White Mountains Insurance (WTM) 10-Q 2 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	9 Months Ended
	Sep. 30, 2016	Oct. 31, 2016	Dec. 31, 2015
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, 2016
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		4,565,240
Document Fiscal Year Focus	2,016
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, 2016	Dec. 31, 2015
Assets
Fixed maturity investments, at fair value	$ 4,131.5	$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	305.6	211.2
Common equity securities, at fair value	425	1,113.9
Other long-term investments	351.4	315.8
Total investments	5,213.5	4,271.1
Cash	244.7	173
Reinsurance recoverable on unpaid losses	169.4	186
Reinsurance recoverable on paid losses	16.4	7.5
Insurance premiums receivable	264.6	220.3
Deferred acquisition costs	109.6	107.6
Deferred tax asset	119.5	112.8
Ceded unearned insurance premiums	35.1	29.5
Accrued investment income	18.9	13.9
Accounts receivable on unsettled investment sales	179	41.9
Goodwill and other intangible assets	59.3	55.4
Other assets	226.3	273.2
Assets held for sale	27	4,790.4
Total assets	6,683.3	10,282.6
Liabilities
Loss and loss adjustment expense reserves	1,362	1,389.8
Unearned insurance premiums	674.1	610.5
Debt	288.5	337.6
Accrued incentive compensation	124.2	140.7
Ceded reinsurance payable	28.2	29.9
Funds held under insurance contracts	156.2	137.8
Accounts payable on unsettled investment purchases	30	0
Other liabilities	210.9	220.9
Total liabilities held for sale	7.8	3,047.4
Total liabilities	2,881.9	5,914.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares	4.6	5.6
Paid-in surplus	805.2	972.6
Retained earnings	2,846.7	3,084.9
Accumulated other comprehensive loss, after tax:
Net unrealized foreign currency translation losses	(0.6)	(145.6)
Pension liability and other	(3.7)	(4.3)
Total White Mountains’s common shareholders’ equity	3,652.2	3,913.2
Non-controlling interests
Non-controlling interests	149.2	454.8
Total equity	3,801.4	4,368
Total liabilities and equity	6,683.3	10,282.6
OneBeacon [Member]
Non-controlling interests
Non-controlling interests	$ 247	$ 245.6

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Cash	$ 0	$ 5.8
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	4,578,698	5,623,735
Common shares, outstanding shares	4,578,698	5,623,735

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues:
Earned insurance premiums	$ 281.3	$ 284.9	$ 838.1	$ 896
Net investment income	22.5	16.8	59.6	42.6
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84	(33.9)
Other revenue	34.8	38.3	110.8	114.4
Total revenues	365	296.1	1,092.5	1,019.1
Expenses:
Loss and loss adjustment expenses	165	170	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4	55.3	159.2	166.1
Other Underwriting Expense	49.6	56.5	156	165.5
General and administrative expenses	70.2	97.3	230.4	231.6
Interest expense	3.8	3.9	12.4	10.8
Total expenses	345	383	1,066.1	1,108.1
Pre-tax income (loss) from continuing operations	20	(86.9)	26.4	(89)
Income tax benefit (expense)	6.7	1.6	22.4	(0.8)
Net income (loss) from continuing operations	26.7	(85.3)	48.8	(89.8)
Net gain on sale of discontinued operations			414.5	18.2
Net income (loss) from discontinued operations, net of tax	14	(3.8)	7.1	59.1
Income (loss) before equity in earnings of unconsolidated affiliates	88.6	(78.8)	470.4	(12.5)
Equity in earnings of unconsolidated affiliates, net of tax	0	3.9	0	18
Net income	88.6	(74.9)	470.4	5.5
Net loss (income) attributable to non-controlling interests	3.1	16	(24.6)	24.2
Net income (loss) attributable to White Mountains’s common shareholders	91.7	(58.9)	445.8	29.7
Other comprehensive income (loss), net of tax:
Change in equity in net unrealized gains (losses) from investments in Symetra common shares, net of tax	0	3.5	0	(29.4)
Change in foreign currency translation, pension liability and other, net of tax	0.2	0.1	0.3	0.3
Comprehensive income (loss)	91.9	(73.8)	591.4	(61.5)
Other comprehensive income (loss) attributable to non-controlling interests	0.1	(0.1)	0.1	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 92	$ (73.9)	$ 591.5	$ (61.6)
Basic income (loss) per share
Continuing operations	$ 6.12	$ (11.10)	$ 4.67	$ (8.01)
Discontinued operations	12.72	1.09	81.60	12.98
Total consolidated operations	18.84	(10.01)	86.27	4.97
Diluted income (loss) per share
Continuing operations	6.11	(11.10)	4.66	(8.01)
Discontinued operations	12.69	1.09	81.47	12.98
Total consolidated operations	18.80	(10.01)	86.13	4.97
Dividends declared per White Mountains’s common share	$ 0	$ 0	$ 1	$ 1
Sale of Tranzact [Member]
Expenses:
Net gain on sale of discontinued operations	$ 51.9	$ 0	$ 51.9	$ 0
Sirius Group
Revenues:
Net realized and unrealized investment gains (losses)		0.8	3.7	3.5
Expenses:
Net gain on sale of discontinued operations	(4)	0	362.6	0
Other comprehensive income (loss), net of tax:
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease	0	(18.5)	32	(62.1)
Esurance
Expenses:
Net gain on sale of discontinued operations		10.3		18.2
Sale of Sirius Group [Member]
Other comprehensive income (loss), net of tax:
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease	$ 0	$ 0	$ 113.3	$ 0

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss)Common shares and paid-in surplus	Foreign Currency Gain (Loss)Retained earnings	Foreign Currency Gain (Loss)AOCI, after tax	Foreign Currency Gain (Loss)Total	Foreign Currency Gain (Loss)Non-controlling interest	Foreign Currency Gain (Loss)Total Equity	Pension liability and other accumulated comprehensive itemsCommon shares and paid-in surplus	Pension liability and other accumulated comprehensive itemsRetained earnings	Pension liability and other accumulated comprehensive itemsAOCI, after tax	Pension liability and other accumulated comprehensive itemsTotal	Pension liability and other accumulated comprehensive itemsNon-controlling interest	Pension liability and other accumulated comprehensive itemsTotal Equity	Prospector Turtle FundCommon shares and paid-in surplus	Prospector Turtle FundRetained earnings	Prospector Turtle FundAOCI, after tax	Prospector Turtle FundTotal	Prospector Turtle FundNon-controlling interest	Prospector Turtle FundTotal Equity	Sale of Sirius Group [Member]Common shares and paid-in surplus	Sale of Sirius Group [Member]Retained earnings	Sale of Sirius Group [Member]AOCI, after tax	Sale of Sirius Group [Member]Total	Sale of Sirius Group [Member]Non-controlling interest	Sale of Sirius Group [Member]Total Equity	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)Common shares and paid-in surplus	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)Retained earnings	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)AOCI, after tax	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)Total	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)Non-controlling interest	Sale of Sirius Group [Member]Foreign Currency Gain (Loss)Total Equity	Sale of Tranzact [Member]Common shares and paid-in surplus	Sale of Tranzact [Member]Retained earnings	Sale of Tranzact [Member]AOCI, after tax	Sale of Tranzact [Member]Total	Sale of Tranzact [Member]Non-controlling interest	Sale of Tranzact [Member]Total Equity	Buzzmove [Member]Common shares and paid-in surplus	Buzzmove [Member]Retained earnings	Buzzmove [Member]AOCI, after tax	Buzzmove [Member]Total	Buzzmove [Member]Non-controlling interest	Buzzmove [Member]Total Equity
Balances at Dec. 31, 2014		$ 1,034.7	$ 3,010.5	$ (49.5)	$ 3,995.7	$ 542.7	$ 4,538.4
Increase (Decrease) in Shareholders' Equity
Net income	$ 5.5	0	29.7	0	29.7	(24.2)	5.5
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	(29.4)	0	0	(29.4)	(29.4)	0	(29.4)
Net change in foreign currency translation	0.3							$ 0	$ 0	$ (62.6)	$ (62.6)	$ 0	$ (62.6)
Net change in pension liability and other accumulated comprehensive items														$ 0	$ 0	$ 0.7	$ 0.7	$ 0.1	$ 0.8
Total comprehensive income (loss)	(61.5)	0	29.7	(91.3)	(61.6)	(24.1)	(85.7)
Dividends declared on common shares		0	(6)	0	(6)	0	(6)
Dividends to non-controlling interests	9.1	0	0	0	0	(35.4)	(35.4)
Repurchases and retirements of common shares		(46)	(145.9)	0	(191.9)	0	(191.9)
Issuances of common shares		0.9	0	0	0.9	0	0.9
Acquisition from non-controlling interests		(2.2)	0	0	(2.2)	(2.7)	(4.9)													$ 0	$ 0	$ 0	$ 0	$ (31.5)	$ (31.5)
Net contributions from non-controlling interests		0	0	0	0	12.1	12.1
Amortization of restricted share awards		10.6	0	0	10.6	(0.3)	10.3
Balances at Sep. 30, 2015		998	2,888.3	(140.8)	3,745.5	460.8	4,206.3
Balances at Dec. 31, 2015	4,368	978.2	3,084.9	(149.9)	3,913.2	454.8	4,368
Increase (Decrease) in Shareholders' Equity
Net income	470.4	0	445.8	0	445.8	24.6	470.4
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	0
Net change in foreign currency translation	0.3							$ 0	$ 0	$ 32	$ 32	$ 0	$ 32																			$ 0	$ 0	$ 113.3	$ 113.3	$ 0	$ 113.3
Net change in pension liability and other accumulated comprehensive items														$ 0	$ 0	$ 0.3	$ 0.3	$ 0.1	$ 0.4
Total comprehensive income (loss)	591.4	0	445.8	145.6	591.4	24.7	616.1
Dividends declared on common shares		0	(5.4)	0	(5.4)	0	(5.4)
Dividends to non-controlling interests	1	0	0	0	0	(17.4)	(17.4)
Repurchases and retirements of common shares		(188.1)	(678.6)	0	(866.7)	0	(866.7)
Issuances of common shares		9.1	0	0	9.1	0	9.1
Noncontrolling Interest, Decrease from Deconsolidation																										$ 0	$ 0	$ 0	$ 0	$ (250)	$ (250)							$ 0	$ 0	$ 0	$ 0	$ (78.4)	$ (78.4)
Noncontrolling Interest, Increase from Business Combination																																												$ 0	$ 0	$ 0	$ 0	$ 3.4	$ 3.4
Acquisition from non-controlling 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
Balances at Sep. 30, 2016	$ 3,801.4	$ 809.8	$ 2,846.7	$ (4.3)	$ 3,652.2	$ 149.2	$ 3,801.4

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operations:
Net income	$ 470.4	$ 5.5
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses	(84)	33.9
Deferred income tax expense (benefit)	12.5	(20.2)
Net income from discontinued operations	(7.1)	(59.1)
Net gain on sale of discontinued operations	(414.5)	(18.2)
Net gain on sale of unconsolidated affiliates and consolidated subsidiaries	(0.5)	(16.1)
Amortization and depreciation	34.7	31.8
Undistributed equity in earnings from unconsolidated affiliates, net of tax	0	(18)
Other operating items:
Net change in loss and loss adjustment expense reserves	(27.8)	83.3
Net change in reinsurance recoverable on paid and unpaid losses	7.6	(63.3)
Net change in unearned insurance premiums	63	54.6
Net change in deferred acquisition costs	(2)	(3.6)
Net change in funds held by ceding entities	0	32.5
Net change in ceded unearned premiums	(5.6)	(23.5)
Net change in funds held under insurance treaties	18.4	19.3
Net change in insurance premiums receivable	(43.3)	(32)
Net change in ceded reinsurance payable	(2.8)	12.4
Net change in restricted cash	5.8	6.9
Net change in other assets and liabilities, net	(150.9)	38.4
Net cash (used for) provided from operations - continuing operations	(126.1)	64.6
Net cash (used for) provided from operations	(142.3)	87.3
Cash flows from investing activities:
Net change in short-term investments	(88.7)	26.6
Sales of fixed maturity and convertible investments	2,433.4	865
Maturities, calls and paydowns of fixed maturity and convertible investments	530.3	249.5
Sales of common equity securities	961.1	370.6
Distributions and redemptions of other long-term investments	19.8	56.6
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	2,657.2	24
Proceeds from (Payments to) Noncontrolling Interests	(141.6)	0
Distribution from (Contributions to) Discontinued Operations for Investing Activities	(396.6)	17.5
Purchases of other long-term investments	(37.3)	(30.5)
Purchases of common equity securities	(249.7)	(329.4)
Purchases of fixed maturity and convertible investments	(4,435)	(1,300.2)
Purchases of unconsolidated affiliates and consolidated subsidiaries, net of cash acquired	(13.7)	(2.4)
Net change in unsettled investment purchases and sales	(107.1)	73.8
Net acquisitions of property and equipment	(3.2)	37
Net cash provided from investing activities - continuing operations	1,128.9	58.1
Net cash provided from investing activities	1,342.2	60.9
Cash flows from financing activities:
Draw down of debt and revolving line of credit	352.5	98.5
Repayment of debt and revolving line of credit	(401.8)	(84.5)
Proceeds from Issuance of Common Stock	3.7	0
Payments on capital lease obligation	(1.4)	(3.5)
Cash dividends paid to the Company’s common shareholders	(5.4)	(6)
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders	(14.2)	(14.8)
Common shares repurchased	(861)	(168.6)
OneBeacon Ltd. common shares repurchased and retired	(11.5)	(1.7)
Distribution to non-controlling interest shareholders	(1)	(9.1)
Contributions to discontinued operations	(3)	(4.1)
Acquisition of additional shares from non-controlling interest	0	(9.1)
Payments of contingent consideration related to purchases of consolidated subsidiaries	(7.8)	0
Capital contributions from BAM members	28.2	20.3
Net cash used for financing activities - continuing operations	(922.7)	(182.6)
Net cash used for financing activities	(922.9)	(163)
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Net change in cash during the period - continuing operations	80.1	(59.9)
Cash balances at beginning of period	167.2	232.9
Cash held for sale at the beginning of period	1.2	2.1
Cash held for sale at the end of period	3.8	2.1
Cash balances at end of period	244.7	173
Supplemental cash flows information:
Interest paid	(7)	(6.3)
Net income tax refund from national governments	13.5	8
Discontinued Operations [Member]
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses	1.5	(31.4)
Net income from discontinued operations	9.8	(81.3)
Net gain on sale of discontinued operations	(7.1)	(59.1)
Other operating items:
Net cash (used for) provided from operations - discontinued operations	(16.2)	22.7
Cash flows from investing activities:
Net cash provided from investing activities - discontinued operations	213.3	2.8
Cash flows from financing activities:
Net cash (used for) provided from financing activities - discontinued operations	(0.2)	19.6
Cash balances at beginning of period	150.1	116.6
Cash balances at end of period	0	157.6
Supplemental cash flows information:
Net income tax refund from national governments	$ 31.8	$ 30.3

CONSOLIDATED STATEMENTS OF CAS7

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash and Cash Equivalents, at Carrying Value	$ 244.7	$ 173
Restricted cash balances	0	16.8
Effect of Exchange Rate on Cash and Cash Equivalents, Discontinued Operations	0	(4.1)
Discontinued Operations [Member]
Cash and Cash Equivalents, at Carrying Value	$ 0	$ 157.6

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2016
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 and reinsurance subsidiaries and affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, HG Global/BAM and Other Operations. As discussed further in Note 2 - “Significant Transactions” , on April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (See Note 2 - “Significant Transactions” ). On June 9, 2016, White Mountains entered into an agreement to sell Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC, which was completed on July 21, 2016. For the three and nine months ended September 30, 2016 , Sirius Group and Tranzact have been presented as discontinued operations in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. The assets and liabilities of Sirius Group and Tranzact are classified as held for sale in the balance sheet as of December 31, 2015. (See Note 17 - “Held for Sale and Discontinued Operations” ). The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of September 30, 2016 and December 31, 2015 , White Mountains owned 76.1% and 75.5% of OneBeacon Ltd.’s outstanding common shares. The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is a municipal bond insurer domiciled in New York that was established in 2012 to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). HG Global, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), also provides 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. As of September 30, 2016 and December 31, 2015 , White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM, which is a mutual insurance company owned by its members. However, GAAP requires White Mountains to consolidate BAM’s results in its financial statements. BAM’s results are attributed to non-controlling interests. White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), White Mountains’s variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s ownership positions in QL Holdings, LLC (“MediaAlpha”) and Wobi Insurance Agency Ltd. (“Wobi”). On August 4, 2016, White Mountains acquired a 70.9% ownership share in Removal Stars Ltd (“Buzzmove”) for a purchase price of GBP 6.0 million (approximately $8.0 million based upon the foreign exchange spot rate at the date of acquisition), which is included in the Other Operations segment at September 30, 2016. The Other Operations segment also includes Abiaka Holdings, LLC, Star & Shield Services LLC, Star & Shield Risk Management LLC (“SSRM”), and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. White Mountains holds surplus notes issued by SSIE (the “SSIE Surplus Notes”) but does not have an ownership interest in SSIE. However, because SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE, White Mountains is required to consolidate SSIE in its GAAP financial statements. SSIE’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, White Mountains has presented Star & Shield's and SSIE's assets and liabilities as held for sale as of September 30, 2016 and December 31, 2015. 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 2015 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 2015 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles 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, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. Fair Value Measurements On January 1, 2016, White Mountains adopted ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820), which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures. Amendments to Consolidation Analysis On January 1, 2016, White Mountains adopted ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718) . The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of September 30, 2016 , the unamortized debt issuance costs included in debt is $1.9 million . Recently Issued Accounting Pronouncements Short-Duration Contracts In May 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures. 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). The new guidance 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. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash, including reinsurance receivables, 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. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance. Stock Compensation In March 2016, the Board issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which is intended to simplify certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. The new guidance is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods with early adoption permitted. White Mountains does not expect a significant effect upon adoption. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this new guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new guidance requires all equity securities with readily determinable fair values 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 investments without readily determinable fair values to identify impairment, and for impaired equity security investments 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 has taken the fair value election for its portfolio of equity security investments 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. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as third party investment management fees charged by White Mountains Advisors as well as commissions and other non-insurance revenues. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (ASC 606) , which delayed the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows.

Significant Transactions

Significant Transactions	9 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions 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. In the second quarter of 2016, White Mountains recorded $ 366.6 million of gain from sale of Sirius Group in discontinued operations and $113.3 million in other comprehensive income from discontinued operations. In the third quarter of 2016, White Mountains recorded a $4.0 million reduction to the gain from sale of Sirius Group in discontinued operations as a result of the final true-up of Sirius Group's closing date tangible common shareholder's equity. 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 do not include White Mountains’s investment in OneBeacon and certain other investments that were held in the Sirius Group legal entities as of December 31, 2015. The value of these investments, net of related tax effects, was approximately $ 686.2 million , of which $ 528.6 million was Symetra common shares, as of December 31, 2015. As part of the sale of Sirius Group, White Mountains purchased Ashmere Insurance Company (“Ashmere”), a subsidiary formerly owned by Sirius Group. Ashmere was under an agreement to be sold as of September 30, 2016 and the sale was completed on October 12, 2016. As of September 30, 2016, Ashmere's assets and liabilities are presented as held for sale. The transactions to purchase the investments in OneBeacon and the other investments held by Sirius Group prior to the closing are presented in the statement of cash flows as net settlement of investment cash flows with discontinued operations. Symetra On February 1, 2016, White Mountains sold its investment in Symetra Financial Corporation (“Symetra”) for proceeds of $ 658.0 million , or $32.00 per share. (See Note 14 - “Investments in Unconsolidated Affiliates” ). OneBeacon Crop Business On July 31, 2015, OneBeacon exited its multiple peril crop insurance (“MPCI”) and its related crop-hail business (collectively, “Crop Business”) as its exclusive managing general agency, Climate Crop Insurance Agency (“CCIA”), exited the business through a sale of the agency to an affiliate of AmTrust. As a result of the transaction, OneBeacon and CCIA agreed to an early termination of the existing five-year agreement. In connection with the termination of the agreement, OneBeacon received a payment of $ 3.0 million . Also related to the transaction, OneBeacon withdrew its 2016 Plan of Operations, which previously authorized it to write MPCI for the 2016 Reinsurance Year, and affiliates of AmTrust agreed to reinsure the Company’s remaining net Crop Business exposure for the 2015 Reinsurance Year under a related 100% quota share reinsurance agreement which, coupled with other transfer and assignment agreements as well as communications with policyholders and agents, had the effect of assumption reinsurance. As a result of this transaction, the Company has no material net exposure related to the Crop Business. 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. White Mountains recorded a $ 51.9 million gain from the sale of Tranzact in discontinued operations, which included a $ 30.2 million tax expense for the reversal of a tax valuation allowance that is offset by a tax benefit recorded in continuing operations. (See Note 8 - “Income Taxes” ). The increase to White Mountains’s book value from the sale of Tranzact was $82.1 million . A reconciliation of the gain reported in discontinued operations to the impact to White Mountains's book value is as follows: Gain from sale of Tranzact reported in discontinued operations $ 51.9 Add back reclassification from continuing operations for the release of a tax valuation allowance 30.2 Increase to White Mountains book value from sale of Tranzact $ 82.1 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. Wobi On February 19, 2014, White Mountains acquired 54% of the outstanding common shares of Wobi for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition). During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $9.0 million based upon the foreign exchange spot rate at the dates of acquisition) of newly-issued convertible preferred shares of Wobi. Wobi is the only price comparison/aggregation business in Israel, with an insurance carrier panel that represents 80% of the premiums written in the Israeli insurance market. Wobi sells four insurance lines of business, primarily personal auto, and operates as an agency, charging upfront commissions on all insurance policy sales. Wobi also offers a pension products comparison service for Israeli customers and is paid transaction fees when customers use the service to connect to companies that sell those pension products. During the three months ended September 30, 2016 , White Mountains purchased NIS 11.5 million (approximately $ 3.1 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. During the nine months ended September 30, 2016 , White Mountains purchased NIS 27.9 million (approximately $7.5 million based upon the foreign exchange spot rates at the dates of acquisitions) of convertible preferred shares of Wobi, increasing its ownership share to 96.7% on a fully converted basis. During 2015, Wobi acquired Tnuva Finansit Ltd. (“Cashboard”) in two transactions for a total of NIS 35.9 million (approximately $8.9 million based upon the foreign exchange spot rate at the dates of acquisition). The acquisition of Cashboard accelerated Wobi’s development of its pension products comparison service. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million , including $0.3 million of goodwill and $2.8 million of other intangible assets and total liabilities assumed of $1.2 million at their estimated acquisition date fair values. Star & Shield On January 31, 2014, White Mountains acquired certain assets and liabilities of Star & Shield Holdings LLC, including SSRM, the attorney-in-fact for SSIE, for a purchase price of $1.8 million . SSIE is a Florida-domiciled reciprocal insurance exchange providing private passenger auto insurance to the public safety community and their families. White Mountains invested $21.0 million of surplus notes issued by SSIE. Principal and interest on the surplus notes are payable to White Mountains only with approval from the Florida Office of Insurance Regulation. SSIE is a variable interest entity (“VIE”). As a result of SSRM’s role as the attorney-in-fact to SSIE and the investment in SSIE’s Surplus Notes, White Mountains is required to consolidate SSIE. In response to the significant increases in both frequency and severity of accidents in the private passenger automobile insurance market in the State of Florida, SSIE voluntarily ceased writing new policies effective July 1, 2016 and will voluntarily cease renewing policies at a future date, subject to regulatory approval. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, during the first nine months of 2016, White Mountains wrote off its investment in SSIE surplus notes. White Mountains consolidates the issuer of the surplus notes, Star & Shield Insurance Exchange and, accordingly the impact of the write down is eliminated in pre-tax income. However, the write down resulted in a $ 21.0 million total decrease to net income attributable to White Mountains's common shareholders and a corresponding increase to net income attributable to non-controlling interests. At September 30, 2016 and December 31, 2015 , consolidated amounts included total assets of $11.4 million and $14.2 million and total liabilities of $7.2 million and $30.3 million , respectively, of SSIE. For the three months ended September 30, 2016 and 2015, SSIE had pre-tax income of $0.4 million and pre-tax loss of $0.8 million that were recorded in net gain (loss) attributable to non-controlling interests. For the nine months ended September 30, 2016 and 2015, SSIE had pre-tax income of $20.3 million and pre-tax loss of $0.9 million that were recorded in net gain (loss) attributable to non-controlling interests. The decrease in total liabilities and increase in pre-tax income in the current year is due to the $ 21.0 million write down of the surplus notes at SSIE.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2016
Insurance Loss Reserves [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Gross beginning balance $ 1,376.6 $ 1,350.8 $ 1,389.8 $ 1,342.2 Less beginning reinsurance recoverable on unpaid losses (162.8 ) (147.1 ) (186.0 ) (161.6 ) Net loss and LAE reserves 1,213.8 1,203.7 1,203.8 1,180.6 SSIE reserves held for sale at beginning of the period (1) 5.3 6.1 5.5 7.7 Loss and LAE incurred relating to: Current year losses 164.9 170.4 492.8 536.7 Prior year losses 0.1 (.4 ) 15.3 (2.6 ) Total incurred losses and LAE 165.0 170.0 508.1 534.1 Loss and LAE paid relating to: Current year losses (55.0 ) (53.0 ) (116.2 ) (120.2 ) Prior year losses (131.1 ) (108.1 ) (403.2 ) (383.5 ) Total loss and LAE payments (186.1 ) (161.1 ) (519.4 ) (503.7 ) SSIE reserves held for sale at end of the period (1) (5.4 ) (5.5 ) (5.4 ) (5.5 ) Net ending balance 1,192.6 1,213.2 1,192.6 1,213.2 Plus ending reinsurance recoverable on unpaid losses 169.4 214.2 169.4 214.2 Gross ending balance $ 1,362.0 $ 1,427.4 $ 1,362.0 $ 1,427.4 (1) See Note 17 - “Held for Sale and Discontinued Operations” . Loss and LAE incurred relating to prior year losses for the three and nine months ended September 30, 2016 For the three and nine months ended September 30, 2016 , White Mountains experienced net unfavorable loss reserve development of $0.1 million and $15.3 million . During the three months ended September 30, 2016 , OneBeacon did not experience any net loss reserve development. During the nine months ended September 30, 2016 , OneBeacon experienced $15.4 million of net unfavorable loss reserve development, primarily related to OneBeacon's Healthcare business described below as well as Other Professional Lines and Programs businesses, partially offset by favorable loss reserve development in its Technology, Accident and Financial Services businesses. During the second quarter of 2016, OneBeacon recognized $20.0 million of unfavorable loss reserve development and also increased the current accident year provision related to its Healthcare business. Paid and incurred activity in the second quarter for extended care facilities coverage within OneBeacon's Healthcare business increased above expectations and above what was experienced in the first quarter of 2016. Extended care facilities coverage provides liability insurance for assisted living, memory care and continuing care facilities, among others. In addition, there were two large claims from the managed care errors and omissions coverage within OneBeacon's Healthcare business that contributed to the adverse loss reserve development. For the three months ended September 30, 2016 , SSIE had net unfavorable loss reserve development of $0.1 million . For the nine months ended September 30, 2016 , SSIE had net favorable loss reserve development of $0.1 million . Loss and LAE incurred relating to prior year losses for the three and nine months ended September 30, 2015 For the three and nine months ended September 30, 2015 , White Mountains experienced net favorable loss reserve development of $0.4 million and $2.6 million . During the three months ended September 30, 2015 , OneBeacon experienced no net loss reserve development on prior accident year reserves. For the nine months ended September 30, 2015 , OneBeacon had net favorable loss reserve development of $1.8 million . For the three and nine months ended September 30, 2015 , SSIE had net favorable loss reserve development of $ 0.4 million and $ 0.8 million.

