White Mountains Insurance (WTM) 10-K 29 Feb 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Feb. 25, 2016	Jun. 30, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	776,867
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 3,639,859,832
Entity Common Stock, Shares Outstanding		5,553,407
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed maturity investments, at fair value	$ 2,639.7	$ 2,422
Short-term investments, at amortized cost (which approximates fair value)	211.3	376.8
Common equity securities, at fair value	1,113.9	611.7
Other long-term investments	315.8	331.9
Total investments	4,280.7	3,742.4
Cash (restricted $5.8 and $23.7)	179.3	261.7
Reinsurance recoverable on unpaid losses	186.5	161.7
Reinsurance recoverable on paid losses	7.5	12.2
Insurance premiums receivable	223.3	241.1
Investments in unconsolidated affiliates	0	414.4
Deferred acquisition costs	107.6	107.2
Deferred Income Tax Assets, Net	112.8	114.6
Ceded unearned insurance premiums	29.5	17.8
Accrued investment income	14	14.3
Accounts receivable on unsettled investment sales	41.9	37.8
Goodwill and intangible assets	375.7	351.2
Other assets	318.7	348.7
Assets held for sale	4,407	4,630.6
Total assets	10,284.5	10,455.7
Liabilities
Loss and loss adjustment expense reserves	1,395.8	1,350
Unearned insurance premiums	612.6	616.7
Debt	442.4	343.1
Deferred tax liability	7	0
Accrued incentive compensation	141.7	108.1
Ceded reinsurance payable	30.5	34.2
Funds held under insurance contracts	137.8	81
Accounts payable on unsettled investment purchases	0	0.5
Other liabilities	264.7	278.4
Liabilities held for sale	2,884	3,105.3
Total liabilities	5,916.5	5,917.3
White Mountains’s common shareholders’ equity
White Mountains’s common shares at $1 par value per share—authorized 50,000,000 shares; issued and outstanding 5,623,735 and 5,986,214 shares	5.6	6
Paid-in surplus	972.6	1,028.7
Retained earnings	3,084.9	3,010.5
Accumulated other comprehensive income (loss), after-tax:
Equity in net unrealized gains from investments in Symetra common shares	0	34.9
Net unrealized foreign currency translation losses	(145.6)	(79.8)
Pension liability and other	(4.3)	(4.6)
Total White Mountains’s common shareholders’ equity	3,913.2	3,995.7
Non-controlling interests
Non-controlling interests	454.8	542.7
Total equity	4,368	4,538.4
Total liabilities and equity	$ 10,284.5	$ 10,455.7

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Restricted Cash and Cash Equivalents, Current	$ 5.8	$ 23.7
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	5,623,735	5,986,214
Common shares, outstanding shares	5,623,735	5,986,214

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Earned insurance premiums	$ 1,188.2	$ 1,185	$ 1,120.9
Net investment income	60.8	59.5	59.8
Net realized and unrealized investment gains	225.4	78.5	133.9
Other revenue	334.2	131.1	47
Total revenues	1,808.6	1,454.1	1,361.6
Expenses
Loss and loss adjustment expenses	708.9	824	622.1
Insurance and reinsurance acquisition expenses	220.1	206.2	210.4
Other underwriting expenses	218.6	179.6	205.2
General and administrative expenses	487.5	257.7	149.1
Interest expense on debt	18.6	15.6	16.2
Total expenses	1,653.7	1,483.1	1,203
Pre-tax income (loss)	154.9	(29)	158.6
Income tax (expense) benefit	0.7	14.8	(32.6)
Net income (loss) from continuing operations	155.6	(14.2)	126
Gain (loss) on sale of discontinued operations, net of tax	18.2	(1.6)	46.6
Net income from discontinued operations, net of tax	80.6	260.2	99.9
Income before equity in earnings of unconsolidated affiliates	254.4	244.4	272.5
Equity in earnings of unconsolidated affiliates, net of tax	25.1	45.6	36.6
Net income	279.5	290	309.1
Net loss attributable to non-controlling interests	18.1	22.2	12.5
Net income attributable to White Mountains’s common shareholders	297.6	312.2	321.6
Other comprehensive income, net of tax:
Change in equity in net unrealized (losses) gains from investments in Symetra common shares, net of tax	(34.9)	75.3	(98.1)
Change in foreign currency translation, net of tax	(65.8)	(168.5)	2.7
Net change in pension liability and other, net of tax	0.3	(11.7)	20.8
Comprehensive income	197.2	207.3	247
Comprehensive loss (income) attributable to non-controlling interests	0	3.3	(5.2)
Comprehensive income attributable to White Mountains’s common shareholders	$ 197.2	$ 210.6	$ 241.8
Basic earnings per share
Continuing operations	$ 33.80	$ 8.77	$ 28.22
Discontinued operations	16.80	42.36	23.63
Total consolidated operations	50.60	51.13	51.85
Diluted earnings per share
Continuing operations	33.80	8.77	28.22
Discontinued operations	16.80	42.36	23.63
Total consolidated operations	50.60	51.13	51.85
Dividends declared and paid per White Mountains’s common share	$ 1	$ 1	$ 1

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	Parent	Non-controlling Interests	Total Equity includes Noncontrolling Interest [Member]	Foreign Currency Gain (Loss)Common shares and paid-in surplus	Foreign Currency Gain (Loss)Retained earnings	Foreign Currency Gain (Loss)AOCI, after-tax	Foreign Currency Gain (Loss)Parent	Foreign Currency Gain (Loss)Non-controlling Interests	Foreign Currency Gain (Loss)Total Equity includes Noncontrolling Interest [Member]	Pension liability and other accumulated comprehensive 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 itemsParent	Pension liability and other accumulated comprehensive itemsNon-controlling Interests	Pension liability and other accumulated comprehensive itemsTotal Equity includes Noncontrolling Interest [Member]	Prospector Turtle FundCommon shares and paid-in surplus	Prospector Turtle FundRetained earnings	Prospector Turtle FundAOCI, after-tax	Prospector Turtle FundParent	Prospector Turtle FundNon-controlling Interests	Prospector Turtle FundTotal Equity includes Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2012		$ 1,057.2	$ 2,542.5	$ 131.9	$ 3,731.6	$ 526.3	$ 4,257.9
Increase (Decrease) in Shareholders' Equity
Net income	$ 309.1	0	321.6	0	321.6	(12.5)	309.1
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax	(98.1)	0	0	(98.1)	(98.1)	0	(98.1)
Other comprehensive income (loss), after-tax														$ 0	$ 0	$ 15.6	$ 15.6	$ 5.2	$ 20.8
Change in foreign currency translation, net of tax	2.7							$ 0	$ 0	$ 2.7	$ 2.7	$ 0	$ 2.7
Net change in pension liability and other accumulated comprehensive items	20.8
Comprehensive income	247	0	321.6	(79.8)	241.8	(7.3)	234.5
Dividends declared on common shares		0	(6.2)	0	(6.2)	0	(6.2)
Dividends/distributions to non-controlling interests		0	0	0	0	(39.7)	(39.7)
Issuances of common shares		1	0	0	1	0	1
Repurchases and retirements of common shares	(79.8)	(23.8)	(56)	0	(79.8)	0	(79.8)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0	0	0	0	11.5	11.5
Amortization of restricted share and option awards		16.7	0	0	16.7	0.9	17.6
Ending balance at Dec. 31, 2013		1,051.1	2,801.9	52.1	3,905.1	491.7	4,396.8
Increase (Decrease) in Shareholders' Equity
Net income	290	0	312.2	0	312.2	(22.2)	290
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax	75.3	0	0	75.3	75.3	0	75.3
Other comprehensive income (loss), after-tax														0	0	(8.7)	(8.7)	(3)	(11.7)
Change in foreign currency translation, net of tax	(168.5)							0	0	(168.2)	(168.2)	(0.3)	(168.5)
Net change in pension liability and other accumulated comprehensive items	(11.7)
Comprehensive income	207.3	0	312.2	(101.6)	210.6	(25.5)	185.1
Dividends declared on common shares		0	(6.2)	0	(6.2)	0	(6.2)
Dividends/distributions to non-controlling interests		0	0	0	0	(40.7)	(40.7)
Issuances of common shares		4.8	0	0	4.8	0	4.8
Repurchases and retirements of common shares	(134.5)	(37.1)	(97.4)	0	(134.5)	0	(134.5)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0	0	0	0	8.3	8.3
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		0	0	0	0	(122.1)	(122.1)													$ 0	$ 0	$ 0	$ 0	$ 14	$ 14
Amortization of restricted share and option awards		15.9	0	0	15.9	0.8	16.7
Ending balance at Dec. 31, 2014	3,995.7	1,034.7	3,010.5	(49.5)	3,995.7	542.7	4,538.4
Increase (Decrease) in Shareholders' Equity
Net income	279.5	0	297.6	0	297.6	(18.1)	279.5
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax	(34.9)	0	0	(34.9)	(34.9)	0	(34.9)
Other comprehensive income (loss), after-tax														$ 0	$ 0	$ 0.3	$ 0.3	$ 0	$ 0.3
Change in foreign currency translation, net of tax	(65.8)							$ 0	$ 0	$ (65.8)	$ (65.8)	$ 0	$ (65.8)
Net change in pension liability and other accumulated comprehensive items	0.3
Comprehensive income	197.2	0	297.6	(100.4)	197.2	(18.1)	179.1
Dividends declared on common shares		0	(6)	0	(6)	0	(6)
Dividends/distributions to non-controlling interests		0	0	0	0	(51.1)	(51.1)
Issuances of common shares		0.9	0	0	0.9	0	0.9
Repurchases and retirements of common shares	(284.2)	(67)	(217.2)	0	(284.2)	0	(284.2)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0	0	0	0	17.5	17.5
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(5.3)	0	0	(5.3)	(2.7)	(8)													$ 0	$ 0	$ 0	$ 0	$ (31.5)	$ (31.5)
Amortization of restricted share and option awards		14.9	0	0	14.9	(2)	12.9
Ending balance at Dec. 31, 2015	$ 3,913.2	$ 978.2	$ 3,084.9	$ (149.9)	$ 3,913.2	$ 454.8	$ 4,368

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operations:
Net income	$ 279.5	$ 290	$ 309.1
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains	(225.4)	(78.5)	(133.9)
Net gain on sale of consolidated and unconsolidated affiliates	(16.1)	0	(21.3)
Amortization and depreciation	67.5	38.2	28.6
Deferred income tax expense (benefit)	5.1	(22.5)	10.1
Undistributed equity in earnings from unconsolidated affiliates, after-tax	(25.1)	(45.6)	(36.6)
Net income from discontinued operations	(80.6)	(260.2)	(99.9)
Net (gain) loss on sale of other discontinued operations	(18.2)	1.6	(46.6)
Other operating items:
Net change in loss and loss adjustment expense reserves	(45.8)	(282.2)	(54.3)
Net change in reinsurance recoverable on paid and unpaid losses	(20.1)	(83.2)	20.7
Net change in unearned insurance premiums	4.1	(57.6)	15.8
Net change in ceded reinsurance premiums payable	(3.7)	11.5	10.2
Net change in ceded unearned insurance premiums	(11.6)	(3.5)	(2.8)
Net change in insurance premiums receivable	17.8	(8.8)	(2.6)
Net change in variable annuity benefit guarantee liabilities	(0.4)	(53.5)	(388.7)
Net change in variable annuity benefit derivative instruments	36.3	12.7	29.1
Net change in deferred acquisition costs	(0.4)	(1.8)	18.5
Net change in funds held under insurance contracts	56.7	17.7	57.4
Net change in restricted cash	17.9	32.4	193.7
Net change in other assets and liabilities, net	40.1	(127.5)	(23)
Net cash provided from (used for) continuing operations	161	58.8	(39.5)
Net cash provided from (used for) discontinued operations	14.7	60	(61.5)
Net cash provided from (used for) operations	175.7	118.8	(101)
Cash flows from investing activities:
Net change in short-term investments	140	(138)	64.6
Sales of fixed maturity and convertible investments	1,281.7	2,210.8	1,925.3
Maturities, calls and paydowns of fixed maturity and convertible investments	335	452.4	280.9
Sales of common equity securities	461.4	609.8	435
Distributions and redemptions of other long-term investments	84.7	64.5	31.6
Sales of unconsolidated affiliates, net of cash sold and held in escrow	24	0	32.4
Contributions to other long-term investments	(78)	(114.7)	(22)
Funding of operational cash flows for discontinued operations	17.5	(88.1)	(72.3)
Purchases of common equity securities	(409.9)	(289.7)	(366.5)
Purchases of fixed maturity and convertible investments	(1,867.2)	(2,456.1)	(2,047.9)
Purchases of consolidated and unconsolidated affiliates (net of cash acquired)	(37.1)	(190.1)	0
Net change in unsettled investment purchases and sales	(4.6)	(44.7)	3.7
Net dispositions (acquisitions) of property and equipment	32.5	(4)	(11.7)
Net cash (used for) provided from investing activities — continuing operations	(20)	12.1	253.1
Net cash provided from investing activities — discontinued operations	31.3	35.8	148.7
Net cash provided from investing activities	11.3	47.9	401.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit	195.5	133.6	200
Repayment of debt and revolving line of credit	(95.6)	(66.5)	(275)
Change in capital lease obligation	(5.4)	(5.3)	(5.7)
Cash dividends paid to the Company’s common shareholders	(6)	(6.2)	(6.2)
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders	(19.7)	(19.8)	(19.9)
Distribution to non-controlling interest shareholders of Tranzact and MediaAlpha	(9.1)	0	0
Distributions from contributions to discontinued operations	(12.1)	37.9	69.6
Common shares repurchased	(268.6)	(128.2)	(79.8)
OneBeacon Ltd. common shares repurchased and retired	(3.7)	(1.7)	0
Capital contributions from non-controlling interest of consolidated LPs	0	4.9	1.6
Redemptions paid to non-controlling interest of consolidated LPs	0	(5.5)	(0.7)
Payments of distributions from subsidiaries to shareholders	(10)	(25.8)	0
Capital contributions from BAM members	29.2	16.2	17.1
Net cash used for financing activities — continuing operations	(205.5)	(66.4)	(99)
Net cash (used for) provided from financing activities — discontinued operations	(9.1)	(63.2)	(87.5)
Net cash used for financing activities	(214.6)	(129.6)	(186.5)
Effect of exchange rate changes on cash (excludes (4.5), (14.3) and (.2) related to discontinued operations	0	0	0
Net change in cash during the period - continuing operations	(64.5)	4.5	114.6
Cash balance at beginning of year	238	233.5	118.9
Cash balance at end of year	$ 173.5	$ 238	$ 233.5

CONSOLIDATED STATEMENTS OF CAS7

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash restricted	$ 5.8	$ 23.7	$ 56.1
Cash Including Restricted Cash	179.3	261.7
Effect of Exchange Rate on Cash	(4.5)	(14.3)	(0.2)
Cash reclassified to assets held-for-sale	0	0	3.5
Sirius Group
Cash Including Restricted Cash	$ 143.9	$ 111.5	$ 93.2

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively with the Company, “White Mountains”) and 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, reinsurance and insurance services subsidiaries and affiliates. The Company’s headquarters is located at 14 Wesley 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, Sirius Group, HG Global/BAM and Other Operations. On July 24, 2015, White Mountains signed a definitive agreement to sell Sirius International Insurance Group, Ltd, an exempted Bermuda limited liability company, and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd., the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (“CMI”). (See Note 2 - “Significant Transactions”). Accordingly, effective for December 31, 2015, Sirius Group has been presented as discontinued operations and assets and liabilities held for sale in the financial statements. Prior year amounts have been reclassified to conform to the current year’s presentation. (See Note 22 - “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 primarily through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of December 31, 2015 and 2014 , White Mountains owned 75.5% and 75.3% of OneBeacon Ltd.’s outstanding common shares. As discussed further in Note 2 - “Significant Transactions” , in December 2014, OneBeacon completed the sale of its of runoff business consisting of assets, liabilities and capital related principally to OneBeacon’s non-specialty business, including the vast majority of its asbestos and environmental reserves (“Runoff Business”) to an affiliate of Armour Group Holdings Limited (the “Runoff Transaction”). Accordingly, OneBeacon’s runoff business is presented as discontinued operations. In the second quarter of 2015, OneBeacon completed the sale of its building in Canton, MA for $58.0 million . The building was presented as held for sale at December 31, 2014. (See Note 22- “Discontinued Operations” ). The HG Global/BAM segment consists of White Mountains’s investment in HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is 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 December 31, 2015 and 2014, 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 is in runoff with all of its contracts maturing by June 30, 2016, and Life Re Bermuda’s U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), White Mountains’s ownership positions in Tranzact Holdings, LLC, a New Jersey-based provider of comprehensive direct-to-consumer customer acquisition solutions to leading insurance carriers (together with its subsidiaries “Tranzact”), QL Holdings, LLC (together with its subsidiaries “MediaAlpha”), a California-based advertising technology company that develops programmatic platforms that bring transparency and efficiency to the buying and selling of insurance and other vertical-specific performance media, Wobi Insurance Agency Ltd. (“Wobi”), the only price comparison/aggregation business in Israel, as well as various other entities and investments. The Other Operations segment also includes Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. As of December 31, 2015 and 2014, White Mountains held $21.0 million and $17.0 million of surplus notes issued by SSIE (the “SSIE Surplus Notes”) but does not have an ownership interest in SSIE. However, as a result of Star & Shield’s role as the attorney-in-fact to SSIE and the investment in SSIE Surplus Notes, 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. All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Significant Accounting Policies Investment Securities As of December 31, 2015 , White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments and common equity securities held for general investment purposes are classified as trading and are reported at fair value as of the balance sheet date. Changes in unrealized gains and losses are reported pre-tax in revenues. Realized investment gains and losses are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common and preferred equity securities, convertible fixed maturity and preferred investments and other long-term investments, such as interests in hedge funds, private equity funds, direct investments in privately held common and convertible securities and the surplus notes issued in connection with the Runoff Transaction (“OneBeacon Surplus Notes”). In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs. As of December 31, 2015 and 2014 , approximately 91% and 88% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equity securities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs are comprised of fixed maturity investments including corporate debt, state and other governmental debt, convertible fixed maturity and preferred investments and mortgage and asset-backed securities. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include White Mountains’s investments in hedge funds and private equity funds, as well as investments in certain debt securities, including asset-backed securities, where quoted market prices are unavailable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services used by White Mountains have indicated that if no observable inputs are available for a security, they will not provide a price. In those circumstances, White Mountains estimates the fair value using industry standard pricing models and observable inputs such as benchmark interest rates, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs. White Mountains performs procedures to validate the market prices obtained from the outside pricing sources. Such procedures, which cover substantially all of its fixed maturity investments include, but are not limited to, evaluation of model pricing methodologies and a review of the pricing services’ quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices, and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Prices that have not changed from period to period and prices that have trended unusually compared to market conditions are also considered outliers. In circumstances where the results of White Mountains’s review process do not appear to support the market price provided by the pricing services, White Mountains challenges the price. During the past year, 8 securities fell outside White Mountains’s expected results, thereby triggering the challenge with the pricing service. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The fair values of such securities are considered to be Level 3 measurements. White Mountains’s investments in debt securities, including asset-backed securities, are generally valued using matrix and other pricing models. 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. Short-term investments consist of 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 cost, which approximated fair value as of December 31, 2015 and 2014 . Other Long-term Investments Other long-term investments consist primarily of hedge funds, private equity funds, other investments in limited partnerships, private equity securities and surplus note investments (see Note 5 - “Investment Securities” ). White Mountains has taken the fair value option for most of its investments in hedge funds, private equity funds, direct investments in privately held common and convertible securities, the OneBeacon surplus notes and various other long-term investments. For the investments for which White Mountains has taken the fair value option, changes in fair value are reported in revenues on a pre-tax basis. For those long-term investments for which White Mountains has not made the fair value election, White Mountains accounts for its interests under the equity method. Derivative Financial Instruments White Mountains holds a variety of derivative financial instruments for both risk management and investment purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income. Warrants As of December 31, 2012, White Mountains held warrants to purchase 9.49 million common shares of Symetra, which were included as investments in unconsolidated affiliates. The Symetra warrants held by White Mountains were entitled to dividends declared to common shareholders. On June 20, 2013, White Mountains exercised its warrants in a cashless transaction and received 2.65 million common shares of Symetra in exchange for the warrants. White Mountains also holds warrants that it has received in the restructuring (e.g., securities received from bankruptcy proceedings) of certain of its common equity and/or fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives. Derivatives—Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan through its wholly owned subsidiary, WM Life Re. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. Guaranteed minimum accumulation benefits (“GMABs”) are paid to an annuitant for any shortfall between accumulated account value at the end of the accumulation period and the annuitant’s total deposit, less any withdrawal payments made to the annuitant during the accumulation period. GMABs meet the definition of a derivative for accounting purposes. Therefore, GMABs are carried at fair value, with changes thereon recognized in income in the period of the change. The liability for the reinsured GMAB contracts has been determined using internal valuation models that use assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates. If an annuitant dies during the accumulation period of an annuity contract, guaranteed minimum death benefits (“GMDBs”) are paid to the annuitant’s beneficiary for shortfalls between accumulated account value at the time of an annuitant’s death and the annuitant’s total deposit, less any living benefit payments or withdrawal payments previously made to the annuitant. White Mountains has elected to measure its GMDB liabilities at fair value. The valuation of these liabilities involves significant judgment and is subject to change based upon changes in capital market assumptions and emerging surrender and mortality experience of the underlying contracts in force. WM Life Re has entered into derivative contracts that are designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. The derivatives include futures and over-the-counter option contracts on interest rates, major bond and equity indices, and foreign currencies. All WM Life Re’s derivative instruments are recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments do not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value are recognized in the current period as gains or losses in the income statement within other revenues. WM Life Re includes the effect of counterparty credit risk when determining the fair value of its derivative contracts and its GMAB and GMDB liabilities. Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash (See Note 9 - “Derivatives” ). Insurance and Reinsurance Operations White Mountains accounts for insurance and reinsurance policies that it writes in accordance with ASC 944. Premiums written are recognized as revenues and are earned ratably over the term of the related policy or reinsurance treaty. Unearned premiums represent the portion of premiums written that are applicable to future insurance or reinsurance coverage provided by policies or treaties in force. White Mountains charges fees on certain of its insurance policies. Refundable fees are classified with premiums and recognized in earnings over the policy term. Fees that represent a reimbursement of expenses, such as installment fees, are recorded as a reduction of underwriting expenses. Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Amortization of deferred acquisition costs are presented within insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. Losses and LAE are charged against income as incurred. Unpaid insurance losses and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance losses and LAE are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any resulting adjustments are reflected in current operations. The process of estimating loss and LAE involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. OneBeacon discounts certain of its long-term workers compensation loss and LAE reserves when such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual claim basis. OneBeacon discounts these reserves using an average discount rate which is determined based on the various assumptions including consideration of when the claims will be settled ( 2.5% as of both December 31, 2015 and 2014 ). As of December 31, 2015 and 2014, the discount on OneBeacon’s workers compensation loss and LAE reserves amounted to $1.1 million and $1.0 million . White Mountains’s insurance and reinsurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. Such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies White Mountains for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. White Mountains has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro-rata basis. The amount of each risk ceded by White Mountains is subject to maximum limits which vary by line of business and type of coverage. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectability of reinsurance recoverables is subject to the solvency of the reinsurers. White Mountains is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with a strong financial condition, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis. Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly. Funds held by ceding companies represent amounts due to White Mountains in connection with certain assumed reinsurance agreements in which the ceding company retains a portion of the premium to provide security against future loss payments. The funds held by ceding companies are generally invested by the ceding company and a contractually agreed interest amount is credited to the Company and recognized as investment income. Funds held under insurance and reinsurance contracts represent contractual payments due to White Mountains that have been retained to secure such obligations. Such amounts are recorded as liabilities in the consolidated financial statements. Accruals for contingent commission liabilities are established for reinsurance contracts that provide for the stated commission percentage to increase or decrease based on the loss experience of the contract. Changes in the estimated liability for such arrangements are recorded as contingent commissions. Accruals for contingent commission liabilities are determined through the review of the contracts that have these adjustable features and are estimated based on expected loss and LAE. Municipal Bond Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. Funds Held Funds held under reinsurance contracts primarily represent amounts due to White Mountains in connection with the Standard Reinsurance Agreement (“SRA”) with the Federal Crop Insurance Corporation (“FCIC”), which is managed by an agency of the U.S. Department of Agriculture. The SRA governs the relationship, including the exchange of funds, between private insurance companies, including White Mountains, and the FCIC relating to our MPCI crop insurance business. Funds held under insurance contracts represents unrestricted collateral held by White Mountains primarily relating to the surety business. Mandatory Shared Market Mechanisms As a condition to its licenses to do business in certain states, White Mountains’s insurance operations must participate in various mandatory shared market mechanisms commonly referred to as “residual” or “involuntary” markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer’s market share of voluntary business written within that state. In certain cases, White Mountains is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms is accounted for as direct insurance business or as assumed reinsurance depending upon the structure of the mechanism. OneBeacon’s market assignments are typically required to be written in the current period, however, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated. Insurance-related Assessments Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains records guaranty fund assessments when it is probable that an assessment will be made and the amount can be reasonably estimated. Deferred Software Costs White Mountains capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll-related costs. White Mountains begins amortization of these costs once the project is completed and ready for its intended use. Amortization is on a straight-line basis and over a useful life of eighteen months to five years. Costs related to software developed for sale to third parties are expensed until technological feasibility has been established. Once technological feasibility has been established, software development costs are capitalized and reported at their net realizable value. Upon product release, the amortization of software development costs is determined annually as the greater of the amount calculated using the ratio of current gross revenues to the total of current and expected gross revenues for the product or the straight-line method over the estimated economic life, which is generally between 18 to 36 months. As of December 31, 2015 and 2014 , White Mountains had unamortized deferred software costs of $18.4 million and $15.4 million . Amortization expense of software development costs of $6.0 million , $4.0 million and $1.3 million were recognized for the years ended December 31, 2015, 2014 and 2013, respectively. Commission and Other Revenue Recognition White Mountains recognizes agent commissions and other revenues when it has fulfilled all of its obligations necessary to earn the revenue and when it can reliably estimate both the amount of revenue, net of any amounts expected to be uncollectible, and any amounts associated with expected cancellations. Federal and Foreign Income Taxes A significant portion of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized. Foreign Currency Exchange The U.S. dollar is the functional currency for all of White Mountains’s businesses except for Sirius International, the Canadian reinsurance operations of Sirius America and certain other smaller international activities. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the average exchange rates for the period. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss. Assets and liabilities relating to foreign operations are translated into the functional currency using current exchange rates; revenues and expenses are translated into the functional currency using the weighted average exchange rate for the period. The resulting exchange gains and losses are reported as a component of net income in the period in which they arise. As of December 31, 2015 and 2014 , White Mountains had unrealized foreign currency translation losses of $145.6 million and $79.8 million recorded in accumulated other comprehensive income on its consolidated balance sheet. The following rates of exchange for the U.S. dollar have been used for the most significant operations: Currency Opening Rate 2015 Closing Rate 2015 Opening Rate 2014 Closing Rate 2014 Swedish kronor 7.7737 8.4247 6.4339 7.7737 British pound 0.6426 0.6757 0.6044 0.6426 Euro 0.8245 0.9189 0.7259 0.8245 Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses. Finite-life intangible assets are measured at their acquisition date fair values, are amortized

