White Mountains Insurance (WTM) 10-Q 8 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2017	Nov. 07, 2017	Dec. 31, 2016
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	776,867
Document Type	10-Q
Document Period End Date	Sep. 30, 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		3,749,971
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Common shares, par value per share (in dollars per share)	$ 1		$ 1

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets
Fixed maturity investments, at fair value	$ 1,490	$ 2,081.1
Short-term investments, at amortized cost (which approximates fair value)	786.5	174.9
Common equity securities, at fair value	774.4	285.6
Other long-term investments	229.6	172.8
Total investments	3,280.5	2,714.4
Cash	48.6	80.2
Insurance premiums receivable	4.5	1.6
Deferred acquisition costs	14.1	10.6
Accrued investment income	15.4	14.8
Accounts receivable on unsettled investment sales	190.1	4.8
Goodwill and other intangible assets	37.8	45.2
Other assets	46.1	49.1
Assets held for sale	0	3,599.5
Total assets	3,637.1	6,520.2
Liabilities
Unearned insurance premiums	118.5	82.9
Debt	9.4	12.7
Accrued incentive compensation	76.9	95.7
Accounts payable on unsettled investment purchases	49.7	0
Other liabilities	45.4	43.6
Total liabilities held for sale	0	2,569.3
Total liabilities	299.9	2,804.2
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.7	4.6
Paid-in surplus	665.6	806.1
Retained earnings	2,800.8	2,776.6
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(1.3)	(1.4)
Accumulated other comprehensive loss from net change in benefit plan assets and obligations	0	(3.2)
Total White Mountains’s common shareholders’ equity	3,468.8	3,582.7
Non-controlling interests
Non-controlling interests	(131.6)	133.3
Total equity	3,337.2	3,716
Total liabilities and equity	$ 3,637.1	$ 6,520.2

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash	$ 0	$ 0
Common shares, par value per share (in dollars per share)	$ 1	$ 1
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	3,749,971	4,563,814
Common shares, outstanding shares	3,749,971	4,563,814

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
Earned insurance premiums	$ 2.4	$ 3.4	$ 7.6	$ 10.2
Net investment income	12.2	9.6	39.7	18.2
Net realized and unrealized investment gains	32.5	10.9	102.5	27.2
Advertising and commission revenues	38.8	28.2	103.9	89.6
Other revenue	1.6	4.7	6.1	18
Total revenues	87.5	56.8	259.8	163.2
Expenses:
Loss and loss adjustment expenses	0	2.2	1.1	6.8
Insurance acquisition expenses	0.9	1.3	3.1	4.4
Other underwriting expenses	0.1	0.2	0.3	0.4
Cost of sales	33.1	24	88.7	76.9
General and administrative expenses	44.1	42.4	162.2	144.9
Interest expense	0.9	0.5	1.8	2.6
Total expenses	79.1	70.6	257.2	236
Pre-tax income (loss) from continuing operations	8.4	(13.8)	2.6	(72.8)
Income tax benefit	4	17.1	5.3	22.7
Net income (loss) from continuing operations	12.4	3.3	7.9	(50.1)
Gain from sale of other discontinued operations, net of tax	554.3	47.9	552.7	414.5
Net (loss) income from discontinued operations, net of tax	(15.2)	36.5	20.5	100.9
Net income	551.5	87.7	581.1	465.3
Net loss (income) attributable to non-controlling interests	10.6	3.1	23.6	(24.6)
Net income attributable to White Mountains’s common shareholders	562.1	90.8	604.7	440.7
Comprehensive income, net of tax:
Change in foreign currency translation, net of tax	0	0.2	0.2	(0.4)
Comprehensive income	565.1	90.7	608.1	586.1
Other comprehensive income attributable to non-controlling interests	0	0.1	(0.1)	0.1
Comprehensive income attributable to White Mountains’s common shareholders	$ 565.1	$ 90.8	$ 608	$ 586.2
Basic income (loss) per share
Continuing operations (in usd per share)	$ 5.36	$ 1.31	$ 7.03	$ (14.47)
Discontinued operations (in usd per share)	125.45	17.34	128.03	99.75
Total consolidated operations (in usd per share)	130.81	18.65	135.06	85.28
Diluted income (loss) per share
Continuing operations (in usd per share)	5.36	1.31	7.03	(14.47)
Discontinued operations (in usd per share)	125.45	17.30	128.03	99.60
Total consolidated operations (in usd per share)	130.81	18.61	135.06	85.13
Dividends declared per White Mountains’s common share	$ 0	$ 0	$ 1	$ 1
OneBeacon
Comprehensive income, net of tax:
Comprehensive income from discontinued operations, net of tax	$ 3	$ (0.3)	$ 3.2	$ 145.8
Discontinued Operations
Expenses:
Net income attributable to White Mountains’s common shareholders	$ 562.1	$ 90.8	$ 604.7	440.7
Comprehensive income, net of tax:
Change in foreign currency translation, net of tax				$ 32.5
Basic income (loss) per share
Total consolidated operations (in usd per share)	$ 125.45	$ 17.34	$ 128.03	$ 99.75
Diluted income (loss) per share
Total consolidated operations (in usd per share)	$ 125.45	$ 17.30	$ 128.03	$ 99.60

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY Unaudited - 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	Recognition of foreign currency translation and other accumulated comprehensive items [Member]Common shares and paid-in surplus	Recognition of foreign currency translation and other accumulated comprehensive items [Member]Retained earnings	Recognition of foreign currency translation and other accumulated comprehensive items [Member]Non-controlling interest	Star & Shield LLCCommon shares and paid-in surplus	Star & Shield LLCRetained earnings	Star & Shield LLCAOCI, after tax	Star & Shield LLCTotal	Star & Shield LLCNon-controlling interest	Star & Shield LLCTotal Equity	OneBeaconNon-controlling interest	OneBeaconTotal Equity	OneBeaconRecognition of foreign currency translation and other accumulated comprehensive items [Member]AOCI, after tax	OneBeaconRecognition of foreign currency translation and other accumulated comprehensive items [Member]Total	OneBeaconRecognition of foreign currency translation and other accumulated comprehensive items [Member]Total Equity	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Common shares and paid-in surplus	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Retained earnings	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]AOCI, after tax	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Total	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Non-controlling interest	Sirius GroupRecognition of foreign currency translation and other accumulated comprehensive items [Member]Total Equity	Sale of Sirius Group	Sale of Sirius GroupCommon shares and paid-in surplus	Sale of Sirius GroupRetained earnings	Sale of Sirius GroupAOCI, after tax	Sale of Sirius GroupTotal	Sale of Sirius GroupNon-controlling interest	Sale of Sirius GroupTotal Equity	Sale of TranzactCommon shares and paid-in surplus	Sale of TranzactRetained earnings	Sale of TranzactAOCI, after tax	Sale of TranzactTotal	Sale of TranzactNon-controlling interest	Sale of TranzactTotal Equity	OneBeacon Insurance CompanyCommon shares and paid-in surplus	OneBeacon Insurance CompanyRetained earnings	OneBeacon Insurance CompanyAOCI, after tax	OneBeacon Insurance CompanyTotal	OneBeacon Insurance CompanyNon-controlling interest	OneBeacon Insurance CompanyTotal Equity
Ending Balances at Dec. 31, 2015		$ 978.2	$ 3,075	$ (149.9)	$ 3,903.3	$ 454.3	$ 4,357.6
Increase (Decrease) in Shareholders' Equity
Net income	$ 465.3	0	440.7	0	440.7	24.6	465.3
Net change in foreign currency translation and benefit plan assets and obligations	(0.4)							$ 0	$ 0	$ 32.1	$ 32.1	$ 0.1	$ 32.2
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary																												$ 0	$ 0	$ 113.3	$ 113.3	$ 0	$ 113.3
Total comprehensive income	586.1	0	440.7	145.4	586.1	24.7	610.8
Dividends declared on common shares		0	(5.4)	0	(5.4)	0	(5.4)
Dividends to non-controlling interests		0	0	0	0	(17.4)	(17.4)
Repurchases and retirements of common shares		(188.1)	(678.6)	0	(866.7)	0	(866.7)
Issuance of common shares		9.1	0	0	9.1	0	9.1
Deconsolidation of non-controlling interests																																			$ 0	$ 0	$ 0	$ 0	$ (250)	$ (250)	$ 0	$ 0	$ 0	$ 0	$ (78.4)	$ (78.4)
Acquisition of subsidiary		0	0	0	0	3.4	3.4
Non-controlling interests, decrease from purchase of interests																																															$ (2.7)	$ 0	$ 0	$ (2.7)	$ (8.8)	$ (11.5)
Issuance of shares to non-controlling interests		0	0	0	0	0.3	0.3
Net contributions from non-controlling interests		0	0	0	0	20.1	20.1
Amortization of restricted share awards		13.3	0	0	13.3	0.5	13.8
Ending Balances at Sep. 30, 2016		809.8	2,831.7	(4.5)	3,637	148.7	3,785.7
Beginning Balances at Dec. 31, 2015		978.2	3,075	(149.9)	3,903.3	454.3	4,357.6
Increase (Decrease) in Shareholders' Equity
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary																																		$ 113.3
Ending Balances at Dec. 31, 2016	3,716	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716
Beginning Balances at Dec. 31, 2016	3,716	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716
Increase (Decrease) in Shareholders' Equity
Net income	581.1	0	604.7	0	604.7	(23.6)	581.1
Net change in foreign currency translation and benefit plan assets and obligations	0.2							$ 0	$ 0	$ 0.4	$ 0.4	$ 0.1	$ 0.5
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary														$ 0	$ 0	$ 0									$ 2.9	$ 2.9	$ 2.9
Total comprehensive income	608.1	0	604.7	3.3	608	(23.5)	584.5
Dividends declared on common shares		0	(4.6)	0	(4.6)	0	(4.6)
Dividends to non-controlling interests		0	0	0	0	(17.7)	(17.7)
Repurchases and retirements of common shares		(147.9)	(575.9)	0	(723.8)	(5.2)	(729)
Issuance of common shares		1.7	0	0	1.7	0	1.7
Deconsolidation of non-controlling interests																	$ 0	$ 0	$ 0	$ 0	$ (4.4)	$ (4.4)	$ (238.3)	$ (238.3)
Non-controlling interests, decrease from purchase of interests		0	0	0	0	0	0
Issuance of shares to non-controlling interests		(4.1)	0	0	(4.1)	4.7	0.6
Net contributions from non-controlling interests		(1.7)	0	0	(1.7)	18.7	17
Amortization of restricted share awards		10.6	0	0	10.6	0.8	11.4
Ending Balances at Sep. 30, 2017	3,337.2	669.3	2,800.8	(1.3)	3,468.8	(131.6)	3,337.2
Increase (Decrease) in Shareholders' Equity
Net income	551.5
Net change in foreign currency translation and benefit plan assets and obligations	0
Total comprehensive income	565.1
Ending Balances at Sep. 30, 2017	$ 3,337.2	$ 669.3	$ 2,800.8	$ (1.3)	$ 3,468.8	$ (131.6)	$ 3,337.2

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Cash flows from operations:
Net income	$ 551.5	$ 87.7	$ 581.1	$ 465.3
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(32.5)	(10.9)	(102.5)	(27.2)
Deferred income benefit			(6.9)	(16.2)
Net income from discontinued operations	15.2	(36.5)	(20.5)	(100.9)
Net gain from sale of discontinued operations, net of tax	(554.3)	(47.9)	(552.7)	(414.5)
Amortization and depreciation			25.9	13.3
Other operating items:
Net change in unearned insurance premiums			36.2	20.1
Net change in deferred acquisition costs			(3.5)	(2.8)
Net change in restricted cash			0	5.8
Net change in other assets and liabilities, net			(21.5)	(117.6)
Net cash used for operations - continuing operations			(64.4)	(174.7)
Net cash provided from (used for) operations			92.6	(136.6)
Cash flows from investing activities:
Net change in short-term investments			(612.2)	(82.3)
Sales of fixed maturity and convertible investments			1,631.2	2,009.4
Maturities, calls and paydowns of fixed maturity and convertible investments			178.6	184.4
Sales of common equity securities			407.8	683
Distributions, settlements and redemptions of other long-term investments			1.3	10.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			1,131	2,657.2
Proceeds paid to non-controlling common shareholders from the sale of consolidated subsidiaries			0	(141.6)
Net settlement of investment cash flows and contributions with discontinued operations			167.7	(396.6)
Purchases of other long-term investments			(67.7)	(36.6)
Purchases of common equity securities			(818.6)	(94.7)
Purchases of fixed maturity and convertible investments			(1,189.8)	(3,591.5)
Purchases of unconsolidated affiliates and consolidated subsidiaries, net of cash acquired			0	(13.7)
Net change in unsettled investment purchases and sales			(135.6)	(113.7)
Net acquisitions of property and equipment			1	(0.9)
Net cash provided from investing activities - continuing operations			694.7	1,072.5
Net cash provided from investing activities			697.7	1,342.2
Cash flows from financing activities:
Draw down of debt and revolving line of credit			361	352.5
Repayment of debt and revolving line of credit			(364.6)	(401.8)
Proceeds from issuances of common shares			0	3.7
Cash dividends paid to the Company’s common shareholders			(4.6)	(5.4)
Common shares repurchased			(714.6)	(860.9)
Distribution to non-controlling interest shareholders			(0.9)	(0.9)
Contributions from discontinued operations			45.2	42.2
Payments of contingent consideration related to purchases of consolidated subsidiaries			0	(7.8)
Capital contributions from BAM members			25.7	28.2
Acquisition of additional subsidiary shares from non-controlling interest			(0.7)	0
Other financing activities, net			(9.3)	(5.8)
Net cash used for financing activities - continuing operations			(662.8)	(856)
Net cash used for financing activities			(724.7)	(928.7)
Net change in cash during the period - continuing operations			(32.5)	41.8
Cash balances at beginning of period			80.2	72	$ 72
Add: cash held for sale, excluding discontinued operations, at the beginning of period			0.9	1.2	1.2
Less: cash held for sale, excluding discontinued operations, at the end of period	0	3.8	0	3.8	0.9
Cash balances at end of period	48.6	111.2	48.6	111.2	80.2
Supplemental cash flows information:
Interest paid			(0.5)	(0.7)
Net income tax refund from national governments			0	0
Discontinued Operations
Other operating items:
Net cash provided from operations - discontinued operations			157	38.1
Cash flows from investing activities:
Net cash provided from investing activities - discontinued operations			3	269.7
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations			(61.9)	(72.7)
Cash balances at beginning of period			70.5	245.4	245.4
Cash balances at end of period	$ 0	$ 137.3	0	137.3	$ 70.5
Supplemental cash flows information:
Net income tax refund from national governments			$ 0	$ 18.3

CONSOLIDATED STATEMENTS OF CAS7

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash balance	$ 48.6	$ 80.2	$ 111.2	$ 72
Restricted cash balances	0	0	0	5.8
Discontinued Operations
Cash balance	$ 0	$ 70.5	$ 137.3	$ 245.4

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 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 HG Global/BAM, MediaAlpha and Other Operations. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). 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 September 30, 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. The MediaAlpha segment consists of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is an advertising technology company that develops transparent and efficient platforms for the buying and selling of insurance and other vertical-specific performance media (i.e., clicks, calls and leads). MediaAlpha’s exchange technology, machine learning and analytical tools facilitate transparent, real-time transactions between advertisers (buyers of advertising inventory) and publishers (sellers of advertising inventory). MediaAlpha works with 330 advertisers and 280 publishers across a number of insurance (auto, motorcycle, home, renter, health and life) and non-insurance (travel, education, personal finance and home services) verticals. White Mountains’s Other Operations segment consists of the Company 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 Wobi Insurance Agency Ltd. (“Wobi”) and Removal Stars Ltd. (“Buzzmove”). During the third quarter of 2017, White Mountains revised certain of its previously issued financial statements for amounts relating to Wobi. See Note 16 — “Financial Statement Revisions” . 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 fully matured on June 30, 2016 and which was liquidated in the third quarter of 2017, and its U.S.-based service provider, White Mountains Financial Services LLC, which was liquidated in the second quarter of 2017 (collectively, “WM Life Re”). On September 28, 2017, Intact Financial Corporation completed its previously announced acquisition of OneBeacon in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). OneBeacon Ltd., an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”), offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. On July 21, 2016, White Mountains completed its sale of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC. On April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. White Mountains has presented the results of OneBeacon, Tranzact and Sirius Group as discontinued operations in the statement of operations and comprehensive income for all periods prior to each transaction’s completion date. White Mountains has presented OneBeacon’s assets and liabilities as held for sale as of December 31, 2016 . On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. Star & Shield provides management services for a fee to 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. White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016. See Note 15 — “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 $9.3 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 nine-month periods ended September 30, 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. 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 September 30, 2017 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 any 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. As of September 30, 2017 , there was an insignificant amount of unamortized debt issuance costs included in debt. Recently Issued Accounting Pronouncements Stock Compensation In May 2017, the FASB issued ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. The new guidance becomes effective for fiscal years beginning after December 15, 2017. White Mountains does not expect a material impact from implementation of this guidance. Cash Flow Statement In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230) . Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented. The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains does not expect a material impact from implementation of this 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 requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current U.S. GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include 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 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 guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and accordingly, does not expect 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. Revenue from insurance contracts, investment income and investments gains and losses are excluded from the scope of 2014-09. 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 does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s revenues from customers.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions OneBeacon Transaction On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a gain of $554.6 million , net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 15 — “Held for Sale and Discontinued Operations” . Sale of Star & Shield On March 7, 2017, White Mountains completed its sale of Star & Shield and its investment in SSIE surplus notes to K2 Insurances LLC. White Mountains did not recognize any gain or loss on the sale. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains’s GAAP financial statements. See Note 15 — “Held for Sale and Discontinued Operations” . Acquisition of 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. On August 1, 2017, White Mountains purchased 37,409 newly-issued preferred shares of Buzzmove for GBP 4.0 million (approximately $5.0 million based upon the foreign exchange spot rate at the date of acquisition) and 5,808 common shares from the company founders for GBP $0.5 million (approximately $0.7 million based upon the spot rate at the date of acquisition). White Mountains ownership share in Buzzmove as of September 30, 2017 was 77.1% . 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 6 — “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’s 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 state tax expense. 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 15 — “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 from Sirius Group. During the third quarter of 2017, White Mountains recorded a $0.8 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees. Through April 18, 2016, Sirius Group’s results are reported as discontinued operations and assets and liabilities held for sale within White Mountains’s GAAP financial statements. The transactions to purchase the 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 15 — “Held for Sale and Discontinued Operations” . Sale of Symetra On February 1, 2016, Symetra Financial Corporation (“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 12 — “Investment in Symetra”.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investments Securities White Mountains’s portfolio of investment securities held for general investment purposes consists of fixed maturity investments, short-term investments, common equity securities and other-long term investments, which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Net realized and unrealized investment gains (losses) on trading securities are reported in pre-tax revenues. White Mountains’s fixed maturity investments are generally valued using industry standard pricing methodologies. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Realized investment gains (losses) resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of interest-bearing money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of September 30, 2017 and December 31, 2016 . Other long-term investments consist primarily of hedge funds, private equity funds, unconsolidated private capital investments and foreign currency forward contracts. Net Investment Income White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments and short-term investments and dividend income from its common equity securities and other long- term investments. Pre-tax net investment income for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Investment income: Fixed maturity investments $ 9.8 $ 9.3 $ 32.9 $ 17.4 Short-term investments .3 .2 .6 .7 Common equity securities 2.7 .5 7.7 1.0 Other long-term investments .1 — .5 .4 Total investment income 12.9 10.0 41.7 19.5 Third-party investment expenses (.7 ) (.4 ) (2.0 ) (1.3 ) Net investment income, pre-tax $ 12.2 $ 9.6 $ 39.7 $ 18.2 Net Realized and Unrealized Investment Gains (Losses) Net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Net realized investment gains, pre-tax $ 6.8 $ .7 $ 20.8 $ 265.0 Net unrealized investment gains (losses), pre-tax 25.7 10.2 81.7 (237.8 ) Net realized and unrealized investment gains, pre-tax 32.5 10.9 102.5 27.2 Income tax expense attributable to net realized and unrealized investment gains (3.9 ) — (9.5 ) (4.0 ) Net realized and unrealized investment gains, after tax $ 28.6 $ 10.9 $ 93.0 $ 23.2 Net Realized Investment Gains (Losses) Net realized investment gains (losses) for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ .6 $ 1.2 $ 1.8 $ .3 $ — $ .3 Short-term investments (.1 ) — (.1 ) .2 — .2 Common equity securities 4.9 5.4 10.3 .2 — .2 Other long-term investments 2.0 (7.2 ) (5.2 ) — — — Net realized investment gains (losses), pre-tax 7.4 (.6 ) 6.8 .7 — .7 Income tax expense attributable to net realized investment gains (.6 ) — (.6 ) (.1 ) — (.1 ) Net realized investment gains (losses), after tax $ 6.8 $ (.6 ) $ 6.2 $ .6 $ — $ .6 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (.4 ) $ 2.7 $ 2.3 $ 2.0 $ — $ 2.0 Short-term investments (.1 ) — (.1 ) .4 — .4 Common equity securities 18.5 6.0 24.5 262.6 — 262.6 Other long-term investments 3.0 (8.9 ) (5.9 ) — — — Net realized investment gains (losses), pre-tax 21.0 (.2 ) 20.8 265.0 — 265.0 Income tax expense attributable to net realized investment gains (3.6 ) — (3.6 ) (45.1 ) — (45.1 ) Net realized investment gains (losses), after tax $ 17.4 $ (.2 ) $ 17.2 $ 219.9 $ — $ 219.9 Net Unrealized Investment Gains (Losses) Net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions Net unrealized gains (losses) Net gains (losses) Total net unrealized gains (losses) reflected in earnings Net unrealized (losses) gains Net Total net unrealized (losses) gains reflected in earnings Fixed maturity investments $ 2.0 $ 5.1 $ 7.1 $ (2.0 ) $ — $ (2.0 ) Common equity securities 26.6 (3.1 ) 23.5 8.5 .2 8.7 Other long-term investments (2.5 ) (2.4 ) (4.9 ) 3.4 .1 3.5 Net unrealized investment gains (losses), pre-tax 26.1 (.4 ) 25.7 9.9 .3 10.2 Income tax (expense) benefit attributable to net unrealized investment gains (losses) (3.3 ) — (3.3 ) .1 — .1 Net unrealized investment gains (losses), after tax $ 22.8 $ (.4 ) $ 22.4 $ 10.0 $ .3 $ 10.3 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions Net unrealized gains Net gains (losses) Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 19.4 $ 12.6 $ 32.0 $ 13.3 $ — $ 13.3 Common equity securities 53.5 — 53.5 (256.1 ) 2.6 (253.5 ) Other long-term investments 9.2 (13.0 ) (3.8 ) 2.1 .3 2.4 Net unrealized investment gains (losses), pre-tax 82.1 (.4 ) 81.7 (240.7 ) 2.9 (237.8 ) Income tax (expense) benefit attributable to net unrealized investment gains (losses) (5.9 ) — (5.9 ) 41.1 — 41.1 Net unrealized investment gains (losses), after tax $ 76.2 $ (.4 ) $ 75.8 $ (199.6 ) $ 2.9 $ (196.7 ) Total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Fixed maturity investments $ — $ — $ — $ .1 Other long-term investments (.7 ) (.9 ) (2.2 ) .7 Total unrealized investment (losses) gains, pre-tax - Level 3 investments $ (.7 ) $ (.9 ) $ (2.2 ) $ .8 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 September 30, 2017 and December 31, 2016 were as follows: September 30, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 218.1 $ .1 $ (.4 ) $ — $ 217.8 Debt securities issued by corporations 608.3 3.2 (1.0 ) 14.5 625.0 Mortgage and asset-backed securities 389.1 1.2 (2.4 ) — 387.9 Municipal obligations 252.1 3.1 (.5 ) — 254.7 Foreign government, agency and provincial obligations 4.5 — (.1 ) .2 4.6 Total fixed maturity investments $ 1,472.1 $ 7.6 $ (4.4 ) $ 14.7 $ 1,490.0 December 31, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 112.1 $ — $ (1.1 ) $ — $ 111.0 Debt securities issued by corporations 752.0 2.3 (10.1 ) 2.1 746.3 Mortgage and asset-backed securities 986.9 .8 (7.9 ) — 979.8 Municipal obligations 238.7 1.1 (1.3 ) — 238.5 Foreign government, agency and provincial obligations 12.0 .1 — — 12.1 Total fixed maturity investments $ 2,101.7 $ 4.3 $ (20.4 ) $ 2.1 $ 2,087.7 Less: fixed maturity investments reclassified to assets held for sale related to SSIE (6.6 ) Total fixed maturity investments $ 2,081.1 The 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 September 30, 2017 and December 31, 2016 were as follows: September 30, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 693.8 $ 82.6 $ (2.0 ) $ — $ 774.4 Other long-term investments $ 254.5 $ 13.1 $ (21.2 ) $ (16.8 ) $ 229.6 December 31, 2016 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 258.6 $ 29.0 $ (2.0 ) $ — $ 285.6 Other long-term investments $ 194.0 $ 7.9 $ (25.2 ) $ (3.9 ) $ 172.8 Other Long-term Investments Other long-term investments as of September 30, 2017 and December 31, 2016 were as follows: Carrying Value at Millions September 30, 2017 December 31, 2016 Hedge funds and private equity funds, at fair value $ 154.9 $ 82.6 Private equity securities and limited liability companies, at fair value (1)(2) 59.1 57.6 Private convertible preferred securities, at fair value (1) 27.2 30.6 Forward Contracts (15.4 ) (1.2 ) Other 3.8 3.2 Total other-long term investments $ 229.6 $ 172.8 (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 September 30, 2017 , no funds had been drawn on the surplus note facility. 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 is generally estimated using the net asset value (“NAV”) of the funds. As of September 30, 2017 , White Mountains held investments in two hedge funds and ten private equity funds. The largest investment in a single fund was $54.7 million as of September 30, 2017 and $21.5 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 September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financials $ 54.7 $ — $ 21.5 $ — Long/short equity REIT 19.1 — 19.9 — Total hedge funds 73.8 — 41.4 — Private equity funds Manufacturing/Industrial 42.7 12.1 19.4 22.9 Aerospace/Defense/Government 28.7 15.6 19.4 25.9 Direct lending 6.7 23.3 1.4 28.6 Financial Services 3.0 13.0 1.0 5.0 Insurance — 41.2 — 41.2 Total private equity funds 81.1 105.2 41.2 123.6 Total hedge funds and private equity funds included in other long-term investments $ 154.9 $ 105.2 $ 82.6 $ 123.6 Redemption of investments in certain hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. As of September 30, 2017 , one hedge fund with a fair value of $54.7 million was subject to a lock-up period that expires on September 1, 2018. The following table summarizes the fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds as of September 30, 2017 : Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice Total Semi-annual $ 54.7 $ 19.1 $ 73.8 White Mountains has submitted a redemption request for its investment in a long/short equity REIT hedge fund. As of September 30, 2017, the redemption of $19.1 million is outstanding and is subject to market fluctuation. The bulk of the redemption proceeds are expected to be received in the first quarter of 2018 with the balance expected in the second quarter of 2018. Investments in private equity funds are generally subject to a lock-up period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds provide an option to extend the lock-up period at either, the sole discretion of the fund manager or upon agreement between the fund and the investors. The following table summarizes investments in private equity funds that were subject to lock-up periods as of September 30, 2017 : Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $4.0 $18.2 $33.5 $25.4 $81.1 Fair Value Measurements as of September 30, 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 September 30, 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 include fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, mortgage and asset-backed securities, municipal obligations, and foreign government, agency and provincial obligations. Investments valued using Level 2 inputs also include certain passive exchange traded funds (“ETFs”) that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges, which management values using the fund manager’s published NAV to account for the difference in market close times. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Investments valued using Level 3 fair value estimates are based upon unobservable inputs and include investments in certain fixed maturity investments, equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Transfers between levels are based on investments held as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, the evaluation of pricing methodologies and a review of the pricing services’ quality control procedures on at least an annual basis, a comparison of its invested asset prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and a review of the underlying assumptions utilized by the pricing services for select measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these assessment procedures are considered outliers. Also considered outliers are prices that have not changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains’s review process does not appear to support the market price provided by the pricing services, White Mountains challenges the vendor provided price. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The valuation process described above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follows: Debt securities issued by corporations: The fair value of debt securities issued by corporations is determined from a pricing evaluation technique that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Mortgage and asset-backed securities: The fair value of mortgage and asset-backed securities is determined from a pricing evaluation technique that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Municipal obligations: The fair value of municipal obligations is determined from a pricing evaluation technique that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Foreign government, agency and provincial obligations: The fair value of foreign government, agency and provincial obligations is determined from a pricing evaluation technique that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2. 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 investment in the hedge fund or private equity fund. As of September 30, 2017 and December 31, 2016, White Mountains did not have any adjustments to the reported NAV of its 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 September 30, 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 7 . September 30, 2017 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 217.8 $ 217.8 $ — $ — Debt securities issued by corporations: Consumer 121.9 — 121.9 — Utilities 109.3 — 109.3 — Health Care 85.6 — 85.6 — Communications 81.7 — 81.7 — Materials 68.6 — 68.6 — Financials 55.6 — 50.0 5.6 Technology 52.0 — 52.0 — Industrial 38.3 — 38.3 — Energy 12.0 — 12.0 — Total debt securities issued by corporations 625.0 — 619.4 5.6 Mortgage and asset-backed securities 387.9 — 387.9 — Municipal obligations 254.7 — 254.7 — Foreign government, agency and provincial obligations 4.6 — 4.6 — Total fixed maturity investments 1,490.0 217.8 1,266.6 5.6 Short-term investments (1) 786.5 764.5 22.0 — Common equity securities: Exchange traded funds (2) 492.5 434.6 57.9 — Health Care 17.1 17.1 — — Financials 14.9 14.9 — — Consumer 13.0 13.0 — — Technology 11.7 11.7 — — Communications 10.3 10.3 — — Industrial 10.2 10.2 — — Energy 4.0 4.0 — — Other (3) 200.7 — 200.7 — Total common equity securities 774.4 515.8 258.6 — Other long-term investments (4)(5) 90.1 — — 90.1 Total investments $ 3,141.0 $ 1,498.1 $ 1,547.2 $ 95.7 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4) Excludes carrying value of $(15.4) related to foreign currency forward contracts. (5) Excludes carrying value of $154.9 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. December 31, 2016 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 111.0 $ 101.5 $ 9.5 $ — Debt securities issued by corporations: Consumer 190.8 — 190.8 — Utilities 140.8 — 140.8 — Health Care 114.9 — 114.9 — Financials 79.7 — 79.7 — Communications 72.0 — 72.0 — Materials 65.0 — 65.0 — Technology 48.8 — 48.8 — Industrial 28.2 — 28.2 — Energy 6.1 — 6.1 — Total debt securities issued by corporations 746.3 — 746.3 — Mortgage and asset-backed securities 979.8 — 979.8 — Municipal obligations 238.5 — 238.5 — Foreign government, agency and provincial obligations 12.1 — 12.1 — Total fixed maturity investments (1) 2,087.7 101.5 1,986.2 — Short-term investments (1)(2) 175.0 162.3 12.7 — Common equity securities: Exchange traded funds (3) 157.2 129.4 27.8 — Health Care 13.9 13.9 — — Consumer 8.6 8.6 — — Financials 7.7 7.7 — — Technology 7.3 7.3 — — Communications 7.0 7.0 — — Energy 2.5 2.5 — — Industrial 1.5 1.5 — — Other (4) 79.9 — 79.9 — Total common equity securities 285.6 177.9 107.7 — Other long-term investments (5)(6) 91.4 — — 91.4 Total investments (1) $ 2,639.7 $ 441.7 $ 2,106.6 $ 91.4 (1) Includes carrying value of $6.6 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE. (2) Short-term investments are measured at amortized cost, which approximates fair value. (3) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of one investment in a unit trust that primarily invests in international equities. (5) Excludes carrying value of $(1.2) related to foreign currency forward contracts. (6) Excludes carrying value of $82.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. Debt Securities Issued by Corporations The following table summarizes the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2017 and December 31, 2016 : Fair Value at Millions September 30, 2017 December 31, 2016 AA $ 26.7 $ 37.3 A 119.4 212.8 BBB 291.9 335.6 BB 165.4 143.2 B 21.6 17.4 Debt securities issued by corporations (1)(2) $ 625.0 $ 746.3 (1) Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC ("Standard & Poor's") and (2) Moody's Investor Service, Inc. ("Moody’s"). (2) Includes carrying value of $4.2 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE. Mortgage and Asset-backed Securities White Mountains purchases commercial mortgage-backed securities (“CMBS”) and residential mortgage-backed securities (“RMBS”) with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics). White Mountains did not hold any RMBS categorized as sub-prime as of September 30, 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 September 30, 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 or any collateralized debt obligations. The following table summarizes the carrying value of White Mountains’s mortgage and asset-backed securities as of September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 51.0 $ 51.0 $ — $ 70.3 $ 70.3 $ — FNMA 88.9 88.9 — 235.5 235.5 — FHLMC 59.7 59.7 — 59.5 59.5 — Total Agency (1) 199.6 199.6 — 365.3 365.3 — Non-agency: Residential 70.1 70.1 — 70.3 70.3 — Commercial 39.0 39.0 — 3.9 3.9 — Total Non-agency 109.1 109.1 — 74.2 74.2 — Total mortgage-backed securities 308.7 308.7 — 439.5 439.5 — Other asset-backed securities: Credit card receivables 40.5 40.5 — 214.2 214.2 — Vehicle receivables 22.7 22.7 — 205.9 205.9 — Other 16.0 16.0 — 120.2 120.2 — Total other asset-backed securities 79.2 79.2 — 540.3 540.3 — Total mortgage and asset-backed securities $ 387.9 $ 387.9 $ — $ 979.8 $ 979.8 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC). Non-agency Mortgage-backed Securities The following table summarizes the security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of September 30, 2017 : Security Issuance Year Millions Fair Value 2004 2013 2014 2015 2016 2017 Non-agency RMBS $ 70.1 $ .3 $ 1.3 $ 20.6 $ 47.9 $ — $ — Non-agency CMBS 39.0 — — — — 3.7 35.3 Total $ 109.1 $ .3 $ 1.3 $ 20.6 $ 47.9 $ 3.7 $ 35.3 Non-agency Residential Mortgage-backed Securities The following table summarizes the classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities as of September 30, 2017 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 70.1 $ 58.4 $ 11.7 $ — Non-prime — — — — Sub-prime — — — — Total $ 70.1 $ 58.4 $ 11.7 $ — (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings, Inc. (“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