Third Party Reinsurance

Third Party Reinsurance	9 Months Ended
	Sep. 30, 2016
Reinsurance Disclosures [Abstract]
Third Party Reinsurance	Third Party Reinsurance In the normal course of business, White Mountains’s insurance subsidiaries may seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. OneBeacon At September 30, 2016 , OneBeacon had $16.4 million and $169.4 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2015 , OneBeacon had $7.5 million and $186.0 million of reinsurance recoverables on paid and unpaid losses. Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant. In the first quarter of 2016, OneBeacon entered into a 50% quota share reinsurance agreement related to its Financial Institutions business. Effective May 1, 2016, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2017. The program provides coverage for OneBeacon's property business as well as certain acts of terrorism. Under the program, the first $ 20.0 million of losses resulting from any single catastrophe are retained, with 100.0% of the next $ 110.0 million of losses resulting from the catastrophe being reinsured. Any part of a catastrophe loss in excess of $ 130.0 million would be retained in full. In the event of a catastrophe, OneBeacon's property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investments White Mountains’s invested assets consist of investment securities and other long-term investments held for general investment purposes. The portfolio of investment securities includes fixed maturity investments, short-term investments, common equity securities, and other-long term investments which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Realized and unrealized investment gains and 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-backed 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 and 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, 2016 and December 31, 2015 . Other long-term investments consist primarily of hedge funds, private equity funds, direct investments in privately held common and convertible securities and the OneBeacon surplus notes. Net Investment Income White Mountains’s net investment income is comprised primarily of interest income associated with White Mountains’s fixed maturity investments, dividend income from its equity investments and interest income from its short-term investments. Pre-tax net investment income for the three and nine months ended September 30, 2016 and 2015 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Investment income: Fixed maturity investments $ 21.3 $ 13.7 $ 53.7 $ 38.5 Short-term investments .2 — .8 .1 Common equity securities 1.3 1.4 3.4 5.7 Other long-term investments .1 2.0 3.5 2.9 Total investment income 22.9 17.1 61.4 47.2 Third-party investment expenses (.4 ) (.3 ) (1.8 ) (4.6 ) Net investment income, pre-tax $ 22.5 $ 16.8 $ 59.6 $ 42.6 Net Realized and Unrealized Investment Gains (Losses) Net realized and unrealized investment gains (losses) consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Net realized investment gains (losses), pre-tax $ 8.0 $ (7.4 ) $ 267.5 $ 49.7 Net unrealized investment gains (losses), pre-tax 18.4 (36.5 ) (183.5 ) (83.6 ) Net realized and unrealized investment gains (losses), pre-tax 26.4 (43.9 ) 84.0 (33.9 ) Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) (5.4 ) 9.4 (23.8 ) 6.9 Net realized and unrealized investment gains (losses), after tax $ 21.0 $ (34.5 ) $ 60.2 $ (27.0 ) Net realized investment gains (losses) Net realized investment gains (losses) for the three and nine months ended September 30, 2016 and 2015 consisted of the following: Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 2.8 $ — $ 2.8 $ (.1 ) $ — $ (.1 ) Short-term investments .2 — .2 — — — Common equity securities 6.4 — 6.4 (5.9 ) — (5.9 ) Other long-term investments (1.4 ) — (1.4 ) (1.4 ) — (1.4 ) Net realized investment gains (losses), pre-tax 8.0 — 8.0 (7.4 ) — (7.4 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (2.7 ) — (2.7 ) 1.5 — 1.5 Net realized investment gains (losses), after tax $ 5.3 $ — $ 5.3 $ (5.9 ) $ — $ (5.9 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 3.5 $ — $ 3.5 $ 2.0 $ — $ 2.0 Short-term investments .4 — .4 — — — Common equity securities 267.7 — 267.7 41.6 .4 42.0 Other long-term investments (4.1 ) — (4.1 ) 5.7 — 5.7 Net realized investment gains, pre-tax 267.5 — 267.5 49.3 .4 49.7 Income tax expense attributable to net realized investment gains (46.0 ) — (46.0 ) (15.7 ) — (15.7 ) Net realized investment gains, after tax $ 221.5 $ — $ 221.5 $ 33.6 $ .4 $ 34.0 Net unrealized investment gains (losses) The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value: Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Net unrealized (losses) gains Net gains Total net unrealized (losses) gains reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (4.4 ) $ — $ (4.4 ) $ 3.3 $ — $ 3.3 Common equity securities 12.5 .2 12.7 (30.6 ) (.2 ) (30.8 ) Other long-term investments 10.0 .1 10.1 (9.0 ) — (9.0 ) Net unrealized investment gains (losses), pre-tax 18.1 .3 18.4 (36.3 ) (.2 ) (36.5 ) Income tax (expense) benefit (2.7 ) — (2.7 ) 7.9 — 7.9 Net unrealized investment $ 15.4 $ .3 $ 15.7 $ (28.4 ) $ (.2 ) $ (28.6 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Net unrealized gains (losses) Net gains Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 38.0 $ — $ 38.0 $ 1.3 $ — $ 1.3 Common equity securities (247.9 ) 2.6 (245.3 ) (56.0 ) (2.5 ) (58.5 ) Other long-term investments 23.5 .3 23.8 (25.6 ) (.8 ) (26.4 ) Net unrealized investment (losses) gains, pre-tax (186.4 ) 2.9 (183.5 ) (80.3 ) (3.3 ) (83.6 ) Income tax benefit attributable to net unrealized investment (losses) 22.2 — 22.2 22.6 — 22.6 Net unrealized investment $ (164.2 ) $ 2.9 $ (161.3 ) $ (57.7 ) $ (3.3 ) $ (61.0 ) The following table summarizes the amount of total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Fixed maturity investments $ .3 $ (1.0 ) $ 1.6 $ (.9 ) Common equity securities — 1.1 — 4.0 Other long-term investments 7.8 (6.8 ) 20.7 (6.6 ) Total unrealized investment gains (losses), pre-tax - Level 3 investments $ 8.1 $ (6.7 ) $ 22.3 $ (3.5 ) 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, 2016 and December 31, 2015 were as follows: September 30, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value U.S. Government and agency obligations $ 96.6 $ .4 $ — $ — $ 97.0 Debt securities issued by corporations 1,212.2 18.7 (1.5 ) — 1,229.4 Municipal obligations 306.3 6.2 (.2 ) — 312.3 Mortgage and asset-backed securities 2,406.6 9.6 (1.7 ) — 2,414.5 Foreign government, agency and provincial obligations 1.0 .2 — — 1.2 Preferred stocks 78.3 7.1 — — 85.4 Total fixed maturity investments $ 4,101.0 $ 42.2 $ (3.4 ) $ — $ 4,139.8 Less: fixed maturity investments reclassified to assets held for sale related to SSIE 8.3 Total fixed maturity investments $ 4,131.5 December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value U.S. Government and agency obligations $ 160.4 $ — $ (.4 ) $ — $ 160.0 Debt securities issued by corporations 1,001.0 4.3 (5.3 ) — 1,000.0 Municipal obligations 227.8 2.2 (1.2 ) — 228.8 Mortgage and asset-backed securities 1,170.6 2.0 (5.6 ) — 1,167.0 Foreign government, agency and provincial obligations 1.0 .2 — — 1.2 Preferred stocks 78.3 4.4 — — 82.7 Total fixed maturity investments $ 2,639.1 $ 13.1 $ (12.5 ) $ — $ 2,639.7 Less: fixed maturity investments reclassified to assets held for sale related to SSIE 9.5 Total fixed maturity investments $ 2,630.2 The cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of White Mountains’s common equity securities and other long-term investments as of September 30, 2016 and December 31, 2015 were as follows: September 30, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 378.9 $ 50.7 $ (4.6 ) $ — $ 425.0 Other long-term investments $ 317.1 $ 42.7 $ (6.4 ) $ (2.0 ) $ 351.4 December 31, 2015 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 822.5 $ 302.8 $ (11.4 ) $ — $ 1,113.9 Other long-term investments $ 304.5 $ 32.0 $ (18.4 ) $ (2.3 ) $ 315.8 Other Long-term Investments Other long-term investments consist of the following as of September 30, 2016 and December 31, 2015 : Carrying Value at Millions September 30, 2016 December 31, 2015 Hedge funds and private equity funds, at fair value (1) $ 143.0 $ 127.8 Private equity securities and limited liability companies, at fair value (1) 84.6 82.1 Surplus note investments, at fair value (1) 71.5 51.5 Private convertible preferred securities (1) 33.1 32.7 Tax advantaged federal affordable housing development fund (2) 12.8 14.7 Partnership investments accounted for under the equity method 3.2 3.8 Other 3.2 3.2 Total other-long term investments $ 351.4 $ 315.8 (1) See Fair Value Measurements by Level table. (2) Fund accounted for using the proportional amortization method. Hedge Funds and Private Equity Funds White Mountains holds investments in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments are generally estimated using the net asset value of the funds. As of September 30, 2016 , White Mountains held investments in 5 hedge funds and 24 private equity funds. The largest investment in a single fund was $34.6 million as of September 30, 2016 . The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of September 30, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financial $ 34.6 $ — $ 12.8 $ — Long/short equity REIT 20.4 — 20.6 — Other 3.4 — 3.6 — Total hedge funds 58.4 — 37.0 — Private equity funds Manufacturing/Industrial 27.7 21.8 24.9 2.5 Aerospace/Defense/Government 23.1 21.8 19.8 30.3 Energy infrastructure & services 14.2 3.2 20.7 3.4 Multi-sector 10.4 2.0 14.8 2.1 Healthcare 3.8 .4 3.8 .4 Private equity secondaries 3.7 2.1 4.4 2.1 Insurance .8 41.3 2.0 41.3 Direct lending/Mezzanine debt .6 36.9 — — Real estate .3 .1 .4 .1 Total private equity funds 84.6 129.6 90.8 82.2 Total hedge funds and private equity funds included in other long-term investments $ 143.0 $ 129.6 $ 127.8 $ 82.2 Redemption of investments in certain hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. The following summarizes the September 30, 2016 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds: Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice 90-119 days notice Total Monthly $ — $ — $ — $ — Quarterly 35.6 — — 35.6 Semi-annual — 20.4 — 20.4 Annual — — 2.4 2.4 Total $ 35.6 $ 20.4 $ 2.4 $ 58.4 Certain of White Mountains’s investments in hedge funds are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund’s underlying investments are liquidated. As of September 30, 2016 , the fair value of hedge funds in liquidation was $1.0 million . The actual amount of the final distribution remittances remain subject to market fluctuations. The date at which such remittances will be received is not determinable as of September 30, 2016 . White Mountains has also submitted redemption requests for certain of its investments in active hedge funds. As of September 30, 2016 , redemptions of $2.4 million are outstanding that would be subject to market fluctuations. The date at which such redemptions will be received is not determinable as of September 30, 2016 . Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations. Investments in private equity funds are generally subject to a “lock-up” period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds provide an option to extend the lock-up period at either the sole discretion of the fund manager or upon agreement between the fund and the investors. As of September 30, 2016 , investments in private equity funds were subject to lock-up periods as follows: Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $23.5 $3.8 $56.3 $1.0 $84.6 OneBeacon Surplus Notes In the fourth quarter of 2014, in conjunction with OneBeacon's sale of its runoff business to an affiliate of Armour Group Holdings Limited (the "OneBeacon Runoff Transaction"), OneBeacon provided financing in the form of surplus notes with a par value of $101.0 million , which had a fair value of $71.5 million and $ 51.5 million as of September 30, 2016 and December 31, 2015, respectively. Subsequent to closing, the surplus notes are included in OneBeacon’s investment portfolio, classified within other long-term investments. The internal valuation model used to estimate the fair value is based on discounted expected cash flows. The estimated fair value of the surplus notes is sensitive to changes in U.S. treasury rates and public corporate debt credit spreads, as well as changes in estimates with respect to other variables including a discount to reflect the private nature of the notes (and the related lack of liquidity), the credit quality of the notes based on the financial performance of the Issuer relative to expectations, and the timing, amount, and likelihood of interest and principal payments on the notes, which are subject to regulatory approval and therefore may vary from the contractual terms. An interest payment of $2.4 million was received in the nine months ended September 30, 2016. OneBeacon has assumed for valuation purposes that subsequent interest payments will begin in year five (2020) and principal repayments begin on a graduated basis in year ten (2025) for the seller priority note and year fifteen (2030) for the pari passu note. Although these variables involve considerable judgment, the Company does not currently expect any resulting changes in the estimated value of the surplus notes to be material to its financial position. Below is a table illustrating the valuation adjustments taken to arrive at estimated fair value of the surplus notes as of September 30, 2016 and December 31, 2015: Type of Surplus Note Total as of September 30, 2016 Total as of December 31, 2015 Millions Seller Priority Pari Passu Par Value $ 57.9 $ 43.1 $ 101.0 $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 5.2 (4.4 ) .8 (15.1 ) Regulatory approval (2) (4.2 ) (11.7 ) (15.9 ) (24.2 ) Liquidity adjustment (3) (9.9 ) (4.5 ) (14.4 ) (10.2 ) Total adjustments (8.9 ) (20.6 ) (29.5 ) (49.5 ) Fair value (4) $ 49.0 $ 22.5 $ 71.5 $ 51.5 (1) Represents the value of the surplus notes, at current market yields on comparable publicly traded debt, and assuming issuer is allowed to make principal and interest payments when its financial capacity is available, as measured by statutory capital in excess of a 250% RBC score. (2) Represents anticipated timing of securing regulatory approvals for interest and principal payments to reflect graduated changes in Issuer’s statutory surplus. The monetary impact of the anticipated timing is measured based on credit spreads of publicly traded debt with roughly equivalent percentages of discounted payments missed. (3) Represents impact of liquidity spread to account for OneBeacon’s sole ownership of the surplus notes, lack of a trading market and ongoing regulatory approval risk. (4) The increase in the fair value of the surplus notes during the nine months ended September 30, 2016 was driven primarily by the narrowing of non-investment grade credit spreads as well as the time value of money benefit generated by moving nine months closer to modeled cash receipts, partially offset by the impact of an interest payment received in the first quarter of 2016. Fair value measurements as of September 30, 2016 Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity’s internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets or liabilities have the highest priority (“Level 1”), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (“Level 2”) and unobservable inputs, including the reporting entity’s estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). White Mountains used quoted market prices or other observable inputs to determine fair value for 91% of its investment portfolio as of September 30, 2016. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equity securities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs are primarily comprised of fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government, agency and provincial obligations and preferred stocks. 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 2 inputs also include certain ETFs that track U.S. stock indices such as the S&P 500 but are traded on foreign exchanges and that management values using the fund’s published NAV to account for the difference in market close times. Level 3 fair value estimates based upon unobservable inputs include White Mountains’s investments in surplus notes, as well as certain investments in fixed maturity investments, common equity securities and other long-term investments where quoted market prices are unavailable or are not considered reasonable. Transfers between levels are based on investments held as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing methodologies and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, credit ratings, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains’s process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, evaluation of pricing methodologies, and review of the pricing services’ quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from alternate independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Also considered outliers are prices that haven’t 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 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 above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follow: Debt securities issued by corporations: The fair value of debt securities issued by corporations is determined from an evaluated pricing process 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 an evaluated pricing process 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 process 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 an evaluated pricing process that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Preferred stocks: The fair value of preferred stocks is determined from an evaluated pricing process that calculates the appropriate spread over a comparable security for each issue. Key inputs include exchange prices (underlying and common stock of same issuer), benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for its other long-term investments, including obtaining and reviewing periodic and audited annual financial statements of hedge funds and private equity funds and discussing each fund’s pricing with the fund manager throughout the year. However, since the fund managers do not provide sufficient information to evaluate the pricing inputs and methods for each underlying investment, the inputs are considered to be unobservable. The fair value of White Mountains’s investments in hedge funds and private equity funds has generally been determined using net asset value. Fair Value Measurements by Level The following tables summarize White Mountains’s fair value measurements for investments as of September 30, 2016 and December 31, 2015 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage-backed or 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. September 30, 2016 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 97.0 $ 87.5 $ 9.5 $ — Debt securities issued by corporations: Consumer 340.5 — 340.5 — Health Care 193.3 — 193.3 — Financials 170.8 — 170.8 — Industrial 132.2 — 132.2 — Utilities 97.1 — 97.1 — Communications 89.4 — 89.4 — Technology 79.6 — 79.6 — Materials 72.9 — 72.9 — Energy 53.6 — 53.6 — Total debt securities issued by corporations 1,229.4 — 1,229.4 — Mortgage and asset-backed securities 2,414.5 — 2,378.5 36.0 Municipal obligations 312.3 — 312.3 — Foreign government, agency and provincial obligations 1.2 .6 .6 — Preferred stocks 85.4 — 13.8 71.6 Total fixed maturity investments (4) 4,139.8 88.1 3,944.1 107.6 Short-term investments (4)(5) 305.6 278.6 27.0 — Common equity securities: Exchange traded funds (1) 113.2 90.5 22.7 — Consumer 111.2 111.2 — — Communications 33.3 33.3 — — Health Care 33.2 33.2 — — Technology 28.1 28.1 — — Financials 11.4 11.4 — — Industrial 11.1 11.1 — — Energy 5.5 5.5 — — Other 78.0 — 78.0 — Total common equity securities 425.0 324.3 100.7 — Other long-term investments (2)(3) 192.4 — — 192.4 Total investments (4) $ 5,062.8 $ 691.0 $ 4,071.8 $ 300.0 (1) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (2) Excludes carrying value of $3.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $12.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (3) Excludes carrying value of $143.0 associated with hedge funds and private equity funds for which fair value is measured at net asset value using the practical expedient. (4) Includes carrying value of $8.3 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE. (5) Short-term investments are measured at amortized cost, which approximates fair value. December 31, 2015 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 160.0 $ 133.4 $ 26.6 $ — Debt securities issued by corporations: Consumer 253.3 — 253.3 — Financials 175.9 — 175.9 — Health Care 151.3 — 151.3 — Industrial 135.6 — 135.6 — Energy 82.0 — 82.0 — Utilities 61.5 — 61.5 — Technology 60.0 — 60.0 — Communications 49.2 — 49.2 — Materials 31.2 — 31.2 — Total debt securities issued by corporations 1,000.0 — 1,000.0 — Mortgage and asset-backed securities 1,167.0 — 1,167.0 — Municipal obligations 228.8 — 228.8 — Foreign government, agency and provincial obligations 1.2 .6 .6 — Preferred stocks 82.7 — 12.7 70.0 Total fixed maturity investments (4) 2,639.7 134.0 2,435.7 70.0 Short-term investments (4)(5) 211.3 211.3 — — Common equity securities: Financials 653.2 653.2 — — Exchange traded funds (1) 183.3 162.0 21.3 — Consumer 70.0 70.0 — — Communications 43.7 43.7 — — Health Care 35.7 35.7 — — Technology 27.0 27.0 — — Industrial 26.6 26.6 — — Other 74.4 — 74.4 — Total common equity securities 1,113.9 1,018.2 95.7 — Other long-term investments (2)(3) 169.5 — — 169.5 Total investments (4) $ 4,134.4 $ 1,363.5 $ 2,531.4 $ 239.5 (1) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (2) Excludes carrying value of $3.8 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $14.7 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (3) Excludes carrying value of $127.8 associated with hedge funds and private equity funds for which fair value is measured at net asset value using the practical expedient. (4) Includes carrying value of $9.5 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE. (5) Short-term investments

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 30, 2016
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. During 2015, Wobi acquired Cashboard in two transactions for a total of NIS 35.9 million (approximately $8.9 million based upon the foreign exchange spot rate at the dates of acquisition), representing a controlling financial interest. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million , including $0.3 million of goodwill and $2.8 million of other intangible assets, and total liabilities assumed of $1.2 million at their estimated fair values. On August 4, 2016, White Mountains acquired a 70.9% ownership share in Buzzmove for a purchase price of GBP 6.0 million (approximately $8.0 million based upon the foreign exchange spot rate at the date of acquisition). White Mountains recognized total assets acquired related to Buzzmove of $11.7 million , including $9.0 million of other intangible assets, total liabilities assumed of $0.1 million , and non-controlling interests of $2.6 million , reflecting acquisition date fair values. The following table shows the change in goodwill and other intangible assets: Three Months Ended September 30, Millions 2016 2015 Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 24.1 $ 29.2 53.3 $ 24.1 $ 36.4 $ 60.5 Amounts held for sale at beginning of the period (1) — — — — .6 .6 Amortization, including foreign currency translation — (3.0 ) (3.0 ) — (2.6 ) (2.6 ) Less: Amounts held for sale at end of the period (1) — — — — .5 .5 Subtotal 24.1 26.2 50.3 24.1 33.9 58.0 Acquisition of business 9.0 (2) — Ending balance $ 59.3 $ 58.0 Nine Months Ended September 30, Millions 2016 2015 Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 24.1 $ 31.3 $ 55.4 $ 23.8 $ 38.7 $ 62.5 Amounts held for sale at beginning of the period (1) — .4 .4 — .8 .8 Acquisitions of businesses — — — .3 2.8 3.1 Acquisitions of other intangible assets — 3.9 3.9 — — — Amortization, including foreign currency translation — (9.4 ) (9.4 ) — (7.9 ) (7.9 ) Less: Amounts held for sale at end of the period (1) — — — — .5 .5 Subtotal 24.1 26.2 50.3 24.1 33.9 58.0 Acquisition of business 9.0 (2) — Ending balance $ 59.3 $ 58.0 (1) See Note 17 - “Held for Sale and Discontinued Operations” . (2) The relative fair values of goodwill and of the other intangible assets recognized in connection with the acquisition of Buzzmove had not yet been determined at September 30, 2016.

Debt

Debt	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Debt	Debt White Mountains’s debt outstanding as of September 30, 2016 and December 31, 2015 consisted of the following: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ 50.0 3.9% OBH Senior Notes, at face value 275.0 4.7% 275.0 4.7% Unamortized original issue discount and debt issuance costs (1.9 ) (2.1 ) OBH Senior Notes, carrying value 273.1 272.9 OneBeacon Bank Facility — N/A — N/A MediaAlpha Bank Facility 15.6 5.8% 15.0 5.5% Unamortized issuance cost (.2 ) (.3 ) MediaAlpha Bank Facility, carrying value 15.4 14.7 Total debt $ 288.5 $ 337.6 (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 three months ended September 30, 2016 , White Mountains borrowed and repaid $ 250.0 million under the WTM Bank Facility. During the nine months ended September 30, 2016 , White Mountains borrowed $350.0 million and repaid $400.0 million under the WTM Bank Facility at a blended interest rate of 3.85% . As of September 30, 2016 , the WTM Bank Facility was undrawn. The WTM Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. OBH Senior Notes In November 2012, OneBeacon U.S. Holdings, Inc. (“OBH”) issued $275.0 million face value of senior unsecured notes (“OBH Senior Notes”) through a public offering, at an issue price of 99.9% and received $272.9 million of proceeds. The OBH Senior Notes bear an annual interest rate of 4.6% payable semi-annually in arrears on May 9 and November 9, until maturity on November 9, 2022, and are fully and unconditionally guaranteed as to the payment of principal and interest by OneBeacon Ltd. Taking into effect the amortization of the original issue discount and all underwriting and issuance expenses, the OBH Senior Notes have an effective yield to maturity of approximately 4.7% per annum. The OBH Senior Notes were issued under indentures that contain restrictive covenants which, among other things, limit the ability of OneBeacon Ltd., OBH, and their respective subsidiaries to create liens and enter into sale and leaseback transactions and limits the ability of OneBeacon Ltd. and OBH to consolidate, merge or transfer its properties and assets. The indentures do not contain any financial ratios or specified levels of net worth or liquidity to which the OneBeacon Ltd. or OBH must adhere. In addition, a failure by OneBeacon Ltd. or OBH or their respective subsidiaries to pay principal and interest on covered debt, where such failure results in the acceleration of at least $75.0 million of the principal amount of covered debt, could trigger the acceleration of the OBH Senior Notes. OneBeacon Bank Facility On September 29, 2015, OneBeacon Ltd. and OneBeacon U.S. Holdings, Inc. (“OBH”), as co-borrowers and co-guarantors, entered into a revolving credit facility administered by U.S. Bank N.A. and also including BMO Harris Bank N.A., which has a total commitment of $65.0 million and has a maturity date of September 29, 2019 (the “OneBeacon Bank Facility”). As of September 30, 2016, the OneBeacon Bank Facility was undrawn. The OneBeacon Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. MediaAlpha Bank Facility On July 23, 2015, MediaAlpha entered into a secured credit facility with Opus Bank, which has a total commitment of $20.0 million and has a maturity date of July 23, 2019 (the “MediaAlpha Bank Facility”). The MediaAlpha Bank Facility consists of a $14.2 million term loan facility and a revolving loan facility for $5.0 million , which had an outstanding balance of $1.5 million as of September 30, 2016 . During the three months ended September 30, 2016 , MediaAlpha repaid $0.8 million under the term loan facility and $0.5 million under the revolving loan facility. During the nine months ended September 30, 2016 , MediaAlpha repaid $0.8 million under the term loan facility and borrowed $2.5 million and repaid $1.0 million under the revolving loan facility. The MediaAlpha Bank Facility carries a variable interest rate that is based on the Prime Rate, as published by the Wall Street Journal, plus a spread of 1.5% as of September 30, 2016 . The MediaAlpha Bank Facility is secured by intellectual property and the common stock of MediaAlpha’s subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a maximum leverage ratio. Compliance At September 30, 2016 , White Mountains was in compliance with the covenants under all of its debt instruments.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2016
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 consolidated subsidiaries and branches are subject to tax are Barbados, Gibraltar, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’s effective tax rate for the three months ended September 30, 2016 was (33.5)% . White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by 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. White Mountains’s effective tax rate for the nine months ended September 30, 2016 was (84.9)% . White Mountains’s income tax benefit related to pre-tax income from continuing operations for the nine months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by a $ 3.5 million favorable settlement of the 2010-2012 IRS exam, by a $ 12.8 million favorable settlement of the 2007-2009 IRS exam and by 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 as described above. 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. White Mountains’s income tax benefit from continuing operations for the first nine months of 2015 was near zero. The effective rates for the three and nine months ended September 30, 2015 represented an effective tax rate that was lower than the U.S. statutory rate of 35% due to income forecasted in jurisdictions with lower tax rates than the United States. In arriving at the effective tax rate for the three and nine months ended September 30, 2016 and 2015, 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, 2016 and 2015. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense. With few exceptions, White Mountains is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2012. On January 19, 2016, White Mountains received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Overassessment) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved with the Joint Committee on Taxation. As the receipt of the Form 870-AD described above represents formal settlement, White Mountains recorded a tax benefit of $ 12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009. On May 27, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax years 2010, 2011 and 2012 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded a tax benefit of $ 3.5 million in the second quarter of 2016 related to tax years 2010, 2011 and 2012. On June 22, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax year 2012 for Guilford Holdings, Inc. and subsidiaries. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded an increase to net operating loss carryforwards of $ 1.7 million related to tax year 2012.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2016
Derivatives	Derivatives
Variable Annuity
Derivatives	Variable Annuity Reinsurance White Mountains entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. During the third quarter of 2015, the variable annuity contracts reinsured by WM Life Re began to mature and were fully runoff by June 30, 2016. At December 31, 2015 , the total guarantee value was approximately ¥50.7 billion (approximately $0.4 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 109% of the guarantee value at December 31, 2015 . The following table summarizes the pre-tax operating results of WM Life Re for the three and nine months ended September 30, 2016 and 2015 . Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Fees, included in other revenue $ — $ 2.1 $ 1.2 $ 7.7 Change in fair value of variable annuity liability, included in other revenue — (2.8 ) (.3 ) (1.7 ) Change in fair value of derivatives, included in other revenue — 1.5 (2.0 ) (6.3 ) Foreign exchange, included in other revenue — .4 1.4 (.8 ) Other investment income and losses — (.2 ) — (.3 ) Total revenue — 1.0 .3 (1.4 ) Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses — — (.3 ) — Death benefit claims paid, included in general and administrative expenses — — — — General and administrative expenses (.5 ) (.8 ) (2.4 ) (3.1 ) Pre-tax income (loss) $ (.5 ) $ .2 $ (2.4 ) $ (4.5 ) The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and nine months ended September 30, 2016 and 2015 and the carrying values, included in other assets, as of December 31, 2015 by type of instrument: Gains (losses) Three Months Ended Nine Months Ended Carrying Value September 30, September 30, As of Millions 2016 2015 2016 2015 December 31, 2015 Fixed income/interest rate $ — $ 2.6 $ 1.8 $ 6.0 $ .5 Foreign exchange — (3.1 ) (4.8 ) (6.0 ) 14.8 Equity — 2.0 1.0 (6.3 ) 4.8 Total $ — $ 1.5 $ (2.0 ) $ (6.3 ) $ 20.1 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 and the three and nine months ended September 30, 2015. Nine Months Ended September 30, 2016 Variable Annuity Assets (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 gains (losses) (.3 ) 2.9 (.7 ) (4.2 ) (2.0 ) Transfers in — — — — — Sales/settlements — (5.6 ) (15.8 ) 3.3 (18.1 ) End of period $ — $ — $ — $ — $ — Three Months Ended September 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Purchases — — — — — Realized and unrealized gains (losses) (2.8 ) 3.4 3.2 (5.1 ) 1.5 Transfers in — — — — — Sales/settlements — (.5 ) (7.8 ) 8.1 (.2 ) End of period $ (1.0 ) $ 11.2 $ 27.9 $ (2.1 ) $ 37.0 Nine Months Ended September 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized gains (losses) (1.7 ) (5.5 ) (2.7 ) 1.9 (6.3 ) Transfers in — — — — — Sales/settlements — (2.2 ) (3.2 ) (7.7 ) (13.1 ) End of period $ (1.0 ) $ 11.2 $ 27.9 $ (2.1 ) $ 37.0 (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. In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties. The total collateral includes the following: Millions December 31, 2015 September 30, 2015 Cash $ 5.8 $ 16.8 Fixed maturity investments — — Total $ 5.8 $ 16.8 Collateral in the form of fixed maturity securities consisted of Government of Japan Bonds, recorded at fair value. Collateral in the form of short-term investments consisted of money-market instruments, carried at amortized cost, which approximated fair value. All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements. The fair value of White Mountains’s variable annuity reinsurance liabilities were estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, were generally unobservable inputs and significantly impacted the fair value estimates. Generally, the liabilities associated with these guarantees increased with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains used derivative instruments to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policyholder behavior and risk margins, were generally the same as those used to estimate the fair value of variable annuity liabilities. There were no open derivatives instruments remaining as of September 30, 2016. WM Life Re entered into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee. In the case of OTC derivatives, WM Life Re had exposure to credit risk for amounts that were uncollateralized by counterparties. WM Life Re’s internal risk management guidelines established net counterparty exposure thresholds that took into account OTC counterparties’ credit ratings. The OTC derivative contracts were subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements. In the case of exchange traded instruments, WM Life Re had exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re had master netting agreements with certain of its counterparties whereby the collateral provided (held) was calculated on a net basis. The following summarizes amounts offset under master netting agreements: December 31, 2015 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Interest rate contracts OTC $ 2.4 $ (2.1 ) $ .3 Exchange traded .1 (.1 ) — Foreign exchange contracts OTC 15.0 — 15.0 Exchange traded .1 (.3 ) (.2 ) Equity contracts OTC 4.4 (.6 ) 3.8 Exchange traded 1.2 — 1.2 Total (2) $ 23.2 $ (3.1 ) $ 20.1 (1) Amount equal to fair value of instrument as recognized in other assets (2) All derivative instruments held by WM Life Re were subject to master netting arrangements. There were no open derivative instruments as of September 30, 2016 and no exposure to credit losses on OTC and exchanged traded derivatives as of that date. The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets as of December 31, 2015 : December 31, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counter-party - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 8.5 $ — $ — $ 8.5 $ — $ — $ 5.5 $ 3.0 A + Bank of America .7 — — .7 — — — .7 A Citigroup - OTC 9.9 — — 9.9 — — 1.4 8.5 A Citigroup - Exchange Traded 1.0 — — 1.0 5.8 — — 6.8 A Total $ 20.1 $ — $ — $ 20.1 $ 5.8 $ — $ 6.9 $ 19.0 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).