Significant Transactions

Significant Transactions	12 Months Ended
	Dec. 31, 2015
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions Acquisitions The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The total assets acquired and liabilities assumed have been measured at their acquisition date fair values. Tranzact On October 10, 2014, White Mountains acquired majority ownership of Tranzact. White Mountains acquired 63.2% of Tranzact for a purchase price of $177.7 million , representing an enterprise value of $281.2 million . Immediately following the closing, Tranzact completed a recapitalization that allowed for the return of $44.2 million in capital to White Mountains. As of the acquisition date, White Mountains recognized total assets acquired related to Tranzact of $332.8 million , including $41.4 million of tangible assets, $145.1 million of goodwill, and $146.3 million of other intangible assets; and total liabilities assumed of $108.7 million at their estimated fair values. The liabilities assumed include a contingent consideration liability of $7.4 million associated with a prior acquisition by Tranzact. The contingent consideration is payable if earnings before interest expense, taxes, depreciation and amortization (“EBITDA”) of the acquiree exceed amounts defined in the purchase agreement. As of December 31, 2014, Tranzact held 36.0% ownership interest in Tranzutary Holdings, LLC (“Tranzutary”). White Mountains determined that Tranzutary was a variable interest entity and that Tranzact was the primary beneficiary. At December 31, 2014, consolidated amounts related to Tranzutary included total assets of $29.6 million , which includes other intangible assets of $28.9 million and total liabilities of $4.1 million . On February 11, 2015, Tranzact acquired the remaining interest in Tranzutary for a purchase price of $12.0 million . On September 1, 2015, Tranzact acquired 100.0% of the outstanding share capital of TruBridge. Tranzact paid an initial purchase price of $31.0 million . The purchase price is subject to adjustment linked to the amount of marketing expense reimbursements to be received in 2016 and 2017. At December 31, 2015, Tranzact recognized a liability of $9.7 million for the estimated amount of the purchase price adjustment. Tranzact recognized total assets acquired of $54.5 million , which includes $18.7 million of goodwill and $28.1 million of other intangible assets, and total liabilities assumed of $4.3 million at their estimated acquisition date fair values. On November 6, 2015, Tranzact acquired the domain name CancerInsurance.com for a purchase price of $3.1 million , which is included in other intangible assets. The purchase price included cash of $1.1 million and a liability of $2.0 million for contingent consideration that is payable if EBITDA exceeds amounts defined in the purchase agreement between November 6, 2016 and May 6, 2018. The maximum amount of the contingent consideration is $6.8 million . MediaAlpha On March 14, 2014, White Mountains acquired 60.0% of the outstanding Class A common units of MediaAlpha. White Mountains paid an initial purchase price of $28.1 million . The purchase price is subject to adjustment equal to 62.5% of the 2015 gross profit in excess of the 2013 gross profit. After adjustment for the estimated contingent purchase price adjustment, White Mountains recognized total assets acquired related to MediaAlpha of $70.1 million , including $18.3 million of goodwill and $38.5 million of other intangible assets; and total liabilities assumed of $10.0 million , reflecting acquisition date fair values. As of December 31, 2015 and 2014, White Mountains recognized a contingent liability of $7.8 million and $7.9 million for the contingent purchase price adjustment. Wobi On February 19, 2014, White Mountains acquired 54.0% of the outstanding common shares of Wobi, the only price comparison/aggregation business in Israel, for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition). During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $9.0 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. As of the acquisition date, White Mountains recognized total assets acquired related to Wobi of $13.4 million , including $5.5 million of goodwill and $2.9 million of other intangible assets; and total liabilities assumed of $0.7 million at their estimated acquisition date fair values. During 2015, White Mountains purchased NIS 79.6 million (approximately $20.7 million based upon the foreign exchange spot rate at the dates of acquisition) of convertible preferred shares of Wobi. In addition, during 2015 White Mountains also purchased NIS 11.8 million (approximately $ 3.1 million based upon the foreign exchange spot rate at the date of acquisition) of common shares of Wobi. As of December 31, 2015 and 2014, White Mountains’s ownership share was 96.1% and 63.3% on a fully converted basis. On February 23, 2015, Wobi acquired 56.2% of the outstanding share capital of Tnuva Finansit Ltd. (“Cashboard”) for NIS 9.5 million (approximately $ 2.4 million ). The acquisition of Cashboard accelerated Wobi's development of its pension products comparison service. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million , including $0.3 million of goodwill and $ 2.8 million of other intangible assets; and total liabilities assumed of $ 1.2 million at their estimated acquisition date fair values. During 2015, Wobi purchased the remaining share capital of Cashboard for NIS 26.4 million (approximately $6.5 million ). Star & Shield On January 31, 2014, White Mountains acquired certain assets and liabilities of Star & Shield Holdings LLC, including SSRM, the attorney-in-fact for SSIE, for a purchase price of $1.8 million . During 2015 and 2014, White Mountains also purchased $ 4.0 million and $17.0 million of 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 Florida-domiciled reciprocal insurance exchange providing private passenger auto insurance to the public safety community and their families. SSIE is a VIE. SSRM’s role as the attorney-in-fact for SSIE gives it the power to direct the significant economic activities of SSIE and therefore, White Mountains is required to consolidate SSIE. See Note 18 - “Variable Interest Entities” . Dispositions Symetra During the third quarter of 2015, Symetra Financial Corporation (“Symetra”) announced that it entered into a definitive merger agreement with Sumitomo Life Insurance Company (“Sumitomo Life”) pursuant to which Sumitomo Life will acquire all of the outstanding shares of Symetra. White Mountains expects to receive $ 32.00 per share in cash at closing. White Mountains also received a special dividend of $ .50 per share as part of the transaction that was paid in the third quarter of 2015. The transaction closed in the first quarter of 2016. See Note 23 - “Subsequent Events” . Sale of Sirius Group On July 24, 2015, White Mountains entered into an agreement to sell Sirius Group to CM International Holding PTE Ltd., the Singapore-based investment arm of CMI. The purchase price will be paid in cash in an amount equal to 127.3% of Sirius Group’s closing date tangible common shareholder’s equity, plus $10.0 million . White Mountains has the option to replenish Sirius’s tangible common shareholder’s equity to its December 31, 2014 level should it be below that level at closing. The transaction is expected to close in the first quarter of 2016 and is subject to regulatory approvals and other customary closing conditions. As a result of the transaction, 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 does not include White Mountains's investment in Symetra and certain other investments that are in the Sirius Group legal entities as of December 31, 2015 but will be retained by White Mountains subsequent to the sale. As part of the transaction, White Mountains will transfer assets at closing equal to the value of the investments to be retained. The value of these investments, net of related tax effects, is approximately $686.2 million as of December 31, 2015. In connection with the transaction, White Mountains caused Sirius Group to purchase several industry loss warranty contracts to mitigate the potential impact of major events on Sirius Group's balance sheet pending the close of the sale to CMI (the “ILW Covers”). The cost and potential economic benefit provided by the coverage under the ILW Covers inure to White Mountains. The majority of the contracts expire in May or June 2016. The following summarizes the ILW Covers: Scope Limit Industry Loss Trigger United States first event $75.0 million $40.0 billion United States first event $22.5 million $50.0 billion United States second event $45.0 million $15.0 billion Japan first event $25.0 million $12.5 billion 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. OneBeacon has withdrawn its 2016 Plan of Operations and AmTrust will reinsure OneBeacon’s remaining net Crop Business exposure for the 2015 reinsurance year. As a result of this transaction, OneBeacon has no material net exposure related to the Crop Business. OneBeacon also received a payment of $3.0 million in connection with the termination of its agreement with CCIA, which has been recorded in other revenue. Sale of OneBeacon Runoff Business On December 23, 2014, OneBeacon completed the sale of its Runoff Business to Trebuchet US Holdings, Inc., a wholly-owned subsidiary of Armour Group Holdings Limited (“Armour”). Financing was provided in the form of surplus notes of $101.0 million that had a fair value of $64.9 million on the date of sale. Subsequent to closing, the surplus notes are included in OneBeacon’s investment portfolio, categorized within other long-term investments (see Note 5 - “Investment Securities” ). The difference of $36.1 million between the par value and the fair value of the surplus notes at the date of sale is included in the loss from sale of discontinued operations (see Note 22 - “Discontinued Operations” ). Sale of Essentia Insurance Company Effective January 1, 2013, OneBeacon completed the sale of Essentia Insurance Company (“Essentia”), an indirect wholly-owned subsidiary which wrote the collector cars and boats business, to Markel Corporation. Concurrently therewith, OneBeacon and Hagerty Insurance Agency (“Hagerty”) terminated their underwriting arrangement with respect to the collector cars and boats business. OneBeacon recognized a pre-tax gain on sale of $23.0 million ( $15.0 million after tax) in the first quarter of 2013. In 2015, OneBeacon recognized in other revenues a $3.7 million negative adjustment to the pre-tax gain on sale of Essentia in connection with an assessment from the Michigan Catastrophic Claims Association (“MCCA”) payable to Markel Corporation pursuant to the indemnification provisions in the stock purchase agreement governing the sale of Essentia.

Reserves for Unpaid Losses and

Reserves for Unpaid Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2015
Insurance Loss Reserves [Abstract]
Reserves for Unpaid Losses and Loss Adjustment Expenses	Reserves for Unpaid Losses and Loss Adjustment Expenses Insurance Operations White Mountains establishes loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. The process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain. Loss and LAE reserves are typically comprised of (1) case reserves for claims reported and (2) reserves for losses that have occurred but for which claims have not yet been reported, referred to as incurred but not reported (“IBNR”) reserves, which include a provision for expected future development on case reserves. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are typically derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made. Ultimate loss and LAE are generally determined by extrapolation of claim emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE with respect to any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. White Mountains’s own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its reserves. External data, available from organizations such as statistical bureaus, consulting firms and reinsurance companies, is sometimes used to supplement or corroborate White Mountains’s own experience. External data can be especially useful for estimating costs on new lines of business. Ultimate loss and LAE for major losses and catastrophes are estimated based on the known and expected exposures to the loss event, rather than simply relying on the extrapolation of reported and settled claims. For some lines of business, such as “long-tail” coverages discussed below, claims data reported in the most recent accident or report year is often too limited to provide a meaningful basis for analysis due to the typical delay in reporting and settling of claims. For this type of business, White Mountains uses an expected loss ratio method for the initial accident year or years. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends, for that line of business. Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and eventually settled. This time lag is sometimes referred to as the “claim-tail”. The claim-tail for most property coverages is typically short (usually a few days up to a few months). The claim-tail for liability/casualty coverages, such as automobile liability, general liability, products liability, multiple peril coverage, and workers compensation, can be especially long as claims are often reported and ultimately paid or settled years, even decades, after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior accident years, as well as about actual claims and trends may become known and, as a result, White Mountains may adjust its reserves. If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made in accordance with GAAP. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted, respectively. In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claim costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services and legislated changes in statutory benefits, as well as by the particular, unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claim costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE, and resulting IBNR reserves, is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain. White Mountains’s actuaries use several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. Management places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories: • Historical paid loss development methods: These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company’s claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to equal the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past. • Historical incurred loss development methods: These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters’ evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods. • Expected loss ratio methods: These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing, and are multiplied by the total amount of premiums earned to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tailed lines of business, when little or no paid or incurred loss information is available. • Adjusted historical paid and incurred loss development methods: These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claim payments or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes. White Mountains performs an actuarial review of its recorded reserves each quarter. White Mountains’s actuaries compare the previous quarter’s estimates of paid loss and LAE, case reserves and IBNR to amounts indicated by actual experience. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE should be relied upon to a greater or lesser extent than it had been in the past. While some variance is expected each quarter due to the inherent uncertainty in loss and LAE, persistent or large variances would indicate that prior assumptions and/or reliance on certain reserving methods may need to be revised going forward. Upon completion of each quarterly review, White Mountains’s actuaries select indicated reserve levels based on the results of the actuarial methods described previously, which are the primary consideration in determining management's best estimate of required reserves. However, in making its best estimate, management also considers other qualitative factors that may lead to a difference between held reserves and actuarially indicated reserves levels in the future. Typically, these factors exist when management and our actuaries conclude that there is insufficient historical incurred and paid loss information or that trends included in the historical incurred and paid loss information are unlikely to repeat in the future. Such factors include, among others, recent entry into new markets or new products, improvements in the claims department that are expected to lessen future ultimate loss costs, legal and regulatory developments, or other volatilities that may arise. Loss and Loss Adjustment Expense Reserve Summary The following table summarizes the loss and LAE reserve activities of White Mountains’s insurance subsidiaries for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, Millions 2015 2014 2013 Gross beginning balance $ 1,350.0 $ 1,054.3 $ 1,000.0 Less beginning reinsurance recoverable on unpaid losses (161.7 ) (80.2 ) (107.3 ) Net loss and LAE reserves 1,188.3 974.1 892.7 Loss and LAE reserves consolidated — SSIE — 13.6 — Losses and LAE incurred relating to: Current year losses 712.9 732.0 622.1 Prior year losses (4.0 ) 92.0 — Total incurred losses and LAE 708.9 824.0 622.1 Loss and LAE paid relating to: Current year losses (208.8 ) (202.6 ) (188.6 ) Prior year losses (479.1 ) (420.8 ) (352.1 ) Total loss and LAE payments (687.9 ) (623.4 ) (540.7 ) Net ending balance 1,209.3 1,188.3 974.1 Plus ending reinsurance recoverable on unpaid losses 186.5 161.7 80.2 Gross ending balance $ 1,395.8 $ 1,350.0 $ 1,054.3 Loss and LAE development —2015 During the year ended December 31, 2015, White Mountains experienced $4.0 million of net favorable loss reserve development, which consisted of $1.8 million of net favorable loss reserve development at OneBeacon and $2.2 million of net favorable loss reserve development at SSIE. The OneBeacon net favorable loss reserve development was primarily attributable to favorable loss reserve development from the Technology, Collector Cars and Boats, Specialty Property and Financial Services lines of business, offset by unfavorable net loss reserve development from the Entertainment and Ocean Marine lines of business. Loss and LAE development —2014 During the year ended December 31, 2014, White Mountains experienced $92.0 million of net unfavorable loss reserve development, which consisted of $89.8 million of net unfavorable loss reserve development at OneBeacon, of which $75.5 million related to the 2014 fourth quarter reserve increase described below, and $2.2 million of net unfavorable loss reserve development at SSIE. OneBeacon 2014 Fourth Quarter Loss and LAE Reserve Increase Through the first nine months of 2014, OneBeacon recorded $14.3 million of unfavorable loss and LAE reserve development, driven by greater-than-expected large losses in several underwriting units, primarily in the professional and management liability lines within Professional Insurance. In 2015, Professional Insurance was reorganized into Other Professional Lines, Management Liability, Financial Services and Healthcare. This large loss activity, which occurred mostly during the second and third quarters of 2014, also impacted the current accident year loss and LAE estimates. Additionally, OneBeacon incurred higher-than-usual claim coverage determination costs, a component of LAE expenses, during the first nine months of 2014. Other underwriting units also reported increased claim activity, including the Entertainment, Government Risks, and Accident underwriting units. Since the increased level of loss and LAE activity continued into the early part of the fourth quarter of 2014, the high level of activity in the second and third quarters no longer seemed to be isolated occurrences. As such, during the fourth quarter of 2014, OneBeacon enhanced its actuarial and claims review in several areas. OneBeacon isolated the recent large loss activity in each of its underwriting units and examined the emergence of large losses relative to the timing and amounts of expected large losses. OneBeacon also conducted additional analyses in the lawyers’ professional liability line within Professional Insurance. These new analyses included a claim level review and the application of additional actuarial methods and loss development assumptions. The results of these analyses indicated that the assumed tail risk included in the loss development patterns used to record IBNR reserves for this line were insufficient and needed to be increased for remaining long-tail exposures. OneBeacon’s claims and actuarial staff also conducted an in-depth review of coverage determination, litigation and other claim-specific adjusting expenses as a result of an emerging trend of increased expenses in these areas over recent quarters, particularly coverage determination expenses. This review concluded that the ultimate costs of these loss adjustment expenses were larger than previously estimated, causing management to record an increase in estimated LAE expenses, primarily in Professional Insurance. Finally, OneBeacon also recorded unfavorable prior year development in other underwriting units, including Entertainment and Government Risks. The unfavorable loss development in Entertainment and Government Risks resulted from heavier than expected claim activity during the fourth quarter, predominantly in the general liability and commercial auto liability lines. In order to fully reflect these recent trends, OneBeacon recorded a $109.2 million increase in loss and LAE reserves, which included a $75.5 million increase in prior accident year loss and LAE reserves and a $33.7 million increase in the current accident year loss and LAE reserves recorded at September 30, 2014. The components of the 2014 fourth quarter loss and LAE reserve increase and the net loss and LAE development for the full year are provided below: Underwriting Unit 2014 Fourth Quarter Reserve Increases Full Year 2014 Millions Current Accident Year Prior Accident Year Total Net Prior Year Development Professional Insurance $ 22.9 $ 46.4 $ 69.3 $ 59.1 Specialty Property (1.1 ) 5.7 4.6 1.1 Crop 3.8 — 3.8 — Other 2.8 (.4 ) 2.4 1.6 Specialty Products 28.4 51.7 80.1 61.8 Entertainment 1.5 11.6 13.1 13.5 Accident — 3.5 3.5 6.0 Government Risks 1.2 7.1 8.3 8.5 Other 2.6 1.6 4.2 — Specialty Industries 5.3 23.8 29.1 28.0 Total $ 33.7 $ 75.5 $ 109.2 $ 89.8 As noted above, OneBeacon increased its provision for current accident year losses and LAE by $33.7 million in the fourth quarter of 2014. In making its loss and LAE reserve picks for the 2014 accident year, OneBeacon considered the results of the enhanced actuarial and claim review and the fact that reported large claims were approaching estimated ultimate held reserves for large losses sooner than originally expected. $3.8 million of the increase is related to higher-than-expected reports of crop losses that emerged in the fourth quarter. The remaining $29.9 million of the increase reflects an increase in management’s best estimate of current losses and LAE as of December 31, 2014 from those recorded in the first nine months of 2014. This increase primarily affected the Professional Insurance underwriting unit, which represented $22.9 million of the total provision. Loss and LAE development —2013 During the year ended December 31, 2013, White Mountains experienced no net loss and LAE reserve development on prior accident year reserves. OneBeacon experienced favorable development in its other liability and ocean marine lines, which was offset by unfavorable development primarily related to property, general liability and accident and health lines.

Third Party Reinsurance

Third Party Reinsurance	12 Months Ended
	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Third-Party Reinsurance	Third-Party Reinsurance In the normal course of business, White Mountains’s insurance and reinsurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third-party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. The effects of reinsurance on White Mountains’s insurance and reinsurance subsidiaries’ written and earned premiums and on losses and LAE were as follows (see Note 10 - “Municipal Bond Guarantee Insurance” for balances related to White Mountains financial guarantee business): Year ended December 31, 2015 Millions OneBeacon HG/BAM (1) Other Total Written premiums: Direct $ 1,279.9 $ 25.9 $ 19.9 $ 1,325.7 Assumed 36.0 — — 36.0 Gross written premiums 1,315.9 25.9 19.9 1,361.7 Ceded (179.3 ) (2) — (9.8 ) (189.1 ) Net written premiums $ 1,136.6 $ 25.9 $ 10.1 $ 1,172.6 Earned premiums: Direct $ 1,298.0 $ 3.3 $ 20.7 $ 1,322.0 Assumed 45.9 — — 45.9 Gross earned premiums 1,343.9 3.3 20.7 1,367.9 Ceded (167.7 ) (2) — (12.0 ) (179.7 ) Net earned premiums $ 1,176.2 3.3 $ 8.7 $ 1,188.2 Losses and LAE: Direct $ 783.0 $ — $ 19.5 $ 802.5 Assumed 55.7 — — 55.7 Gross losses and LAE 838.7 — 19.5 858.2 Ceded (138.0 ) (2) — (11.3 ) (149.3 ) Net losses and LAE $ 700.7 $ — $ 8.2 $ 708.9 (1) During 2015, BAM ceded $19.3 in written premiums and $16.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment. (2) During 2015, OneBeacon recorded ceded $ 33.3 in written premiums, $ 33.3 in earned premiums and $33.4 in loss and loss adjustment expenses as a result of the exit of the Crop Business due to the 100% quota share reinsurance agreement with AmTrust. Year ended December 31, 2014 Millions OneBeacon HG/BAM (1) Other (2) Total Written premiums: Direct $ 1,257.5 $ 16.2 $ 22.6 $ 1,296.3 Assumed 65.9 — — 65.9 Gross written premiums 1,323.4 16.2 22.6 1,362.2 Ceded (106.5 ) — (16.7 ) (123.2 ) Net written premiums $ 1,216.9 $ 16.2 $ 5.9 $ 1,239.0 Earned premiums: Direct $ 1,209.1 $ 1.8 $ 22.6 $ 1,233.5 Assumed 70.9 — — 70.9 Gross earned premiums 1,280.0 1.8 22.6 1,304.4 Ceded (102.9 ) — (16.5 ) (119.4 ) Net earned premiums $ 1,177.1 1.8 $ 6.1 $ 1,185.0 Losses and LAE: Direct $ 778.7 $ — $ 24.1 $ 802.8 Assumed 115.7 — — 115.7 Gross losses and LAE 894.4 — 24.1 918.5 Ceded (79.3 ) — (15.2 ) (94.5 ) Net losses and LAE $ 815.1 $ — $ 8.9 $ 824.0 (1) During 2014, BAM ceded $ 12.3 in written premiums and $ 1.4 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment. (2) During 2014, SSIE ceded $16.0 in written premiums, $ 15.7 in earned premiums, and $16.9 in loss and loss adjustment expenses to OneBeacon, which have been eliminated in consolidation. Year ended December 31, 2013 Millions OneBeacon HG/BAM (1) Total Written premiums: Direct $ 1,103.1 $ 13.6 $ 1,116.7 Assumed 59.8 — 59.8 Gross written premiums 1,162.9 13.6 1,176.5 Ceded (74.3 ) — (74.3 ) Net written premiums $ 1,088.6 $ 13.6 $ 1,102.2 Earned premiums: Direct $ 1,043.3 $ .5 $ 1,043.8 Assumed 148.5 — 148.5 Gross earned premiums 1,191.8 .5 1,192.3 Ceded (71.4 ) — (71.4 ) Net earned premiums $ 1,120.4 $ .5 $ 1,120.9 Losses and LAE: Direct $ 584.9 $ — $ 584.9 Assumed 76.3 — 76.3 Gross losses and LAE 661.2 — 661.2 Ceded (39.1 ) — (39.1 ) Net losses and LAE $ 622.1 $ — $ 622.1 (1) During 2013, BAM ceded $ 10.6 in written premiums ($ 10.2 in earned premiums) to HG Global, which have been eliminated within the HG/BAM segment. OneBeacon The timing and size of catastrophe losses are unpredictable and the level of losses experienced in any year could be material to OneBeacon’s operating results and financial condition. Examples of catastrophes include losses caused by earthquakes, wildfires, hurricanes and other types of storms and terrorist acts. The extent of losses caused by a catastrophic event is a function of severity and the amount and type of insured exposure in the affected area. In the normal course of business, OneBeacon's insurance subsidiaries seek to limit losses that may arise from catastrophes or other events through individual risk selection, imposing deductibles and limits, limiting its concentration of insurance in catastrophe-prone areas, such as coastal regions, and reinsuring with third-party reinsurers. OneBeacon uses models (primarily AIR Worldwide (“AIR”) Touchstone version 3.0) to estimate potential losses from catastrophes. OneBeacon uses this model output in conjunction with other data to manage its exposure to catastrophe losses based on a probable maximum loss (“PML”) forecast to quantify its exposure to an extreme catastrophe event. OneBeacon purchases a general catastrophe reinsurance treaty with unaffiliated reinsurers to manage its exposure to large catastrophe losses. Effective May 1, 2015, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2016. The program provides coverage for OneBeacon’s property business as well as certain acts of terrorism. Under the program, the first $20.0 million of losses resulting from any single catastrophe are retained and 95% of the next $10.0 million of losses resulting from the catastrophe are reinsured. The part of a catastrophe loss in excess of $130.0 million would be retained in full. In the event of a catastrophe, OneBeacon’s property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium. OneBeacon's current third party reinsurance programs provide varying degrees of coverage for terrorism events. The Company's overall terrorism exposure is impacted by the Terrorism Risk Insurance Program (the “Terrorism Act”), which is a federal program administered by the Department of the Treasury that provides for a shared system of public and private compensation for commercial property and casualty losses resulting from events that reach the threshold for losses ( $120.0 million in 2016 and increasing $20.0 million in subsequent years until the threshold becomes $200.0 million in 2020) and are certified as an act of terrorism by the U.S. Secretary of the Treasury, in concurrence with the Secretary of Homeland Security and the Attorney General of the United States. The Terrorism Act limits the industry's aggregate liability for losses from certified terrorist acts by requiring the federal government to share a set amount of losses ( 84% in 2016 and decreasing 1% in subsequent years until it reaches a floor of 80% in 2020) once a company meets a specific retention or deductible as determined by its prior year's direct written premiums. It also limits the aggregate liability to be paid by the government and industry without further action by Congress to $100.0 billion . In exchange for this “backstop,” primary insurers are required to make coverage available to commercial insureds for losses from acts of terrorism as specified in the Terrorism Act. The following types of coverage are excluded from the program: commercial automobile, burglary and theft, surety, farmowners multi-peril and all professional liability coverage except directors and officers coverage. All losses that result from a nuclear, biological, chemical or radiological terrorist attack are excluded from the Company's current third party reinsurance program. OneBeacon's property catastrophe treaty also excludes acts of terrorism certified pursuant to the Terrorism Act and committed by an individual or individuals acting on behalf of any foreign person or foreign interest. OneBeacon's casualty clash treaty provides coverage for losses that result from certified and non-certified acts of terrorism, on an aggregated basis, subject to a maximum of one full treaty limit. OneBeacon's property per risk, casualty and workers compensation treaties each provide full coverage for certified acts of terrorism on behalf of a non-foreign person or interest, but are sublimited to one full treaty limit for certified acts of terrorism committed on behalf of any foreign person or foreign interest. OneBeacon's healthcare treaty is sublimited to one full treaty limit of coverage for all acts of terrorism. OneBeacon estimates its individual retention level for commercial policies subject to the Terrorism Act to be approximately $120.0 million in 2016. The federal government will pay 84% of covered terrorism losses that exceed OneBeacon’s or the industry’s retention levels in 2016, up to a total of $100.0 billion . As indicated above, OneBeacon’s 16% copay will increase annually beginning in 2016 by 1% until it reaches a limit of 20% in 2020. In addition to the corporate catastrophe reinsurance protection, OneBeacon also purchases dedicated reinsurance protection for certain lines of business. OneBeacon Specialty Property underwriting unit purchases a dedicated property catastrophe program providing 100% coverage for $34.0 million of loss in excess of $6.0 million , which inures to the benefit of the property catastrophe reinsurance program described previously. This treaty limit cannot be reinstated. OneBeacon also purchases property-per-risk reinsurance coverage to reduce large loss volatility. The property-per-risk reinsurance program reinsures 100% of losses in excess of $3.0 million , which represents a retention decrease from $5.0 million for 2014, up to $100.0 million . Individual risk facultative reinsurance is purchased above $100.0 million . The property-per-risk treaty provides one limit of reinsurance protection for losses in excess of $3.0 million up to $100.0 million on an individual risk basis for certified acts of terrorism committed on behalf of any foreign person or foreign interest. However, any nuclear events, or biological, chemical or radiological terrorist attacks are not covered. OneBeacon also maintains a casualty reinsurance program that provides protection for individual policies involving general liability, automobile liability, professional liability or umbrella liability. OneBeacon's healthcare professional liability treaty covers losses in excess of $3.0 million up to $10.0 million . All other casualty business is covered in a separate treaty covering losses in excess of $3.0 million up to $11.0 million . Losses in excess of $10.0 million for business subject to the healthcare professional liability treaty up to $20.0 million , and losses in excess of $11.0 million for all other casualty business up to $21.0 million are 100% reinsured by a combined Second Casualty Excess of Loss treaty layer. OneBeacon also purchases a treaty to protect against large workers compensation losses that covers 100% of the loss in excess of $2.0 million up to $10.0 million per occurrence. Additionally, for casualty, surety, and/or workers compensation catastrophe losses, OneBeacon maintains a dedicated clash treaty, which provides coverage in the event that one loss event results in two or more claims, that covers losses up to $60.0 million in excess of a $10.0 million retention for workers compensation and for losses up to $ 40.0 million in excess of $ 10.0 million retention for casualty and surety losses. OneBeacon purchases a per-occurrence treaty for marine business, both inland and ocean, that protects against large occurrences, whether a single large claim or a catastrophe. The marine treaty attaches at $2.5 million per occurrence. The first layer of the marine treaty is $7.5 million in excess of $2.5 million , with an annual aggregate deductible of $5.0 million for catastrophe losses. Coverage is provided up to $60.0 million . Retained catastrophe losses are subject to the corporate catastrophe treaty. Individual risk losses from inland marine exceeding $20.0 million are subject to the corporate property per risk treaty. Reinstatement premiums are paid in full or in part depending on the layer and the occurrence if the coverage is attached. OneBeacon also purchases reinsurance for its surety business. Effective October 1, 2015, this treaty covers 100% of losses in excess of $5.0 million up to $50.0 million per bond and up to $100.0 million in aggregate. Additionally, effective January 1, 2015, OneBeacon placed an HMO/Provider Excess reinsurance agreement providing unlimited coverage excess of $5.0 million per member in two layers with no aggregate coverage maximum. Effective June 1, 2015, OneBeacon also purchased reinsurance on its film completion bond business in excess of $2.0 million up to $35.0 million in three layers, with a facultative treaty layer providing coverage up to $60.0 million as needed. On July 31, 2015, OneBeacon reinsured 100% of its net retained losses for both its multi-peril crop insurance (“MPCI”) and crop-hail business by entering into a quota share reinsurance agreement with an insurance operating affiliate of AmTrust for the remaining net exposure to the 2015 Crop reinsurance year. As of December 31, 2015 , OneBeacon had $7.5 million and $186.0 million of reinsurance recoverables on paid and unpaid losses. As reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders, collectability of balances due from reinsurers is important to OneBeacon’s financial strength. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength of its reinsurers on an ongoing basis. Uncollectible amounts historically have not been significant. The following table summarizes Standard & Poor’s Financial Services LLC (“Standard & Poor’s”) ratings for OneBeacon’s reinsurers. Standard & Poor’s Rating (1) $ in millions Balance at December 31, 2015 % of Total AA $ 41.7 22 % A 126.2 65 % BBB, Not rated and other (2) 25.6 13 % Total $ 193.5 100 % (1) Standard & Poor’s ratings as detailed above are: “AA” (Very strong), “A” (Strong) and “BBB” (Adequate). (2) Includes $20.4 related to OBIC, an unrated entity sold to Armour as part of the Runoff Transaction.