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	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. 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. 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. The following table shows the change in goodwill and other intangible assets: Three Months Ended September 30, 2017 2016 Millions Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 25.9 $ 14.4 $ 40.3 $ 18.3 $ 23.3 $ 41.6 Acquisition of businesses — — — 7.6 1.1 8.7 Amortization, including foreign currency translation — (2.5 ) (2.5 ) — (2.6 ) (2.6 ) Ending balance $ 25.9 $ 11.9 $ 37.8 $ 25.9 $ 21.8 $ 47.7 Nine Months Ended September 30, 2017 2016 Millions Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 25.9 $ 19.3 $ 45.2 $ 18.6 $ 26.9 $ 45.5 Add: Amounts held for sale at beginning of the period (1) — — — — .4 .4 Acquisition of businesses — 7.6 5.0 12.6 Wobi write off — — — (.3 ) (2.5 ) (2.8 ) Amortization, including foreign currency translation — (7.4 ) (7.4 ) — (8.0 ) (8.0 ) Ending balance $ 25.9 $ 11.9 $ 37.8 $ 25.9 $ 21.8 $ 47.7 (1) See Note 15 — “Held for Sale and Discontinued Operations” . The following table is a summary of goodwill and other intangible assets as of September 30, 2017 , December 31, 2016 , and September 30, 2016 : Millions September 30, 2017 December 31, 2016 Goodwill MediaAlpha $ 18.3 $ 18.3 Buzzmove 7.6 7.6 Total goodwill 25.9 25.9 Other intangible assets MediaAlpha 11.0 18.3 Buzzmove .9 1.0 Total other intangible assets 11.9 19.3 Total goodwill and other intangible assets 37.8 45.2 Goodwill and other intangible assets held for sale — 1.2 Goodwill and other intangible assets attributed to non-controlling interests (13.7 ) (17.1 ) Goodwill and other intangible assets included in White Mountains's $ 24.1 $ 29.3

Debt

Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt	Debt White Mountains’s debt outstanding as of September 30, 2017 and December 31, 2016 consisted of the following: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ — N/A Unamortized issue costs — — WTM Bank Facility, carrying value — — MediaAlpha Bank Facility 9.4 5.5% — N/A Unamortized issuance cost — — MediaAlpha Bank Facility, carrying value 9.4 — Previous MediaAlpha Bank Facility — N/A 12.9 5.7% Unamortized issuance cost — (.2 ) Previous MediaAlpha Bank Facility, carrying value — 12.7 Total debt $ 9.4 $ 12.7 (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”). During the third quarter of 2017, White Mountains borrowed $350.0 million under the WTM Bank Facility to partially fund a tender offer and subsequently repaid the $350 million after receiving the proceeds from the OneBeacon Transaction. As of September 30, 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. MediaAlpha Bank Facility On May 12, 2017, MediaAlpha entered into a secured credit facility (the “MediaAlpha Bank Facility”) with Western Alliance Bank, which has a total commitment of $20.0 million and has a maturity date of May 12, 2020. The MediaAlpha Bank Facility replaced MediaAlpha’s previous credit facility (the “Previous MediaAlpha Bank Facility”), which had a total commitment of $20.0 million . The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.5% on the term loan facility and 0.25% on the revolving credit facility as of September 30, 2017 . The MediaAlpha Bank Facility consists of a $5.0 million term loan facility, which has an outstanding balance of $3.4 million as of September 30, 2017 , and a revolving loan facility for $15.0 million , which has an outstanding balance of $6.0 million as of September 30, 2017 . During the nine months ended September 30, 2017 , MediaAlpha borrowed $5.0 million on the term loan and $6.0 million on the revolving loan under the MediaAlpha Bank Facility. During the nine months ended September 30, 2017, MediaAlpha repaid $12.9 million under the Previous MediaAlpha Bank Facility and $1.6 million on the term loan under the MediaAlpha Bank Facility. The MediaAlpha Bank Facility is secured by intellectual property and the common stock of MediaAlpha’s subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a fixed charge coverage ratio and an asset coverage ratio. Compliance At September 30, 2017 , White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 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, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three and nine months ended September 30, 2017 represented effective tax rates of (47.6)% and (203.9)% . The effective tax rates were different from the U.S. statutory rate of 35% , primarily due to a full valuation allowance on all U.S. operations, a tax benefit recorded at BAM and consolidated pre-tax income being near break-even. For the three and nine months ended September 30, 2017, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2017, BAM recorded a tax benefit of $2.1 million and $6.9 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2016 represented effective tax rates of (123.9)% and (31.2)% . The effective tax rates were different from the U.S. statutory rate of 35% , primarily due to a tax benefit recorded at BAM and a $14.0 million tax benefit recognized in continuing operations related to the reversal of a valuation allowance that resulted from income that was recognized within discontinued operations. ASC 740 includes an exception to the general principle of intra-period tax allocations that requires a company with a current period loss from continuing operations to consider income recorded in other categories, including discontinued operations, in determining the tax benefit that is allocated to continuing operations. The valuation allowance reversal relates to a consolidated tax group within White Mountains that has a current period loss within continuing operations. Accordingly, the tax benefit resulting from the valuation allowance reversal was recorded in continuing operations with an offsetting tax expense for the same amount in discontinued operations. For the three and nine months ended September 30, 2016, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2016, BAM recorded a tax benefit of $3.3 million and $8.2 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity. In arriving at the effective tax rate for the three and nine months ended September 30, 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. 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

Derivatives	9 Months Ended
	Sep. 30, 2017
Derivatives	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 September 30, 2016 . The reinsurance agreement was commuted in December 2016. WM Life Re was liquidated in the third quarter of 2017. The following table summarizes the pre-tax operating results of WM Life Re for the three and nine months ended September 30, 2016 . Three Months Ended Nine Months Ended Millions September 30, 2016 September 30, 2016 Fees, included in other revenue $ — $ 1.2 Change in fair value of variable annuity liability, included in other revenue — (.3 ) Change in fair value of derivatives, included in other revenue — (2.0 ) Foreign exchange, included in other revenue — 1.4 Total revenue — .3 Death benefit claims paid, included in general and administrative expenses — (.3 ) General and administrative expenses (.5 ) (2.4 ) Pre-tax loss $ (.5 ) $ (2.4 ) The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and nine months ended September 30, 2016 and the carrying values, included in other assets, as of December 31, 2016 by type of instrument: Gains (losses) Carrying Value Three Months Ended Nine Months Ended As of Millions September 30, 2016 September 30, 2016 December 31, 2016 Fixed income/interest rate $ — $ 1.8 $ — Foreign exchange — (4.8 ) — Equity — 1.0 — Total $ — $ (2.0 ) $ — The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the nine months ended September 30, 2016 . Nine Months Ended September 30, 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 (.3 ) 2.9 (.7 ) (4.2 ) (2.0 ) Transfers in — — — — — Sales/settlements — (5.6 ) (15.8 ) 3.3 (18.1 ) End of period $ — $ — $ — $ — $ — (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.
Forward Contracts
Derivatives	Forward Contracts White Mountains’s investment portfolio includes certain investment grade fixed maturity investments denominated in British Pound Sterling (GBP) and common equity securities denominated in Japanese Yen (JPY), Euro (EUR), GBP and other foreign currencies. White Mountains has entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. The contracts do not meet the criteria to be accounted for as a hedge. White Mountains actively manages its net foreign currency exposure and adjusts its foreign currency positions within ranges established by senior management. Mismatches between currency driven movements in foreign denominated investments and foreign currency forward contracts may result in net foreign currency positions being outside pre-defined ranges and/or may result in net foreign currency gains/(losses). At September 30, 2017 , White Mountains held $261.8 million (GBP 160.7 million , EUR 18.9 million and JPY 2,646.4 million ) 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 has been estimated using OTC quotes for similar instruments and accordingly, the measurements have been classified as Level 2 measurements as of September 30, 2017 . The net realized derivative loss recognized in net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2017 was $7.2 million and $8.9 million . The net unrealized derivative loss recognized in net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2017 was $2.8 million and $14.2 million . White Mountains’s forward contracts are subject to master netting agreements. As of September 30, 2017 and December 31, 2016, there were no offsets to the gross liability amount under the master netting agreement and the resulting net amount recognized in other long-term investments was $15.4 million and $1.2 million . White Mountains does not hold or provide any collateral under its forward contracts. The following table summarizes the gross notional amount associated with the forward currency contracts as of September 30, 2017 : September 30, 2017 Millions Notional Amount Carrying Value Standard & Poor's Rating (1) Barclays Bank PLC $ 201.3 $ (14.1 ) A- JP Morgan Chase Bank N.A. 60.5 (1.3 ) A+ Total $ 261.8 $ (15.4 ) (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings) and “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings).

Municipal Bond Guarantee

Municipal Bond Guarantee	9 Months Ended
	Sep. 30, 2017
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	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 September 30, 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, 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 and nine months ended September 30, 2017 , HG Global had pre-tax income of $7.0 million and $20.3 million , which included $4.8 million and $14.3 million of interest income on the BAM Surplus Notes. For the three and nine months ended September 30, 2016 , HG Global had pre-tax income of $5.2 million and $18.4 million , which included $4.5 million and $13.4 million of interest income on the BAM Surplus Notes. For the three and nine months ended September 30, 2017 , White Mountains reported pre-tax losses of $11.9 million and $35.6 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.8 million and $14.3 million of interest expense on the BAM Surplus Notes. For the three and nine months ended September 30, 2016 , White Mountains reported pre-tax losses of $13.7 million and $30.3 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.5 million and $13.4 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 8.0% 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 8.0% . No payment of interest or principal on the BAM Surplus Notes may be made without the approval of the New York State Department of Financial Services. BAM has stated its intention to seek regulatory approval to pay interest and principal on its surplus notes only to the extent that (1) its remaining qualified statutory capital (“QSC”) exceeds $500 million and (2) its remaining QSC and other capital resources continue to support its outstanding obligations, business plan and its AA stable rating from S&P. During the second quarter of 2017, in order to further support BAM’s long-term capital position and business prospects, HG Global agreed to contribute the $203.0 million Series A BAM Surplus Notes (“Series A Notes”) into the supplemental collateral trust (the “Supplemental Trust”) at HG Re, HG Global’s wholly owned reinsurance subsidiary. The Supplemental Trust already holds the $300.0 million Series B BAM Surplus Notes (“Series B Notes” and, collectively with the Series A Notes, the “BAM Surplus Notes”). Assets held in the Supplemental Trust serve to collateralize HG Re’s obligations to BAM under the first loss reinsurance treaty between BAM and HG Re. HG Global and BAM also agreed to change the payment terms of the Series B Notes, so that payments will reduce principal and accrued interest on a pro rata basis, consistent with the payment terms on the Series A Notes. The terms of the Series B Notes had previously stipulated that payments would first reduce interest owed, then reduce principal owed once all accrued interest had been paid. The New York Department of Financial Services approved the change during the third quarter of 2017. During the second quarter of 2017, HG Global and BAM also made certain changes to the ceding commission arrangements under the reinsurance treaty between HG Re and BAM. These changes will accelerate growth in BAM’s statutory capital but will not impact the net risk premium ceded from BAM to HG Re. Under GAAP, if the terms of a debt instrument are amended, unless there is a greater than 10% change in the expected discounted future cash flows of such instrument, a change in the instrument’s carrying value is not permitted. White Mountains has determined that the impact of the changes made in the second quarter of 2017 to the terms of the BAM Surplus Notes on the expected discounted future cash flows is not greater than 10% . 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: September 30, 2017 December 31, 2016 Contracts outstanding 5,907 4,807 Remaining weighted average contract period outstanding (in years) 10.9 10.8 Contractual debt service outstanding (in millions): Principal $ 39,207.5 $ 33,057.3 Interest 19,681.0 16,396.6 Total debt service outstanding $ 58,888.5 $ 49,453.9 Gross unearned insurance premiums $ 118.5 $ 82.9 The following table is a schedule of BAM’s future premium revenues as of September 30, 2017 : Millions September 30, 2017 October 1, 2017 - December 31, 2017 $ 2.5 January 1, 2018 - March 31, 2018 2.5 April 1, 2018 - June 30, 2018 2.5 July 1, 2018 - September 30, 2018 2.5 October 1, 2018 - December 31, 2018 2.4 9.9 2019 9.5 2020 9.2 2021 8.7 2022 and thereafter 78.7 Total gross unearned insurance premiums $ 118.5

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share White Mountains calculates earnings per share using the two-class method, which allocates earnings between common shares and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by the net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share from continuing operations for the three and nine months ended September 30, 2017 and 2016 . See Note 15 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 562.1 $ 90.8 $ 604.7 $ 440.7 Less: total income from discontinued operations, net of tax (539.1 ) (84.4 ) (573.2 ) (515.4 ) Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 23.0 $ 6.4 $ 31.5 $ (74.7 ) Allocation of earnings to participating restricted common shares (1) (.3 ) (.1 ) (.4 ) .8 Basic and diluted earnings per share numerators $ 22.7 $ 6.3 $ 31.1 $ (73.9 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,297.2 4,867.4 4,477.0 5,166.6 Average unvested restricted common shares (2) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Basic earnings per share denominator 4,243.5 4,799.3 4,422.5 5,103.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (3) 4,297.2 4,879.4 4,477.0 5,174.8 Average unvested restricted common shares (2) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Diluted earnings per share denominator (3) 4,243.5 4,811.3 4,422.5 5,111.9 Basic earnings per share (in dollars) - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders $ 5.36 $ 1.31 $ 7.03 $ (14.47 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings (loss) $ 5.36 $ 1.31 $ 6.03 $ (15.47 ) Diluted earnings per share (in dollars) - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders $ 5.36 $ 1.31 $ 7.03 $ (14.47 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings (loss) $ 5.36 $ 1.31 $ 6.03 $ (15.47 ) (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 13 — “Employee Share-Based Compensation Plans” . (3) The diluted earnings per share denominator for the three and nine months ended September 30, 2016 includes the impact of 120,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 11,943 and 8,208 incremental shares outstanding over the period. The following table summarizes the undistributed net earnings (loss) from continuing operations for the three and nine months ended September 30, 2017 and 2016 . See Note 15 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 22.7 $ 6.3 $ 31.1 $ (73.9 ) Dividends declared net of restricted common share amounts (1) — — (4.5 ) (5.9 ) Total undistributed net earnings (loss), net of restricted common share amounts $ 22.7 $ 6.3 $ 26.6 $ (79.8 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

Non-controlling Interest

Non-controlling Interest	9 Months Ended
	Sep. 30, 2017
Noncontrolling Interest [Abstract]
Non-controlling Interests	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 September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. — % $ — 23.9 % $ 244.6 Other, excluding mutuals and reciprocals HG Global 3.1 16.1 3.1 16.6 MediaAlpha 40.0 9.5 40.0 11.7 Buzzmove 22.9 3.0 29.1 2.9 Wobi 5.0 — 5.0 .1 Dewar (1) — — 18.8 3.9 Total other, excluding mutuals and reciprocals 28.6 35.2 Mutuals and reciprocals BAM 100.0 (160.2 ) 100.0 (150.9 ) SSIE — — 100.0 4.4 Total mutuals and reciprocals (160.2 ) (146.5 ) Total non-controlling interests $ (131.6 ) $ 133.3 (1) Dewar is a subsidiary of OneBeacon.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are HG Global/BAM, MediaAlpha and Other Operations. As a result of the Sirius Group and Tranzact sales and the OneBeacon Transaction, the results of operations for Sirius Group and OneBeacon, previously reported in their own respective segments, and Tranzact, previously reported in the Other Operations segment, have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of operations and comprehensive income. Beginning in the second quarter of 2017, MediaAlpha’s results have been presented as a separate segment within White Mountains’s consolidated financial statements. Prior year amounts have been reclassified to conform to the current period’s presentation. See Note 15 — “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 HG Global BAM (1) MediaAlpha Other Operations Total Three Months Ended September 30, 2017 Earned insurance premiums $ 1.8 $ .6 $ — $ — $ 2.4 Net investment income 1.0 2.3 — 8.9 12.2 Net investment income (loss) - BAM Surplus Note interest 4.8 (4.8 ) — — — Net realized and unrealized investment gains .1 .7 — 31.7 32.5 Advertising and commission revenues — — 37.9 .9 38.8 Other revenue — .2 — 1.4 1.6 Total revenues 7.7 (1.0 ) 37.9 42.9 87.5 Insurance acquisition expenses .4 .5 — — .9 Other underwriting expenses — .1 — — .1 Cost of sales — — 32.2 .9 33.1 General and administrative expenses .3 10.3 6.1 27.4 44.1 Interest expense — — .1 .8 .9 Total expenses .7 10.9 38.4 29.1 79.1 Pre-tax income (loss) $ 7.0 $ (11.9 ) $ (.5 ) $ 13.8 $ 8.4 (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2017 Earned insurance premiums $ 5.0 $ 1.6 $ — $ 1.0 $ 7.6 Net investment income 2.4 6.5 — 30.8 39.7 Net investment income (loss) - BAM Surplus Note interest 14.3 (14.3 ) — — — Net realized and unrealized investment gains .4 2.8 — 99.3 102.5 Advertising and commission revenues — — 101.2 2.7 103.9 Other revenue — .8 — 5.3 6.1 Total revenues 22.1 (2.6 ) 101.2 139.1 259.8 Losses and loss adjustment expenses — — — 1.1 1.1 Insurance acquisition expenses 1.0 2.0 — .1 3.1 Other underwriting expenses — .3 — — .3 Cost of sales — — 86.0 2.7 88.7 General and administrative expenses .8 30.7 17.9 112.8 162.2 Interest expense — — .6 1.2 1.8 Total expenses 1.8 33.0 104.5 117.9 257.2 Pre-tax income (loss) $ 20.3 $ (35.6 ) $ (3.3 ) $ 21.2 $ 2.6 (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Three Months Ended September 30, 2016 Earned insurance premiums $ 1.2 $ .3 $ — $ 1.9 $ 3.4 Net investment income .6 1.7 — 7.3 9.6 Net investment income (loss) - BAM Surplus Note interest 4.5 (4.5 ) — — — Net realized and unrealized investment (losses) gains (.3 ) (1.6 ) — 12.8 10.9 Advertising and commission revenues — — 27.6 .6 28.2 Other revenue — .4 — 4.3 4.7 Total revenues 6.0 (3.7 ) 27.6 26.9 56.8 Losses and loss adjustment expenses — — — 2.2 2.2 Insurance acquisition expenses .2 .6 — .5 1.3 Other underwriting expenses — .1 — .1 .2 Cost of sales — — 23.0 1.0 24.0 General and administrative expenses .6 9.3 5.7 26.8 42.4 Interest expense — — .2 .3 .5 Total expenses .8 10.0 28.9 30.9 70.6 Pre-tax income (loss) $ 5.2 $ (13.7 ) $ (1.3 ) $ (4.0 ) $ (13.8 ) (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2016 Earned insurance premiums $ 3.1 $ 1.0 $ — $ 6.1 $ 10.2 Net investment income 1.6 5.1 — 11.5 18.2 Net investment income (loss) - BAM Surplus Note interest 13.4 (13.4 ) — — — Net realized and unrealized investment gains 2.3 6.5 — 18.4 27.2 Advertising and commission revenues — — 88.4 1.2 89.6 Other revenue — .8 — 17.2 18.0 Total revenues 20.4 — 88.4 54.4 163.2 Losses and loss adjustment expenses — — — 6.8 6.8 Insurance acquisition expenses .6 1.9 — 1.9 4.4 Other underwriting expenses — .3 — .1 .4 Cost of sales — — 74.0 2.9 76.9 General and administrative expenses 1.4 28.1 16.3 99.1 144.9 Interest expense — — .7 1.9 2.6 Total expenses 2.0 30.3 91.0 112.7 236.0 Pre-tax income (loss) $ 18.4 $ (30.3 ) $ (2.6 ) $ (58.3 ) $ (72.8 ) (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.

Investments in Symetra Investme

Investments in Symetra Investments in Symetra	9 Months Ended
	Sep. 30, 2017
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in Symetra	Investment in Symetra White Mountains’s investment in Symetra represented an investment in which White Mountains had a significant voting and economic interest but did not control the entity. In August 2015, Symetra announced it had entered into a definitive merger agreement with Sumitomo Life pursuant to which Sumitomo Life would acquire all of the outstanding shares of Symetra. Following the announcement and Symetra shareholders’ November 5, 2015 meeting to approve the transaction, White Mountains relinquished its representation on Symetra’s board of directors. As a result, White Mountains changed its accounting for Symetra common shares from the equity method to fair value 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

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Share-Based Incentive Compensation Plans	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 of White Mountains. As of September 30, 2017, White Mountains’s share-based compensation incentive awards consist of performance shares and restricted shares. 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 approved for payment. The following table summarizes performance share activity for the three and nine months ended September 30, 2017 and 2016 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 50,575 $ 31.2 73,297 $ 32.5 80,353 $ 42.4 93,654 $ 57.7 Shares paid (1) — — — — (30,838 ) (21.6 ) (36,294 ) (41.0 ) New grants — — 6,400 — 17,510 — 22,615 — Forfeitures and cancellations (2) (256 ) (.4 ) (160 ) (.1 ) (16,706 ) (9.4 ) (438 ) .2 Expense recognized — 7.0 — 5.0 — 26.4 — 20.5 End of period (3) 50,319 $ 37.8 79,537 $ 37.4 50,319 $ 37.8 79,537 $ 37.4 (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, which were paid in April 2016, 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 September 30, 2017 and 2016 exclude 2,195 and 7,315 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 September 30, 2017 , the total additional compensation cost to be recognized would have been $28.7 million , based on accrual factors (common share price and payout assumptions) at September 30, 2017 . 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 September 30, 2017 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 18,370 $ 20.6 2016 – 2018 16,235 12.1 2017 – 2019 16,480 5.7 Sub-total 51,085 38.4 Assumed forfeitures (766 ) (.6 ) September 30, 2017 50,319 $ 37.8 Restricted Shares Granted Under the WTM Incentive Plan The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 53,815 $ 23.5 66,470 $ 24.2 70,620 $ 19.7 70,675 $ 15.7 Issued — — 4,150 3.4 17,785 16.7 25,365 19.7 Vested (260 ) — — — (28,846 ) — (24,620 ) — Forfeited — — — — (6,004 ) (3.5 ) (800 ) .2 Expense recognized — (3.8 ) — (3.8 ) — (13.2 ) — (11.8 ) End of period (1) 53,555 $ 19.7 70,620 $ 23.8 53,555 $ 19.7 70,620 $ 23.8 (1) Restricted share awards outstanding as of September 30, 2017 and 2016 include 2,195 and 5,235 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations. During the second quarter of 2017, White Mountains issued 550 restricted shares that vest on January 1, 2020, 250 restricted shares that vest on January 1, 2019 and 250 restricted shares that vest on January 1, 2018. During the first quarter of 2017, White Mountains issued 16,735 restricted shares that vest on January 1, 2020. During the first nine months of 2016, White Mountains issued 24,615 restricted shares that vest on January 1, 2019 and 750 restricted shares that vest on January 1, 2018. The unamortized issue date fair value at September 30, 2017 is expected to be recognized ratably over the remaining vesting periods. Stock Options Non-Qualified Options As January 20, 2017, the 125,000 Non-Qualified options issued to the Company’s former Chairman and CEO had been exercised. During the first quarter of 2017, 40,000 Non-Qualified Options, with an intrinsic value of $4.4 million , were exercised in exchange for 5,142 common shares with an equal total market value. During 2016, 5,000 Non-Qualified Options, with an intrinsic value of $0.4 million , were exercised at $742 per common share and 80,000 Non-Qualified Options, with an intrinsic value of $8.4 million , were exercised in exchange for 9,930 common shares with an equal total market value. Intrinsic value represents the difference between the market price of the Company’s common shares at the date of exercise and the fixed strike price of $742 per common share. The Non-Qualified Options were fully amortized as of 2011.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments White Mountains accounts for its financial instruments at fair value with the exception of the WTM Bank Facility, which was undrawn at September 30, 2017 and December 31, 2016 , the MediaAlpha Bank Facility and the Previous MediaAlpha Bank Facility, which is 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 September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Carrying Value Fair Value Carrying Value MediaAlpha Bank Facility $ 9.4 $ 9.4 $ — $ — Previous MediaAlpha Bank Facility — — 13.0 12.7 The fair value estimate for the MediaAlpha Bank Facility and the Previous MediaAlpha Bank Facility has been determined based on a discounted cash flows approach and is considered to be a Level 3 measurement.