Municipal Bond Guarantee

Municipal Bond Guarantee	9 Months Ended
	Sep. 30, 2016
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, 2016 , White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. Through HG Re, which had statutory capital of $476.5 million at September 30, 2016 , HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. For the three 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, 2015 , HG Global had pre-tax income of $5.2 million and $13.8 million , which included $4.0 million and $11.9 million of interest income 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. For the three and nine months ended September 30, 2015 , White Mountains reported pre-tax losses of $10.6 million and $33.5 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.0 million and $11.9 million of interest expense on the BAM Surplus Notes. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM Surplus Notes for the five years ending December 31, 2018 from a fixed rate of 8% to a variable rate equal to the one -year U.S. treasury rate plus 300 basis points, set annually, which is 3.54% and 3.15% for 2016 and 2015. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8% . BAM is required to seek regulatory approval to pay interest and principal on its surplus notes only when adequate capital resources have accumulated beyond BAM’s initial capitalization and a level that continues to support its outstanding obligations, business plan and ratings. All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are received upfront and an unearned premium revenue liability, equal to the amount of the cash received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. The following table provides a schedule of BAM’s insured obligations: September 30, 2016 December 31, 2015 Contracts outstanding 4,357 3,103 Remaining weighted average contract period outstanding (in years) 10.9 12.8 Contractual debt service outstanding (in millions): Principal $ 30,470.0 $ 22,556.0 Interest 15,258.1 11,984.4 Total debt service outstanding $ 45,728.1 $ 34,540.4 Gross unearned insurance premiums $ 71.0 $ 50.2 The following table is a schedule of BAM’s future premium revenues as of September 30, 2016 : Millions September 30, 2016 October 1, 2016 - December 31, 2016 $ 1.6 January 1, 2017 - March 31, 2017 1.6 April 1, 2017 - June 30, 2017 1.6 July 1, 2017 - September 30, 2017 1.6 October 1, 2017 - December 31, 2017 1.5 6.3 2018 6.0 2019 5.7 2020 5.4 2021 and thereafter 46.0 Total $ 71.0

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share from continuing operations for the three and nine months ended September 30, 2016 and 2015 . (See Note 17 - “Held for Sale and Discontinued Operations” ). Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Basic and diluted earnings per share numerators (in millions): Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 29.8 $ (65.4 ) $ 24.2 $ (47.6 ) Allocation of income for unvested restricted common shares (.4 ) — (.3 ) — Dividends declared on participating restricted common shares (1) — — (.1 ) (.1 ) Total allocation to restricted common shares (.4 ) — (.4 ) (.1 ) Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 23.8 $ (47.7 ) Undistributed net earnings (in millions): Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 23.8 $ (47.7 ) Dividends declared net of restricted common share amounts (1) — — (5.4 ) (5.9 ) Total undistributed net earnings, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 18.4 $ (53.6 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,867.4 5,890.1 5,166.6 5,951.1 Average unvested restricted shares (2) (68.1 ) — (62.9 ) — Basic earnings per share denominator 4,799.3 5,890.1 5,103.7 5,951.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (3) 4,879.4 5,890.1 5,174.8 5,951.1 Average unvested restricted common shares (2) (68.1 ) — (62.9 ) — Diluted earnings per share denominator (3) 4,811.3 5,890.1 5,111.9 5,951.1 Basic earnings per share (in dollars): Net income (loss) attributable to White Mountains’s common shareholders $ 6.12 $ (11.10 ) $ 4.67 $ (8.01 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ 6.12 $ (11.10 ) $ 3.67 $ (9.01 ) Diluted earnings per share (in dollars): Net income (loss) attributable to White Mountains’s common shareholders $ 6.11 $ (11.10 ) $ 4.66 $ (8.01 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ 6.11 $ (11.10 ) $ 3.66 $ (9.01 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans”) . (3) The diluted earnings per share denominator for the three 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 results in 11,943 and 8,208 incremental shares outstanding over the periods.

Non-controlling Interest

Non-controlling Interest	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 23.9 % $ 247.0 24.5 % $ 245.6 SIG Preference Shares — — 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 16.9 3.1 17.1 MediaAlpha 40.0 12.5 40.0 14.4 Tranzact — — 36.8 79.4 Wobi 3.3 .5 3.9 .6 Dewar 18.8 3.7 19.0 3.7 Buzzmove 29.1 3.3 — — Total other, excluding mutuals and reciprocals 36.9 115.2 Mutuals and reciprocals BAM 100.0 (138.9 ) 100.0 (140.0 ) SSIE 100.0 4.2 100.0 (16.0 ) Total mutuals and reciprocals (134.7 ) (156.0 ) Total non-controlling interests $ 149.2 $ 454.8

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2016
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are OneBeacon, HG Global/BAM and Other Operations. As a result of the Sirius Group and Tranzact sales, the results of operations for Sirius Group, previously reported in its own segment, and Tranzact, previously reported in the Other Operations segment, have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation (See Note 17 - “Held for Sale and Discontinued Operations” ). White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Three Months Ended September 30, 2016 Earned insurance premiums $ 277.9 $ 1.2 $ .3 $ 1.9 $ 281.3 Net investment income 11.8 .6 1.7 8.4 22.5 Net investment income (loss) - surplus note interest — 4.5 (4.5 ) — — Net realized and unrealized investment gains (losses) 15.5 (.3 ) (1.6 ) 12.8 26.4 Other revenue 1.8 — .4 32.6 (2) 34.8 Total revenues 307.0 6.0 (3.7 ) 55.7 365.0 Losses and LAE 162.8 — — 2.2 165.0 Insurance acquisition expenses 55.1 .2 .6 .5 56.4 Other underwriting expenses 49.4 — .1 .1 49.6 General and administrative expenses 3.2 .6 9.3 54.1 (3) 67.2 Amortization of other intangible assets .3 — — 2.7 3.0 Interest expense 3.3 — — .5 3.8 Total expenses 274.1 .8 10.0 60.1 345.0 Pre-tax income (loss) $ 32.9 $ 5.2 $ (13.7 ) $ (4.4 ) $ 20.0 HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Nine Months Ended September 30, 2016 Earned insurance premiums $ 827.9 $ 3.1 $ 1.0 $ 6.1 $ 838.1 Net investment income 38.3 1.6 5.1 14.6 59.6 Net investment income (loss) - surplus note interest — 13.4 (13.4 ) — — Net realized and unrealized investment gains 56.8 2.3 6.5 18.4 84.0 Other revenue 3.5 — .8 106.5 (2) 110.8 Total revenues 926.5 20.4 — 145.6 1,092.5 Losses and LAE 501.3 — — 6.8 508.1 Insurance acquisition expenses 154.8 .6 1.9 1.9 159.2 Other underwriting expenses 155.6 — .3 .1 156.0 General and administrative expenses 10.0 1.4 28.1 181.5 (3) 221.0 Amortization of other intangible assets .9 — — 8.5 9.4 Interest expense 9.8 — — 2.6 12.4 Total expenses 832.4 2.0 30.3 201.4 1,066.1 Pre-tax income (loss) $ 94.1 $ 18.4 $ (30.3 ) $ (55.8 ) $ 26.4 HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Three Months Ended September 30, 2015 Earned insurance premiums $ 281.4 $ .6 $ .3 $ 2.6 $ 284.9 Net investment income 12.4 .5 1.1 2.8 16.8 Net investment income (loss) - surplus note interest — 4.0 (4.0 ) — — Net realized and unrealized investment (losses) gains (29.9 ) .6 1.7 (16.3 ) (43.9 ) Other revenue 3.9 — .2 34.2 (2) 38.3 Total revenues 267.8 5.7 (.7 ) 23.3 296.1 Losses and LAE 167.5 — — 2.5 170.0 Insurance acquisition expenses 53.8 .1 .4 1.0 55.3 Other underwriting expenses 56.4 — .1 — 56.5 General and administrative expenses 3.4 .4 9.4 81.5 (3) 94.7 Amortization of other intangible assets .3 — — 2.3 2.6 Interest expense 3.2 — — .7 3.9 Total expenses 284.6 .5 9.9 88.0 383.0 Pre-tax (loss) income $ (16.8 ) $ 5.2 $ (10.6 ) $ (64.7 ) $ (86.9 ) HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Nine Months Ended September 30, 2015 Earned insurance premiums $ 887.3 $ 1.7 $ .6 $ 6.4 $ 896.0 Net investment income 33.0 1.4 2.9 5.3 42.6 Net investment income (loss) - surplus note interest — 11.9 (11.9 ) — — Net realized and unrealized investment (losses) gains (29.7 ) .3 2.8 (7.3 ) (33.9 ) Other (loss) revenue (.4 ) — .5 114.3 (2) 114.4 Total revenues 890.2 15.3 (5.1 ) 118.7 1,019.1 Losses and LAE 527.9 — — 6.2 534.1 Insurance acquisition expenses 161.2 .3 1.8 2.8 166.1 Other underwriting expenses 165.2 — .3 — 165.5 General and administrative expenses 11.0 1.2 26.3 185.2 (3) 223.7 Amortization of other intangible assets 1.0 — — 6.9 7.9 Interest expense 9.7 — — 1.1 10.8 Total expenses 876.0 1.5 28.4 202.2 1,108.1 Pre-tax income (loss) $ 14.2 $ 13.8 $ (33.5 ) $ (83.5 ) $ (89.0 ) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services. (2) Includes $ 27.6 and $ 88.4 from MediaAlpha for the three months and nine months ended September 30, 2016 , and $ 28.3 and $ 81.2 from MediaAlpha for the three months and nine months ended September 30, 2015 . (3) Includes $ 26.2 and $ 82.7 from MediaAlpha for the three months and nine months ended September 30, 2016 , and $ 26.2 and $ 75.8 from MediaAlpha for the three months and nine months ended September 30, 2015 .

Investments in Unconsolidated A

Investments in Unconsolidated Affiliates	9 Months Ended
	Sep. 30, 2016
Equity Method Investments and Joint Ventures [Abstract]
Investments in Unconsolidated Affiliates	Investments in Unconsolidated Affiliates White Mountains’s investments in unconsolidated affiliates represent investments in other companies in which White Mountains has a significant voting and economic interest but does not control the entity. Symetra In August 2015, Symetra announced it had entered into a definitive merger agreement with Sumitomo Life 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. 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. During the three and nine months ended September 30, 2015 , White Mountains received cash dividends from Symetra of $12.5 million and $16.9 million , which included a special dividend of $10.3 million . The dividends were recorded as a reduction of White Mountains’s investment in Symetra in accordance with equity method accounting. As of December 31, 2011, White Mountains concluded that its investment in Symetra common shares was other-than-temporarily impaired and wrote down the GAAP book value of the investment to its estimated fair value of or $15 per share. This impairment, as well as the effect of Symetra capital transactions, resulted in a basis difference between the GAAP carrying value of White Mountains’s investment in Symetra common shares under the equity method and the amount derived by multiplying the percentage of White Mountains common share ownership by Symetra’s total GAAP equity. As of September 30, 2015, the pre-tax unamortized basis difference was $160.8 million , of which $32.6 million is attributable to equity in earnings of unconsolidated affiliates and $128.2 million is attributable to equity in net unrealized gains of unconsolidated affiliates. The pre-tax basis difference was amortized over a 30 -year period, based on estimated future cash flows associated with Symetra’s underlying assets and liabilities to which the basis differences was attributed. White Mountains recognized the amortization of the basis difference through equity in earnings of unconsolidated affiliates and equity in net unrealized gains (losses) from investments in unconsolidated affiliates consistent with the original attribution of the basis differences between equity in earnings and equity in net unrealized gains (losses). For the three and nine months ended September 30, 2015, White Mountains recognized after-tax amortization of $0.6 million and $2.0 million and through equity in earnings of unconsolidated affiliates and $2.6 million and $8.1 million through equity in net unrealized gains from investments in unconsolidated affiliates. The following table summarizes amounts recorded by White Mountains under the equity method relating to its investment in Symetra for the three and nine months ended September 30, 2015 : Three Months Ended Nine Months Ended Millions September 30, 2015 September 30, 2015 Equity method carrying value of investment in Symetra at the beginning of the period $ 397.2 $ 411.4 Equity in earnings (1)(2) 4.2 17.5 Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio (3) 3.7 (31.8 ) Dividends received (12.5 ) (16.9 ) Distribution from Prospector Offshore Fund — 12.4 Equity method carrying value of investment in Symetra at the end of the period (4) $ 392.6 $ 392.6 (1) For the three and nine months ended September 30, 2015 , equity in earnings excludes tax expense of $0.3 and $1.1 . (2) For the three and nine months ended September 30, 2015 , equity in earnings includes $0.7 and $2.1 increase relating to the pre-tax amortization of the Symetra common share basis difference. (3) For the three and nine months ended September 30, 2015 , net unrealized gains includes $2.9 and $8.6 increase relating to the pre-tax amortization of the Symetra common share basis difference. (4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $5.7 as of September 30, 2015 , which excludes tax expense of $0.2 . Hamer On May 27, 2015, White Mountains sold its interest in Hamer LLC, which resulted in a gain of $20.0 million recorded in other revenue. Prior to the sale, White Mountains recorded equity in earnings of $1.1 million and $1.6 million for the three and six months ended June 30, 2015.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2016
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 and service providers of White Mountains. White Mountains’s share-based compensation incentive awards consist of performance shares, restricted shares and stock options. Share-Based Compensation Based on White Mountains Common Shares WTM Performance Shares Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. The following table summarizes performance share activity for the three and nine months ended September 30, 2016 and 2015 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 73,297 $ 32.5 106,572 $ 28.8 93,654 $ 57.7 111,257 $ 44.4 Shares paid or expired (1) (8,958 ) (5.5 ) (36,294 ) (41.0 ) (42,959 ) (30.4 ) New grants (2) 6,400 — — 22,615 — 29,195 — Forfeitures and cancellations (3) (160 ) (.1 ) (3,960 ) (.8 ) (438 ) .2 (3,839 ) (.3 ) Expense recognized — 5.0 — 30.0 — 20.5 — 38.8 End of period (4) 79,537 $ 37.4 93,654 $ 52.5 79,537 $ 37.4 93,654 $ 52.5 (1) 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. WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 88% to 131.5% of target. (2) In the third quarter of 2016 White Mountains granted 3,000 WTM performance shares for the 2015-2017 performance cycle and 3,400 WTM performance shares for the 2016-2018 performance cycle. (3) Amounts include changes in assumed forfeitures, as required under GAAP. (4) Outstanding performance share awards as of September 30, 2016 and 2015 both exclude 7,315 and 10,825 performance share awards granted to employees of Sirius Group. For performance shares earned in the 2013-2015 performance cycle, the Company issued 5,000 common shares and settled the remainder in cash. For the 2012-2014 performance cycle, all performance shares earned were settled in cash. If all the outstanding WTM performance shares had vested on September 30, 2016 , the total additional compensation cost to be recognized would have been $26.9 million , based on accrual factors at September 30, 2016 (common share price and payout assumptions). Performance Shares granted under the WTM Incentive Plan The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at September 30, 2016 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2016 – 2018 19,615 $ 3.4 2015 – 2017 31,795 15.1 2014 – 2016 30,167 19.9 Sub-total 81,577 38.4 Assumed forfeitures (2,040 ) (1.0 ) Total at September 30, 2016 79,537 $ 37.4 Restricted Shares The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 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 66,470 $ 24.2 71,875 $ 23.2 70,675 $ 15.7 83,314 $ 14.3 Issued 4,150 3.4 25,365 19.7 23,640 15.7 Vested — — (1,200 ) — (24,620 ) — (36,279 ) — Forfeited — — — — (800 ) .2 — — Expense recognized (1) — (3.8 ) — (3.7 ) — (11.8 ) — (10.5 ) End of period (2) 70,620 $ 23.8 70,675 $ 19.5 70,620 $ 23.8 70,675 $ 19.5 (1) In connection with the sale of Sirius Group in the second quarter of 2016, White Mountains recognized $ 1.4 from accelerating the amortization of the issue date fair value for shares issued to Sirius Group employees, who no longer have a service obligation to the Company. (2) Restricted share awards outstanding as of September 30, 2016 and 2015 include 2,512 and 3,572 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations. During the three and nine months ended September 30, 2016, White Mountains issued 4,150 and 25,365 restricted shares that vest on January 1, 2019. During the first nine months of 2015, White Mountains issued 23,640 restricted shares that vest on January 1, 2018. The unrecognized compensation cost at September 30, 2016 is expected to be recognized ratably over the remaining vesting periods. Stock Options Non-Qualified Options As September 30, 2016 and December 31, 2015 , the Company’s Chairman and CEO held 120,000 and 125,000 Non-Qualified Options, which are exercisable at a fixed price of $742 per common share and expire on January 20, 2017. The potential in-the-money value of the Non-Qualified Options in excess of $100 million is limited to 50% of the excess amount. The Non-Qualified Options were fully amortized as of the first quarter in 2011. On May 5, 2016, the Company’s Chairman and CEO exercised 5,000 Non-Qualified Options at $742 per common share. Share-Based Compensation Based on OneBeacon Ltd. Common Shares The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based and non share-based incentive awards. OneBeacon’s share-based incentive awards include OneBeacon performance shares and restricted shares. OneBeacon Performance Shares OneBeacon performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. OneBeacon performance share awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of OneBeacon Ltd. common shares at the time awards are paid. The following table summarizes performance share activity for the three and nine months ended September 30, 2016 and 2015 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 441,206 $ 1.0 462,147 $ 2.4 449,435 $ 1.4 517,470 $ 3.4 Shares paid or expired (1) — — — — (167,300 ) (.7 ) (181,290 ) (1.5 ) New grants — — — — 163,150 — 154,887 — Assumed forfeitures and cancellations (2) — — — — (4,079 ) — (28,920 ) — Expense recognized — .3 — (.3 ) — .6 — .2 End of period 441,206 $ 1.3 462,147 $ 2.1 441,206 $ 1.3 462,147 $ 2.1 (1) OneBeacon performance share payments in 2016 for the 2013-2015 performance cycle were at 24.3% of target. OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. If the outstanding OneBeacon performance shares had been vested on September 30, 2016 , the total additional compensation cost to be recognized would have been $2.3 million , based on accrual factors at September 30, 2016 (common share price, accumulated dividends and payout assumptions). The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at September 30, 2016 for each performance cycle: Millions, except share amounts Target Accrued Expense Performance cycle: 2016 – 2018 163,150 $ .6 2015 – 2017 146,659 .7 2014 – 2016 142,710 — Sub-total 452,519 1.3 Assumed forfeitures (11,313 ) — Total at September 30, 2016 441,206 $ 1.3 OneBeacon Restricted Shares The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 395,872 $ 3.5 390,950 $ 3.5 382,722 $ 2.5 612,500 $ 3.5 Issued — — — — 170,650 2.3 75,950 1.1 Vested — — — — (157,500 ) — (296,000 ) — Forfeited — — — — — — (1,500 ) — Expense recognized — (.7 ) — (.5 ) — (2.0 ) — (1.6 ) End of period 395,872 $ 2.8 390,950 $ 3.0 395,872 $ 2.8 390,950 $ 3.0 On February 24, 2016, OneBeacon issued 170,650 shares of restricted stock, of which 92,500 restricted shares vest on February 24, 2018 and 78,150 vest on January 1, 2019. On February 24, 2015, OneBeacon issued 75,950 restricted shares, which vest on January 1, 2018. On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments. The first installment vested on February 22, 2014. Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent five years were reduced by 35,000 shares. As of September 30, 2016, unrecognized compensation expense of $ 2.8 million related to restricted stock awards is expected to be recognized over the remaining vesting periods. OneBeacon Restricted Stock Units During the first nine months of 2016, the OneBeacon Compensation Committee awarded to certain employees 233,461 restricted stock units (“RSUs”) that are scheduled to vest on December 31, 2018, of which, net of forfeiture assumptions, 219,112 were outstanding as of September 30, 2016. During the first nine months of 2015, the OneBeacon Compensation Committee awarded to certain employees 215,698 RSUs that are scheduled to vest on December 31, 2017, of which, net of forfeiture assumptions, 177,409 were outstanding as of September 30, 2016. The RSUs will be paid out in cash or shares at the discretion of the OneBeacon Compensation Committee. For the three and nine months ended September 30, 2016 , the expense associated with the RSUs was $0.5 million and $1.4 million . For the three and nine months ended September 30, 2015 , the expense associated with the RSUs was $0.2 million and $0.6 million .

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2016
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 OBH Senior Notes and MediaAlpha Bank Facility, which are recorded as debt at face value less unamortized original issue discount. The following table summarizes the fair value and carrying value of these financial instruments as of September 30, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 277.8 $ 273.1 $ 276.4 $ 272.9 MediaAlpha Bank Facility 15.7 15.4 15.0 14.7 The fair value estimate for the OBH Senior Notes has been determined using quoted market prices. The OBH Senior Notes are considered a Level 2 measurement. The fair value estimate for the MediaAlpha Bank Facility has been determined based on a discounted cash flows approach and is considered to be a Level 3 measurement.

Discontinued Operations

Discontinued Operations	9 Months Ended
	Sep. 30, 2016
Discontinued Operations	Held for Sale and Discontinued Operations Sirius Group On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion . $161.8 million of this amount was used to purchase certain assets to be retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. In the second quarter of 2016, White Mountains recorded $ 366.6 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. In the third quarter of 2016, White Mountains recorded a $ 4.0 million reduction to the gain from sale of Sirius Group in discontinued operations as a result of the final true-up of Sirius Group's closing date tangible common shareholder's equity. For the nine months ended September 30, 2016, White Mountains recorded a $ 4.3 million net loss from discontinued operations from Sirius Group and $ 32.0 million of other comprehensive income. 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 income (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 nine months ended September 30, 2016, $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 income (loss) from discontinued operations. For the three and nine months ended September 30, 2015, $0.8 million and $3.5 million of net investment income and realized and unrealized investment gains and losses, net of related tax effects, that had been previously included in the Sirius Group legal entities have been excluded from net income (loss) from discontinued operations. As part of the sale of Sirius Group, White Mountains purchased Ashmere, a subsidiary formerly owned by Sirius Group. Ashmere was under an agreement to be sold as of September 30, 2016 and the sale was completed on October 12, 2016. As of September 30, 2016, Ashmere's assets and liabilities are presented as held for sale. Tranzact On June 9, 2016, White Mountains announced that it had entered into a definitive agreement for the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC. On July 21, 2016, White Mountains completed the sale of Tranzact and received net proceeds of $ 221.3 million . During the three and nine months ended September 30, 2016, White Mountains recorded $51.9 million of gain from the sale of Tranzact in discontinued operations in the statement of operations. (See Note 2 - “Significant Transactions” ). Through July 21, 2016, Tranzact's results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains's GAAP financial statements. For the stub period ended July 21, 2016 and the nine months ended September 30, 2016, White Mountains recorded a net loss from discontinued operations of $ 2.1 million and $ 2.6 million from Tranzact. For the three and nine months ended September 30, 2015, White Mountains recorded a net loss from discontinued operations of $ 7.7 million and $ 14.2 million from Tranzact. At September 30, 2016, White Mountains also recorded a liability of $ 2.8 million for escrow funds associated with the sale payable to the non-controlling interests. 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 related to the valuation allowance reversal was recorded to gain from sale of Tranzact in discontinued operations and a $ 16.1 million tax benefit related to the valuation allowance reversal was recorded to net income (loss) from discontinued operations. Star & Shield SSIE voluntarily ceased writing new policies effective July 1, 2016 and will voluntarily cease renewing policies at a future date, subject to regulatory approval. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, White Mountains has presented Star & Shield's and SSIE's assets and liabilities as held for sale as of September 30, 2016 and December 31, 2015. OneBeacon Runoff For the nine months ended September 30, 2015, White Mountains recorded $0.3 million to the gain from sale of discontinued operations related to an adjustment on the estimated loss on sale, which included the final settlement of certain post-closing items. For the three and nine months ended September 30, 2015, the net loss from discontinued operations relating to the Runoff Business, net of tax, was $0.1 million and $0.4 million . The transaction to sell the Runoff Business was completed in December 2014. Esurance For the three and nine months ended September 30, 2015, White Mountains recorded a net gain from the sale of discontinued operations of $10.3 million and $17.9 million , which primarily related to an installment payment from Allstate for the favorable development on loss reserves. (See Note 18 - “Contingencies” ).
Financials
Discontinued Operations	Summary of Reclassified Balances and Related Items Net Assets Held for Sale The following summarizes the assets and liabilities associated with business classified as held for sale. At December 31, 2015, amounts presented relate to Sirius Group, Tranzact, Star and Shield and SSIE. At September 30, 2016, the amounts presented relate to Star and Shield, SSIE and Ashmere, a subsidiary formerly owned by Sirius Group. Millions September 30, 2016 December 31, 2015 Assets held for sale Fixed maturity investments, at fair value $ 13.4 $ 2,383.5 Short-term investments, at amortized cost (which approximates fair value) 1.3 352.1 Common equity securities, at fair value — 174.4 Other long-term investments — 72.2 Total investments 14.7 2,982.2 Cash 3.8 150.1 Reinsurance recoverable on unpaid losses .6 283.6 Reinsurance recoverable on paid losses — 10.2 Insurance and reinsurance premiums receivable 2.0 326.6 Deferred acquisition costs — 74.6 Deferred tax asset — 303.1 Ceded unearned insurance and reinsurance premiums — 87.7 Accounts receivable on unsettled investment sales — 29.0 Goodwill and other intangible assets 5.2 330.5 Other assets .7 212.8 Total assets held for sale $ 27.0 $ 4,790.4 Liabilities held for sale Loss and loss adjustment expense reserves $ 6.0 $ 1,650.4 Unearned insurance and reinsurance premiums 1.6 344.3 Debt — 506.4 Deferred tax liability — 270.6 Accrued incentive compensation — 64.2 Ceded reinsurance payable (.5 ) 67.7 Funds held under reinsurance treaties — 52.9 Other liabilities .7 90.9 Total liabilities held for sale 7.8 3,047.4 Net assets held for sale $ 19.2 $ 1,743.0 Net Income (Loss) from Discontinued Operations The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2016, the amounts presented relate to Sirius Group and Tranzact. For the three and nine months ended September 30, 2015, the amounts presented relate to Sirius Group, Tranzact, Esurance, and the OneBeacon's runoff business. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains. Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ — $ — $ — $ 202.8 $ — $ 202.8 Net investment income — — — 10.5 — 10.5 Net realized and unrealized losses — — — (.9 ) — (.9 ) Other revenue — 14.8 14.8 1.8 38.5 40.3 Total revenues — 14.8 14.8 214.2 38.5 252.7 Expenses Loss and loss adjustment expenses — — — 121.7 — 121.7 Insurance and reinsurance acquisition expenses — — — 45.4 — 45.4 Other underwriting expenses — — — 24.8 — 24.8 General and administrative expenses — 16.2 16.2 9.4 45.3 54.7 Interest expense — .5 .5 6.6 1.0 7.6 Total expenses — 16.7 16.7 207.9 46.3 254.2 Pre-tax (loss) income — (1.9 ) (1.9 ) 6.3 (7.8 ) (1.5 ) Income tax expense (benefit) — 15.9 15.9 (2.3 ) — (2.3 ) Net income (loss) from discontinued operations — 14.0 14.0 4.0 (7.8 ) (3.8 ) Net loss from sale of discontinued operations - Sirius (4.0 ) — (4.0 ) — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - Esurance — — — — 10.3 10.3 Total (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ 4.0 $ 2.5 $ 6.5 Change in foreign currency translation and other from discontinued operations — — — (18.5 ) — (18.5 ) Comprehensive (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ (14.5 ) $ 2.5 $ (12.0 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 240.1 $ — $ 240.1 $ 623.3 $ — $ 623.3 Net investment income 14.4 — 14.4 28.6 — 28.6 Net realized and unrealized (losses) gains (1.5 ) — (1.5 ) 31.4 — 31.4 Other revenue .6 119.6 120.2 (20.2 ) 110.6 90.4 Total revenues 253.6 119.6 373.2 663.1 110.6 773.7 Expenses Loss and loss adjustment expenses 154.9 — 154.9 311.3 — 311.3 Insurance and reinsurance acquisition expenses 59.0 — 59.0 135.5 — 135.5 Other underwriting expenses 30.9 — 30.9 78.0 — 78.0 General and administrative expenses 10.4 116.7 127.1 22.4 122.5 144.9 Interest expense 7.9 3.2 11.1 20.0 2.7 22.7 Total expenses 263.1 119.9 383.0 567.2 125.2 692.4 Pre-tax (loss) income (9.5 ) (.3 ) (9.8 ) 95.9 (14.6 ) 81.3 Income tax benefit (expense) 3.1 13.8 16.9 (22.2 ) — (22.2 ) Net (loss) income from discontinued operations (6.4 ) 13.5 7.1 73.7 (14.6 ) 59.1 Net gain from sale of discontinued operations - Sirius 362.6 — 362.6 — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - OneBeacon — — — — .3 .3 Net gain from sale of discontinued operations - Esurance — — — — 17.9 17.9 Total income from discontinued operations $ 356.2 $ 65.4 $ 421.6 $ 73.7 $ 3.6 $ 77.3 Change in foreign currency translation and other from discontinued operations 32.0 — 32.0 (62.1 ) — (62.1 ) Change in foreign currency translation and other from sale of Sirius Group 113.3 — 113.3 — — — Comprehensive income from discontinued operations $ 501.5 $ 65.4 $ 566.9 $ 11.6 $ 3.6 $ 15.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax (payment to) refund from national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, (Millions) 2016 2015 Net cash (used for) provided from operations $ (16.2 ) $ 22.7 Net cash provided from investing activities 213.3 2.8 Net cash (used for) provided from financing activities (.2 ) 19.6 Effect of exchange rate changes on cash — (4.1 ) Net change in cash during the period 196.9 41.0 Cash balances at beginning of period 150.1 116.6 Net change in cash held for sale 2.6 — Cash sold as part of sale of consolidated subsidiaries 345.8 — Cash balances at end of period $ 3.8 $ 157.6 Supplemental cash flows information: Interest paid $ (1.4 ) $ (28.1 ) Net income tax payment to national governments $ (31.8 ) $ (30.3 ) Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 61.9 $ 6.5 $ 421.6 $ 77.3 Allocation of income for participating unvested restricted common shares (1) (.9 ) (.1 ) (5.1 ) (.9 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ 61.0 $ 6.4 $ 416.5 $ 76.4 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,867.4 5,890.1 5,166.6 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Basic earnings per share denominator 4,799.3 5,819.2 5,103.7 5,884.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,879.4 5,890.1 5,174.8 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Diluted earnings per share denominator (4) 4,811.3 5,819.2 5,111.9 5,884.0 Basic earnings per share (in dollars): $ 12.72 $ 1.09 $ 81.60 $ 12.98 Diluted earnings per share (in dollars): $ 12.69 $ 1.09 $ 81.47 $ 12.98 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2016 and 2015 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans” ). (4) ) The diluted earnings per share denominator for the three 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 results in 11,943 and 8,208 incremental shares outstanding over the periods.

