8
Cash					0.9
Reinsurance recoverable on unpaid losses					0.3
Insurance premiums receivable					1.5
Other assets					(0.6)
Assets held for sale					10.1
Loss and loss adjustment expense reserves					5.0
Unearned insurance premiums					1.2
Other liabilities					1.1
Total liabilities held for sale					5.1
Discontinued Operations [Member] \| Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Net Assets					5.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0.0		202.4
Net investment income	0.0		12.2
Net realized and unrealized investment gains	0.0		(8.8)
Other revenue	0.0		(4.1)
Total revenues	0.0		201.7
Loss and loss adjustment expenses	0.0		113.7
Insurance Acquisition Expenses	0.0		48.4
Other Underwriting Expense	0.0		26.7
General and administrative expenses	0.0		8.1
Interest expense	0.0		6.6
Total expenses	0.0		203.5
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0		(1.8)
Discontinued Operation, Tax Effect of Discontinued Operation	0.0		0.9
Net (loss) income from discontinued operations	0.0		0.9
Net loss from discontinued operations, net of tax	0.0		(0.9)
Net change in foreign currency translation	0.0		37.2
Total comprehensive income	0.0		36.3
Discontinued Operations [Member] \| Tranzact [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation					(8.8)
Net (loss) income from discontinued operations			2.0
Discontinued Operations [Member] \| Tranzact [Member] \| Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain on sale of discontinued operations, net of tax	(1.0)
Discontinued Operations [Member] \| Other Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
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 revenue	0.0		57.8
Total revenues	0.0		57.8
Loss and loss adjustment expenses	0.0		0.0
Insurance Acquisition Expenses	0.0		0.0
Other Underwriting Expense	0.0		0.0
General and administrative expenses	0.0		53.4
Interest expense	0.0		1.4
Total expenses	0.0		54.8
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0		3.0
Discontinued Operation, Tax Effect of Discontinued Operation	0.0		(1.0)
Net (loss) income from discontinued operations	0.0		(2.0)
Net loss from discontinued operations, net of tax	(1.0)		2.0
Total comprehensive income	(1.0)		2.0
Discontinued Operations [Member] \| OneBeacon Runoff [Member] \| Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain on sale of discontinued operations, net of tax				$ 0.0
Discontinued Operations, Held-for-sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	0.9		150.2		150.2
Cash balances at end of period	$ 0.0		$ 139.9		$ 0.9

Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements			882
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	$ 0.4		$ 0.1
Net Income (Loss) Available to Common Stockholders, Basic	$ 34.3		$ 13.0
Total average common shares outstanding during the period	4,564,600		5,539,600
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Average outstanding dilutive options to acquire common shares	4,564,600		5,540,500
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Earnings Per Share, Diluted	$ 7.50		$ 2.34
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	$ (1.0)		$ 1.1
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	0.0		0.0
Net Income (Loss) Available to Common Stockholders, Basic	$ (1.0)		$ 1.1
Total average common shares outstanding during the period	4,564,600		5,540,500
Average unvested restricted common shares	(52,500)		(54,000)
Basic earnings per share denominator	4,512,100		5,485,600
Diluted earnings (loss) per share denominator	4,512,100		5,486,500
Earnings Per Share, Basic	$ (0.22)		$ 0.20
Earnings Per Share, Diluted	$ (0.22)		$ 0.20
Employee Stock Option [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		120,000

Contingencies (Details) $ in Millions	1 Months Ended
Contingencies (Details) $ in Millions	Jun. 30, 2007 USD ($)
Tribune Company Litigation \| Pending Litigation \| OneBeacon [Member]
Loss Contingencies [Line Items]
Proceeds from Sale, Maturity and Collection of Investments	$ 32.0

OneBeacon Pension Plan (Details) - USD ($) $ in Millions	3 Months Ended
OneBeacon Pension Plan (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Transfer of assets	$ 47.1
Qualified Plan
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 13.8
Principal Financial Group
Defined Contribution Plan Disclosure [Line Items]
Pre-tax settlement charge		$ 0.3

Subsequent Event (Details) $ / shares in Units, $ in Billions		3 Months Ended
	May 02, 2017 $ / shares	Dec. 31, 2017 USD ($)	Mar. 31, 2017
Scenario, Forecast
Subsequent Event [Line Items]
Proceeds from acquisition \| $		$ 1.3
Intact Financial Corporation \| OneBeacon [Member] \| Subsequent Event
Subsequent Event [Line Items]
Acquisition price per share (in USD per share) \| $ / shares	$ 18.10
Transaction book value multiple	1.65
OneBeacon [Member]
Subsequent Event [Line Items]
Percentage of voting interest acquired			96.90%
Ownership interest (as a percent)			75.70%