Discontinued Operations

Discontinued Operations	9 Months Ended
	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operations	Held for Sale and Discontinued Operations OneBeacon On September 28, 2017, Intact Financial Corporation completed its previously announced acquisition of OneBeacon in an all-cash transaction for $18.10 per share. White Mountains received total proceeds of $1.3 billion and recorded a gain of $554.6 million , net of transaction costs. Net income (loss) from discontinued operations related to OneBeacon was $20.5 million and $(15.2) million though the closing date of the nine and three months ended and September 30, 2017. Net income from discontinued operations related to OneBeacon was $93.8 million and $22.5 million for the nine and three months ended September 30, 2016. Star & Shield On March 7, 2017, White Mountains completed the sale of Star & Shield and its investment in SSIE surplus notes to K2 Insurances LLC. White Mountains did not recognize any gain or loss on the sale. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains's GAAP financial statements. Tranzact On July 21, 2016, White Mountains completed the sale of Tranzact to Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million at closing. On October 5, 2016, White Mountains received additional proceeds of $1.2 million following the release of the post-closing purchase price adjustment escrow. 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. 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. Net loss from discontinued operations related to Tranzact, prior to the tax benefit described below, was $2.1 million and $2.6 million for the three and nine months ended September 30, 2016. During the third quarter of 2016, White Mountains recognized a $14.0 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 $16.1 million tax benefit was recorded to net income from discontinued operations. In the nine months ended September 30, 2017, White Mountains recorded a $1.0 million reduction to the gain from sale of Tranzact in discontinued operations as a result of 2016 state tax payments. 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 and six ended June 30, 2016, $0.4 million and $3.7 million of net investment income and 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. White Mountains recorded $(4.0) million and $356.2 million of total income from discontinued operations for the three and nine months ended September 30, 2016 and $145.3 million of other comprehensive income for the nine months ended September 30, 2016. During the third quarter of 2017, White Mountains recorded a $0.8 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees.
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operations	Net Assets Held for Sale The following table summarizes the assets and liabilities associated with businesses classified as held for sale. Amounts presented relate to OneBeacon, Star & Shield and SSIE. Millions December 31, 2016 Assets held for sale Fixed maturity investments, at fair value $ 2,175.7 Short-term investments, at amortized cost (which approximates fair value) 112.3 Common equity securities, at fair value 188.7 Other long-term investments 150.5 Total investments 2,627.2 Cash 70.5 Reinsurance recoverable on unpaid and paid losses 179.8 Insurance and reinsurance premiums receivable 229.8 Deferred acquisition costs 96.3 Deferred tax asset 126.7 Ceded unearned insurance and reinsurance premiums 44.2 Accounts receivable on unsettled investment sales 1.4 Goodwill and other intangible assets 1.2 Accrued investment income 11.3 Other assets 211.1 Total assets held for sale $ 3,599.5 Liabilities held for sale Loss and loss adjustment expense reserves $ 1,370.6 Unearned insurance and reinsurance premiums 576.3 Debt 273.2 Accrued incentive compensation 44.3 Funds held under reinsurance treaties 153.0 Other liabilities 151.9 Total liabilities held for sale 2,569.3 Net assets held for sale $ 1,030.2 Net Income (Loss) from Discontinued Operations The following table summarizes the results of operations, including related income taxes, associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2017 , the amounts presented relate to OneBeacon and Sirius Group. For the three and nine months ended September 30, 2016 , the amounts presented relate to OneBeacon, Sirius Group and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 277.9 $ — $ — $ 277.9 Net investment income 13.0 — 13.0 11.8 — — 11.8 Net realized and unrealized gains 11.5 — 11.5 15.5 — — 15.5 Other revenue 2.2 — 2.2 1.8 — 14.8 16.6 Total revenues 295.1 — 295.1 307.0 — 14.8 321.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 162.8 — — 162.8 Insurance and reinsurance acquisition expenses 51.9 — 51.9 55.1 — — 55.1 Other underwriting expenses 44.9 — 44.9 49.4 — — 49.4 General and administrative expenses 7.4 — 7.4 3.5 — 16.2 19.7 Interest expense 3.4 — 3.4 3.3 — .5 3.8 Total expenses 314.4 — 314.4 274.1 — 16.7 290.8 Pre-tax (loss) income (19.3 ) — (19.3 ) 32.9 — (1.9 ) 31.0 Income tax benefit (expense) 4.1 — 4.1 (10.4 ) — 15.9 5.5 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 22.5 — 14.0 36.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 — (4.0 ) 51.9 47.9 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 22.5 (4.0 ) 65.9 84.4 Change in foreign currency translation and other from discontinued operations .1 — .1 (.3 ) — — (.3 ) Recognition of benefit plan assets and obligations 2.9 — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 22.2 $ (4.0 ) $ 65.9 $ 84.1 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Tranzact Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 807.6 $ — $ — $ 807.6 $ 827.9 $ 240.1 $ — $ 1,068.0 Net investment income 39.7 — — 39.7 38.3 14.4 — 52.7 Net realized and unrealized gains (losses) 38.8 — — 38.8 56.8 (1.5 ) — 55.3 Other revenue 7.7 — — 7.7 3.5 .6 119.6 123.7 Total revenues 893.8 — — 893.8 926.5 253.6 119.6 1,299.7 Expenses Loss and loss adjustment expenses 546.0 — — 546.0 501.3 154.9 — 656.2 Insurance and reinsurance acquisition expenses 145.6 — — 145.6 154.8 59.0 — 213.8 Other underwriting expenses 156.2 — — 156.2 155.6 30.9 — 186.5 General and administrative expenses 21.2 — — 21.2 10.9 10.4 116.7 138.0 Interest expense 10.0 — — 10.0 9.8 7.9 3.2 20.9 Total expenses 879.0 — — 879.0 832.4 263.1 119.9 1,215.4 Pre-tax income (loss) 14.8 — — 14.8 94.1 (9.5 ) (.3 ) 84.3 Income tax benefit (expense) 5.7 — — 5.7 (.3 ) 3.1 13.8 16.6 Net income (loss) from discontinued operations 20.5 — — 20.5 93.8 (6.4 ) 13.5 100.9 Net gain (loss) from sale of discontinued operations 554.5 (.8 ) (1.0 ) 552.7 — 362.6 51.9 414.5 Total income (loss) from discontinued operations 575.0 (.8 ) (1.0 ) 573.2 93.8 356.2 65.4 515.4 Change in foreign currency translation and other from discontinued operations .3 — — .3 .5 32.0 — 32.5 Recognition of foreign currency translation and other from sale of Sirius Group, net of tax — — — — — 113.3 113.3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 $ 94.3 $ 501.5 $ 65.4 $ 661.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax payment to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2017 2016 Net cash provided from operations $ 157.0 $ 38.1 Net cash provided from investing activities 3.0 269.7 Net cash used for financing activities (61.9 ) (72.7 ) Net change in cash during the period 98.1 235.1 Cash balances at beginning of period 70.5 245.4 Net change in cash held for sale, excluding discontinued operations (.9 ) 2.6 Cash sold as part of sale of consolidated subsidiaries 167.7 345.8 Cash balances at end of period $ — $ 137.3 Supplemental cash flows information: Interest paid $ — $ (7.7 ) Net income tax payment to national governments $ — $ (18.3 ) Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 562.1 $ 90.8 $ 604.7 $ 440.7 Less: total income from continuing operations, net of tax (23.0 ) (6.4 ) (31.5 ) 74.7 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ 539.1 $ 84.4 $ 573.2 $ 515.4 Allocation of earnings to participating restricted common shares (1) (6.7 ) (1.2 ) (7.0 ) (6.3 ) Basic and diluted earnings per share numerators $ 532.4 $ 83.2 $ 566.2 $ 509.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,297.2 4,867.4 4,477.0 5,166.6 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Basic earnings per share denominator 4,243.5 4,799.3 4,422.5 5,103.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,297.2 4,879.4 4,477.0 5,174.8 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Diluted earnings per share denominator (4) 4,243.5 4,811.3 4,422.5 5,111.9 Basic earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.34 $ 128.03 $ 99.75 Diluted earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.30 $ 128.03 $ 99.60 (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 and nine months ended September 30, 2017 and 2016 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Compensation Plans” . (4) The diluted earnings per share denominator for the three and nine months ended September 30, 2016 includes the impact of 120,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 11,943 and 8,208 incremental shares outstanding over the period. Fair Value of Financial Instruments in Liabilities Held for Sale The OBH Senior Notes are recorded as debt at face value less unamortized original issue discount. The following table summarizes the fair value and carrying value of this financial instrument as of December 31, 2016 : December 31, 2016 Millions Fair Value Carrying Value OBH Senior Notes $ 274.2 $ 273.2 The fair value estimate for the OBH Senior Notes has been determined using quoted market prices. The OBH Senior Notes are considered a Level 2 measurement. OneBeacon Surplus Notes in Assets Held for Sale In the fourth quarter of 2014, in conjunction with OneBeacon’s sale of its runoff business to an affiliate of Armour Group Holdings Limited (the “OneBeacon Runoff Transaction”), OneBeacon provided financing in the form of surplus notes (the “OneBeacon Surplus Notes”) with a par value of $101.0 million which had a fair value of $71.9 million as of December 31, 2016 . The OneBeacon Surplus Notes, issued by one of the transferred entities, Bedivere Insurance Company (the “Issuer”) were in the form of both seller priority and pari passu notes. Subsequent to 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. Below is a table illustrating the valuation adjustments taken to arrive at estimated fair value of the OneBeacon Surplus Notes as of December 31, 2016 : Millions Total as of December 31, 2016 Par Value $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 5.1 Regulatory approval (2) (15.6 ) Liquidity adjustment (3) (18.6 ) Total adjustments (29.1 ) Fair value $ 71.9 (1) Represents the value of the surplus notes, at current market yields on comparable publicly traded debt, and assuming issuer is allowed to make principal and interest payments when its financial capacity is available, as measured by statutory capital in excess of a 250% RBC score under the National Association of Insurance Commissioners’ risk-based capital standards for property and casualty companies. The favorable year-to-date change in impact is due principally to the narrowing of non-investment grade credit spreads as well as the time value of money benefit from moving three months closer to modeled cash receipts. (2) Represents anticipated delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer's statutory surplus. The monetary impact of the anticipated delay is measured based on credit spreads of public securities with roughly equivalent percentages of discounted payments missed. The favorable year-to-date change in impact is driven primarily by the narrowing of non-investment grade credit spreads, which causes negative valuation impact from the anticipated delay in securing regulatory approval to be lower. (3) Represents impact of liquidity spread to account for OneBeacon's sole ownership of the notes, lack of a trading market, and unique nature of the ongoing regulatory approval process.

Financial Statement Revisions

Financial Statement Revisions	9 Months Ended
	Sep. 30, 2017
Accounting Changes and Error Corrections [Abstract]
Financial Statement Revisions	Financial Statement Revisions In October 2017, White Mountains discovered that the former CEO of Wobi, one of its overseas portfolio companies, had been reporting overstated commission revenues and related receivables to White Mountains. As a result, White Mountains has revised certain of its previously issued financial statements. The revisions resulted in reductions to commission revenues (included in advertising and commission revenues) and commissions receivable (included in other assets). In addition, the overstatements led White Mountains to write down the goodwill and other intangible assets related to Wobi to zero. This write down is recorded in general and administrative expenses in the years ended December 31, 2016 and 2015. White Mountains evaluated the impact of the misstatements resulting from the overstatements at Wobi on prior periods’ financial statements in accordance with SEC Staff Accounting Bulletin (“SAB”) Topic 1.M, Materiality , and concluded the misstatements were not material to any previously reported financial statements. However, while not material to any previously reported annual or quarterly period, the aggregate amount of prior period misstatements could be material to White Mountains's results for the full fiscal year ended December 31, 2017. White Mountains has therefore revised all periods impacted including its consolidated balance sheet as of June 30, 2017 and December 31, 2016 and 2015; and its consolidated statements of operations and comprehensive income, consolidated statements of changes in equity, cash flows and earnings per share for the years ended December 31, 2016, 2015 and 2014, the six month period ended June 30, 2017, and the three and nine month periods ended September 30, 2016. The revisions also reflect a previously recorded out of period adjustment in 2015 and 2014. The impact of these revisions to each of the previously reported consolidated statements are disclosed below. Amounts previously reported reflect the reclassification of OneBeacon, Sirius Group and Tranzact to discontinued operations for all applicable periods presented. CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited) Three months ended Nine months ended September 30, 2016 Millions, except for per share amounts As previously reported Adjustments As revised As previously reported Adjustments As revised Revenues: Earned insurance premiums $ 3.4 $ — 3.4 $ 10.2 $ — $ 10.2 Net investment income 9.6 — 9.6 18.2 — 18.2 Net realized and unrealized investment gains 10.9 — 10.9 27.2 — 27.2 Advertising and commission revenues 29.4 (1.2 ) 28.2 92.4 (2.8 ) 89.6 Other revenue 4.7 — 4.7 18.0 — 18.0 Total revenues 58.0 (1.2 ) 56.8 166.0 (2.8 ) 163.2 Expenses: Loss and loss adjustment expenses 2.2 — 2.2 6.8 — 6.8 Insurance and acquisition expenses 1.3 — 1.3 4.4 — 4.4 Other underwriting expenses .2 — .2 .4 — .4 Cost of sales 24.0 — 24.0 76.9 — 76.9 General and administrative expenses 42.7 (.3 ) 42.4 142.6 2.3 144.9 Interest expense on debt .5 — .5 2.6 — 2.6 Total expenses 70.9 (.3 ) 70.6 233.7 2.3 236.0 Pre-tax loss (12.9 ) (.9 ) (13.8 ) (67.7 ) (5.1 ) (72.8 ) Income tax benefit 17.1 — 17.1 22.7 — 22.7 Net income (loss) from continuing operations 4.2 (.9 ) 3.3 (45.0 ) (5.1 ) (50.1 ) Gain on sale of discontinued operations 47.9 — 47.9 414.5 — 414.5 Net income from discontinued operations 36.5 — 36.5 100.9 — 100.9 Net income (loss) (1) 88.6 (.9 ) 87.7 470.4 (5.1 ) 465.3 Net loss (income) attributable to non-controlling interests 3.1 — 3.1 (24.6 ) — (24.6 ) Net income (loss) attributable to White Mountains's common shareholders 91.7 (.9 ) 90.8 445.8 (5.1 ) 440.7 Other comprehensive income (loss), net of tax .2 (.2 ) — 145.6 (.2 ) 145.4 Comprehensive income (loss) 91.9 (1.1 ) 90.8 591.4 (5.3 ) 586.1 Comprehensive loss attributable to non-controlling interests .1 — .1 .1 — .1 Comprehensive income (loss) attributable to White Mountains's common shareholders $ 92.0 $ (1.1 ) $ 90.9 $ 591.5 $ (5.3 ) $ 586.2 Basic and diluted earnings per share - continuing operations $ 1.50 $ (.18 ) $ 1.32 $ (13.49 ) $ (1.00 ) $ (14.49 ) (1) The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities. CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited) Six months ended June 30, 2017 Millions, except for per share amounts As previously reported Adjustments As revised Revenues: Earned insurance premiums $ 5.2 $ — $ 5.2 Net investment income 27.5 — 27.5 Net realized and unrealized investment gains 70.0 — 70.0 Advertising and commission revenues 71.7 (6.6 ) 65.1 Other revenue 4.5 — 4.5 Total revenues 178.9 (6.6 ) 172.3 Expenses: Loss and loss adjustment expenses 1.1 — 1.1 Insurance and acquisition expenses 2.2 — 2.2 Other underwriting expenses .2 — .2 Cost of sales 55.6 — 55.6 General and administrative expenses 117.9 .1 118.0 Interest expense on debt .9 — .9 Total expenses 177.9 .1 178.0 Pre-tax income (loss) 1.0 (6.7 ) (5.7 ) Income tax benefit 1.3 — 1.3 Net income (loss) from continuing operations 2.3 (6.7 ) (4.4 ) Loss on sale of discontinued operations (1.6 ) — (1.6 ) Net income from discontinued operations 35.7 — 35.7 Net income (loss) 36.4 (6.7 ) 29.7 Net loss (income) attributable to non-controlling interests 13.4 (.4 ) 13.0 Net income (loss) attributable to White Mountains's common shareholders 49.8 (7.1 ) 42.7 Other comprehensive income (loss), net of tax 1.7 (1.3 ) .4 Comprehensive income (loss) 51.5 (8.4 ) 43.1 Comprehensive income attributable to non-controlling interests (.1 ) — (.1 ) Comprehensive income (loss) attributable to White Mountains's common shareholders $ 51.4 $ (8.4 ) $ 43.0 Basic and diluted earnings per share - continuing operations $ 3.42 $ (1.59 ) $ 1.83 CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME As previously reported Adjustments As revised Years ended December 31, Millions, except for per share amounts 2016 2015 2014 2016 2015 2014 2016 2015 2014 Revenues: Earned insurance premiums $ 13.4 $ 12.0 $ 7.9 $ — $ — $ — $ 13.4 $ 12.0 $ 7.9 Net investment income 32.1 10.9 12.2 — — — 32.1 10.9 12.2 Net realized and unrealized investment gains (27.4 ) 260.5 38.1 — — — (27.4 ) 260.5 38.1 Advertising and commission revenues 126.9 110.1 65.7 (8.6 ) (2.7 ) (.2 ) 118.3 107.4 65.5 Other revenue (1) 21.3 41.8 20.5 — 7.4 (6.9 ) 21.3 49.2 13.6 Total revenues 166.3 435.3 144.4 (8.6 ) 4.7 (7.1 ) 157.7 440.0 137.3 Expenses: Loss and loss adjustment expenses 8.0 8.2 8.9 — — — 8.0 8.2 8.9 Insurance and acquisition expenses 5.6 6.3 2.9 — — — 5.6 6.3 2.9 Other underwriting expenses .5 .4 .4 — — — .5 .4 .4 Cost of sales 102.0 93.6 57.8 — — — 102.0 93.6 57.8 General and administrative expenses 183.7 193.2 144.9 2.2 7.5 — 185.9 200.7 144.9 Interest expense on debt 3.0 1.6 1.2 — — — 3.0 1.6 1.2 Total expenses 302.8 303.3 216.1 2.2 7.5 — 305.0 310.8 216.1 Pre-tax income (136.5 ) 132.0 (71.7 ) (10.8 ) (2.8 ) (7.1 ) (147.3 ) 129.2 (78.8 ) Income benefit (expense) 32.9 (12.7 ) 2.5 — — — 32.9 (12.7 ) 2.5 Net income from continuing operations (103.6 ) 119.3 (69.2 ) (10.8 ) (2.8 ) (7.1 ) (114.4 ) 116.5 (76.3 ) Gain on sale of discontinued operations 415.1 17.9 17.2 — — — 415.1 17.9 17.2 Net income from discontinued operations 108.3 117.2 296.4 — — — 108.3 117.2 296.4 Income (loss) before equity in earnings of unconsolidated affiliates 419.8 254.4 244.4 (10.8 ) (2.8 ) (7.1 ) 409.0 251.6 237.3 Equity in earnings of unconsolidated affiliates — 25.1 45.6 — — — 25.1 45.6 Net income (loss) (2) 419.8 279.5 290.0 (10.8 ) (2.8 ) (7.1 ) 409.0 276.7 282.9 Net (income) loss attributable to non-controlling interests (7.3 ) 18.1 22.2 .1 .4 .1 (7.2 ) 18.5 22.3 Net income (loss) attributable to White Mountains's common shareholders 412.5 297.6 312.2 (10.7 ) (2.4 ) (7.0 ) 401.8 295.2 305.2 Other comprehensive income, net of tax (1) 145.6 (100.4 ) (104.9 ) — (7.4 ) 6.9 145.6 (107.8 ) (98.0 ) Comprehensive income (loss) 558.1 197.2 207.3 (10.7 ) (9.8 ) (.1 ) 547.4 187.4 207.2 Comprehensive (income) loss attributable to non-controlling interests (.3 ) — 3.3 — — — (.3 ) — 3.3 Comprehensive income (loss) attributable to White Mountains's common shareholders $ 557.8 $ 197.2 $ 210.6 $ (10.7 ) $ (9.8 ) $ (.1 ) $ 547.1 $ 187.4 $ 210.5 Basic and diluted earnings per share - continuing operations $ (22.13 ) $ 27.63 $ (.24 ) $ (2.15 ) $ (.42 ) $ (1.16 ) $ (24.28 ) $ 27.21 $ (1.40 ) (1) In 2015 and 2014, White Mountains recorded a foreign currency translation gain related to its investment in Symetra in net income when it should have been recorded through other comprehensive income. The correction to properly reflect the translation amount through other comprehensive income did not have any impact on comprehensive income attributable to White Mountains's common shareholders or to book value per share. (2) The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities. CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY As previously reported Adjustments As revised Years ended December 31, Millions 2016 2015 2014 2016 2015 2014 2016 2015 2014 Common shares and paid-in surplus $ 810.7 $ 978.2 $ 1,034.7 $ — $ — $ — $ 810.7 $ 978.2 $ 1,034.7 Retained earnings, beginning of year 3,084.9 3,010.5 2,801.9 (9.9 ) (7.5 ) (.5 ) 3,075.0 3,003.0 2,801.4 Share repurchases (694.8 ) (217.2 ) (97.4 ) — — — (694.8 ) (217.2 ) (97.4 ) Net income (loss) 412.5 297.6 312.2 (10.7 ) (2.4 ) (7.0 ) 401.8 295.2 305.2 Dividends (5.4 ) (6.0 ) (6.2 ) — — — (5.4 ) (6.0 ) (6.2 ) Retained earnings, end of year 2,797.2 3,084.9 3,010.5 (20.6 ) (9.9 ) (7.5 ) 2,776.6 3,075.0 3,003.0 Accumulated other comprehensive income, after tax, beginning of year (149.9 ) (49.5 ) 52.1 — 7.4 .5 (149.9 ) (42.1 ) 52.6 Net change in foreign currency translation 31.4 (65.8 ) (168.2 ) — (7.4 ) 6.9 31.4 (73.2 ) (161.3 ) Net other changes in AOCI 113.9 (34.6 ) 66.6 — — — 113.9 (34.6 ) 66.6 Accumulated other comprehensive income, after tax, end of year (4.6 ) (149.9 ) (49.5 ) — — 7.4 (4.6 ) (149.9 ) (42.1 ) Total White Mountains Common Shareholders' Equity 3,603.3 3,913.2 3,995.7 (20.6 ) (9.9 ) (.1 ) 3,582.7 3,903.3 3,995.6 Non-controlling interests, beginning of year 454.8 542.7 491.7 (.5 ) (.1 ) — 454.3 542.6 491.7 Net income (loss) 7.3 (18.1 ) (22.2 ) (.1 ) (.4 ) (.1 ) 7.2 (18.5 ) (22.3 ) Other changes in NCI (328.2 ) (69.8 ) 73.2 — — — (328.2 ) (69.8 ) 73.2 Non-controlling interests, end of year 133.9 454.8 542.7 (.6 ) (.5 ) (.1 ) 133.3 454.3 542.6 Total equity $ 3,737.2 $ 4,368.0 $ 4,538.4 $ (21.2 ) $ (10.4 ) $ (.2 ) $ 3,716.0 $ 4,357.6 $ 4,538.2 CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) Nine months ended September 30, 2016 Six months ended June 30, 2017 Millions As previously reported Adjustments As revised As previously reported Adjustments As revised Common shares and paid-in surplus $ 809.8 $ — $ 809.8 $ 815.1 $ .7 $ 815.8 Retained earnings, beginning of year 3,084.9 (9.9 ) 3,075.0 2,797.2 (20.6 ) 2,776.6 Share repurchases (678.6 ) — (678.6 ) (7.2 ) — (7.2 ) Net income (loss) 445.8 (5.1 ) 440.7 49.8 (7.1 ) 42.7 Dividends (5.4 ) — (5.4 ) (4.6 ) — (4.6 ) Retained earnings, end of period 2,846.7 (15.0 ) 2,831.7 2,835.2 (27.7 ) 2,807.5 Accumulated other comprehensive income, after tax, beginning of year (149.9 ) — (149.9 ) (4.6 ) — (4.6 ) Net change in foreign currency translation 32.0 (.2 ) 31.8 1.6 (1.3 ) .3 Net other changes in AOCI 113.6 — 113.6 — — — Accumulated other comprehensive income, after tax, end of year (4.3 ) (.2 ) (4.5 ) (3.0 ) (1.3 ) (4.3 ) Total White Mountains Common Shareholders' Equity 3,652.2 (15.2 ) 3,637.0 3,647.3 (28.3 ) 3,619.0 Non-controlling interests, beginning of year 454.8 (.5 ) 454.3 133.9 (.6 ) 133.3 Net income (loss) 24.6 — 24.6 (13.4 ) .4 (13.0 ) Other changes in NCI (330.2 ) — (330.2 ) (3.7 ) (.7 ) (4.4 ) Non-controlling interests, end of period 149.2 (.5 ) 148.7 116.8 (.9 ) 115.9 Total equity $ 3,801.4 $ (15.7 ) $ 3,785.7 $ 3,764.1 $ (29.2 ) $ 3,734.9 CONSOLIDATED BALANCE SHEETS December 31, 2016 December 31, 2015 Millions As previously reported Adjustments As revised As previously reported Adjustments As revised Assets Investments $ 2,714.4 $ — $ 2,714.4 $ 1,679.7 $ — $ 1,679.7 Cash 80.2 — 80.2 77.8 — 77.8 Insurance premiums receivable 1.6 — 1.6 1.3 — 1.3 Deferred acquisition costs 10.6 — 10.6 6.9 — 6.9 Deferred tax asset — — — — — — Accrued investment income 14.8 — 14.8 3.8 — 3.8 Accounts receivable on unsettled investment sales 4.8 — 4.8 11.4 — 11.4 Goodwill 31.7 (5.8 ) 25.9 24.1 (5.5 ) 18.6 Intangible assets 23.0 (3.7 ) 19.3 28.9 (2.0 ) 26.9 Assets held for sale 3,599.5 — 3,599.5 8,365.6 — 8,365.6 Other assets 64.1 (15.0 ) 49.1 83.1 (3.9 ) 79.2 Total assets $ 6,544.7 $ (24.5 ) $ 6,520.2 $ 10,282.6 $ (11.4 ) $ 10,271.2 Liabilities Unearned insurance premiums $ 82.9 $ — $ 82.9 $ 50.2 $ — $ 50.2 Debt 12.7 — 12.7 64.7 — 64.7 Deferred tax liability — — — 27.4 — 27.4 Accrued incentive compensation 95.7 — 95.7 96.2 — 96.2 Liabilities held for sale 2,569.3 — 2,569.3 5,618.1 — 5,618.1 Other liabilities 46.9 (3.3 ) 43.6 58.0 (1.0 ) 57.0 Total liabilities 2,807.5 (3.3 ) 2,804.2 5,914.6 (1.0 ) 5,913.6 Equity White Mountains's common shares 4.6 — 4.6 5.6 — 5.6 Paid in surplus 806.1 — 806.1 972.6 — 972.6 Retained earnings 2,797.2 (20.6 ) 2,776.6 3,084.9 (9.9 ) 3,075.0 Accumulated other comprehensive income, net of tax (4.6 ) — (4.6 ) (149.9 ) — (149.9 ) Total White Mountains's common shareholders' equity 3,603.3 (20.6 ) 3,582.7 3,913.2 (9.9 ) 3,903.3 Non-controlling interests 133.9 (.6 ) 133.3 454.8 (.5 ) 454.3 Total equity 3,737.2 (21.2 ) 3,716.0 4,368.0 (10.4 ) 4,357.6 Total liabilities and equity $ 6,544.7 $ (24.5 ) $ 6,520.2 $ 10,282.6 $ (11.4 ) $ 10,271.2 CONSOLIDATED BALANCE SHEET (Unaudited) June 30, 2017 Millions As previously reported Adjustments As revised Assets Investments $ 2,692.9 $ — $ 2,692.9 Cash 53.3 — 53.3 Insurance and reinsurance premiums receivable 2.8 — 2.8 Deferred acquisition costs 13.0 — 13.0 Accrued investment income 16.0 — 16.0 Accounts receivable on unsettled investment sales 199.5 — 199.5 Goodwill 31.7 (5.8 ) 25.9 Intangible assets 17.8 (3.4 ) 14.4 Assets held for sale 3,696.4 — 3,696.4 Other assets 62.8 (19.7 ) 43.1 Total assets $ 6,786.2 $ (28.9 ) $ 6,757.3 Liabilities Unearned insurance premiums $ 109.9 $ — $ 109.9 Debt 10.6 — 10.6 Accrued incentive compensation 63.3 — 63.3 Accounts payable for unsettled investment purchases 114.6 — 114.6 Liabilities held for sale 2,678.8 — 2,678.8 Other liabilities 44.9 .3 45.2 Total liabilities 3,022.1 .3 3,022.4 Equity White Mountains's common shares 4.6 — 4.6 Paid in surplus 810.5 .7 811.2 Retained earnings 2,835.2 (27.7 ) 2,807.5 Accumulated other comprehensive income, net of tax (3.0 ) (1.3 ) (4.3 ) Total White Mountains's common shareholders' equity 3,647.3 (28.3 ) 3,619.0 Non-controlling interests 116.8 (.9 ) 115.9 Total equity 3,764.1 (29.2 ) 3,734.9 Total liabilities and equity $ 6,786.2 $ (28.9 ) $ 6,757.3

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Note 17. Contingencies Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 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 $9.3 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 nine-month periods ended September 30, 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. 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 September 30, 2017 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 any 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. As of September 30, 2017 , there was an insignificant amount of unamortized debt issuance costs included in debt. Recently Issued Accounting Pronouncements Stock Compensation In May 2017, the FASB issued ASU 2017-09, Stock Compensation: Scope of Modification Accounting (ASC 718), which narrows the scope of transactions subject to modification accounting to changes in terms of an award that result in a change in the award’s fair value, vesting conditions or classification. The new guidance becomes effective for fiscal years beginning after December 15, 2017. White Mountains does not expect a material impact from implementation of this guidance. Cash Flow Statement In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230) . Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented. The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains does not expect a material impact from implementation of this 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 requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. This differs from current U.S. GAAP, which delays recognition until it is probable a loss has been incurred. The new guidance is expected to accelerate recognition of credit losses. The types of assets within the scope of the new guidance include 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 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 guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and accordingly, does not expect 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. Revenue from insurance contracts, investment income and investments gains and losses are excluded from the scope of 2014-09. 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 does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s revenues from customers.