Contingencies

Contingencies	9 Months Ended
	Sep. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. (See Note 3 - “Loss and Loss Adjustment Expense Reserves”) . White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of September 30, 2016 : Sale of Sirius Group On April 18, 2016, the Company completed the sale of Sirius Group to CMI pursuant a Stock Purchase Agreement dated as of July 24, 2015 (filed as an exhibit to the Company’s current report on Form 8-K on July 27, 2015, the (“Agreement”)). White Mountains has provided indemnification to CMI for certain representations and warranties made in the Agreement. Additionally, the final purchase price payable by CMI under the Agreement, which is based upon the tangible book value of Sirius Group at the closing date, was subject to a true-up process that was completed in the third quarter of 2016 and resulted in a reduction to the gain from sale of Sirius Group in discontinued operation of $ 4.0 million . Sale of Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (filed as an exhibit to the Company’s current report on Form 8-K on May 18, 2011. Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) specified litigation matters, (b) losses of the Transferred Subsidiaries arising from extra-contractual claims and claims in excess of policy limits (“ECO/EPL losses”), (c) certain corporate reorganizations effected to remove entities from the Transferred Subsidiaries that were not being sold in the transaction, and (d) certain tax matters, including certain net operating losses being less than stated levels. In addition, the Company retained 90% of positive or negative development in the loss reserves of the Transferred Subsidiaries as of the closing date (net of ECO/EPL losses) through December 31, 2014 (the “Reserve Settlement”). For the three and nine months ended September 30, 2015, White Mountains recorded a net gain from the sale of discontinued operations of $10.3 million and an additional $ 17.9 million related to the final settlement with Allstate for favorable development on loss reserves transferred in the sale of Esurance and Answer Financial. Since the closing of the transaction, White Mountains received a net amount of $ 28.3 million from Allstate, primarily related to the favorable development on loss reserves. Tribune Company In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune’s leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. OneBeacon received approximately $32.0 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and the plaintiffs appealed. On March 29, 2016, a three judge panel of the U.S. Second Circuit Court of Appeals affirmed the dismissal of the Noteholders Action. On July 22, 2016, the Plaintiff's petition to the Second Circuit for reconsideration or for a rehearing en banc was denied in full. On September 9, 2016, the Plaintiffs filed for a writ of certiorari, seeking review in the United States Supreme Court. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014. No amount has been accrued in connection with this matter as of September 30, 2016 , as the amount of loss, if any, cannot be reasonably estimated.

OneBeacon Pension Plan

OneBeacon Pension Plan	9 Months Ended
	Sep. 30, 2016
Compensation Related Costs [Abstract]
OneBeacon Pension Plan	Note 19. OneBeacon Pension Plan OneBeacon previously sponsored the OneBeacon qualified pension plan (the “Qualified Plan”). During the nine months ended September 30, 2016, the Qualified Plan finalized its termination by purchasing group annuity contracts from the Principal Financial Group (“Principal Financial”), and making lump sum distributions to Qualified Plan participants electing such payments, which eliminated the remaining Qualified Plan liability. In the third quarter of 2016, OneBeacon received a refund from Principal Financial related to the final settlement of the Qualified Plan and recognized a pre-tax settlement gain of $ 3.2 million . As of September 30, 2016, OneBeacon no longer has a projected benefit obligation with respect to the Qualified Plan. During 2016, OneBeacon transferred $ 47.1 million of excess invested assets from the Qualified Plan into the trust supporting the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (“KSOP”), which OneBeacon determined to be the Qualified Replacement Plan (“QRP”). As of September 30, 2016, an additional $ 12.6 million of excess invested assets remain in the Qualified Plan trust for potential post-termination obligations of that plan, as approved in a private letter ruling from the IRS received in the first quarter of 2016. The invested assets related to both the Qualified Plan and the QRP are included in other assets and are accounted for at fair value with related income recognized in net other revenues (expenses). OneBeacon continues to sponsor a non-qualified, non-contributory, defined benefit pension plan (“Non-qualified Plan”) covering certain employees who were employed as of December 31, 2001 and former employees who had met the eligibility requirements, as well as retirees. The Non-qualified Plan was frozen and curtailed in 2002, resulting in the pension benefit obligation being equal to the accumulated benefit obligation. The benefits are based primarily on years of service and employees’ compensation through December 31, 2002. OneBeacon’s funding policy is generally to contribute amounts to satisfy actual disbursements for the calendar year.

Summary of Significant Accoun27

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles 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, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. Fair Value Measurements On January 1, 2016, White Mountains adopted ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820), which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures. Amendments to Consolidation Analysis On January 1, 2016, White Mountains adopted ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and, with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718) . The new guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of September 30, 2016 , the unamortized debt issuance costs included in debt is $1.9 million . Recently Issued Accounting Pronouncements Short-Duration Contracts In May 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures. 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). The new guidance 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. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash, including reinsurance receivables, 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. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance. Stock Compensation In March 2016, the Board issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which is intended to simplify certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. The new guidance is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods with early adoption permitted. White Mountains does not expect a significant effect upon adoption. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this new guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new guidance requires all equity securities with readily determinable fair values 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 investments without readily determinable fair values to identify impairment, and for impaired equity security investments 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 has taken the fair value election for its portfolio of equity security investments 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. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as third party investment management fees charged by White Mountains Advisors as well as commissions and other non-insurance revenues. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (ASC 606) , which delayed the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows.

Significant Transactions (Table

Significant Transactions (Tables)	9 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reconciliation to the increase in book value	A reconciliation of the gain reported in discontinued operations to the impact to White Mountains's book value is as follows: Gain from sale of Tranzact reported in discontinued operations $ 51.9 Add back reclassification from continuing operations for the release of a tax valuation allowance 30.2 Increase to White Mountains book value from sale of Tranzact $ 82.1

Loss and Loss Adjustment Expe29

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2016
Insurance Loss Reserves [Abstract]
Loss and loss adjustment expense ("LAE") reserve activities	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Gross beginning balance $ 1,376.6 $ 1,350.8 $ 1,389.8 $ 1,342.2 Less beginning reinsurance recoverable on unpaid losses (162.8 ) (147.1 ) (186.0 ) (161.6 ) Net loss and LAE reserves 1,213.8 1,203.7 1,203.8 1,180.6 SSIE reserves held for sale at beginning of the period (1) 5.3 6.1 5.5 7.7 Loss and LAE incurred relating to: Current year losses 164.9 170.4 492.8 536.7 Prior year losses 0.1 (.4 ) 15.3 (2.6 ) Total incurred losses and LAE 165.0 170.0 508.1 534.1 Loss and LAE paid relating to: Current year losses (55.0 ) (53.0 ) (116.2 ) (120.2 ) Prior year losses (131.1 ) (108.1 ) (403.2 ) (383.5 ) Total loss and LAE payments (186.1 ) (161.1 ) (519.4 ) (503.7 ) SSIE reserves held for sale at end of the period (1) (5.4 ) (5.5 ) (5.4 ) (5.5 ) Net ending balance 1,192.6 1,213.2 1,192.6 1,213.2 Plus ending reinsurance recoverable on unpaid losses 169.4 214.2 169.4 214.2 Gross ending balance $ 1,362.0 $ 1,427.4 $ 1,362.0 $ 1,427.4 (1) See Note 17 - “Held for Sale and Discontinued Operations” .

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Investment income: Fixed maturity investments $ 21.3 $ 13.7 $ 53.7 $ 38.5 Short-term investments .2 — .8 .1 Common equity securities 1.3 1.4 3.4 5.7 Other long-term investments .1 2.0 3.5 2.9 Total investment income 22.9 17.1 61.4 47.2 Third-party investment expenses (.4 ) (.3 ) (1.8 ) (4.6 ) Net investment income, pre-tax $ 22.5 $ 16.8 $ 59.6 $ 42.6
Schedule of Realized and Unrealized Gain (Loss) on Investments	Net realized and unrealized investment gains (losses) consisted of the following: Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Net realized investment gains (losses), pre-tax $ 8.0 $ (7.4 ) $ 267.5 $ 49.7 Net unrealized investment gains (losses), pre-tax 18.4 (36.5 ) (183.5 ) (83.6 ) Net realized and unrealized investment gains (losses), pre-tax 26.4 (43.9 ) 84.0 (33.9 ) Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) (5.4 ) 9.4 (23.8 ) 6.9 Net realized and unrealized investment gains (losses), after tax $ 21.0 $ (34.5 ) $ 60.2 $ (27.0 )
Realized Gain (Loss) on Investments	Net realized investment gains (losses) for the three and nine months ended September 30, 2016 and 2015 consisted of the following: Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 2.8 $ — $ 2.8 $ (.1 ) $ — $ (.1 ) Short-term investments .2 — .2 — — — Common equity securities 6.4 — 6.4 (5.9 ) — (5.9 ) Other long-term investments (1.4 ) — (1.4 ) (1.4 ) — (1.4 ) Net realized investment gains (losses), pre-tax 8.0 — 8.0 (7.4 ) — (7.4 ) Income tax (expense) benefit attributable to net realized investment gains (losses) (2.7 ) — (2.7 ) 1.5 — 1.5 Net realized investment gains (losses), after tax $ 5.3 $ — $ 5.3 $ (5.9 ) $ — $ (5.9 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 3.5 $ — $ 3.5 $ 2.0 $ — $ 2.0 Short-term investments .4 — .4 — — — Common equity securities 267.7 — 267.7 41.6 .4 42.0 Other long-term investments (4.1 ) — (4.1 ) 5.7 — 5.7 Net realized investment gains, pre-tax 267.5 — 267.5 49.3 .4 49.7 Income tax expense attributable to net realized investment gains (46.0 ) — (46.0 ) (15.7 ) — (15.7 ) Net realized investment gains, after tax $ 221.5 $ — $ 221.5 $ 33.6 $ .4 $ 34.0
Schedule of unrealized gain (loss) on investments	The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value: Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Net unrealized (losses) gains Net gains Total net unrealized (losses) gains reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (4.4 ) $ — $ (4.4 ) $ 3.3 $ — $ 3.3 Common equity securities 12.5 .2 12.7 (30.6 ) (.2 ) (30.8 ) Other long-term investments 10.0 .1 10.1 (9.0 ) — (9.0 ) Net unrealized investment gains (losses), pre-tax 18.1 .3 18.4 (36.3 ) (.2 ) (36.5 ) Income tax (expense) benefit (2.7 ) — (2.7 ) 7.9 — 7.9 Net unrealized investment $ 15.4 $ .3 $ 15.7 $ (28.4 ) $ (.2 ) $ (28.6 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Net unrealized gains (losses) Net gains Total net unrealized gains (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ 38.0 $ — $ 38.0 $ 1.3 $ — $ 1.3 Common equity securities (247.9 ) 2.6 (245.3 ) (56.0 ) (2.5 ) (58.5 ) Other long-term investments 23.5 .3 23.8 (25.6 ) (.8 ) (26.4 ) Net unrealized investment (losses) gains, pre-tax (186.4 ) 2.9 (183.5 ) (80.3 ) (3.3 ) (83.6 ) Income tax benefit attributable to net unrealized investment (losses) 22.2 — 22.2 22.6 — 22.6 Net unrealized investment $ (164.2 ) $ 2.9 $ (161.3 ) $ (57.7 ) $ (3.3 ) $ (61.0 )
Net unrealized investment gains (losses) for Level 3 investments	The following table summarizes the amount of total gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Fixed maturity investments $ .3 $ (1.0 ) $ 1.6 $ (.9 ) Common equity securities — 1.1 — 4.0 Other long-term investments 7.8 (6.8 ) 20.7 (6.6 ) Total unrealized investment gains (losses), pre-tax - Level 3 investments $ 8.1 $ (6.7 ) $ 22.3 $ (3.5 )
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, 2016 and December 31, 2015 were as follows: September 30, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value U.S. Government and agency obligations $ 96.6 $ .4 $ — $ — $ 97.0 Debt securities issued by corporations 1,212.2 18.7 (1.5 ) — 1,229.4 Municipal obligations 306.3 6.2 (.2 ) — 312.3 Mortgage and asset-backed securities 2,406.6 9.6 (1.7 ) — 2,414.5 Foreign government, agency and provincial obligations 1.0 .2 — — 1.2 Preferred stocks 78.3 7.1 — — 85.4 Total fixed maturity investments $ 4,101.0 $ 42.2 $ (3.4 ) $ — $ 4,139.8 Less: fixed maturity investments reclassified to assets held for sale related to SSIE 8.3 Total fixed maturity investments $ 4,131.5 December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value U.S. Government and agency obligations $ 160.4 $ — $ (.4 ) $ — $ 160.0 Debt securities issued by corporations 1,001.0 4.3 (5.3 ) — 1,000.0 Municipal obligations 227.8 2.2 (1.2 ) — 228.8 Mortgage and asset-backed securities 1,170.6 2.0 (5.6 ) — 1,167.0 Foreign government, agency and provincial obligations 1.0 .2 — — 1.2 Preferred stocks 78.3 4.4 — — 82.7 Total fixed maturity investments $ 2,639.1 $ 13.1 $ (12.5 ) $ — $ 2,639.7 Less: fixed maturity investments reclassified to assets held for sale related to SSIE 9.5 Total fixed maturity investments $ 2,630.2 The cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses), and carrying values of White Mountains’s common equity securities and other long-term investments as of September 30, 2016 and December 31, 2015 were as follows: September 30, 2016 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 378.9 $ 50.7 $ (4.6 ) $ — $ 425.0 Other long-term investments $ 317.1 $ 42.7 $ (6.4 ) $ (2.0 ) $ 351.4 December 31, 2015 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 822.5 $ 302.8 $ (11.4 ) $ — $ 1,113.9 Other long-term investments $ 304.5 $ 32.0 $ (18.4 ) $ (2.3 ) $ 315.8 Other Long-term Investments Other long-term investments consist of the following as of September 30, 2016 and December 31, 2015 : Carrying Value at Millions September 30, 2016 December 31, 2015 Hedge funds and private equity funds, at fair value (1) $ 143.0 $ 127.8 Private equity securities and limited liability companies, at fair value (1) 84.6 82.1 Surplus note investments, at fair value (1) 71.5 51.5 Private convertible preferred securities (1) 33.1 32.7 Tax advantaged federal affordable housing development fund (2) 12.8 14.7 Partnership investments accounted for under the equity method 3.2 3.8 Other 3.2 3.2 Total other-long term investments $ 351.4 $ 315.8 (1) See Fair Value Measurements by Level table. (2) Fund accounted for using the proportional amortization method.
Other long-term investments	The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of September 30, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short banks and financial $ 34.6 $ — $ 12.8 $ — Long/short equity REIT 20.4 — 20.6 — Other 3.4 — 3.6 — Total hedge funds 58.4 — 37.0 — Private equity funds Manufacturing/Industrial 27.7 21.8 24.9 2.5 Aerospace/Defense/Government 23.1 21.8 19.8 30.3 Energy infrastructure & services 14.2 3.2 20.7 3.4 Multi-sector 10.4 2.0 14.8 2.1 Healthcare 3.8 .4 3.8 .4 Private equity secondaries 3.7 2.1 4.4 2.1 Insurance .8 41.3 2.0 41.3 Direct lending/Mezzanine debt .6 36.9 — — Real estate .3 .1 .4 .1 Total private equity funds 84.6 129.6 90.8 82.2 Total hedge funds and private equity funds included in other long-term investments $ 143.0 $ 129.6 $ 127.8 $ 82.2
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following summarizes the September 30, 2016 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds: Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice 90-119 days notice Total Monthly $ — $ — $ — $ — Quarterly 35.6 — — 35.6 Semi-annual — 20.4 — 20.4 Annual — — 2.4 2.4 Total $ 35.6 $ 20.4 $ 2.4 $ 58.4
Fair Value of private equity funds subject to lock-up periods	As of September 30, 2016 , investments in private equity funds were subject to lock-up periods as follows: Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $23.5 $3.8 $56.3 $1.0 $84.6
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 surplus notes as of September 30, 2016 and December 31, 2015: Type of Surplus Note Total as of September 30, 2016 Total as of December 31, 2015 Millions Seller Priority Pari Passu Par Value $ 57.9 $ 43.1 $ 101.0 $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 5.2 (4.4 ) .8 (15.1 ) Regulatory approval (2) (4.2 ) (11.7 ) (15.9 ) (24.2 ) Liquidity adjustment (3) (9.9 ) (4.5 ) (14.4 ) (10.2 ) Total adjustments (8.9 ) (20.6 ) (29.5 ) (49.5 ) Fair value (4) $ 49.0 $ 22.5 $ 71.5 $ 51.5 (1) Represents the value of the surplus notes, at current market yields on comparable publicly traded debt, and assuming issuer is allowed to make principal and interest payments when its financial capacity is available, as measured by statutory capital in excess of a 250% RBC score. (2) Represents anticipated timing of securing regulatory approvals for interest and principal payments to reflect graduated changes in Issuer’s statutory surplus. The monetary impact of the anticipated timing is measured based on credit spreads of publicly traded debt with roughly equivalent percentages of discounted payments missed. (3) Represents impact of liquidity spread to account for OneBeacon’s sole ownership of the surplus notes, lack of a trading market and ongoing regulatory approval risk. (4) The increase in the fair value of the surplus notes during the nine months ended September 30, 2016 was driven primarily by the narrowing of non-investment grade credit spreads as well as the time value of money benefit generated by moving nine months closer to modeled cash receipts, partially offset by the impact of an interest payment received in the first quarter of 2016.
Fair value measurements by level, investment securities	The following tables summarize White Mountains’s fair value measurements for investments as of September 30, 2016 and December 31, 2015 by level. The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing mortgage-backed or 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. September 30, 2016 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 97.0 $ 87.5 $ 9.5 $ — Debt securities issued by corporations: Consumer 340.5 — 340.5 — Health Care 193.3 — 193.3 — Financials 170.8 — 170.8 — Industrial 132.2 — 132.2 — Utilities 97.1 — 97.1 — Communications 89.4 — 89.4 — Technology 79.6 — 79.6 — Materials 72.9 — 72.9 — Energy 53.6 — 53.6 — Total debt securities issued by corporations 1,229.4 — 1,229.4 — Mortgage and asset-backed securities 2,414.5 — 2,378.5 36.0 Municipal obligations 312.3 — 312.3 — Foreign government, agency and provincial obligations 1.2 .6 .6 — Preferred stocks 85.4 — 13.8 71.6 Total fixed maturity investments (4) 4,139.8 88.1 3,944.1 107.6 Short-term investments (4)(5) 305.6 278.6 27.0 — Common equity securities: Exchange traded funds (1) 113.2 90.5 22.7 — Consumer 111.2 111.2 — — Communications 33.3 33.3 — — Health Care 33.2 33.2 — — Technology 28.1 28.1 — — Financials 11.4 11.4 — — Industrial 11.1 11.1 — — Energy 5.5 5.5 — — Other 78.0 — 78.0 — Total common equity securities 425.0 324.3 100.7 — Other long-term investments (2)(3) 192.4 — — 192.4 Total investments (4) $ 5,062.8 $ 691.0 $ 4,071.8 $ 300.0 (1) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (2) Excludes carrying value of $3.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $12.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (3) Excludes carrying value of $143.0 associated with hedge funds and private equity funds for which fair value is measured at net asset value using the practical expedient. (4) Includes carrying value of $8.3 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE. (5) Short-term investments are measured at amortized cost, which approximates fair value. December 31, 2015 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 160.0 $ 133.4 $ 26.6 $ — Debt securities issued by corporations: Consumer 253.3 — 253.3 — Financials 175.9 — 175.9 — Health Care 151.3 — 151.3 — Industrial 135.6 — 135.6 — Energy 82.0 — 82.0 — Utilities 61.5 — 61.5 — Technology 60.0 — 60.0 — Communications 49.2 — 49.2 — Materials 31.2 — 31.2 — Total debt securities issued by corporations 1,000.0 — 1,000.0 — Mortgage and asset-backed securities 1,167.0 — 1,167.0 — Municipal obligations 228.8 — 228.8 — Foreign government, agency and provincial obligations 1.2 .6 .6 — Preferred stocks 82.7 — 12.7 70.0 Total fixed maturity investments (4) 2,639.7 134.0 2,435.7 70.0 Short-term investments (4)(5) 211.3 211.3 — — Common equity securities: Financials 653.2 653.2 — — Exchange traded funds (1) 183.3 162.0 21.3 — Consumer 70.0 70.0 — — Communications 43.7 43.7 — — Health Care 35.7 35.7 — — Technology 27.0 27.0 — — Industrial 26.6 26.6 — — Other 74.4 — 74.4 — Total common equity securities 1,113.9 1,018.2 95.7 — Other long-term investments (2)(3) 169.5 — — 169.5 Total investments (4) $ 4,134.4 $ 1,363.5 $ 2,531.4 $ 239.5 (1) ETFs traded on foreign exchanges are priced using the fund's published NAV to account for the difference in market close times and are therefore designated a level 2 measurement. (2) Excludes carrying value of $3.8 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $14.7 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (3) Excludes carrying value of $127.8 associated with hedge funds and private equity funds for which fair value is measured at net asset value using the practical expedient. (4) Includes carrying value of $9.5 in fixed maturity investments and $0.1 in short-term investments that are classified as assets held for sale related to SSIE.
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, 2016 and December 31, 2015 : Fair Value at Millions September 30, 2016 December 31, 2015 AAA $ — $ — AA 106.4 95.2 A 331.3 397.7 BBB 602.3 507.1 BB 173.0 — B 16.4 — Debt securities issued by corporations (1) $ 1,229.4 $ 1,000.0 (1) Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor’s and 2) Moody’s.
Mortgage-backed, asset-backed securities	September 30, 2016 December 31, 2015 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 301.1 $ 301.1 $ — $ 265.5 $ 265.5 $ — FNMA 239.4 239.4 — 42.2 42.2 — FHLMC 35.5 35.5 — 22.8 22.8 — Total Agency (1) 576.0 576.0 — 330.5 330.5 — Non-agency: Residential 213.4 177.4 36.0 133.2 133.2 — Commercial 113.7 113.7 — 140.4 140.4 — Total Non-agency 327.1 291.1 36.0 273.6 273.6 — Total mortgage-backed securities 903.1 867.1 36.0 604.1 604.1 — Other asset-backed securities: Credit card receivables 534.0 534.0 — 217.7 217.7 — Vehicle receivables 721.8 721.8 — 269.7 269.7 — Other 255.6 255.6 — 75.5 75.5 — Total other asset-backed securities 1,511.4 1,511.4 — 562.9 562.9 — Total mortgage and asset-backed securities $ 2,414.5 $ 2,378.5 $ 36.0 $ 1,167.0 $ 1,167.0 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of September 30, 2016 are as follows: Security Issuance Year Millions Fair Value 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Non-agency RMBS $ 213.4 $ 20.6 $ 6.0 $ 3.2 $ — $ 3.0 $ — $ 8.6 $ 11.0 $ 6.0 $ 11.4 $ 59.6 $ 48.0 $ 36.0 Non-agency CMBS 113.7 — — — — — — 4.6 — 18.3 11.6 23.5 45.9 9.8 Total $ 327.1 $ 20.6 $ 6.0 $ 3.2 $ — $ 3.0 $ — $ 13.2 $ 11.0 $ 24.3 $ 23.0 $ 83.1 $ 93.9 $ 45.8
Non-agency residential mortgage securities, collateral quality and tranche levels	The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of September 30, 2016 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 213.4 $ 122.8 $ 90.6 $ — Non-prime — — — — Sub-prime — — — — Total $ 213.4 $ 122.8 $ 90.6 $ — (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to other “AAA” or “Aaa” bonds. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.
Non-agency commercial mortgage securities, type of interest rate and tranche levels	The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of September 30, 2016 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Fixed rate CMBS $ 102.3 $ 8.9 $ 55.2 $ 38.2 Floating rate CMBS 11.4 — — 11.4 Total $ 113.7 $ 8.9 $ 55.2 $ 49.6 (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings (“Fitch”) and were senior to other “AAA” or “Aaa” bonds. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds.
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, 2016 and 2015 : Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Common equity securities Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (3) Total Balance at January 1, 2016 $ 1,152.2 $ 2,531.4 $ 70.0 $ — $ 169.5 $ 127.8 $ 4,050.9 (1)(2)(4) Total realized and unrealized gains (losses) 20.0 42.7 1.6 — 20.7 (1.0 ) 84.0 Amortization/Accretion — (13.2 ) — — — — (13.2 ) Purchases 1,577.5 2,989.4 120.8 — 2.2 33.0 4,722.9 Sales (2,337.3 ) (1,587.5 ) (2.8 ) — — (16.8 ) (3,944.4 ) Transfers in — 82.0 — — — — 82.0 Transfers out — — (82.0 ) — — — (82.0 ) Balance at $ 412.4 $ 4,044.8 $ 107.6 $ — $ 192.4 $ 143.0 $ 4,900.2 (1)(2)(4) (1) Excludes carrying value of $3.8 and $3.2 at January 1, 2016 and September 30, 2016 associated with other long-term investments accounted for using the equity method. Excludes carrying value of $14.7 and $12.8 at January 1, 2016 and September 30, 2016 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (2) Excludes carrying value of $ 211.3 and $ 305.6 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 . (3) Investments for which fair value is measured at net asset value using the practical expedient are no longer classified within the fair value hierarchy. (See Note 1 - “Summary of Significant Accounting Policies” ). (4) Includes carrying value of $9.5 and $ 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. Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Common equity securities Other long-term investments Hedge Funds and Private Equity Funds measured at NAV (4) Total Balance at January 1, 2015 $ 550.6 $ 2,372.9 $ 76.4 $ 39.5 $ 126.9 $ 177.3 $ 3,343.6 (1)(2)(5) Total realized and unrealized (losses) gains (19.5 ) .6 (.9 ) 7.7 (13.5 ) (7.1 ) (32.7 ) (3) Amortization/Accretion — (15.2 ) — — — — (15.2 ) Purchases 550.0 1,056.0 35.3 — 22.6 8.1 1,672.0 Sales (618.2 ) (894.4 ) — (9.4 ) (2.3 ) (16.6 ) (1,540.9 ) Effect of redemption of Prospector hedge funds (43.5 ) — — (3.6 ) — (14.3 ) (61.4 ) Transfers in — 41.7 — — — — 41.7 Transfers out (1.2 ) — (40.5 ) — — — (41.7 ) Balance at $ 418.2 $ 2,561.6 $ 70.3 $ 34.2 $ 133.7 $ 147.4 $ 3,365.4 (1)(2)(5) (1) Excludes carrying value of $ 5.2 and $3.9 at January 1, 2015 and September 30, 2015 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $16.8 and $15.2 at January 1, 2015 and September 30, 2015 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (2) Excludes carrying value of $376.8 and $ 325.0 at January 1, 2015 and September 30, 2015 associated with short-term investments of which $1.9 and $0.6 is classified as held for sale at January 1, 2015 and September 30, 2015 . (3) Excludes $0.8 realized and unrealized losses associated with the Prospector Offshore Fund consolidation of investment-related liabilities. (4) Investments for which fair value is measured at net asset value using the practical expedient are no longer classified within the fair value hierarchy. (See Note 1 - “Summary of Significant Accounting Policies” ). (5) Includes carrying value of $10.1 and $9.7 of fixed maturity investments at January 1, 2015 and September 30, 2015 that is classified as assets held for sale related to SSIE.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 as of September 30, 2016 and December 31, 2015 . The fair value of investments in hedge funds and private equity funds are generally estimated using the net asset value of the funds. Description September 30, 2016 $ in millions, except share price Rating (2) Valuation Technique(s) Fair (3) Unobservable Input Preferred Stock (1) NR Discounted cash flow $71.6 Discount yield - 6.25% Non-agency residential mortgage-backed securities (1) AAA Broker pricing $36.0 Broker quote Private equity security (1) NR Share price of most recent transaction $21.0 Share price - $1.00 Private equity security (1) NR Share price of most recent transaction $33.8 Share price - $1.03 Private equity security (1) NR Share price of most recent transaction $3.2 Share price - $2.52 Convertible preferred security (1) NR Multiple of EBITDA $6.1 EBITDA multiple - 6.00 Convertible preferred security (1) NR Share price of most recent transaction $27.0 Share price - $3.83 Private equity security (1) NR Option pricing method $9.9 Time until expiration - 4 years Volatility/Standard deviation - 60.0% Risk free rate - 1.15% Surplus notes: NR - Seller priority Discounted cash flow $49.0 Discount rate (4) - 9.5% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 10 years - Pari passu Discounted cash flow $22.5 Discount rate (5) - 15.8% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 15 years (1) As of September 30, 2016 , each 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 unrealized gains and losses associated with foreign currency; foreign currency effects based on observable inputs. (4) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note. (5) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note. (6) OneBeacon has assumed for valuation purposes that subsequent interest payments will begin in year five (2020) and principal repayments begin on a graduated basis in year ten (2025) for the seller priority note and year fifteen (2030) for the pari passu note. Description December 31, 2015 $ in millions, except share price Rating (2) Valuation Technique(s) Fair (3) Unobservable Input Preferred Stock (1)(7) NR Par value (7) $70.0 Issuer’s intent to call - $70.0 Private equity security (1) NR Share price of most recent transaction $21.0 Share price - $1.00 Private equity security (1) NR Share price of most recent transaction $33.8 Share price - $1.03 Convertible preferred security (1) NR Multiple of EBITDA $5.7 EBITDA multiple - 6.00 Convertible preferred security (1) NR Share price of most recent transaction $27.0 Share price - $3.83 Private equity security (1) NR Option pricing method $9.6 Time until expiration - 4 years Volatility/Standard deviation - 60.0% Risk free rate - 1.15% Surplus notes: NR - Seller priority Discounted cash flow $38.0 Discount rate (4) - 13.0% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 10 years - Pari passu Discounted cash flow $13.5 Discount rate (5) - 22.4% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 15 years (1) As of December 31, 2015 each 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 unrealized gains and losses associated with foreign currency; foreign currency effects based on observable inputs. (4) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note. (5) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note. (6) OneBeacon has assumed for valuation purposes that subsequent interest payments will begin in year five (2020) and principal repayments begin on a graduated basis in year ten (2025) for the seller priority note and year fifteen (2030) for the pari passu note. (7) Valuation based on the issuer’s intent as of December 31, 2015 to call the security in the near term.