Investment Securities

Investment Securities	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investment Securities 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 2015, 2014 and 2013 consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Investment income: Fixed maturity investments $ 52.6 $ 51.3 $ 48.1 Short-term investments .2 .1 .8 Common equity securities 10.1 16.6 16.5 Other long-term investments 3.3 4.4 6.8 Total investment income 66.2 72.4 72.2 Third-party investment expenses (5.4 ) (12.9 ) (12.4 ) Net investment income, pre-tax $ 60.8 $ 59.5 $ 59.8 Net Realized and Unrealized Investment Gain (Losses) Net realized and unrealized investment gains (losses) consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Net realized investment gains, pre-tax $ 77.3 $ 166.8 $ 95.5 Net unrealized investment gains (losses), pre-tax 148.1 (88.3 ) 38.4 Net realized and unrealized investment gains, pre-tax 225.4 78.5 133.9 Income tax expense attributable to net realized and unrealized investment gains (35.2 ) (18.0 ) (12.8 ) Net realized and unrealized investment gains, after tax $ 190.2 $ 60.5 $ 121.1 Net realized investment gains Net realized investment gains for 2015, 2014 and 2013 consisted of the following: Year ended December 31, 2015 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ 1.9 $ — $ 1.9 Common equity securities 64.4 .4 64.8 Other long-term investments 10.6 — 10.6 Net realized investment gains, pre-tax 76.9 .4 77.3 Income tax expense attributable to realized investment gains (22.8 ) — (22.8 ) Net realized investment gains, after-tax $ 54.1 $ .4 $ 54.5 Year ended December 31, 2014 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ 5.8 $ — $ 5.8 Common equity securities 138.0 — 138.0 Other long-term investments 23.0 — 23.0 Net realized investment gains, pre-tax 166.8 — 166.8 Income tax expense attributable to realized investment gains (28.4 ) — (28.4 ) Net realized investment gains, after-tax $ 138.4 $ — $ 138.4 Year ended December 31, 2013 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ (.2 ) $ (.2 ) $ (.4 ) Short-term investments .1 — .1 Common equity securities 89.4 — 89.4 Other long-term investments 6.4 — 6.4 Net realized investment gains (losses), pre-tax 95.7 (.2 ) 95.5 Income tax (expense) benefit attributable to realized investment gains (losses) (16.2 ) .1 (16.1 ) Net realized investment gains (losses), after-tax $ 79.5 $ (.1 ) $ 79.4 Net unrealized investment gains (losses) The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value: Year ended December 31, 2015 Millions Net unrealized gains (losses) Net foreign exchange gains (losses) Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (15.6 ) $ — $ (15.6 ) Common equity securities 207.6 (3.7 ) 203.9 Other long-term investments (39.1 ) (1.1 ) (40.2 ) Net unrealized investment gains (losses), pre-tax 152.9 (4.8 ) 148.1 Income tax expense attributable to unrealized investment gains (losses) (12.3 ) (.1 ) (12.4 ) Net unrealized investment gains (losses), after-tax $ 140.6 $ (4.9 ) $ 135.7 Year ended December 31, 2014 Millions Net Net Total net unrealized Fixed maturity investments $ 11.3 $ — $ 11.3 Common equity securities (83.0 ) (7.7 ) (90.7 ) Other long-term investments (7.6 ) (1.3 ) (8.9 ) Net unrealized investment losses, pre-tax (79.3 ) (9.0 ) (88.3 ) Income tax benefit attributable to unrealized investment losses 9.9 .5 10.4 Net unrealized investment losses, after-tax $ (69.4 ) $ (8.5 ) $ (77.9 ) Year ended December 31, 2013 Millions Net unrealized gains (losses) Net foreign exchange gains (losses) Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (49.9 ) $ — $ (49.9 ) Common equity securities 86.1 (1.9 ) 84.2 Other long-term investments 4.1 — 4.1 Net unrealized investment gains (losses), pre-tax 40.3 (1.9 ) 38.4 Income tax benefit attributable to unrealized investment gains (losses) 3.3 — 3.3 Net unrealized investment gains (losses), after-tax $ 43.6 $ (1.9 ) $ 41.7 White Mountains recognized gross realized investment gains of $112.9 million , $194.0 million and $152.8 million and gross realized investment losses of $35.6 million , $27.2 million and $57.3 million on sales of investment securities during 2015, 2014 and 2013. 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 years ended December 31, 2015, 2014 and 2013 . Year Ended December 31, Millions 2015 2014 2013 Fixed maturity investments $ (1.1 ) $ 1.9 $ (1.6 ) Common equity securities (9.0 ) 5.8 .9 Other long-term investments (40.1 ) — 2.1 Total net unrealized investment (losses) gains, pre-tax - Level 3 investments $ (50.2 ) $ 7.7 $ 1.4 The components of White Mountains’s net realized and unrealized investment gains (losses), after-tax, as recorded on the statements of operations and comprehensive income were as follows: Year Ended December 31, Millions 2015 2014 2013 Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates $ (39.2 ) $ 81.2 $ (106.4 ) Income tax benefit (expense) 2.9 (5.9 ) 8.3 Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax (36.3 ) 75.3 (98.1 ) Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra 1.4 — — Total investment (losses) gains through accumulated other comprehensive income (34.9 ) 75.3 (98.1 ) Net realized and unrealized investment gains, after-tax 190.2 60.5 121.1 Total investment gains recorded during the period, after-tax $ 155.3 $ 135.8 $ 23.0 Investment Holdings The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s fixed maturity investments as of December 31, 2015 and 2014 , were as follows: December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value US Government and agency obligations $ 160.4 $ — $ (.4 ) $ — $ 160.0 Debt securities issued by corporations 1,001.0 4.3 (5.3 ) — 1,000.0 Municipal obligations 227.8 2.2 (1.2 ) — 228.8 Mortgage-backed 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 December 31, 2014 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value US Government and agency obligations $ 105.4 $ .1 $ (.3 ) $ — $ 105.2 Debt securities issued by corporations 1,162.0 13.1 (3.4 ) — 1,171.7 Municipal obligations 81.0 1.4 (.2 ) — 82.2 Mortgage-backed and asset-backed securities 967.5 2.8 (2.4 ) — 967.9 Foreign government, agency and provincial obligations 11.5 .3 (1.0 ) — 10.8 Preferred stocks 78.3 5.9 — — 84.2 Total fixed maturity investments $ 2,405.7 $ 23.6 $ (7.3 ) $ — $ 2,422.0 The weighted average duration of White Mountains’s fixed income portfolio as of December 31, 2015 was approximately 2.2 years, including short-term investments, and approximately 2.4 years excluding short-term investments. The cost or amortized cost and carrying value of White Mountains’s fixed maturity investments as of December 31, 2015 is presented below by contractual maturity. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties. December 31, 2015 Millions Cost or amortized cost Carrying value Due in one year or less $ 241.6 $ 242.0 Due after one year through five years 982.3 981.5 Due after five years through ten years 96.4 96.1 Due after ten years 69.9 70.4 Mortgage-backed and asset-backed securities 1,170.6 1,167.0 Preferred stocks 78.3 82.7 Total $ 2,639.1 $ 2,639.7 The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of December 31, 2015 and 2014 were as follows: December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value Common equity securities $ 822.5 $ 302.8 $ (11.4 ) $ — $ 1,113.9 Other long-term investments $ 304.5 $ 32.0 $ (18.4 ) $ (2.3 ) $ 315.8 December 31, 2014 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value Common equity securities $ 495.3 $ 127.4 $ (3.6 ) $ (7.4 ) $ 611.7 Other long-term investments $ 286.7 $ 53.3 $ (6.8 ) $ (1.3 ) $ 331.9 Proceeds from the sales and maturities of investments, excluding short-term investments, totaled $2.2 billion , $3.3 billion and $2.7 billion for the years ended December 31, 2015, 2014 and 2013 . Investments Held on Deposit or as Collateral As of December 31, 2015 and 2014 , investments of $104.8 million and $103.4 million , respectively, were held in trusts required to be maintained in relation to various reinsurance agreements. White Mountains’s consolidated insurance and reinsurance operations are required to maintain deposits with certain insurance regulatory agencies in order to maintain their insurance licenses. The fair value of such deposits which are included within total investments totaled $88.0 million and $68.9 million as of December 31, 2015 and 2014 . As of December 31, 2015 and 2014 , OneBeacon held unrestricted collateral from its customers, which is included in cash and invested assets, relating to its surety business of $137.7 million and $81.0 million . The obligation to return these funds is included in funds held under insurance and reinsurance contracts in the consolidated balance sheets. As of December 31, 2014, White Mountains held $9.5 million of restricted collateral in the form of fixed maturity investments associated with variable annuity reinsurance agreements. See Note 9 - “Derivatives”. Fair value measurements as of December 31, 2015 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. 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 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 hedge funds and private equity funds, surplus notes, as well as certain investments in fixed maturity investments, common equity securities, and convertible fixed and preferred maturity investments where quoted market prices are unavailable or are not considered reasonable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services 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 models and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, 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 model pricing methodologies, review of the pricing services' quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and review of 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 model 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 model 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 an evaluated pricing model 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 obligations: The fair value of foreign government obligations is determined from an evaluated pricing model 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 model 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 the audited annual financial statements of hedge funds and private equity funds and periodically discussing each fund’s pricing with the fund manager. 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. Accordingly, the fair values of White Mountains’s investments in hedge funds and private equity funds have been classified as Level 3 measurements. The fair value of White Mountains’s investments in hedge funds and private equity funds has been determined using net asset value. Prior to the liquidation of the Prospector Offshore Fund, in addition to the investments described above, White Mountains had $31.6 million of investment-related liabilities recorded at fair value and included in other liabilities as of December 31, 2014. These liabilities relate to securities that have been sold short by limited partnerships in which White Mountains has investments and was required to consolidate under GAAP. These liabilities have a Level 1 designation. Fair Value Measurements by Level The following tables summarize White Mountains’s fair value measurements for investments as of December 31, 2015 and 2014 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 asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and 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 Barclays Intermediate Aggregate and S&P 500 indices. The fair value measurements for derivative assets associated with White Mountains’s variable annuity business are presented in Note 9 . December 31, 2015 Millions Fair value Level 1 Inputs Level 2 Inputs Level 3 Inputs 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 — Other — — — — Total debt securities issued by corporations: 1,000.0 — 1,000.0 — Mortgage-backed 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 2,639.7 134.0 2,435.7 70.0 Short-term investments 211.3 211.3 — — Common equity securities: Exchange traded funds 141.8 120.5 21.3 — Financials 694.7 694.7 — — 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 (1) 297.3 — — 297.3 Total investments (1) $ 4,262.2 $ 1,363.5 $ 2,531.4 $ 367.3 (1) 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. December 31, 2014 Millions Fair value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fixed maturity investments: U.S. Government and agency obligations $ 105.2 $ 51.2 $ 54.0 $ — Debt securities issued by corporations: Consumer 259.2 — 259.2 — Financials 254.7 — 254.7 — Health Care 181.8 — 181.8 — Industrial 107.3 — 102.0 5.3 Communications 71.5 — 71.5 — Energy 65.6 — 65.6 — Utilities 84.1 — 84.1 — Technology 88.3 — 88.3 — Materials 51.7 — 51.7 — Other 7.5 — 7.5 — Total debt securities issued by corporations: 1,171.7 — 1,166.4 5.3 Mortgage-backed and asset-backed securities 967.9 — 967.9 — Municipal obligations 82.2 — 82.2 — Foreign government, agency and provincial obligations 10.8 .6 10.2 — Preferred stocks 84.2 — 13.1 71.1 Total fixed maturity investments 2,422.0 51.8 2,293.8 76.4 Short-term investments 376.8 376.3 .5 — Common equity securities: Financials 180.7 141.2 — 39.5 Consumer 116.0 115.9 .1 — Health Care 74.4 74.4 — — Industrial 57.1 57.1 — — Technology 44.8 44.8 — — Communications 28.5 28.5 — — Energy 18.1 18.1 — — Materials 12.0 12.0 — — Utilities 4.9 4.9 — — Other 75.2 1.9 73.3 — Total common equity securities 611.7 498.8 73.4 39.5 Other long-term investments (1) 309.9 — 5.7 304.2 Total investments (1) $ 3,720.4 $ 926.9 $ 2,373.4 $ 420.1 (1) Excludes the carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method and the carrying value of $16.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. Debt securities issued by corporations The following table summarizes the ratings of the corporate debt securities held in White Mountains’s investment portfolio as of December 31, 2014 and 2013: Fair Value at December 31, Millions 2015 2014 AA $ 95.2 $ 144.9 A 397.7 594.9 BBB 507.1 426.5 Other — 5.4 Debt securities issued by corporations (1) $ 1,000.0 $ 1,171.7 (1) Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor’s and 2) Moody’s. Mortgage-backed, Asset-backed Securities White Mountains purchases commercial mortgage-backed securities (“CMBS”) and residential mortgage-backed securities (“RMBS”) with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics). White Mountains categorizes mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. White Mountains’s non-agency residential mortgage-backed portfolio is generally moderate-term and structurally senior. White Mountains does not own any collateralized loan obligations. White Mountains does not own any collateralized debt obligations, with the exception of $39.6 million of non-agency residential mortgage resecuritization tranches, each a senior tranche in its own right and each collateralized by a single earlier vintage Super Senior or Senior non-agency residential mortgage backed security. December 31, 2015 December 31, 2014 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 265.5 $ 265.5 $ — $ 308.7 $ 308.7 $ — FNMA 42.2 42.2 — 33.4 33.4 — FHLMC 22.8 22.8 — 28.9 28.9 — Total Agency (1) 330.5 330.5 — 371.0 371.0 — Non-agency: Residential 133.2 133.2 — 71.0 71.0 — Commercial 140.4 140.4 — 109.3 109.3 — Total Non-agency 273.6 273.6 — 180.3 180.3 — Total mortgage-backed securities 604.1 604.1 — 551.3 551.3 — Asset-backed securities: Credit card receivables 217.7 217.7 — 218.1 218.1 — Vehicle receivables 269.7 269.7 — 152.8 152.8 — Other 75.5 75.5 — 45.7 45.7 — Total asset-backed securities 562.9 562.9 — 416.6 416.6 — Total mortgage and asset-backed securities $ 1,167.0 $ 1,167.0 $ — $ 967.9 $ 967.9 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC). Non-agency Mortgage-backed Securities The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2015 are as follows: Security Issuance Year Millions Fair Value 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Non-agency RMBS $ 133.2 $ 17.3 $ 8.6 $ 3.9 $ — $ 3.6 $ — $ 15.2 $ 13.5 $ 9.8 $ 14.0 $ 47.3 $ — Non-agency CMBS 140.4 — — — — — — 5.4 — 18.0 17.2 55.0 $ 44.8 Total $ 273.6 $ 17.3 $ 8.6 $ 3.9 $ — $ 3.6 $ — $ 20.6 $ 13.5 $ 27.8 $ 31.2 $ 102.3 $ 44.8 Non-agency Residential Mortgage-backed Securities The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of December 31, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 133.1 $ 65.6 $ 67.5 $ — Non-prime .1 — .1 — Sub-prime — — — — Total $ 133.2 $ 65.6 $ 67.6 $ — (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-backed Securities White Mountains’s non-agency CMBS portfolio is generally short-term and structurally subordinate, with more than 25 points of subordination on average for both fixed rate CMBS and floating rate CMBS as of December 31, 2015 . In general, subordination represents the percentage principal loss on the underlying collateral that would be absorbed by other securities lower in the capital structure before the more senior security incurs a loss. As of December 31, 2015 , on average less than 1% of the underlying loans were reported as non-performing for all non-agency CMBS held by White Mountains. The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of December 31, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Fixed rate CMBS $ 101.1 $ 8.7 $ 48.6 $ 43.8 Floating rate CMBS 39.3 — — 39.3 Total $ 140.4 $ 8.7 $ 48.6 $ 83.1 (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to other “AAA” or “Aaa” bonds. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds. Other Long-term Investments Other long-term investments consist of the following as of December 31, 2015 and 2014: Carrying Value at Millions December 31, 2015 December 31, 2014 Hedge funds and private equity funds, at fair value (1) $ 127.8 $ 178.5 Private equity securities and limited liability companies, at fair value (1) 82.1 45.3 Surplus notes investments, at fair value (1) 51.5 65.1 Convertible preferred securities (1) 32.7 8.3 Tax advantaged federal affordable housing development fund (2) 14.7 16.8 Partnership investments accounted for under the equity method 3.8 5.2 Convertible fixed maturity investments (1) — 5.6 Other (1) 3.2 7.1 Total other-long term investments $ 315.8 $ 331.9 (1) See Fair Value Measurements by Level table. (2) Fund accounted for using the proportional amortization method. Hedge Funds and Private Equity Funds White Mountains holds investments in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments has been estimated using the net asset value of the funds. As of December 31, 2015 , White Mountains held investments in 5 hedge funds and 22 private equity funds. The largest investment in a single fund was $22.7 million as of December 31, 2015 and $22.0 million as of December 31, 2014 . The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2015 and 2014 : December 31, 2015 December 31, 2014 Millions Fair Value Unfunded Fair Value Unfunded Hedge funds Long/short equity REIT $ 20.6 $ — $ 20.4 $ — Long/short credit & distressed — — 8.4 — Long/short banks and financial 12.8 — 29.9 — Other 3.6 — 15.7 — Total hedge funds 37.0 — 74.4 — Private equity funds Energy infrast

Debt and Standby Letter of Cred

Debt and Standby Letter of Credit Facilities	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Debt and Standby Letter of Credit Facilities	Debt White Mountains’s debt outstanding as of December 31, 2015 and 2014 consisted of the following: December 31, Millions 2015 2014 WTM Bank Facility $ 50.0 $ — OBH Senior Notes, at face value 275.0 275.0 Unamortized original issue discount (.2 ) (.3 ) OBH Senior Notes, carrying value 274.8 274.7 OneBeacon Bank Facility — — Tranzact Bank Facility 104.7 68.7 Unamortized issuance cost (1.8 ) (1.3 ) Tranzact Bank Facility, carrying value 102.9 67.4 MediaAlpha Bank Facility 15.0 — Unamortized issuance cost (.3 ) — MediaAlpha Bank Facility, carrying value 14.7 — Other debt — 1.0 Total debt $ 442.4 $ 343.1 A schedule of contractual repayments of White Mountains’s debt as of December 31, 2015 , follows: Millions December 31, Due in one year or less $ 74.9 Due in two to three years 43.3 Due in four to five years 51.5 Due after five years 275.0 Total $ 444.7 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 (the “WTM Bank Facility”) and has a maturity date of August 14, 2018 (the “WTM Bank Facility”). The WTM Bank Facility replaced White Mountains’s previous revolving credit facility, which had a total commitment of $375.0 million (the “Previous WTM Bank Facility”). During 2015, White Mountains borrowed a total of $125.0 million and repaid a total of $75.0 million under the WTM Bank Facility at a blended interest rate of 3.74% . During 2014, White Mountains borrowed and repaid a total of $65.0 million under the WTM Bank Facility at a blended interest rate of 3.65% . As of December 31, 2015 , the WTM Bank Facility had an outstanding balance of $50.0 million . White Mountains recorded $0.1 million , $0.3 million, $0.3 million of interest expense on the WTM Bank Facility for the years ended December 31, 2015, 2014 and 2013 . The WTM Bank Facility contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. These covenants can restrict White Mountains in several ways, including its ability to incur additional indebtedness. OBH Senior Notes In November 2012, OneBeacon U.S. Holdings, Inc. (“OBH”), an intermediate holding company of OneBeacon, issued $275.0 million face value of senior unsecured notes (“OBH Senior Notes”) through a public offering, at an issue price of 99.9% and received $272.9 million of proceeds. The OBH Senior Notes bear an annual interest rate of 4.6% payable semi-annually in arrears on May 9 and November 9, until maturity on November 9, 2022, and are fully and unconditionally guaranteed as to the payment of principal and interest by OneBeacon Ltd. OBH incurred $2.8 million in expenses related to the issuance of the OBH Senior Notes (including $1.8 million in underwriting fees), which have been deferred and are being recognized as interest expense over the life of the OBH Senior Notes. Taking into effect the amortization of the original issue discount and all underwriting and issuance expenses, the OBH Senior Notes have an effective yield to maturity of approximately 4.7% per annum. In December 2012, the proceeds from the OBH Senior Notes were utilized to repurchase the remaining $269.8 million of outstanding senior notes issued in 2003 by OBH with an annual interest rate of 5.875% for $275.9 million . The repurchase resulted in a $6.3 million loss, including transaction fees and the write-off of the remaining $0.2 million in unamortized deferred costs and original issue discount at the time of repurchase. OBH recorded $13.0 million in interest expense on the OBH Senior Notes for each of the years ended December 31, 2015, 2014 and 2013 . 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 to consolidate, merge or transfer their properties and assets. The indentures do not contain any financial ratios or specified levels of net worth or liquidity to which OneBeacon Ltd. or OBH must adhere. OneBeacon Bank Facility On September 29, 2015, OneBeacon Ltd. and OBH, as co-borrowers and co-guarantors, entered into a revolving credit facility administered by U.S. Bank N.A. and also including BMO Harris Bank N.A., which has a total commitment of $65.0 million and has a maturity date of September 29, 2019 (the “OneBeacon Bank Facility”). As of December 31, 2015 , the OneBeacon Bank Facility was undrawn. The OneBeacon Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. These covenants can restrict White Mountains in several ways, including its ability to incur additional indebtedness. Tranzact Bank Facility On October 10, 2014, Tranzact entered into a secured credit facility with a syndicate of lenders administered by The PrivateBank and Trust Company (the “Tranzact Bank Facility”). The term of the credit facility ends on October 10, 2019. During 2015, Tranzact amended the Tranzact Bank Facility, which now has a total commitment of $116.0 million , consisting of a $101.0 million term loan facility and a $15.0 million revolving facility. The amendment increased the term loan facility by $31.0 million , the proceeds of which were used by Tranzact to finance the acquisition of TruBridge. During 2015, Tranzact borrowed $24.0 million and repaid $13.0 million under the revolving facility. Also during 2015, Tranzact repaid a total of $6.5 million under the term loan portion. As of December 31, 2015 , the total amount outstanding under the Tranzact Bank Facility was $104.7 million . Tranzact has entered into an interest rate swap agreement to effectively fix the rate it pays on $70.0 million of the $101.0 million term loan. (See Note 9 - “Derivatives“ ) White Mountains recorded $4.0 million and $0.7 million of interest expense on the Tranzact Bank Facility for the years ended December 31, 2015 and 2014. The Tranzact Bank Facility carries a variable interest rate, at a margin over the U.S. dollar LIBOR rate. The margin over LIBOR may vary between 2.5% to 4.5% . At December 31, 2015, the variable interest rate on the debt was 4.7% , including a margin over LIBOR of 4.5% . The Tranzact Bank Facility, which is secured by the intellectual property and the common stock of Tranzact’s subsidiaries, contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including a minimum fixed charge coverage ratio and a maximum leverage ratio. Failure to meet one or more of these covenants could result in an event of default, which ultimately could eliminate availability under these facilities and result in acceleration of principal repayment on any amounts outstanding. MediaAlpha Bank Facility On July 23, 2015, MediaAlpha entered into a credit facility with Opus Bank, which has a total commitment of $20.0 million and has a maturity date of July 23, 2019 (the “MediaAlpha Bank Facility”). The MediaAlpha Bank Facility consists of a $15.0 million term loan facility, which was fully drawn as of December 31, 2015, and a revolving credit facility for an additional $5.0 million , which was undrawn as of December 31, 2015. The MediaAlpha Bank Facility carries a variable interest rate that is based on the U.S. dollar LIBOR rate. The MediaAlpha Bank Facility is secured by the intellectual property and the common stock of MediaAlpha's subsidiaries, and contains various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including a maximum leverage ratio. Failure to meet one or more of these covenants could result in an event of default, which ultimately could eliminate availability under these facilities and result in acceleration of principal repayment on any amounts outstanding. Debt Covenants As of December 31, 2015 , White Mountains was in compliance with all of the covenants under all of its debt facilities. Interest Total interest expense incurred by White Mountains for its indebtedness was $18.6 million , $15.6 million and $16.2 million in 2015, 2014 and 2013 . Total interest paid by White Mountains for its indebtedness was $17.6 million , $15.6 million , and $16.2 million in 2015, 2014 and 2013 .

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	12 Months Ended
	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets White Mountains has recognized goodwill and other identifiable intangible assets at the acquisition date fair values in connection with its purchases of subsidiaries. The following table shows the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, for each company acquired: $ in millions Weighted Average Economic life (in years) December 31, 2015 December 31, 2014 Acquisition date fair value Accumulated amortization Net carrying value Acquisition date fair value Accumulated amortization Net carrying value Tranzact: Domain and trade names 7 $ 30.5 $ 5.4 $ 25.1 $ 25.6 $ .9 $ 24.7 Customer relationships 11 132.3 13.1 119.2 107.5 2.1 105.4 Information technology 6 12.7 2.8 9.9 12.7 .5 12.2 Other 8 2.0 .1 1.9 .5 — .5 Subtotal 177.5 21.4 156.1 146.3 3.5 142.8 MediaAlpha: Customer relationships 4 6.5 2.9 3.6 6.5 1.2 5.3 Information technology 5 32.0 11.2 20.8 32.0 4.8 27.2 Subtotal 38.5 14.1 24.4 38.5 6.0 32.5 Wobi: Trademark 8 2.1 .5 1.6 2.1 .2 1.9 Information technology 7 3.6 .7 2.9 .8 .2 .6 Subtotal 5.7 1.2 4.5 2.9 .4 2.5 Star & Shield: Customer relationships 3 1.2 .8 .4 1.2 .4 .8 OneBeacon (EBI) 10 9.4 7.0 2.4 9.4 5.7 3.7 Total other intangible assets 232.3 44.5 187.8 198.3 16.0 182.3 Goodwill: Tranzact N/A 163.8 — 163.8 145.1 — 145.1 Quote Lab N/A 18.3 — 18.3 18.3 — 18.3 Wobi N/A 5.8 — 5.8 5.5 — 5.5 Total goodwill 187.9 — 187.9 168.9 — 168.9 Total goodwill and other intangible assets $ 420.2 $ 44.5 $ 375.7 $ 367.2 $ 16.0 $ 351.2 The goodwill recognized for the above acquisitions is attributed to expected future cash flows. The acquisition date fair values of other intangible assets with finite lives are estimated using income approach techniques, which use future expected cash flows to develop a discounted present value amount. The multi-period-excess-earnings method estimates fair value using the present value of the incremental after-tax cash flows attributable solely to the intangible asset over its remaining life. This approach was used to estimate the fair value of intangible assets associated with trademarks, brand names, customer relationships and contracts and information technology. The relief-from-royalty method was used to estimate fair value for intangible assets that relate to rights that could be obtained via a license from a third-party owner. Under this method, the fair value is estimated using the present value of license fees avoided by owning rather than leasing the asset. This technique was used to estimate the fair value of domain names. The with-or-without method estimates the fair value of an intangible asset that provides an incremental benefit. Under this method, the fair value of the intangible asset is calculated by comparing the value of the entity with and without the intangible asset. This approach was used to estimate the fair value of favorable lease terms, which is included in other. The following table shows the change in goodwill and other intangible assets: December 31, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 168.9 $ 182.3 $ — $ 5.1 Acquisitions of businesses 19.0 34.0 168.9 188.9 Dispositions of businesses — — — — Amortization, including foreign currency translation — (28.5 ) — (11.7 ) Ending balance $ 187.9 $ 187.8 $ 168.9 $ 182.3 Amortization expense was $28.5 million , $11.7 million and $1.4 million for the years ended December 31, 2015, 2014 and 2013 , respectively. White Mountains expects to recognize amortization expense in each of the next five years as follows: Millions Amortization expense 2016 $ 30.4 2017 29.6 2018 27.2 2019 19.4 2020 16.1 Total $ 122.7