Significant Transactions (Table

Significant Transactions (Tables)	9 Months Ended
	Sep. 30, 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’s book value from sale of Tranzact $ 82.1

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	Pre-tax net investment income for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Investment income: Fixed maturity investments $ 9.8 $ 9.3 $ 32.9 $ 17.4 Short-term investments .3 .2 .6 .7 Common equity securities 2.7 .5 7.7 1.0 Other long-term investments .1 — .5 .4 Total investment income 12.9 10.0 41.7 19.5 Third-party investment expenses (.7 ) (.4 ) (2.0 ) (1.3 ) Net investment income, pre-tax $ 12.2 $ 9.6 $ 39.7 $ 18.2
Schedule of Realized and Unrealized Gain (Loss) on Investments	Net realized and unrealized investment gains (losses) for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Net realized investment gains, pre-tax $ 6.8 $ .7 $ 20.8 $ 265.0 Net unrealized investment gains (losses), pre-tax 25.7 10.2 81.7 (237.8 ) Net realized and unrealized investment gains, pre-tax 32.5 10.9 102.5 27.2 Income tax expense attributable to net realized and unrealized investment gains (3.9 ) — (9.5 ) (4.0 ) Net realized and unrealized investment gains, after tax $ 28.6 $ 10.9 $ 93.0 $ 23.2
Schedule of realized gain (loss) on investments	Net Realized Investment Gains (Losses) Net realized investment gains (losses) for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ .6 $ 1.2 $ 1.8 $ .3 $ — $ .3 Short-term investments (.1 ) — (.1 ) .2 — .2 Common equity securities 4.9 5.4 10.3 .2 — .2 Other long-term investments 2.0 (7.2 ) (5.2 ) — — — Net realized investment gains (losses), pre-tax 7.4 (.6 ) 6.8 .7 — .7 Income tax expense attributable to net realized investment gains (.6 ) — (.6 ) (.1 ) — (.1 ) Net realized investment gains (losses), after tax $ 6.8 $ (.6 ) $ 6.2 $ .6 $ — $ .6 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ (.4 ) $ 2.7 $ 2.3 $ 2.0 $ — $ 2.0 Short-term investments (.1 ) — (.1 ) .4 — .4 Common equity securities 18.5 6.0 24.5 262.6 — 262.6 Other long-term investments 3.0 (8.9 ) (5.9 ) — — — Net realized investment gains (losses), pre-tax 21.0 (.2 ) 20.8 265.0 — 265.0 Income tax expense attributable to net realized investment gains (3.6 ) — (3.6 ) (45.1 ) — (45.1 ) Net realized investment gains (losses), after tax $ 17.4 $ (.2 ) $ 17.2 $ 219.9 $ — $ 219.9
Schedule of unrealized gain (loss) on investments	Net Unrealized Investment Gains (Losses) Net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions Net unrealized gains (losses) Net gains (losses) Total net unrealized gains (losses) reflected in earnings Net unrealized (losses) gains Net Total net unrealized (losses) gains reflected in earnings Fixed maturity investments $ 2.0 $ 5.1 $ 7.1 $ (2.0 ) $ — $ (2.0 ) Common equity securities 26.6 (3.1 ) 23.5 8.5 .2 8.7 Other long-term investments (2.5 ) (2.4 ) (4.9 ) 3.4 .1 3.5 Net unrealized investment gains (losses), pre-tax 26.1 (.4 ) 25.7 9.9 .3 10.2 Income tax (expense) benefit attributable to net unrealized investment gains (losses) (3.3 ) — (3.3 ) .1 — .1 Net unrealized investment gains (losses), after tax $ 22.8 $ (.4 ) $ 22.4 $ 10.0 $ .3 $ 10.3 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions Net unrealized gains Net gains (losses) Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 19.4 $ 12.6 $ 32.0 $ 13.3 $ — $ 13.3 Common equity securities 53.5 — 53.5 (256.1 ) 2.6 (253.5 ) Other long-term investments 9.2 (13.0 ) (3.8 ) 2.1 .3 2.4 Net unrealized investment gains (losses), pre-tax 82.1 (.4 ) 81.7 (240.7 ) 2.9 (237.8 ) Income tax (expense) benefit attributable to net unrealized investment gains (losses) (5.9 ) — (5.9 ) 41.1 — 41.1 Net unrealized investment gains (losses), after tax $ 76.2 $ (.4 ) $ 75.8 $ (199.6 ) $ 2.9 $ (196.7 )
Net unrealized investment gains (losses) for Level 3 investments	Total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2017 and 2016 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Fixed maturity investments $ — $ — $ — $ .1 Other long-term investments (.7 ) (.9 ) (2.2 ) .7 Total unrealized investment (losses) gains, pre-tax - Level 3 investments $ (.7 ) $ (.9 ) $ (2.2 ) $ .8
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	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 September 30, 2017 and December 31, 2016 were as follows: September 30, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 218.1 $ .1 $ (.4 ) $ — $ 217.8 Debt securities issued by corporations 608.3 3.2 (1.0 ) 14.5 625.0 Mortgage and asset-backed securities 389.1 1.2 (2.4 ) — 387.9 Municipal obligations 252.1 3.1 (.5 ) — 254.7 Foreign government, agency and provincial obligations 4.5 — (.1 ) .2 4.6 Total fixed maturity investments $ 1,472.1 $ 7.6 $ (4.4 ) $ 14.7 $ 1,490.0 December 31, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains Carrying value U.S. Government and agency obligations $ 112.1 $ — $ (1.1 ) $ — $ 111.0 Debt securities issued by corporations 752.0 2.3 (10.1 ) 2.1 746.3 Mortgage and asset-backed securities 986.9 .8 (7.9 ) — 979.8 Municipal obligations 238.7 1.1 (1.3 ) — 238.5 Foreign government, agency and provincial obligations 12.0 .1 — — 12.1 Total fixed maturity investments $ 2,101.7 $ 4.3 $ (20.4 ) $ 2.1 $ 2,087.7 Less: fixed maturity investments reclassified to assets held for sale related to SSIE (6.6 ) Total fixed maturity investments $ 2,081.1 The 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 September 30, 2017 and December 31, 2016 were as follows: September 30, 2017 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 693.8 $ 82.6 $ (2.0 ) $ — $ 774.4 Other long-term investments $ 254.5 $ 13.1 $ (21.2 ) $ (16.8 ) $ 229.6 December 31, 2016 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 258.6 $ 29.0 $ (2.0 ) $ — $ 285.6 Other long-term investments $ 194.0 $ 7.9 $ (25.2 ) $ (3.9 ) $ 172.8 Other Long-term Investments Other long-term investments as of September 30, 2017 and December 31, 2016 were as follows: Carrying Value at Millions September 30, 2017 December 31, 2016 Hedge funds and private equity funds, at fair value $ 154.9 $ 82.6 Private equity securities and limited liability companies, at fair value (1)(2) 59.1 57.6 Private convertible preferred securities, at fair value (1) 27.2 30.6 Forward Contracts (15.4 ) (1.2 ) Other 3.8 3.2 Total other-long term investments $ 229.6 $ 172.8 (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 September 30, 2017 , no funds had been drawn on the surplus note facility.
Other long-term investments	The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financials $ 54.7 $ — $ 21.5 $ — Long/short equity REIT 19.1 — 19.9 — Total hedge funds 73.8 — 41.4 — Private equity funds Manufacturing/Industrial 42.7 12.1 19.4 22.9 Aerospace/Defense/Government 28.7 15.6 19.4 25.9 Direct lending 6.7 23.3 1.4 28.6 Financial Services 3.0 13.0 1.0 5.0 Insurance — 41.2 — 41.2 Total private equity funds 81.1 105.2 41.2 123.6 Total hedge funds and private equity funds included in other long-term investments $ 154.9 $ 105.2 $ 82.6 $ 123.6
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following table summarizes the fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds as of September 30, 2017 : Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice Total Semi-annual $ 54.7 $ 19.1 $ 73.8
Fair Value of private equity funds subject to lock-up periods	The following table summarizes investments in private equity funds that were subject to lock-up periods as of September 30, 2017 : Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $4.0 $18.2 $33.5 $25.4 $81.1
Fair value measurements by level, investment securities	The following tables summarize White Mountains’s fair value measurements for investments as of September 30, 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 7 . September 30, 2017 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 217.8 $ 217.8 $ — $ — Debt securities issued by corporations: Consumer 121.9 — 121.9 — Utilities 109.3 — 109.3 — Health Care 85.6 — 85.6 — Communications 81.7 — 81.7 — Materials 68.6 — 68.6 — Financials 55.6 — 50.0 5.6 Technology 52.0 — 52.0 — Industrial 38.3 — 38.3 — Energy 12.0 — 12.0 — Total debt securities issued by corporations 625.0 — 619.4 5.6 Mortgage and asset-backed securities 387.9 — 387.9 — Municipal obligations 254.7 — 254.7 — Foreign government, agency and provincial obligations 4.6 — 4.6 — Total fixed maturity investments 1,490.0 217.8 1,266.6 5.6 Short-term investments (1) 786.5 764.5 22.0 — Common equity securities: Exchange traded funds (2) 492.5 434.6 57.9 — Health Care 17.1 17.1 — — Financials 14.9 14.9 — — Consumer 13.0 13.0 — — Technology 11.7 11.7 — — Communications 10.3 10.3 — — Industrial 10.2 10.2 — — Energy 4.0 4.0 — — Other (3) 200.7 — 200.7 — Total common equity securities 774.4 515.8 258.6 — Other long-term investments (4)(5) 90.1 — — 90.1 Total investments $ 3,141.0 $ 1,498.1 $ 1,547.2 $ 95.7 (1) Short-term investments are measured at amortized cost, which approximates fair value. (2) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of two investments in unit trusts that primarily invest in international equities. (4) Excludes carrying value of $(15.4) related to foreign currency forward contracts. (5) Excludes carrying value of $154.9 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient. December 31, 2016 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 111.0 $ 101.5 $ 9.5 $ — Debt securities issued by corporations: Consumer 190.8 — 190.8 — Utilities 140.8 — 140.8 — Health Care 114.9 — 114.9 — Financials 79.7 — 79.7 — Communications 72.0 — 72.0 — Materials 65.0 — 65.0 — Technology 48.8 — 48.8 — Industrial 28.2 — 28.2 — Energy 6.1 — 6.1 — Total debt securities issued by corporations 746.3 — 746.3 — Mortgage and asset-backed securities 979.8 — 979.8 — Municipal obligations 238.5 — 238.5 — Foreign government, agency and provincial obligations 12.1 — 12.1 — Total fixed maturity investments (1) 2,087.7 101.5 1,986.2 — Short-term investments (1)(2) 175.0 162.3 12.7 — Common equity securities: Exchange traded funds (3) 157.2 129.4 27.8 — Health Care 13.9 13.9 — — Consumer 8.6 8.6 — — Financials 7.7 7.7 — — Technology 7.3 7.3 — — Communications 7.0 7.0 — — Energy 2.5 2.5 — — Industrial 1.5 1.5 — — Other (4) 79.9 — 79.9 — Total common equity securities 285.6 177.9 107.7 — Other long-term investments (5)(6) 91.4 — — 91.4 Total investments (1) $ 2,639.7 $ 441.7 $ 2,106.6 $ 91.4 (1) Includes carrying value of $6.6 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE. (2) Short-term investments are measured at amortized cost, which approximates fair value. (3) ETFs traded on foreign exchanges are priced using the fund’s published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (3) Consists of one investment in a unit trust that primarily invests in international equities. (5) Excludes carrying value of $(1.2) related to foreign currency forward contracts. (6) Excludes carrying value of $82.6 associated with hedge funds and private equity funds for which fair value is measured at NAV using the practical expedient.
Debt securities issued by corporations, credit ratings	The following table summarizes the ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of September 30, 2017 and December 31, 2016 : Fair Value at Millions September 30, 2017 December 31, 2016 AA $ 26.7 $ 37.3 A 119.4 212.8 BBB 291.9 335.6 BB 165.4 143.2 B 21.6 17.4 Debt securities issued by corporations (1)(2) $ 625.0 $ 746.3 (1) Credit ratings are assigned based on the following hierarchy: (1) Standard & Poor’s Financial Services LLC ("Standard & Poor's") and (2) Moody's Investor Service, Inc. ("Moody’s"). (2) Includes carrying value of $4.2 of fixed maturity investments at December 31, 2016 that is classified as assets held for sale related to SSIE.
Mortgage-backed, asset-backed securities	The following table summarizes the carrying value of White Mountains’s mortgage and asset-backed securities as of September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 51.0 $ 51.0 $ — $ 70.3 $ 70.3 $ — FNMA 88.9 88.9 — 235.5 235.5 — FHLMC 59.7 59.7 — 59.5 59.5 — Total Agency (1) 199.6 199.6 — 365.3 365.3 — Non-agency: Residential 70.1 70.1 — 70.3 70.3 — Commercial 39.0 39.0 — 3.9 3.9 — Total Non-agency 109.1 109.1 — 74.2 74.2 — Total mortgage-backed securities 308.7 308.7 — 439.5 439.5 — Other asset-backed securities: Credit card receivables 40.5 40.5 — 214.2 214.2 — Vehicle receivables 22.7 22.7 — 205.9 205.9 — Other 16.0 16.0 — 120.2 120.2 — Total other asset-backed securities 79.2 79.2 — 540.3 540.3 — Total mortgage and asset-backed securities $ 387.9 $ 387.9 $ — $ 979.8 $ 979.8 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	The following table summarizes the security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of September 30, 2017 : Security Issuance Year Millions Fair Value 2004 2013 2014 2015 2016 2017 Non-agency RMBS $ 70.1 $ .3 $ 1.3 $ 20.6 $ 47.9 $ — $ — Non-agency CMBS 39.0 — — — — 3.7 35.3 Total $ 109.1 $ .3 $ 1.3 $ 20.6 $ 47.9 $ 3.7 $ 35.3
Non-agency residential mortgage securities, collateral quality and tranche levels	The following table summarizes the classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities as of September 30, 2017 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 70.1 $ 58.4 $ 11.7 $ — Non-prime — — — — Sub-prime — — — — Total $ 70.1 $ 58.4 $ 11.7 $ — (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings, Inc. (“Fitch”) and were senior to other “AAA” or “Aaa” bonds. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.
Non-agency commercial mortgage securities, type of interest rate and tranche levels	The following table summarizes the amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities as of September 30, 2017 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Fixed rate CMBS $ 17.8 $ — $ 16.2 $ 1.6 Floating rate CMBS 21.2 — — 21.2 Total $ 39.0 $ — $ 16.2 $ 22.8 (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.
Rollforward of fair value investments by level	The following tables summarize the changes in White Mountains’s fair value measurements by level for the nine months ended September 30, 2017 and 2016 : Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at January 1, 2017 $ 279.5 $ 2,093.8 $ — $ 91.4 $ 82.6 $ 2,547.3 (1)(2)(4) Net realized and unrealized gains (losses) 52.4 59.8 — (2.2 ) 15.6 125.6 Amortization/Accretion — (6.6 ) — — — (6.6 ) Purchases 940.7 1,038.5 31.2 2.9 64.9 2,078.2 Sales (539.0 ) (1,668.2 ) (12.5 ) (2.0 ) (8.2 ) (2,229.9 ) Deconsolidation of SSIE — (5.2 ) — — — (5.2 ) Transfers in — 13.1 — — — 13.1 Transfers out — — (13.1 ) — — (13.1 ) Balance at September 30, 2017 $ 733.6 $ 1,525.2 $ 5.6 $ 90.1 $ 154.9 $ 2,509.4 (1)(2) (1) Excludes carrying value of $(1.2) and $(15.4) as of January 1, 2017 and September 30, 2017 associated with foreign currency forward contracts. (2) Excludes carrying value of $ 175.0 and $ 786.5 at January 1, 2017 and September 30, 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 Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (2) Total Balance at January 1, 2016 $ 789.0 $ 585.6 $ — $ 103.6 $ 65.3 $ 1,543.5 (1)(3) Net realized and unrealized gains 7.2 17.4 .1 .8 1.7 27.2 Amortization/Accretion .1 (3.8 ) — — — (3.7 ) Purchases 1,387.8 2,228.5 70.0 2.2 38.4 3,726.9 Sales (1,992.7 ) (884.1 ) — (.1 ) (10.1 ) (2,887.0 ) Transfers in — 68.0 — — — 68.0 Transfers out — — (68.0 ) — — (68.0 ) Balance at September 30, 2016 $ 191.4 $ 2,011.6 $ 2.1 $ 106.5 $ 95.3 $ 2,406.9 (1)(3) (1) Excludes carrying value of $ 142.0 and $ 230.0 at January 1, 2016 and September 30, 2016 associated with short-term investments of which $0.1 and $0.1 is classified as held for sale at January 1, 2016 and September 30, 2016 . (2) 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” . (3) Includes carrying value of $9.5 and $8.3 of fixed maturity investments at January 1, 2016 and September 30, 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	The following table 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 September 30, 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 September 30, 2017 $ in millions, except share price Rating (2) Valuation Technique(s) Fair (3) Unobservable Input Debt securities issued by corporations (1) BBB Broker pricing $5.6 Broker quote - 133.792 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 $3.6 Share price - $2.52 Private convertible preferred security NR Multiple of EBITDA $0.2 EBITDA multiple - 6.00 Private convertible preferred security NR Share price of most recent transaction $27.0 Share price - $3.83 Private equity security NR Discounted cash flow/ Option pricing method $10.4 Discount rate - 21.0% Time until expiration - 4 years Volatility/Standard deviation - 50.0% Risk free rate - 1.00% (1) As of September 30, 2017, asset type consists of one security. (2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s. (3) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs. 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 Private equity security Discounted cash flow/ $9.3 Discount rate - 21.0% Time until expiration - 4 years Volatility/Standard deviation - 50.0% Risk free rate - 1.00% (1) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible28

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in Goodwill and Intangible Assets	The following table shows the change in goodwill and other intangible assets: Three Months Ended September 30, 2017 2016 Millions Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 25.9 $ 14.4 $ 40.3 $ 18.3 $ 23.3 $ 41.6 Acquisition of businesses — — — 7.6 1.1 8.7 Amortization, including foreign currency translation — (2.5 ) (2.5 ) — (2.6 ) (2.6 ) Ending balance $ 25.9 $ 11.9 $ 37.8 $ 25.9 $ 21.8 $ 47.7 Nine Months Ended September 30, 2017 2016 Millions Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 25.9 $ 19.3 $ 45.2 $ 18.6 $ 26.9 $ 45.5 Add: Amounts held for sale at beginning of the period (1) — — — — .4 .4 Acquisition of businesses — 7.6 5.0 12.6 Wobi write off — — — (.3 ) (2.5 ) (2.8 ) Amortization, including foreign currency translation — (7.4 ) (7.4 ) — (8.0 ) (8.0 ) Ending balance $ 25.9 $ 11.9 $ 37.8 $ 25.9 $ 21.8 $ 47.7 (1) See Note 15 — “Held for Sale and Discontinued Operations” .
Schedule of Intangible Assets and Goodwill	The following table is a summary of goodwill and other intangible assets as of September 30, 2017 , December 31, 2016 , and September 30, 2016 : Millions September 30, 2017 December 31, 2016 Goodwill MediaAlpha $ 18.3 $ 18.3 Buzzmove 7.6 7.6 Total goodwill 25.9 25.9 Other intangible assets MediaAlpha 11.0 18.3 Buzzmove .9 1.0 Total other intangible assets 11.9 19.3 Total goodwill and other intangible assets 37.8 45.2 Goodwill and other intangible assets held for sale — 1.2 Goodwill and other intangible assets attributed to non-controlling interests (13.7 ) (17.1 ) Goodwill and other intangible assets included in White Mountains's $ 24.1 $ 29.3

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Schedule of debt outstanding	White Mountains’s debt outstanding as of September 30, 2017 and December 31, 2016 consisted of the following: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ — N/A Unamortized issue costs — — WTM Bank Facility, carrying value — — MediaAlpha Bank Facility 9.4 5.5% — N/A Unamortized issuance cost — — MediaAlpha Bank Facility, carrying value 9.4 — Previous MediaAlpha Bank Facility — N/A 12.9 5.7% Unamortized issuance cost — (.2 ) Previous MediaAlpha Bank Facility, carrying value — 12.7 Total debt $ 9.4 $ 12.7 (1) Effective rate considers the effect of the debt issuance costs.

Derivatives (Tables)

Derivatives (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Schedule of Collateral	White Mountains does not hold or provide any collateral under its forward contracts. The following table summarizes the gross notional amount associated with the forward currency contracts as of September 30, 2017 : September 30, 2017 Millions Notional Amount Carrying Value Standard & Poor's Rating (1) Barclays Bank PLC $ 201.3 $ (14.1 ) A- JP Morgan Chase Bank N.A. 60.5 (1.3 ) A+ Total $ 261.8 $ (15.4 ) (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings) and “A-” (Strong, which is the seventh highest of twenty-three 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 and nine months ended September 30, 2016 . Three Months Ended Nine Months Ended Millions September 30, 2016 September 30, 2016 Fees, included in other revenue $ — $ 1.2 Change in fair value of variable annuity liability, included in other revenue — (.3 ) Change in fair value of derivatives, included in other revenue — (2.0 ) Foreign exchange, included in other revenue — 1.4 Total revenue — .3 Death benefit claims paid, included in general and administrative expenses — (.3 ) General and administrative expenses (.5 ) (2.4 ) Pre-tax loss $ (.5 ) $ (2.4 )
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and nine months ended September 30, 2016 and the carrying values, included in other assets, as of December 31, 2016 by type of instrument: Gains (losses) Carrying Value Three Months Ended Nine Months Ended As of Millions September 30, 2016 September 30, 2016 December 31, 2016 Fixed income/interest rate $ — $ 1.8 $ — Foreign exchange — (4.8 ) — Equity — 1.0 — Total $ — $ (2.0 ) $ —
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 nine months ended September 30, 2016 . Nine Months Ended September 30, 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 (.3 ) 2.9 (.7 ) (4.2 ) (2.0 ) Transfers in — — — — — Sales/settlements — (5.6 ) (15.8 ) 3.3 (18.1 ) End of period $ — $ — $ — $ — $ — (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.

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	9 Months Ended
	Sep. 30, 2017
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table provides a schedule of BAM’s insured obligations: September 30, 2017 December 31, 2016 Contracts outstanding 5,907 4,807 Remaining weighted average contract period outstanding (in years) 10.9 10.8 Contractual debt service outstanding (in millions): Principal $ 39,207.5 $ 33,057.3 Interest 19,681.0 16,396.6 Total debt service outstanding $ 58,888.5 $ 49,453.9 Gross unearned insurance premiums $ 118.5 $ 82.9
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 September 30, 2017 : Millions September 30, 2017 October 1, 2017 - December 31, 2017 $ 2.5 January 1, 2018 - March 31, 2018 2.5 April 1, 2018 - June 30, 2018 2.5 July 1, 2018 - September 30, 2018 2.5 October 1, 2018 - December 31, 2018 2.4 9.9 2019 9.5 2020 9.2 2021 8.7 2022 and thereafter 78.7 Total gross unearned insurance premiums $ 118.5

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2017 and 2016 . See Note 15 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 562.1 $ 90.8 $ 604.7 $ 440.7 Less: total income from discontinued operations, net of tax (539.1 ) (84.4 ) (573.2 ) (515.4 ) Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 23.0 $ 6.4 $ 31.5 $ (74.7 ) Allocation of earnings to participating restricted common shares (1) (.3 ) (.1 ) (.4 ) .8 Basic and diluted earnings per share numerators $ 22.7 $ 6.3 $ 31.1 $ (73.9 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,297.2 4,867.4 4,477.0 5,166.6 Average unvested restricted common shares (2) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Basic earnings per share denominator 4,243.5 4,799.3 4,422.5 5,103.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (3) 4,297.2 4,879.4 4,477.0 5,174.8 Average unvested restricted common shares (2) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Diluted earnings per share denominator (3) 4,243.5 4,811.3 4,422.5 5,111.9 Basic earnings per share (in dollars) - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders $ 5.36 $ 1.31 $ 7.03 $ (14.47 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings (loss) $ 5.36 $ 1.31 $ 6.03 $ (15.47 ) Diluted earnings per share (in dollars) - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders $ 5.36 $ 1.31 $ 7.03 $ (14.47 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings (loss) $ 5.36 $ 1.31 $ 6.03 $ (15.47 ) (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 13 — “Employee Share-Based Compensation Plans” . (3) The diluted earnings per share denominator for the three and nine months ended September 30, 2016 includes the impact of 120,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 11,943 and 8,208 incremental shares outstanding over the period.
Disposal Groups, Including Discontinued Operations	The following table summarizes the undistributed net earnings (loss) from continuing operations for the three and nine months ended September 30, 2017 and 2016 . See Note 15 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 22.7 $ 6.3 $ 31.1 $ (73.9 ) Dividends declared net of restricted common share amounts (1) — — (4.5 ) (5.9 ) Total undistributed net earnings (loss), net of restricted common share amounts $ 22.7 $ 6.3 $ 26.6 $ (79.8 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. Net Assets Held for Sale The following table summarizes the assets and liabilities associated with businesses classified as held for sale. Amounts presented relate to OneBeacon, Star & Shield and SSIE. Millions December 31, 2016 Assets held for sale Fixed maturity investments, at fair value $ 2,175.7 Short-term investments, at amortized cost (which approximates fair value) 112.3 Common equity securities, at fair value 188.7 Other long-term investments 150.5 Total investments 2,627.2 Cash 70.5 Reinsurance recoverable on unpaid and paid losses 179.8 Insurance and reinsurance premiums receivable 229.8 Deferred acquisition costs 96.3 Deferred tax asset 126.7 Ceded unearned insurance and reinsurance premiums 44.2 Accounts receivable on unsettled investment sales 1.4 Goodwill and other intangible assets 1.2 Accrued investment income 11.3 Other assets 211.1 Total assets held for sale $ 3,599.5 Liabilities held for sale Loss and loss adjustment expense reserves $ 1,370.6 Unearned insurance and reinsurance premiums 576.3 Debt 273.2 Accrued incentive compensation 44.3 Funds held under reinsurance treaties 153.0 Other liabilities 151.9 Total liabilities held for sale 2,569.3 Net assets held for sale $ 1,030.2 Net Income (Loss) from Discontinued Operations The following table summarizes the results of operations, including related income taxes, associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2017 , the amounts presented relate to OneBeacon and Sirius Group. For the three and nine months ended September 30, 2016 , the amounts presented relate to OneBeacon, Sirius Group and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 277.9 $ — $ — $ 277.9 Net investment income 13.0 — 13.0 11.8 — — 11.8 Net realized and unrealized gains 11.5 — 11.5 15.5 — — 15.5 Other revenue 2.2 — 2.2 1.8 — 14.8 16.6 Total revenues 295.1 — 295.1 307.0 — 14.8 321.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 162.8 — — 162.8 Insurance and reinsurance acquisition expenses 51.9 — 51.9 55.1 — — 55.1 Other underwriting expenses 44.9 — 44.9 49.4 — — 49.4 General and administrative expenses 7.4 — 7.4 3.5 — 16.2 19.7 Interest expense 3.4 — 3.4 3.3 — .5 3.8 Total expenses 314.4 — 314.4 274.1 — 16.7 290.8 Pre-tax (loss) income (19.3 ) — (19.3 ) 32.9 — (1.9 ) 31.0 Income tax benefit (expense) 4.1 — 4.1 (10.4 ) — 15.9 5.5 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 22.5 — 14.0 36.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 — (4.0 ) 51.9 47.9 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 22.5 (4.0 ) 65.9 84.4 Change in foreign currency translation and other from discontinued operations .1 — .1 (.3 ) — — (.3 ) Recognition of benefit plan assets and obligations 2.9 — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 22.2 $ (4.0 ) $ 65.9 $ 84.1 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Tranzact Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 807.6 $ — $ — $ 807.6 $ 827.9 $ 240.1 $ — $ 1,068.0 Net investment income 39.7 — — 39.7 38.3 14.4 — 52.7 Net realized and unrealized gains (losses) 38.8 — — 38.8 56.8 (1.5 ) — 55.3 Other revenue 7.7 — — 7.7 3.5 .6 119.6 123.7 Total revenues 893.8 — — 893.8 926.5 253.6 119.6 1,299.7 Expenses Loss and loss adjustment expenses 546.0 — — 546.0 501.3 154.9 — 656.2 Insurance and reinsurance acquisition expenses 145.6 — — 145.6 154.8 59.0 — 213.8 Other underwriting expenses 156.2 — — 156.2 155.6 30.9 — 186.5 General and administrative expenses 21.2 — — 21.2 10.9 10.4 116.7 138.0 Interest expense 10.0 — — 10.0 9.8 7.9 3.2 20.9 Total expenses 879.0 — — 879.0 832.4 263.1 119.9 1,215.4 Pre-tax income (loss) 14.8 — — 14.8 94.1 (9.5 ) (.3 ) 84.3 Income tax benefit (expense) 5.7 — — 5.7 (.3 ) 3.1 13.8 16.6 Net income (loss) from discontinued operations 20.5 — — 20.5 93.8 (6.4 ) 13.5 100.9 Net gain (loss) from sale of discontinued operations 554.5 (.8 ) (1.0 ) 552.7 — 362.6 51.9 414.5 Total income (loss) from discontinued operations 575.0 (.8 ) (1.0 ) 573.2 93.8 356.2 65.4 515.4 Change in foreign currency translation and other from discontinued operations .3 — — .3 .5 32.0 — 32.5 Recognition of foreign currency translation and other from sale of Sirius Group, net of tax — — — — — 113.3 113.3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 $ 94.3 $ 501.5 $ 65.4 $ 661.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax payment to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2017 2016 Net cash provided from operations $ 157.0 $ 38.1 Net cash provided from investing activities 3.0 269.7 Net cash used for financing activities (61.9 ) (72.7 ) Net change in cash during the period 98.1 235.1 Cash balances at beginning of period 70.5 245.4 Net change in cash held for sale, excluding discontinued operations (.9 ) 2.6 Cash sold as part of sale of consolidated subsidiaries 167.7 345.8 Cash balances at end of period $ — $ 137.3 Supplemental cash flows information: Interest paid $ — $ (7.7 ) Net income tax payment to national governments $ — $ (18.3 ) Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 562.1 $ 90.8 $ 604.7 $ 440.7 Less: total income from continuing operations, net of tax (23.0 ) (6.4 ) (31.5 ) 74.7 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ 539.1 $ 84.4 $ 573.2 $ 515.4 Allocation of earnings to participating restricted common shares (1) (6.7 ) (1.2 ) (7.0 ) (6.3 ) Basic and diluted earnings per share numerators $ 532.4 $ 83.2 $ 566.2 $ 509.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,297.2 4,867.4 4,477.0 5,166.6 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Basic earnings per share denominator 4,243.5 4,799.3 4,422.5 5,103.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,297.2 4,879.4 4,477.0 5,174.8 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Diluted earnings per share denominator (4) 4,243.5 4,811.3 4,422.5 5,111.9 Basic earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.34 $ 128.03 $ 99.75 Diluted earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.30 $ 128.03 $ 99.60 (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 and nine months ended September 30, 2017 and 2016 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Compensation Plans” . (4) The diluted earnings per share denominator for the three and nine months ended September 30, 2016 includes the impact of 120,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 11,943 and 8,208 incremental shares outstanding over the period. Fair Value of Financial Instruments in Liabilities Held for Sale The OBH Senior Notes are recorded as debt at face value less unamortized original issue discount. The following table summarizes the fair value and carrying value of this financial instrument as of December 31, 2016 : December 31, 2016 Millions Fair Value Carrying Value OBH Senior Notes $ 274.2 $ 273.2 The fair value estimate for the OBH Senior Notes has been determined using quoted market prices. The OBH Senior Notes are considered a Level 2 measurement.