Goodwill and Other Intangible31

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	The following table shows the change in goodwill and other intangible assets: Three Months Ended September 30, Millions 2016 2015 Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 24.1 $ 29.2 53.3 $ 24.1 $ 36.4 $ 60.5 Amounts held for sale at beginning of the period (1) — — — — .6 .6 Amortization, including foreign currency translation — (3.0 ) (3.0 ) — (2.6 ) (2.6 ) Less: Amounts held for sale at end of the period (1) — — — — .5 .5 Subtotal 24.1 26.2 50.3 24.1 33.9 58.0 Acquisition of business 9.0 (2) — Ending balance $ 59.3 $ 58.0 Nine Months Ended September 30, Millions 2016 2015 Goodwill Other intangible assets Total Goodwill Other intangible assets Total Beginning balance $ 24.1 $ 31.3 $ 55.4 $ 23.8 $ 38.7 $ 62.5 Amounts held for sale at beginning of the period (1) — .4 .4 — .8 .8 Acquisitions of businesses — — — .3 2.8 3.1 Acquisitions of other intangible assets — 3.9 3.9 — — — Amortization, including foreign currency translation — (9.4 ) (9.4 ) — (7.9 ) (7.9 ) Less: Amounts held for sale at end of the period (1) — — — — .5 .5 Subtotal 24.1 26.2 50.3 24.1 33.9 58.0 Acquisition of business 9.0 (2) — Ending balance $ 59.3 $ 58.0 (1) See Note 17 - “Held for Sale and Discontinued Operations” . (2) The relative fair values of goodwill and of the other intangible assets recognized in connection with the acquisition of Buzzmove had not yet been determined at September 30, 2016.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Schedule of debt outstanding	White Mountains’s debt outstanding as of September 30, 2016 and December 31, 2015 consisted of the following: Millions September 30, Effective Rate (1) December 31, Effective Rate (1) WTM Bank Facility $ — N/A $ 50.0 3.9% OBH Senior Notes, at face value 275.0 4.7% 275.0 4.7% Unamortized original issue discount and debt issuance costs (1.9 ) (2.1 ) OBH Senior Notes, carrying value 273.1 272.9 OneBeacon Bank Facility — N/A — N/A MediaAlpha Bank Facility 15.6 5.8% 15.0 5.5% Unamortized issuance cost (.2 ) (.3 ) MediaAlpha Bank Facility, carrying value 15.4 14.7 Total debt $ 288.5 $ 337.6 (1) Effective rate considers the effect of the debt issuance costs.

Derivatives (Tables)

Derivatives (Tables) - Variable Annuity	9 Months Ended
	Sep. 30, 2016
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 and 2015 . Three Months Ended Nine Months Ended September 30, September 30, Millions 2016 2015 2016 2015 Fees, included in other revenue $ — $ 2.1 $ 1.2 $ 7.7 Change in fair value of variable annuity liability, included in other revenue — (2.8 ) (.3 ) (1.7 ) Change in fair value of derivatives, included in other revenue — 1.5 (2.0 ) (6.3 ) Foreign exchange, included in other revenue — .4 1.4 (.8 ) Other investment income and losses — (.2 ) — (.3 ) Total revenue — 1.0 .3 (1.4 ) Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses — — (.3 ) — Death benefit claims paid, included in general and administrative expenses — — — — General and administrative expenses (.5 ) (.8 ) (2.4 ) (3.1 ) Pre-tax income (loss) $ (.5 ) $ .2 $ (2.4 ) $ (4.5 )
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 2015 and the carrying values, included in other assets, as of December 31, 2015 by type of instrument: Gains (losses) Three Months Ended Nine Months Ended Carrying Value September 30, September 30, As of Millions 2016 2015 2016 2015 December 31, 2015 Fixed income/interest rate $ — $ 2.6 $ 1.8 $ 6.0 $ .5 Foreign exchange — (3.1 ) (4.8 ) (6.0 ) 14.8 Equity — 2.0 1.0 (6.3 ) 4.8 Total $ — $ 1.5 $ (2.0 ) $ (6.3 ) $ 20.1
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 and the three and nine months ended September 30, 2015. Nine Months Ended September 30, 2016 Variable Annuity Assets (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 gains (losses) (.3 ) 2.9 (.7 ) (4.2 ) (2.0 ) Transfers in — — — — — Sales/settlements — (5.6 ) (15.8 ) 3.3 (18.1 ) End of period $ — $ — $ — $ — $ — Three Months Ended September 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Purchases — — — — — Realized and unrealized gains (losses) (2.8 ) 3.4 3.2 (5.1 ) 1.5 Transfers in — — — — — Sales/settlements — (.5 ) (7.8 ) 8.1 (.2 ) End of period $ (1.0 ) $ 11.2 $ 27.9 $ (2.1 ) $ 37.0 Nine Months Ended September 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized gains (losses) (1.7 ) (5.5 ) (2.7 ) 1.9 (6.3 ) Transfers in — — — — — Sales/settlements — (2.2 ) (3.2 ) (7.7 ) (13.1 ) End of period $ (1.0 ) $ 11.2 $ 27.9 $ (2.1 ) $ 37.0 (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.
Assets Held under Reinsurance Counterparty Trust Agreement	The total collateral includes the following: Millions December 31, 2015 September 30, 2015 Cash $ 5.8 $ 16.8 Fixed maturity investments — — Total $ 5.8 $ 16.8
Exchange traded and OTC derivative instruments	The following summarizes amounts offset under master netting agreements: December 31, 2015 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Interest rate contracts OTC $ 2.4 $ (2.1 ) $ .3 Exchange traded .1 (.1 ) — Foreign exchange contracts OTC 15.0 — 15.0 Exchange traded .1 (.3 ) (.2 ) Equity contracts OTC 4.4 (.6 ) 3.8 Exchange traded 1.2 — 1.2 Total (2) $ 23.2 $ (3.1 ) $ 20.1 (1) Amount equal to fair value of instrument as recognized in other assets (2) All derivative instruments held by WM Life Re were subject to master netting arrangements.
Net exposure to collateral	The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets as of December 31, 2015 : December 31, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counter-party - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 8.5 $ — $ — $ 8.5 $ — $ — $ 5.5 $ 3.0 A + Bank of America .7 — — .7 — — — .7 A Citigroup - OTC 9.9 — — 9.9 — — 1.4 8.5 A Citigroup - Exchange Traded 1.0 — — 1.0 5.8 — — 6.8 A Total $ 20.1 $ — $ — $ 20.1 $ 5.8 $ — $ 6.9 $ 19.0 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	9 Months Ended
	Sep. 30, 2016
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table provides a schedule of BAM’s insured obligations: September 30, 2016 December 31, 2015 Contracts outstanding 4,357 3,103 Remaining weighted average contract period outstanding (in years) 10.9 12.8 Contractual debt service outstanding (in millions): Principal $ 30,470.0 $ 22,556.0 Interest 15,258.1 11,984.4 Total debt service outstanding $ 45,728.1 $ 34,540.4 Gross unearned insurance premiums $ 71.0 $ 50.2
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, 2016 : Millions September 30, 2016 October 1, 2016 - December 31, 2016 $ 1.6 January 1, 2017 - March 31, 2017 1.6 April 1, 2017 - June 30, 2017 1.6 July 1, 2017 - September 30, 2017 1.6 October 1, 2017 - December 31, 2017 1.5 6.3 2018 6.0 2019 5.7 2020 5.4 2021 and thereafter 46.0 Total $ 71.0

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Computation of earnings per share	The following table outlines the Company’s computation of earnings per share from continuing operations for the three and nine months ended September 30, 2016 and 2015 . (See Note 17 - “Held for Sale and Discontinued Operations” ). Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Basic and diluted earnings per share numerators (in millions): Net income (loss) from continuing operations attributable to White Mountains’s common shareholders $ 29.8 $ (65.4 ) $ 24.2 $ (47.6 ) Allocation of income for unvested restricted common shares (.4 ) — (.3 ) — Dividends declared on participating restricted common shares (1) — — (.1 ) (.1 ) Total allocation to restricted common shares (.4 ) — (.4 ) (.1 ) Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 23.8 $ (47.7 ) Undistributed net earnings (in millions): Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 23.8 $ (47.7 ) Dividends declared net of restricted common share amounts (1) — — (5.4 ) (5.9 ) Total undistributed net earnings, net of restricted common share amounts $ 29.4 $ (65.4 ) $ 18.4 $ (53.6 ) Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,867.4 5,890.1 5,166.6 5,951.1 Average unvested restricted shares (2) (68.1 ) — (62.9 ) — Basic earnings per share denominator 4,799.3 5,890.1 5,103.7 5,951.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (3) 4,879.4 5,890.1 5,174.8 5,951.1 Average unvested restricted common shares (2) (68.1 ) — (62.9 ) — Diluted earnings per share denominator (3) 4,811.3 5,890.1 5,111.9 5,951.1 Basic earnings per share (in dollars): Net income (loss) attributable to White Mountains’s common shareholders $ 6.12 $ (11.10 ) $ 4.67 $ (8.01 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ 6.12 $ (11.10 ) $ 3.67 $ (9.01 ) Diluted earnings per share (in dollars): Net income (loss) attributable to White Mountains’s common shareholders $ 6.11 $ (11.10 ) $ 4.66 $ (8.01 ) Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ 6.11 $ (11.10 ) $ 3.66 $ (9.01 ) (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans”) . (3) The diluted earnings per share denominator for the three 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 results in 11,943 and 8,208 incremental shares outstanding over the periods.

Non-controlling Interest (Table

Non-controlling Interest (Tables)	9 Months Ended
	Sep. 30, 2016
Noncontrolling Interest [Abstract]
Noncontrolling Interest Disclosure	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, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 23.9 % $ 247.0 24.5 % $ 245.6 SIG Preference Shares — — 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 16.9 3.1 17.1 MediaAlpha 40.0 12.5 40.0 14.4 Tranzact — — 36.8 79.4 Wobi 3.3 .5 3.9 .6 Dewar 18.8 3.7 19.0 3.7 Buzzmove 29.1 3.3 — — Total other, excluding mutuals and reciprocals 36.9 115.2 Mutuals and reciprocals BAM 100.0 (138.9 ) 100.0 (140.0 ) SSIE 100.0 4.2 100.0 (16.0 ) Total mutuals and reciprocals (134.7 ) (156.0 ) Total non-controlling interests $ 149.2 $ 454.8

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2016
Segment Reporting [Abstract]
Financial information for White Mountains' segments	Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Three Months Ended September 30, 2016 Earned insurance premiums $ 277.9 $ 1.2 $ .3 $ 1.9 $ 281.3 Net investment income 11.8 .6 1.7 8.4 22.5 Net investment income (loss) - surplus note interest — 4.5 (4.5 ) — — Net realized and unrealized investment gains (losses) 15.5 (.3 ) (1.6 ) 12.8 26.4 Other revenue 1.8 — .4 32.6 (2) 34.8 Total revenues 307.0 6.0 (3.7 ) 55.7 365.0 Losses and LAE 162.8 — — 2.2 165.0 Insurance acquisition expenses 55.1 .2 .6 .5 56.4 Other underwriting expenses 49.4 — .1 .1 49.6 General and administrative expenses 3.2 .6 9.3 54.1 (3) 67.2 Amortization of other intangible assets .3 — — 2.7 3.0 Interest expense 3.3 — — .5 3.8 Total expenses 274.1 .8 10.0 60.1 345.0 Pre-tax income (loss) $ 32.9 $ 5.2 $ (13.7 ) $ (4.4 ) $ 20.0 HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Nine Months Ended September 30, 2016 Earned insurance premiums $ 827.9 $ 3.1 $ 1.0 $ 6.1 $ 838.1 Net investment income 38.3 1.6 5.1 14.6 59.6 Net investment income (loss) - surplus note interest — 13.4 (13.4 ) — — Net realized and unrealized investment gains 56.8 2.3 6.5 18.4 84.0 Other revenue 3.5 — .8 106.5 (2) 110.8 Total revenues 926.5 20.4 — 145.6 1,092.5 Losses and LAE 501.3 — — 6.8 508.1 Insurance acquisition expenses 154.8 .6 1.9 1.9 159.2 Other underwriting expenses 155.6 — .3 .1 156.0 General and administrative expenses 10.0 1.4 28.1 181.5 (3) 221.0 Amortization of other intangible assets .9 — — 8.5 9.4 Interest expense 9.8 — — 2.6 12.4 Total expenses 832.4 2.0 30.3 201.4 1,066.1 Pre-tax income (loss) $ 94.1 $ 18.4 $ (30.3 ) $ (55.8 ) $ 26.4 HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Three Months Ended September 30, 2015 Earned insurance premiums $ 281.4 $ .6 $ .3 $ 2.6 $ 284.9 Net investment income 12.4 .5 1.1 2.8 16.8 Net investment income (loss) - surplus note interest — 4.0 (4.0 ) — — Net realized and unrealized investment (losses) gains (29.9 ) .6 1.7 (16.3 ) (43.9 ) Other revenue 3.9 — .2 34.2 (2) 38.3 Total revenues 267.8 5.7 (.7 ) 23.3 296.1 Losses and LAE 167.5 — — 2.5 170.0 Insurance acquisition expenses 53.8 .1 .4 1.0 55.3 Other underwriting expenses 56.4 — .1 — 56.5 General and administrative expenses 3.4 .4 9.4 81.5 (3) 94.7 Amortization of other intangible assets .3 — — 2.3 2.6 Interest expense 3.2 — — .7 3.9 Total expenses 284.6 .5 9.9 88.0 383.0 Pre-tax (loss) income $ (16.8 ) $ 5.2 $ (10.6 ) $ (64.7 ) $ (86.9 ) HG Global/BAM Millions OneBeacon HG BAM (1) Other Operations Total Nine Months Ended September 30, 2015 Earned insurance premiums $ 887.3 $ 1.7 $ .6 $ 6.4 $ 896.0 Net investment income 33.0 1.4 2.9 5.3 42.6 Net investment income (loss) - surplus note interest — 11.9 (11.9 ) — — Net realized and unrealized investment (losses) gains (29.7 ) .3 2.8 (7.3 ) (33.9 ) Other (loss) revenue (.4 ) — .5 114.3 (2) 114.4 Total revenues 890.2 15.3 (5.1 ) 118.7 1,019.1 Losses and LAE 527.9 — — 6.2 534.1 Insurance acquisition expenses 161.2 .3 1.8 2.8 166.1 Other underwriting expenses 165.2 — .3 — 165.5 General and administrative expenses 11.0 1.2 26.3 185.2 (3) 223.7 Amortization of other intangible assets 1.0 — — 6.9 7.9 Interest expense 9.7 — — 1.1 10.8 Total expenses 876.0 1.5 28.4 202.2 1,108.1 Pre-tax income (loss) $ 14.2 $ 13.8 $ (33.5 ) $ (83.5 ) $ (89.0 ) (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services. (2) Includes $ 27.6 and $ 88.4 from MediaAlpha for the three months and nine months ended September 30, 2016 , and $ 28.3 and $ 81.2 from MediaAlpha for the three months and nine months ended September 30, 2015 . (3) Includes $ 26.2 and $ 82.7 from MediaAlpha for the three months and nine months ended September 30, 2016 , and $ 26.2 and $ 75.8 from MediaAlpha for the three months and nine months ended September 30, 2015 .

Investments in Unconsolidated38

Investments in Unconsolidated Affiliates (Tables)	9 Months Ended
	Sep. 30, 2016
Equity Method Investments and Joint Ventures [Abstract]
Schedule of Changes in Investments in Unconsolidated Affiliates	The following table summarizes amounts recorded by White Mountains under the equity method relating to its investment in Symetra for the three and nine months ended September 30, 2015 : Three Months Ended Nine Months Ended Millions September 30, 2015 September 30, 2015 Equity method carrying value of investment in Symetra at the beginning of the period $ 397.2 $ 411.4 Equity in earnings (1)(2) 4.2 17.5 Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio (3) 3.7 (31.8 ) Dividends received (12.5 ) (16.9 ) Distribution from Prospector Offshore Fund — 12.4 Equity method carrying value of investment in Symetra at the end of the period (4) $ 392.6 $ 392.6 (1) For the three and nine months ended September 30, 2015 , equity in earnings excludes tax expense of $0.3 and $1.1 . (2) For the three and nine months ended September 30, 2015 , equity in earnings includes $0.7 and $2.1 increase relating to the pre-tax amortization of the Symetra common share basis difference. (3) For the three and nine months ended September 30, 2015 , net unrealized gains includes $2.9 and $8.6 increase relating to the pre-tax amortization of the Symetra common share basis difference. (4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $5.7 as of September 30, 2015 , which excludes tax expense of $0.2 .

Employee Share-Based Incentiv39

Employee Share-Based Incentive Compensation Plans (Tables)	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of performance share activity	The following table summarizes performance share activity for the three and nine months ended September 30, 2016 and 2015 for performance shares granted under the WTM Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 73,297 $ 32.5 106,572 $ 28.8 93,654 $ 57.7 111,257 $ 44.4 Shares paid or expired (1) (8,958 ) (5.5 ) (36,294 ) (41.0 ) (42,959 ) (30.4 ) New grants (2) 6,400 — — 22,615 — 29,195 — Forfeitures and cancellations (3) (160 ) (.1 ) (3,960 ) (.8 ) (438 ) .2 (3,839 ) (.3 ) Expense recognized — 5.0 — 30.0 — 20.5 — 38.8 End of period (4) 79,537 $ 37.4 93,654 $ 52.5 79,537 $ 37.4 93,654 $ 52.5 (1) 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. WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 88% to 131.5% of target. (2) In the third quarter of 2016 White Mountains granted 3,000 WTM performance shares for the 2015-2017 performance cycle and 3,400 WTM performance shares for the 2016-2018 performance cycle. (3) Amounts include changes in assumed forfeitures, as required under GAAP. (4) Outstanding performance share awards as of September 30, 2016 and 2015 both exclude 7,315 and 10,825 performance share awards granted to employees of Sirius Group.
Summary of performance shares outstanding and accrued expense for performance shares awarded 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, 2016 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2016 – 2018 19,615 $ 3.4 2015 – 2017 31,795 15.1 2014 – 2016 30,167 19.9 Sub-total 81,577 38.4 Assumed forfeitures (2,040 ) (1.0 ) Total at September 30, 2016 79,537 $ 37.4
Summary of restricted share 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, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 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 66,470 $ 24.2 71,875 $ 23.2 70,675 $ 15.7 83,314 $ 14.3 Issued 4,150 3.4 25,365 19.7 23,640 15.7 Vested — — (1,200 ) — (24,620 ) — (36,279 ) — Forfeited — — — — (800 ) .2 — — Expense recognized (1) — (3.8 ) — (3.7 ) — (11.8 ) — (10.5 ) End of period (2) 70,620 $ 23.8 70,675 $ 19.5 70,620 $ 23.8 70,675 $ 19.5 (1) In connection with the sale of Sirius Group in the second quarter of 2016, White Mountains recognized $ 1.4 from accelerating the amortization of the issue date fair value for shares issued to Sirius Group employees, who no longer have a service obligation to the Company. (2) Restricted share awards outstanding as of September 30, 2016 and 2015 include 2,512 and 3,572 restricted shares issued to employees of Sirius Group, which was accounted for as discontinued operations.
Summary of performance share activity for OneBeacon performance shares granted under OneBeacon Incentive Plan	The following table summarizes performance share activity for the three and nine months ended September 30, 2016 and 2015 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 441,206 $ 1.0 462,147 $ 2.4 449,435 $ 1.4 517,470 $ 3.4 Shares paid or expired (1) — — — — (167,300 ) (.7 ) (181,290 ) (1.5 ) New grants — — — — 163,150 — 154,887 — Assumed forfeitures and cancellations (2) — — — — (4,079 ) — (28,920 ) — Expense recognized — .3 — (.3 ) — .6 — .2 End of period 441,206 $ 1.3 462,147 $ 2.1 441,206 $ 1.3 462,147 $ 2.1 (1) OneBeacon performance share payments in 2016 for the 2013-2015 performance cycle were at 24.3% of target. OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of OneBeacon performance shares outstanding and accrued expense for OneBeacon performance shares awarded under the OneBeacon Incentive Plan	The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at September 30, 2016 for each performance cycle: Millions, except share amounts Target Accrued Expense Performance cycle: 2016 – 2018 163,150 $ .6 2015 – 2017 146,659 .7 2014 – 2016 142,710 — Sub-total 452,519 1.3 Assumed forfeitures (11,313 ) — Total at September 30, 2016 441,206 $ 1.3
Summary of OneBeacon restricted shares activity	The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 395,872 $ 3.5 390,950 $ 3.5 382,722 $ 2.5 612,500 $ 3.5 Issued — — — — 170,650 2.3 75,950 1.1 Vested — — — — (157,500 ) — (296,000 ) — Forfeited — — — — — — (1,500 ) — Expense recognized — (.7 ) — (.5 ) — (2.0 ) — (1.6 ) End of period 395,872 $ 2.8 390,950 $ 3.0 395,872 $ 2.8 390,950 $ 3.0

Fair Value of Financial Instr40

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 277.8 $ 273.1 $ 276.4 $ 272.9 MediaAlpha Bank Facility 15.7 15.4 15.0 14.7

Discontinued Operations (Tables

Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Financial Statements of Discontinued Operations	Held for Sale and Discontinued Operations Sirius Group On April 18, 2016, White Mountains completed the sale of Sirius Group to CMI for approximately $2.6 billion . $161.8 million of this amount was used to purchase certain assets to be retained by White Mountains out of Sirius Group, including shares of OneBeacon. The amount paid at closing was based on an estimate of Sirius Group’s closing date tangible common shareholder’s equity. In the second quarter of 2016, White Mountains recorded $ 366.6 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. In the third quarter of 2016, White Mountains recorded a $ 4.0 million reduction to the gain from sale of Sirius Group in discontinued operations as a result of the final true-up of Sirius Group's closing date tangible common shareholder's equity. For the nine months ended September 30, 2016, White Mountains recorded a $ 4.3 million net loss from discontinued operations from Sirius Group and $ 32.0 million of other comprehensive income. 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 income (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 nine months ended September 30, 2016, $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 income (loss) from discontinued operations. For the three and nine months ended September 30, 2015, $0.8 million and $3.5 million of net investment income and realized and unrealized investment gains and losses, net of related tax effects, that had been previously included in the Sirius Group legal entities have been excluded from net income (loss) from discontinued operations. As part of the sale of Sirius Group, White Mountains purchased Ashmere, a subsidiary formerly owned by Sirius Group. Ashmere was under an agreement to be sold as of September 30, 2016 and the sale was completed on October 12, 2016. As of September 30, 2016, Ashmere's assets and liabilities are presented as held for sale. Tranzact On June 9, 2016, White Mountains announced that it had entered into a definitive agreement for the sale of Tranzact to an affiliate of Clayton, Dubilier & Rice, LLC. On July 21, 2016, White Mountains completed the sale of Tranzact and received net proceeds of $ 221.3 million . During the three and nine months ended September 30, 2016, White Mountains recorded $51.9 million of gain from the sale of Tranzact in discontinued operations in the statement of operations. (See Note 2 - “Significant Transactions” ). Through July 21, 2016, Tranzact's results of operations are reported as discontinued operations and assets and liabilities held for sale within White Mountains's GAAP financial statements. For the stub period ended July 21, 2016 and the nine months ended September 30, 2016, White Mountains recorded a net loss from discontinued operations of $ 2.1 million and $ 2.6 million from Tranzact. For the three and nine months ended September 30, 2015, White Mountains recorded a net loss from discontinued operations of $ 7.7 million and $ 14.2 million from Tranzact. At September 30, 2016, White Mountains also recorded a liability of $ 2.8 million for escrow funds associated with the sale payable to the non-controlling interests. 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 related to the valuation allowance reversal was recorded to gain from sale of Tranzact in discontinued operations and a $ 16.1 million tax benefit related to the valuation allowance reversal was recorded to net income (loss) from discontinued operations. Star & Shield SSIE voluntarily ceased writing new policies effective July 1, 2016 and will voluntarily cease renewing policies at a future date, subject to regulatory approval. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, White Mountains has presented Star & Shield's and SSIE's assets and liabilities as held for sale as of September 30, 2016 and December 31, 2015. OneBeacon Runoff For the nine months ended September 30, 2015, White Mountains recorded $0.3 million to the gain from sale of discontinued operations related to an adjustment on the estimated loss on sale, which included the final settlement of certain post-closing items. For the three and nine months ended September 30, 2015, the net loss from discontinued operations relating to the Runoff Business, net of tax, was $0.1 million and $0.4 million . The transaction to sell the Runoff Business was completed in December 2014. Esurance For the three and nine months ended September 30, 2015, White Mountains recorded a net gain from the sale of discontinued operations of $10.3 million and $17.9 million , which primarily related to an installment payment from Allstate for the favorable development on loss reserves. (See Note 18 - “Contingencies” ).
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet And Additional Disclosures	Summary of Reclassified Balances and Related Items Net Assets Held for Sale The following summarizes the assets and liabilities associated with business classified as held for sale. At December 31, 2015, amounts presented relate to Sirius Group, Tranzact, Star and Shield and SSIE. At September 30, 2016, the amounts presented relate to Star and Shield, SSIE and Ashmere, a subsidiary formerly owned by Sirius Group. Millions September 30, 2016 December 31, 2015 Assets held for sale Fixed maturity investments, at fair value $ 13.4 $ 2,383.5 Short-term investments, at amortized cost (which approximates fair value) 1.3 352.1 Common equity securities, at fair value — 174.4 Other long-term investments — 72.2 Total investments 14.7 2,982.2 Cash 3.8 150.1 Reinsurance recoverable on unpaid losses .6 283.6 Reinsurance recoverable on paid losses — 10.2 Insurance and reinsurance premiums receivable 2.0 326.6 Deferred acquisition costs — 74.6 Deferred tax asset — 303.1 Ceded unearned insurance and reinsurance premiums — 87.7 Accounts receivable on unsettled investment sales — 29.0 Goodwill and other intangible assets 5.2 330.5 Other assets .7 212.8 Total assets held for sale $ 27.0 $ 4,790.4 Liabilities held for sale Loss and loss adjustment expense reserves $ 6.0 $ 1,650.4 Unearned insurance and reinsurance premiums 1.6 344.3 Debt — 506.4 Deferred tax liability — 270.6 Accrued incentive compensation — 64.2 Ceded reinsurance payable (.5 ) 67.7 Funds held under reinsurance treaties — 52.9 Other liabilities .7 90.9 Total liabilities held for sale 7.8 3,047.4 Net assets held for sale $ 19.2 $ 1,743.0 Net Income (Loss) from Discontinued Operations The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2016, the amounts presented relate to Sirius Group and Tranzact. For the three and nine months ended September 30, 2015, the amounts presented relate to Sirius Group, Tranzact, Esurance, and the OneBeacon's runoff business. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains. Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ — $ — $ — $ 202.8 $ — $ 202.8 Net investment income — — — 10.5 — 10.5 Net realized and unrealized losses — — — (.9 ) — (.9 ) Other revenue — 14.8 14.8 1.8 38.5 40.3 Total revenues — 14.8 14.8 214.2 38.5 252.7 Expenses Loss and loss adjustment expenses — — — 121.7 — 121.7 Insurance and reinsurance acquisition expenses — — — 45.4 — 45.4 Other underwriting expenses — — — 24.8 — 24.8 General and administrative expenses — 16.2 16.2 9.4 45.3 54.7 Interest expense — .5 .5 6.6 1.0 7.6 Total expenses — 16.7 16.7 207.9 46.3 254.2 Pre-tax (loss) income — (1.9 ) (1.9 ) 6.3 (7.8 ) (1.5 ) Income tax expense (benefit) — 15.9 15.9 (2.3 ) — (2.3 ) Net income (loss) from discontinued operations — 14.0 14.0 4.0 (7.8 ) (3.8 ) Net loss from sale of discontinued operations - Sirius (4.0 ) — (4.0 ) — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - Esurance — — — — 10.3 10.3 Total (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ 4.0 $ 2.5 $ 6.5 Change in foreign currency translation and other from discontinued operations — — — (18.5 ) — (18.5 ) Comprehensive (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ (14.5 ) $ 2.5 $ (12.0 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 240.1 $ — $ 240.1 $ 623.3 $ — $ 623.3 Net investment income 14.4 — 14.4 28.6 — 28.6 Net realized and unrealized (losses) gains (1.5 ) — (1.5 ) 31.4 — 31.4 Other revenue .6 119.6 120.2 (20.2 ) 110.6 90.4 Total revenues 253.6 119.6 373.2 663.1 110.6 773.7 Expenses Loss and loss adjustment expenses 154.9 — 154.9 311.3 — 311.3 Insurance and reinsurance acquisition expenses 59.0 — 59.0 135.5 — 135.5 Other underwriting expenses 30.9 — 30.9 78.0 — 78.0 General and administrative expenses 10.4 116.7 127.1 22.4 122.5 144.9 Interest expense 7.9 3.2 11.1 20.0 2.7 22.7 Total expenses 263.1 119.9 383.0 567.2 125.2 692.4 Pre-tax (loss) income (9.5 ) (.3 ) (9.8 ) 95.9 (14.6 ) 81.3 Income tax benefit (expense) 3.1 13.8 16.9 (22.2 ) — (22.2 ) Net (loss) income from discontinued operations (6.4 ) 13.5 7.1 73.7 (14.6 ) 59.1 Net gain from sale of discontinued operations - Sirius 362.6 — 362.6 — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - OneBeacon — — — — .3 .3 Net gain from sale of discontinued operations - Esurance — — — — 17.9 17.9 Total income from discontinued operations $ 356.2 $ 65.4 $ 421.6 $ 73.7 $ 3.6 $ 77.3 Change in foreign currency translation and other from discontinued operations 32.0 — 32.0 (62.1 ) — (62.1 ) Change in foreign currency translation and other from sale of Sirius Group 113.3 — 113.3 — — — Comprehensive income from discontinued operations $ 501.5 $ 65.4 $ 566.9 $ 11.6 $ 3.6 $ 15.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax (payment to) refund from national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, (Millions) 2016 2015 Net cash (used for) provided from operations $ (16.2 ) $ 22.7 Net cash provided from investing activities 213.3 2.8 Net cash (used for) provided from financing activities (.2 ) 19.6 Effect of exchange rate changes on cash — (4.1 ) Net change in cash during the period 196.9 41.0 Cash balances at beginning of period 150.1 116.6 Net change in cash held for sale 2.6 — Cash sold as part of sale of consolidated subsidiaries 345.8 — Cash balances at end of period $ 3.8 $ 157.6 Supplemental cash flows information: Interest paid $ (1.4 ) $ (28.1 ) Net income tax payment to national governments $ (31.8 ) $ (30.3 ) Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 61.9 $ 6.5 $ 421.6 $ 77.3 Allocation of income for participating unvested restricted common shares (1) (.9 ) (.1 ) (5.1 ) (.9 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ 61.0 $ 6.4 $ 416.5 $ 76.4 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,867.4 5,890.1 5,166.6 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Basic earnings per share denominator 4,799.3 5,819.2 5,103.7 5,884.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,879.4 5,890.1 5,174.8 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Diluted earnings per share denominator (4) 4,811.3 5,819.2 5,111.9 5,884.0 Basic earnings per share (in dollars): $ 12.72 $ 1.09 $ 81.60 $ 12.98 Diluted earnings per share (in dollars): $ 12.69 $ 1.09 $ 81.47 $ 12.98 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2016 and 2015 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans” ). (4) ) The diluted earnings per share denominator for the three 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 results in 11,943 and 8,208 incremental shares outstanding over the periods.
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, 2016 and December 31, 2015 : September 30, 2016 December 31, 2015 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 277.8 $ 273.1 $ 276.4 $ 272.9 MediaAlpha Bank Facility 15.7 15.4 15.0 14.7
Financials
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Financial Statements of Discontinued Operations	Summary of Reclassified Balances and Related Items Net Assets Held for Sale The following summarizes the assets and liabilities associated with business classified as held for sale. At December 31, 2015, amounts presented relate to Sirius Group, Tranzact, Star and Shield and SSIE. At September 30, 2016, the amounts presented relate to Star and Shield, SSIE and Ashmere, a subsidiary formerly owned by Sirius Group. Millions September 30, 2016 December 31, 2015 Assets held for sale Fixed maturity investments, at fair value $ 13.4 $ 2,383.5 Short-term investments, at amortized cost (which approximates fair value) 1.3 352.1 Common equity securities, at fair value — 174.4 Other long-term investments — 72.2 Total investments 14.7 2,982.2 Cash 3.8 150.1 Reinsurance recoverable on unpaid losses .6 283.6 Reinsurance recoverable on paid losses — 10.2 Insurance and reinsurance premiums receivable 2.0 326.6 Deferred acquisition costs — 74.6 Deferred tax asset — 303.1 Ceded unearned insurance and reinsurance premiums — 87.7 Accounts receivable on unsettled investment sales — 29.0 Goodwill and other intangible assets 5.2 330.5 Other assets .7 212.8 Total assets held for sale $ 27.0 $ 4,790.4 Liabilities held for sale Loss and loss adjustment expense reserves $ 6.0 $ 1,650.4 Unearned insurance and reinsurance premiums 1.6 344.3 Debt — 506.4 Deferred tax liability — 270.6 Accrued incentive compensation — 64.2 Ceded reinsurance payable (.5 ) 67.7 Funds held under reinsurance treaties — 52.9 Other liabilities .7 90.9 Total liabilities held for sale 7.8 3,047.4 Net assets held for sale $ 19.2 $ 1,743.0 Net Income (Loss) from Discontinued Operations The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations. For the three and nine months ended September 30, 2016, the amounts presented relate to Sirius Group and Tranzact. For the three and nine months ended September 30, 2015, the amounts presented relate to Sirius Group, Tranzact, Esurance, and the OneBeacon's runoff business. The results of discontinued operations from Sirius Group and Tranzact up to the closing date of the transaction inure to White Mountains. Three Months Ended Three Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ — $ — $ — $ 202.8 $ — $ 202.8 Net investment income — — — 10.5 — 10.5 Net realized and unrealized losses — — — (.9 ) — (.9 ) Other revenue — 14.8 14.8 1.8 38.5 40.3 Total revenues — 14.8 14.8 214.2 38.5 252.7 Expenses Loss and loss adjustment expenses — — — 121.7 — 121.7 Insurance and reinsurance acquisition expenses — — — 45.4 — 45.4 Other underwriting expenses — — — 24.8 — 24.8 General and administrative expenses — 16.2 16.2 9.4 45.3 54.7 Interest expense — .5 .5 6.6 1.0 7.6 Total expenses — 16.7 16.7 207.9 46.3 254.2 Pre-tax (loss) income — (1.9 ) (1.9 ) 6.3 (7.8 ) (1.5 ) Income tax expense (benefit) — 15.9 15.9 (2.3 ) — (2.3 ) Net income (loss) from discontinued operations — 14.0 14.0 4.0 (7.8 ) (3.8 ) Net loss from sale of discontinued operations - Sirius (4.0 ) — (4.0 ) — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - Esurance — — — — 10.3 10.3 Total (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ 4.0 $ 2.5 $ 6.5 Change in foreign currency translation and other from discontinued operations — — — (18.5 ) — (18.5 ) Comprehensive (loss) income from discontinued operations $ (4.0 ) $ 65.9 $ 61.9 $ (14.5 ) $ 2.5 $ (12.0 ) Nine Months Ended Nine Months Ended September 30, 2016 September 30, 2015 Millions Sirius Group Other Disc Ops Total Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 240.1 $ — $ 240.1 $ 623.3 $ — $ 623.3 Net investment income 14.4 — 14.4 28.6 — 28.6 Net realized and unrealized (losses) gains (1.5 ) — (1.5 ) 31.4 — 31.4 Other revenue .6 119.6 120.2 (20.2 ) 110.6 90.4 Total revenues 253.6 119.6 373.2 663.1 110.6 773.7 Expenses Loss and loss adjustment expenses 154.9 — 154.9 311.3 — 311.3 Insurance and reinsurance acquisition expenses 59.0 — 59.0 135.5 — 135.5 Other underwriting expenses 30.9 — 30.9 78.0 — 78.0 General and administrative expenses 10.4 116.7 127.1 22.4 122.5 144.9 Interest expense 7.9 3.2 11.1 20.0 2.7 22.7 Total expenses 263.1 119.9 383.0 567.2 125.2 692.4 Pre-tax (loss) income (9.5 ) (.3 ) (9.8 ) 95.9 (14.6 ) 81.3 Income tax benefit (expense) 3.1 13.8 16.9 (22.2 ) — (22.2 ) Net (loss) income from discontinued operations (6.4 ) 13.5 7.1 73.7 (14.6 ) 59.1 Net gain from sale of discontinued operations - Sirius 362.6 — 362.6 — — — Net gain from sale of discontinued operations - Tranzact — 51.9 51.9 — — — Net gain from sale of discontinued operations - OneBeacon — — — — .3 .3 Net gain from sale of discontinued operations - Esurance — — — — 17.9 17.9 Total income from discontinued operations $ 356.2 $ 65.4 $ 421.6 $ 73.7 $ 3.6 $ 77.3 Change in foreign currency translation and other from discontinued operations 32.0 — 32.0 (62.1 ) — (62.1 ) Change in foreign currency translation and other from sale of Sirius Group 113.3 — 113.3 — — — Comprehensive income from discontinued operations $ 501.5 $ 65.4 $ 566.9 $ 11.6 $ 3.6 $ 15.2 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash, including income tax (payment to) refund from national governments and interest paid associated with the business classified as discontinued operations: Nine Months Ended September 30, (Millions) 2016 2015 Net cash (used for) provided from operations $ (16.2 ) $ 22.7 Net cash provided from investing activities 213.3 2.8 Net cash (used for) provided from financing activities (.2 ) 19.6 Effect of exchange rate changes on cash — (4.1 ) Net change in cash during the period 196.9 41.0 Cash balances at beginning of period 150.1 116.6 Net change in cash held for sale 2.6 — Cash sold as part of sale of consolidated subsidiaries 345.8 — Cash balances at end of period $ 3.8 $ 157.6 Supplemental cash flows information: Interest paid $ (1.4 ) $ (28.1 ) Net income tax payment to national governments $ (31.8 ) $ (30.3 ) Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 61.9 $ 6.5 $ 421.6 $ 77.3 Allocation of income for participating unvested restricted common shares (1) (.9 ) (.1 ) (5.1 ) (.9 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ 61.0 $ 6.4 $ 416.5 $ 76.4 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 4,867.4 5,890.1 5,166.6 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Basic earnings per share denominator 4,799.3 5,819.2 5,103.7 5,884.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period (4) 4,879.4 5,890.1 5,174.8 5,951.1 Average unvested restricted common shares (3) (68.1 ) (70.9 ) (62.9 ) (67.1 ) Diluted earnings per share denominator (4) 4,811.3 5,819.2 5,111.9 5,884.0 Basic earnings per share (in dollars): $ 12.72 $ 1.09 $ 81.60 $ 12.98 Diluted earnings per share (in dollars): $ 12.69 $ 1.09 $ 81.47 $ 12.98 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and nine months ended September 30, 2016 and 2015 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans” ). (4) ) The diluted earnings per share denominator for the three 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 results in 11,943 and 8,208 incremental shares outstanding over the periods.