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Barbados, Germany, Gibraltar, Israel, Luxembourg, the Netherlands, Peru, Sweden, Switzerland, the United Kingdom and the United States. The total income tax (expense) benefit for the years ended December 31, 2015, 2014 and 2013 consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Current tax (expense) benefit: U.S. federal $ 9.1 $ (4.3 ) $ (20.3 ) State (1.8 ) (2.0 ) (1.0 ) Non-U.S. (1.5 ) (1.4 ) (1.2 ) Total current tax expense 5.8 (7.7 ) (22.5 ) Deferred tax (expense) benefit: U.S. federal (5.1 ) 22.5 (10.1 ) Total deferred tax benefit (expense) (5.1 ) 22.5 (10.1 ) Total income tax (expense) benefit $ .7 $ 14.8 $ (32.6 ) Effective Rate Reconciliation A reconciliation of taxes calculated using the 35% U.S. statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income follows: Year Ended December 31, Millions 2015 2014 2013 Tax (expense) benefit at the U.S. statutory rate $ (54.2 ) $ 10.2 $ (55.5 ) Differences in taxes resulting from: Non-U.S. earnings, net of foreign taxes 76.7 37.8 75.1 Change in valuation allowance (18.7 ) (36.0 ) (49.9 ) Tax exempt interest and dividends 2.6 2.5 2.4 Tax reserve adjustments (1.7 ) 5.2 (1.2 ) Withholding tax (1.2 ) (2.4 ) (1.0 ) Other, net (2.8 ) (2.5 ) (2.5 ) Total income tax (expense) benefit on pre-tax income $ .7 $ 14.8 $ (32.6 ) The non-U.S. component of pre-tax income was $209.5 million , $60.4 million and $163.7 million for the years ended December 31, 2015, 2014 and 2013 . Tax Payments and Receipts Net income tax payments to (receipts from) national governments (primarily the United States) totaled $(6.5) million , $2.7 million , and $3.6 million for the years ended December 31, 2015, 2014 and 2013 . Deferred Tax Inventory Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes. An outline of the significant components of White Mountains’s deferred tax assets and liabilities follows: December 31, Millions 2015 2014 Deferred income tax assets related to: U.S. federal net operating and capital loss carryforwards $ 186.6 $ 152.3 Incentive compensation 45.4 40.2 Unearned premiums 37.5 38.3 Non-U.S. net operating loss carryforwards 33.8 33.1 Loss reserve discount 26.8 41.4 Tax credit carryforwards 16.9 13.4 Runoff Transaction 12.6 12.6 Deferred compensation 6.2 6.7 Fixed assets 1.7 2.3 Accrued interest 1.3 5.0 Other items 3.0 2.8 Total gross deferred income tax assets 371.8 348.1 Less: valuation allowances (156.8 ) (140.5 ) Total net deferred income tax assets 215.0 207.6 Deferred income tax liabilities related to: Deferred acquisition costs 34.9 32.0 Net unrealized investment gains 28.2 31.1 Members surplus contributions 19.0 10.3 Investment basis difference 13.8 17.0 Purchase accounting 9.3 — Pension and benefit accruals 3.5 2.5 Other items .5 .1 Total deferred income tax liabilities 109.2 93.0 Net deferred tax asset $ 105.8 $ 114.6 White Mountains’s deferred tax assets are net of U.S. federal, state, and non-U.S. valuation allowances and, to the extent they relate to non-U.S. jurisdictions, they are shown at year-end exchange rates. Valuation Allowance White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense. Of the $156.8 million valuation allowance as of December 31, 2015 , $28.3 million relates to deferred tax assets on net operating losses in Luxembourg subsidiaries, $122.9 million relates to deferred tax assets on U.S. losses and other federal deferred tax benefits, $5.5 million relates to net operating losses in the Israel subsidiaries and $0.1 million relates to net operating losses in the Netherlands subsidiary. Of the $140.5 million valuation allowance as of December 31, 2014 , $31.0 million relates to deferred tax assets on net operating losses in Luxembourg subsidiaries, $107.5 million relates to deferred tax assets on U.S. losses and other federal deferred tax benefits; and $1.9 million relates to net operating losses in the Israel subsidiaries and $0.1 million relates to net operating losses in the Netherlands subsidiary. Non-U.S. jurisdictions During 2015 and 2014, White Mountains recorded tax expense of $0.3 million and $15.7 million to establish a valuation allowance against deferred tax assets at certain Luxembourg-domiciled subsidiaries, as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets. During 2015 and 2014, White Mountains recorded tax expense of $3.0 million and $1.2 million to establish a valuation allowance against deferred tax assets at certain Israeli-domiciled subsidiaries, as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets. United States During 2015 and 2014, White Mountains recorded tax expense of $8.2 million and $7.2 million to establish a valuation allowance against deferred tax assets of Guilford Holdings, Inc. and subsidiaries (“Guilford”), as White Mountains management does not anticipate sufficient taxable income to utilize the deferred tax assets. Guilford consists of service companies and certain other investments that are included in the Other Operations segment. During 2015 and 2014, White Mountains recorded tax expense of $6.3 million and $8.5 million to establish valuation allowances against deferred tax assets of BAM, as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. Also during 2015 and 2014, BAM has income in other comprehensive income that is available to offset its loss from continuing operations, as a result, BAM recorded a tax benefit of $8.7 million and $4.9 million , in continuing operations, with an offsetting tax expense in other comprehensive income. However, since BAM is a mutual insurance company that is owned by its members, its results do not affect White Mountains’s common shareholders’ equity, as they are attributable to non-controlling interests. During 2015, Houston General Insurance Exchange (“Houston General Insurance”) recorded a tax benefit of $0.5 million to reduce a valuation allowance, primarily due to the restructuring of a surplus note. During 2014, Houston General Insurance recorded tax expense of $0.1 million as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. Houston General Insurance is a reciprocal, which is included in the Company’s consolidated results as a variable interest entity. See Note 18 - “Variable Interest Entities” . During 2015 and 2014, SSIE recorded tax expense of $1.2 million and $3.2 million , as it is uncertain if it will have sufficient taxable income to utilize its deferred tax assets. SSIE is a reciprocal, which is included in the Company’s consolidated results as a variable interest entity. See Note 18 - “Variable Interest Entities” . Net Operating Loss and Capital Loss Carryforwards Net operating loss and capital loss carryforwards as of December 31, 2015 , the expiration dates and the deferred tax assets thereon are as follows: December 31, 2015 Millions United States Luxembourg Netherlands Israel Total 2016 - 2020 $ .8 $ — $ — $ — $ .8 2021 - 2025 2.0 — .3 — 2.3 2026 - 2035 540.6 — — — 540.6 No expiration date — 96.8 — 21.8 118.6 Total $ 543.4 $ 96.8 $ .3 $ 21.8 $ 662.3 Gross deferred tax asset $ 186.6 $ 28.3 $ .1 $ 5.5 $ 220.5 Valuation allowance (121.2 ) (28.3 ) (.1 ) (5.5 ) (155.1 ) Net deferred tax asset $ 65.4 $ — $ — $ — $ 65.4 Included in the U.S. net operating loss carryforwards are losses of $22.6 million subject to an annual limitation on utilization under Internal Revenue Code Section 382. These loss carryforwards will begin to expire in 2025. Also included in the U.S. net operating loss carryforwards are losses of $9.3 million due to additional deductions related to equity compensation. These loss carryforwards begin to expire in 2030. As of December 31, 2015 , there are U.S. foreign tax credit carryforwards available of $0.4 million , which begin to expire in 2023. As of December 31, 2015 , there are U.S. alternative minimum tax credit carryforwards of $1.9 million which do not expire. As of December 31, 2015, there are U.S. low income housing credit carryforwards of $14.6 million , which begin to expire in 2031. Uncertain Tax Positions Recognition of the benefit of a given tax position is based upon whether a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more-likely-than-not recognition threshold, White Mountains must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: Millions Permanent Differences (1) Temporary Differences (2) Interest and Penalties (3) Total Balance at January 1, 2013 $ 8.4 $ 23.6 $ 8.0 $ 40.0 Changes in prior year tax positions — — 1.0 1.0 Tax positions taken during the current year — (7.0 ) — (7.0 ) Lapse in statute of limitations — — — — Settlements with tax authorities — — (.3 ) (.3 ) Balance at December 31, 2013 8.4 16.6 8.7 33.7 Changes in prior year tax positions (2.2 ) (.8 ) (1.9 ) (4.9 ) Tax positions taken during the current year — 7.3 — 7.3 Lapse in statute of limitations (.8 ) — (.3 ) (1.1 ) Balance at December 31, 2014 5.4 23.1 6.5 35.0 Changes in prior year tax positions — (12.4 ) 1.7 (10.7 ) Tax positions taken during the current year — — — — Lapse in statute of limitations — — — — Settlements with tax authorities — — — — Balance at December 31, 2015 $ 5.4 $ 10.7 $ 8.2 $ 24.3 (1) Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate. (2) Represents the amount of unrecognized tax benefits that, if recognized would create a temporary difference between the reported amount of an item in the Company’s Consolidated Balance Sheet and its tax basis. (3) Net of tax benefit. If White Mountains determines in the future that its reserves for unrecognized tax benefits on permanent differences and interest and penalties are not needed, the reversal of $13.6 million of such reserves as of December 31, 2015 would be recorded as an income tax benefit and would impact the effective tax rate. If White Mountains determines in the future that its reserves for unrecognized tax benefits on temporary differences are not needed, the reversal of $10.7 million of such reserves as of December 31, 2015 would not impact the effective tax rate due to deferred tax accounting but would accelerate the payment of cash to the taxing authority. The vast majority of White Mountains’s reserves for unrecognized tax benefits on temporary differences relate to deductions for loss reserves where the timing of the deductions is uncertain. White Mountains classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2015, 2014 and 2013 , White Mountains recognized $1.7 million , $(2.2) million , $0.7 million in net interest (income) expense. The balance of accrued interest as of December 31, 2015 and 2014 is $8.2 million and $6.5 million , net of any tax benefit. Tax Examinations With few exceptions, White Mountains is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2007. The IRS concluded an examination of income tax returns for 2005 and 2006 for certain U.S. subsidiaries of OneBeacon. On February 14, 2014, White Mountains received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Over Assessment) from the IRS Appeals Office relating to the examination of tax years 2005 and 2006. All disputed items have now been agreed to and resolved with the Joint Committee. The total assessment, including interest, is $3.3 million . As the receipt of the Form 870-AD described above represents formal settlement, White Mountains recorded a tax benefit of approximately $5.0 million in the first quarter of 2014 relating to the settlement of the IRS examination for tax years 2005 and 2006. On July 28, 2011, the IRS commenced an examination of the income tax returns for 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon. On July 17, 2013, White Mountains received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. Subsequent to the year end 2015, 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. All disputed items have now been agreed to and resolved with the Joint Committee on Taxation. The estimated total overpayment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $4.0 million . However, $2.7 million of the adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the deduction in the exam period would not affect the effective tax rate. As the receipt of the Form 870-AD described above represents formal settlement, White Mountains expects to record a tax benefit of approximately $12 million in the first quarter of 2016 related to the tax settlement of the IRS examination for tax years 2007, 2008 and 2009. On September 2, 2013, the IRS commenced an examination of the income tax returns for 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. On November 18, 2015, White Mountains received Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2010, 2011 and 2012. The estimated total overpayment is $1.1 million . White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows. On December 18, 2014, the IRS commenced an examination of the 2012 income tax return for Guilford Holdings, Inc. and subsidiaries. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.

Derivatives

Derivatives	12 Months Ended
	Dec. 31, 2015
Derivative [Line Items]
Derivatives	Derivatives
Variable Annuity Reinsurance
Derivative [Line Items]
Derivatives	Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. As of December 31, 2015 , the total guarantee value was approximately ¥50.7 billion (approximately $0.4 billion at exchange rates on that date). The collective account values of the underlying variable annuities were approximately 109% of the guarantee value as of December 31, 2015 . The following table summarizes the pre-tax operating results of WM Life Re for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, Millions 2015 2014 2013 Fees, included in other revenue $ 9.3 $ 18.6 $ 25.0 Change in fair value of variable annuity liability, included in other revenue (.4 ) 52.9 378.5 Change in fair value of derivatives, included in other revenue (8.8 ) (72.4 ) (402.0 ) Foreign exchange, included in other revenue (1.3 ) (3.2 ) (14.5 ) Other investment income and (losses) gains (.4 ) (1.4 ) (7.1 ) Total revenues (1.6 ) (5.5 ) (20.1 ) Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses — .6 10.2 Death benefit claims paid, included in general and administrative expenses (.1 ) (.1 ) (1.9 ) General and administrative expenses (4.0 ) (4.3 ) (4.9 ) Pre-tax loss $ (5.7 ) $ (9.3 ) $ (16.7 ) During 2013, the ratio of annuitants’ aggregate account values to the aggregate guarantee value provided by WM Life Re increased, and as a result, annuitants surrendered their policies at higher rates than WM Life Re observed in the past. In response to this trend, WM Life Re adjusted the projected surrender assumptions used in the valuation of its variable annuity reinsurance liability upward. In 2014, surrender rates continued to outpace assumptions and WM Life Re again adjusted the projected surrender assumptions. For the year ended December 31, 2014, the change in the fair value of the variable annuity liability included $0.2 million of losses associated with changes in projected surrender assumptions. There was no change in projected surrender assumptions in 2015. All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements as of December 31, 2015 . The following summarizes realized and unrealized gains (losses) recognized in other revenues for the years ended December 31, 2015, 2014 and 2013 and the carrying values as of December 31, 2015 and 2014 by type of derivative instrument: Carrying Value Year Ended December 31, December 31, Millions 2015 2014 2013 2015 2014 Fixed income/interest rate $ 6.4 $ (33.7 ) $ (108.7 ) $ .5 $ (1.7 ) Foreign exchange (7.3 ) (1.3 ) (96.7 ) 14.8 44.1 Equity (7.9 ) (37.4 ) (196.6 ) 4.8 14.0 Total $ (8.8 ) $ (72.4 ) $ (402.0 ) $ 20.1 $ 56.4 The following table summarizes the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2015, 2014 and 2013 : Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2015 $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized (losses) gains (.4 ) (5) (9.7 ) (7.5 ) 8.4 (8.8 ) Transfers in (out) — — — — — Sales/settlements — (6.5 ) (9.8 ) (11.2 ) (27.5 ) Balance at December 31, 2015 $ .3 $ 2.7 $ 16.5 $ 0.9 $ 20.1 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2014 $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 Purchases — — — — — Realized and unrealized gains (losses) 53.5 (5) (38.6 ) (71.0 ) 37.2 (72.4 ) Transfers in (out) — — — — — Sales/settlements — (5.9 ) 100.1 (34.6 ) 59.6 Balance at December 31, 2014 $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2013 $ (441.5 ) $ 140.5 $ (20.5 ) $ (21.7 ) $ 98.3 Purchases — 59.4 — — 59.4 Realized and unrealized gains (losses) 388.7 (5) (136.5 ) (196.1 ) (69.4 ) (402.0 ) Transfers in (out) — — — — — Sales/settlements — — 221.3 92.2 313.5 Balance at December 31, 2013 $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 (1) Consists of over-the-counter instruments. (2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index. (3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded. (4) In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $5.8 , $33.2 and $81.3 as of December 31, 2015, 2014 and 2013 posted as collateral to its reinsurance counterparties. (5) There was no changes in the fair value of variable annuity death benefit liabilities for the year ended December 31, 2015. In 2014 and 2013, $0.6 and $10.2 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses. (6) Consists of foreign currency options and equity options. In addition, WM Life Re held cash and short-term investments posted as collateral to its variable annuity reinsurance and derivatives counterparties. The total collateral includes the following: December 31, Millions 2015 2014 Cash $ 5.8 $ 23.7 Fixed maturity investments — 9.5 Total $ 5.8 $ 33.2 Collateral in the form of fixed maturity securities consists of Government of Japan Bonds, which are recorded at fair value. Collateral in the form of short-term investments consists of money-market instruments, carried at amortized cost, which approximates fair value. All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements as of December 31, 2015 and 2014 . The fair value of White Mountains’s variable annuity reinsurance liabilities are estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Assumptions regarding future policyholder behavior, including surrender and lapse rates, are generally unobservable inputs and significantly impact the fair value estimates. Market conditions including, but not limited to, changes in interest rates, equity indices, market volatility and foreign currency exchange rates as well as the variations in actuarial assumptions regarding policyholder behavior may result in significant fluctuations in the fair value estimates. Generally, the liabilities associated with these guarantees increase with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains uses derivative instruments, including put options, interest rate swaps, total return swaps on bond and equity indices and forwards and futures contracts on major equity indices, currency pairs and government bonds, 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, are generally the same as those used to estimate the fair value of variable annuity liabilities. The following summarizes quantitative information about significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments that have been classified as Level 3 measurements: Description December 31, 2015 $ in millions Fair Value Valuation Technique(s) Unobservable Input Range Weighted Average Variable annuity benefit guarantee asset $ (.3 ) Discounted cash flows Surrenders 0.1 % - 40.0 % 40.0 % Mortality 0.0 % - 6.4% 1.0 % Foreign exchange volatilities 0-1 year 11.5 % - 18.7% 13.1 % Index volatilities 0-1 year 25.0 % - 27.5% 26.4 % Foreign Exchange Options $ 1.9 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations (0.5 )% - (7.3)% (5.9 )% Equity Index Options $ .8 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations 0.7 % - (6.7)% (2.5 )% WM Life Re enters 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 has exposure to credit risk for amounts that are uncollateralized by counterparties. WM Life Re’s internal risk management guidelines establish net counterparty exposure thresholds that take into account OTC counterparties’ credit ratings. The OTC derivative contracts are 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 has exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re has entered into master netting agreements with certain of its counterparties whereby the collateral provided (held) is calculated on a net basis. The following summarizes amounts offset under master netting agreements: December 31, 2015 December 31, 2014 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets 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 ) $ 0.3 $ 1.0 $ (5.4 ) $ (4.4 ) Exchange traded .1 (.1 ) — 2.8 (.1 ) 2.7 Foreign exchange contracts OTC 15.0 — 15.0 45.5 — 45.5 Exchange traded .1 (.3 ) (0.2 ) — (1.4 ) (1.4 ) Equity contracts OTC 4.4 (.6 ) 3.8 11.7 (.2 ) 11.5 Exchange traded 1.2 — 1.2 3.4 (.9 ) 2.5 Total (2) $ 23.2 $ (3.1 ) $ 20.1 $ 64.4 $ (8.0 ) $ 56.4 (1) Amount equal to fair value of instrument as recognized in other assets. (2) All derivative instruments held by WM Life Re are subject to master netting arrangements. 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: December 31, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re - Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 8.5 $ — $ — $ 8.5 $ — $ — $ 5.5 $ 3.0 A + Bank of America .7 — — .7 — — — .7 A Citigroup - OTC 9.9 — — 9.9 — — 1.4 8.5 A Citigroup - Exchange Traded 1.0 — — 1.0 5.8 — — 6.8 A Total $ 20.1 $ — $ — $ 20.1 $ 5.8 $ — $ 6.9 $ 19.0 December 31, 2014 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 24.3 $ — $ — $ 24.3 $ — $ — $ 8.8 $ 15.5 A + Bank of America 5.6 — — 5.6 — — — 5.6 A Nomura (3.5 ) 3.5 — — 1.7 9.5 — 11.2 BBB + Citigroup - OTC 22.2 — — 22.2 — — 1.1 21.1 A Citigroup - Exchange Traded 3.7 — — 3.7 16.0 — — 19.7 A Royal Bank of Scotland 4.0 — — 4.0 — — — 4.0 A - Barclays .1 — — .1 — — — .1 A Total $ 56.4 $ 3.5 $ — $ 59.9 $ 17.7 $ 9.5 $ 9.9 $ 77.2 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-one creditworthiness ratings),“A” (which is the sixth highest of twenty-one creditworthiness ratings), “A-” (which is the seventh highest of twenty-one creditworthiness ratings), and BBB+ (which is the eighth highest of twenty-one creditworthiness ratings).
Tranzact [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivatives	Tranzact Interest Rate Swap Tranzact has a $101.0 million term loan facility (see Note 7 - “Debt” ) that carries a variable rate equal to the U.S. dollar LIBOR rate, plus an applicable margin. At December 31, 2015 , the variable interest rate on the term loan was 4.73% , including a margin over LIBOR of 4.50% . Effective October 22, 2014, to effectively fix the rate it pays on this term loan, Tranzact entered into an interest rate swap agreement with a notional amount of $70.0 million . The notional amount decreases over the term of the swap by amounts equivalent to scheduled principal repayments made on Tranzact’s term loan. Under the terms of the swap agreement, Tranzact pays a fixed rate of 1.34% and receives a variable rate, which is reset monthly, based on the then-current U.S. dollar LIBOR rate. The variable rate received by Tranzact under the swap agreement was 0.15% at inception and 0.23% at December 31, 2015 . The total current effective rate on Tranzact's debt was 6.07% at December 31, 2015 . The swap is measured at fair value with changes therein recognized within other revenues and is accounted for as a non-hedge derivative instrument. As of December 31, 2015 , the estimated fair value of the swap was $(0.2) million . There are no collateral arrangements associated with the swap.

Municipal Bond Guarantee Insura

Municipal Bond Guarantee Insurance	12 Months Ended
	Dec. 31, 2015
Insurance [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 December 31, 2015 , White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM surplus notes. HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding on a per policy basis through HG Re, which had statutory capital of $460.8 million and $449.1 million as of December 31, 2015 and 2014. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. At December 31, 2015 and 2014, the collateral trusts held assets of $136.2 million and $120.0 million . As of December 31, 2015 and 2014 , HG Global had pre-tax income of $17.9 million and $18.3 million , which included $15.8 million and $15.7 million of interest income on the BAM surplus notes. As of December 31, 2015 and 2014 , BAM had a pre-tax loss of $47.3 million and $40.5 million that was recorded in net loss attributable to non-controlling interests, which included $15.8 million and $15.7 million of interest expense on the BAM surplus notes. As of December 31, 2015, HG Global has accrued $90.2 million of interest receivable on the BAM surplus notes. The following table provides a schedule of BAM’s insured obligations: December 31, 2015 December 31, 2014 Contracts outstanding 3,103 1,750 Remaining weighted average contract period (in years) 12.8 12.8 Contractual debt service outstanding (in millions): Principal $ 22,556.0 $ 12,362.5 Interest $ 11,984.4 $ 7,086.9 Gross unearned insurance premiums $ 50.2 $ 27.6 The following table is a schedule of BAM’s future premium revenues as of December 31, 2015 : Millions December 31, 2015 January 1, 2016 - March 31, 2016 $ 1.1 April 1, 2016 - June 30, 2016 1.1 July 1, 2016 - September 30, 2016 1.1 October 1, 2016 - December 31, 2016 1.1 4.4 2017 4.3 2018 4.1 2019 3.8 2020 3.6 2020 and thereafter 30.0 Total $ 50.2

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2015
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 years ended December 31, 2015, 2014 and 2013 (see Note 22 - Discontinued Operations” for earnings per share amounts for discontinued operations): Year Ended December 31, 2015 2014 2013 Basic and diluted earnings per share numerators (in millions): Net income from continuing operations attributable to White Mountains’s common shareholders $ 198.8 $ 53.6 $ 175.1 Allocation of income for unvested restricted common shares (1) (2.3 ) (.7 ) (2.6 ) Dividends declared on participating restricted common shares (1) (.1 ) (.1 ) (.1 ) Total allocation to restricted common shares (2.4 ) (.8 ) (2.7 ) Net income attributable to White Mountains’s common shareholders, $ 196.4 $ 52.8 $ 172.4 Undistributed net earnings (in millions): Net income attributable to White Mountains’s common shareholders, $ 196.4 $ 52.8 $ 172.4 Dividends declared net of restricted common share amounts (1) (5.9 ) (6.1 ) (6.1 ) Total undistributed net earnings, net of restricted common share amounts $ 190.5 $ 46.7 $ 166.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (2) (68.0 ) (78.9 ) (91.4 ) Basic earnings per share denominator 5,811.2 6,026.0 6,109.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (2) (68.0 ) (78.9 ) (91.4 ) Average outstanding dilutive options to acquire common shares (3) — — — Diluted earnings per share denominator 5,811.2 6,026.0 6,109.0 Basic earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ 33.80 $ 8.77 $ 28.22 Dividends declared and paid (1.00 ) (1.00 ) (1.00 ) Undistributed earnings $ 32.80 $ 7.77 $ 27.22 Diluted earnings per share (in dollars) Net income attributable to White Mountains’s common shareholders $ 33.80 $ 8.77 $ 28.22 Dividends declared and paid (1.00 ) (1.00 ) (1.00 ) Undistributed earnings $ 32.80 $ 7.77 $ 27.22 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13 - “Employee Share-Based Incentive Compensation Plans” ). (3) The diluted earnings per share denominator for the years ended December 31, 2015, 2014 and 2013 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Retirement and Postretirement P