Non-controlling Interest (Table

Non-controlling Interest (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. — % $ — 23.9 % $ 244.6 Other, excluding mutuals and reciprocals HG Global 3.1 16.1 3.1 16.6 MediaAlpha 40.0 9.5 40.0 11.7 Buzzmove 22.9 3.0 29.1 2.9 Wobi 5.0 — 5.0 .1 Dewar (1) — — 18.8 3.9 Total other, excluding mutuals and reciprocals 28.6 35.2 Mutuals and reciprocals BAM 100.0 (160.2 ) 100.0 (150.9 ) SSIE — — 100.0 4.4 Total mutuals and reciprocals (160.2 ) (146.5 ) Total non-controlling interests $ (131.6 ) $ 133.3 (1) Dewar is a subsidiary of OneBeacon.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 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 HG Global BAM (1) MediaAlpha Other Operations Total Three Months Ended September 30, 2017 Earned insurance premiums $ 1.8 $ .6 $ — $ — $ 2.4 Net investment income 1.0 2.3 — 8.9 12.2 Net investment income (loss) - BAM Surplus Note interest 4.8 (4.8 ) — — — Net realized and unrealized investment gains .1 .7 — 31.7 32.5 Advertising and commission revenues — — 37.9 .9 38.8 Other revenue — .2 — 1.4 1.6 Total revenues 7.7 (1.0 ) 37.9 42.9 87.5 Insurance acquisition expenses .4 .5 — — .9 Other underwriting expenses — .1 — — .1 Cost of sales — — 32.2 .9 33.1 General and administrative expenses .3 10.3 6.1 27.4 44.1 Interest expense — — .1 .8 .9 Total expenses .7 10.9 38.4 29.1 79.1 Pre-tax income (loss) $ 7.0 $ (11.9 ) $ (.5 ) $ 13.8 $ 8.4 (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2017 Earned insurance premiums $ 5.0 $ 1.6 $ — $ 1.0 $ 7.6 Net investment income 2.4 6.5 — 30.8 39.7 Net investment income (loss) - BAM Surplus Note interest 14.3 (14.3 ) — — — Net realized and unrealized investment gains .4 2.8 — 99.3 102.5 Advertising and commission revenues — — 101.2 2.7 103.9 Other revenue — .8 — 5.3 6.1 Total revenues 22.1 (2.6 ) 101.2 139.1 259.8 Losses and loss adjustment expenses — — — 1.1 1.1 Insurance acquisition expenses 1.0 2.0 — .1 3.1 Other underwriting expenses — .3 — — .3 Cost of sales — — 86.0 2.7 88.7 General and administrative expenses .8 30.7 17.9 112.8 162.2 Interest expense — — .6 1.2 1.8 Total expenses 1.8 33.0 104.5 117.9 257.2 Pre-tax income (loss) $ 20.3 $ (35.6 ) $ (3.3 ) $ 21.2 $ 2.6 (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Three Months Ended September 30, 2016 Earned insurance premiums $ 1.2 $ .3 $ — $ 1.9 $ 3.4 Net investment income .6 1.7 — 7.3 9.6 Net investment income (loss) - BAM Surplus Note interest 4.5 (4.5 ) — — — Net realized and unrealized investment (losses) gains (.3 ) (1.6 ) — 12.8 10.9 Advertising and commission revenues — — 27.6 .6 28.2 Other revenue — .4 — 4.3 4.7 Total revenues 6.0 (3.7 ) 27.6 26.9 56.8 Losses and loss adjustment expenses — — — 2.2 2.2 Insurance acquisition expenses .2 .6 — .5 1.3 Other underwriting expenses — .1 — .1 .2 Cost of sales — — 23.0 1.0 24.0 General and administrative expenses .6 9.3 5.7 26.8 42.4 Interest expense — — .2 .3 .5 Total expenses .8 10.0 28.9 30.9 70.6 Pre-tax income (loss) $ 5.2 $ (13.7 ) $ (1.3 ) $ (4.0 ) $ (13.8 ) (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. HG Global/BAM Millions HG Global BAM (1) MediaAlpha Other Operations Total Nine Months Ended September 30, 2016 Earned insurance premiums $ 3.1 $ 1.0 $ — $ 6.1 $ 10.2 Net investment income 1.6 5.1 — 11.5 18.2 Net investment income (loss) - BAM Surplus Note interest 13.4 (13.4 ) — — — Net realized and unrealized investment gains 2.3 6.5 — 18.4 27.2 Advertising and commission revenues — — 88.4 1.2 89.6 Other revenue — .8 — 17.2 18.0 Total revenues 20.4 — 88.4 54.4 163.2 Losses and loss adjustment expenses — — — 6.8 6.8 Insurance acquisition expenses .6 1.9 — 1.9 4.4 Other underwriting expenses — .3 — .1 .4 Cost of sales — — 74.0 2.9 76.9 General and administrative expenses 1.4 28.1 16.3 99.1 144.9 Interest expense — — .7 1.9 2.6 Total expenses 2.0 30.3 91.0 112.7 236.0 Pre-tax income (loss) $ 18.4 $ (30.3 ) $ (2.6 ) $ (58.3 ) $ (72.8 ) (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.

Employee Share-Based Incentiv35

Employee Share-Based Incentive Compensation Plans (Tables) - White Mountains	9 Months Ended
	Sep. 30, 2017
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 and nine months ended September 30, 2017 and 2016 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Millions, except share amounts Target Performance Accrued Expense Target Performance Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 50,575 $ 31.2 73,297 $ 32.5 80,353 $ 42.4 93,654 $ 57.7 Shares paid (1) — — — — (30,838 ) (21.6 ) (36,294 ) (41.0 ) New grants — — 6,400 — 17,510 — 22,615 — Forfeitures and cancellations (2) (256 ) (.4 ) (160 ) (.1 ) (16,706 ) (9.4 ) (438 ) .2 Expense recognized — 7.0 — 5.0 — 26.4 — 20.5 End of period (3) 50,319 $ 37.8 79,537 $ 37.4 50,319 $ 37.8 79,537 $ 37.4 (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, which were paid in April 2016, 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 September 30, 2017 and 2016 exclude 2,195 and 7,315 performance share awards granted to employees of Sirius Group.
Share-based Compensation, Performance Shares Award Outstanding Activity	The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at September 30, 2017 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 18,370 $ 20.6 2016 – 2018 16,235 12.1 2017 – 2019 16,480 5.7 Sub-total 51,085 38.4 Assumed forfeitures (766 ) (.6 ) September 30, 2017 50,319 $ 37.8
Summary of restricted shares activity	The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 53,815 $ 23.5 66,470 $ 24.2 70,620 $ 19.7 70,675 $ 15.7 Issued — — 4,150 3.4 17,785 16.7 25,365 19.7 Vested (260 ) — — — (28,846 ) — (24,620 ) — Forfeited — — — — (6,004 ) (3.5 ) (800 ) .2 Expense recognized — (3.8 ) — (3.8 ) — (13.2 ) — (11.8 ) End of period (1) 53,555 $ 19.7 70,620 $ 23.8 53,555 $ 19.7 70,620 $ 23.8 (1) Restricted share awards outstanding as of September 30, 2017 and 2016 include 2,195 and 5,235 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.

Fair Value of Financial Instr36

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017 and December 31, 2016 : September 30, 2017 December 31, 2016 Millions Fair Value Carrying Value Fair Value Carrying Value MediaAlpha Bank Facility $ 9.4 $ 9.4 $ — $ — Previous MediaAlpha Bank Facility — — 13.0 12.7

Discontinued Operations (Tables

Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2017
Discontinued Operations and Disposal Groups [Abstract]
Disposal Groups, Including Discontinued Operations	The following table summarizes the undistributed net earnings (loss) from continuing operations for the three and nine months ended September 30, 2017 and 2016 . See Note 15 — “Held for Sale and Discontinued Operations” . Three Months Ended Nine Months Ended September 30, September 30, Millions 2017 2016 2017 2016 Undistributed net earnings - continuing operations: Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 22.7 $ 6.3 $ 31.1 $ (73.9 ) Dividends declared net of restricted common share amounts (1) — — (4.5 ) (5.9 ) Total undistributed net earnings (loss), net of restricted common share amounts $ 22.7 $ 6.3 $ 26.6 $ (79.8 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. Net Assets Held for Sale The following table summarizes the assets and liabilities associated with businesses classified as held for sale. Amounts presented relate to OneBeacon, Star & Shield and SSIE. Millions December 31, 2016 Assets held for sale Fixed maturity investments, at fair value $ 2,175.7 Short-term investments, at amortized cost (which approximates fair value) 112.3 Common equity securities, at fair value 188.7 Other long-term investments 150.5 Total investments 2,627.2 Cash 70.5 Reinsurance recoverable on unpaid and paid losses 179.8 Insurance and reinsurance premiums receivable 229.8 Deferred acquisition costs 96.3 Deferred tax asset 126.7 Ceded unearned insurance and reinsurance premiums 44.2 Accounts receivable on unsettled investment sales 1.4 Goodwill and other intangible assets 1.2 Accrued investment income 11.3 Other assets 211.1 Total assets held for sale $ 3,599.5 Liabilities held for sale Loss and loss adjustment expense reserves $ 1,370.6 Unearned insurance and reinsurance premiums 576.3 Debt 273.2 Accrued incentive compensation 44.3 Funds held under reinsurance treaties 153.0 Other liabilities 151.9 Total liabilities held for sale 2,569.3 Net assets held for sale $ 1,030.2 Net Income (Loss) from Discontinued Operations The following table summarizes the results of operations, including related income taxes, associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2017 , the amounts presented relate to OneBeacon and Sirius Group. For the three and nine months ended September 30, 2016 , the amounts presented relate to OneBeacon, Sirius Group and Tranzact. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inured to White Mountains. Given the fixed price nature of the OneBeacon Transaction, OneBeacon’s results were economically transferred to the buyer at signing. Three Months Ended Three Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 268.4 $ — $ 268.4 $ 277.9 $ — $ — $ 277.9 Net investment income 13.0 — 13.0 11.8 — — 11.8 Net realized and unrealized gains 11.5 — 11.5 15.5 — — 15.5 Other revenue 2.2 — 2.2 1.8 — 14.8 16.6 Total revenues 295.1 — 295.1 307.0 — 14.8 321.8 Expenses Loss and loss adjustment expenses 206.8 — 206.8 162.8 — — 162.8 Insurance and reinsurance acquisition expenses 51.9 — 51.9 55.1 — — 55.1 Other underwriting expenses 44.9 — 44.9 49.4 — — 49.4 General and administrative expenses 7.4 — 7.4 3.5 — 16.2 19.7 Interest expense 3.4 — 3.4 3.3 — .5 3.8 Total expenses 314.4 — 314.4 274.1 — 16.7 290.8 Pre-tax (loss) income (19.3 ) — (19.3 ) 32.9 — (1.9 ) 31.0 Income tax benefit (expense) 4.1 — 4.1 (10.4 ) — 15.9 5.5 Net (loss) income from discontinued operations (15.2 ) — (15.2 ) 22.5 — 14.0 36.5 Net gain (loss) from sale of discontinued operations 554.5 (.2 ) 554.3 — (4.0 ) 51.9 47.9 Total income (loss) from discontinued operations 539.3 (.2 ) 539.1 22.5 (4.0 ) 65.9 84.4 Change in foreign currency translation and other from discontinued operations .1 — .1 (.3 ) — — (.3 ) Recognition of benefit plan assets and obligations 2.9 — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 542.3 $ (.2 ) $ 542.1 $ 22.2 $ (4.0 ) $ 65.9 $ 84.1 Nine Months Ended Nine Months Ended September 30, 2017 September 30, 2016 Millions OneBeacon Sirius Group Tranzact Total OneBeacon Sirius Group Tranzact Total Revenues Earned insurance premiums $ 807.6 $ — $ — $ 807.6 $ 827.9 $ 240.1 $ — $ 1,068.0 Net investment income 39.7 — — 39.7 38.3 14.4 — 52.7 Net realized and unrealized gains (losses) 38.8 — — 38.8 56.8 (1.5 ) — 55.3 Other revenue 7.7 — — 7.7 3.5 .6 119.6 123.7 Total revenues 893.8 — — 893.8 926.5 253.6 119.6 1,299.7 Expenses Loss and loss adjustment expenses 546.0 — — 546.0 501.3 154.9 — 656.2 Insurance and reinsurance acquisition expenses 145.6 — — 145.6 154.8 59.0 — 213.8 Other underwriting expenses 156.2 — — 156.2 155.6 30.9 — 186.5 General and administrative expenses 21.2 — — 21.2 10.9 10.4 116.7 138.0 Interest expense 10.0 — — 10.0 9.8 7.9 3.2 20.9 Total expenses 879.0 — — 879.0 832.4 263.1 119.9 1,215.4 Pre-tax income (loss) 14.8 — — 14.8 94.1 (9.5 ) (.3 ) 84.3 Income tax benefit (expense) 5.7 — — 5.7 (.3 ) 3.1 13.8 16.6 Net income (loss) from discontinued operations 20.5 — — 20.5 93.8 (6.4 ) 13.5 100.9 Net gain (loss) from sale of discontinued operations 554.5 (.8 ) (1.0 ) 552.7 — 362.6 51.9 414.5 Total income (loss) from discontinued operations 575.0 (.8 ) (1.0 ) 573.2 93.8 356.2 65.4 515.4 Change in foreign currency translation and other from discontinued operations .3 — — .3 .5 32.0 — 32.5 Recognition of foreign currency translation and other from sale of Sirius Group, net of tax — — — — — 113.3 113.3 Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax 2.9 — — 2.9 — — — — Comprehensive income (loss) from discontinued operations $ 578.2 $ (.8 ) $ (1.0 ) $ 576.4 $ 94.3 $ 501.5 $ 65.4 $ 661.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax payment to national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, Millions 2017 2016 Net cash provided from operations $ 157.0 $ 38.1 Net cash provided from investing activities 3.0 269.7 Net cash used for financing activities (61.9 ) (72.7 ) Net change in cash during the period 98.1 235.1 Cash balances at beginning of period 70.5 245.4 Net change in cash held for sale, excluding discontinued operations (.9 ) 2.6 Cash sold as part of sale of consolidated subsidiaries 167.7 345.8 Cash balances at end of period $ — $ 137.3 Supplemental cash flows information: Interest paid $ — $ (7.7 ) Net income tax payment to national governments $ — $ (18.3 ) Earnings Per Share from Discontinued Operations White Mountains calculates earnings per share using the two-class method, which allocates earnings between common and unvested restricted common shares. Both classes of shares participate equally in earnings on a per share basis. Basic earnings per share amounts are based on the weighted average number of common shares outstanding adjusted for unvested restricted common shares. Diluted earnings per share amounts are also impacted by net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 562.1 $ 90.8 $ 604.7 $ 440.7 Less: total income from continuing operations, net of tax (23.0 ) (6.4 ) (31.5 ) 74.7 Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders $ 539.1 $ 84.4 $ 573.2 $ 515.4 Allocation of earnings to participating restricted common shares (1) (6.7 ) (1.2 ) (7.0 ) (6.3 ) Basic and diluted earnings per share numerators $ 532.4 $ 83.2 $ 566.2 $ 509.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,297.2 4,867.4 4,477.0 5,166.6 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Basic earnings per share denominator 4,243.5 4,799.3 4,422.5 5,103.7 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,297.2 4,879.4 4,477.0 5,174.8 Average unvested restricted common shares (3) (53.7 ) (68.1 ) (54.5 ) (62.9 ) Diluted earnings per share denominator (4) 4,243.5 4,811.3 4,422.5 5,111.9 Basic earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.34 $ 128.03 $ 99.75 Diluted earnings per share (in dollars) - discontinued operations: $ 125.45 $ 17.30 $ 128.03 $ 99.60 (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 and nine months ended September 30, 2017 and 2016 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. See Note 13 — “Employee Share-Based Compensation Plans” . (4) The diluted earnings per share denominator for the three and nine months ended September 30, 2016 includes the impact of 120,000 common shares issuable upon exercise of the non-qualified options outstanding, which resulted in 11,943 and 8,208 incremental shares outstanding over the period. Fair Value of Financial Instruments in Liabilities Held for Sale The OBH Senior Notes are recorded as debt at face value less unamortized original issue discount. The following table summarizes the fair value and carrying value of this financial instrument as of December 31, 2016 : December 31, 2016 Millions Fair Value Carrying Value OBH Senior Notes $ 274.2 $ 273.2 The fair value estimate for the OBH Senior Notes has been determined using quoted market prices. The OBH Senior Notes are considered a Level 2 measurement.
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 December 31, 2016 : Millions Total as of December 31, 2016 Par Value $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 5.1 Regulatory approval (2) (15.6 ) Liquidity adjustment (3) (18.6 ) Total adjustments (29.1 ) Fair value $ 71.9 (1) Represents the value of the surplus notes, at current market yields on comparable publicly traded debt, and assuming issuer is allowed to make principal and interest payments when its financial capacity is available, as measured by statutory capital in excess of a 250% RBC score under the National Association of Insurance Commissioners’ risk-based capital standards for property and casualty companies. The favorable year-to-date change in impact is due principally to the narrowing of non-investment grade credit spreads as well as the time value of money benefit from moving three months closer to modeled cash receipts. (2) Represents anticipated delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer's statutory surplus. The monetary impact of the anticipated delay is measured based on credit spreads of public securities with roughly equivalent percentages of discounted payments missed. The favorable year-to-date change in impact is driven primarily by the narrowing of non-investment grade credit spreads, which causes negative valuation impact from the anticipated delay in securing regulatory approval to be lower. (3) Represents impact of liquidity spread to account for OneBeacon's sole ownership of the notes, lack of a trading market, and unique nature of the ongoing regulatory approval process.

Financial Statement Revisions (

Financial Statement Revisions (Tables)	9 Months Ended
	Sep. 30, 2017
Accounting Changes and Error Corrections [Abstract]
Schedule of Error Corrections and Prior Period Adjustments	CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited) Three months ended Nine months ended September 30, 2016 Millions, except for per share amounts As previously reported Adjustments As revised As previously reported Adjustments As revised Revenues: Earned insurance premiums $ 3.4 $ — 3.4 $ 10.2 $ — $ 10.2 Net investment income 9.6 — 9.6 18.2 — 18.2 Net realized and unrealized investment gains 10.9 — 10.9 27.2 — 27.2 Advertising and commission revenues 29.4 (1.2 ) 28.2 92.4 (2.8 ) 89.6 Other revenue 4.7 — 4.7 18.0 — 18.0 Total revenues 58.0 (1.2 ) 56.8 166.0 (2.8 ) 163.2 Expenses: Loss and loss adjustment expenses 2.2 — 2.2 6.8 — 6.8 Insurance and acquisition expenses 1.3 — 1.3 4.4 — 4.4 Other underwriting expenses .2 — .2 .4 — .4 Cost of sales 24.0 — 24.0 76.9 — 76.9 General and administrative expenses 42.7 (.3 ) 42.4 142.6 2.3 144.9 Interest expense on debt .5 — .5 2.6 — 2.6 Total expenses 70.9 (.3 ) 70.6 233.7 2.3 236.0 Pre-tax loss (12.9 ) (.9 ) (13.8 ) (67.7 ) (5.1 ) (72.8 ) Income tax benefit 17.1 — 17.1 22.7 — 22.7 Net income (loss) from continuing operations 4.2 (.9 ) 3.3 (45.0 ) (5.1 ) (50.1 ) Gain on sale of discontinued operations 47.9 — 47.9 414.5 — 414.5 Net income from discontinued operations 36.5 — 36.5 100.9 — 100.9 Net income (loss) (1) 88.6 (.9 ) 87.7 470.4 (5.1 ) 465.3 Net loss (income) attributable to non-controlling interests 3.1 — 3.1 (24.6 ) — (24.6 ) Net income (loss) attributable to White Mountains's common shareholders 91.7 (.9 ) 90.8 445.8 (5.1 ) 440.7 Other comprehensive income (loss), net of tax .2 (.2 ) — 145.6 (.2 ) 145.4 Comprehensive income (loss) 91.9 (1.1 ) 90.8 591.4 (5.3 ) 586.1 Comprehensive loss attributable to non-controlling interests .1 — .1 .1 — .1 Comprehensive income (loss) attributable to White Mountains's common shareholders $ 92.0 $ (1.1 ) $ 90.9 $ 591.5 $ (5.3 ) $ 586.2 Basic and diluted earnings per share - continuing operations $ 1.50 $ (.18 ) $ 1.32 $ (13.49 ) $ (1.00 ) $ (14.49 ) (1) The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities. CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited) Six months ended June 30, 2017 Millions, except for per share amounts As previously reported Adjustments As revised Revenues: Earned insurance premiums $ 5.2 $ — $ 5.2 Net investment income 27.5 — 27.5 Net realized and unrealized investment gains 70.0 — 70.0 Advertising and commission revenues 71.7 (6.6 ) 65.1 Other revenue 4.5 — 4.5 Total revenues 178.9 (6.6 ) 172.3 Expenses: Loss and loss adjustment expenses 1.1 — 1.1 Insurance and acquisition expenses 2.2 — 2.2 Other underwriting expenses .2 — .2 Cost of sales 55.6 — 55.6 General and administrative expenses 117.9 .1 118.0 Interest expense on debt .9 — .9 Total expenses 177.9 .1 178.0 Pre-tax income (loss) 1.0 (6.7 ) (5.7 ) Income tax benefit 1.3 — 1.3 Net income (loss) from continuing operations 2.3 (6.7 ) (4.4 ) Loss on sale of discontinued operations (1.6 ) — (1.6 ) Net income from discontinued operations 35.7 — 35.7 Net income (loss) 36.4 (6.7 ) 29.7 Net loss (income) attributable to non-controlling interests 13.4 (.4 ) 13.0 Net income (loss) attributable to White Mountains's common shareholders 49.8 (7.1 ) 42.7 Other comprehensive income (loss), net of tax 1.7 (1.3 ) .4 Comprehensive income (loss) 51.5 (8.4 ) 43.1 Comprehensive income attributable to non-controlling interests (.1 ) — (.1 ) Comprehensive income (loss) attributable to White Mountains's common shareholders $ 51.4 $ (8.4 ) $ 43.0 Basic and diluted earnings per share - continuing operations $ 3.42 $ (1.59 ) $ 1.83 CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME As previously reported Adjustments As revised Years ended December 31, Millions, except for per share amounts 2016 2015 2014 2016 2015 2014 2016 2015 2014 Revenues: Earned insurance premiums $ 13.4 $ 12.0 $ 7.9 $ — $ — $ — $ 13.4 $ 12.0 $ 7.9 Net investment income 32.1 10.9 12.2 — — — 32.1 10.9 12.2 Net realized and unrealized investment gains (27.4 ) 260.5 38.1 — — — (27.4 ) 260.5 38.1 Advertising and commission revenues 126.9 110.1 65.7 (8.6 ) (2.7 ) (.2 ) 118.3 107.4 65.5 Other revenue (1) 21.3 41.8 20.5 — 7.4 (6.9 ) 21.3 49.2 13.6 Total revenues 166.3 435.3 144.4 (8.6 ) 4.7 (7.1 ) 157.7 440.0 137.3 Expenses: Loss and loss adjustment expenses 8.0 8.2 8.9 — — — 8.0 8.2 8.9 Insurance and acquisition expenses 5.6 6.3 2.9 — — — 5.6 6.3 2.9 Other underwriting expenses .5 .4 .4 — — — .5 .4 .4 Cost of sales 102.0 93.6 57.8 — — — 102.0 93.6 57.8 General and administrative expenses 183.7 193.2 144.9 2.2 7.5 — 185.9 200.7 144.9 Interest expense on debt 3.0 1.6 1.2 — — — 3.0 1.6 1.2 Total expenses 302.8 303.3 216.1 2.2 7.5 — 305.0 310.8 216.1 Pre-tax income (136.5 ) 132.0 (71.7 ) (10.8 ) (2.8 ) (7.1 ) (147.3 ) 129.2 (78.8 ) Income benefit (expense) 32.9 (12.7 ) 2.5 — — — 32.9 (12.7 ) 2.5 Net income from continuing operations (103.6 ) 119.3 (69.2 ) (10.8 ) (2.8 ) (7.1 ) (114.4 ) 116.5 (76.3 ) Gain on sale of discontinued operations 415.1 17.9 17.2 — — — 415.1 17.9 17.2 Net income from discontinued operations 108.3 117.2 296.4 — — — 108.3 117.2 296.4 Income (loss) before equity in earnings of unconsolidated affiliates 419.8 254.4 244.4 (10.8 ) (2.8 ) (7.1 ) 409.0 251.6 237.3 Equity in earnings of unconsolidated affiliates — 25.1 45.6 — — — 25.1 45.6 Net income (loss) (2) 419.8 279.5 290.0 (10.8 ) (2.8 ) (7.1 ) 409.0 276.7 282.9 Net (income) loss attributable to non-controlling interests (7.3 ) 18.1 22.2 .1 .4 .1 (7.2 ) 18.5 22.3 Net income (loss) attributable to White Mountains's common shareholders 412.5 297.6 312.2 (10.7 ) (2.4 ) (7.0 ) 401.8 295.2 305.2 Other comprehensive income, net of tax (1) 145.6 (100.4 ) (104.9 ) — (7.4 ) 6.9 145.6 (107.8 ) (98.0 ) Comprehensive income (loss) 558.1 197.2 207.3 (10.7 ) (9.8 ) (.1 ) 547.4 187.4 207.2 Comprehensive (income) loss attributable to non-controlling interests (.3 ) — 3.3 — — — (.3 ) — 3.3 Comprehensive income (loss) attributable to White Mountains's common shareholders $ 557.8 $ 197.2 $ 210.6 $ (10.7 ) $ (9.8 ) $ (.1 ) $ 547.1 $ 187.4 $ 210.5 Basic and diluted earnings per share - continuing operations $ (22.13 ) $ 27.63 $ (.24 ) $ (2.15 ) $ (.42 ) $ (1.16 ) $ (24.28 ) $ 27.21 $ (1.40 ) (1) In 2015 and 2014, White Mountains recorded a foreign currency translation gain related to its investment in Symetra in net income when it should have been recorded through other comprehensive income. The correction to properly reflect the translation amount through other comprehensive income did not have any impact on comprehensive income attributable to White Mountains's common shareholders or to book value per share. (2) The adjustment to net income resulted in a corresponding adjustment in the statement of cash flows, with an offsetting adjustment to the change in other assets and liabilities within the operating cash flows section. There was no change to cash flows from operations, cash flows from investing activities or cash flows from financing activities. CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY As previously reported Adjustments As revised Years ended December 31, Millions 2016 2015 2014 2016 2015 2014 2016 2015 2014 Common shares and paid-in surplus $ 810.7 $ 978.2 $ 1,034.7 $ — $ — $ — $ 810.7 $ 978.2 $ 1,034.7 Retained earnings, beginning of year 3,084.9 3,010.5 2,801.9 (9.9 ) (7.5 ) (.5 ) 3,075.0 3,003.0 2,801.4 Share repurchases (694.8 ) (217.2 ) (97.4 ) — — — (694.8 ) (217.2 ) (97.4 ) Net income (loss) 412.5 297.6 312.2 (10.7 ) (2.4 ) (7.0 ) 401.8 295.2 305.2 Dividends (5.4 ) (6.0 ) (6.2 ) — — — (5.4 ) (6.0 ) (6.2 ) Retained earnings, end of year 2,797.2 3,084.9 3,010.5 (20.6 ) (9.9 ) (7.5 ) 2,776.6 3,075.0 3,003.0 Accumulated other comprehensive income, after tax, beginning of year (149.9 ) (49.5 ) 52.1 — 7.4 .5 (149.9 ) (42.1 ) 52.6 Net change in foreign currency translation 31.4 (65.8 ) (168.2 ) — (7.4 ) 6.9 31.4 (73.2 ) (161.3 ) Net other changes in AOCI 113.9 (34.6 ) 66.6 — — — 113.9 (34.6 ) 66.6 Accumulated other comprehensive income, after tax, end of year (4.6 ) (149.9 ) (49.5 ) — — 7.4 (4.6 ) (149.9 ) (42.1 ) Total White Mountains Common Shareholders' Equity 3,603.3 3,913.2 3,995.7 (20.6 ) (9.9 ) (.1 ) 3,582.7 3,903.3 3,995.6 Non-controlling interests, beginning of year 454.8 542.7 491.7 (.5 ) (.1 ) — 454.3 542.6 491.7 Net income (loss) 7.3 (18.1 ) (22.2 ) (.1 ) (.4 ) (.1 ) 7.2 (18.5 ) (22.3 ) Other changes in NCI (328.2 ) (69.8 ) 73.2 — — — (328.2 ) (69.8 ) 73.2 Non-controlling interests, end of year 133.9 454.8 542.7 (.6 ) (.5 ) (.1 ) 133.3 454.3 542.6 Total equity $ 3,737.2 $ 4,368.0 $ 4,538.4 $ (21.2 ) $ (10.4 ) $ (.2 ) $ 3,716.0 $ 4,357.6 $ 4,538.2 CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) Nine months ended September 30, 2016 Six months ended June 30, 2017 Millions As previously reported Adjustments As revised As previously reported Adjustments As revised Common shares and paid-in surplus $ 809.8 $ — $ 809.8 $ 815.1 $ .7 $ 815.8 Retained earnings, beginning of year 3,084.9 (9.9 ) 3,075.0 2,797.2 (20.6 ) 2,776.6 Share repurchases (678.6 ) — (678.6 ) (7.2 ) — (7.2 ) Net income (loss) 445.8 (5.1 ) 440.7 49.8 (7.1 ) 42.7 Dividends (5.4 ) — (5.4 ) (4.6 ) — (4.6 ) Retained earnings, end of period 2,846.7 (15.0 ) 2,831.7 2,835.2 (27.7 ) 2,807.5 Accumulated other comprehensive income, after tax, beginning of year (149.9 ) — (149.9 ) (4.6 ) — (4.6 ) Net change in foreign currency translation 32.0 (.2 ) 31.8 1.6 (1.3 ) .3 Net other changes in AOCI 113.6 — 113.6 — — — Accumulated other comprehensive income, after tax, end of year (4.3 ) (.2 ) (4.5 ) (3.0 ) (1.3 ) (4.3 ) Total White Mountains Common Shareholders' Equity 3,652.2 (15.2 ) 3,637.0 3,647.3 (28.3 ) 3,619.0 Non-controlling interests, beginning of year 454.8 (.5 ) 454.3 133.9 (.6 ) 133.3 Net income (loss) 24.6 — 24.6 (13.4 ) .4 (13.0 ) Other changes in NCI (330.2 ) — (330.2 ) (3.7 ) (.7 ) (4.4 ) Non-controlling interests, end of period 149.2 (.5 ) 148.7 116.8 (.9 ) 115.9 Total equity $ 3,801.4 $ (15.7 ) $ 3,785.7 $ 3,764.1 $ (29.2 ) $ 3,734.9 CONSOLIDATED BALANCE SHEETS December 31, 2016 December 31, 2015 Millions As previously reported Adjustments As revised As previously reported Adjustments As revised Assets Investments $ 2,714.4 $ — $ 2,714.4 $ 1,679.7 $ — $ 1,679.7 Cash 80.2 — 80.2 77.8 — 77.8 Insurance premiums receivable 1.6 — 1.6 1.3 — 1.3 Deferred acquisition costs 10.6 — 10.6 6.9 — 6.9 Deferred tax asset — — — — — — Accrued investment income 14.8 — 14.8 3.8 — 3.8 Accounts receivable on unsettled investment sales 4.8 — 4.8 11.4 — 11.4 Goodwill 31.7 (5.8 ) 25.9 24.1 (5.5 ) 18.6 Intangible assets 23.0 (3.7 ) 19.3 28.9 (2.0 ) 26.9 Assets held for sale 3,599.5 — 3,599.5 8,365.6 — 8,365.6 Other assets 64.1 (15.0 ) 49.1 83.1 (3.9 ) 79.2 Total assets $ 6,544.7 $ (24.5 ) $ 6,520.2 $ 10,282.6 $ (11.4 ) $ 10,271.2 Liabilities Unearned insurance premiums $ 82.9 $ — $ 82.9 $ 50.2 $ — $ 50.2 Debt 12.7 — 12.7 64.7 — 64.7 Deferred tax liability — — — 27.4 — 27.4 Accrued incentive compensation 95.7 — 95.7 96.2 — 96.2 Liabilities held for sale 2,569.3 — 2,569.3 5,618.1 — 5,618.1 Other liabilities 46.9 (3.3 ) 43.6 58.0 (1.0 ) 57.0 Total liabilities 2,807.5 (3.3 ) 2,804.2 5,914.6 (1.0 ) 5,913.6 Equity White Mountains's common shares 4.6 — 4.6 5.6 — 5.6 Paid in surplus 806.1 — 806.1 972.6 — 972.6 Retained earnings 2,797.2 (20.6 ) 2,776.6 3,084.9 (9.9 ) 3,075.0 Accumulated other comprehensive income, net of tax (4.6 ) — (4.6 ) (149.9 ) — (149.9 ) Total White Mountains's common shareholders' equity 3,603.3 (20.6 ) 3,582.7 3,913.2 (9.9 ) 3,903.3 Non-controlling interests 133.9 (.6 ) 133.3 454.8 (.5 ) 454.3 Total equity 3,737.2 (21.2 ) 3,716.0 4,368.0 (10.4 ) 4,357.6 Total liabilities and equity $ 6,544.7 $ (24.5 ) $ 6,520.2 $ 10,282.6 $ (11.4 ) $ 10,271.2 CONSOLIDATED BALANCE SHEET (Unaudited) June 30, 2017 Millions As previously reported Adjustments As revised Assets Investments $ 2,692.9 $ — $ 2,692.9 Cash 53.3 — 53.3 Insurance and reinsurance premiums receivable 2.8 — 2.8 Deferred acquisition costs 13.0 — 13.0 Accrued investment income 16.0 — 16.0 Accounts receivable on unsettled investment sales 199.5 — 199.5 Goodwill 31.7 (5.8 ) 25.9 Intangible assets 17.8 (3.4 ) 14.4 Assets held for sale 3,696.4 — 3,696.4 Other assets 62.8 (19.7 ) 43.1 Total assets $ 6,786.2 $ (28.9 ) $ 6,757.3 Liabilities Unearned insurance premiums $ 109.9 $ — $ 109.9 Debt 10.6 — 10.6 Accrued incentive compensation 63.3 — 63.3 Accounts payable for unsettled investment purchases 114.6 — 114.6 Liabilities held for sale 2,678.8 — 2,678.8 Other liabilities 44.9 .3 45.2 Total liabilities 3,022.1 .3 3,022.4 Equity White Mountains's common shares 4.6 — 4.6 Paid in surplus 810.5 .7 811.2 Retained earnings 2,835.2 (27.7 ) 2,807.5 Accumulated other comprehensive income, net of tax (3.0 ) (1.3 ) (4.3 ) Total White Mountains's common shareholders' equity 3,647.3 (28.3 ) 3,619.0 Non-controlling interests 116.8 (.9 ) 115.9 Total equity 3,764.1 (29.2 ) 3,734.9 Total liabilities and equity $ 6,786.2 $ (28.9 ) $ 6,757.3