Summary of Significant Accoun42

Summary of Significant Accounting Policies (Basis of Presentation) (Details) £ in Millions, $ in Millions	Aug. 04, 2016GBP (£)	Aug. 04, 2016USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)	Jul. 31, 2012USD ($)
Basis of Presentation
Debt Issuance Cost					$ 1.9
Net investment income			$ 22.5	$ 16.8	59.6	$ 42.6
Income Tax Expense (Benefit)			(6.7)	(1.6)	(22.4)	0.8
Equity in earnings of unconsolidated affiliates, net of tax			$ 0	$ 3.9	0	18
Payments to Acquire Businesses and Interest in Affiliates					$ 13.7	$ 2.4
OneBeacon Ltd.
Basis of Presentation
Ownership interest (as a percent)			76.10%		76.10%		75.50%
HG Global
Basis of Presentation
Surplus Notes								$ 503
Percentage of par value of policy reinsured			15.00%		15.00%
Noncontrolling equity percentage			3.10%		3.10%		3.10%
Buzzmove [Member]
Basis of Presentation
Ownership interest (as a percent)	70.90%	70.90%
Noncontrolling equity percentage			29.10%		29.10%		0.00%
Star & Shield Insurance Exchange
Basis of Presentation
Noncontrolling equity percentage			100.00%		100.00%		100.00%
OneBeacon [Member]
Basis of Presentation
Debt Issuance Cost							$ 1.9
Noncontrolling equity percentage			23.90%		23.90%		24.50%
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)			96.90%		96.90%
Noncontrolling equity percentage			96.90%		96.90%		96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)			88.40%		88.40%
Noncontrolling equity percentage			88.40%		88.40%		88.40%
United Kingdom, Pounds | Buzzmove [Member]
Basis of Presentation
Payments to Acquire Businesses and Interest in Affiliates	£ 6	$ 6
United States of America, Dollars | Buzzmove [Member]
Basis of Presentation
Payments to Acquire Businesses and Interest in Affiliates		$ 8

Significant Transactions (Detai

Significant Transactions (Details) $ / shares in Units, ₪ in Millions, $ in Millions	Apr. 18, 2016USD ($)	Feb. 01, 2016USD ($)$ / shares	Jan. 31, 2014USD ($)	Feb. 23, 2015ILS (₪)	Feb. 23, 2015USD ($)	Feb. 19, 2014ILS (₪)	Feb. 19, 2014USD ($)	Sep. 30, 2016ILS (₪)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2016USD ($)	Sep. 30, 2016ILS (₪)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Aug. 04, 2016USD ($)	Dec. 31, 2015USD ($)	Jun. 30, 2015USD ($)	Dec. 31, 2014USD ($)	Mar. 31, 2014USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt															$ 401.8	$ 84.5
Payments to Acquire Other Investments															37.3	30.5
Investments									$ 5,213.5						5,213.5			$ 4,271.1
Sales of common equity securities															961.1	370.6
Payments to Acquire Businesses and Interest in Affiliates															13.7	2.4
Other Income									34.8			$ 38.3			110.8	114.4
Assets									6,683.3						6,683.3			10,282.6
Goodwill										$ 24.1			$ 24.1					24.1	$ 24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)										29.2			$ 29.2					31.3	$ 36.4	$ 38.7
Liabilities									2,881.9						2,881.9			5,914.6
Net Income (Loss) Attributable to Noncontrolling Interest									3.1			16			(24.6)	24.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															2,657.2	24
Net gain on sale of discontinued operations															414.5	18.2
Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of Surplus Notes																					$ 21
Other Income															(21)
Assets									11.4						11.4			14.2
Liabilities									$ 7.2						7.2			30.3
Net Income (Loss) Attributable to Noncontrolling Interest															$ 20.3	(0.9)
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Business in the Insurance Market													80.00%
Star & Shield LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates			$ 1.8
Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						54.00%	54.00%		96.70%						96.70%
Non-controlling interest | Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest									$ 0.4			(0.8)
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪								₪ 11.5						₪ 27.9
Israel, New Shekels | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪						₪ 31.5
Israel, New Shekels | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪				₪ 35.9
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪						₪ 14.4
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries									3.1						$ 7.5
United States of America, Dollars | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates							$ 9
United States of America, Dollars | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates					$ 8.9
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates							$ 4.1
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets					5.5												$ 11.7
Goodwill					0.3
Intangible Assets, Net (Excluding Goodwill)					2.8												9
Liabilities					$ 1.2												$ 0.1
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Other Investments	$ 161.8									161.8
Investments																		686.2
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600									2,600
Net gain on sale of discontinued operations									(4)	366.6		0			362.6	0
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease									0	$ 113.3		(18.5)			32	(62.1)
Sale of Sirius Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease									0			$ 0	$ 113.3		$ 113.3	$ 0
Tranzact [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of Debt									56.3
Increase of Book Value									82.1
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold									221.3
Net gain on sale of discontinued operations									51.9
Discontinued Operation, Tax Effect of Discontinued Operation									$ 30.2
Crop insurance [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related from termination of agreement																		3
Percentage of Quota Share Reinsurance Agreement														100.00%	100.00%
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities		$ 658
Dividends Payable, Amount Per Share | $ / shares		$ 32
Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales of common equity securities											$ 658
Symetra | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments																		$ 528.6

Loss and Loss Adjustment Expe44

Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9			$ 0	$ 18
Gross beginning balance	1,376.6	1,350.8	$ 1,389.8	$ 1,342.2	1,389.8	1,342.2
Less beginning reinsurance recoverable on unpaid losses	(162.8)	(147.1)	(186)	(161.6)	(186)	(161.6)
Net loss and LAE reserves	1,213.8	1,203.7	1,203.8	1,180.6	1,203.8	1,180.6
Loss and LAE incurred relating to current year losses	164.9	170.4			492.8	536.7
Net (favorable) and unfavorable loss reserve development	0.1	(0.4)			15.3	(2.6)
Total incurred losses and LAE	165	170			508.1	534.1
Current year losses	(55)	(53)			(116.2)	(120.2)
Prior year losses	(131.1)	(108.1)			(403.2)	(383.5)
Total loss and LAE payments	(186.1)	(161.1)			(519.4)	(503.7)
Net ending balance	1,192.6	1,213.2	1,213.8	1,203.7	1,192.6	1,213.2
Plus ending reinsurance recoverable on unpaid losses	169.4	214.2	162.8	147.1	169.4	214.2
Gross ending balance	1,362	1,427.4	$ 1,376.6	$ 1,350.8	1,362	1,427.4
Star & Shield Insurance Exchange
Net ending balance	5.4	5.5			5.4	5.5
Star & Shield Insurance Exchange
Liability for Unpaid Claims and Claims Adjustment Expense, Consolidated Claims Current Year	5.3	6.1			5.5	7.7
Net (favorable) and unfavorable loss reserve development	$ 0.1	$ (0.4)			$ (0.1)	$ (0.8)

Loss and Loss Adjustment Expe45

Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Loss and Loss Adjustment, Expense Reserves [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0			$ 3.9		$ 0	$ 18
Net (favorable) and unfavorable loss reserve development	0.1			(0.4)		15.3	(2.6)
Loss and LAE incurred relating to current year losses	164.9			170.4		492.8	536.7
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ 0.1			(0.4)		$ (0.1)	(0.8)
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development		$ 20		$ 15.4			$ 1.8
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Percentage of Quota Share Reinsurance Agreement			50.00%
Net (favorable) and unfavorable loss reserve development					$ 15.4

Third Party Reinsurance (Narrat

Third Party Reinsurance (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses		$ 16.4	$ 7.5
Reinsurance recoverable on unpaid losses		169.4	186
OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses		16.4	7.5
Reinsurance recoverable on unpaid losses		$ 169.4	$ 186
OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Percentage of Quota Share Reinsurance Agreement	50.00%
Property Catastrophe Program [Member] | OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation		$ 110
Property Catastrophe Program [Member] | Minimum | OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained		20
Property Catastrophe Program [Member] | Maximum [Member] | OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured		$ 130

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Investment income
Total investment income	22.9	17.1	61.4	47.2
Third-party investment expenses	(0.4)	(0.3)	(1.8)	(4.6)
Net investment income, pre-tax	22.5	16.8	59.6	42.6
Fixed maturity investments.
Investment income
Total investment income	21.3	13.7	53.7	38.5
Short-term investments
Investment income
Total investment income	0.2	0	0.8	0.1
Common Stock
Investment income
Total investment income	1.3	1.4	3.4	5.7
Other long-term investments
Investment income
Total investment income	$ 0.1	$ 2	$ 3.5	$ 2.9

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Net realized investment gains (losses), pre-tax	8	(7.4)	267.5	49.7
Net unrealized investment gains (losses), pre-tax	18.4	(36.5)	(183.5)	(83.6)
Net realized and unrealized investment gains (losses), pre-tax	26.4	(43.9)	84	(33.9)
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(5.4)	9.4	(23.8)	6.9
Net realized and unrealized investment gains (losses), after tax	21	(34.5)	60.2	(27)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	2.8	(0.1)	3.5	2
Net unrealized investment gains (losses), pre-tax	(4.4)	3.3	38	1.3
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	0.2	0	0.4	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	6.4	(5.9)	267.7	42
Net unrealized investment gains (losses), pre-tax	12.7	(30.8)	(245.3)	(58.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	(1.4)	(1.4)	(4.1)	5.7
Net unrealized investment gains (losses), pre-tax	$ 10.1	$ (9)	$ 23.8	$ (26.4)

Investment Securities (Net Re49

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 8	$ (7.4)	$ 267.5	$ 49.3
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(2.7)	1.5	(46)	(15.7)
Trading securities realized holding gain (loss) on investments after tax	5.3	(5.9)	221.5	33.6
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0.4
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	0	0	0.4
Net realized investment gains (losses), pre-tax	8	(7.4)	267.5	49.7
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(2.7)	1.5	(46)	(15.7)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	5.3	(5.9)	221.5	34
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2.8	(0.1)	3.5	2
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Net realized investment gains (losses), pre-tax	2.8	(0.1)	3.5	2
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.2	0	0.4	0
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Net realized investment gains (losses), pre-tax	0.2	0	0.4	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	6.4	(5.9)	267.7	41.6
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0.4
Net realized investment gains (losses), pre-tax	6.4	(5.9)	267.7	42
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(1.4)	(1.4)	(4.1)	5.7
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Net realized investment gains (losses), pre-tax	$ (1.4)	$ (1.4)	$ (4.1)	$ 5.7

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 18.1	$ (36.3)	$ (186.4)	$ (80.3)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(2.7)	7.9	22.2	22.6
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	15.4	(28.4)	(164.2)	(57.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0.3	(0.2)	2.9	(3.3)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0	0	0	0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	0.3	(0.2)	2.9	(3.3)
Net unrealized investment gains (losses), pre-tax	18.4	(36.5)	(183.5)	(83.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(2.7)	7.9	22.2	22.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	15.7	(28.6)	(161.3)	(61)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(4.4)	3.3	38	1.3
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0	0
Net unrealized investment gains (losses), pre-tax	(4.4)	3.3	38	1.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	12.5	(30.6)	(247.9)	(56)
Foreign Currency Transaction Gain (Loss), Unrealized	0.2	(0.2)	2.6	(2.5)
Net unrealized investment gains (losses), pre-tax	12.7	(30.8)	(245.3)	(58.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	10	(9)	23.5	(25.6)
Foreign Currency Transaction Gain (Loss), Unrealized	0.1	0	0.3	(0.8)
Net unrealized investment gains (losses), pre-tax	$ 10.1	$ (9)	$ 23.8	$ (26.4)

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Unrealized gains (losses) on investments	18.1	(36.3)	(186.4)	(80.3)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(4.4)	3.3	38	1.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	12.5	(30.6)	(247.9)	(56)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	10	(9)	23.5	(25.6)
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	8.1	(6.7)	22.3	(3.5)
Level 3 Inputs | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.3	(1)	1.6	(0.9)
Level 3 Inputs | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	1.1	0	4
Level 3 Inputs | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 7.8	$ (6.8)	$ 20.7	$ (6.6)

Investment Securities (Invest52

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,131.5	$ 2,630.2
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	96.6	160.4
Trading Securities, Unrealized Holding Gain	0.4	0
Trading Securities, Unrealized Holding Loss	0	(0.4)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	97	160
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,212.2	1,001
Trading Securities, Unrealized Holding Gain	18.7	4.3
Trading Securities, Unrealized Holding Loss	(1.5)	(5.3)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1,229.4	1,000
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	306.3	227.8
Trading Securities, Unrealized Holding Gain	6.2	2.2
Trading Securities, Unrealized Holding Loss	(0.2)	(1.2)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	312.3	228.8
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,406.6	1,170.6
Trading Securities, Unrealized Holding Gain	9.6	2
Trading Securities, Unrealized Holding Loss	(1.7)	(5.6)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	2,414.5	1,167
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1	1
Trading Securities, Unrealized Holding Gain	0.2	0.2
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1.2	1.2
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	78.3	78.3
Trading Securities, Unrealized Holding Gain	7.1	4.4
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	85.4	82.7
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,101	2,639.1
Trading Securities, Unrealized Holding Gain	42.2	13.1
Trading Securities, Unrealized Holding Loss	(3.4)	(12.5)
Net foreign currency gains (losses)	0	0
Total fixed maturity investments including assets held for sale	4,139.8	2,639.7
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	$ 9.5	$ 9.7	$ 10.1

Investment Securities (Invest53

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,131.5	$ 2,630.2
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	378.9	822.5
Trading Securities, Unrealized Holding Gain	50.7	302.8
Trading Securities, Unrealized Holding Loss	(4.6)	(11.4)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	425	1,113.9
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	317.1	304.5
Trading Securities, Unrealized Holding Gain	42.7	32
Trading Securities, Unrealized Holding Loss	(6.4)	(18.4)
Net foreign currency gains (losses)	(2)	(2.3)
Fixed maturity investments, at fair value	$ 351.4	$ 315.8

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments	$ 37.3	$ 30.5
Other Investments	351.4		$ 315.8
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	143		127.8
Limited liability companies and private equity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	84.6		82.1
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	71.5		51.5
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	33.1		32.7
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8	$ 15.2	14.7	$ 16.8
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.2		3.8
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.2		3.2
Hedge Funds Equity R E I T [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.4		20.6
Unfunded Commitments	$ 0		$ 0

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Sep. 30, 2016USD ($)fund	Dec. 31, 2015USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments | $	$ 351.4	$ 315.8
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	5
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	24
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 | $	$ 34.6

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 58.4
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.6
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	143	$ 127.8
Unfunded Commitments	129.6	82.2
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	14.2	20.7
Unfunded Commitments	3.2	3.4
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	10.4	14.8
Unfunded Commitments	2	2.1
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.7	4.4
Unfunded Commitments	2.1	2.1
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.8	3.8
Unfunded Commitments	0.4	0.4
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.8	2
Unfunded Commitments	41.3	41.3
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	23.1	19.8
Unfunded Commitments	21.8	30.3
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	27.7	24.9
Unfunded Commitments	21.8	2.5
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.6	90.8
Unfunded Commitments	129.6	82.2
Private equity funds | Real Estate Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	0.4
Unfunded Commitments	0.1	0.1
Private equity funds | Venture Capital Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.6	0
Unfunded Commitments	36.9	0
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	34.6	12.8
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity R E I T [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.4	20.6
Unfunded Commitments	0	0
Hedge funds | Hedge Funds, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.4	3.6
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.4	37
Unfunded Commitments	$ 0	$ 0

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Sep. 30, 2016USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Distributions from Inactive Hedge Funds	$ 1
Other long-term investments	58.4
Redemption requests	2.4
30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	35.6
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.4
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.4
Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	35.6
Quarterly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	35.6
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.4
Semi-annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.4
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.4
Annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 2.4

Investment Securities (Invest58

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	$ (3.1)	$ (16)	$ 24.6	$ (24.2)
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.6		84.6
1-3 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	23.5		23.5
3-5 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.8		3.8
5-10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	56.3		56.3
Greater than 10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 1		$ 1

Investment Securities OneBeacon

Investment Securities OneBeacon Surplus Notes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 23, 2014
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Interest Paid			7	$ 6.3
OneBeacon [Member]
Assets, Fair Value Adjustment			(29.5)		$ (49.5)
OneBeacon [Member] | Par Value [Member]
Surplus Notes	101		101		101	$ 101
OneBeacon [Member] | Fair Value [Member]
Surplus Notes	71.5		71.5		51.5
OneBeacon [Member] | Current market rates on repayments [Member]
Assets, Fair Value Adjustment			0.8		(15.1)
OneBeacon [Member] | Regulatory Approval [Member]
Assets, Fair Value Adjustment			(15.9)		(24.2)
OneBeacon [Member] | Liquidity Adjustment [Member]
Assets, Fair Value Adjustment			(14.4)		$ (10.2)
Seller Priority Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment			(8.9)
Seller Priority Surplus Note [Member] | OneBeacon [Member] | Par Value [Member]
Surplus Notes	57.9		57.9
Seller Priority Surplus Note [Member] | OneBeacon [Member] | Fair Value [Member]
Surplus Notes	49		49
Seller Priority Surplus Note [Member] | OneBeacon [Member] | Current market rates on repayments [Member]
Assets, Fair Value Adjustment			5.2
Seller Priority Surplus Note [Member] | OneBeacon [Member] | Regulatory Approval [Member]
Assets, Fair Value Adjustment			(4.2)
Seller Priority Surplus Note [Member] | OneBeacon [Member] | Liquidity Adjustment [Member]
Assets, Fair Value Adjustment			(9.9)
Surplus Note
Interest Paid			2.4
Pari Passu Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment			(20.6)
Pari Passu Surplus Note [Member] | OneBeacon [Member] | Par Value [Member]
Surplus Notes	43.1		43.1
Pari Passu Surplus Note [Member] | OneBeacon [Member] | Fair Value [Member]
Surplus Notes	$ 22.5		22.5
Pari Passu Surplus Note [Member] | OneBeacon [Member] | Current market rates on repayments [Member]
Assets, Fair Value Adjustment			(4.4)
Pari Passu Surplus Note [Member] | OneBeacon [Member] | Regulatory Approval [Member]
Assets, Fair Value Adjustment			(11.7)
Pari Passu Surplus Note [Member] | OneBeacon [Member] | Liquidity Adjustment [Member]
Assets, Fair Value Adjustment			$ (4.5)
Surplus Note
RBC Score			250.00%		250.00%

Investment Securities (Fair V60

Investment Securities (Fair Value Measurements) (Details) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016USD ($)
Investments, Debt and Equity Securities [Abstract]
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1

Investment Securities (Fair V61

Investment Securities (Fair Value Measurement by Level) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of Investments Recorded at Fair Value			91.00%
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Fair value investments	4,900.2	3,365.4	4,900.2	3,365.4	$ 4,050.9	$ 3,343.6
Other Investments	351.4		351.4		315.8
Short-term investments, at amortized cost (which approximates fair value)	305.6		305.6		211.2
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	412.4	418.2	412.4	418.2	1,152.2	550.6
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,044.8	2,561.6	4,044.8	2,561.6	2,531.4	2,372.9
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,229.4		1,229.4		1,000
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,414.5		2,414.5		1,167
Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,378.5		2,378.5		1,167
Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36		36		0
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	107.6	70.3	107.6	70.3	70	76.4
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	34.2	0	34.2	0	39.5
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	192.4	133.7	192.4	133.7	169.5	126.9
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	143		143		127.8
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	143	147.4	143	147.4	127.8	177.3
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5,062.8		5,062.8		4,134.4
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	691		691		1,363.5
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,071.8		4,071.8		2,531.4
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	300		300		239.5
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	97		97		160
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	87.5		87.5		133.4
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	9.5		9.5		26.6
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		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	1,229.4		1,229.4		1,000
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		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	1,229.4		1,229.4		1,000
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	0		0		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	340.5		340.5		253.3
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		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	340.5		340.5		253.3
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		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	132.2		132.2		135.6
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		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	132.2		132.2		135.6
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		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	170.8		170.8		175.9
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		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	170.8		170.8		175.9
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	0		0		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	89.4		89.4		49.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	89.4		89.4		49.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	193.3		193.3		151.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	193.3		193.3		151.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		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	53.6		53.6		82
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		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	53.6		53.6		82
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		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	79.6		79.6		60
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		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	79.6		79.6		60
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		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	97.1		97.1		61.5
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		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	97.1		97.1		61.5
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		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	72.9		72.9		31.2
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		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	72.9		72.9		31.2
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		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	2,414.5		2,414.5		1,167
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		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	2,378.5		2,378.5		1,167
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	36		36		0
Fair value measured on a recurring basis | Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	312.3		312.3		228.8
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		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	312.3		312.3		228.8
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		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	1.2		1.2		1.2
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	0.6		0.6		0.6
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	0.6		0.6		0.6
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		0
Fair value measured on a recurring basis | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	85.4		85.4		82.7
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13.8		13.8		12.7
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	71.6		71.6		70
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,139.8		4,139.8		2,639.7
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	88.1		88.1		134
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	3,944.1		3,944.1		2,435.7
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	107.6		107.6		70
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	305.6		305.6		211.3
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	278.6		278.6		211.3
Short-term investments, at amortized cost (which approximates fair value)		325		325	211.3	376.8
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	27		27		0
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		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	111.2		111.2		70
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	111.2		111.2		70
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		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		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	11.1		11.1		26.6
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	11.1		11.1		26.6
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		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		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	11.4		11.4		653.2
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	11.4		11.4		653.2
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		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		0
Fair value measured on a recurring basis | Common equity securities | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.3		33.3		43.7
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.3		33.3		43.7
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.2		33.2		35.7
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.2		33.2		35.7
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		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	5.5		5.5
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	5.5		5.5
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 | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	28.1		28.1		27
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	28.1		28.1		27
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		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		0
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	113.2		113.2		183.3
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.5		90.5		162
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	22.7		22.7		21.3
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Common equity securities | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78		78		74.4
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		0		0
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78		78		74.4
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0		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	425		425		1,113.9
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	324.3		324.3		1,018.2
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	100.7		100.7		95.7
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		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	192.4		192.4		169.5
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		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		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	192.4		192.4		169.5
Carrying value of investment accounted for using the equity method	3.2	3.9	3.2	3.9	3.8	5.2
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8	15.2	12.8	15.2	14.7	16.8
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8		12.8		14.7
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	$ 9.7	8.3	$ 9.7	9.5	$ 10.1
Star & Shield Insurance Exchange | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale	$ 0.1		$ 0.1		$ 0.1
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
RBC Score			250.00%		250.00%