Retirement and Postretirement Plans	12 Months Ended
	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Retirement and Postretirement Plans	Retirement and Postretirement Plans OneBeacon sponsors qualified and non-qualified, non-contributory, defined benefit pension plans covering substantially all employees who were employed as of December 31, 2001 and former employees who had met the eligibility requirements, as well as retirees. Current plans include the OneBeacon qualified pension plan (the “Qualified Plan”) and the OneBeacon non-qualified pension plan (the “Non-qualified Plan”) (collectively the “Plans”). The Plans were frozen and curtailed in 2002 and, as a result, the projected benefit obligation is equal to the accumulated benefit obligation. In April 2014, the board of managers of the Qualified Plan's plan sponsor, a wholly-owned indirect subsidiary of OneBeacon, voted to terminate the Qualified Plan as of June 30, 2014. The plan sponsor applied for a determination letter from the Internal Revenue Service (“IRS”) with respect to the Qualified Plan's tax qualified status at termination and received a favorable determination letter from the IRS on July 23, 2015. OneBeacon anticipates the majority of plan assets to be distributed within 12 months of the receipt of the favorable determination letter by way of annuity contract purchases and/or lump sum payments to plan participants. As part of the normal remeasurement process as of December 31, 2015, OneBeacon considered the significant progress made throughout the year on the termination and determined that using assumptions consisting of the termination value were appropriate for that Qualified Plan as the likelihood is remote that the execution of the plan termination will be blocked by third parties and that the plan will return from termination. Therefore, as of December 31, 2015, OneBeacon has estimated the projected benefit obligation of the Qualified Plan on a termination valuation basis based on the combination of the estimated cost of annuity contract purchases plus lump sum payments, which are anticipated to be made in the first quarter of 2016. OneBeacon notes that the purchase of annuities, which is currently in final stages of the bidding process, is subject to availability of such contracts, with a creditworthy insurance company, at negotiated market terms deemed acceptable to the plan Sponsor. OneBeacon does not expect the impact of the final termination to be material on the OneBeacon’s financial position. The benefits for the Plans are based primarily on years of service and employees’ compensation through December 31, 2002. OneBeacon’s funding policy is consistent with the funding requirements of U.S. federal laws and regulations. The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans as of December 31, 2015 and 2014 : Pension Benefits December 31, Millions 2015 2014 Change in projected benefit obligation: Projected benefit obligation at beginning of year $ 119.7 $ 102.9 Service cost .9 .6 Interest cost 4.6 4.7 Settlement loss (1) (1.8 ) — Special termination benefits expense — .3 Assumption changes (3.5 ) 17.5 Actuarial loss .9 1.1 Benefits and expenses paid with plan assets (13.7 ) (5.2 ) Benefits paid directly by OneBeacon (2.2 ) (2.2 ) Remeasurement due to plan termination (2) 4.6 — Projected benefit obligation at end of year $ 109.5 $ 119.7 Change in plan assets: Fair value of plan assets at beginning of year $ 146.0 $ 142.8 Actual return on plan assets 7.5 8.4 Benefits and expenses paid (13.7 ) (5.2 ) Fair value of plan assets at end of year $ 139.8 $ 146.0 Funded status at end of year $ 30.3 $ 26.3 (1) During the fourth quarter of 2015, OneBeacon triggered settlement accounting for the Qualified Plan as the total lump sum payments exceeded the service plus interest costs, resulting in a $1.8 adverse effect on accumulated other comprehensive income. (2) As noted in the paragraph preceding this table, the projected benefit obligation was valued on a plan termination basis as of December 31, 2015. The funded status of the consolidated pension plans as of December 31, 2015 was $30.3 million , which represents an over-funding of $55.8 million related to the Qualified Plan and an under-funding of $25.5 million related to the Non-qualified Plan. The Non-qualified Plan, which is unfunded, does not hold any assets. OneBeacon has set aside $18.6 million in an irrevocable rabbi trust for the benefit of Non-qualified Plan participants. Assets held in the rabbi trust are not reflected in the funded status of the consolidated pension plans as presented but are included in other assets in the consolidated balance sheet. OneBeacon currently anticipates transferring some or all of the excess invested assets of the Qualified Plan into a qualified replacement plan subsequent to the plan termination. OneBeacon has also applied for a private letter ruling from the IRS related to the wind-down of potential post-termination obligations of the Qualified Plan. Amounts recognized in the financial statements as of December 31, 2015 and 2014 consist of: December 31, Millions 2015 2014 Net assets of the Qualified Plan recorded in other assets $ 55.8 $ 53.4 Net liabilities of the Non-qualified Plan recorded in other liabilities (25.5 ) (27.1 ) Net amount accrued in the financial statements $ 30.3 $ 26.3 Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows: December 31, Millions 2015 2014 Projected benefit obligation $ 25.5 $ 27.1 Accumulated benefit obligation $ 25.5 $ 27.1 Fair value of plan assets $ — $ — Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets, was as follows: December 31, Millions 2015 2014 Projected benefit obligation $ 84.0 (1) $ 92.6 Accumulated benefit obligation $ 84.0 (1) $ 92.6 Fair value of plan net assets $ 139.8 $ 146.0 (1) Measured on a plan termination basis The amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis and before non-controlling interest for the years ended December 31, 2015 and 2014 were as follows: December 31, Millions 2015 2014 Accumulated other comprehensive (loss) income beginning balance $ (8.0 ) $ 10.5 Increase (decrease) in accumulated other comprehensive income (loss): Amortization of net actuarial gains recognized during the year 1.3 .3 Net actuarial losses occurring during the year (1) (1.3 ) (18.8 ) Accumulated other comprehensive loss ending balance $ (8.0 ) $ (8.0 ) (1) Net actuarial gains (losses) resulted from investment returns and demographic experience different than assumed. The 2015 amount reflects the valuation of the Qualified Plan on a termination basis. The amount in accumulated other comprehensive loss, on a pre-tax basis, that has not yet been recognized as a component of net periodic benefit cost for the year ended December 31, 2015 is attributable to net losses. During 2016, OneBeacon expects $1.1 million will be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost. The components of net periodic benefit (income) cost for the years ended December 31, 2015, 2014 and 2013 were as follows: Year Ended December 31, Millions 2015 2014 2013 Service cost $ .9 $ .6 $ .8 Interest cost 4.6 4.7 4.2 Expected return on plan assets (8.7 ) (8.5 ) (7.1 ) Amortization of unrecognized loss 1.3 .3 .9 Net periodic pension income before settlements, curtailments and special termination benefits (1.9 ) (2.9 ) (1.2 ) Special termination benefits expense (1) — .3 .3 Total net periodic benefit income $ (1.9 ) $ (2.6 ) $ (.9 ) (1) Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force. Assumptions The weighted average discount rate assumptions used to determine benefit obligations was 3.22% and 3.91% as of December 31, 2015 and 2014 . The weighted average assumptions used to determine net periodic benefit cost included a 3.91% discount rate and 6.00% expected long-term rate of return on plan assets for the year ended December 31, 2015 . The weighted average assumptions used to determine net periodic benefit cost included a 4.66% discount rate and 6.00% expected long-term rate of return on plan assets for the year ended December 31, 2014 . OneBeacon’s discount rate assumptions used to account for the Plans reflect the rates at which the benefit obligations could be effectively settled. The December 31, 2015 discount rate used to determine benefit obligations also reflects the Qualified Plan valued on a plan termination basis. In addition to consideration of published yields for high quality long-term corporate bonds, U.S. Treasuries and insurance company annuity contract pricings, consideration was given to cash flow matching analyses. OneBeacon performed an analysis of expected long-term rates of return based on the allocation of its Qualified Plan assets as of December 31, 2014 to develop expected rates of return for 2015 and 2014 for each significant asset class or economic indicator. A range of returns was developed based both on forecasts and on broad market historical benchmarks for expected return, correlation, and volatility for each asset class. Plan Assets The Qualified Plan's assets are managed internally by OneBeacon. Given the anticipated Plan termination and considering the expected payments to purchase annuities or for lump sum distributions, the invested assets were reallocated during 2015 to consist substantially of U.S. Treasury securities. The Qualified Plan’s investments are stated at fair value. Many factors are considered in arriving at fair market value. In general, fixed maturity investments such as corporate bonds and government securities are valued based on yields currently available on comparable securities of issuers with similar credit ratings. U.S. Treasury securities and shares of common and preferred stock are valued at quoted market prices when available. Convertible fixed maturity investments are valued based on quoted market prices, analysis of listed markets and use of sensitivity analysis. Registered investment companies are valued at the net asset value as reported by the fund at year-end. The fair value of the Qualified Plan’s assets and their related inputs as of December 31, 2015 and 2014 by asset category were as follows: December 31, 2015 December 31, 2014 Millions Fair Value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fair Value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fixed maturity investments $ 132.4 $ 132.4 $ — $ — $ 24.2 $ — $ 24.2 $ — Common equity securities — — — — 101.0 101.0 — — Convertible fixed maturity investments — — — — 12.8 — 12.8 — Cash and short-term investments 7.0 7.0 — — 5.2 4.8 .4 — Total $ 139.4 $ 139.4 $ — $ — $ 143.2 $ 105.8 $ 37.4 $ — There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2015 and 2014 . The Qualified Plan’s asset allocations as of December 31, 2015 and 2014 , by asset category were as follows: Plan Assets at December 31, Asset Category 2015 2014 Fixed maturity investments 95.0 % 16.9 % Common equity securities — 70.6 Convertible fixed maturity investments — 8.9 Cash and short-term investments 5.0 3.6 Total 100.0 % 100.0 % As described above, the Qualified Plan's investment securities are exposed to various risks such as interest rate, market, and credit risks. However as a result of the Qualified Plan termination, substantially all assets as of December 31, 2015 are invested in United States Treasury securities which significantly limits the exposure to these risks. Cash Flows OneBeacon does not expect to make a contribution to its Qualified Plan in 2016. As noted above, the termination will result in significant cash payments for the purchase of annuity and lump sum payments. OneBeacon anticipates contributing $2.1 million to the Non-qualified Plan in 2016, for which OneBeacon has assets held in a rabbi trust. The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid: Millions Expected Benefit Payments 2016 $ 86.8 2017 2.1 2018 2.1 2019 2.0 2020 2.0 2021-2025 8.9 (1) Primarily reflects the anticipated payout related to annuity purchases and lump sum payments resulting from the termination of the Qualified Plan Other Benefit Plans OneBeacon sponsors a defined contribution plan, the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (“KSOP”), covering the majority of its employees. The contributory plan provides qualifying employees with matching contributions of 50% up to the first six percent of salary (subject to U.S. federal limits on allowable contributions in a given year). Total expense for matching contributions to the plan was $3.0 million , $2.7 million and $2.3 million in 2015, 2014 and 2013 . The employee stock ownership component of the KSOP provides all of its participants with an annual base contribution in common shares equal to 3% of their salary, up to the applicable Social Security wage base ( $118,500 for 2015 ). Additionally, those participants not otherwise eligible to receive certain other OneBeacon benefits can earn a variable contribution of up to 6% of their salary, subject to the applicable IRS annual covered compensation limits ( $265,000 for 2015 ) and contingent upon OneBeacon’s performance. White Mountains recorded $5.7 million , $4.4 million and $6.3 million in compensation expense to pay benefits and allocate common shares to participant’s accounts for the years ended 2015, 2014 and 2013 . OneBeacon had a post-employment benefit liability, which primarily relates to disability and health benefits available to former employees, of $4.1 million and $4.8 million as of December 31, 2015 and 2014 . OneBeacon also had a post-employment benefit liability related to death benefits to beneficiaries of former executives of $12.5 million and $12.9 million as of December 31, 2015 and 2014. OneBeacon has set aside funds to satisfy its obligation in a Rabbi Trust of $33.9 million and $33.2 million as of December 31, 2015 and 2014.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s share-based incentive compensation plans are designed to incentivize key employees and service providers to maximize shareholder value over long periods of time. White Mountains believes that this is best pursued by utilizing a pay-for-performance program that closely aligns the financial interests of management with those of its shareholders. White Mountains accomplishes this by emphasizing highly variable long-term compensation that is contingent on performance over a number of years rather than entitlements. White Mountains expenses all its share-based compensation. As a result, White Mountains’s calculation of its owners’ returns includes the expense of all outstanding share-based compensation awards. Incentive Compensation Plans White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non-share-based incentive awards to key employees and service providers of White Mountains. The WTM Incentive Plan was adopted by the Board, was approved by the Company’s sole shareholder in 1985 and was subsequently amended by its shareholders in 1995, 2001, 2003, 2005, 2010 and 2013. Share-based incentive awards that may be granted under the plan include performance shares, restricted shares, incentive stock options and non-qualified stock options (“Non-Qualified Options”). Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. Performance shares earned under the WTM Incentive Plan are typically paid in cash but may be paid in common shares. Compensation expense is recognized on a pro rata basis over the vesting period of the awards. The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based incentive awards, including performance shares, restricted shares, restricted stock units and Non-Qualified Options. White Mountains offers certain types of share-based compensation under qualified retirement plans. The defined contribution plan of OneBeacon (the “401(k) Plan”) offers its U.S.-domiciled participants the ability to invest their balances in several different investment options, including the Company’s or OneBeacon’s common shares. OneBeacon’s KSOP is an employer-funded benefit plan that provides all of its participants with an annual base contribution in common shares equal to 3% of their salary, up to the applicable Social Security wage base ( $118,500 for 2015 ). Additionally, those participants not otherwise eligible to receive certain other OneBeacon benefits can earn a variable contribution of up to 6% of their salary, subject to the applicable IRS annual covered compensation limits ( $265,000 for 2015 ) and contingent upon OneBeacon’s performance. Performance Shares Performance shares are designed to reward company-wide performance. The level of payout ranges from zero to two times the number of shares initially granted, depending on White Mountains’s financial performance. Performance shares become payable at the conclusion of a performance cycle (typically three years) if pre-defined financial targets are met. The principal performance measure used for determining performance share payouts is after-tax growth in White Mountains’s intrinsic business value per share. The Compensation Committee historically has considered the growth in intrinsic business value per share to be based equally on the growth of economic value per share and growth in adjusted book value per share, both inclusive of dividends. Economic value is calculated by adjusting the GAAP book value per share for differences between the GAAP carrying values of certain assets and liabilities and White Mountains’s estimate of their underlying economic values (for example, the time value discount in loss reserves). The following summarizes performance share activity for the years ended December 31, 2015, 2014 and 2013 for performance shares granted under the WTM Incentive Plan and the WTM Phantom Share Plan: Year Ended December 31, 2015 2014 2013 $ in millions Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 113,198 $ 44.4 108,605 $ 54.9 108,065 $ 26.5 Shares paid or expired (1) (42,958 ) (30.8 ) (33,730 ) (24.5 ) (43,100 ) (10.1 ) New grants 29,828 — 41,580 — 43,660 — Assumed forfeitures and cancellations (2)(3) (3,857 ) (.3 ) (3,257 ) (.1 ) (20 ) (.6 ) Expense recognized — 44.4 — 14.1 — 39.1 End of period (4) 96,211 $ 57.7 113,198 $ 44.4 108,605 $ 54.9 (1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance shares payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target. WTM performance shares payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target. (2) On November 19, 2014, the Compensation Committee canceled 3,040 WTM performance shares for the 2014-2016 performance cycle and issued the same number of WTM restricted shares. (3) Amounts include changes in assumed forfeitures, as required under GAAP. (4) Outstanding performance share awards as of December 31, 2015, 2014 and 2013 exclude 7,997 , 10,351 and 10,615 performance shares awards, net of assumed forfeitures, granted to employees of Sirius Group, which is accounted for as discontinued operation. For the 2011-2013 performance cycle, the Company issued common shares for 3,570 performance shares earned and all other performance shares earned were settled in cash. For the 2012-2014 and 2010-2012 performance cycles, all performance shares earned were settled in cash. If the outstanding WTM performance shares had vested on December 31, 2015 , the total additional compensation cost to be recognized would have been $22.5 million , based on accrual factors as of December 31, 2015 (common share price and payout assumptions). Performance shares granted under the WTM Incentive Plan The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2015 for each performance cycle: $ in millions Target WTM Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 29,828 $ 8.3 2014 – 2016 32,557 12.9 2013 – 2015 36,293 38.0 Sub-total 98,678 59.2 Assumed forfeitures (2,467 ) (1.5 ) Total at December 31, 2015 96,211 $ 57.7 The targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel for the 2015-2017 performance cycles is an 8% growth in intrinsic business value per share. Growth of 2% or less would result in no payout and growth of 14% or more would result in a payout of 200% . The targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel for the 2014-2016 performance cycles is a 9% growth in intrinsic business value per share. Growth of 3% or less would result in no payout and growth of 15% or more would result in a payout of 200% . The targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan to non-investment personnel for the 2013-2015 performance cycles is an 8% growth in intrinsic business value per share. Growth of 2% or less would result in no payout and growth of 14% or more would result in a payout of 200% . For investment personnel, the targeted performance goal for full payment of outstanding performance shares granted under the WTM Incentive Plan is based one-third on growth in intrinsic business value per share (as described above), one-third on achieving a total return on invested assets as measured against metrics based on U.S. Treasury Note returns and one-third on achieving a total return on invested assets as measured against metrics based on the Barclays U.S. Intermediate Aggregate Index returns. Restricted Shares The following outlines the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, 2015 2014 2013 $ in millions Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 83,314 $ 14.3 94,130 $ 17.0 69,910 $ 16.8 Issued (1) 23,640 15.7 23,440 13.1 25,720 14.4 Vested (36,279 ) — (33,205 ) — (1,500 ) — Forfeited — — (1,051 ) (.5 ) — — Expense recognized — (14.3 ) — (15.3 ) — (14.2 ) End of period (2) 70,675 $ 15.7 83,314 $ 14.3 94,130 $ 17.0 (1) On November 19, 2014, the Compensation Committee canceled 3,040 WTM performance shares for the 2014-2016 performance cycle and issued the same number of WTM restricted shares. (2) Outstanding restricted share awards as of December 31, 2015, 2014 and 2013 include 3,571 , 3,265 , and 3,789 restricted shares issued to employees of Sirius Group, which is accounted for as discontinued operations. During 2015, White Mountains issued 23,640 restricted shares that vest on January 1, 2018. During 2014, White Mountains issued 23,440 restricted shares that vest on January 1, 2017. During 2013, White Mountains issued 25,720 restricted shares that vest on January 1, 2016. The unrecognized compensation cost as of December 31, 2015 is expected to be recognized ratably over the remaining vesting periods. Stock Options Non-Qualified Options The Company’s Chairman and CEO holds 125,000 Non-Qualified Options, which are exercisable at $742 per common share and expire on January 20, 2017. The Non-Qualified Options have been fully amortized. Share-Based Compensation Based on OneBeacon Ltd. Common Shares OneBeacon Performance Shares The following summarizes activity for the years ended December 31, 2015, 2014 and 2013 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Year Ended December 31, 2015 2014 2013 $ in millions Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 517,470 $ 3.4 493,421 $ 4.0 563,190 $ 1.2 Payments and deferrals (1) (181,290 ) (1.5 ) (142,138 ) (1.0 ) (238,658 ) — New awards 154,887 — 165,800 — 179,000 — Forfeitures and cancellations (2) (41,632 ) (.1 ) 387 — (10,111 ) (.1 ) Expense recognized — (.4 ) — .4 — 2.9 End of period 449,435 $ 1.4 517,470 $ 3.4 493,421 $ 4.0 (1) OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. No payments were made in 2013 for the 2010-2012 OneBeacon performance cycle as the performance factor was zero. (2) Amounts include changes in assumed forfeitures, as required under GAAP. The following summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan as of December 31, 2015 for each performance cycle: $ in millions Target OneBeacon Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 146,659 $ .7 2014 – 2016 142,710 .1 2013 – 2015 167,300 .6 Sub-total 456,669 1.4 Assumed forfeitures (7,234 ) — Total at December 31, 2015 449,435 $ 1.4 If the outstanding OneBeacon performance shares had been vested on December 31, 2015 , the total additional compensation cost to be recognized would have been $1.3 million , based on December 31, 2015 accrual factors (common share price, accumulated dividends and payout assumptions). The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2015 is growth in book value per share of 13% . At a growth in book value per share of 6% or less, no performance shares would be earned and at a growth in book value per share of 20% or more, 200% of performance shares would be earned. The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2014 is growth in book value per share of 14% . At a growth in book value per share of 7% or less, no performance shares would be earned and at a growth in book value per share of 21% or more, 200% of performance shares would be earned. The targeted performance goal for full payment of the outstanding OneBeacon performance shares granted during 2013 is growth in book value per share of 13% . At a growth in book value per share of 6% or less, no performance shares would be earned and at a growth in book value per share of 20% or more, 200% of performance shares would be earned. OneBeacon Restricted Shares The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the years ended December 31, 2015, 2014 and 2013 . Year Ended December 31, 2015 2014 2013 $ in millions Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 612,500 $ 3.5 915,000 $ 6.5 927,000 $ 9.6 Issued 75,950 1.1 — — — — Vested (296,000 ) — (300,000 ) — (9,000 ) — Forfeited (9,728 ) (.1 ) (2,500 ) — (3,000 ) — Expense recognized — (2.0 ) — (3.0 ) — (3.1 ) End of period 382,722 $ 2.5 612,500 $ 3.5 915,000 $ 6.5 On February 24, 2015, OneBeacon issued to certain employees 75,950 shares of restricted stock having a grant date fair value of $1.1 million , which are scheduled to cliff vest on January 1, 2018. On March 1, 2012, OneBeacon issued 300,000 restricted shares to key employees that vest in two equal installments on February 28, 2014 and 2015. On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments beginning on February 22, 2014, 2015, 2016 and 2017. Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent 5 years have been or will also be reduced by 35,000 shares. The restricted shares contain dividend participation features, and therefore, are considered participating securities. The unrecognized compensation cost associated with outstanding restricted share awards as of December 31, 2015 is expected to be recognized ratably over the remaining vesting periods. OneBeacon Restricted Stock Units During the year ended December 31, 2015, 226,778 RSUs were issued, with 200,194 RSUs, net of forfeitures, outstanding as of December 31, 2015. The RSUs are scheduled to cliff vest on January 1, 2018, at which time the RSUs will be paid out in cash or shares at the discretion of the OneBeacon Compensation Committee. The expense associated with the RSUs, which is being amortized over the vesting period, was $0.8 million for the year ended December 31, 2015. Share-based Compensation Under Qualified Retirement Plans Contributions to the KSOP with respect to the years ended 2015, 2014 and 2013 were made with either the Company’s or OneBeacon Ltd.’s common shares, dependent on the employer. The variable contribution amounts for eligible participants of the KSOP constituted approximately 3% , 2% and 4% of salary for the years ended 2015, 2014 and 2013 . White Mountains recorded $5.7 million , $4.4 million and $6.3 million in compensation expense to pay benefits and allocate common shares to participant’s accounts for the years ended 2015, 2014 and 2013 . As of December 31, 2015 and 2014 , the plans owned 3% or less of either of the Company’s or OneBeacon Ltd.’s total common shares outstanding. All White Mountains common shares held by the KSOP are considered outstanding for earnings (loss) per share computations.

Common Shareholders_ Equity and

Common Shareholders’ Equity and Non-controlling Interests	12 Months Ended
	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Common Shareholder's Equity	Common Shareholders’ Equity and Non-controlling Interests Common Shares Repurchased and Retired During the past several years, White Mountains’s board of directors authorized the Company to repurchase its common shares, from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. The repurchase authorizations do not have a stated expiration date. As of December 31, 2015 , White Mountains may repurchase an additional 476,253 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through tender offers that were separately approved by its board of directors. During 2015, the Company repurchased 387,495 common shares for $284.2 million at an average share price of $733 , which were comprised of 361,839 common shares repurchased under the board authorization for $267.4 million at an average share price of $739 and 10,802 common shares repurchased pursuant to employee benefit plans. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above. During 2014, the Company repurchased 217,879 common shares for $134.5 million at an average share price of $617 , which were comprised of 207,404 common shares repurchased under the board authorization for $128.2 million at an average share price of $618 and 10,475 common shares repurchased pursuant to employee benefit plans. During 2013, the Company repurchased 141,535 common shares for $79.8 million at an average share price of $564 , which were comprised of 140,000 common shares repurchased in a private transaction under the board authorization for $78.9 million at an average share price of $564 and 1,535 common shares repurchased pursuant to employee benefit plans. Common Shares Issued During 2015 , the Company issued a total of 25,016 common shares, which consisted of 23,640 restricted shares issued to key management personnel and 1,376 shares issued to directors of the Company. During 2014 , the Company issued a total of 28,405 common shares, which consisted of 23,440 restricted shares issued to key management personnel, 3,570 shares issued in satisfaction of performance shares and 1,395 shares issued to directors of the Company. During 2013 , the Company issued a total of 27,310 common shares, which consisted of 25,720 restricted shares to key personnel and 1,590 shares issued to directors of the Company. Dividends on Common Shares For the years ended December 31, 2015, 2014 and 2013 , the Company declared and paid cash dividends totaling $6.0 million , $6.2 million and $6.2 million (or $1.00 per common share). Non-controlling Interests Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated entities and are presented separately on the balance sheet. The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of December 31, 2015 and 2014: December 31, 2015 December 31, 2014 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 24.5 % $ 245.6 24.7 % $ 258.4 SIG Preference Shares 100.0 250.0 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 17.1 3.1 17.9 MediaAlpha 40.0 14.4 40.0 22.6 Tranzact 36.8 79.4 36.8 88.2 Wobi 3.9 .6 36.7 5.4 Prospector Offshore Fund — — 23.4 31.1 Dewar 19.0 3.7 18.0 3.4 Total other, excluding mutuals and reciprocals 115.2 168.6 Mutuals and reciprocals BAM 100.0 (140.0 ) 100.0 (121.9 ) SSIE 100.0 (16.0 ) 100.0 (12.4 ) Total non-controlling interests $ 454.8 $ 542.7

Statutory Capital and Surplus

Statutory Capital and Surplus	12 Months Ended
	Dec. 31, 2015
Insurance [Abstract]
Statutory Capital and Surplus	Statutory Capital and Surplus White Mountains’s insurance and reinsurance operations are subject to regulation and supervision in each of the jurisdictions where they are domiciled and licensed to conduct business. Generally, regulatory authorities have broad supervisory and administrative powers over such matters as licenses, standards of solvency, premium rates, policy forms, investments, security deposits, methods of accounting, form and content of financial statements, reserves for unpaid loss and LAE, reinsurance, minimum capital and surplus requirements, dividends and other distributions to shareholders, periodic examinations and annual and other report filings. In general, such regulation is for the protection of policyholders rather than shareholders. The NAIC uses risk-based capital (“RBC”) standards for U.S. property and casualty insurers as a means of monitoring certain aspects affecting the overall financial condition of insurance companies. As of December 31, 2015 , White Mountains’s active U.S. insurance and reinsurance operating subsidiaries exceeded their respective RBC requirements. The Insurance Act 1978 of Bermuda and related regulations, as amended (“Insurance Act”), regulates the insurance business of Bermuda-domiciled insurers and reinsurers. Under the Insurance Act, insurers and reinsurers are required to maintain available statutory capital and surplus at a level equal to or in excess of its enhanced capital requirement which is established by reference to either a Bermuda Solvency Capital Requirement (“BSCR”) model or an approved internal capital model. Generally, the Bermuda Monetary Authority (“BMA”) has broad supervisory and administrative powers over such matters as licenses, standards of solvency, investments, methods of accounting, form and content of financial statements, minimum capital and surplus requirements, and annual and other report filings. OneBeacon: OneBeacon’s U.S. combined statutory surplus was $622.3 million and $721.5 million as of December 31, 2015 and 2014 . OneBeacon’s combined U.S. statutory net income (loss) for the years ended December 31, 2015, 2014 and 2013 was $44.5 million , $(14.2) million and $100.1 million . The minimum policyholders' surplus necessary to satisfy OneBeacon’s top tier regulated U.S. insurance operating subsidiary, Atlantic Specialty Insurance Company (“ASIC”), regulatory requirements was $117.7 million as of December 31, 2015 , which equals the authorized control level of the NAIC risk-based capital of ASIC’s policyholders’ surplus. Split Rock’s statutory capital and surplus was $249.5 million and $122.6 million as of December 31, 2015 and 2014, which met Bermuda’s statutory capital and surplus requirements. Split Rock reported $45.5 million , $46.2 million and $(38.7) million of statutory net income (loss) for the years ended December 31, 2015, 2014 and 2013. The principal differences between OneBeacon’s combined U.S. and Split Rock’s statutory amounts, and the amounts reported in accordance with GAAP include deferred acquisition costs, deferred taxes, and market value adjustments for debt securities. OneBeacon’s insurance subsidiaries' statutory policyholders’ surplus as of December 31, 2015 was in excess of the minimum requirements of relevant state and Bermuda insurance regulations. HG Global/BAM: HG Re is a Special Purpose Insurer under Bermuda insurance regulations and is subject to regulation and supervision by the BMA. As of December 31, 2015 , HG Re had statutory capital of $460.8 million . As a Special Purpose Insurer, HG Re does not have minimum regulatory capital requirements. BAM is domiciled in New York and is subject to regulation by the New York State Department of Financial Services (“NYDFS”). New York financial guarantee insurance law establishes single risk and aggregate limits with respect to insured obligations insured by financial guarantee insurers. BAM’s statutory net loss for the years ended December 31, 2015, 2014 and 2013 was $32.0 million , $31.8 million and $29.3 million . BAM’s members’ surplus, as reported to regulatory authorities as of December 31, 2015 was $437.3 million , which exceeds the minimum members’ surplus necessary for BAM to maintain its New York State financial guarantee insurance license of $66.0 million . Other Operations: WM Life Re is subject to regulation and supervision by the BMA. As of December 31, 2015 and 2014, WM Life Re had statutory capital and surplus of $77.5 million and $76.0 million . The minimum regulatory capital held by WM Life Re necessary to satisfy the requirements established by the BMA was $0.5 million as of December 31, 2015 . SSIE’s policyholders’ surplus, as reported to regulatory authorities as of December 31, 2015 and 2014, was $5.0 million and $6.5 million . SSIE’s statutory net loss for the year ended December 31, 2015 and 2014 was $3.5 million and $9.5 million . The minimum policyholders’ surplus necessary to satisfy SSIE’s regulatory requirements was $1.5 million as of December 31, 2015, which equals the authorized control level of the NAIC risk-based capital based on SSIE’s policyholders’ surplus. Dividend Capacity There are no restrictions under Bermuda law or the law of any other jurisdiction on the payment of dividends from retained earnings by White Mountains. However, under the insurance laws of the states and jurisdictions under which White Mountains’s insurance and reinsurance operating subsidiaries are domiciled, an insurer is restricted with respect to the timing and the amount of dividends it may pay without prior approval by regulatory authorities. As of December 31, 2015 , White Mountains’s top tier insurance and reinsurance subsidiaries have approximately $554.0 million of GAAP shareholders’ equity (net of $180 million of non-controlling interest at OneBeacon), $48.0 million of which can be distributed to White Mountains without prior regulatory approval. As a result, as of December 31, 2015 , $505.0 million of White Mountains’s GAAP shareholders’ equity held in its insurance and reinsurance subsidiaries was not available for the payment of dividends without prior regulatory approval, and approximately $2.6 billion of White Mountains’s retained earnings is unrestricted with respect to the payment of dividends to White Mountains’s common shareholders. When determining whether to make distributions from its insurance and reinsurance operating subsidiaries, White Mountains also considers factors such as internal capital targets and rating agency capital requirements. Accordingly, there can be no assurance regarding the amount of such dividends that may be paid by such subsidiaries in the future. Following is a description of the dividend capacity of White Mountains’s insurance and reinsurance operating subsidiaries: OneBeacon: On December 23, 2014, OneBeacon Insurance Company (subsequently renamed Bedivere) distributed ASIC to its immediate parent at a value of $700.5 million as part of the Runoff Transaction. OBIC also distributed $151.2 million of cash and other securities to its immediate parent in accordance with the prescribed minimum capital to be included in the company at the time of its sale to Armour, as approved by the PID. OneBeacon’s top-tier regulated U.S. insurance operating subsidiary, ASIC, has the ability to pay dividends to its immediate parent during any twelve-month period without the prior approval of regulatory authorities in an amount set by formula based on the lesser of net investment income, as defined by statute, or 10% of statutory surplus, in both cases as most recently reported to regulatory authorities, subject to the availability of earned surplus and subject to dividends paid in prior periods. Based upon the formula above, most recently calculated as of December 31, 2015, ASIC has the ability to pay $26.6 million of dividends in 2016 without prior approval of regulatory authorities. As of December 31, 2015 , ASIC had $622.3 million of statutory surplus and $68.3 million of earned surplus. During 2015, ASIC paid $44.9 million of dividends to its immediate parent and redeemed and retired common shares held by its immediate parent at an aggregate price of $66.0 million . OneBeacon is in the process of seeking regulatory approval of the redomestication of ASIC from New York to Pennsylvania. Under the exiting dividend capacity formula for the Commonwealth of Pennsylvania, which is the greater of 10% of policyholder surplus or net income of the previous year, ASIC would have the ability to pay $62.2 million in dividends in 2016. OneBeacon expects that any distributions paid by ASIC prior to its redomestication to Pennsylvania would reduce the $62.2 million allowed under the dividend capacity formula. During 2016, Split Rock has the ability to pay dividends and make capital distributions without the prior approval of regulatory authorities of up to $37.4 million , which is equal to 15% of its December 31, 2015 statutory capital and surplus, subject to meeting all appropriate liquidity and solvency requirements. During the year ended December 31, 2015, Split Rock made no capital distributions and paid no dividends to its immediate parent. During the year ended December 31, 2014, Split Rock paid $10.0 million of capital distributions and $10.0 million of dividends to its immediate parent. During the year ended December 31, 2015, OneBeacon Ltd., through an intermediary holding company, contributed $85.0 million to Split Rock as additional paid-in capital. During 2015 and 2014, OneBeacon’s unregulated insurance operating subsidiaries paid $5.3 million and $4.8 million of dividends to their immediate parent. As of December 31, 2015 , OneBeacon’s unregulated insurance operating subsidiaries had $104.4 million of net unrestricted cash, short-term investments and fixed maturity investments and surplus notes classified as other investments that had a par value of $101.0 million and a fair value of $51.5 million . During 2015 and 2014, OneBeacon Ltd. paid $80.0 million of regular quarterly dividends to its common shareholders. For both 2015 and 2014, White Mountains received $60.3 million of these dividends. During 2013, OneBeacon Ltd. declared and paid regular quarterly dividends to its common shareholders totaling $80.2 million . As of December 31, 2015 , OneBeacon Ltd. and its intermediate holding companies held $65.3 million of net unrestricted cash, short-term investments and fixed maturity investments, $54.6 million of common equity securities and $51.5 million of other long-term investments outside of its regulated and unregulated insurance operating subsidiaries. HG Global/BAM: As of December 31, 2015 , HG Global had $613.0 million face value of preferred shares outstanding, of which White Mountains owned 96.9% . Holders of the HG Global preferred shares receive cumulative dividends at a fixed annual rate of 6.0% on a quarterly basis, when and if declared by HG Global. HG Global did not declare or pay any preferred dividends in 2015. As of December 31, 2015 , HG Global has accrued $139.7 million of dividends payable to holders of its preferred shares, $135.3 million of which is payable to White Mountains and eliminated in consolidation. HG Re is a Special Purpose Insurer subject to regulation and supervision by the BMA, but does not require regulatory approval to pay dividends. However, HG Re’s dividend capacity is limited by amounts held in the collateral trusts pursuant to the first loss reinsurance treaty (“FLRT”) with BAM. As of December 31, 2015 , HG Re had statutory capital of $460.8 million , of which $401.9 million was held as collateral in the supplemental trust pursuant to the FLRT with BAM and $53.8 million relates to accrued interest on the BAM Surplus Notes held by HG Re. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM surplus notes for the five years ending December 31, 2018 from a fixed rate of 8.0% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which is 3.15% for 2015 and is 3.54% for 2016. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8.0% . BAM is required to seek regulatory approval to pay interest and principal on 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. BAM did not pay any interest on the BAM Surplus Notes in 2015, 2014 or 2013. Other Operations: During 2015 , WM Advisors did not pay any dividends to its immediate parent. As of December 31, 2015 , WM Advisors held $14.4 million of net unrestricted cash and short-term investments. During 2015, White Mountains paid a $6.0 million common share dividend. As of December 31, 2015 , the Company and its intermediate holding companies held $73.7 million of net unrestricted cash, short-term investments and fixed maturity investments, $292.3 million of common equity securities and $17.6 million of other long-term investments included in its Other Operations segment.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
Segment Information	Segment Information White Mountains has determined that its reportable segments are OneBeacon, HG Global/BAM and Other Operations. As a result of the Sirius sale, the results of operations for Sirius Group have been classified as discontinued operations and are now presented as such, net of related income taxes, in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation (See Note 22 - “Discontinued Operations” ). White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors. OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products through independent agencies, regional and national brokers, wholesalers and managing general agencies. The HG Global/BAM segment consists of White Mountains’s investment in HG Global and the consolidated results of BAM. BAM is a municipal bond insurer domiciled in New York that was established to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of BAM Surplus Notes. HG Global also provides 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. BAM's results are attributed to non-controlling interests. Other Operations consists of the Company, the Company’s intermediate holding companies, WM Advisors, WM Life Re, Tranzact, Wobi, MediaAlpha, Star & Shield, SSIE, White Mountains’s investment in Symetra and various other entities and investments. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. SSIE’s results are attributed to non-controlling interests. Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2015 Earned insurance and reinsurance premiums $ 1,176.2 $ 2.5 $ .8 $ 8.7 $ 1,188.2 Net investment income 45.9 1.9 4.2 8.8 60.8 Net investment income (loss) - surplus note interest — 15.8 (15.8 ) — — Net realized and unrealized investment (losses) gains (35.1 ) (.3 ) .9 259.9 (2) 225.4 Other (expense) revenue (.6 ) — .7 334.1 (4) 334.2 Total revenues (3) 1,186.4 19.9 (9.2 ) 611.5 1,808.6 Losses and LAE 700.7 — — 8.2 708.9 Insurance and reinsurance acquisition expenses 213.8 .6 2.3 3.4 220.1 Other underwriting expenses 218.2 — .4 — 218.6 General and administrative expenses 14.1 1.4 35.4 408.0 (5) 458.9 Amortization of intangible assets 1.3 — — 27.3 28.6 Interest expense 13.0 — — 5.6 18.6 Total expenses 1,161.1 2.0 38.1 452.5 1,653.7 Pre-tax income (loss) $ 25.3 $ 17.9 $ (47.3 ) $ 159.0 $ 154.9 HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2014 Earned insurance and reinsurance premiums $ 1,177.1 $ 1.4 $ .4 $ 6.1 $ 1,185.0 Net investment income 43.4 1.4 5.7 9.0 59.5 Net investment income (loss) - surplus note interest — 15.7 (15.7 ) — — Net realized and unrealized investment gains 40.4 1.7 6.6 29.8 78.5 Other revenue 5.8 — .6 124.7 (4) 131.1 Total revenues (3) 1,266.7 20.2 (2.4 ) 169.6 1,454.1 Losses and LAE 815.1 — — 8.9 824.0 Insurance and reinsurance acquisition expenses 203.3 .3 1.8 .8 206.2 Other underwriting expenses 179.2 — .4 — 179.6 General and administrative expenses 12.4 1.6 35.9 196.1 246.0 Amortization of intangible assets 1.4 — — 10.3 11.7 Interest expense 13.0 — — 2.6 15.6 Total expenses 1,224.4 1.9 38.1 218.7 1,483.1 Pre-tax income (loss) $ 42.3 $ 18.3 $ (40.5 ) $ (49.1 ) $ (29.0 ) HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2013 Earned insurance and reinsurance premiums $ 1,120.4 $ .4 $ .1 $ — $ 1,120.9 Net investment income 43.0 1.0 4.7 11.1 59.8 Net investment income (loss) - surplus note interest — 40.2 (40.2 ) — — Net realized and unrealized investment gains (losses) 49.4 (2.0 ) (9.3 ) 95.8 133.9 Other revenue 31.2 — .4 15.4 47.0 Total revenues (3) 1,244.0 39.6 (44.3 ) 122.3 1,361.6 Losses and LAE 622.1 — — — 622.1 Insurance and reinsurance acquisition expenses 208.9 .1 1.4 — 210.4 Other underwriting expenses 204.8 — .4 — 205.2 General and administrative expenses 10.6 1.4 32.5 103.2 147.7 Amortization of intangible assets 1.4 — — — 1.4 Interest expense 13.0 — — 3.2 16.2 Total expenses 1,060.8 1.5 34.3 106.4 1,203.0 Pre-tax income (loss) $ 183.2 $ 38.1 $ (78.6 ) $ 15.9 $ 158.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services. (2) Net realized and unrealized investment gains (losses) in the Other Operations segment includes the unrealized gain of $258.8 related to the investment in Symetra common shares, representing the difference between the carrying value under the equity method at November 5, 2015 and the fair value at December 31, 2015. (See Note 17 - “Investments in Unconsolidated Affiliates” ). (3) Total revenues includes both revenues from customers as well as investment performance. (4) Includes $186.9 from Tranzact and $105.5 from MediaAlpha for the year ended December 31, 2015; and $43.2 from Tranzact and $65.3 from MediaAlpha for the year ended December 31, 2014. (5) Includes $167.3 from Tranzact and $99.0 from MediaAlpha for the year ended December 31, 2015; and $37.4 from Tranzact and $60.6 from MediaAlpha for the year ended December 31, 2014. HG Global/BAM Selected Balance Sheet Data Millions OneBeacon HG Global BAM Other Operations Discontinued Operations Total December 31, 2015: Total investments $ 2,591.4 $ 136.2 $ 460.3 $ 1,092.8 $ — $ 4,280.7 Reinsurance recoverable on paid and unpaid losses 193.5 — — .5 — 194.0 Assets held for sale — — — — 4,407.0 4,407.0 Total assets 3,602.7 739.0 (91.1 ) (1 ) 1,626.9 4,407.0 10,284.5 Loss and LAE reserves 1,389.8 — — 6.0 — 1,395.8 Liabilities held for sale — — — — 2,884.0 2,884.0 Total liabilities 2,598.2 181.2 48.9 204.2 2,884.0 5,916.5 Total White Mountains’s common shareholders’ 755.2 540.7 — 1,344.3 1,273.0 3,913.2 Non-controlling interest 249.3 17.1 (140.0 ) 78.4 — (2) 204.8 December 31, 2014: Total investments $ 2,525.8 $ 121.0 $ 454.2 $ 641.4 $ — $ 3,742.4 Reinsurance recoverable on paid and unpaid losses 173.8 — — .1 — 173.9 Assets held for sale 58.1 — — — 4,572.5 4,630.6 Total assets 3,559.8 704.4 (89.4 ) (1 ) 1,708.4 4,572.5 10,455.7 Loss and LAE reserves 1,342.2 — — 7.8 — 1,350.0 Liabilities held for sale — — — — 3,105.3 3,105.3 Total liabilities 2,510.5 121.0 32.5 148.0 3,105.3 5,917.3 Total White Mountains’s common shareholders’ 787.5 565.5 — 1,425.5 1,217.2 3,995.7 Non-controlling interest 261.8 17.9 (121.9 ) 134.9 — (2) 292.7 (1) BAM total assets reflect the elimination of $503.0 in surplus notes issued to HG Global and its subsidiaries, and $90.2 and $74.4 in accrued interest related to those surplus notes as of December 31, 2015 and 2014. (2) Excludes $250.0 related to SIG Preference Shares (See Note 14 - “Common Shareholders’ Equity and Non-controlling Interests” )