Summary of Significant Accoun39

Summary of Significant Accounting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)publisheradvertiser	Sep. 30, 2016USD ($)	Sep. 28, 2017$ / shares	Jun. 30, 2017	Dec. 31, 2016	Jul. 31, 2012USD ($)
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes						$ 503
MediaAlpha
Basis of Presentation
Number of advertisers | advertiser	330
Number of publishers | publisher	280
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%			96.90%	96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%			88.40%	88.40%
Restricted Stock
Basis of Presentation
Payments related to tax withholding for share-based compensation	$ 9.3	$ 5.8
OneBeacon
Basis of Presentation
Business acquisition, share price | $ / shares			$ 18.10

Significant Transactions (Detai

Significant Transactions (Details) - USD ($)	Sep. 28, 2017	Aug. 01, 2017	Oct. 05, 2016	Aug. 04, 2016	Jul. 21, 2016	Apr. 18, 2016	Feb. 01, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold												$ 1,131,000,000	$ 2,657,200,000
Gain from sale of other discontinued operations, net of tax								$ 554,300,000		$ 47,900,000		552,700,000	414,500,000
Assets								3,637,100,000				3,637,100,000		$ 6,520,200,000
Goodwill								25,900,000		25,900,000		25,900,000	25,900,000	25,900,000	$ 25,900,000	$ 18,300,000	$ 18,600,000
Liabilities								$ 299,900,000				299,900,000		2,804,200,000
Repayments of debt												364,600,000	401,800,000
Payments to acquire other investments												67,700,000	36,600,000
Sales of common equity securities												$ 407,800,000	683,000,000
Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				70.90%				77.10%				77.10%
Assets				$ 11,500,000
Goodwill				7,600,000
Intangible Assets, Net (Excluding Goodwill)				1,100,000
Liabilities				100,000
United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary				6,100,000
United States of America, Dollars | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary				$ 8,100,000
OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300,000,000
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 1,200,000		$ 221,300,000
Gain from sale of other discontinued operations, net of tax			51,900,000						$ (1,000,000)					51,900,000
Repayments of debt					56,300,000
Discontinued operation, tax effect of discontinued operation			30,200,000							15,900,000			13,800,000	30,200,000
Increase of book value			$ 82,100,000		$ 82,100,000
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold						$ 2,600,000,000
Gain from sale of other discontinued operations, net of tax										(4,000,000)				363,200,000
Payments to acquire other investments						$ 161,800,000
Sale of Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary														113,300,000
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities							$ 658,000,000
Dividends payable, amount per share							$ 32
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities							$ 658,000,000
Common stock, dividends, per share, cash paid							$ 0.50				$ 0.50
Preferred Stock | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 5,000,000
Stock issued during period, shares, acquisitions		37,409
Preferred Stock | United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 4,000,000
Common Stock | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 700,000
Stock issued during period, shares, acquisitions		5,808
Common Stock | United Kingdom, Pounds | Buzzmove
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisition of subsidiary		$ 500,000
Discontinued Operations | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, tax effect of discontinued operation								$ 4,100,000		(10,400,000)		$ 5,700,000	(300,000)
Discontinued Operations | Tranzact
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, tax effect of discontinued operation														$ 30.2
Discontinued Operations | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of other discontinued operations, net of tax								(800,000)				$ 1,000,000
Discontinued operation, tax effect of discontinued operation								$ 0		$ 0			$ 3,100,000

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investment income
Total investment income	$ 12.9	$ 10	$ 41.7	$ 19.5
Third-party investment expenses	(0.7)	(0.4)	(2)	(1.3)
Net investment income, pre-tax	12.2	9.6	39.7	18.2
Fixed maturity investments
Investment income
Total investment income	9.8	9.3	32.9	17.4
Short-term investments
Investment income
Total investment income	0.3	0.2	0.6	0.7
Common Stock
Investment income
Total investment income	2.7	0.5	7.7	1
Other long-term investments
Investment income
Total investment income	$ 0.1	$ 0	$ 0.5	$ 0.4

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 6.8	$ 0.7	$ 20.8	$ 265
Net unrealized investment gains (losses), pre-tax	25.7	10.2	81.7	(237.8)
Net realized and unrealized investment gains, pre-tax	32.5	10.9	102.5	27.2
Income tax expense attributable to net realized and unrealized investment gains	(3.9)	0	(9.5)	(4)
Net realized and unrealized investment gains, after tax	$ 28.6	$ 10.9	$ 93	$ 23.2

Investment Securities (Net Re43

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 7.4	$ 0.7	$ 21	$ 265
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.6)	(0.1)	(3.6)	(45.1)
Trading securities realized holding gain (loss) on investments after tax	6.8	0.6	17.4	219.9
Foreign Currency Transaction Gain (Loss), Realized	(0.6)	0	(0.2)	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	(0.6)	0	(0.2)	0
Net realized investment gains, pre-tax	6.8	0.7	20.8	265
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.6)	(0.1)	(3.6)	(45.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	6.2	0.6	17.2	219.9
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.6	0.3	(0.4)	2
Foreign Currency Transaction Gain (Loss), Realized	1.2	0	2.7	0
Net realized investment gains, pre-tax	1.8	0.3	2.3	2
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.1)	0.2	(0.1)	0.4
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Net realized investment gains, pre-tax	(0.1)	0.2	(0.1)	0.4
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	4.9	0.2	18.5	262.6
Foreign Currency Transaction Gain (Loss), Realized	5.4	0	6	0
Net realized investment gains, pre-tax	10.3	0.2	24.5	262.6
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2	0	3	0
Foreign Currency Transaction Gain (Loss), Realized	(7.2)	0	(8.9)	0
Net realized investment gains, pre-tax	$ (5.2)	$ 0	$ (5.9)	$ 0

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 26.1	$ 9.9	$ 82.1	$ (240.7)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(3.3)	0.1	(5.9)	41.1
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	22.8	10	76.2	(199.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.4)	0.3	(0.4)	2.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0	0	0	0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.4)	0.3	(0.4)	2.9
Net unrealized investment gains (losses), pre-tax	25.7	10.2	81.7	(237.8)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(3.3)	0.1	(5.9)	41.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	22.4	10.3	75.8	(196.7)
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	2	(2)	19.4	13.3
Foreign Currency Transaction Gain (Loss), Unrealized	5.1	0	12.6	0
Net unrealized investment gains (losses), pre-tax	7.1	(2)	32	13.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	26.6	8.5	53.5	(256.1)
Foreign Currency Transaction Gain (Loss), Unrealized	(3.1)	0.2	0	2.6
Net unrealized investment gains (losses), pre-tax	23.5	8.7	53.5	(253.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(2.5)	3.4	9.2	2.1
Foreign Currency Transaction Gain (Loss), Unrealized	(2.4)	0.1	(13)	0.3
Net unrealized investment gains (losses), pre-tax	$ (4.9)	$ 3.5	$ (3.8)	$ 2.4

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 26.1	$ 9.9	$ 82.1	$ (240.7)
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	2	(2)	19.4	13.3
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(2.5)	3.4	9.2	2.1
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(0.7)	(0.9)	(2.2)	0.8
Level 3 Inputs | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	0	0	0.1
Level 3 Inputs | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (0.7)	$ (0.9)	$ (2.2)	$ 0.7

Investment Securities (Invest46

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,490	$ 2,081.1
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	218.1	112.1
Trading Securities, Unrealized Holding Gain	0.1	0
Trading Securities, Unrealized Holding Loss	(0.4)	(1.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	217.8	111
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	608.3	752
Trading Securities, Unrealized Holding Gain	3.2	2.3
Trading Securities, Unrealized Holding Loss	(1)	(10.1)
Net foreign currency gains (losses)	14.5	2.1
Fixed maturity investments, at fair value	625	746.3
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	252.1	238.7
Trading Securities, Unrealized Holding Gain	3.1	1.1
Trading Securities, Unrealized Holding Loss	(0.5)	(1.3)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	254.7	238.5
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	389.1	986.9
Trading Securities, Unrealized Holding Gain	1.2	0.8
Trading Securities, Unrealized Holding Loss	(2.4)	(7.9)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	387.9	979.8
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4.5	12
Trading Securities, Unrealized Holding Gain	0	0.1
Trading Securities, Unrealized Holding Loss	(0.1)	0
Net foreign currency gains (losses)	0.2	0
Fixed maturity investments, at fair value	4.6	12.1
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,472.1	2,101.7
Trading Securities, Unrealized Holding Gain	7.6	4.3
Trading Securities, Unrealized Holding Loss	(4.4)	(20.4)
Net foreign currency gains (losses)	14.7	2.1
Total fixed maturity investments including assets held for sale	$ 1,490	2,087.7
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)	$ (8.3)	$ (9.5)

Investment Securities (Invest47

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,490	$ 2,081.1
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	693.8	258.6
Trading Securities, Unrealized Holding Gain	82.6	29
Trading Securities, Unrealized Holding Loss	(2)	(2)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	774.4	285.6
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	254.5	194
Trading Securities, Unrealized Holding Gain	13.1	7.9
Trading Securities, Unrealized Holding Loss	(21.2)	(25.2)
Net foreign currency gains (losses)	(16.8)	(3.9)
Fixed maturity investments, at fair value	$ 229.6	$ 172.8

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 229.6	$ 172.8
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	154.9	82.6
Limited liability companies and private equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	59.1	57.6
Convertible Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	27.2	30.6
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	(15.4)	(1.2)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 3.8	$ 3.2
OneTitle
Schedule of Trading Securities and Other Trading Assets [Line Items]
Ownership interest (as a percent)	20.00%
Surplus notes	$ 10

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Sep. 30, 2017USD ($)fund	Dec. 31, 2016USD ($)
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	2
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	10
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 | $	$ 54.7	$ 21.5

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 81.1
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	154.9	$ 82.6
Unfunded Commitments	105.2	123.6
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.7	21.5
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity R E I T
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.1	19.9
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	73.8	41.4
Unfunded Commitments	0	0
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	42.7	19.4
Unfunded Commitments	12.1	22.9
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	28.7	19.4
Unfunded Commitments	15.6	25.9
Private equity funds | Direct Lending
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.7	1.4
Unfunded Commitments	23.3	28.6
Private equity funds | Financial Services Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3	1
Unfunded Commitments	13	5
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0	0
Unfunded Commitments	41.2	41.2
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	81.1	41.2
Unfunded Commitments	$ 105.2	$ 123.6

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	Sep. 30, 2017USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Outstanding redemptions	$ 19.1
Semi-annual | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	73.8
Semi-annual | 30-59 days notice | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.7
Semi-annual | 60-89 days notice | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 19.1
September 2018 | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of hedge funds	1
Other long-term investments	$ 54.7

Investment Securities (Invest52

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity funds $ in Millions	Sep. 30, 2017USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 81.1
1-3 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4
3-5 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.2
5-10 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	33.5
Greater than 10 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 25.4

Investment Securities (Fair V53

Investment Securities (Fair Value Measurements) (Details) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Percentage of investments recorded at fair value	93.00%	94.00%
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1

Investment Securities (Fair V54

Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	Sep. 30, 2017USD ($)Investment	Dec. 31, 2016USD ($)Investment	Sep. 30, 2016USD ($)Investment	Dec. 31, 2015USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	733.6	279.5	191.4	789
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 1,525.2	$ 2,093.8	2,011.6	585.6
Common Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of hedge funds | Investment	2	1
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 625	$ 746.3
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5.6	0	$ 2.1	0
Number of hedge funds | Investment	2		2
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 90.1	91.4	$ 106.5	103.6
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	154.9	82.6
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 investments	154.9	82.6	$ 95.3	$ 65.3
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3,141	2,639.7
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,498.1	441.7
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,547.2	2,106.6
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	95.7	91.4
Fair value measured on a recurring basis | U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	217.8	111
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	217.8	101.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	9.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	625	746.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	619.4	746.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.6	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	121.9	190.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	121.9	190.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109.3	140.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109.3	140.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	85.6	114.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	85.6	114.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	81.7	72
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	81.7	72
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	68.6	65
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	68.6	65
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.6	79.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	50	79.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.6	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	52	48.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	52	48.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.3	28.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.3	28.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12	6.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12	6.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	254.7	238.5
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	254.7	238.5
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.6	12.1
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.6	12.1
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,490	2,087.7
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	217.8	101.5
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,266.6	1,986.2
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.6	0
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	786.5	175
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	764.5	162.3
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22	12.7
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13	8.6
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13	8.6
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17.1	13.9
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17.1	13.9
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.3	7
Fair value measured on a recurring basis | Common equity securities | Communications | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.3	7
Fair value measured on a recurring basis | Common equity securities | Communications | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.9	7.7
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.9	7.7
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7	7.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7	7.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.2	1.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.2	1.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4	2.5
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4	2.5
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	492.5	157.2
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	434.6	129.4
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57.9	27.8
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other(3)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	200.7	79.9
Fair value measured on a recurring basis | Common equity securities | Other(3) | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other(3) | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	200.7	79.9
Fair value measured on a recurring basis | Common equity securities | Other(3) | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	774.4	285.6
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	515.8	177.9
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	258.6	107.7
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.1	91.4
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	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]
Fair value investments	0	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]
Fair value investments	90.1	91.4
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	$ (15.4)	(1.2)
Star & Shield Insurance Exchange | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		6.6
Star & Shield Insurance Exchange | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		$ 0.1

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,490,000,000	$ 2,081,100,000
Fair value investments	2,509,400,000	2,547,300,000	$ 2,406,900,000	$ 1,543,500,000
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	625,000,000	746,300,000
Fair value investments	625,000,000	746,300,000
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26,700,000	37,300,000
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	119,400,000	212,800,000
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	291,900,000	335,600,000
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	165,400,000	143,200,000
B | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 21,600,000	17,400,000
Star & Shield Insurance Exchange | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value		4.2
Fair value investments		$ 6,600,000

Investment Securities (Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5

Investment Securities (Mortga57

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	51	70.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	88.9	235.5
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	59.7	59.5
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	199.6	365.3
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1	70.3
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39	3.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109.1	74.2
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	308.7	439.5
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.5	214.2
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22.7	205.9
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16	120.2
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.2	540.3
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,525.2	2,093.8	$ 2,011.6	$ 585.6
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	51	70.3
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	88.9	235.5
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	59.7	59.5
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	199.6	365.3
Level 2 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1	70.3
Level 2 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39	3.9
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109.1	74.2
Level 2 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	308.7	439.5
Level 2 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.5	214.2
Level 2 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22.7	205.9
Level 2 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16	120.2
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.2	540.3
Level 2 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	387.9	979.8
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
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
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
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
Level 3 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	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	0	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	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
Level 3 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0	$ 0

Investment Securities (Non-agen

Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1	70.3
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39	3.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109.1	$ 74.2
Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.3
Securities Issued in 2004 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.3
Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.3
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.3
Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.6
Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.6
Securities Issued in 2015 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47.9
Securities Issued in 2015 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2015 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47.9
Securities Issued in 2016 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2016 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.7
Securities Issued in 2016 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.7
Securities Issued in 2017 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2017 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.3
Securities Issued in 2017 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 35.3

Investment Securities (Non-ag59

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1	$ 70.3
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.4
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.1
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.4
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.7
Prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0

Investment Securities (Non-ag60

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2015USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5
Commercial Mortgage Backed Securities Fixed Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average basis points of subordination	0.3
Fair value investments	$ 17.8
Commercial Mortgage Backed Securities Fixed Rate | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Commercial Mortgage Backed Securities Fixed Rate | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.2
Commercial Mortgage Backed Securities Fixed Rate | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 1.6
Commercial Mortgage Backed Securities Floating Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average basis points of subordination	0.3
Fair value investments	$ 21.2
Commercial Mortgage Backed Securities Floating Rate | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Commercial Mortgage Backed Securities Floating Rate | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Commercial Mortgage Backed Securities Floating Rate | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39	$ 3.9
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16.2
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 22.8

Investment Securities (Rollforw

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)Investment	Sep. 30, 2016USD ($)Investment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 2,547.3	$ 1,543.5
Net realized and unrealized gains (losses)	(125.6)	27.2
Amortization/Accretion	(6.6)	(3.7)
Purchases	2,078.2	3,726.9
Sales	2,229.9	2,887
Transfers in	13.1	68
Transfers out	(13.1)	(68)
Ending balance	2,509.4	2,406.9
Short-term investments, at amortized cost (which approximates fair value)	786.5		$ 174.9
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	279.5	789
Net realized and unrealized gains (losses)	52.4	7.2
Amortization/Accretion	0	0.1
Purchases	940.7	1,387.8
Sales	539	1,992.7
Transfers in	0	0
Transfers out	0	0
Ending balance	733.6	191.4
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,093.8	585.6
Net realized and unrealized gains (losses)	59.8	17.4
Amortization/Accretion	(6.6)	(3.8)
Purchases	1,038.5	2,228.5
Sales	1,668.2	884.1
Transfers in	13.1	68
Transfers out	0	0
Ending balance	1,525.2	2,011.6
Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0	0
Net realized and unrealized gains (losses)	0	0.1
Amortization/Accretion	0	0
Purchases	31.2	70
Sales	12.5
Transfers in	0	0
Transfers out	13.1	(68)
Ending balance	$ 5.6	$ 2.1
Number of investments | Investment	2	2
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 91.4	$ 103.6
Net realized and unrealized gains (losses)	(2.2)	0.8
Amortization/Accretion	0	0
Purchases	2.9	2.2
Sales	2	0.1
Transfers in	0	0
Transfers out	0	0
Ending balance	90.1	106.5
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	82.6	65.3
Net realized and unrealized gains (losses)	15.6	1.7
Amortization/Accretion	0	0
Purchases	64.9	38.4
Sales	8.2	10.1
Transfers in	0	0
Transfers out	0	0
Ending balance	154.9	95.3
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,639.7
Ending balance	3,141
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	441.7
Ending balance	1,498.1
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,106.6
Ending balance	1,547.2
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	91.4
Ending balance	95.7
Fair value measured on a recurring basis | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,087.7
Ending balance	1,490
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	101.5
Ending balance	217.8
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	1,986.2
Ending balance	1,266.6
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	5.6
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	91.4
Ending balance	90.1
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	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]
Beginning balance	0
Ending balance	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]
Beginning balance	91.4
Ending balance	90.1
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	175
Ending balance	786.5
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	162.3
Ending balance	764.5
Short-term investments, at amortized cost (which approximates fair value)		230	175	$ 142
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	12.7
Ending balance	22
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0
Ending balance	0
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)		0.1	0.1	0.1
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale		$ 8.3	6.6	$ 9.5
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	(15.4)		$ (1.2)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	5.2
Star & Shield Insurance Exchange | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0
Star & Shield Insurance Exchange | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	5.2
Star & Shield Insurance Exchange | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	6.6
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0
Star & Shield Insurance Exchange | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 0.1

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015input
BBB | Debt securities issued by corporations | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 5.6
Valuation techniques	133.792
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%
NR | Private equity funds | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 22.1	$ 22.1
Valuation techniques	0.25	0.25
NR | Private equity funds | Option pricing method
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 10.4	$ 9.3
Valuation techniques	0.21	0.21
NR | Convertible Preferred Stock | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 0.2	$ 3.6
Valuation techniques	6	6
Unobservable inputs | input			0
Investment A | NR | Private equity funds | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21	$ 21
Valuation techniques	1	1
Investment B | NR | Private equity funds | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 3.6	$ 3.2
Valuation techniques	2.521	2.52
Investment E | NR | Convertible Preferred Stock | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 27	$ 27
Valuation techniques	3.83	3.83

Goodwill and Other Intangible63

Goodwill and Other Intangible Assets - Changes In Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	Aug. 04, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to acquire businesses and interest in affiliates			$ 0	$ 13.7
Assets			3,637.1			$ 6,520.2
Goodwill			25.9	$ 25.9	$ 25.9	25.9	$ 18.3	$ 18.6
Liabilities			$ 299.9			$ 2,804.2
Oversee.net
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to acquire businesses and interest in affiliates		$ 3.9
Buzzmove
Acquired Finite-Lived Intangible Assets [Line Items]
Ownership interest (as a percent)	70.90%		77.10%
Assets	$ 11.5
Goodwill	7.6
Intangible Assets, Net (Excluding Goodwill)	1.1
Liabilities	0.1
United Kingdom, Pounds | Buzzmove
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of subsidiary	6.1
United States of America, Dollars | Buzzmove
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of subsidiary	$ 8.1

Goodwill and Other Intangible64

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Liabilities	$ 299.9		$ 299.9		$ 2,804.2
Goodwill [Roll Forward]
Goodwill, beginning balance	25.9	$ 18.3	25.9	$ 18.6
Intangible assets, net (including Goodwill), beginning balance	40.3	41.6	45.2	45.5
Add: Amounts held for sale at beginning of the period					0	$ 0.4
Acquisition of businesses	0	1.1	0
Wobi write off				(0.3)
Wobi write off				(2.8)
Acquisitions of goodwill		7.6		7.6
Amortization, including foreign currency translation	(2.5)	(2.6)	(7.4)	(8)
Goodwill, ending balance	25.9	25.9	25.9	25.9
Intangible assets, net (including Goodwill), ending balance	37.8	47.7	37.8	47.7
Assets	3,637.1		3,637.1		6,520.2
Other Intangible Assets
Goodwill [Roll Forward]
Other intangible assets, beginning balance	14.4	23.3	19.3	26.9
Add: Amounts held for sale at beginning of the period					$ 0	$ 0.4
Acquisition of businesses				5
Wobi write off				(2.5)
Amortization, including foreign currency translation	(2.5)	(2.6)	(7.4)	(8)
Other intangible assets, ending balance	$ 11.9	21.8	$ 11.9	21.8
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Acquisition of businesses		$ 8.7		$ 12.6

Goodwill and Other Intangible65

Goodwill and Other Intangible Assets - Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 25.9	$ 25.9	$ 25.9	$ 25.9	$ 18.3	$ 18.6
Goodwill and other intangible assets	37.8	40.3	45.2	47.7	41.6	45.5
MediaAlpha
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	18.3		18.3
Buzzmove
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	7.6		7.6
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	11.9	$ 14.4	19.3	$ 21.8	$ 23.3	$ 26.9
Other Intangible Assets | MediaAlpha
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	11		18.3
Other Intangible Assets | Buzzmove
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Net (Excluding Goodwill)	0.9		1
Discontinued Operations, Held-for-sale
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	0		1.2
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	(13.7)		(17.1)
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	$ 24.1		$ 29.3

Debt (Details)