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,229.4	1,000
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	106.4	95.2
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	331.3	397.7
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	602.3	507.1
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	173	0
Standard & Poor's, B Rating [Member] | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 16.4	$ 0

Investment Securities (Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Non-Agency Residential Mortgage [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	27
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 113.7	$ 140.4

Investment Securities (Mortga64

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	102.3
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301.1	265.5
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	239.4	42.2
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.5	22.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	576	330.5
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	213.4	133.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	113.7	140.4
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	327.1	273.6
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	903.1	604.1
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	534	217.7
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	721.8	269.7
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	255.6	75.5
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,511.4	562.9
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,414.5	1,167
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,044.8	2,531.4	$ 2,561.6	$ 2,372.9
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301.1	265.5
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	239.4	42.2
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	35.5	22.8
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	576	330.5
Level 2 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	177.4	133.2
Level 2 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	113.7	140.4
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	291.1	273.6
Level 2 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	867.1	604.1
Level 2 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	534	217.7
Level 2 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	721.8	269.7
Level 2 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	255.6	75.5
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,511.4	562.9
Level 2 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,378.5	1,167
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	36	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	36	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36	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	$ 36	$ 0

Investment Securities (Non-agen

Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	102.3
Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.4
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	213.4	133.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	113.7	140.4
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	327.1	$ 273.6
Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.6
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	20.6
Securities Issued in 2005 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6
Securities Issued in 2005 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6
Securities Issued In 2006 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.2
Securities Issued In 2006 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.2
Securities Issued In 2007 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3
Securities Issued In 2008 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3
Securities Issued In 2009 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2010 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.6
Securities Issued In 2010 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.6
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13.2
Securities Issued In 2011 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11
Securities Issued In 2011 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11
Securities Issued in 2012 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6
Securities Issued in 2012 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.3
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	24.3
Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.4
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.6
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23
Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	59.6
Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23.5
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	83.1
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	48
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	45.9
Securities Issued in 2015 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	93.9
Securities Issued in 2016 [Member] | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	36
Securities Issued in 2016 [Member] | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.8
Securities Issued in 2016 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	45.8
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.2
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.4
Subordinate | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Subordinate | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	49.6
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9
Super Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Super Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	122.8
Super Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.2
Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.6
Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 55.2

Investment Securities (Non-ag66

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	213.4	$ 133.2
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	122.8
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.6
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	213.4
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	122.8
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.6
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-ag67

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2014USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 4,900.2	$ 4,050.9	$ 3,365.4	$ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average Basis Points of Subordination	0.0030
Fair value investments	$ 102.3
Commercial Mortgage Backed Securities Fixed Rate [Member] | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9
Commercial Mortgage Backed Securities Fixed Rate [Member] | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.2
Commercial Mortgage Backed Securities Fixed Rate [Member] | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	113.7	$ 140.4
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.9
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	55.2
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 49.6

Rollforward of Fair Value Measu

Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			$ 4,050.9	$ 3,343.6
Total realized and unrealized gains (losses)			84	(32.7)
Amortization/Accretion			(13.2)	(15.2)
Purchases			4,722.9	1,672
Sales			(3,944.4)	(1,540.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				61.4
Transfers in			82	41.7
Transfers out			(82)	(41.7)
Balance at September 30, 2016	$ 4,900.2	$ 3,365.4	4,900.2	3,365.4
Other Investments	351.4		351.4		$ 315.8
Net realized and unrealized investment gains (losses), after tax	21	(34.5)	60.2	(27)
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84	(33.9)
Short-term investments, at amortized cost (which approximates fair value)	305.6		$ 305.6		211.2
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			1,152.2	550.6
Total realized and unrealized gains (losses)			20	(19.5)
Amortization/Accretion			0	0
Purchases			1,577.5	550
Sales			(2,337.3)	(618.2)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				43.5
Transfers in			0	0
Transfers out			0	(1.2)
Balance at September 30, 2016	412.4	418.2	412.4	418.2
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,531.4	2,372.9
Total realized and unrealized gains (losses)			42.7	0.6
Amortization/Accretion			(13.2)	(15.2)
Purchases			2,989.4	1,056
Sales			(1,587.5)	(894.4)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0
Transfers in			82	41.7
Transfers out			0	0
Balance at September 30, 2016	4,044.8	2,561.6	4,044.8	2,561.6
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			70	76.4
Total realized and unrealized gains (losses)			1.6	(0.9)
Amortization/Accretion			0	0
Purchases			120.8	35.3
Sales			(2.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0
Transfers in			0	0
Transfers out			(82)	(40.5)
Balance at September 30, 2016	107.6	70.3	107.6	70.3
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0	39.5
Total realized and unrealized gains (losses)			0	7.7
Amortization/Accretion			0	0
Purchases			0	0
Sales			0	(9.4)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				3.6
Transfers in			0	0
Transfers out			0	0
Balance at September 30, 2016	0	34.2	0	34.2
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			169.5	126.9
Total realized and unrealized gains (losses)			20.7	(13.5)
Amortization/Accretion			0	0
Purchases			2.2	22.6
Sales			0	(2.3)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0
Transfers in			0	0
Transfers out			0	0
Balance at September 30, 2016	192.4	133.7	192.4	133.7
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			127.8	177.3
Total realized and unrealized gains (losses)			(1)	(7.1)
Amortization/Accretion			0	0
Purchases			33	8.1
Sales			(16.8)	(16.6)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				(14.3)
Transfers in			0	0
Transfers out			0	0
Balance at September 30, 2016	143	147.4	143	147.4
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			4,134.4
Balance at September 30, 2016	5,062.8		5,062.8
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			1,363.5
Balance at September 30, 2016	691		691
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,531.4
Balance at September 30, 2016	4,071.8		4,071.8
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			239.5
Balance at September 30, 2016	300		300
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,639.7
Balance at September 30, 2016	4,139.8		4,139.8
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			134
Balance at September 30, 2016	88.1		88.1
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,435.7
Balance at September 30, 2016	3,944.1		3,944.1
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			70
Balance at September 30, 2016	107.6		107.6
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			169.5
Balance at September 30, 2016	192.4		192.4
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0
Balance at September 30, 2016	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]
Balance at January 1, 2016			0
Balance at September 30, 2016	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]
Balance at January 1, 2016			169.5
Balance at September 30, 2016	192.4		192.4
Carrying value of investment accounted for using the equity method	3.2	3.9	3.2	3.9	3.8	$ 5.2
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			211.3
Balance at September 30, 2016	305.6		305.6
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			211.3
Balance at September 30, 2016	278.6		278.6
Short-term investments, at amortized cost (which approximates fair value)		325		325	211.3	376.8
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0
Balance at September 30, 2016	27		27
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0
Balance at September 30, 2016	0		0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains (losses)	0	(0.9)	(1.5)	31.4
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	0.6	0.1	0.6	0.1	1.9
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains (losses)				0.8
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8	15.2	12.8	15.2	14.7	16.8
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8		12.8		14.7
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	$ 9.7	$ 8.3	$ 9.7	$ 9.5	$ 10.1

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016USD ($)Investment	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)Investment	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)inputInvestment
Quantitative information for Level 3 Fair Value Measurements Assets
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Number of Investments with Significant Unobseravble Inputs | Investment	1		1		1
NR | Preferred Stock | Par Value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure					$ 70
NR | Preferred Stock | Issuer's intent to call [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Measurements, Valuation Techniques					70
NR | Convertible Preferred Stock [Member] | Multiple of EBITA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs | input					0
Fair Value Measurements, Valuation Techniques					6
NR | Convertible Preferred Stock [Member] | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure					$ 5.7
NR | Seller Priority Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 49		$ 49		$ 38
Timing of interest payments			5 years		5 years
Timing of principal payments			10 years		10 years
Fair Value Measurements, Valuation Techniques			0.095		0.13
NR | Pari Passu Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	22.5		$ 22.5		$ 13.5
Timing of interest payments			5 years		5 years
Timing of principal payments			15 years		15 years
Fair Value Measurements, Valuation Techniques			0.158		0.224
AAA | Mortgage-backed and asset-backed securities | Broker Pricing [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	36		$ 36
Standard & Poor's, AA+ Rating [Member] | Residential Mortgage Backed Securities | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Conditional Prepayment Rate			1.50%
MSA [Member] | NR | Preferred Stock | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	71.6		$ 71.6
Fair Value Measurements, Valuation Techniques			0.06245
PassportCard [Member] | NR | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	21		$ 21		$ 21
PassportCard [Member] | NR | Private equity funds | Average Share Price [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Measurements, Valuation Techniques			1		1
Compare.com [Member] | NR | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	33.8		$ 33.8
Compare.com [Member] | NR | Private equity funds | Average Share Price [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Measurements, Valuation Techniques			1.03
Compare.com [Member] | NR | Private equity funds | Recent Transaction Two [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure					$ 33.8
Fair Value Measurements, Valuation Techniques					1.03
OneTitle [Member] | NR | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	3.2		$ 3.2
OneTitle [Member] | NR | Private equity funds | Average Share Price [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Measurements, Valuation Techniques			2.521
Galvanic [Member] | NR | Convertible Preferred Stock [Member] | Multiple of EBITA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Measurements, Valuation Techniques			6
Galvanic [Member] | NR | Convertible Preferred Stock [Member] | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	6.1		$ 6.1
Captricity [Member] | NR | Convertible Preferred Stock [Member] | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 27		$ 27		$ 27
Fair Value Measurements, Valuation Techniques			3.83		3.83
durchblicker.at [Member] | NR | Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Time Until Expiration of Security			4 years		4 years
Volatility Rate	60.00%		60.00%		60.00%
Risk Free Rate	1.15%		1.15%		1.15%
durchblicker.at [Member] | NR | Private equity funds | Option pricing method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 9.9		$ 9.9		$ 9.6
Standard & Poor's, B Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points			2.50%
Standard & Poor's, CCC Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points					2.50%

Goodwill and Other Intangible70

Goodwill and Other Intangible Assets (Details) ₪ in Millions, £ in Millions, $ in Millions	Aug. 04, 2016GBP (£)	Aug. 04, 2016USD ($)	Feb. 23, 2015ILS (₪)	Feb. 23, 2015USD ($)	Mar. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2016USD ($)	Dec. 31, 2015USD ($)	Jun. 30, 2015USD ($)	Dec. 31, 2014USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Acquired During Period							$ 0.3
Payments to Acquire Businesses and Interest in Affiliates						$ 13.7	$ 2.4
Assets						6,683.3			$ 10,282.6
Goodwill								$ 24.1	24.1	$ 24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)								$ 29.2	31.3	$ 36.4	$ 38.7
Liabilities						2,881.9			5,914.6
Non-controlling interests						149.2			454.8
Buzzmove [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Ownership interest (as a percent)		70.90%
Non-controlling interests		$ 2.6				$ 3.3			$ 0
Lowfares.com [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates					$ 3.9
United States of America, Dollars | Buzzmove [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		8
United Kingdom, Pounds | Buzzmove [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates	£ 6	6
Common Stock | Israel, New Shekels | Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪			₪ 35.9
Common Stock | United States of America, Dollars | Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				$ 8.9
Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Assets		11.7		5.5
Goodwill				0.3
Intangible Assets, Net (Excluding Goodwill)		9		2.8
Liabilities		$ 0.1		$ 1.2

Goodwill and Other Intangible71

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill					$ 24.1	$ 24.1	$ 24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)					29.2	31.3	36.4	38.7
Goodwill and other intangible assets	$ 59.3	$ 58	$ 59.3	$ 58	53.3	55.4	60.5	62.5
Other Intangible Assets, Net	9		9
Disposal Group, Including Discontinued Operation, Intangible Assets		0.5		0.5	$ 0	0.4	0.6	0.8
Goodwill, Acquired During Period				0.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			9	2.8
Finite-lived Intangible Assets Acquired			3.9
Amortization of Intangible Assets	(3)	(2.6)	(9.4)	(7.9)
WTM entities except Buzzmove [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	24.1	24.1	24.1	24.1
Intangible Assets, Net (Excluding Goodwill)	26.2	33.9	26.2	33.9
Goodwill and other intangible assets	50.3	58	50.3	58
Intangible Asset and Goodwill [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Disposal Group, Including Discontinued Operation, Intangible Assets		0.5		0.5		$ 0.4	$ 0.6	$ 0.8
Acquisition of goodwill and intangible assets				3.1
Finite-lived Intangible Assets Acquired			3.9
Amortization of Intangible Assets	$ (3)	$ (2.6)	$ (9.4)	$ (7.9)

Debt (Details)

Debt (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Nov. 30, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		11,943			8,208
Total debt		$ 288.5			$ 288.5		$ 337.6
Repayments of Debt					401.8	$ 84.5
Draw down of debt and revolving line of credit					$ 352.5	$ 98.5
MediaAlpha Bank Facility [Member]
Debt Instrument
Debt Instrument, Basis Spread on Variable Rate					1.50%
2012 OBH Senior Notes
Debt Instrument
Proceeds from Issuance of Debt				$ 275
WTM Bank Facility
Debt Instrument
Debt Instrument, Interest Rate, Effective Percentage							3.90%
Total commitment under revolving credit facility									$ 425
Debt, Weighted Average Interest Rate		3.85%			3.85%
Repayments of Debt		$ 250			$ 400
Proceeds from Issuance of Debt			$ 250		350
Term Loan [Member]
Debt Instrument
Repayments of Debt		0.8			0.8
Revolving Credit Facility [Member]
Debt Instrument
Repayments of Debt		0.5			1
Proceeds from Issuance of Debt					2.5
OB Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility		65			65
One Beacon Holdings Inc [Member] | 2012 OBH Senior Notes
Debt Instrument
Debt Instrument, Interest Rate, Effective Percentage	4.70%
Proceeds from Issuance of Debt	$ 272.9
Debt Instrument, Interest Rate, Stated Percentage	4.60%
Debt Instrument, Issue Price as Percentage of Par Value	99.90%
OneBeacon [Member] | OneBeacon Bank Facility [Member]
Debt Instrument
Long-term Line of Credit		0			0
OneBeacon [Member] | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value		275			275		$ 275
Unamortized original issue discount		(1.9)			(1.9)		(2.1)
Total debt		$ 273.1			$ 273.1		$ 272.9
Debt Instrument, Interest Rate, Effective Percentage		4.70%			4.70%		4.70%
White Mountains | WTM Bank Facility
Debt Instrument
Total debt		$ 0			$ 0		$ 50
Tranzact [Member] | OneBeacon Bank Facility [Member]
Debt Instrument
Long-term Line of Credit							0
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total debt		1.5			1.5
Long-term Line of Credit		$ 15.6			$ 15.6		$ 15
Debt Instrument, Interest Rate, Effective Percentage		5.80%			5.80%		5.50%
Unamortized Debt Issuance Expense		$ (0.2)			$ (0.2)		$ (0.3)
Line of Credit Facility, Fair Value of Amount Outstanding		15.4			15.4		$ 14.7
Total commitment under revolving credit facility								$ 20
Notional amount		14.2			14.2
Line of Credit Facility, Remaining Borrowing Capacity		5			5
Failure to pay principal and interest [Member] | OneBeacon [Member] | 2012 OBH Senior Notes
Debt Instrument
Total debt		$ 75			$ 75

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Percent	(33.50%)				(84.90%)
U.S. statutory rate (as a percent)	35.00%				35.00%	35.00%
Income Tax Expense (Benefit)	$ (6.7)			$ (1.6)	$ (22.4)	$ 0.8
Equity in earnings of unconsolidated affiliates, net of tax	0			$ 3.9	0	$ 18
Tax Year 2012 [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards		$ 1.7
Tax Assessment Year 2010 to 2012 [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)		$ 3.5			3.5
Tax Assessment Year 2007 to 2009 [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)			$ 12.8		12.8
Discontinued Operations [Member] | Tranzact [Member]
Income Tax Contingency [Line Items]
Income Tax Expense (Benefit)	$ (14)				$ (14)

Derivatives (Details)

Derivatives (Details) ¥ in Billions, $ in Billions	Dec. 31, 2015USD ($)	Dec. 31, 2015JPY (¥)
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%	113.00%
White Mountains Life Re
Notional amount	$ 0.4	¥ 50.7
Collective account values of underlying variable annuities as a percentage of the guarantee value	109.00%	109.00%

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Pre-tax operating results of WM Life Re
Total revenues	365	296.1	1,092.5	1,019.1
General and administrative expenses	(70.2)	(97.3)	(230.4)	(231.6)
Pre-tax income (loss) from continuing operations	20	(86.9)	26.4	(89)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	0	2.1	1.2	7.7
Change in fair value of variable annuity liability, included in other revenue	0	(2.8)	(0.3)	(1.7)
Realized and unrealized gains (losses)	0	1.5	(2)	(6.3)
Foreign exchange, included in other revenue	0	0.4	1.4	(0.8)
Other investment income and losses	0	(0.2)	0	(0.3)
Total revenues	0	1	0.3	(1.4)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	0	(0.3)	0
Death benefit claims paid, included in general and administrative expenses	0	0	0	0
General and administrative expenses	(0.5)	(0.8)	(2.4)	(3.1)
Pre-tax income (loss) from continuing operations	$ (0.5)	$ 0.2	$ (2.4)	$ (4.5)

Derivatives Realized and Unreal

Derivatives Realized and Unrealized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net					$ 20.1
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	1.5	(2)	(6.3)
Derivative, fair value, net		37		37	20.1	$ 35.7	$ 56.4
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Derivative Transfers In (Out), Net		0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		0.2	18.1	13.1
Fixed income/interest rate | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	2.6	1.8	6
Derivative, fair value, net					0.5
Foreign exchange | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0	(3.1)	(4.8)	(6)
Derivative, fair value, net					14.8
Equity | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	$ 0	2	1	(6.3)
Derivative, fair value, net					4.8
Level 2 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		3.2	(0.7)	(2.7)
Derivative, fair value, net		27.9		27.9	16.5	32.5	33.8
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Derivative Transfers In (Out), Net		0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		7.8	15.8	3.2
Level 1 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		(5.1)	(4.2)	1.9
Derivative, fair value, net		(2.1)		(2.1)	0.9	(5.1)	3.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Derivative Transfers In (Out), Net		0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		(8.1)	(3.3)	7.7
Fair Value, Inputs, Level 3 [Member] | Variable Annuity | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		1		1	(0.3)	(1.8)	(0.7)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(2.8)	(0.3)	(1.7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers		0	0	0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements		0	0	0
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		3.4	2.9	(5.5)
Derivative, fair value, net		11.2		11.2	$ 2.7	$ 8.3	$ 18.9
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Derivative Transfers In (Out), Net		0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		$ 0.5	$ 5.6	$ 2.2

Derivatives Variable Annuity Re

Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2015JPY (¥)
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value					113.00%	113.00%
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Notional amount					$ 400	¥ 50.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			$ 0
Changes in derivative instruments
Beginning of period		$ 35.7	20.1	$ 56.4
Purchases		0		0
Realized and unrealized gains (losses)	$ 0	1.5	(2)	(6.3)
Transfers in		0	0	0
Sales/settlements		(0.2)	(18.1)	(13.1)
End of period		37		37
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0	0	(0.3)	0
Collective account values of underlying variable annuities as a percentage of the guarantee value					109.00%	109.00%
White Mountains Life Re | Level 2 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Changes in derivative instruments
Beginning of period		32.5	16.5	33.8
Purchases		0		0
Realized and unrealized gains (losses)		3.2	(0.7)	(2.7)
Transfers in		0	0	0
Sales/settlements		(7.8)	(15.8)	(3.2)
End of period		27.9		27.9
White Mountains Life Re | Level 1 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Changes in derivative instruments
Beginning of period		(5.1)	0.9	3.7
Purchases		0		0
Realized and unrealized gains (losses)		(5.1)	(4.2)	1.9
Transfers in		0	0	0
Sales/settlements		8.1	3.3	(7.7)
End of period		(2.1)		(2.1)
Variable Annuity | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Changes in derivative instruments
Beginning balance		(1.8)	(0.3)	(0.7)
Realized and unrealized gains (losses)		(2.8)	(0.3)	(1.7)
Transfers in		0	0	0
Sales/settlements		0	0	0
Ending balance		1		1
Purchases		0		0
Derivative [Member] | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0
Changes in derivative instruments
Beginning of period		8.3	2.7	18.9
Purchases		0		0
Realized and unrealized gains (losses)		3.4	2.9	(5.5)
Transfers in		0	0	0
Sales/settlements		(0.5)	$ (5.6)	(2.2)
End of period		$ 11.2		$ 11.2

Derivatives Additional Collater

Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015
Credit risk on derivative instruments
Collateral cash and investments	$ 5.8	$ 16.8
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	5.8	16.8
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 0	$ 0

Derivatives (Value, Collateral

Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) - White Mountains Life Re $ in Millions	Dec. 31, 2015USD ($)
Offsetting Liabilities [Line Items]
Derivative Assets	$ 23.2
Derivative Liabilities	(3.1)
Net amounts recognized in Other assets	20.1
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.4
Derivative Liabilities	(2.1)
Net amounts recognized in Other assets	0.3
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	15
Derivative Liabilities	0
Net amounts recognized in Other assets	15
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	4.4
Derivative Liabilities	(0.6)
Net amounts recognized in Other assets	3.8
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.1
Derivative Liabilities	(0.1)
Net amounts recognized in Other assets	0
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.1
Derivative Liabilities	(0.3)
Net amounts recognized in Other assets	(0.2)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.2
Derivative Liabilities	0
Net amounts recognized in Other assets	$ 1.2

Derivatives (Uncollateralized A

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Dec. 31, 2015USD ($)Rating
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 20,100
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	20,100
Excess collateral provided to counterparty in cash	5,800
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	6,900
Net amount of exposure to counter-party	$ 19,000
S and P Credit Ratings | Rating	23
Standard & Poor's, A Rating [Member] | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 700
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	700
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	700
Standard & Poor's, A Rating [Member] | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	9,900
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	9,900
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	1,400
Net amount of exposure to counter-party	8,500
Standard & Poor's, A Rating [Member] | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	1,000
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,000
Excess collateral provided to counterparty in cash	5,800
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	6,800
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	8,500
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	8,500
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	5,500
Net amount of exposure to counter-party	$ 3,000

Municipal Bond Guarantee (Detai

Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Jul. 31, 2012USD ($)	Sep. 30, 2016USD ($)Contract	Mar. 31, 2016	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)yrContract	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)Contract
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ 20		$ (86.9)	$ 26.4	$ (89)
Net investment income (loss) - surplus note interest		$ 0		0	$ 0	0
BAM
Guarantor Obligations [Line Items]
Municipal bond contract | Contract		4,357			4,357		3,103
Insured Financial Obligations, Remaining Weighted Average Contract Period					10 years 11 months		12 years 10 months
Municipal bond outstanding principal		$ 30,470			$ 30,470		$ 22,556
Municipal bond outstanding interest		15,258.1			15,258.1		11,984.4
Municipal bond debt service outstanding		45,728.1			$ 45,728.1		34,540.4
Year of U.S. Treasury Rate | yr					1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Remainder of Fiscal Year		1.6			$ 1.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Second Quarter		1.6			1.6
Financial Guarantee Insurance Contracts, Premium Receivable, Premium Expected to be Collected, Third Quarter		1.6			1.6
Financial Guarantee Insurance Contracts, Premium Receivable, Premium Expected to be Collected, Next Quarter		1.6			1.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months		1.5			1.5
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		6.3			6.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three		6			6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five		5.4			5.4
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four		5.7			5.7
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten		46			46
Financial Guarantee Insurance Contracts, Unearned Premium Revenue		71			71		$ 50.2
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 594.5
Surplus Notes	503
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ 5.2			$ 18.4
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		96.90%			96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		88.40%			88.40%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ (13.7)			$ (30.3)
BAM
Guarantor Obligations [Line Items]
Pre-tax (loss) income		(13.7)		(10.6)	(30.3)	(33.5)
Net investment income (loss) - surplus note interest		(4.5)		(4)	(13.4)	(11.9)
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance		476.5			476.5
HG Global
Guarantor Obligations [Line Items]
Pre-tax (loss) income		5.2		5.2	18.4	13.8
Net investment income (loss) - surplus note interest		4.5		$ 4	13.4	$ 11.9
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points			3.00%
Interest Income, Other		$ 4.5			$ 13.4
Derivative, Fixed Interest Rate		3.54%			3.54%		3.15%
Surplus Note | Build America Mutual
Guarantor Obligations [Line Items]
Interest expense		$ 4.5			$ 13.4
Combination One [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr					5
Combination Two [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr					3
Financial Guarantee [Member]
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured					15.00%
Fixed interest rate [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate		8.00%			8.00%

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 29.8	$ (65.4)	$ 24.2	$ (47.6)
Allocation of income for unvested restricted common shares	(0.4)	0	(0.3)	0
Dividends declared on participating restricted common shares	0	0	(0.1)	(0.1)
Total allocation to restricted common shares	(0.4)	0	(0.4)	(0.1)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	29.4	(65.4)	23.8	(47.7)
Undistributed net earnings (in millions):
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	29.4	(65.4)	23.8	(47.7)
Dividends declared net of restricted common share amounts	0	0	(5.4)	(5.9)
Total undistributed net earnings, net of restricted common share amounts	$ 29.4	$ (65.4)	$ 18.4	$ (53.6)
Basic earnings per share denominators
Total average common shares outstanding during the period	4,867,400	5,890,100	5,166,600	5,951,100
Average unvested restricted common shares	(68,100)	0	(62,900)	0
Basic earnings per share denominator	4,799,300	5,890,100	5,103,700	5,951,100
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,867,400	5,890,100	5,166,600	5,951,100
Average unvested restricted common shares	(68,100)	0	(62,900)	0
Average outstanding dilutive options to acquire common shares	4,879,400	5,890,100	5,174,800	5,951,100
Diluted earnings (loss) per share denominator	4,811,300	5,890,100	5,111,900	5,951,100
Basic earnings per share (in dollars):
Continuing operations	$ 6.12	$ (11.10)	$ 4.67	$ (8.01)
Undistributed earnings (loss) (in dollars per share)	6.12	(11.10)	3.67	(9.01)
Earnings Per Share, Diluted [Abstract]
Continuing operations	6.11	(11.10)	4.66	(8.01)
Dividends declared	0	0	(1)	(1)
Undistributed earnings (loss) (in dollars per share)	$ 6.11	$ (11.10)	$ 3.66	$ (9.01)
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	11,943		8,208
Discontinued Operations [Member]
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	$ 0.9	$ 0.1	$ (5.1)	$ 0.9
Basic earnings per share denominators
Total average common shares outstanding during the period	4,879,400	5,890,100	5,174,800	5,951,100
Average unvested restricted common shares	(68,100)	(70,900)	(62,900)	(67,100)
Basic earnings per share denominator	4,799,300	5,819,200	5,103,700	5,884,000
Diluted earnings per share denominator
Total average common shares outstanding during the period	4,879,400	5,890,100	5,174,800	5,951,100
Average unvested restricted common shares	(68,100)	(70,900)	(62,900)	(67,100)
Diluted earnings (loss) per share denominator	4,811,300	5,819,200	5,111,900	5,884,000
Employee Stock Option [Member]
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	120,000		120,000