Investments in Unconsolidated A

Investments in Unconsolidated Affiliates	12 Months Ended
	Dec. 31, 2015
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. November 5, December 31, Millions 2015 2014 Symetra common shares $ 394.4 $ 373.8 Unrealized (losses) gains from Symetra’s fixed maturity portfolio (1.6 ) 37.6 Carrying value of Symetra common shares 392.8 411.4 Hamer — 3.0 Total investments in unconsolidated affiliates $ 392.8 $ 414.4 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. 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. During 2015, pursuant to the redemption of White Mountains's investments in the Prospector Funds, 513,500 common shares of Symetra were distributed to White Mountains. As of November 5, 2015 and December 31, 2014, White Mountains owned 20,562,379 and 20,048,879 common shares of Symetra, a 17.70% and 17.31% common share ownership, respectively. During the period of January 1, 2015 through November 5, 2015 , White Mountains received cash dividends from Symetra of $16.9 million on its common share investment which was accounted for as a reduction of White Mountains’s investment in Symetra in accordance with equity accounting. White Mountains previously owned warrants to acquire an additional 9.49 million common shares of Symetra at $11.49 per share. On June 20, 2013, White Mountains exercised these warrants in a cashless transaction and received 2.65 million common shares of Symetra in exchange for the warrants. Prior to their exercise, White Mountains accounted for its Symetra warrants as derivatives with changes in fair value recognized as a gain or loss recognized through other revenue in the income statement. White Mountains used a Black Scholes valuation model to determine the fair value of the Symetra warrants. During the six months ended June 30, 2013, White Mountains recognized a $10.8 million increase in the value of the warrants through other revenues based on the final Black Scholes valuation that was agreed upon between Symetra and White Mountains. During the six months ended June 30, 2013, White Mountains also received $1.5 million of dividends from Symetra on the warrants that were recorded in net investment income. As of December 31, 2011, White Mountains concluded that its investment in Symetra common shares was other-than-temporarily impaired and wrote down the GAAP book value of the investment to its estimated fair value of $261.0 million or $15 per share. This impairment, as well as the effect of Symetra capital transactions, has resulted in a basis difference between the GAAP carrying value of White Mountains’s investment in Symetra common shares and the amount derived by multiplying the percentage of White Mountains common share ownership by Symetra’s total GAAP equity. As of November 5, 2015, the pre-tax unamortized basis difference was $159.9 million , of which $32.6 million is attributable to equity in earnings of unconsolidated affiliates and $127.3 million is attributable to equity in net unrealized gains of unconsolidated affiliates. As of December 31, 2014, the pre-tax unamortized basis difference was $170.4 million , of which $36.4 million is attributable to equity in earnings of unconsolidated affiliates and $134.0 million is attributable to equity in net unrealized gains of unconsolidated affiliates. The pre-tax basis differences were amortized over a 30 -year period, based on estimated future cash flows associated with Symetra’s underlying assets and liabilities to which the basis differences were attributed. White Mountains continued to record its equity in Symetra's earnings and net unrealized gains (losses) under the equity method through November 5, 2015. In addition, White Mountains recognized the amortization of the basis differences 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 period ended November 5, 2015, White Mountains recognized after-tax amortization of $2.2 million through equity in earnings of unconsolidated affiliates and $8.9 million through equity in net unrealized gains from investments in unconsolidated affiliates. For the period ended December 31, 2014, White Mountains recognized after-tax amortization of $2.8 million through equity in earnings of unconsolidated affiliates and $11.7 million through equity in net unrealized gains from investments in unconsolidated affiliates. For the period ended December 31, 2013, White Mountains recognized after-tax amortization of $2.7 million through equity in earnings of unconsolidated affiliates and $10.8 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 through November 5, 2015: Millions Common shares Warrants Total Equity method carrying value of investment in Symetra as of December 31, 2012 (2) $ 351.2 $ 30.3 $ 381.5 Equity in earnings (1)(3)(6) 37.8 — 37.8 Equity in net unrealized gains from Symetra’s fixed maturity portfolio (4)(5) (106.4 ) — (106.4 ) Dividends received (6.4 ) — (6.4 ) Increase in value of warrants — 10.8 10.8 Exercise of warrants 41.1 (41.1 ) — Equity method carrying value of investment in Symetra as of December 31, 2013 (2) 317.3 — 317.3 Equity in earnings (1)(3)(6) 47.0 — 47.0 Equity in net unrealized losses from Symetra’s fixed maturity portfolio (4)(5) 81.2 — 81.2 Dividends received (34.1 ) — (34.1 ) Equity method carrying value of investment in Symetra as of December 31, 2014 (2) 411.4 — 411.4 Equity in earnings (1)(3)(6) 25.1 — 25.1 Equity in net unrealized gains from Symetra’s fixed maturity portfolio (4)(5) (39.2 ) — (39.2 ) Distribution from Prospector Offshore Fund 12.4 — 12.4 Dividends received (16.9 ) — (16.9 ) Equity method carrying value of investment in Symetra as of November 5, 2015 (2)(7) $ 392.8 $ — $ 392.8 (1) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 excludes tax expense of $1.6 , $3.3 , and $2.8 (2) Includes White Mountains’s equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $(1.6) , $37.6 , and $(43.6) as of November 5, 2015 and December 31, 2014 and 2013, which excludes tax benefit (expense) of $0.2 , $(2.7) and $3.2 (3) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $2.3 , $3.0 and $3.0 increases relating to the pre-tax amortization of the Symetra common share basis difference. (4) Net unrealized gains for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $9.4 , $12.7 and $11.8 increases relating to the pre-tax amortization of the Symetra common share basis difference. (5) Net unrealized gains (losses) from Symetra’s fixed maturity portfolio excludes tax benefit (expense) of $2.9 , $(5.9) and $8.3 for the period ended November 5, 2015 and years ended December 31, 2014 and 2013. (6) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $(0.1) , $(0.1) , and $0.2 (gain) loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra (7) The aggregate value of White Mountains’s investment in common shares of Symetra was $651.2 based upon the quoted market price of $31.67 per share as of November 5, 2015. The following table summarizes financial information for Symetra as of September 30, 2015 and December 31, 2014: September 30, December 31, Millions 2015 2014 Symetra balance sheet data: Total investments $ 32,409.2 $ 30,634.3 Separate account assets 885.9 949.8 Total assets 34,962.8 33,001.7 Policyholder liabilities 29,492.0 27,276.0 Long-term debt 697.5 697.2 Separate account liabilities 885.9 949.8 Total liabilities 31,836.7 29,641.1 Common shareholders’ equity 3,126.1 3,360.6 The following table summarizes financial information for Symetra for the nine months ended September 30, 2015 and years ended December 31, 2014 and 2013: Nine months ended September 30, Year ended December 31, Millions 2015 2014 2013 Symetra income statement data: Net premiums earned $ 539.3 $ 629.1 $ 627.2 Net investment income 994.3 1,320.5 1,285.0 Total revenues 1,605.9 2,182.4 2,139.5 Policy benefits 1,143.7 1,399.7 1,394.9 Total expenses 1,543.6 1,882.5 1,865.4 Net income 89.6 254.4 220.7 Comprehensive net (loss) income (234.1 ) 397.0 (777.6 ) Hamer and Bri-Mar 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 $1.6 million for the six months ended June 30, 2015 and $1.9 million and $0.9 million for the years ended December 31, 2014 and 2013. On October 10, 2013, White Mountains sold its interest in Bri-Mar under an asset purchase agreement. For the year ended December 31, 2013, White Mountains recorded $1.1 million of cash proceeds from the sale and a $1.7 million loss on sale. Prior to the sale, White Mountains recorded equity in earnings of $0.9 million for Bri-Mar for the nine months ended September 30, 2013.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2015
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities BAM As a mutual insurance company, BAM is owned by its members and a portion of each member’s policy payments represents a contribution to member’s surplus. During 2012, White Mountains capitalized HG Global to fund BAM through the purchase of $503.0 million of BAM Surplus Notes. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional subordinated financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE. BAM and HG Global, through its wholly-owned subsidiary, HG Re, entered into a first loss reinsurance treaty (“FLRT”), under which HG Re will provide first loss protection up to 15% of par outstanding on each bond insured by BAM in exchange for 60% of the premium, net of a ceding commission, charged by BAM. HG Re’s obligations under the FLRT are satisfied by the assets in two collateral trusts: a Regulation 114 Trust and a Supplemental Trust. Losses required to be reimbursed to BAM by HG Re are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, HG Global has the right to designate two directors for election to BAM’s board of directors. White Mountains is required to consolidate the results of BAM. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. Reciprocals Reciprocals are policyholder-owned insurance carriers organized as unincorporated associations. Each policyholder insured by the reciprocal shares risk with the other policyholders. Policyholders share profits and losses in the same proportion as the amount of insurance purchased but are not subject to assessment for net losses of the reciprocal. Houston General Insurance Exchange Houston General Management Company, a wholly-owned indirect subsidiary of OneBeacon provides management services for a fee to a reciprocal, Houston General Insurance Exchange (“HGIE”). As of December 31, 2015, OneBeacon holds a surplus note issued by HGIE, the remaining balance of which is $5.0 million as of December 31, 2015. The principal and interest on the surplus note is repayable to OneBeacon only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. OneBeacon has no ownership interest in the reciprocal. OneBeacon has determined that HGIE qualifies as a VIE. Furthermore, OneBeacon has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates HGIE. As of December 31, 2015 and 2014 , consolidated amounts related to HGIE included total assets of $5.1 million and $2.4 million and total liabilities of $5.0 million and $4.0 million . As of December 31, 2015 , the net amount of capital at risk is equal to the surplus note of $5.0 million less the accumulated losses of $0.2 million which includes accrued interest on the surplus note of $0.3 million that eliminates in consolidation. Star & Shield Insurance Exchange Star & Shield Risk Management LLC, a wholly-owned indirect subsidiary of White Mountains provides management and other services for a fee to a reciprocal, Star & Shield Insurance Exchange (“SSIE”). As of December 31, 2015, White Mountains held $21.0 million of surplus notes issued by SSIE. The principal and interest on the surplus notes are repayable to White Mountains only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. Because SSIE is consolidated in White Mountains’s financial statements, the SSIE Surplus Notes and accrued interest are classified as intercompany notes, carried at face value and eliminated in consolidation. White Mountains has no ownership interest in SSIE. White Mountains has determined that SSIE qualifies as a VIE. Furthermore, White Mountains has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, White Mountains consolidates SSIE. As of December 31, 2015 and 2014, consolidated amounts related to SSIE included total assets of $14.2 million and $13.5 million and total liabilities of $30.3 million and $25.9 million . For both December 31, 2015 and 2014, the net amount of capital at risk is equal to the surplus notes of $21.0 million less the accumulated losses of $16.0 million . Prospector Offshore Fund In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. of which White Mountains’s owned 67.6% prior to the redemption on June 30, 2015. The consolidated results of the Prospector Offshore Fund are included in Other Operations from January 1, 2015 through June 30, 2015, at which point the results of the Prospector Offshore Fund were no longer consolidated by White Mountains. Prior to the redemption, White Mountains determined that the Prospector Offshore Fund was a VIE of which White Mountains was the primary beneficiary and was required to consolidate.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2015
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, which are recorded as debt liabilities at face value less unamortized original issue discount, and the Tranzact Bank Facility and MediaAlpha Bank Facility, both of which are carried at face value less unamortized issuance costs. The following table summarizes the fair values and book values as of December 31, 2015 and 2014 : December 31, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 276.4 $ 274.8 $ 286.0 $ 274.7 Tranzact Bank Facility 102.8 102.9 68.7 67.4 MediaAlpha Bank Facility 15.0 14.7 — — The fair value estimate for the 2012 OBH Senior Notes has been determined using quoted market prices. The 2012 OBH Senior Notes are considered a Level 2 measurement based upon the volume and frequency of observable transactions. The fair value estimate for the Tranzact Bank Facility and MediaAlpha Bank Facility have been determined based on a discounted cash flows approach and is considered to be a Level 3 measurement.

Transactions with Related Perso

Transactions with Related Persons	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie, the founder and Managing Member of Prospector, retired from the WTM Board of Directors in May 2015. Until June 30, 2015, Prospector served as a discretionary adviser with respect to specified assets, primarily common equity securities and convertible fixed maturity investments, managed directly or through WM Advisors on behalf of White Mountains and other clients of WM Advisors. At that time, the investment management agreements between WM Advisors and Prospector and OneBeacon and Prospector and the Consulting Agreement described below were terminated. Pursuant to an investment management agreements with WM Advisors (the “WMA Agreement”) and OneBeacon (the “OneBeacon Agreement”), Prospector charged WM Advisors and OneBeacon fees based on the following schedule: 100 basis points on the first $200 million of assets under management; 50 basis points on the next $200 million ; and 25 basis points on amounts over $400 million . As of December 31, 2014, Prospector managed $195.7 million of assets under the WMA Agreement and $431.2 million of assets under the OneBeacon Agreement, including $202.9 million of ERISA assets. During 2015, 2014 and 2013, Prospector earned $2.1 million , $6.5 million , and $6.5 million in total fees pursuant to the investment management agreements with WM Advisors and OneBeacon. Prospector also had a separate investment management agreement with Symetra that was terminated in the fourth quarter of 2015. Prospector also advised White Mountains on matters including capital management, asset allocation, private equity investments and mergers and acquisitions. Pursuant to a Consulting Agreement for those services, Prospector was granted 6,250 performance shares for the 2014-2016 cycle and 7,000 performance shares for the 2013-2015 cycle. Under the terms of the Consulting Agreement, Prospector earned a prorated portion of the outstanding performance share grants at the time of the termination of the Consulting Agreement and was paid $5.5 million in respect thereof. Pursuant to a pre-existing revenue sharing agreement (the “Revenue Agreement”), Prospector paid White Mountains 6% of the annual revenues in excess of $500,000 of certain of Prospector’s funds in return for White Mountains having made a founding investment in 1997. White Mountains and Prospector mutually agreed to terminate the Revenue Agreement as of June 30, 2015. During 2014 and 2013, White Mountains earned $0.4 million , and $0.9 million under the Revenue Agreement. No amount was earned under the Revenue Agreement in 2015. Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector (the “Funds”). As of December 31, 2014, White Mountains had $82.4 million invested in these limited partnerships. This total included $15.8 million of OneBeacon assets. Under the limited partnership agreements, Prospector served as general partner and general manager of the Funds and was paid management and incentive performance fees by White Mountains and OneBeacon. For the years ended December 31, 2015, 2014 and 2013, White Mountains and OneBeacon incurred $0.4 million , $1.1 million and $1.3 million in management fees and $0.1 million , $0.7 million and $3.2 million in incentive fees. Oakum Bay Capital Mr. Kernan Oberting, a Managing Director of White Mountains Capital, founded Oakum Bay Capital (“OBC”). OBC serves as the general manager and owns general partnership interests in KVO Capital Partners (“KCP”) and the Trimarc Capital Fund (“TCF”) hedge funds. Mr. Oberting is a limited partner in KCP and TCF. In connection with Mr. Oberting commencing employment with White Mountains Capital, on July 16, 2012, White Mountains purchased $2.0 million of preferred stock and received 7.5% of the common equity of OBC. After giving effect to the White Mountains’s investment, Mr. Oberting and his family beneficially owned 67.5% of the common equity of OBC. During 2013, Mr. Oberting and his family assigned all of their common equity interests to unaffiliated OBC management with effect from January 1, 2013. In September 2013, White Mountains exchanged its preferred stock and common equity interest in OBC for $500,000 in cash and a portion of OBC’s revenue for the next ten years. During 2012, White Mountains invested $9.5 million in the TCF hedge funds, which were redeemed in the fourth quarter of 2015 for $8.9 million . Other Relationships and Transactions WM Advisors provides investment advisory and management services to Symetra. As of December 31, 2015 and 2014 , WMA had $22.1 billion and $30.9 billion of assets under management from Symetra. During 2015, 2014 and 2013 , WMA earned $8.0 million , $18.4 million and $16.5 million in fees from Symetra. During 2015, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company, in two transactions. On June 1, 2015, the Company repurchased 19,688 WTM common shares for $650.03 per share, the market price at the time the agreement was reached. On September 17, 2015, the Company repurchased 26,300 WTM common shares for $761.50 , the market price at the time the agreement was reached.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office space under non-cancellable operating leases that expire on various dates through 2021. Rental expense for all of White Mountains’s locations was $15.5 million , $11.8 million and $12.1 million for the years ended December 31, 2015, 2014 and 2013 . White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $14.3 million , $13.4 million , $11.4 million , and $50.6 million for 2016, 2017, 2018 and 2019 and thereafter, respectively. White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $82.2 million , do not have fixed funding dates. Capital Lease In December 2011, OneBeacon sold the majority of its fixed assets and capitalized software. OneBeacon entered into lease financing arrangements with US Bancorp and Fifth Third whereby OneBeacon sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. OneBeacon then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. OneBeacon received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions. In December 2015, the lease agreement with Fifth Third expired and OneBeacon purchased the leased assets under the agreement with a remaining book value of $1.3 million for a nominal fee. At the end of the lease term with US Bancorp on December 31, 2016, OneBeacon will purchase the leased furniture and equipment assets for a nominal fee, and all rights, title and interest will transfer back to it. As of December 31, 2015 and 2014 , OneBeacon had a capital lease obligation of $1.7 million and $7.1 million , respectively, included within other liabilities and a capital lease asset of $2.4 million and $7.0 million included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OneBeacon’s future annual minimum rental payments are $1.7 million for the year ended December 31, 2016. Assigned Risks As a condition of its license to do business in certain states, White Mountains’s insurance operations are required to participate in mandatory shared market mechanisms. Each state dictates the types of insurance and the level of coverage that must be provided. The total amount of business an insurer is required to accept is based on its market share of voluntary business in the state. In certain cases, White Mountains is obligated to write business from mandatory shared market mechanisms at some time in the future based on the market share of voluntary policies it is currently writing. Underwriting results related to assigned risk plans are typically adverse and are not subject to the predictability associated with White Mountains’s voluntarily written business. Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains accrues any significant insolvencies when the loss is probable and the assessment amount can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years. As of December 31, 2015 , the reserve for such assessments totaled $8.5 million . 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 - “Reserves for Unpaid Losses and Loss Adjustment Expenses” . White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or it there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of December 31, 2015 : 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 (the “Agreement”). 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”). Runoff Transaction Subsequent to the closing of the Runoff Transaction, on January 22, 2015, three holders of insurance policies issued by the companies OneBeacon sold to Armour in the Runoff Transaction filed a Petition for Review with the Commonwealth Court of Pennsylvania (Commonwealth Court) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (Department) orders approving the Runoff Transaction and denying their right to intervene in the Department’s regulatory review of the Runoff Transaction. On January 14, 2016, the Commonwealth Court upheld the Department’s December 23, 2014 order approving the Runoff Transaction. The objectors did not appeal the Commonwealth Court's decision and the time for appeal has expired. 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 received approximately $32.0 million and Sirius Group, which is now included in discontinued operations, received $6.1 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs’ appeal is pending. 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 December 31, 2015, as the amount of loss, if any, cannot be reasonably estimated. OneTitle White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OTH”) and has provided a $10.0 million surplus note facility under which OTH’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may draw funds. At December 31, 2015, no funds had been drawn on the surplus note facility.

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivatives	Derivatives
Discontinued Operations	Discontinued Operations Sirius Group On July 24, 2015, White Mountains entered into an agreement to sell Sirius Group to CM International Holding PTE Ltd., the Singapore-based investment arm of CMI. The transaction is expected to close in the first quarter of 2016 and is subject to regulatory approvals and other customary closing conditions. The results of operations for Sirius Group have been classified as discontinued operations in the statement of operations and comprehensive income and the assets and liabilities of Sirius Group have been presented in the balance sheet as held for sale. Prior year amounts have been reclassified to conform to the current period’s presentation. The amounts reflected within discontinued operations differ from amounts previously presented within the Sirius Group segment. The segment results of operations for Sirius Group reported in previous periods included investment income and realized and unrealized investment gains and losses from certain investments that are to be retained by White Mountains after the completion of the sale. For the years ended December 31, 2015, 2014 and 2013, $225.0 million , of which $218.5 million is related to Symetra, $4.0 million and $1.0 million of net investment income and realized and unrealized investment gains and losses that had been previously included in the Sirius Group legal entities have been excluded from net income (loss) from discontinued operations. OneBeacon Runoff In December 2014, OneBeacon completed the Runoff Transaction. The results of operations for the Runoff Business have been classified as discontinued operations and are presented, net of related income taxes, in the statement of operations and comprehensive income. The amounts classified as discontinued operations exclude investing and financing activities that are conducted on an overall consolidated level and, accordingly, there were no separately identifiable investments associated with the Runoff Business. As part of closing the Runoff Transaction on December 23, 2014, OneBeacon provided financing in the form of surplus notes with a par value of $101.0 million issued by OneBeacon Insurance Company (“OBIC”), one of the entities that were transferred from OneBeacon to Armour as part of the transaction (the “Surplus Notes”). At closing, the Surplus Notes had a fair value of $64.9 million , based on a discounted cash flow model, resulting in a total valuation adjustment of $36.1 million pre-tax ( $23.5 million after-tax) included in loss from sale of discontinued operations. Subsequent to closing, the Surplus Notes are included in OneBeacon’s investment portfolio, categorized within other long-term investments. (See Note 5 - “Investment Securities” for further disclosures regarding the Surplus Notes). Under the contractual terms of both the seller priority and pari passu notes, scheduled interest payments accrue at 6.0% until the scheduled maturity date of March 15, 2020 and at a floating interest rate thereafter, should any principal remain outstanding. As required by the PID, interest on the notes does not compound. The notes restrict the Issuer’s ability to make distributions to holders of its equity interest. All such distributions are prohibited while the seller priority note is outstanding, and while the pari passu note is outstanding, distributions are permitted only if the Issuer concurrently repays a pro rata amount of any outstanding principal on the pari passu note. Pursuant to the notes, the Issuer shall seek to redeem the notes annually each March 15 at a requested redemption amount such that the Issuer’s total adjusted capital following the proposed redemption payment would equal 200% of the Issuer’s “authorized control level RBC”, as such term is defined by the insurance laws of the Commonwealth of Pennsylvania and as prescribed by the PID. All redemptions or repayments of principal and payments of interest on the notes are subject to approval by the PID. Below is a table illustrating the valuation adjustments taken to arrive at the estimated fair value of the OneBeacon Surplus Notes as of December 23, 2014: Type of Surplus Note Millions Seller Priority Pari Passu Total Par Value $ 57.9 $ 43.1 $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 1.6 (8.4 ) (6.8 ) Regulatory approval (2) (4.6 ) (8.0 ) (12.6 ) Liquidity adjustment (3) (11.0 ) (5.7 ) (16.7 ) Total adjustments (14.0 ) (22.1 ) (36.1 ) Fair value $ 43.9 $ 21.0 $ 64.9 (1) Represents the value of the OneBeacon Surplus Notes, at current market yields on 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 delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer’s statutory surplus. T he monetary impact of the anticipated delay is measured based on credit spreads of public securities 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. Esurance For the years ended December 31, 2015 and 2014, White Mountains recorded a gain in discontinued operations of $17.9 million and $3.2 million , which primarily related to a payments from Allstate for favorable development on loss reserves. For the year ended December 31, 2013, White Mountains recorded income from discontinued operations of $4.5 million . Since the closing of the transaction through December 31, 2015, White Mountains has received a net amount of $28.3 million from Allstate, primarily related to the favorable development on loss reserves. (See Note 21 - “Commitments and Contingencies” ) Fireman's Fund During 2014, White Mountains recorded a gain in discontinued operations of $14.0 million from a payment received from Allianz, the purchaser of White Mountains's former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991. Summary of Reclassified Balances and Related Items Net Assets Held for Sale The following summarizes the assets and liabilities associated with the business classified as held for sale, which all relate to Sirius Group with the exception of the building held by OneBeacon at December 31, 2014: December 31, Millions 2015 2014 Assets held for sale Fixed maturity investments, at fair value $ 2,374.0 $ 2,362.3 Short term 352.0 494.9 Common Equities 174.4 189.9 Other 72.2 95.6 Total Investments 2,972.6 3,142.7 Cash 143.9 111.5 Reinsurance recoverable on unpaid losses 283.1 322.2 Reinsurance recoverable on paid losses 10.2 11.4 Insurance premiums receivable 323.6 306.6 Deferred acquisition costs 74.6 69.9 Deferred tax asset 303.1 341.5 Ceded unearned insurance and reinsurance premiums 87.7 76.2 Accounts receivable on unsettled investment sales 29.0 18.7 Intangible assets 10.2 15.2 Other assets 169.0 156.6 Other assets - OneBeacon building — 58.1 Total assets held for sale $ 4,407.0 $ 4,630.6 Liabilities held for sale Loss and loss adjustment expense reserves $ 1,644.4 $ 1,809.8 Unearned insurance premiums 342.2 338.6 Debt 403.5 403.5 Deferred tax liability 263.6 282.8 Accrued incentive compensation 63.2 76.5 Ceded reinsurance payable 67.1 71.5 Funds held under reinsurance treaties 52.9 57.9 Accounts payable on unsettled investment purchases — 2.1 Other liabilities 47.1 62.6 Total liabilities held for sale 2,884.0 3,105.3 Net assets held for sale $ 1,523.0 $ 1,525.3 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. The results of Sirius Group through the closing date of the transaction inures to White Mountains. Year Ended December 31, 2015 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 847.0 $ — $ 847.0 Net investment income 40.7 — 40.7 Net realized and unrealized investment gains 15.1 — 15.1 Other revenue (20.6 ) (.7 ) (21.3 ) Total revenues 882.2 (.7 ) 881.5 Expenses Loss and loss adjustment expenses 422.7 — 422.7 Insurance and reinsurance acquisition expenses 189.8 — 189.8 Other underwriting expenses 107.9 — 107.9 Interest expense on debt 26.6 — 26.6 General and administrative expenses 27.0 — 27.0 Total expenses 774.0 — 774.0 Pre-tax income (loss) 108.2 (.7 ) 107.5 Income tax expense (27.1 ) .2 (26.9 ) Net income (loss) from discontinued operations 81.1 (.5 ) 80.6 Net gain from sales of discontinued operations - OneBeacon — .3 .3 Net gain from sales of discontinued operations - Esurance — 17.9 17.9 Total income from discontinued operations 81.1 17.7 98.8 Change in foreign currency translation and other from discontinued operations (65.0 ) — (65.0 ) Comprehensive income from discontinued operations $ 16.1 $ 17.7 $ 33.8 Year Ended December 31, 2014 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 873.9 $ .1 $ 874.0 Net investment income 40.9 — 40.9 Net realized and unrealized investment gains 205.4 — 205.4 Other revenue (62.4 ) — (62.4 ) Total revenues 1,057.8 .1 1,057.9 Expenses Loss and loss adjustment expenses 345.3 (.7 ) 344.6 Insurance and reinsurance acquisition expenses 193.6 .1 193.7 Other underwriting expenses 129.7 3.5 133.2 Interest expense on debt 26.3 — 26.3 General and administrative expenses 30.5 — 30.5 Total expenses 725.4 2.9 728.3 Pre-tax income (loss) 332.4 (2.8 ) 329.6 Income tax (expense) benefit (70.4 ) 1.0 (69.4 ) Net income (loss) from discontinued operations 262.0 (1.8 ) 260.2 Net gain from sales of discontinued operations - FFIC — 14.0 14.0 Net losses from sales of discontinued operations - OneBeacon — (18.8 ) (18.8 ) Net gain from sales of discontinued operations - Esurance — 3.2 3.2 Total income from discontinued operations 262.0 (3.4 ) 258.6 Change in foreign currency translation and other from discontinued operations (169.5 ) — (169.5 ) Comprehensive income (loss) from discontinued operations $ 92.5 $ (3.4 ) $ 89.1 Year Ended December 31, 2013 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 866.4 $ .8 $ 867.2 Net investment income 46.7 — 46.7 Net realized and unrealized investment gains 27.8 — 27.8 Other revenue 16.8 10.8 27.6 Total revenues 957.7 11.6 969.3 Expenses Loss and loss adjustment expenses 418.4 78.9 497.3 Insurance and reinsurance acquisition expenses 166.5 — 166.5 Other underwriting expenses 126.1 (.2 ) 125.9 Interest expense on debt 26.3 — 26.3 General and administrative expenses 32.2 — 32.2 Total expenses 769.5 78.7 848.2 Pre-tax income (loss) 188.2 (67.1 ) 121.1 Income tax (expense) benefit (46.2 ) 25.0 (21.2 ) Net income (loss) from discontinued operations 142.0 (42.1 ) 99.9 Net gain from sales of discontinued operations — 46.6 46.6 Total income from discontinued operations 142.0 4.5 146.5 Change in foreign currency translation and other from discontinued operations 6.1 — 6.1 Comprehensive income from discontinued operations $ 148.1 $ 4.5 $ 152.6 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash associated with the businesses classified as discontinued operations: Year ended December 31, Millions 2015 2014 2013 Net cash provided from (used for) operations $ 14.7 $ 60.0 $ (61.5 ) Net cash provided from investing activities 31.3 35.8 148.7 Net cash used for financing activities (9.1 ) (63.2 ) (87.5 ) Effect of exchange rate changes on cash (4.5 ) (14.3 ) (.2 ) Net change in cash during the period 32.4 18.3 (.5 ) Cash balances at beginning of period 111.5 93.2 93.7 Cash balances at end of period $ 143.9 $ 111.5 $ 93.2 Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, 2015 2014 2013 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 98.8 $ 258.6 $ 146.5 Allocation of income for participating unvested restricted common shares (1) (1.1 ) (3.3 ) (2.2 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ 97.7 $ 255.3 $ 144.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (3) (68.0 ) (78.9 ) (91.4 ) Basic earnings per share denominator 5,811.2 6,026.0 6,109.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (3) (68.0 ) (78.9 ) (91.4 ) Average outstanding dilutive options to acquire common shares (4) — — — Diluted earnings per share denominator 5,811.2 6,026.0 6,109.0 Basic and diluted earnings per share (in dollars): $ 16.80 $ 42.36 $ 23.63 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net income (loss) attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2015, 2014 and 2013 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13 - “Employee Share-Based Incentive Compensation Plans” ). (4) The diluted earnings (loss) per share denominator for the years ended December 31, 2015, 2014 and 2013 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operations	Fair Value of Financial Instruments The following table summarizes the fair value and carrying value of the Sirius Group financials instruments, the SIG Senior Notes and the SIG Preference Shares, as of December 31, 2015 and 2014: December 31, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value SIG Senior Notes $ 410.0 $ 399.8 $ 437.8 $ 399.7 SIG Preference Shares 255.0 250.0 260.0 250.0 The fair value estimates for the SIG Senior Notes and the SIG Preference Shares have been determined based on indicative broker quotes and are considered to be Level 3 measurements.
Interest Rate Cap | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivatives	Interest Rate Cap In May 2007, SIG issued the SIG Preference Shares, with an initial fixed annual dividend rate of 7.506% . In June 2017, the fixed rate will move to a floating rate equal to the greater of (i) 7.506% or (ii) 3 month LIBOR plus 320 bps. In July 2013, SIG executed an interest rate cap for the period from June 2017 to June 2022 to protect against a significant increase in interest rates during that 5 -year period (the “Interest Rate Cap”). The Interest Rate Cap economically fixes the annual dividend rate on the SIG Preference Shares from June 2017 to June 2022 at 8.30% . The cost of the Interest Rate Cap was an upfront premium of 395 bps of the $250.0 million notional value, or approximately $9.9 million for the full notional amount. The Interest Rate Cap does not qualify for hedge accounting. It is recorded in other assets at fair value. Changes in fair value are recognized as unrealized gains or losses and are presented within other revenues. Collateral held is recorded within short-term investments with an equal amount recognized as a liability to return collateral. The fair value of the interest rate cap has been estimated using a single broker quote and accordingly, has been classified as a Level 3 measurement as of December 31, 2015 . The following table summarizes the change in the fair value of the Interest Rate Cap for the year ended December 31, 2015 : December 31, Millions 2015 2014 Beginning of period $ 4.1 $ 11.1 Net realized and unrealized losses (2.2 ) (7.0 ) End of period $ 1.9 $ 4.1 White Mountains does not provide any collateral to the interest rate counterparties. Under the terms of the Interest Rate Cap, White Mountains holds collateral in respect of future amounts due. White Mountains’s liability to return that collateral is based on the amounts provided by the counterparties and investment earnings thereon. The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty: December 31, 2015 Millions Collateral Balances Held Standard & Poor’s Rating (1) Barclays Bank Plc $ 1.3 A- Nordea Bank Finland Plc .6 AA- Total $ 1.9 (1) Standard & Poor’s ratings as detailed above are: “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings) and “AA-” (Very Strong, which is the fourth highest of twenty-one creditworthiness ratings).