Debt (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	May 12, 2017	Dec. 31, 2016	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument
Total debt	$ 9.4	$ 9.4			$ 12.7
Repayments of debt		364.6	$ 401.8
Draw down of debt and revolving line of credit		361	$ 352.5
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 425
White Mountains | WTM Bank Facility
Debt Instrument
Total debt	0	0			0
MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Debt Instrument, Face Amount	9.4	9.4
Unamortized issuance cost	$ 0	$ 0
Debt instrument, interest rate, effective percentage	5.50%	5.50%
Total commitment under revolving credit facility				$ 20
Long-term line of credit	$ 9.4	$ 9.4
Draw down of debt and revolving line of credit		6
MediaAlpha | Previous MediaAlpha Bank Facility
Debt Instrument
Debt Instrument, Face Amount	0	0			12.9
Unamortized issuance cost	0	0			$ (0.2)
Debt instrument, interest rate, effective percentage					5.70%
Total commitment under revolving credit facility						$ 20
Repayments of debt		12.9
Long-term line of credit	0	0			$ 12.7
OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Total debt	0	0			0
Unamortized issuance cost	0	$ 0			$ 0
Term Loan | MediaAlpha Bank Facility
Debt Instrument
Basis points		1.50%
Term Loan | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility	5	$ 5
Repayments of debt		1.6
Long-term line of credit	3.4	3.4
Proceeds from issuance of debt		5
Revolving Credit Facility [Member] | MediaAlpha | MediaAlpha Bank Facility
Debt Instrument
Total commitment under revolving credit facility	15	15
Long-term line of credit	6	$ 6
Line of Credit | MediaAlpha Bank Facility
Debt Instrument
Basis points		0.25%
Line of Credit | White Mountains | WTM Bank Facility
Debt Instrument
Repayments of debt	$ 350	$ 350

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)			35.00%	35.00%
Income tax expense (benefit)	$ (4)	$ (17.1)	$ (5.3)	$ (22.7)
Effective income tax rate reconciliation, percent	(47.60%)	(123.90%)	(203.90%)	(31.20%)
Tax Year 2012
Income Tax Contingency [Line Items]
Deferred tax assets, gross					$ 0.6
Operating loss carryforwards, valuation allowance					$ 0.6
Continuing Operations
Income Tax Contingency [Line Items]
Income tax expense (benefit)				$ (14)
Continuing Operations | BAM
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (2.1)	$ (3.3)	$ (6.9)	$ (8.2)

Derivatives Pre-Tax Operating R

Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 87.5	$ 56.8	$ 259.8	$ 163.2
General and administrative expenses	(44.1)	(42.4)	(162.2)	(144.9)
Pre-tax income (loss) from continuing operations	$ 8.4	(13.8)	$ 2.6	(72.8)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue		0		1.2
Change in fair value of variable annuity liability, included in other revenue		0		(0.3)
Change in fair value of derivatives, included in other revenue		0		(2)
Foreign exchange, included in other revenue		0		1.4
Total revenues		0		0.3
Death benefit claims paid, included in general and administrative expenses		0		(0.3)
General and administrative expenses		(0.5)		(2.4)
Pre-tax income (loss) from continuing operations		$ (0.5)		$ (2.4)

Derivatives Realized and Unreal

Derivatives Realized and Unrealized Gains (Losses) (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	$ 0	$ (2)
Derivative, fair value, net	0	0	$ 0	$ 20.1
Purchases		0
Derivative Transfers In (Out), Net		0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		18.1
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	1.8
Derivative, fair value, net			0
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	(4.8)
Derivative, fair value, net			0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	1
Derivative, fair value, net			$ 0
Level 3 Inputs | Variable Annuity
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0	0		(0.3)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(0.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers		0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements		0
Level 3 Inputs | Derivative
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		2.9
Derivative, fair value, net	0	0		2.7
Purchases		0
Derivative Transfers In (Out), Net		0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		5.6
Level 2 Inputs
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		16.5
Purchases		0
Derivative Transfers In (Out), Net		0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		15.8
Level 1 Inputs
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		$ 0.9
Purchases		0
Derivative Transfers In (Out), Net		0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		$ (3.3)

Derivatives Variable Annuity Re

Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in derivative instruments
Beginning of period		$ 0	$ 20.1
Purchases			0
Realized and unrealized gains (losses)	$ 0		(2)
Transfers in			0
Sales/settlements			(18.1)
End of period	0		0
Level 2 Inputs
Changes in derivative instruments
Beginning of period			16.5
Purchases			0
Realized and unrealized gains (losses)			(0.7)
Transfers in			0
Sales/settlements			(15.8)
End of period	0		0
Level 1 Inputs
Changes in derivative instruments
Beginning of period			0.9
Purchases			0
Realized and unrealized gains (losses)			(4.2)
Transfers in			0
Sales/settlements			3.3
End of period	0		0
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance			(0.3)
Realized and unrealized gains (losses)			(0.3)
Transfers in			0
Sales/settlements			0
Ending balance	0		0
Purchases			0
Derivative | Level 3 Inputs
Changes in derivative instruments
Beginning of period			2.7
Purchases			0
Realized and unrealized gains (losses)			2.9
Transfers in			0
Sales/settlements			(5.6)
End of period	$ 0		$ 0

Derivatives Forward Contracts (

Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Forward Contracts
Derivative [Line Items]
Notional amount	$ 261.8		$ 261.8
Realized and unrealized gains (losses)	7.2	$ 1.7	8.9
Unrealized gain (loss) on derivatives	2.8		14.2
Derivative, fair value, net	(15.4)		(15.4)
Other Investments
Derivative [Line Items]
Derivative liability	15.4		15.4	$ 1.2
United States of America, Dollars
Derivative [Line Items]
Notional amount	261.8		261.8
United Kingdom, Pounds
Derivative [Line Items]
Notional amount	160.7		160.7
Euro Member Countries, Euro
Derivative [Line Items]
Notional amount	18.9		18.9
Japan, Yen
Derivative [Line Items]
Notional amount	2,646.4		2,646.4
Barclays | Standard & Poor's, A- Rating | Forward Contracts
Derivative [Line Items]
Notional amount	201.3		201.3
Derivative, fair value, net	(14.1)		(14.1)
JP Morgan | Standard & Poor's, A plus | Forward Contracts
Derivative [Line Items]
Notional amount	60.5		60.5
Derivative, fair value, net	$ (1.3)		$ (1.3)

Municipal Bond Guarantee (Detai

Municipal Bond Guarantee (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017USD ($)Contract	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)yrContract	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)Contract	Dec. 31, 2012USD ($)	Jun. 30, 2017USD ($)	Jul. 31, 2012USD ($)
Guarantor Obligations [Line Items]
Pre-tax income (loss)	$ 8.4	$ (13.8)	$ 2.6	$ (72.8)
Net investment income (loss) - BAM Surplus Note interest	0	0	0	0
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution						$ 594.5
Surplus notes								$ 503
Income (loss) from continuing operations before income taxes, domestic	$ 7		$ 20.3
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	96.90%		96.90%		96.90%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%		88.40%		88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Surplus notes	$ 500		$ 500
Year of U.S. Treasury rate | yr			1
Municipal bond contract | Contract	5,907		5,907		4,807
Remaining weighted average contract period outstanding (in years)			10 years 11 months		10 years 10 months
Municipal bond outstanding principal	$ 39,207.5		$ 39,207.5		$ 33,057.3
Municipal bond outstanding interest	19,681		19,681		16,396.6
Municipal bond debt service outstanding	58,888.5		58,888.5		49,453.9
October 1, 2017 - December 31, 2017	2.5		2.5
2,018	9.9		9.9
2,019	9.5		9.5
2,020	9.2		9.2
2,021	8.7		8.7
2022 and thereafter	78.7		78.7
Total gross unearned insurance premiums	118.5		118.5		$ 82.9
Build America Mutual
Guarantor Obligations [Line Items]
Income (loss) from continuing operations before income taxes, domestic	(11.9)		(35.6)
HG Global
Guarantor Obligations [Line Items]
Pre-tax income (loss)	7	5.2	20.3	18.4
Net investment income (loss) - BAM Surplus Note interest	4.8	4.5	$ 14.3	13.4
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured			15.00%
BAM
Guarantor Obligations [Line Items]
Pre-tax income (loss)	(11.9)	(13.7)	$ (35.6)	(30.3)
Net investment income (loss) - BAM Surplus Note interest	(4.8)	$ (4.5)	(14.3)	$ (13.4)
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution						$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Interest income, other	$ 4.8		$ 14.3
Derivative, fixed interest rate (as a percent)	3.78%		3.78%		3.54%
Basis points			30000.00%
Surplus Note | Build America Mutual
Guarantor Obligations [Line Items]
Interest expense	$ 4.8		$ 14.3
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 | Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points			3.00%
Fixed interest rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)	8.00%		8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,018	$ 2.5		$ 2.5
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,018	2.5		2.5
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,018	2.5		2.5
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2,018	$ 2.4		$ 2.4
Series A BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes							$ 203
Series B BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes							$ 300

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 562.1	$ 90.8	$ 604.7	$ 440.7
Total income (loss) from discontinued operations	(539.1)	(84.4)	(573.2)	(515.4)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	23	6.4	31.5	(74.7)
Allocation of earnings to participating restricted common shares(1)	(0.3)	(0.1)	(0.4)	0.8
Basic and diluted earnings per share numerators	$ 22.7	$ 6.3	$ 31.1	$ (73.9)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,297,200	4,867,400	4,477,000	5,166,600
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Basic earnings per share denominator	4,243,500	4,799,300	4,422,500	5,103,700
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,297,200	4,879,400	4,477,000	5,174,800
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Diluted earnings per share denominator	4,243,500	4,811,300	4,422,500	5,111,900
Basic earnings per share (in dollars) - continuing operations:
Continuing operations (in usd per share)	$ 5.36	$ 1.31	$ 7.03	$ (14.47)
Dividends declared	0	0	(1)	(1)
Undistributed earnings (loss) (in dollars per share)	5.36	1.31	6.03	(15.47)
Earnings Per Share, Diluted [Abstract]
Continuing operations (in usd per share)	5.36	1.31	7.03	(14.47)
Dividends declared	0	0	(1)	(1)
Undistributed earnings (loss) (in dollars per share)	$ 5.36	$ 1.31	$ 6.03	$ (15.47)
Incremental common shares attributable to dilutive effect of share-based payment arrangements		11,943		8,208
Employee Stock Option
Earnings Per Share, Diluted [Abstract]
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000

Earnings Per Share - Undistribu

Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Undistributed net earnings - continuing operations:	$ 22.7	$ 6.3	$ 31.1	$ (73.9)
Dividends declared net of restricted common share amounts	0	0	(4.5)	(5.9)
Total undistributed net earnings (loss), net of restricted common share amounts	$ 22.7	$ 6.3	$ 26.6	$ (79.8)

Non-controlling Interest (Detai

Non-controlling Interest (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (131.6)	$ 133.3
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 28.6	$ 35.2
OneBeacon
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	23.90%
Total non-controlling interests	$ 0	$ 244.6
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 16.1	$ 16.6
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 9.5	$ 11.7
Buzzmove
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	22.90%	29.10%
Total non-controlling interests	$ 3	$ 2.9
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	5.00%	5.00%
Total non-controlling interests	$ 0	$ 0.1
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	18.80%
Total non-controlling interests	$ 0	$ 3.9
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (160.2)	$ (150.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	100.00%
Total non-controlling interests	$ 0	$ 4.4
Star & Shield Insurance Exchange | Total mutual and reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (160.2)	$ (146.5)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment reporting information
Earned insurance premiums	$ 2.4	$ 3.4	$ 7.6	$ 10.2
Net investment income	12.2	9.6	39.7	18.2
Net investment income (loss) - BAM Surplus Note interest	0	0	0	0
Net realized and unrealized investment gains	32.5	10.9	102.5	27.2
Advertising and commission revenues	38.8	28.2	103.9	89.6
Other revenue	1.6	4.7	6.1	18
Total revenues	87.5	56.8	259.8	163.2
Loss and loss adjustment expenses	0	2.2	1.1	6.8
Insurance acquisition expenses	0.9	1.3	3.1	4.4
Other underwriting expenses	0.1	0.2	0.3	0.4
Cost of sales	33.1	24	88.7	76.9
General and administrative expenses	44.1	42.4	162.2	144.9
Interest expense	0.9	0.5	1.8	2.6
Total expenses	79.1	70.6	257.2	236
Pre-tax income (loss)	8.4	(13.8)	2.6	(72.8)
HG Global
Segment reporting information
Earned insurance premiums	1.8	1.2	5	3.1
Net investment income	1	0.6	2.4	1.6
Net investment income (loss) - BAM Surplus Note interest	4.8	4.5	14.3	13.4
Net realized and unrealized investment gains	0.1	(0.3)	0.4	2.3
Advertising and commission revenues	0	0	0	0
Other revenue	0	0	0	0
Total revenues	7.7	6	22.1	20.4
Loss and loss adjustment expenses		0	0	0
Insurance acquisition expenses	0.4	0.2	1	0.6
Other underwriting expenses	0	0	0	0
Cost of sales	0	0	0	0
General and administrative expenses	0.3	0.6	0.8	1.4
Interest expense	0	0	0	0
Total expenses	0.7	0.8	1.8	2
Pre-tax income (loss)	7	5.2	20.3	18.4
BAM
Segment reporting information
Earned insurance premiums	0.6	0.3	1.6	1
Net investment income	2.3	1.7	6.5	5.1
Net investment income (loss) - BAM Surplus Note interest	(4.8)	(4.5)	(14.3)	(13.4)
Net realized and unrealized investment gains	0.7	(1.6)	2.8	6.5
Advertising and commission revenues	0	0	0	0
Other revenue	0.2	0.4	0.8	0.8
Total revenues	(1)	(3.7)	(2.6)	0
Loss and loss adjustment expenses		0	0	0
Insurance acquisition expenses	0.5	0.6	2	1.9
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	0	0	0	0
General and administrative expenses	10.3	9.3	30.7	28.1
Interest expense	0	0	0	0
Total expenses	10.9	10	33	30.3
Pre-tax income (loss)	(11.9)	(13.7)	(35.6)	(30.3)
MediaAlpha
Segment reporting information
Earned insurance premiums	0	0	0	0
Net investment income	0	0	0	0
Net investment income (loss) - BAM Surplus Note interest	0	0	0	0
Net realized and unrealized investment gains	0	0	0	0
Advertising and commission revenues	37.9	27.6	101.2	88.4
Other revenue	0	0	0	0
Total revenues	37.9	27.6	101.2	88.4
Loss and loss adjustment expenses			0	0
Insurance acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
Cost of sales	32.2	23	86	74
General and administrative expenses	6.1	5.7	17.9	16.3
Interest expense	0.1	0.2	0.6	0.7
Total expenses	38.4	28.9	104.5	91
Pre-tax income (loss)	(0.5)	(1.3)	(3.3)	(2.6)
Other Operations
Segment reporting information
Earned insurance premiums	0	1.9	1	6.1
Net investment income	8.9	7.3	30.8	11.5
Net investment income (loss) - BAM Surplus Note interest	0	0	0	0
Net realized and unrealized investment gains	31.7	12.8	99.3	18.4
Advertising and commission revenues	0.9	0.6	2.7	1.2
Other revenue	1.4	4.3	5.3	17.2
Total revenues	42.9	26.9	139.1	54.4
Loss and loss adjustment expenses		2.2	1.1	6.8
Insurance acquisition expenses	0	0.5	0.1	1.9
Other underwriting expenses	0	0.1	0	0.1
Cost of sales	0.9	1	2.7	2.9
General and administrative expenses	27.4	26.8	112.8	99.1
Interest expense	0.8	0.3	1.2	1.9
Total expenses	29.1	30.9	117.9	112.7
Pre-tax income (loss)	$ 13.8	$ (4)	$ 21.2	$ (58.3)

Investments in Symetra (Symetra

Investments in Symetra (Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 01, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments		$ 26.1	$ 9.9				$ 82.1	$ (240.7)
Trading securities unrealized holding gain on investment after tax		$ 22.8	10				76.2	(199.6)
Sales of common equity securities							$ 407.8	683
Common shares, par value per share (in dollars per share)		$ 1					$ 1		$ 1
Net realized and unrealized investment gains		$ 32.5	$ 10.9				$ 102.5	$ 27.2
Symetra
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments					$ 258.8
Trading securities unrealized holding gain on investment after tax					$ 241.1
Sales of common equity securities	$ 658
Common shares, par value per share (in dollars per share)	$ 32
Net realized and unrealized investment gains				$ 4.7
Symetra
Investments in unconsolidated affiliates
Common stock, dividends, per share, cash paid	$ 0.50					$ 0.50
Sales of common equity securities	$ 658

Employee Share-Based Incentiv78

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accrued Expense
Accrued expense at the beginning of period		$ 95.7		$ 95.7
Accrued expense at the end of the period	$ 76.9			$ 76.9		$ 95.7
Common shares, issued shares	3,749,971			3,749,971		4,563,814
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2014-2016
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
Accrued Expense
Percentage of target payout - minimum actual						140.00%
Percentage of target payout - maximum actual						142.00%
Common shares, issued shares						5,000
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period				3 years
Target Performance Share Activity
Beginning of period (in shares)	50,575	80,353	73,297	80,353	93,654	93,654
Shares paid or expired (in shares)	0		0	(30,838)	(36,294)
New grants (in shares)	0		6,400	17,510	22,615
Assumed forfeitures and cancellations	(256)		(160)	(16,706)	(438)
Ending of period (in shares)	50,319		79,537	50,319	79,537	80,353
Accrued Expense
Accrued expense at the beginning of period	$ 31.2	$ 42.4	$ 32.5	$ 42.4	$ 57.7	$ 57.7
Payments and deferrals	0		0	(21.6)	(41)
Assumed forfeitures and cancellations	(0.4)		(0.1)	(9.4)	0.2
Expense recognized	7		5	26.4	20.5
Accrued expense at the end of the period	37.8		$ 37.4	37.8	$ 37.4	$ 42.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 28.7			$ 28.7
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	2,195		7,315	2,195	7,315

Employee Share-Based Incentiv79

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 76.9		$ 95.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	51,085
Accrued Expense	$ 38.4
Target Performance Shares Outstanding, Assumed forfeitures	(766)
Accrued Expense, Assumed forfeitures	(0.6)
Total Target Performance Shares Outstanding	50,319	50,575	80,353	79,537	73,297	93,654
Total Accrued Expense	$ 37.8	$ 31.2	$ 42.4	$ 37.4	$ 32.5	$ 57.7
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	18,370
Accrued Expense	$ 20.6
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	16,235
Accrued Expense	$ 12.1
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	16,480
Accrued Expense	$ 5.7

Employee Share-Based Incentiv80

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	53,815		70,620	66,470	70,620	70,675
Issued (in shares)	0			4,150	17,785	25,365
Vested (in shares)	(260)			0	(28,846)	(24,620)
Assumed forfeitures and cancellations	0			0	(6,004)	(800)
Ending of period (in shares)	53,555	53,815		70,620	53,555	70,620
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 23.5		$ 19.7	$ 24.2	$ 19.7	$ 15.7
Issued	0			3.4	16.7	19.7
Assumed forfeitures and cancellations	0			0	(3.5)	0.2
Expense recognized	(3.8)			(3.8)	(13.2)	(11.8)
Unamortized grant date fair value, ending balance	$ 19.7	$ 23.5		$ 23.8	$ 19.7	$ 23.8
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		250				750
White Mountains | Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)		250				24,615
White Mountains | Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)		550	16,735
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	2,195			5,235	2,195	5,235

Employee Share-Based Incentiv81

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

Fair Value of Financial Instr82

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 9.4	$ 12.7
MediaAlpha | Book Value | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	9.4	0
MediaAlpha | Book Value | Previous MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	0	12.7
MediaAlpha | Estimate of Fair Value Measurement | MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	9.4	0
MediaAlpha | Estimate of Fair Value Measurement | Previous MediaAlpha Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt instrument, fair value disclosure	$ 0	$ 13

Discontinued Operations - (Deta

Discontinued Operations - (Details) - USD ($)	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold										$ 1,131,000,000	$ 2,657,200,000
Gain from sale of other discontinued operations, net of tax					$ 554,300,000		$ 47,900,000			552,700,000	414,500,000
Net (loss) income from discontinued operations					(15,200,000)		36,500,000			20,500,000	100,900,000
Income tax expense (benefit)					(4,000,000)		(17,100,000)			(5,300,000)	(22,700,000)
Payments to acquire other investments										67,700,000	36,600,000
Investments					3,280,500,000					3,280,500,000		$ 2,714,400,000
Net realized and unrealized investment gains					32,500,000		10,900,000			102,500,000	27,200,000
Net income attributable to White Mountains’s common shareholders					(539,100,000)		(84,400,000)			(573,200,000)	(515,400,000)
Net change in foreign currency translation and benefit plan assets and obligations					0		200,000			200,000	(400,000)
OneBeacon
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300,000,000
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
Net (loss) income from discontinued operations					(15,200,000)		22,500,000			20,500,000	93,800,000
Tranzact
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1,200,000	$ 221,300,000
Gain from sale of other discontinued operations, net of tax		51,900,000				$ (1,000,000)						51,900,000
Net (loss) income from discontinued operations							14,000,000				13,500,000
Discontinued operation, tax effect of discontinued operation		$ 30,200,000					15,900,000				13,800,000	30,200,000
Net realized and unrealized investment gains							0				0
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600,000,000
Gain from sale of other discontinued operations, net of tax							(4,000,000)					363,200,000
Payments to acquire other investments				$ 161,800,000
Investments													$ 686,200,000
Net realized and unrealized investment gains								$ 400,000	$ 3,700,000
Sale of Sirius Group
Discontinued Operations [Line Items}
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary												113,300,000
Discontinued Operations
Discontinued Operations [Line Items}
Net income attributable to White Mountains’s common shareholders					539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations											32,500,000
Discontinued Operations | OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations					(15,200,000)		22,500,000			20,500,000	93,800,000
Discontinued operation, tax effect of discontinued operation					4,100,000		(10,400,000)			5,700,000	(300,000)
Net realized and unrealized investment gains					11,500,000		15,500,000			38,800,000	56,800,000
Net income attributable to White Mountains’s common shareholders					539,300,000		22,500,000			575,000,000	93,800,000
Net change in foreign currency translation and benefit plan assets and obligations					100,000		(300,000)			300,000	500,000
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations							2,100,000				2,600,000
Discontinued operation, tax effect of discontinued operation												30.2
Income tax expense (benefit)												$ 16.1
Net income attributable to White Mountains’s common shareholders							65,900,000			(1,000,000)	65,400,000
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					(800,000)					1,000,000
Net (loss) income from discontinued operations					0		0				(6,400,000)
Discontinued operation, tax effect of discontinued operation					0		0				3,100,000
Net realized and unrealized investment gains					0		0				(1,500,000)
Net income attributable to White Mountains’s common shareholders					$ (200,000)		(4,000,000)			$ (800,000)	356,200,000
Other comprehensive income											145,300,000
Net change in foreign currency translation and benefit plan assets and obligations							0				32,000,000
Discontinued Operations | Sale of Sirius Group
Discontinued Operations [Line Items}
Net change in foreign currency translation and benefit plan assets and obligations							0				113,300,000
Continuing Operations
Discontinued Operations [Line Items}
Income tax expense (benefit)											$ (14,000,000)
Continuing Operations | Tranzact
Discontinued Operations [Line Items}
Income tax expense (benefit)							$ 14,000,000
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments													$ 528,600,000
Intact Financial Corporation [Member] | OneBeacon
Discontinued Operations [Line Items}
Sale of Stock, Price Per Share	$ 18.10

Discontinued Operations - Finan

Discontinued Operations - Financial Statement Disclosures (Details) - USD ($)	Sep. 28, 2017	Oct. 05, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jun. 30, 2017	Dec. 31, 2015
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value			$ 1,490,000,000					$ 1,490,000,000		$ 2,081,100,000
Short-term investments, at amortized cost (which approximates fair value)			786,500,000					786,500,000		174,900,000
Common equity securities, at fair value			774,400,000					774,400,000		285,600,000
Other long-term investments			229,600,000					229,600,000		172,800,000
Total investments			3,280,500,000					3,280,500,000		2,714,400,000
Cash			48,600,000					48,600,000		80,200,000
Insurance premiums receivable			4,500,000					4,500,000		1,600,000
Deferred acquisition costs			14,100,000					14,100,000		10,600,000
Accounts receivable on unsettled investment sales			190,100,000					190,100,000		4,800,000
Goodwill and other intangible assets			37,800,000		$ 47,700,000	$ 41,600,000	$ 41,600,000	37,800,000	$ 47,700,000	45,200,000	$ 40,300,000	$ 45,500,000
Accrued investment income			15,400,000					15,400,000		14,800,000
Other assets			46,100,000					46,100,000		49,100,000
Total assets held for sale			0					0		3,599,500,000
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Unearned insurance premiums			118,500,000					118,500,000		82,900,000
Debt			9,400,000					9,400,000		12,700,000
Accrued incentive compensation			76,900,000					76,900,000		95,700,000
Accounts payable on unsettled investment purchases			49,700,000					49,700,000		0
Other liabilities			45,400,000					45,400,000		43,600,000
Total liabilities held for sale			0					0		2,569,300,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			2,400,000		3,400,000			7,600,000	10,200,000
Net investment income			12,200,000		9,600,000			39,700,000	18,200,000
Net realized and unrealized investment gains			32,500,000		10,900,000			102,500,000	27,200,000
Total revenues			87,500,000		56,800,000			259,800,000	163,200,000
Loss and loss adjustment expenses			0		2,200,000			1,100,000	6,800,000
Insurance acquisition expenses			900,000		1,300,000			3,100,000	4,400,000
Other underwriting expenses			100,000		200,000			300,000	400,000
General and administrative expenses			44,100,000		42,400,000			162,200,000	144,900,000
Interest expense			900,000		500,000			1,800,000	2,600,000
Total expenses			79,100,000		70,600,000			257,200,000	236,000,000
Net (loss) income from discontinued operations, net of tax			(15,200,000)		36,500,000			20,500,000	100,900,000
Gain from sale of other discontinued operations, net of tax			554,300,000		47,900,000			552,700,000	414,500,000
Total income (loss) from discontinued operations			(539,100,000)		(84,400,000)			(573,200,000)	(515,400,000)
Net change in foreign currency translation and benefit plan assets and obligations			0		200,000			200,000	(400,000)
Total comprehensive income			565,100,000		90,700,000			608,100,000	586,100,000
Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Goodwill and other intangible assets			0					0		1,200,000
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments												$ 686,200,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains						$ 400,000	$ 3,700,000
Gain from sale of other discontinued operations, net of tax					(4,000,000)					363,200,000
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax			(15,200,000)		22,500,000			20,500,000	93,800,000
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums					0				0
Net investment income					0				0
Net realized and unrealized investment gains					0				0
Other revenue					14,800,000				119,600,000
Total revenues					14,800,000				119,600,000
Loss and loss adjustment expenses					0				0
Insurance acquisition expenses					0				0
Other underwriting expenses					0				0
General and administrative expenses					16,200,000				116,700,000
Interest expense					500,000				3,200,000
Total expenses					16,700,000				119,900,000
Pre-tax (loss) income					(1,900,000)				(300,000)
Income tax benefit (expense)		$ 30,200,000			15,900,000				13,800,000	30,200,000
Net (loss) income from discontinued operations, net of tax					14,000,000				13,500,000
Gain from sale of other discontinued operations, net of tax		$ 51,900,000		$ (1,000,000)						51,900,000
Total comprehensive income					65,900,000
Discontinued Operations
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales										1,400,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations			539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations									32,500,000
Discontinued Operations | Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value										2,175,700,000
Short-term investments, at amortized cost (which approximates fair value)										112,300,000
Common equity securities, at fair value										188,700,000
Other long-term investments										150,500,000
Total investments										2,627,200,000
Cash										70,500,000
Reinsurance recoverable on unpaid and paid losses										179,800,000
Insurance premiums receivable										229,800,000
Deferred acquisition costs										96,300,000
Deferred tax asset										126,700,000
Ceded unearned insurance and reinsurance premiums										44,200,000
Goodwill and other intangible assets										1,200,000
Accrued investment income										11,300,000
Other assets										211,100,000
Total assets held for sale										3,599,500,000
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Loss and loss adjustment expense reserves										1,370,600,000
Unearned insurance premiums										576,300,000
Debt										273,200,000
Accrued incentive compensation										44,300,000
Funds held under reinsurance treaties										153,000,000
Other liabilities										151,900,000
Total liabilities held for sale										2,569,300,000
Discontinued Operations | Sirius Group
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Net Assets										1,030,200,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			0		0				240,100,000
Net investment income			0		0				14,400,000
Net realized and unrealized investment gains			0		0				(1,500,000)
Other revenue			0		0				600,000
Total revenues			0		0				253,600,000
Loss and loss adjustment expenses			0		0				154,900,000
Insurance acquisition expenses			0		0				59,000,000
Other underwriting expenses			0		0				30,900,000
General and administrative expenses			0		0				10,400,000
Interest expense			0		0				7,900,000
Total expenses			0		0				263,100,000
Pre-tax (loss) income			0		0				(9,500,000)
Income tax benefit (expense)			0		0				3,100,000
Net (loss) income from discontinued operations, net of tax			0		0				(6,400,000)
Gain from sale of other discontinued operations, net of tax			(800,000)					1,000,000
Total income (loss) from discontinued operations			(200,000)		(4,000,000)			(800,000)	356,200,000
Net change in foreign currency translation and benefit plan assets and obligations					0				32,000,000
Total comprehensive income			(200,000)		(4,000,000)			(800,000)	501,500,000
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax			(200,000)		(4,000,000)			(800,000)	362,600,000
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268,400,000		277,900,000			807,600,000	827,900,000
Net investment income			13,000,000		11,800,000			39,700,000	38,300,000
Net realized and unrealized investment gains			11,500,000		15,500,000			38,800,000	56,800,000
Other revenue			2,200,000		1,800,000			7,700,000	3,500,000
Total revenues			295,100,000		307,000,000			893,800,000	926,500,000
Loss and loss adjustment expenses			206,800,000		162,800,000			546,000,000	501,300,000
Insurance acquisition expenses			51,900,000		55,100,000			145,600,000	154,800,000
Other underwriting expenses			44,900,000		49,400,000			156,200,000	155,600,000
General and administrative expenses			7,400,000		3,500,000			21,200,000	10,900,000
Interest expense			3,400,000		3,300,000			10,000,000	9,800,000
Total expenses			314,400,000		274,100,000			879,000,000	832,400,000
Pre-tax (loss) income			(19,300,000)		32,900,000			14,800,000	94,100,000
Income tax benefit (expense)			4,100,000		(10,400,000)			5,700,000	(300,000)
Net (loss) income from discontinued operations, net of tax			(15,200,000)		22,500,000			20,500,000	93,800,000
Total income (loss) from discontinued operations			539,300,000		22,500,000			575,000,000	93,800,000
Net change in foreign currency translation and benefit plan assets and obligations			100,000		(300,000)			300,000	500,000
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2,900,000					2,900,000
Total comprehensive income			542,300,000		22,200,000			578,200,000	94,300,000
Discontinued Operations | OneBeacon | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax			554,500,000					554,500,000
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268,400,000		277,900,000			807,600,000	1,068,000,000
Net investment income			13,000,000		11,800,000			39,700,000	52,700,000
Net realized and unrealized investment gains			11,500,000		15,500,000			38,800,000	55,300,000
Other revenue			2,200,000		16,600,000			7,700,000	123,700,000
Total revenues			295,100,000		321,800,000			893,800,000	1,299,700,000
Loss and loss adjustment expenses			206,800,000		162,800,000			546,000,000	656,200,000
Insurance acquisition expenses			51,900,000		55,100,000			145,600,000	213,800,000
Other underwriting expenses			44,900,000		49,400,000			156,200,000	186,500,000
General and administrative expenses			7,400,000		19,700,000			21,200,000	138,000,000
Interest expense			3,400,000		3,800,000			10,000,000	20,900,000
Total expenses			314,400,000		290,800,000			879,000,000	1,215,400,000
Pre-tax (loss) income			(19,300,000)		31,000,000			14,800,000	84,300,000
Income tax benefit (expense)			4,100,000		5,500,000			5,700,000	16,600,000
Net (loss) income from discontinued operations, net of tax			(15,200,000)		36,500,000			20,500,000	100,900,000
Total income (loss) from discontinued operations			539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations			100,000		(300,000)			300,000	113,300,000
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2,900,000					2,900,000
Total comprehensive income			542,100,000		84,100,000			576,400,000	661,200,000
Discontinued Operations | Total | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax			$ 554,300,000		47,900,000			552,700,000	414,500,000
Discontinued Operations | Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit (expense)										$ 30.2
Net (loss) income from discontinued operations, net of tax					2,100,000				2,600,000
Total income (loss) from discontinued operations					65,900,000			(1,000,000)	65,400,000
Total comprehensive income								(1,000,000)	65,400,000
Discontinued Operations | Tranzact | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax					51,900,000			$ (1,000,000)	51,900,000
Discontinued Operations | Sale of Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net change in foreign currency translation and benefit plan assets and obligations					$ 0				$ 113,300,000