Non-controlling Interest (Detai

Non-controlling Interest (Details) - USD ($) $ in Millions	Sep. 30, 2016	Aug. 04, 2016	Dec. 31, 2015
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 149.2		$ 454.8
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 36.9		$ 115.2
OneBeacon [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	23.90%		24.50%
Total non-controlling interests	$ 247		$ 245.6
SIG Preference Shares
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage			100.00%
Total non-controlling interests	$ 0		$ 250
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%		3.10%
Total non-controlling interests	$ 16.9		$ 17.1
MediaAlpha [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%		40.00%
Total non-controlling interests	$ 12.5		$ 14.4
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%		36.80%
Total non-controlling interests	$ 0		$ 79.4
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.30%		3.90%
Total non-controlling interests	$ 0.5		$ 0.6
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	18.80%		19.00%
Total non-controlling interests	$ 3.7		$ 3.7
Buzzmove [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	29.10%		0.00%
Total non-controlling interests	$ 3.3	$ 2.6	$ 0
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%		100.00%
Total non-controlling interests	$ (138.9)		$ (140)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%		100.00%
Total non-controlling interests	$ 4.2		$ (16)
Star & Shield Insurance Exchange | Total mutual and reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (134.7)		$ (156)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment reporting information
Premiums Earned, Net	$ 281.3	$ 284.9	$ 838.1	$ 896
Net investment income	22.5	16.8	59.6	42.6
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84	(33.9)
Other Income	34.8	38.3	110.8	114.4
Total revenues	365	296.1	1,092.5	1,019.1
Loss and loss adjustment expenses	165	170	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4	55.3	159.2	166.1
Other Underwriting Expense	49.6	56.5	156	165.5
General and administrative expenses	70.2	97.3	230.4	231.6
Amortization of Intangible Assets	3	2.6	9.4	7.9
Interest expense	3.8	3.9	12.4	10.8
Total expenses	345	383	1,066.1	1,108.1
Pre-tax (loss) income	20	(86.9)	26.4	(89)
OneBeacon [Member]
Segment reporting information
Premiums Earned, Net	277.9	281.4	827.9	887.3
Net investment income	11.8	12.4	38.3	33
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	15.5	(29.9)	56.8	(29.7)
Other Income	1.8	3.9	3.5	(0.4)
Total revenues	307	267.8	926.5	890.2
Loss and loss adjustment expenses	162.8	167.5	501.3	527.9
Insurance and Reinsurance Acquisition Expenses	55.1	53.8	154.8	161.2
Other Underwriting Expense	49.4	56.4	155.6	165.2
Interest expense	3.3	3.2	9.8	9.7
Total expenses	274.1	284.6	832.4	876
Pre-tax (loss) income	32.9	(16.8)	94.1	14.2
HG Global
Segment reporting information
Premiums Earned, Net	1.2	0.6	3.1	1.7
Net investment income	0.6	0.5	1.6	1.4
Net investment income (loss) - surplus note interest	4.5	4	13.4	11.9
Net realized and unrealized investment gains (losses)	(0.3)	0.6	2.3	0.3
Other Income	0	0	0	0
Total revenues	6	5.7	20.4	15.3
Loss and loss adjustment expenses	0	0	0	0
Insurance and Reinsurance Acquisition Expenses	0.2	0.1	0.6	0.3
Other Underwriting Expense	0	0	0	0
Interest expense	0	0	0	0
Total expenses	0.8	0.5	2	1.5
Pre-tax (loss) income	5.2	5.2	18.4	13.8
BAM
Segment reporting information
Premiums Earned, Net	0.3	0.3	1	0.6
Net investment income	1.7	1.1	5.1	2.9
Net investment income (loss) - surplus note interest	(4.5)	(4)	(13.4)	(11.9)
Net realized and unrealized investment gains (losses)	(1.6)	1.7	6.5	2.8
Other Income	0.4	0.2	0.8	0.5
Total revenues	(3.7)	(0.7)	0	(5.1)
Loss and loss adjustment expenses	0	0	0	0
Insurance and Reinsurance Acquisition Expenses	0.6	0.4	1.9	1.8
Other Underwriting Expense	0.1	0.1	0.3	0.3
Interest expense	0	0	0	0
Total expenses	10	9.9	30.3	28.4
Pre-tax (loss) income	(13.7)	(10.6)	(30.3)	(33.5)
Other Operations
Segment reporting information
Premiums Earned, Net	1.9	2.6	6.1	6.4
Net investment income	8.4	2.8	14.6	5.3
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	12.8	(16.3)	18.4	(7.3)
Other Income	32.6	34.2	106.5	114.3
Total revenues	55.7	23.3	145.6	118.7
Loss and loss adjustment expenses	2.2	2.5	6.8	6.2
Insurance and Reinsurance Acquisition Expenses	0.5	1	1.9	2.8
Other Underwriting Expense	0.1	0	0.1	0
Interest expense	0.5	0.7	2.6	1.1
Total expenses	60.1	88	201.4	202.2
Pre-tax (loss) income	(4.4)	(64.7)	(55.8)	(83.5)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	67.2	94.7	221	223.7
Segment Reconciling Items [Member] | OneBeacon [Member]
Segment reporting information
General and administrative expenses	3.2	3.4	10	11
Segment Reconciling Items [Member] | HG Global
Segment reporting information
General and administrative expenses	0.6	0.4	1.4	1.2
Segment Reconciling Items [Member] | BAM
Segment reporting information
General and administrative expenses	9.3	9.4	28.1	26.3
Segment Reconciling Items [Member] | Other Operations
Segment reporting information
General and administrative expenses	54.1	81.5	181.5	185.2
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	3	2.6	9.4	7.9
General and Administrative Expense [Member] | OneBeacon [Member]
Segment reporting information
Amortization of Intangible Assets	0.3	0.3	0.9	1
General and Administrative Expense [Member] | HG Global
Segment reporting information
Amortization of Intangible Assets	0	0	0	0
General and Administrative Expense [Member] | BAM
Segment reporting information
Amortization of Intangible Assets	0	0	0	0
General and Administrative Expense [Member] | Other Operations
Segment reporting information
Amortization of Intangible Assets	2.7	2.3	8.5	6.9
Star & Shield Insurance Exchange
Segment reporting information
Other Income			(21)
MediaAlpha [Member] | Other Operations
Segment reporting information
Other Income	27.6	28.3	88.4	81.2
General and administrative expenses	$ 26.2	$ 26.2	$ 82.7	$ 75.8

Investments in Unconsolidated85

Investments in Unconsolidated Affiliates (Investment in Unconsolidated Affiliates Table) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9			$ 0	$ 18
Symetra | Common Stock
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax		$ 4.2				$ 17.5
Hamer, LLC [Member]
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax			$ 1.1	$ 1.6

Investments in Unconsolidated86

Investments in Unconsolidated Affiliates (Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 01, 2016	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2011
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax		$ 0			$ 3.9	$ 0	$ 18
Unrealized gains (losses) on investments		18.1			(36.3)	(186.4)	(80.3)
Revenues		365			296.1	1,092.5	1,019.1
Total expenses		345			383	1,066.1	1,108.1
Pre-tax (loss) income		20			(86.9)	26.4	(89)
Net income		88.6			(74.9)	470.4	5.5
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax		$ 15.4			(28.4)	(164.2)	(57.7)
Sales of common equity securities						$ 961.1	370.6
Common shares, par value per share (in dollars per share)		$ 1		$ 1		$ 1
Net realized and unrealized investment gains (losses)		$ 26.4			(43.9)	$ 84	(33.9)
Symetra
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments				$ 258.8
Dividends Payable, Amount Per Share	$ 32
Equity Method Investments Fair Value Per Share								$ 15
Term of amortization (in years)						30 years
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					160.8		160.8
Trading Securities Unrealized Holding Gain (Loss) on Investments 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 (losses)			$ 4.7
Common Stock | Symetra
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax					4.2		17.5
Proceeds from Equity Method Investment, Dividends or Distributions					12.5		16.9
equity in earnings of unconsolidated affiliates | Symetra
Investments in unconsolidated affiliates
After tax amortization through equity in earnings					0.6		2
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					32.6		32.6
Net unrealized gains (losses) of unconsolidated affiliates | Symetra
Investments in unconsolidated affiliates
After tax amortization through equity in net unrealized losses					2.6		8.1
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity					128.2		128.2
Amortization of common share basis difference | Common Stock | Symetra
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax					0.7		2.1
Special Dividend [Member] | Common Stock | Symetra
Investments in unconsolidated affiliates
Proceeds from Equity Method Investment, Dividends or Distributions					$ 10.3		$ 10.3

Investments in Unconsolidated87

Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Changes in carrying value of investment in Symetra
Equity in earnings	$ 0	$ 3.9		$ 0	$ 18
Income Tax Expense (Benefit)	$ (6.7)	(1.6)		$ (22.4)	0.8
Symetra | Common Stock
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period		397.2	$ 411.4		411.4
Equity in earnings		4.2			17.5
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio		3.7			(31.8)
Dividends received		(12.5)			(16.9)
Carrying value of a distribution		0			12.4
Carrying value at the end of the period		392.6	$ 397.2		392.6
White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio		5.7
equity in earnings of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)		0.3			1.1
Net unrealized gains (losses) of unconsolidated affiliates | White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)		0.2
Amortization of common share basis difference | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings		0.7			2.1
impairment of net unrealized gains (losses) | Net unrealized gains (losses) of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio		$ 2.9			$ 8.6

Investments in Unconsolidated88

Investments in Unconsolidated Affiliates Hamer (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Realized Gain (Loss) on Disposal			$ 20
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9			$ 0	$ 18
Hamer, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax			$ 1.1	$ 1.6

Employee Share-Based Incentiv89

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	4,578,698			4,578,698		5,623,735
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9		$ 0	$ 18
Accrued Expense
Accrued expense at the beginning of period				140.7
Accrued expense at the end of the period	$ 124.2			$ 124.2		$ 140.7
Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	5,000			5,000
Accrued Expense
Percentage of target payout - minimum actual				140.00%
Percentage of target payout - maximum actual				142.00%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual						88.00%
Percentage of target payout - maximum actual						131.50%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2015 to 2017 [Member]
Target Performance Share Activity
New grants (in shares)	3,000
Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018
Target Performance Share Activity
New grants (in shares)	3,400
White Mountains | Restricted Share Cliff Vesting in January 2019 [Member] | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	4,150			25,365
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	73,297	106,572	111,257	93,654	111,257	111,257
Shares paid or expired (in shares)		(8,958)		(36,294)	(42,959)
New grants (in shares)	6,400			22,615	29,195
Assumed forfeitures and cancellations	(160)	(3,960)		(438)	(3,839)
Ending of period (in shares)	79,537	93,654	106,572	79,537	93,654	93,654
Accrued Expense
Accrued expense at the beginning of period	$ 32.5	$ 28.8	$ 44.4	$ 57.7	$ 44.4	$ 44.4
Payments and deferrals		(5.5)		(41)	(30.4)
Assumed forfeitures and cancellations	(0.1)	(0.8)		(0.2)	(0.3)
Expense recognized	5	30		20.5	38.8
Accrued expense at the end of the period	37.4	$ 52.5	$ 28.8	$ 37.4	$ 52.5	$ 57.7
Vesting Period				3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 26.9			$ 26.9
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	7,315	10,825		7,315	10,825

Employee Share-Based Incentiv90

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 124.2		$ 140.7
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	81,577
Accrued Expense	$ 38.4
Target Performance Shares Outstanding, Assumed forfeitures	(2,040)
Accrued Expense, Assumed forfeitures	(1)
Total Target Performance Shares Outstanding	79,537	73,297	93,654	93,654	106,572	111,257
Total Accrued Expense	$ 37.4	$ 32.5	$ 57.7	$ 52.5	$ 28.8	$ 44.4
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	19,615
Accrued Expense	$ 3.4
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	31,795
Accrued Expense	$ 15.1
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	30,167
Accrued Expense	$ 19.9

Employee Share-Based Incentiv91

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - USD ($) $ / shares in Units, $ in Millions	May 05, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax		$ 0		$ 3.9		$ 0	$ 18
Non-Qualified Options [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value		$ 100				$ 100
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage						50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number Exercised	5,000
Target Restricted Share Activity
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number		120,000				120,000		125,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price	$ 742							$ 742
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)		4,150				25,365
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)							23,640
White Mountains | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)		66,470		71,875	83,314	70,675	83,314
Issued (in shares)		4,150				25,365	23,640
Vested (in shares)		0		(1,200)		(24,620)	(36,279)
Assumed forfeitures and cancellations		0		0		(800)	0
Ending of period (in shares)		70,620	66,470	70,675	71,875	70,620	70,675
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance		$ 24.2		$ 23.2	$ 14.3	$ 15.7	$ 14.3
Issued		3.4				19.7	15.7
Assumed forfeitures and cancellations		0		0		0.2	0
Expense recognized		(3.8)		(3.7)		(11.8)	(10.5)
Unamortized grant date fair value, ending balance		$ 23.8	$ 24.2	$ 19.5	$ 23.2	$ 23.8	$ 19.5
Sirius Group | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,512		3,572		2,512	3,572
Unamortized grant date fair value
Expense recognized			$ 1.4

Employee Share-Based Incentiv92

Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9		$ 0	$ 18
Expense Amortized
Accrued expense at the beginning of period				140.7
Accrued expense at the end of the period	$ 124.2			$ 124.2		$ 140.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period				3 years
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Expense Amortized
Percentage of target payout - minimum actual				24.30%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual						45.70%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	441,206	462,147	517,470	449,435	517,470	517,470
Shares paid or expired (in shares)	0	0		(167,300)	(181,290)
New grants (in shares)	0	0		163,150	154,887
Assumed forfeitures and cancellations	0	0		(4,079)	(28,920)
Ending of period (in shares)	441,206	462,147	462,147	441,206	462,147	449,435
Expense Amortized
Accrued expense at the beginning of period	$ 1	$ 2.4	$ 3.4	$ 1.4	$ 3.4	$ 3.4
Payments and deferrals	0	0		(0.7)	(1.5)
Assumed forfeitures and cancellations	0	0		0	0
Expense recognized	0.3	(0.3)		0.6	0.2
Accrued expense at the end of the period	1.3	$ 2.1	$ 2.4	1.3	$ 2.1	$ 1.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 2.3			$ 2.3

Employee Share-Based Incentiv93

Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 124.2		$ 140.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	452,519
Incentive Compensation Payable before Forfeitures	$ 1.3
Target Performance Shares Outstanding, Assumed forfeitures	(11,313)
Accrued Expense, Assumed forfeitures	0
Total Target Performance Shares Outstanding	441,206	441,206	449,435	462,147	462,147	517,470
Accrued incentive compensation	$ 1.3	$ 1	$ 1.4	$ 2.1	$ 2.4	$ 3.4
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	163,150
Incentive Compensation Payable before Forfeitures	$ 0.6
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	146,659
Incentive Compensation Payable before Forfeitures	$ 0.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	142,710
Incentive Compensation Payable before Forfeitures	$ 0

Employee Share-Based Incentiv94

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) $ in Millions	Feb. 24, 2016shares	Feb. 24, 2015shares	May 31, 2011shares	Sep. 30, 2016USD ($)shares	Sep. 30, 2015USD ($)shares	Jun. 30, 2015USD ($)shares	Sep. 30, 2016USD ($)shares	Sep. 30, 2015USD ($)shares	May 25, 2011Installment
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax | $				$ 0	$ 3.9		$ 0	$ 18
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				395,872	390,950	612,500	382,722	612,500
Issued (in shares)	170,650	75,950		0	0		170,650	75,950
Vested (in shares)				0	0		(157,500)	(296,000)
Assumed forfeitures and cancellations				0	0		0	(1,500)
Ending of period (in shares)				395,872	390,950	390,950	395,872	390,950
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $				$ 3.5	$ 3.5	$ 3.5	$ 2.5	$ 3.5
Issued | $				0	0		2.3	1.1
Vested | $							0
Assumed forfeitures and cancellations | $				0	0		0	0
Expense recognized | $				(0.7)	(0.5)		(2)	(1.6)
Unamortized grant date fair value, ending balance | $				$ 2.8	$ 3	$ 3.5	$ 2.8	$ 3
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)			630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest | Installment									4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years			35,000
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share
Target Restricted Share Activity
Beginning of period (in shares)				441,206	462,147	517,470	449,435	517,470
Assumed forfeitures and cancellations				0	0		(4,079)	(28,920)
Ending of period (in shares)				441,206	462,147	462,147	441,206	462,147
Unamortized grant date fair value
Assumed forfeitures and cancellations | $				$ 0	$ 0		$ 0	$ 0
Expense recognized | $				(0.3)	$ 0.3		(0.6)	$ (0.2)
Unamortized grant date fair value, ending balance | $				$ 2.3			$ 2.3
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares			35,000
Years Performance Shares will be Forfeited			5 years
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock Units (RSUs) [Member]
Target Restricted Share Activity
Issued (in shares)							233,461	215,698
Ending of period (in shares)				219,112	177,409		219,112	177,409
Unamortized grant date fair value
Expense recognized | $				$ (0.5)	$ (0.2)		$ (1.4)	$ (0.6)
Restricted Share Vesting in February 2018 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Issued (in shares)	92,500
Restricted Share Vesting in February 2019 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Issued (in shares)	78,150

Employee Share-Based Incentiv95

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - Restricted Stock Units (RSUs) [Member] - OneBeacon [Member] - One Beacon Long Term Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based compensation arrangement by share-based payment award
Issued (in shares)			233,461	215,698
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	219,112	177,409	219,112	177,409
Expense recognized	$ 0.5	$ 0.2	$ 1.4	$ 0.6

Fair Value of Financial Instr96

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 288.5	$ 337.6
OneBeacon [Member] | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	273.1	272.9
OneBeacon [Member] | Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	273.1	272.9
OneBeacon [Member] | Estimate of Fair Value Measurement [Member] | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt Instrument, Fair Value Disclosure	277.8	276.4
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	1.5
Long-term Line of Credit	15.6	15
MediaAlpha [Member] | Book Value | MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	15.4	14.7
MediaAlpha [Member] | Estimate of Fair Value Measurement [Member] | MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt Instrument, Fair Value Disclosure	$ 15.7	$ 15

Discontinued Operations (Detail

Discontinued Operations (Details) - USD ($) $ in Millions	Apr. 18, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold					$ 2,657.2	$ 24
Payments to Acquire Other Investments					37.3	30.5
Investments		$ 5,213.5			5,213.5		$ 5,213.5	$ 4,271.1
Net realized and unrealized investment gains (losses)		26.4		$ (43.9)	84	(33.9)
Net gain on sale of discontinued operations					414.5	18.2
Net (loss) income from discontinued operations, net of tax		14		(3.8)	7.1	59.1
Other liabilities		210.9			210.9		210.9	220.9
Income Tax Expense (Benefit)		(6.7)		(1.6)	(22.4)	0.8
Net Realized and Unrealized Gain (Loss) on Trading Securities		26.4		(43.9)	84	(33.9)
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 2,600		$ 2,600
Payments to Acquire Other Investments	$ 161.8		161.8
Investments								686.2
Net realized and unrealized investment gains (losses)				0.8	3.7	3.5
Net gain on sale of discontinued operations		(4)	366.6	0	362.6	0
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease		0	113.3	(18.5)	32	(62.1)
Tranzact [Member]
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		221.3
Net gain on sale of discontinued operations		51.9
Discontinued Operation, Tax Effect of Discontinued Operation		30.2
Esurance
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations				10.3		18.2
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments								$ 528.6
Discontinued Operations [Member]
Discontinued Operations [Line Items}
Net realized and unrealized investment gains (losses)		0		(0.9)	(1.5)	31.4
Net gain on sale of discontinued operations		14		(3.8)	7.1	59.1
Discontinued Operation, Tax Effect of Discontinued Operation		15.9		(2.3)	16.9	(22.2)
Discontinued Operations [Member] | Sirius Group
Discontinued Operations [Line Items}
Net realized and unrealized investment gains (losses)			0	(0.9)	(1.5)	31.4
Net gain on sale of discontinued operations			0	4	(6.4)	73.7
Net (loss) income from discontinued operations, net of tax					(4.3)
Discontinued Operation, Tax Effect of Discontinued Operation			$ 0	(2.3)	3.1	(22.2)
Discontinued Operations [Member] | Tranzact [Member]
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations, net of tax		(2.1)		(7.7)	(2.6)	(14.2)
Other liabilities		2.8			2.8		2.8
Income Tax Expense (Benefit)		(14)			$ (14)
Tax Effect of Extraordinary Item, Gain (Loss)		30.2
Discontinued Operation, Tax Effect of Discontinued Operation		(16.1)
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations						0.3
Discontinued Operations [Member] | Esurance
Discontinued Operations [Line Items}
Net gain on sale of discontinued operations		$ 10.3		$ 10.3		$ 17.9	$ 28.3

Discontinued Operations - Finan

Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		60 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	$ 4,131.5			$ 4,131.5		$ 4,131.5	$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	305.6			305.6		305.6	211.2
Common equity securities, at fair value	425			425		425	1,113.9
Other long-term investments	351.4			351.4		351.4	315.8
Total investments	5,213.5			5,213.5		5,213.5	4,271.1
Cash	244.7			244.7		244.7	173
Reinsurance recoverable on unpaid losses	169.4			169.4		169.4	186
Reinsurance recoverable on paid losses	16.4			16.4		16.4	7.5
Insurance premiums receivable	264.6			264.6		264.6	220.3
Deferred acquisition costs	109.6			109.6		109.6	107.6
Deferred tax asset	119.5			119.5		119.5	112.8
Ceded unearned insurance premiums	35.1			35.1		35.1	29.5
Accounts receivable on unsettled investment sales	179			179		179	41.9
Goodwill and other intangible assets	59.3	$ 53.3	$ 58	59.3	$ 58	59.3	55.4	$ 60.5	$ 62.5
Other assets	226.3			226.3		226.3	273.2
Assets held for sale	27			27		27	4,790.4
Loss and loss adjustment expense reserves	1,362	1,376.6	1,427.4	1,362	1,427.4	1,362	1,389.8	$ 1,350.8	$ 1,342.2
Unearned insurance premiums	674.1			674.1		674.1	610.5
Debt	288.5			288.5		288.5	337.6
Accrued incentive compensation	124.2			124.2		124.2	140.7
Ceded reinsurance payable	28.2			28.2		28.2	29.9
Funds held under insurance contracts	156.2			156.2		156.2	137.8
Accounts payable on unsettled investment purchases	30			30		30	0
Other liabilities	210.9			210.9		210.9	220.9
Total liabilities held for sale	7.8			7.8		7.8	3,047.4
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	281.3		284.9	838.1	896
Net investment income	22.5		16.8	59.6	42.6
Net realized and unrealized investment gains (losses)	26.4		(43.9)	84	(33.9)
Other revenue	34.8		38.3	110.8	114.4
Total revenues	365		296.1	1,092.5	1,019.1
Loss and loss adjustment expenses	165		170	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4		55.3	159.2	166.1
Other Underwriting Expense	49.6		56.5	156	165.5
General and administrative expenses	70.2		97.3	230.4	231.6
Interest expense	3.8		3.9	12.4	10.8
Total expenses	345		383	1,066.1	1,108.1
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax				7.1	59.1
Net gain on sale of discontinued operations				414.5	18.2
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period				167.2	232.9
Cash balances at end of period	244.7		173	244.7	173	244.7
Net income tax refund from national governments				(13.5)	(8)
Net change in foreign currency translation	0.2		0.1	0.3	0.3
Total comprehensive income (loss)	91.9		(73.8)	591.4	(61.5)
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments							686.2
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains (losses)			0.8	3.7	3.5
Net gain on sale of discontinued operations	(4)	366.6	0	362.6	0
Tranzact [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation	30.2
Net gain on sale of discontinued operations	51.9
Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations			10.3		18.2
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Reclassified to Assets Held-for-sale				2.6
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash sold as part of sale of subsidiaries	345.8			345.8		345.8
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Reclassified to Assets Held-for-sale	345.8			345.8		345.8
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0		202.8	240.1	623.3
Net investment income	0		10.5	14.4	28.6
Net realized and unrealized investment gains (losses)	0		(0.9)	(1.5)	31.4
Other revenue	14.8		40.3	120.2	90.4
Total revenues	14.8		252.7	373.2	773.7
Loss and loss adjustment expenses	0		121.7	154.9	311.3
Insurance and Reinsurance Acquisition Expenses	0		45.4	59	135.5
Other Underwriting Expense	0		24.8	30.9	78
General and administrative expenses	16.2		54.7	127.1	144.9
Interest expense	0.5		7.6	11.1	22.7
Total expenses	16.7		254.2	383	692.4
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.9)		(1.5)	(9.8)	81.3
Discontinued Operation, Tax Effect of Discontinued Operation	15.9		(2.3)	16.9	(22.2)
Net gain on sale of discontinued operations	14		(3.8)	7.1	59.1
Net loss from discontinued operations, net of tax	61.9		6.5	421.6	77.3
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash (used for) provided from operations - discontinued operations				(16.2)	22.7
Net cash provided from investing activities - discontinued operations				213.3	2.8
Net cash (used for) provided from financing activities - discontinued operations				(0.2)	19.6
Effect of exchange rate changes on cash				0	(4.1)
Net change in cash during the period				196.9	41
Cash balances at beginning of period				150.1	116.6
Cash balances at end of period	0		157.6	0	157.6	0
Interest paid				(1.4)	(28.1)
Net income tax refund from national governments				(31.8)	(30.3)
Net change in foreign currency translation	0		(18.5)		(62.1)
Total comprehensive income (loss)	61.9		(12)	566.9	15.2
Discontinued Operations [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	(4)			362.6
Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	13.4			13.4		13.4	2,383.5
Short-term investments, at amortized cost (which approximates fair value)	1.3			1.3		1.3	352.1
Common equity securities, at fair value	0			0		0	174.4
Other long-term investments	0			0		0	72.2
Total investments	14.7			14.7		14.7	2,982.2
Cash	3.8			3.8		3.8	150.1
Reinsurance recoverable on unpaid losses	0.6			0.6		0.6	283.6
Reinsurance recoverable on paid losses	0			0		0	10.2
Insurance premiums receivable	2			2		2	326.6
Deferred acquisition costs	0			0		0	74.6
Deferred tax asset	0			0		0	303.1
Ceded unearned insurance premiums	0			0		0	87.7
Goodwill and other intangible assets	5.2			5.2		5.2	330.5
Other assets	0.7			0.7		0.7	212.8
Assets held for sale	27			27		27	4,790.4
Loss and loss adjustment expense reserves	6			6		6	1,650.4
Unearned insurance premiums	1.6			1.6		1.6	344.3
Debt	0			0		0	506.4
Deferred tax liability	0			0		0	270.6
Accrued incentive compensation	0			0		0	64.2
Ceded reinsurance payable	(0.5)			(0.5)		(0.5)	67.7
Funds held under insurance contracts	0			0		0	52.9
Other liabilities	0.7			0.7		0.7	90.9
Total liabilities held for sale	7.8			7.8		7.8	3,047.4
Discontinued Operations [Member] | Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales	0			0		0	29
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0	202.8	240.1	623.3
Net investment income		0	10.5	14.4	28.6
Net realized and unrealized investment gains (losses)		0	(0.9)	(1.5)	31.4
Other revenue		0	1.8	0.6	(20.2)
Total revenues		0	214.2	253.6	663.1
Loss and loss adjustment expenses		0	121.7	154.9	311.3
Insurance and Reinsurance Acquisition Expenses		0	45.4	59	135.5
Other Underwriting Expense		0	24.8	30.9	78
General and administrative expenses		0	9.4	10.4	22.4
Interest expense		0	6.6	7.9	20
Total expenses		0	207.9	263.1	567.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax		0	6.3	(9.5)	95.9
Discontinued Operation, Tax Effect of Discontinued Operation		0	(2.3)	3.1	(22.2)
Net gain on sale of discontinued operations		0	4	(6.4)	73.7
Net loss from discontinued operations, net of tax		(4)	4	356.2	73.7
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net Assets	19.2			19.2		19.2	$ 1,743
Net change in foreign currency translation		0	(18.5)	32	(62.1)
Total comprehensive income (loss)		(4)	(14.5)	501.5	11.6
Discontinued Operations [Member] | Sirius Group | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations		$ (4)		362.6
Discontinued Operations [Member] | Tranzact [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Other liabilities	2.8			2.8		2.8
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation	(16.1)
Discontinued Operations [Member] | Tranzact [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations				51.9
Discontinued Operations [Member] | Other Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0		0	0	0
Net investment income	0		0	0	0
Net realized and unrealized investment gains (losses)	0		0	0	0
Other revenue	14.8		38.5	119.6	110.6
Total revenues	14.8		38.5	119.6	110.6
Loss and loss adjustment expenses	0		0	0	0
Insurance and Reinsurance Acquisition Expenses	0		0	0	0
Other Underwriting Expense	0		0	0	0
General and administrative expenses	16.2		45.3	116.7	122.5
Interest expense	0.5		1	3.2	2.7
Total expenses	16.7		46.3	119.9	125.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.9)		(7.8)	(0.3)	(14.6)
Discontinued Operation, Tax Effect of Discontinued Operation	15.9		0	13.8	0
Net gain on sale of discontinued operations	14		(7.8)	13.5	(14.6)
Net loss from discontinued operations, net of tax	65.9		2.5	65.4	3.6
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Total comprehensive income (loss)	65.9		2.5	65.4	3.6
Discontinued Operations [Member] | Sale of Sirius Group [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in foreign currency translation				113.3
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations					0.3
Net loss from discontinued operations, net of tax			(0.1)		(0.4)
Discontinued Operations [Member] | OneBeacon Runoff [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	51.9		0		0.3
Discontinued Operations [Member] | Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	10.3		10.3		17.9	28.3
Discontinued Operations [Member] | Esurance | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	0		10.3		17.9
Discontinued Operations, Held-for-sale [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period				150.1	116.6
Cash balances at end of period	$ 3.8		$ 157.6	$ 3.8	$ 157.6	$ 3.8

Discontinued Operations - EPS (

Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0	$ 3.9	$ 0	$ 18
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	11,943		8,208
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	$ 0.4	0	$ 0.3	0
Net Income (Loss) Available to Common Stockholders, Basic	$ 91.7	$ (58.9)	$ 445.8	$ 29.7
Total average common shares outstanding during the period	4,867,400	5,890,100	5,166,600	5,951,100
Average unvested restricted common shares	(68,100)	0	(62,900)	0
Basic earnings per share denominator	4,799,300	5,890,100	5,103,700	5,951,100
Average outstanding dilutive options to acquire common shares	4,879,400	5,890,100	5,174,800	5,951,100
Diluted earnings (loss) per share denominator	4,811,300	5,890,100	5,111,900	5,951,100
Earnings Per Share, Diluted	$ 18.80	$ (10.01)	$ 86.13	$ 4.97
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	$ 61.9	$ 6.5	$ 421.6	$ 77.3
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	(0.9)	(0.1)	5.1	(0.9)
Net Income (Loss) Available to Common Stockholders, Basic	$ 61	$ 6.4	$ 416.5	$ 76.4
Total average common shares outstanding during the period	4,879,400	5,890,100	5,174,800	5,951,100
Average unvested restricted common shares	(68,100)	(70,900)	(62,900)	(67,100)
Basic earnings per share denominator	4,799,300	5,819,200	5,103,700	5,884,000
Diluted earnings (loss) per share denominator	4,811,300	5,819,200	5,111,900	5,884,000
Earnings Per Share, Basic	$ 12.72	$ 1.09	$ 81.60	$ 12.98
Earnings Per Share, Diluted	$ 12.69	$ 1.09	$ 81.47	$ 12.98
Common Stock [Member] | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax		$ 4.2		$ 17.5
Amortization of common share basis difference | Common Stock [Member] | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax		$ 0.7		$ 2.1
Employee Stock Option [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	120,000		120,000

Discontinued Operations - Fair

Discontinued Operations - Fair Value 2 (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	$ 288.5	$ 337.6

Contingencies (Details)

Contingencies (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended		60 Months Ended
	Jun. 30, 2007	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Oct. 07, 2011
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations					$ 414.5	$ 18.2
Esurance
Loss Contingencies [Line Items]
Percentage of fluctuation in loss reserves of transferred subsidiaries retained								90.00%
Tribune Company Litigation | Pending Litigation | OneBeacon [Member]
Loss Contingencies [Line Items]
Proceeds from Sale, Maturity and Collection of Investments	$ 32
Discontinued Operations [Member]
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations		$ 14		$ (3.8)	7.1	59.1
Esurance
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations				10.3		18.2
Esurance | Discontinued Operations [Member]
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations		10.3		10.3		17.9	$ 28.3
OneBeacon Runoff [Member] | Discontinued Operations [Member]
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations						0.3
Sirius Group
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations		$ (4)	$ 366.6	0	362.6	0
Sirius Group | Discontinued Operations [Member]
Loss Contingencies [Line Items]
Net gain on sale of discontinued operations			$ 0	$ 4	$ (6.4)	$ 73.7

OneBeacon Pension Plan (Details

OneBeacon Pension Plan (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 34.8	$ 38.3	$ 110.8	$ 114.4
Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Assets for Plan Benefits	12.6		12.6
Transfer of assets	47.1		47.1
OneBeacon [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	1.8	$ 3.9	$ 3.5	$ (0.4)
OneBeacon [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 3.2