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event Symetra 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.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Selected Quartely Financial Data (Unaudited)	SELECTED QUARTERLY FINANCIAL DATA (Unaudited) Selected quarterly financial data for 2015 and 2014 is shown in the following table. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. As a result of the Sirius sale, Esurance sale and the Runoff Business sale, the results of operations for Sirius, Esurance and the Runoff Business have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation (see Note 22 - “Discontinued Operations” ). 2015 Three Months Ended 2014 Three Months Ended Millions, except per share amounts Dec. 31 Sept. 30 June 30 Mar. 31 Dec. 31 Sept. 30 June 30 Mar. 31 Revenues $ 678.5 $ 334.7 $ 403.6 $ 391.8 $ 405.6 $ 323.2 $ 393.5 $ 331.8 Expenses 420.4 429.3 412.7 391.3 471.3 358.7 359.9 293.2 Pre-tax income 258.1 (94.6 ) (9.1 ) .5 (65.7 ) (35.5 ) 33.6 38.6 Tax benefit (expense) 1.5 1.6 2.2 (4.6 ) 26.2 4.7 (7.1 ) (9.0 ) Income (loss) from continuing operations 259.6 (93.0 ) (6.9 ) (4.1 ) (39.5 ) (30.8 ) 26.5 29.6 Income from discontinued operations, net of tax 7.3 14.2 3.5 73.8 82.1 63.9 61.1 51.5 Non-controlling interest in consolidated subsidiaries (6.1 ) 16.0 .9 7.3 15.0 11.2 (4.6 ) .6 Equity in earnings of unconsolidated affiliates 7.1 3.9 6.8 7.3 12.3 7.0 12.5 13.8 Income (loss) attributable to White Mountains’s common shareholders $ 267.9 $ (58.9 ) $ 4.3 $ 84.3 $ 69.9 $ 51.3 $ 95.5 $ 95.5 Income (loss) attributable to White Mountains’s common shareholders per share: Basic Continuing operations $ 45.99 $ (12.42 ) $ .13 $ 1.76 $ (2.03 ) $ (2.07 ) $ 5.58 $ 7.13 Discontinued operations 1.29 2.41 .59 12.33 13.68 10.49 9.92 8.35 Total consolidated operations $ 47.28 $ (10.01 ) $ .72 $ 14.09 $ 11.65 $ 8.42 $ 15.50 $ 15.48 Diluted Continuing operations $ 45.96 $ (12.42 ) $ .13 $ 1.76 $ (2.03 ) $ (2.07 ) $ 5.58 $ 7.13 Discontinued operations 1.29 2.41 .59 12.33 13.68 10.49 9.92 8.35 Total consolidated operations $ 47.25 $ (10.01 ) $ .72 $ 14.09 $ 11.65 $ 8.42 $ 15.50 $ 15.48

SCHEDULE I SUMMARY OF INVESTMEN

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2015
Summary of Investments, Other than Investments in Related Parties [Abstract]
SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUMMARY OF INVESTMENTS—OTHER THAN INVESTMENTS IN RELATED PARTIES At December 31, 2015 Millions Cost Carrying Value Fair Value Fixed maturity investments: Bonds: U.S. Government and government agencies and authorities $ 160.4 $ 160.0 $ 160.0 Debt securities issued by corporations 1,001.0 1,000.0 1,000.0 States, municipalities and political subdivisions 227.8 228.8 228.8 Mortgage-backed and asset-backed securities 1,170.6 1,167.0 1,167.0 Foreign governments 1.0 1.2 1.2 Redeemable preferred stocks 78.3 82.7 82.7 Total fixed maturity investments 2,639.1 2,639.7 2,639.7 Short-term investments 211.3 211.3 211.3 Common equity securities: Exchange traded funds 147.2 141.8 141.8 Banks, trust and insurance companies 436.9 694.7 694.7 Public utilities — — — Industrial, miscellaneous and other 238.4 277.4 277.4 Total common equity securities 822.5 1,113.9 1,113.9 Other long-term investments (1) 285.9 297.3 297.3 Total investments $ 3,958.8 $ 4,262.2 $ 4,262.2 (1) 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. Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE II CONDENSED FINANCIAL

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT	12 Months Ended
	Dec. 31, 2015
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT	CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT CONDENSED BALANCE SHEETS December 31, Millions 2015 2014 Assets: Cash $ .3 $ .2 Short-term investments, at amortized cost 23.4 31.0 Other assets 6.3 .2 Investments in consolidated and unconsolidated affiliates 4,350.9 4,118.5 Total assets $ 4,380.9 $ 4,149.9 Liabilities: Revolving line of credit $ 50.0 $ — Payable to subsidiary (2) 400.0 161.6 Other liabilities 17.7 (7.4 ) Total liabilities 467.7 154.2 White Mountains’s common shareholders’ equity 3,913.2 3,995.7 Total liabilities and equity $ 4,380.9 $ 4,149.9 CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) Year Ended December 31, Millions 2015 2014 2013 Revenues (including realized gains and losses) $ 5.0 $ .3 $ 1.2 Expenses 59.9 32.3 39.0 Pre-tax loss (54.9 ) (32.0 ) (37.8 ) Income tax benefit (expense) — .9 (.1 ) Net loss (54.9 ) (31.1 ) (37.9 ) Net gain from discontinued operations, net of tax — 13.9 (1) — Equity in earnings from consolidated and unconsolidated affiliates 352.5 329.4 359.5 Net income attributable to White Mountains’s common shareholders 297.6 312.2 321.6 Other comprehensive (loss) income items, after-tax (100.4 ) (101.6 ) (79.8 ) Comprehensive income attributable to White Mountains’s common $ 197.2 $ 210.6 $ 241.8 Computation of net income available to common shareholders: Net income available to common shareholders $ 297.6 $ 312.2 $ 321.6 (1) During 2014, the Company received a payment from Allianz, the purchaser of White Mountains’s former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991. See Note 22 - “Discontinued Operations” . (2) During 2015 and 2014, the Company used cash proceeds received from the issuance of debt from its direct wholly-owned subsidiary primarily to fund repurchases of its common shares. Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes. SCHEDULE II (continued) CONDENSED STATEMENTS OF CASH FLOWS Year Ended December 31, Millions 2015 2014 2013 Net income attributable to White Mountains’s common shareholders $ 297.6 $ 312.2 $ 321.6 Charges (credits) to reconcile net income to net cash from operations: Net realized and unrealized gain on sales of investments — (.2 ) (.2 ) Undistributed current earnings from subsidiaries (352.5 ) (329.4 ) (359.5 ) Net gain on sale of other discontinued operations (7) — (13.9 ) — Other non-cash reconciling items including restricted share and option amortization (.4 ) 12.9 15.4 Net change in other assets and liabilities (2) 21.4 (7.6 ) (2.7 ) Net cash used for continuing operations (33.9 ) (26.0 ) (25.4 ) Net cash provided from discontinued operations (7) — 13.9 — Net cash used for operations (33.9 ) (12.1 ) (25.4 ) Cash flows from investing activities: Net decrease in short-term investments 7.6 (29.6 ) 7.5 Purchases of investment securities (4) — (7.6 ) (26.2 ) Sales and maturities of investment securities — 39.4 61.1 Issuance of debt from subsidiaries (5) 271.0 144.6 153.9 Repayment of debt to subsidiaries (35.0 ) (30.0 ) (10.3 ) Receipt of cash flows from discontinued operations (7) — 13.9 — Contributions to subsidiaries — — — Distributions from subsidiaries (1)(3)(4) 15.0 29.7 .1 Net cash provided from investing activities — continuing operations 258.6 160.4 186.1 Net cash used for investing activities — discontinued operations — (13.9 ) — Net cash provided from investing activities 258.6 146.5 186.1 Cash flows from financing activities: Draw down of revolving line of credit (6) 125.0 65.0 200.0 Repayment of revolving line of credit (5)(6) (75.0 ) (65.0 ) (275.0 ) Proceeds from issuances of common shares — — — Repurchases and retirement of common shares (5) (268.6 ) (128.2 ) (79.8 ) Dividends paid on common shares (6.0 ) (6.2 ) (6.2 ) Net cash used for financing activities — continuing operations (224.6 ) (134.4 ) (161.0 ) Net cash (used for) provided from financing activities — discontinued operations — — — Net cash used for financing activities (224.6 ) (134.4 ) (161.0 ) Net decrease in cash during the year .1 — (.3 ) Cash balance at beginning of year .2 .2 .5 Cash balance at end of year $ .3 $ .2 $ .2 Supplemental cash flow information: Interest paid $ — $ (.3 ) $ (2.0 ) (1) During 2015, the Company received a distribution of $15.0 from Lone Tree Holdings, Ltd., a direct wholly-owned subsidiary. (2) For 2015, net change in other assets and liabilities includes a $2.4 payable to the Company’s subsidiaries. (3) During 2014, the Company received a distribution of $29.7 from Lone Tree Holdings, Ltd. (4) During 2013, the Company received a distribution of $28.0 from Lone Tree Holdings, Ltd. The distribution included $27.9 of fixed maturities and $.1 of cash. Purchases of investment securities excludes the non-cash distribution of $27.9 . (5) During 2015 and 2014, the Company used cash proceeds received from the issuance of debt from its direct wholly-owned subsidiary primarily to fund repurchases of its common shares. During 2013, the Company used cash proceeds received from the issuance of debt from subsidiaries to repurchase $79.8 of its common shares and repay $75.0 of its revolving line of credit. (6) The WTM Bank Facility presented in Note 7 - “Debt” is a direct obligation of the Registrant. (7) During 2014, the Company received a payment from Allianz, the purchaser of White Mountains’s former subsidiary Fireman’s Fund Insurance Company (“FFIC”), related to the utilization of alternative minimum tax credits associated with the tax loss on the sale of FFIC in 1991. See Note 22 - “Discontinued Operations” . Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE III SUPPLEMENTARY INSU

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2015
Supplementary Insurance Information [Abstract]
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTARY INSURANCE INFORMATION (4) (Millions) Column A Column B Column C Column D Column E Column F Column G Column H Column I Column J Column K Segment Millions Deferred acquisition costs Future policy benefits, losses, claims and loss expenses Unearned premiums Other policy claims and benefits payable Premiums earned Net investment income (1) Benefits, claims, losses, and settlement expenses Amortization of deferred policy acquisition costs Other operating expenses Premiums written Years ended: December 31, 2015 OneBeacon (2) $ 100.7 $ 1,389.8 $ 560.3 $ — $ 1,176.2 $ 45.9 $ 700.7 $ 213.8 $ 218.2 $ 1,136.6 HG Global/BAM 6.9 — 50.2 — 3.3 6.1 — 2.9 .4 25.9 Other operations (3) — 6.0 2.1 — 8.7 .2 8.2 3.4 — 10.1 December 31, 2014 OneBeacon (2) $ 103.2 $ 1,342.2 $ 588.3 $ — $ 1,177.1 $ 43.4 $ 815.1 $ 203.3 $ 179.2 $ 1,216.9 HG Global/BAM 4.0 — 27.6 — 1.8 7.1 — 2.1 .4 16.2 Other operations — 7.8 0.8 — 6.1 .2 8.9 0.8 — 5.9 December 31, 2013 OneBeacon (2) $ 103.7 $ 1,054.3 $ 544.9 $ — $ 1,120.4 $ 43.0 $ 622.1 $ 208.9 $ 204.8 $ 1,088.6 Other operations — — — — — .1 — — — — (1) The amounts shown exclude net investment income relating to non-insurance operations of $8.6 , $8.8 and $11.0 for the twelve months ended December 31, 2015, 2014 and 2013 , respectively. (2) The amounts shown excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012 and to AutoOne as of December 31, 2011. See Note 22 - “Discontinued Operations” . (3) The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests. (4) Schedule excludes activity related to Sirius Group for all periods presented. See Note 22 - “Discontinued Operations” . Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE IV REINSURANCE

SCHEDULE IV REINSURANCE	12 Months Ended
	Dec. 31, 2015
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
SCHEDULE IV REINSURANCE	WHITE MOUNTAINS INSURANCE GROUP, LTD. REINSURANCE (3) (Millions) Column A Column B Column C Column D Column E Column F Premiums earned $ in millions Gross amount Ceded to other companies Assumed from other companies Net amount Percentage of amount assumed to net Years ended: December 31, 2015 OneBeacon (1) $ 1,298.0 $ (167.7 ) $ 45.9 $ 1,176.2 3.9 % HG/BAM 3.3 — — 3.3 — % Other operations (2) 20.7 (12.0 ) — 8.7 — December 31, 2014 OneBeacon (1) $ 1,209.1 $ (102.9 ) $ 70.9 $ 1,177.1 6.0 % HG/BAM 1.8 — — 1.8 — % Other operations (2) 22.6 (16.5 ) — 6.1 — December 31, 2013 OneBeacon (1) $ 1,043.3 $ (71.4 ) $ 148.5 $ 1,120.4 13.3 % (1) The amounts shown excludes balances reclassified to discontinued operations in the consolidated income statement related to the Runoff Transaction as of December 31, 2015, 2014 and 2013. See Note 22 - “Discontinued Operations” . (2) The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests. (3) Schedule excludes activity related to Sirius Group for all periods presented. See Note 22 - “Discontinued Operations” . Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE V VALUATION AND QUALIF

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	WHITE MOUNTAINS INSURANCE GROUP, LTD. VALUATION AND QUALIFYING ACCOUNTS (2) Column A Column B Column C Column D Column E Additions (subtractions) Millions Balance at beginning of period Charged to costs and expenses Charged to other accounts Deductions described (1) Balance at end of period Years ended: December 31, 2015 Reinsurance recoverable on paid losses: Allowance for reinsurance balances $ 2.2 $ (.1 ) $ — $ (1.4 ) $ .7 Property and casualty insurance and reinsurance premiums receivable: Allowance for uncollectible accounts 1.9 .8 (.7 ) — 2.0 Accounts receivable: Allowance for uncollectible accounts 3.3 3.2 — (.7 ) 5.8 December 31, 2014 Reinsurance recoverable on paid losses: Allowance for reinsurance balances $ 14.3 $ (.5 ) $ — $ (11.6 ) $ 2.2 Property and casualty insurance and reinsurance premiums receivable: Allowance for uncollectible accounts 3.1 (.2 ) (1.0 ) — 1.9 Accounts receivable: Allowance for uncollectible accounts — 3.6 — (.3 ) 3.3 December 31, 2013 Reinsurance recoverable on paid losses: Allowance for reinsurance balances $ 25.6 $ — $ — $ (11.3 ) $ 14.3 Property and casualty insurance and reinsurance premiums receivable: Allowance for uncollectible accounts 4.4 .3 — (1.6 ) 3.1 (1) Represents net collections (charge-offs) of balances receivable. (2) Schedule excludes amounts related to Sirius Group for all periods presented. See Note 22 - “Discontinued Operations” . Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

SCHEDULE VI SUPPLEMENTAL INFORM

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS	12 Months Ended
	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS	WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (4) (Millions) Column A Column B Column C Column D Column E Column F Column G Column H Column I Column J Column K Affiliation with registrant Deferred acquisition costs Reserves Expenses Discount, if any, deducted in Column C Unearned Premiums Earned Premiums Net investment income Claims and Claims Adjustment Expenses Incurred Related to Amortization costs Paid Expenses Premiums written Millions Current Year (1) Prior Year OneBeacon (2) : 2015 $ 100.7 $ 1,389.8 $ 1.1 (1) $ 560.3 $ 1,176.2 $ 45.9 $ 702.5 $ (1.8 ) $ (213.8 ) $ 677.5 $ 1,136.6 2014 103.2 1,342.2 1.0 (1) 588.3 1,177.1 43.4 725.3 89.8 (203.3 ) 608.6 1,216.9 2013 103.7 1,054.3 3.0 (1) 544.9 1,120.4 43.0 622.1 — (208.9 ) 540.7 1,088.6 Other operations (3) : 2015 $ — $ 6.0 $ — $ 2.1 $ 8.7 $ .2 $ 10.4 $ (2.2 ) $ — $ 10.4 $ 10.1 2014 — 7.8 — 0.8 6.1 .2 6.7 2.2 — 14.8 5.9 2013 — — — — — — — — — — — (1) The amounts shown represent OneBeacon’s discount on its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using a discount rate which is determined based on the facts and circumstances applicable at the time the claims are settled ( 2.5% , 2.5% and 3.5% as of December 31, 2015, 2014 and 2013 ). (2) The amounts shown excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013. See Note 22 - “Discontinued Operations” . (3) The Other operations amounts shown relate to SSIE. SSIE’s results are attributed to non-controlling interests. (4) Schedule excludes amounts related to Sirius Group for all periods presented. See Note 22 - “Discontinued Operations” . Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