Discontinued Operations - Net C

Discontinued Operations - Net Change Cash From Discontinued Operations (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	$ 80.2	$ 72
Cash balances at end of period	48.6	111.2
Net income tax payment to national governments	0	0
Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from operations - discontinued operations	157	38.1
Net cash provided from investing activities - discontinued operations	3	269.7
Net cash used for financing activities - discontinued operations	(61.9)	(72.7)
Net change in cash during the period	98.1	235.1
Cash balances at beginning of period	70.5	245.4
Cash balances at end of period	0	137.3
Interest paid	0	(7.7)
Net income tax payment to national governments	0	(18.3)
Discontinued Operations, Held-for-sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period	70.5	245.4
Cash balances at end of period	0	137.3
Disposal Group, Held-for-sale, Not Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash held for sale	(0.9)	2.6
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash sold as part of sale of consolidated subsidiaries	$ 167.7	$ 345.8

Discontinued Operations - EPS (

Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 562.1	$ 90.8	$ 604.7	$ 440.7
Less: total income from continuing operations, net of tax	23	6.4	31.5	(74.7)
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	(539.1)	(84.4)	(573.2)	(515.4)
Basic and diluted earnings per share numerators	$ 22.7	$ 6.3	$ 31.1	$ (73.9)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,297,200	4,867,400	4,477,000	5,166,600
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Basic earnings per share denominator	4,243,500	4,799,300	4,422,500	5,103,700
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,297,200	4,879,400	4,477,000	5,174,800
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Diluted earnings per share denominator	4,243,500	4,811,300	4,422,500	5,111,900
Basic earnings per share (in dollars)	$ 130.81	$ 18.65	$ 135.06	$ 85.28
Diluted earnings per share (in dollars)	$ 130.81	$ 18.61	$ 135.06	$ 85.13
Incremental common shares attributable to dilutive effect of share-based payment arrangements		11,943		8,208
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 562.1	$ 90.8	$ 604.7	$ 440.7
Less: total income from continuing operations, net of tax	(23)	(6.4)	(31.5)	74.7
Net income (loss) from discontinued operations attributable to White Mountains’s common shareholders	539.1	84.4	573.2	515.4
Allocation of income for participating unvested restricted common shares	(6.7)	(1.2)	(7)	(6.3)
Basic and diluted earnings per share numerators	$ 532.4	$ 83.2	$ 566.2	$ 509.1
Basic earnings per share denominators
Total average common shares outstanding during the period	4,297,200	4,867,400	4,477,000	5,166,600
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Basic earnings per share denominator	4,243,500	4,799,300	4,422,500	5,103,700
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,297,200	4,879,400	4,477,000	5,174,800
Average unvested restricted common shares	(53,700)	(68,100)	(54,500)	(62,900)
Diluted earnings per share denominator	4,243,500	4,811,300	4,422,500	5,111,900
Basic earnings per share (in dollars)	$ 125.45	$ 17.34	$ 128.03	$ 99.75
Diluted earnings per share (in dollars)	$ 125.45	$ 17.30	$ 128.03	$ 99.60
Employee Stock Option
Diluted earnings per share denominator
Incremental common shares attributable to dilutive effect of share-based payment arrangements		120,000		120,000

Discontinued Operations - Fair

Discontinued Operations - Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	$ 9.4	$ 12.7
2012 OBH Senior Notes | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	$ 0	0
Estimate of Fair Value Measurement | 2012 OBH Senior Notes | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt instrument, fair value disclosure		274.2
Book Value | 2012 OBH Senior Notes | OneBeacon
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt		$ 273.2

Discontinued Operations - Surpl

Discontinued Operations - Surplus Note (Details) - OneBeacon - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Assets, fair value adjustment		$ (29.1)
Par Value
Surplus notes	$ 101	$ 101
Fair Value
Basis points		2.50%
Surplus notes		$ 71.9
Current market rates on repayments
Assets, fair value adjustment		5.1
Current market rates on repayments | Seller Priority Surplus Note
RBC Score	250.00%
Regulatory Approval
Assets, fair value adjustment		(15.6)
Liquidity Adjustment
Assets, fair value adjustment		$ (18.6)

Financial Statement Revisions -

Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Earned insurance premiums	$ 2.4	$ 3.4		$ 7.6	$ 10.2
Net investment income	12.2	9.6		39.7	18.2
Net realized and unrealized investment gains	32.5	10.9		102.5	27.2
Advertising and commission revenues	38.8	28.2		103.9	89.6
Other revenue	1.6	4.7		6.1	18
Total revenues	87.5	56.8		259.8	163.2
Expenses:
Loss and loss adjustment expenses	0	2.2		1.1	6.8
Insurance acquisition expenses	0.9	1.3		3.1	4.4
Other underwriting expenses	0.1	0.2		0.3	0.4
Cost of sales	33.1	24		88.7	76.9
General and administrative expenses	44.1	42.4		162.2	144.9
Interest expense	0.9	0.5		1.8	2.6
Total expenses	79.1	70.6		257.2	236
Pre-tax income (loss) from continuing operations	8.4	(13.8)		2.6	(72.8)
Income tax benefit	4	17.1		5.3	22.7
Net income (loss) from continuing operations	12.4	3.3		7.9	(50.1)
Gain from sale of other discontinued operations, net of tax	554.3	47.9		552.7	414.5
Net (loss) income from discontinued operations, net of tax	(15.2)	36.5		20.5	100.9
Net income	551.5	87.7		581.1	465.3
Net loss (income) attributable to non-controlling interests	10.6	3.1		23.6	(24.6)
Net income attributable to White Mountains’s common shareholders	562.1	90.8		604.7	440.7
Comprehensive income, net of tax:
Comprehensive income	565.1	90.7		608.1	586.1
Other comprehensive income attributable to non-controlling interests	0	0.1		(0.1)	0.1
Comprehensive income attributable to White Mountains’s common shareholders	$ 565.1	$ 90.8		$ 608	$ 586.2
Continuing operations (in usd per share)	$ 5.36	$ 1.31		$ 7.03	$ (14.47)
As previously reported
Revenues:
Earned insurance premiums		$ 3.4	$ 5.2		$ 10.2	$ 13.4	$ 12	$ 7.9
Net investment income		9.6	27.5		18.2	32.1	10.9	12.2
Net realized and unrealized investment gains		10.9	70		27.2	(27.4)	260.5	38.1
Advertising and commission revenues		29.4	71.7		92.4	126.9	110.1	65.7
Other revenue		4.7	4.5		18	21.3	41.8	20.5
Total revenues		58	178.9		166	166.3	435.3	144.4
Expenses:
Loss and loss adjustment expenses		2.2	1.1		6.8	8	8.2	8.9
Insurance acquisition expenses		1.3	2.2		4.4	5.6	6.3	2.9
Other underwriting expenses		0.2	0.2		0.4	0.5	0.4	0.4
Cost of sales		24	55.6		76.9	102	93.6	57.8
General and administrative expenses		42.7	117.9		142.6	183.7	193.2	144.9
Interest expense		0.5	0.9		2.6	3	1.6	1.2
Total expenses		70.9	177.9		233.7	302.8	303.3	216.1
Pre-tax income (loss) from continuing operations		(12.9)	1		(67.7)	(136.5)	132	(71.7)
Income tax benefit		17.1	1.3		22.7	32.9	(12.7)	2.5
Net income (loss) from continuing operations		4.2	2.3		(45)	(103.6)	119.3	(69.2)
Gain from sale of other discontinued operations, net of tax		47.9	(1.6)		414.5	415.1	17.9	17.2
Net (loss) income from discontinued operations, net of tax		36.5	35.7		100.9	108.3	117.2	296.4
Income (loss) before equity in earnings of unconsolidated affiliates						419.8	254.4	244.4
Equity in earnings of unconsolidated affiliates						0	25.1	45.6
Net income		88.6	36.4		470.4	419.8	279.5	290
Net loss (income) attributable to non-controlling interests		3.1	13.4		(24.6)	(7.3)	18.1	22.2
Net income attributable to White Mountains’s common shareholders		91.7	49.8		445.8	412.5	297.6	312.2
Comprehensive income, net of tax:
Other comprehensive income		0.2	1.7		145.6	145.6	(100.4)	(104.9)
Comprehensive income		91.9	51.5		591.4	558.1	197.2	207.3
Other comprehensive income attributable to non-controlling interests		0.1	(0.1)		0.1	(0.3)	0	3.3
Comprehensive income attributable to White Mountains’s common shareholders		$ 92	$ 51.4		$ 591.5	$ 557.8	$ 197.2	$ 210.6
Continuing operations (in usd per share)		$ 1.50	$ 0		$ (13.49)	$ 0	$ 0	$ 0
Adjustments
Revenues:
Earned insurance premiums		$ 0	$ 0		$ 0	$ 0	$ 0	$ 0
Net investment income		0	0		0	0	0	0
Net realized and unrealized investment gains		0	0		0	0	0	0
Advertising and commission revenues		(1.2)	(6.6)		(2.8)	(8.6)	(2.7)	(0.2)
Other revenue		0	0		0	0	7.4	(6.9)
Total revenues		(1.2)	(6.6)		(2.8)	(8.6)	4.7	(7.1)
Expenses:
Loss and loss adjustment expenses		0	0		0	0	0	0
Insurance acquisition expenses		0	0		0	0	0	0
Other underwriting expenses		0	0		0	0	0	0
Cost of sales		0	0		0	0	0	0
General and administrative expenses		(0.3)	0.1		2.3	2.2	7.5	0
Interest expense		0	0		0	0	0	0
Total expenses		(0.3)	0.1		2.3	2.2	7.5	0
Pre-tax income (loss) from continuing operations		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Income tax benefit		0	0		0	0	0	0
Net income (loss) from continuing operations		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Gain from sale of other discontinued operations, net of tax		0	0		0	0	0	0
Net (loss) income from discontinued operations, net of tax		0	0		0	0	0	0
Income (loss) before equity in earnings of unconsolidated affiliates						(10.8)	(2.8)	(7.1)
Equity in earnings of unconsolidated affiliates							0	0
Net income		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Net loss (income) attributable to non-controlling interests		0	(0.4)		0	0.1	0.4	0.1
Net income attributable to White Mountains’s common shareholders		(0.9)	(7.1)		(5.1)	(10.7)	(2.4)	(7)
Comprehensive income, net of tax:
Other comprehensive income		(0.2)	(1.3)		(0.2)	0	(7.4)	6.9
Comprehensive income		(1.1)	(8.4)		(5.3)	(10.7)	(9.8)	(0.1)
Other comprehensive income attributable to non-controlling interests		0	0		0	0	0	0
Comprehensive income attributable to White Mountains’s common shareholders		$ (1.1)	$ (8.4)		$ (5.3)	$ (10.7)	$ (9.8)	$ (0.1)
Continuing operations (in usd per share)		$ (0.18)	$ 0		$ (1)	$ 0	$ 0	$ 0
As revised
Revenues:
Earned insurance premiums		$ 3.4	$ 5.2		$ 10.2	$ 13.4	$ 12	$ 7.9
Net investment income		9.6	27.5		18.2	32.1	10.9	12.2
Net realized and unrealized investment gains		10.9	70		27.2	(27.4)	260.5	38.1
Advertising and commission revenues		28.2	65.1		89.6	118.3	107.4	65.5
Other revenue		4.7	4.5		18	21.3	49.2	13.6
Total revenues		56.8	172.3		163.2	157.7	440	137.3
Expenses:
Loss and loss adjustment expenses		2.2	1.1		6.8	8	8.2	8.9
Insurance acquisition expenses		1.3	2.2		4.4	5.6	6.3	2.9
Other underwriting expenses		0.2	0.2		0.4	0.5	0.4	0.4
Cost of sales		24	55.6		76.9	102	93.6	57.8
General and administrative expenses		42.4	118		144.9	185.9	200.7	144.9
Interest expense		0.5	0.9		2.6	3	1.6	1.2
Total expenses		70.6	178		236	305	310.8	216.1
Pre-tax income (loss) from continuing operations		(13.8)	(5.7)		(72.8)	(147.3)	129.2	(78.8)
Income tax benefit		17.1	1.3		22.7	32.9	(12.7)	2.5
Net income (loss) from continuing operations		3.3	(4.4)		(50.1)	(114.4)	116.5	(76.3)
Gain from sale of other discontinued operations, net of tax		47.9	(1.6)		414.5	415.1	17.9	17.2
Net (loss) income from discontinued operations, net of tax		36.5	35.7		100.9	108.3	117.2	296.4
Income (loss) before equity in earnings of unconsolidated affiliates						409	251.6	237.3
Equity in earnings of unconsolidated affiliates						0	25.1	45.6
Net income		87.7	29.7		465.3	409	276.7	282.9
Net loss (income) attributable to non-controlling interests		3.1	13		(24.6)	(7.2)	18.5	22.3
Net income attributable to White Mountains’s common shareholders		90.8	42.7		440.7	401.8	295.2	305.2
Comprehensive income, net of tax:
Other comprehensive income		0	0.4		145.4	145.6	(107.8)	(98)
Comprehensive income		90.8	43.1		586.1	547.4	187.4	207.2
Other comprehensive income attributable to non-controlling interests		0.1	(0.1)		0.1	(0.3)	0	3.3
Comprehensive income attributable to White Mountains’s common shareholders		$ 90.9	$ 43		$ 586.2	$ 547.1	$ 187.4	$ 210.5
Continuing operations (in usd per share)		$ 1.32	$ 0		$ (14.49)	$ 0	$ 0	$ 0

Financial Statement Revisions90

Financial Statement Revisions - Statements of Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Shareholders' Equity
Beginning Balances			$ 3,716	$ 3,716
Net income	$ 551.5	$ 87.7		581.1	$ 465.3
Change in foreign currency translation, net of tax	0	0.2		0.2	(0.4)
Ending Balances	3,337.2			3,337.2		$ 3,716
As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,764.1		3,737.2	3,737.2	4,368	4,368
Net income		88.6	36.4		470.4	419.8	$ 279.5	$ 290
Ending Balances			3,764.1			3,737.2	4,368
Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(29.2)		(21.2)	(21.2)	(10.4)	(10.4)
Net income		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Ending Balances			(29.2)			(21.2)	(10.4)
As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,734.9		3,716	3,716	4,357.6	4,357.6
Net income		87.7	29.7		465.3	409	276.7	282.9
Ending Balances			3,734.9			3,716	4,357.6
Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Beginning Balances			810.7	810.7	978.2	978.2
Repurchases and retirements of common shares				(147.9)	(188.1)
Net income				0	0
Dividends declared on common shares				0	0
Ending Balances	669.3	809.8		669.3	809.8	810.7	978.2
Common shares and paid-in surplus | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	815.1		810.7	810.7	978.2	978.2	1,034.7
Ending Balances		809.8	815.1		809.8	810.7	978.2	1,034.7
Common shares and paid-in surplus | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	0.7		0	0	0	0	0
Ending Balances		0	0.7		0	0	0	0
Common shares and paid-in surplus | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	815.8		810.7	810.7	978.2	978.2	1,034.7
Ending Balances		809.8	815.8		809.8	810.7	978.2	1,034.7
Retained earnings
Increase (Decrease) in Shareholders' Equity
Beginning Balances			2,776.6	2,776.6	3,075	3,075
Repurchases and retirements of common shares				(575.9)	(678.6)
Net income				604.7	440.7
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	2,800.8	2,831.7		2,800.8	2,831.7	2,776.6	3,075
Retained earnings | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,835.2		2,797.2	2,797.2	3,084.9	3,084.9	3,010.5	2,801.9
Repurchases and retirements of common shares			(7.2)		(678.6)	(694.8)	(217.2)	(97.4)
Net income			49.8		445.8	412.5	297.6	312.2
Dividends declared on common shares			(4.6)		(5.4)	(5.4)	(6)	(6.2)
Net other changes in AOCI			0		113.6	113.9	(34.6)	66.6
Ending Balances		2,846.7	2,835.2		2,846.7	2,797.2	3,084.9	3,010.5
Retained earnings | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(27.7)		(20.6)	(20.6)	(9.9)	(9.9)	(7.5)	(0.5)
Repurchases and retirements of common shares			0		0	0	0	0
Net income			(7.1)		(5.1)	(10.7)	(2.4)	(7)
Dividends declared on common shares			0		0	0	0	0
Net other changes in AOCI			0		0	0	0	0
Ending Balances		(15)	(27.7)		(15)	(20.6)	(9.9)	(7.5)
Retained earnings | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,807.5		2,776.6	2,776.6	3,075	3,075	3,003	2,801.4
Repurchases and retirements of common shares			(7.2)		(678.6)	(694.8)	(217.2)	(97.4)
Net income			42.7		440.7	401.8	295.2	305.2
Dividends declared on common shares			(4.6)		(5.4)	(5.4)	(6)	(6.2)
Net other changes in AOCI			0		113.6	113.9	(34.6)	66.6
Ending Balances		2,831.7	2,807.5		2,831.7	2,776.6	3,075	3,003
AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Beginning Balances			(4.6)	(4.6)	(149.9)	(149.9)
Repurchases and retirements of common shares				0	0
Net income				0	0
Dividends declared on common shares				0	0
Ending Balances	(1.3)	(4.5)		(1.3)	(4.5)	(4.6)	(149.9)
AOCI, after tax | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(3)		(4.6)	(4.6)	(149.9)	(149.9)	(49.5)	52.1
Ending Balances		(4.3)	(3)		(4.3)	(4.6)	(149.9)	(49.5)
AOCI, after tax | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(1.3)		0	0	0	0	7.4	0.5
Ending Balances		(0.2)	(1.3)		(0.2)	0	0	7.4
AOCI, after tax | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(4.3)		(4.6)	(4.6)	(149.9)	(149.9)	(42.1)	52.6
Ending Balances		(4.5)	(4.3)		(4.5)	(4.6)	(149.9)	(42.1)
Total
Increase (Decrease) in Shareholders' Equity
Beginning Balances			3,582.7	3,582.7	3,903.3	3,903.3
Repurchases and retirements of common shares				(723.8)	(866.7)
Net income				604.7	440.7
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	3,468.8	3,637		3,468.8	3,637	3,582.7	3,903.3
Total | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,647.3		3,603.3	3,603.3	3,913.2	3,913.2	3,995.7
Ending Balances		3,652.2	3,647.3		3,652.2	3,603.3	3,913.2	3,995.7
Total | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(28.3)		(20.6)	(20.6)	(9.9)	(9.9)	(0.1)
Ending Balances		(15.2)	(28.3)		(15.2)	(20.6)	(9.9)	(0.1)
Total | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,619		3,582.7	3,582.7	3,903.3	3,903.3	3,995.6
Ending Balances		3,637	3,619		3,637	3,582.7	3,903.3	3,995.6
Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Beginning Balances			133.3	133.3	454.3	454.3
Repurchases and retirements of common shares				(5.2)	0
Net income				(23.6)	24.6
Dividends declared on common shares				0	0
Ending Balances	(131.6)	148.7		(131.6)	148.7	133.3	454.3
Non-controlling interest | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	116.8		133.9	133.9	454.8	454.8	542.7	491.7
Net income			(13.4)		24.6	7.3	(18.1)	(22.2)
Other changes in NCI			(3.7)		(330.2)	(328.2)	(69.8)	73.2
Ending Balances		149.2	116.8		149.2	133.9	454.8	542.7
Non-controlling interest | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(0.9)		(0.6)	(0.6)	(0.5)	(0.5)	(0.1)	0
Net income			0.4		0	(0.1)	(0.4)	(0.1)
Other changes in NCI			(0.7)		0	0	0	0
Ending Balances		(0.5)	(0.9)		(0.5)	(0.6)	(0.5)	(0.1)
Non-controlling interest | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	115.9		133.3	133.3	454.3	454.3	542.6	491.7
Net income			(13)		24.6	7.2	(18.5)	(22.3)
Other changes in NCI			(4.4)		(330.2)	(328.2)	(69.8)	73.2
Ending Balances		148.7	115.9		148.7	133.3	454.3	542.6
Total Equity
Increase (Decrease) in Shareholders' Equity
Beginning Balances			3,716	3,716	4,357.6	4,357.6
Repurchases and retirements of common shares				(729)	(866.7)
Net income				581.1	465.3
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	3,337.2	3,785.7		3,337.2	3,785.7	3,716	4,357.6
Total Equity | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,764.1		3,737.2	3,737.2	4,368	4,368	4,538.4
Ending Balances		3,801.4	3,764.1		3,801.4	3,737.2	4,368	4,538.4
Total Equity | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(29.2)		(21.2)	(21.2)	(10.4)	(10.4)	(0.2)
Ending Balances		(15.7)	(29.2)		(15.7)	(21.2)	(10.4)	(0.2)
Total Equity | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	$ 3,734.9		3,716	3,716	4,357.6	4,357.6	4,538.2
Ending Balances		$ 3,785.7	3,734.9		3,785.7	3,716	4,357.6	4,538.2
Foreign Currency Gain (Loss) | Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0	0
Foreign Currency Gain (Loss) | Retained earnings
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0	0
Foreign Currency Gain (Loss) | AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.4	32.1
Foreign Currency Gain (Loss) | AOCI, after tax | As previously reported
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			1.6		32	31.4	(65.8)	(168.2)
Foreign Currency Gain (Loss) | AOCI, after tax | Adjustments
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			(1.3)		(0.2)	0	(7.4)	6.9
Foreign Currency Gain (Loss) | AOCI, after tax | As revised
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			$ 0.3		31.8	$ 31.4	$ (73.2)	$ (161.3)
Foreign Currency Gain (Loss) | Total
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.4	32.1
Foreign Currency Gain (Loss) | Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.1	0.1
Foreign Currency Gain (Loss) | Total Equity
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				$ 0.5	$ 32.2

Financial Statement Revisions91

Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Investments	$ 3,280.5		$ 2,714.4
Cash	48.6		80.2
Insurance premiums receivable	4.5		1.6
Deferred acquisition costs	14.1		10.6
Accrued investment income	15.4		14.8
Accounts receivable on unsettled investment sales	190.1		4.8
Goodwill	25.9	$ 25.9	25.9	$ 25.9	$ 18.3	$ 18.6
Assets held for sale	0		3,599.5
Other assets	46.1		49.1
Total assets	3,637.1		6,520.2
Liabilities
Unearned insurance premiums	118.5		82.9
Debt	9.4		12.7
Accrued incentive compensation	76.9		95.7
Accounts payable on unsettled investment purchases	49.7		0
Liabilities held for sale	0		2,569.3
Other liabilities	45.4		43.6
Liabilities	299.9		2,804.2
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.7		4.6
Paid-in surplus	665.6		806.1
Retained earnings	2,800.8		2,776.6
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations	0		(3.2)
Total White Mountains’s common shareholders’ equity	3,468.8		3,582.7
Non-controlling interests	(131.6)		133.3
Total equity	3,337.2		3,716
Total liabilities and equity	$ 3,637.1		6,520.2
As previously reported
Assets
Investments		2,692.9	2,714.4			1,679.7
Cash		53.3	80.2			77.8
Insurance premiums receivable		2.8	1.6			1.3
Deferred acquisition costs		13	10.6			6.9
Deferred tax asset			0			0
Accrued investment income		16	14.8			3.8
Accounts receivable on unsettled investment sales		199.5	4.8			11.4
Goodwill		31.7	31.7			24.1
Intangible assets		17.8	23			28.9
Assets held for sale		3,696.4	3,599.5			8,365.6
Other assets		62.8	64.1			83.1
Total assets		6,786.2	6,544.7			10,282.6
Liabilities
Unearned insurance premiums		109.9	82.9			50.2
Debt		10.6	12.7			64.7
Deferred tax liability			0			27.4
Accrued incentive compensation		63.3	95.7			96.2
Accounts payable on unsettled investment purchases		114.6
Liabilities held for sale		2,678.8	2,569.3			5,618.1
Other liabilities		44.9	46.9			58
Liabilities		3,022.1	2,807.5			5,914.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares		4.6	4.6			5.6
Paid-in surplus		810.5	806.1			972.6
Retained earnings		2,835.2	2,797.2			3,084.9
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(3)	(4.6)			(149.9)
Total White Mountains’s common shareholders’ equity		3,647.3	3,603.3			3,913.2
Non-controlling interests		116.8	133.9			454.8
Total equity		3,764.1	3,737.2			4,368
Total liabilities and equity		6,786.2	6,544.7			10,282.6
Adjustments
Assets
Investments		0	0			0
Cash		0	0			0
Insurance premiums receivable		0	0			0
Deferred acquisition costs		0	0			0
Deferred tax asset			0			0
Accrued investment income		0	0			0
Accounts receivable on unsettled investment sales		0	0			0
Goodwill		(5.8)	(5.8)			(5.5)
Intangible assets		(3.4)	(3.7)			(2)
Assets held for sale		0	0			0
Other assets		(19.7)	(15)			(3.9)
Total assets		(28.9)	(24.5)			(11.4)
Liabilities
Unearned insurance premiums		0	0			0
Debt		0	0			0
Deferred tax liability			0			0
Accrued incentive compensation		0	0			0
Accounts payable on unsettled investment purchases		0
Liabilities held for sale		0	0			0
Other liabilities		0.3	(3.3)			(1)
Liabilities		0.3	(3.3)			(1)
White Mountains’s common shareholders’ equity
White Mountains’ common shares		0	0			0
Paid-in surplus		0.7	0			0
Retained earnings		(27.7)	(20.6)			(9.9)
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(1.3)	0			0
Total White Mountains’s common shareholders’ equity		(28.3)	(20.6)			(9.9)
Non-controlling interests		(0.9)	(0.6)			(0.5)
Total equity		(29.2)	(21.2)			(10.4)
Total liabilities and equity		(28.9)	(24.5)			(11.4)
As revised
Assets
Investments		2,692.9	2,714.4			1,679.7
Cash		53.3	80.2			77.8
Insurance premiums receivable		2.8	1.6			1.3
Deferred acquisition costs		13	10.6			6.9
Deferred tax asset			0			0
Accrued investment income		16	14.8			3.8
Accounts receivable on unsettled investment sales		199.5	4.8			11.4
Goodwill		25.9	25.9			18.6
Intangible assets		14.4	19.3			26.9
Assets held for sale		3,696.4	3,599.5			8,365.6
Other assets		43.1	49.1			79.2
Total assets		6,757.3	6,520.2			10,271.2
Liabilities
Unearned insurance premiums		109.9	82.9			50.2
Debt		10.6	12.7			64.7
Deferred tax liability			0			27.4
Accrued incentive compensation		63.3	95.7			96.2
Accounts payable on unsettled investment purchases		114.6
Liabilities held for sale		2,678.8	2,569.3			5,618.1
Other liabilities		45.2	43.6			57
Liabilities		3,022.4	2,804.2			5,913.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares		4.6	4.6			5.6
Paid-in surplus		811.2	806.1			972.6
Retained earnings		2,807.5	2,776.6			3,075
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(4.3)	(4.6)			(149.9)
Total White Mountains’s common shareholders’ equity		3,619	3,582.7			3,903.3
Non-controlling interests		115.9	133.3			454.3
Total equity		3,734.9	3,716			4,357.6
Total liabilities and equity		$ 6,757.3	$ 6,520.2			$ 10,271.2