Summary of Significant Accoun38

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Revenue Recognition, Services, Commissions [Policy Text Block]	Commission and Other Revenue Recognition White Mountains recognizes agent commissions and other revenues when it has fulfilled all of its obligations necessary to earn the revenue and when it can reliably estimate both the amount of revenue, net of any amounts expected to be uncollectible, and any amounts associated with expected cancellations.
Investment securities	Investment Securities As of December 31, 2015 , White Mountains’s invested assets consisted of securities and other investments held for general investment purposes. White Mountains’s portfolio of fixed maturity investments and common equity securities held for general investment purposes are classified as trading and are reported at fair value as of the balance sheet date. Changes in unrealized gains and losses are reported pre-tax in revenues. Realized investment gains and losses are accounted for using the specific identification method and are reported pre-tax in revenues. Premiums and discounts on all fixed maturity investments are amortized and accreted to income over the anticipated life of the investment. White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common and preferred equity securities, convertible fixed maturity and preferred investments and other long-term investments, such as interests in hedge funds, private equity funds, direct investments in privately held common and convertible securities and the surplus notes issued in connection with the Runoff Transaction (“OneBeacon Surplus Notes”). In determining its estimates of fair value, White Mountains uses a variety of valuation approaches and inputs. Whenever possible, White Mountains estimates fair value using valuation methods that maximize the use of quoted prices and other observable inputs. As of December 31, 2015 and 2014 , approximately 91% and 88% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equity securities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs are comprised of fixed maturity investments including corporate debt, state and other governmental debt, convertible fixed maturity and preferred investments and mortgage and asset-backed securities. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include White Mountains’s investments in hedge funds and private equity funds, as well as investments in certain debt securities, including asset-backed securities, where quoted market prices are unavailable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services used by White Mountains have indicated that if no observable inputs are available for a security, they will not provide a price. In those circumstances, White Mountains estimates the fair value using industry standard pricing models and observable inputs such as benchmark interest rates, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs. White Mountains performs procedures to validate the market prices obtained from the outside pricing sources. Such procedures, which cover substantially all of its fixed maturity investments include, but are not limited to, evaluation of model pricing methodologies and a review of the pricing services’ quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices, and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected sales activity to determine whether there are any significant differences between the market price used to value the security prior to sale and the actual sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Prices that have not changed from period to period and prices that have trended unusually compared to market conditions are also considered outliers. In circumstances where the results of White Mountains’s review process do not appear to support the market price provided by the pricing services, White Mountains challenges the price. During the past year, 8 securities fell outside White Mountains’s expected results, thereby triggering the challenge with the pricing service. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The fair values of such securities are considered to be Level 3 measurements. White Mountains’s investments in debt securities, including asset-backed securities, are generally valued using matrix and other pricing models. 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. Short-term investments consist of 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 cost, which approximated fair value as of December 31, 2015 and 2014 .
Other long-term investments	Other Long-term Investments Other long-term investments consist primarily of hedge funds, private equity funds, other investments in limited partnerships, private equity securities and surplus note investments (see Note 5 - “Investment Securities” ). White Mountains has taken the fair value option for most of its investments in hedge funds, private equity funds, direct investments in privately held common and convertible securities, the OneBeacon surplus notes and various other long-term investments. For the investments for which White Mountains has taken the fair value option, changes in fair value are reported in revenues on a pre-tax basis. For those long-term investments for which White Mountains has not made the fair value election, White Mountains accounts for its interests under the equity method.
Derivative financial instruments	Derivative Financial Instruments White Mountains holds a variety of derivative financial instruments for both risk management and investment purposes. White Mountains recognizes all derivatives as either assets or liabilities, measured at fair value, in the consolidated balance sheets. Changes in the fair value of derivative instruments are recognized in current period pre-tax income.
Warrants	Warrants As of December 31, 2012, White Mountains held warrants to purchase 9.49 million common shares of Symetra, which were included as investments in unconsolidated affiliates. The Symetra warrants held by White Mountains were entitled to dividends declared to common shareholders. On June 20, 2013, White Mountains exercised its warrants in a cashless transaction and received 2.65 million common shares of Symetra in exchange for the warrants. White Mountains also holds warrants that it has received in the restructuring (e.g., securities received from bankruptcy proceedings) of certain of its common equity and/or fixed maturity investments. White Mountains accounts for its investments in warrants as derivatives.
Derivatives - Variable annuity reinsurance	Derivatives—Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan through its wholly owned subsidiary, WM Life Re. The accounting for benefit guarantees differs depending on whether or not the guarantee is classified as a derivative or an insurance liability. Guaranteed minimum accumulation benefits (“GMABs”) are paid to an annuitant for any shortfall between accumulated account value at the end of the accumulation period and the annuitant’s total deposit, less any withdrawal payments made to the annuitant during the accumulation period. GMABs meet the definition of a derivative for accounting purposes. Therefore, GMABs are carried at fair value, with changes thereon recognized in income in the period of the change. The liability for the reinsured GMAB contracts has been determined using internal valuation models that use assumptions for interest rates, equity markets, foreign exchange rates and market volatilities at the valuation date, as well as annuitant-related actuarial assumptions, including surrender and mortality rates. If an annuitant dies during the accumulation period of an annuity contract, guaranteed minimum death benefits (“GMDBs”) are paid to the annuitant’s beneficiary for shortfalls between accumulated account value at the time of an annuitant’s death and the annuitant’s total deposit, less any living benefit payments or withdrawal payments previously made to the annuitant. White Mountains has elected to measure its GMDB liabilities at fair value. The valuation of these liabilities involves significant judgment and is subject to change based upon changes in capital market assumptions and emerging surrender and mortality experience of the underlying contracts in force. WM Life Re has entered into derivative contracts that are designed to economically hedge against changes in the fair value of living and death benefit liabilities associated with its variable annuity reinsurance arrangements. The derivatives include futures and over-the-counter option contracts on interest rates, major bond and equity indices, and foreign currencies. All WM Life Re’s derivative instruments are recorded as assets or liabilities at fair value on the balance sheet within other assets. These derivative financial instruments do not meet the criteria for hedge accounting treatment, and accordingly, changes in fair value are recognized in the current period as gains or losses in the income statement within other revenues. WM Life Re includes the effect of counterparty credit risk when determining the fair value of its derivative contracts and its GMAB and GMDB liabilities.
Cash	Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries and exclude changes in amounts of restricted cash (See Note 9 - “Derivatives” ).
Insurance and reinsurance operations	Insurance and Reinsurance Operations White Mountains accounts for insurance and reinsurance policies that it writes in accordance with ASC 944. Premiums written are recognized as revenues and are earned ratably over the term of the related policy or reinsurance treaty. Unearned premiums represent the portion of premiums written that are applicable to future insurance or reinsurance coverage provided by policies or treaties in force. White Mountains charges fees on certain of its insurance policies. Refundable fees are classified with premiums and recognized in earnings over the policy term. Fees that represent a reimbursement of expenses, such as installment fees, are recorded as a reduction of underwriting expenses. Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as insurance and reinsurance acquisition expenses. Amortization of deferred acquisition costs are presented within insurance and reinsurance acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses (“LAE”), expected dividends to policyholders, unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. Losses and LAE are charged against income as incurred. Unpaid insurance losses and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance losses and LAE are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management’s best estimate of ultimate losses and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any resulting adjustments are reflected in current operations. The process of estimating loss and LAE involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements. OneBeacon discounts certain of its long-term workers compensation loss and LAE reserves when such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual claim basis. OneBeacon discounts these reserves using an average discount rate which is determined based on the various assumptions including consideration of when the claims will be settled ( 2.5% as of both December 31, 2015 and 2014 ). As of December 31, 2015 and 2014, the discount on OneBeacon’s workers compensation loss and LAE reserves amounted to $1.1 million and $1.0 million . White Mountains’s insurance and reinsurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. Such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies White Mountains for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. White Mountains has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro-rata basis. The amount of each risk ceded by White Mountains is subject to maximum limits which vary by line of business and type of coverage. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectability of reinsurance recoverables is subject to the solvency of the reinsurers. White Mountains is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with a strong financial condition, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis. Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly. Funds held by ceding companies represent amounts due to White Mountains in connection with certain assumed reinsurance agreements in which the ceding company retains a portion of the premium to provide security against future loss payments. The funds held by ceding companies are generally invested by the ceding company and a contractually agreed interest amount is credited to the Company and recognized as investment income. Funds held under insurance and reinsurance contracts represent contractual payments due to White Mountains that have been retained to secure such obligations. Such amounts are recorded as liabilities in the consolidated financial statements. Accruals for contingent commission liabilities are established for reinsurance contracts that provide for the stated commission percentage to increase or decrease based on the loss experience of the contract. Changes in the estimated liability for such arrangements are recorded as contingent commissions. Accruals for contingent commission liabilities are determined through the review of the contracts that have these adjustable features and are estimated based on expected loss and LAE.
Municipal Bond Insurance	Municipal Bond Insurance All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are generally received upfront and an unearned premium revenue liability, equal to the amount of the premium received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. Deferred acquisition costs represent commissions, premium taxes, excise taxes and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income.
Funds Held Under Reinsurance Contracts Policy [Policy Text Block]	Funds Held Funds held under reinsurance contracts primarily represent amounts due to White Mountains in connection with the Standard Reinsurance Agreement (“SRA”) with the Federal Crop Insurance Corporation (“FCIC”), which is managed by an agency of the U.S. Department of Agriculture. The SRA governs the relationship, including the exchange of funds, between private insurance companies, including White Mountains, and the FCIC relating to our MPCI crop insurance business. Funds held under insurance contracts represents unrestricted collateral held by White Mountains primarily relating to the surety business.
Mandatory Shared Market Mechanisms	Mandatory Shared Market Mechanisms As a condition to its licenses to do business in certain states, White Mountains’s insurance operations must participate in various mandatory shared market mechanisms commonly referred to as “residual” or “involuntary” markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer’s market share of voluntary business written within that state. In certain cases, White Mountains is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms is accounted for as direct insurance business or as assumed reinsurance depending upon the structure of the mechanism. OneBeacon’s market assignments are typically required to be written in the current period, however, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated.
Insurance-Related Assessments	Insurance-related Assessments Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains records guaranty fund assessments when it is probable that an assessment will be made and the amount can be reasonably estimated.
Deferred Software Costs	Deferred Software Costs White Mountains capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll-related costs. White Mountains begins amortization of these costs once the project is completed and ready for its intended use. Amortization is on a straight-line basis and over a useful life of eighteen months to five years. Costs related to software developed for sale to third parties are expensed until technological feasibility has been established. Once technological feasibility has been established, software development costs are capitalized and reported at their net realizable value. Upon product release, the amortization of software development costs is determined annually as the greater of the amount calculated using the ratio of current gross revenues to the total of current and expected gross revenues for the product or the straight-line method over the estimated economic life, which is generally between 18 to 36 months.
Federal and foreign income taxes	Federal and Foreign Income Taxes A significant portion of White Mountains’s subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States. Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.
Foreign currency exchange	Foreign Currency Exchange The U.S. dollar is the functional currency for all of White Mountains’s businesses except for Sirius International, the Canadian reinsurance operations of Sirius America and certain other smaller international activities. White Mountains also invests in securities denominated in foreign currencies. Assets and liabilities recorded in these foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the average exchange rates for the period. Net foreign exchange gains and losses arising from the translation of functional currencies are generally reported in shareholders’ equity, in accumulated other comprehensive income or loss. Assets and liabilities relating to foreign operations are translated into the functional currency using current exchange rates; revenues and expenses are translated into the functional currency using the weighted average exchange rate for the period. The resulting exchange gains and losses are reported as a component of net income in the period in which they arise.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the amount paid to acquire subsidiaries over the fair value of identifiable net assets at the date of acquisition. Other intangible assets consist primarily of trademarks, URL and online names, customer relationships, information technology and insurance licenses. Finite-life intangible assets are measured at their acquisition date fair values, are amortized over their economic lives and presented net of accumulated amortization on the balance sheet. Goodwill is not amortized, but rather is evaluated for impairment on an annual basis, or whenever indications of potential impairment exist. In the absence of any indications of potential impairment, the evaluation of goodwill is performed during the fourth quarter of each year. White Mountains initially evaluates goodwill using a qualitative approach (“step zero”) to determine whether it is more likely than not that the fair value of goodwill is greater than its carrying value. If the results of the qualitative evaluation indicate that it is more likely than not that the carrying value of goodwill exceeds its fair value, White Mountains, performs the two-step quantitative test for impairment. Other intangible assets with finite lives are evaluated for impairment at least annually and when events or changes in circumstances indicate that it is more likely than not that the asset is impaired. White Mountains evaluated the goodwill and other intangible assets associated with Tranzact, MediaAlpha and Wobi and did not recognize any impairment losses for any of the years ended December 31, 2015, 2014 and 2013 (See Note 6 - “Goodwill and Other Intangible Assets” ).
Noncontrolling Interest	Non-controlling Interest Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries, and are presented separately on the balance sheet. The portion of comprehensive income attributable to non-controlling interests is presented net of related income taxes in the statement of operations and comprehensive income (See Note 14 - “Common Shareholders’ Equity and Non-controlling Interests”) .
Variable Interest Entities	Variable Interest Entities White Mountains consolidates a variable interest entity (“VIE”) when it has both the power to direct the activities of the VIE that most significantly impact its economic performance and either the obligation to absorb losses or the right to receive returns from the VIE that could potentially be significant to the VIE (See Note 18 - “Variable Interest Entities” ).
Recent Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity On April 10, 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity (ASC 205 and ASC 360) to reduce diversity in practice for reporting discontinued operations. ASU 2014-08 limits discontinued operations treatment to disposals that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. As discussed further in Note 2, White Mountains has entered into an agreement to sell Sirius Group, which has been classified as discontinued operations in accordance with ASU 2014-08. Qualified Affordable Housing Projects Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects ( “ QAHP ” ) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.4 million . The retrospective adoption resulted in an increase of $1.7 million and $1.9 million to net investment income and a net increase of $2.3 million and $2.2 million to income tax expense, respectively for the years ended December 31, 2014 and 2013. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above. Pushdown Accounting ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014 . The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections . White Mountains has not had any acquisitions for which it has elected to apply pushdown accounting since ASU 2014-17 became effective. Unrecognized Tax Benefits Effective January 1, 2014, White Mountains adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss (“NOL”) carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance became effective for White Mountains on January 1, 2014. Adoption did not have any impact on White Mountains's financial condition, results of operations or cash flows or financial statement presentation. Recent Accounting Pronouncements Financial Instruments - Recognition and Measurement On January 5, 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities . 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 simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment 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. Business Combinations - Measurement Period Adjustments On September 25, 2015, the FASB issued ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments. At the date of an acquisition, fair value of certain assets and liabilities may not be accurately determinable and are therefore recognized at the acquirer's best estimate. Such amounts may be updated as additional information becomes available in periods subsequent to the acquisition for up to one year. Prior to the issuance of this new ASU, subsequent adjustments had to be pushed back to the acquisition date, which required retroactive adjustments to prior period amounts. Under the new guidance, adjustments to provisional amounts that are identified during the measurement period are to be recorded in the reporting period in which the adjustment amounts are determined. ASU 2015-16 is effective for fiscal years beginning after December 15, 2015 and is applied prospectively to adjustments to estimated purchase accounting amounts that occur after the effective date. Early application is permitted. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year ended December 31, 2015. Short-Duration Contracts On May 21, 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 disaggregated 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. Fair Value Measurements On May 1, 2015, the FASB issued 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 and has classified those measurements within Level 3 of the fair value hierarchy. The new guidance is effective for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years. Debt Issuance Costs On April 7, 2015, the FASB issued ASU 2015-03, Imputation of Interest (ASC 835) which requires debt issuance costs related to a recognized debt liability to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. On August 18, 2015, the FASB issued ASU 2015-15, Imputation of Interest (ASC 835), which addresses the presentation and subsequent measurement of debt issuance costs related to line-of-credit arrangements. ASU 2015-03 and ASU 2-15-15 are effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 or ASU 2015-15 to impact its financial position, results of operations, cash flows, presentation and disclosures. Amendments to Consolidation Analysis On February 18, 2015, the FASB issued 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. ASU 2015-02 is effective for annual and interim reporting periods beginning after December 15, 2015 and must be applied retrospectively. White Mountains does not expect ASU 2015-02 to affect the consolidation analysis for any of its existing investments. Share-Based Compensation Awards On June 19, 2014, the FASB issued 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 is intended to eliminate diversity in practice for employee share-based awards containing performance targets that could be achieved after the requisite service period. Some reporting entities account for performance targets that can be achieved after the requisite service period as performance conditions that affect the vesting of the award while other reporting entities treat those performance targets as non-vesting conditions that affect the grant-date fair value of the award. The updated 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 should be recognized in the period it becomes probable that the performance target will be achieved and is attributable to the periods for which service has been rendered. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect adoption to have a significant effect on its financial position, results of operations, cash flows, presentation or disclosures. Revenue Recognition On May 28, 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, commissions and other non-insurance revenues. 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. In August 2015, The FASB issued ASU 2015-14, Revenue from Contracts with Customers (ASC 606) , which delays the effective date of ASU 2014-09, which is now effective for annual and interim reporting periods beginning after December 15, 2017. White Mountains is evaluating the expected impact of this new guidance and available adoption methods.

Summary of Significant Accoun39

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of rates of exchange for the U.S. dollar that have been used for the most significant operations	The following rates of exchange for the U.S. dollar have been used for the most significant operations: Currency Opening Rate 2015 Closing Rate 2015 Opening Rate 2014 Closing Rate 2014 Swedish kronor 7.7737 8.4247 6.4339 7.7737 British pound 0.6426 0.6757 0.6044 0.6426 Euro 0.8245 0.9189 0.7259 0.8245

Reserves for Unpaid Losses an40

Reserves for Unpaid Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2015
Insurance Loss Reserves [Abstract]
Loss and LAE reserve activities	The following table summarizes the loss and LAE reserve activities of White Mountains’s insurance subsidiaries for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, Millions 2015 2014 2013 Gross beginning balance $ 1,350.0 $ 1,054.3 $ 1,000.0 Less beginning reinsurance recoverable on unpaid losses (161.7 ) (80.2 ) (107.3 ) Net loss and LAE reserves 1,188.3 974.1 892.7 Loss and LAE reserves consolidated — SSIE — 13.6 — Losses and LAE incurred relating to: Current year losses 712.9 732.0 622.1 Prior year losses (4.0 ) 92.0 — Total incurred losses and LAE 708.9 824.0 622.1 Loss and LAE paid relating to: Current year losses (208.8 ) (202.6 ) (188.6 ) Prior year losses (479.1 ) (420.8 ) (352.1 ) Total loss and LAE payments (687.9 ) (623.4 ) (540.7 ) Net ending balance 1,209.3 1,188.3 974.1 Plus ending reinsurance recoverable on unpaid losses 186.5 161.7 80.2 Gross ending balance $ 1,395.8 $ 1,350.0 $ 1,054.3
Loss and LAE Reserve and Net Loss and LAE Development	The components of the 2014 fourth quarter loss and LAE reserve increase and the net loss and LAE development for the full year are provided below: Underwriting Unit 2014 Fourth Quarter Reserve Increases Full Year 2014 Millions Current Accident Year Prior Accident Year Total Net Prior Year Development Professional Insurance $ 22.9 $ 46.4 $ 69.3 $ 59.1 Specialty Property (1.1 ) 5.7 4.6 1.1 Crop 3.8 — 3.8 — Other 2.8 (.4 ) 2.4 1.6 Specialty Products 28.4 51.7 80.1 61.8 Entertainment 1.5 11.6 13.1 13.5 Accident — 3.5 3.5 6.0 Government Risks 1.2 7.1 8.3 8.5 Other 2.6 1.6 4.2 — Specialty Industries 5.3 23.8 29.1 28.0 Total $ 33.7 $ 75.5 $ 109.2 $ 89.8

Third Party Reinsurance (Tables

Third Party Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Schedule of direct assumed and ceded amounts	The effects of reinsurance on White Mountains’s insurance and reinsurance subsidiaries’ written and earned premiums and on losses and LAE were as follows (see Note 10 - “Municipal Bond Guarantee Insurance” for balances related to White Mountains financial guarantee business): Year ended December 31, 2015 Millions OneBeacon HG/BAM (1) Other Total Written premiums: Direct $ 1,279.9 $ 25.9 $ 19.9 $ 1,325.7 Assumed 36.0 — — 36.0 Gross written premiums 1,315.9 25.9 19.9 1,361.7 Ceded (179.3 ) (2) — (9.8 ) (189.1 ) Net written premiums $ 1,136.6 $ 25.9 $ 10.1 $ 1,172.6 Earned premiums: Direct $ 1,298.0 $ 3.3 $ 20.7 $ 1,322.0 Assumed 45.9 — — 45.9 Gross earned premiums 1,343.9 3.3 20.7 1,367.9 Ceded (167.7 ) (2) — (12.0 ) (179.7 ) Net earned premiums $ 1,176.2 3.3 $ 8.7 $ 1,188.2 Losses and LAE: Direct $ 783.0 $ — $ 19.5 $ 802.5 Assumed 55.7 — — 55.7 Gross losses and LAE 838.7 — 19.5 858.2 Ceded (138.0 ) (2) — (11.3 ) (149.3 ) Net losses and LAE $ 700.7 $ — $ 8.2 $ 708.9 (1) During 2015, BAM ceded $19.3 in written premiums and $16.0 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment. (2) During 2015, OneBeacon recorded ceded $ 33.3 in written premiums, $ 33.3 in earned premiums and $33.4 in loss and loss adjustment expenses as a result of the exit of the Crop Business due to the 100% quota share reinsurance agreement with AmTrust. Year ended December 31, 2014 Millions OneBeacon HG/BAM (1) Other (2) Total Written premiums: Direct $ 1,257.5 $ 16.2 $ 22.6 $ 1,296.3 Assumed 65.9 — — 65.9 Gross written premiums 1,323.4 16.2 22.6 1,362.2 Ceded (106.5 ) — (16.7 ) (123.2 ) Net written premiums $ 1,216.9 $ 16.2 $ 5.9 $ 1,239.0 Earned premiums: Direct $ 1,209.1 $ 1.8 $ 22.6 $ 1,233.5 Assumed 70.9 — — 70.9 Gross earned premiums 1,280.0 1.8 22.6 1,304.4 Ceded (102.9 ) — (16.5 ) (119.4 ) Net earned premiums $ 1,177.1 1.8 $ 6.1 $ 1,185.0 Losses and LAE: Direct $ 778.7 $ — $ 24.1 $ 802.8 Assumed 115.7 — — 115.7 Gross losses and LAE 894.4 — 24.1 918.5 Ceded (79.3 ) — (15.2 ) (94.5 ) Net losses and LAE $ 815.1 $ — $ 8.9 $ 824.0 (1) During 2014, BAM ceded $ 12.3 in written premiums and $ 1.4 in earned premiums to HG Global, which have been eliminated within the HG/BAM segment. (2) During 2014, SSIE ceded $16.0 in written premiums, $ 15.7 in earned premiums, and $16.9 in loss and loss adjustment expenses to OneBeacon, which have been eliminated in consolidation. Year ended December 31, 2013 Millions OneBeacon HG/BAM (1) Total Written premiums: Direct $ 1,103.1 $ 13.6 $ 1,116.7 Assumed 59.8 — 59.8 Gross written premiums 1,162.9 13.6 1,176.5 Ceded (74.3 ) — (74.3 ) Net written premiums $ 1,088.6 $ 13.6 $ 1,102.2 Earned premiums: Direct $ 1,043.3 $ .5 $ 1,043.8 Assumed 148.5 — 148.5 Gross earned premiums 1,191.8 .5 1,192.3 Ceded (71.4 ) — (71.4 ) Net earned premiums $ 1,120.4 $ .5 $ 1,120.9 Losses and LAE: Direct $ 584.9 $ — $ 584.9 Assumed 76.3 — 76.3 Gross losses and LAE 661.2 — 661.2 Ceded (39.1 ) — (39.1 ) Net losses and LAE $ 622.1 $ — $ 622.1 (1) During 2013, BAM ceded $ 10.6 in written premiums ($ 10.2 in earned premiums) to HG Global, which have been eliminated within the HG/BAM segment.
OneBeacon
Ceded Credit Risk [Line Items]
Ceded Credit Risk	The following table summarizes Standard & Poor’s Financial Services LLC (“Standard & Poor’s”) ratings for OneBeacon’s reinsurers. Standard & Poor’s Rating (1) $ in millions Balance at December 31, 2015 % of Total AA $ 41.7 22 % A 126.2 65 % BBB, Not rated and other (2) 25.6 13 % Total $ 193.5 100 % (1) Standard & Poor’s ratings as detailed above are: “AA” (Very strong), “A” (Strong) and “BBB” (Adequate). (2) Includes $20.4 related to OBIC, an unrated entity sold to Armour as part of the Runoff Transaction.

Investment Securities (Tables)

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Par Value to Fair Value Reconciliation of Surplus Notes [Table Text Block]	Below is a table illustrating the valuation adjustments taken to arrive at the estimated fair value of the OneBeacon surplus notes as of December 31, 2015 and 2014: Type of Surplus Note Total as of December 31, 2015 Total as of December 31, 2014 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) (.4 ) (14.7 ) (15.1 ) (6.6 ) Regulatory approval (2) (11.7 ) (12.5 ) (24.2 ) (12.6 ) Liquidity adjustment (3) (7.8 ) (2.4 ) (10.2 ) (16.7 ) Total adjustments (19.9 ) (29.6 ) (49.5 ) (35.9 ) Fair value (4) $ 38.0 $ 13.5 $ 51.5 $ 65.1 (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 delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer’s statutory surplus. T he monetary impact of the anticipated delay is measured based on credit spreads of public securities 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 decrease in the fair value of the surplus notes during the twelve months ended December 31, 2015 was primarily due to widening of non-investment grade credit spreads.
Pre-tax net investment income	Pre-tax net investment income for 2015, 2014 and 2013 consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Investment income: Fixed maturity investments $ 52.6 $ 51.3 $ 48.1 Short-term investments .2 .1 .8 Common equity securities 10.1 16.6 16.5 Other long-term investments 3.3 4.4 6.8 Total investment income 66.2 72.4 72.2 Third-party investment expenses (5.4 ) (12.9 ) (12.4 ) Net investment income, pre-tax $ 60.8 $ 59.5 $ 59.8
Net Realized and Unrealized Investment Gains and Losses	Net realized and unrealized investment gains (losses) consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Net realized investment gains, pre-tax $ 77.3 $ 166.8 $ 95.5 Net unrealized investment gains (losses), pre-tax 148.1 (88.3 ) 38.4 Net realized and unrealized investment gains, pre-tax 225.4 78.5 133.9 Income tax expense attributable to net realized and unrealized investment gains (35.2 ) (18.0 ) (12.8 ) Net realized and unrealized investment gains, after tax $ 190.2 $ 60.5 $ 121.1
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 years ended December 31, 2015, 2014 and 2013 . Year Ended December 31, Millions 2015 2014 2013 Fixed maturity investments $ (1.1 ) $ 1.9 $ (1.6 ) Common equity securities (9.0 ) 5.8 .9 Other long-term investments (40.1 ) — 2.1 Total net unrealized investment (losses) gains, pre-tax - Level 3 investments $ (50.2 ) $ 7.7 $ 1.4
Net realized and unrealized investment gains, after-tax, as recorded on the statements of operations and comprehensive income (losses)	The components of White Mountains’s net realized and unrealized investment gains (losses), after-tax, as recorded on the statements of operations and comprehensive income were as follows: Year Ended December 31, Millions 2015 2014 2013 Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates $ (39.2 ) $ 81.2 $ (106.4 ) Income tax benefit (expense) 2.9 (5.9 ) 8.3 Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax (36.3 ) 75.3 (98.1 ) Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra 1.4 — — Total investment (losses) gains through accumulated other comprehensive income (34.9 ) 75.3 (98.1 ) Net realized and unrealized investment gains, after-tax 190.2 60.5 121.1 Total investment gains recorded during the period, after-tax $ 155.3 $ 135.8 $ 23.0
Realized Gain (Loss) on Investments	Net realized investment gains for 2015, 2014 and 2013 consisted of the following: Year ended December 31, 2015 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ 1.9 $ — $ 1.9 Common equity securities 64.4 .4 64.8 Other long-term investments 10.6 — 10.6 Net realized investment gains, pre-tax 76.9 .4 77.3 Income tax expense attributable to realized investment gains (22.8 ) — (22.8 ) Net realized investment gains, after-tax $ 54.1 $ .4 $ 54.5 Year ended December 31, 2014 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ 5.8 $ — $ 5.8 Common equity securities 138.0 — 138.0 Other long-term investments 23.0 — 23.0 Net realized investment gains, pre-tax 166.8 — 166.8 Income tax expense attributable to realized investment gains (28.4 ) — (28.4 ) Net realized investment gains, after-tax $ 138.4 $ — $ 138.4 Year ended December 31, 2013 Millions Net realized gains (losses) Net foreign Total changes in Fixed maturity investments $ (.2 ) $ (.2 ) $ (.4 ) Short-term investments .1 — .1 Common equity securities 89.4 — 89.4 Other long-term investments 6.4 — 6.4 Net realized investment gains (losses), pre-tax 95.7 (.2 ) 95.5 Income tax (expense) benefit attributable to realized investment gains (losses) (16.2 ) .1 (16.1 ) Net realized investment gains (losses), after-tax $ 79.5 $ (.1 ) $ 79.4
Unrealized Gain (Loss) on Investments	The following table summarizes net unrealized investment gains (losses) and changes in the carrying value of investments measured at fair value: Year ended December 31, 2015 Millions Net unrealized gains (losses) Net foreign exchange gains (losses) Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (15.6 ) $ — $ (15.6 ) Common equity securities 207.6 (3.7 ) 203.9 Other long-term investments (39.1 ) (1.1 ) (40.2 ) Net unrealized investment gains (losses), pre-tax 152.9 (4.8 ) 148.1 Income tax expense attributable to unrealized investment gains (losses) (12.3 ) (.1 ) (12.4 ) Net unrealized investment gains (losses), after-tax $ 140.6 $ (4.9 ) $ 135.7 Year ended December 31, 2014 Millions Net Net Total net unrealized Fixed maturity investments $ 11.3 $ — $ 11.3 Common equity securities (83.0 ) (7.7 ) (90.7 ) Other long-term investments (7.6 ) (1.3 ) (8.9 ) Net unrealized investment losses, pre-tax (79.3 ) (9.0 ) (88.3 ) Income tax benefit attributable to unrealized investment losses 9.9 .5 10.4 Net unrealized investment losses, after-tax $ (69.4 ) $ (8.5 ) $ (77.9 ) Year ended December 31, 2013 Millions Net unrealized gains (losses) Net foreign exchange gains (losses) Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (49.9 ) $ — $ (49.9 ) Common equity securities 86.1 (1.9 ) 84.2 Other long-term investments 4.1 — 4.1 Net unrealized investment gains (losses), pre-tax 40.3 (1.9 ) 38.4 Income tax benefit attributable to unrealized investment gains (losses) 3.3 — 3.3 Net unrealized investment gains (losses), after-tax $ 43.6 $ (1.9 ) $ 41.7
Investment holdings, fixed maturity investments	The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s fixed maturity investments as of December 31, 2015 and 2014 , were as follows: December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) Carrying value US Government and agency obligations $ 160.4 $ — $ (.4 ) $ — $ 160.0 Debt securities issued by corporations 1,001.0 4.3 (5.3 ) — 1,000.0 Municipal obligations 227.8 2.2 (1.2 ) — 228.8 Mortgage-backed 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 December 31, 2014 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value US Government and agency obligations $ 105.4 $ .1 $ (.3 ) $ — $ 105.2 Debt securities issued by corporations 1,162.0 13.1 (3.4 ) — 1,171.7 Municipal obligations 81.0 1.4 (.2 ) — 82.2 Mortgage-backed and asset-backed securities 967.5 2.8 (2.4 ) — 967.9 Foreign government, agency and provincial obligations 11.5 .3 (1.0 ) — 10.8 Preferred stocks 78.3 5.9 — — 84.2 Total fixed maturity investments $ 2,405.7 $ 23.6 $ (7.3 ) $ — $ 2,422.0
Schedule of contractual maturities of fixed maturities	The cost or amortized cost and carrying value of White Mountains’s fixed maturity investments as of December 31, 2015 is presented below by contractual maturity. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties. December 31, 2015 Millions Cost or amortized cost Carrying value Due in one year or less $ 241.6 $ 242.0 Due after one year through five years 982.3 981.5 Due after five years through ten years 96.4 96.1 Due after ten years 69.9 70.4 Mortgage-backed and asset-backed securities 1,170.6 1,167.0 Preferred stocks 78.3 82.7 Total $ 2,639.1 $ 2,639.7
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s common equity securities and other long-term investments as of December 31, 2015 and 2014 were as follows: December 31, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value Common equity securities $ 822.5 $ 302.8 $ (11.4 ) $ — $ 1,113.9 Other long-term investments $ 304.5 $ 32.0 $ (18.4 ) $ (2.3 ) $ 315.8 December 31, 2014 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign currency losses Carrying value Common equity securities $ 495.3 $ 127.4 $ (3.6 ) $ (7.4 ) $ 611.7 Other long-term investments $ 286.7 $ 53.3 $ (6.8 ) $ (1.3 ) $ 331.9
Fair value measurements by level, investment securities	The following tables summarize White Mountains’s fair value measurements for investments as of December 31, 2015 and 2014 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 asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and 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 Barclays Intermediate Aggregate and S&P 500 indices. The fair value measurements for derivative assets associated with White Mountains’s variable annuity business are presented in Note 9 . December 31, 2015 Millions Fair value Level 1 Inputs Level 2 Inputs Level 3 Inputs 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 — Other — — — — Total debt securities issued by corporations: 1,000.0 — 1,000.0 — Mortgage-backed 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 2,639.7 134.0 2,435.7 70.0 Short-term investments 211.3 211.3 — — Common equity securities: Exchange traded funds 141.8 120.5 21.3 — Financials 694.7 694.7 — — 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 (1) 297.3 — — 297.3 Total investments (1) $ 4,262.2 $ 1,363.5 $ 2,531.4 $ 367.3 (1) 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. December 31, 2014 Millions Fair value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fixed maturity investments: U.S. Government and agency obligations $ 105.2 $ 51.2 $ 54.0 $ — Debt securities issued by corporations: Consumer 259.2 — 259.2 — Financials 254.7 — 254.7 — Health Care 181.8 — 181.8 — Industrial 107.3 — 102.0 5.3 Communications 71.5 — 71.5 — Energy 65.6 — 65.6 — Utilities 84.1 — 84.1 — Technology 88.3 — 88.3 — Materials 51.7 — 51.7 — Other 7.5 — 7.5 — Total debt securities issued by corporations: 1,171.7 — 1,166.4 5.3 Mortgage-backed and asset-backed securities 967.9 — 967.9 — Municipal obligations 82.2 — 82.2 — Foreign government, agency and provincial obligations 10.8 .6 10.2 — Preferred stocks 84.2 — 13.1 71.1 Total fixed maturity investments 2,422.0 51.8 2,293.8 76.4 Short-term investments 376.8 376.3 .5 — Common equity securities: Financials 180.7 141.2 — 39.5 Consumer 116.0 115.9 .1 — Health Care 74.4 74.4 — — Industrial 57.1 57.1 — — Technology 44.8 44.8 — — Communications 28.5 28.5 — — Energy 18.1 18.1 — — Materials 12.0 12.0 — — Utilities 4.9 4.9 — — Other 75.2 1.9 73.3 — Total common equity securities 611.7 498.8 73.4 39.5 Other long-term investments (1) 309.9 — 5.7 304.2 Total investments (1) $ 3,720.4 $ 926.9 $ 2,373.4 $ 420.1 (1) Excludes the carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method and the carrying value of $16.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
Debt securities issued by corporations, credit ratings	The following table summarizes the ratings of the corporate debt securities held in White Mountains’s investment portfolio as of December 31, 2014 and 2013: Fair Value at December 31, Millions 2015 2014 AA $ 95.2 $ 144.9 A 397.7 594.9 BBB 507.1 426.5 Other — 5.4 Debt securities issued by corporations (1) $ 1,000.0 $ 1,171.7 (1) Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor’s and 2) Moody’s.
Mortgage-backed, asset-backed securities	December 31, 2015 December 31, 2014 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 265.5 $ 265.5 $ — $ 308.7 $ 308.7 $ — FNMA 42.2 42.2 — 33.4 33.4 — FHLMC 22.8 22.8 — 28.9 28.9 — Total Agency (1) 330.5 330.5 — 371.0 371.0 — Non-agency: Residential 133.2 133.2 — 71.0 71.0 — Commercial 140.4 140.4 — 109.3 109.3 — Total Non-agency 273.6 273.6 — 180.3 180.3 — Total mortgage-backed securities 604.1 604.1 — 551.3 551.3 — Asset-backed securities: Credit card receivables 217.7 217.7 — 218.1 218.1 — Vehicle receivables 269.7 269.7 — 152.8 152.8 — Other 75.5 75.5 — 45.7 45.7 — Total asset-backed securities 562.9 562.9 — 416.6 416.6 — Total mortgage and asset-backed securities $ 1,167.0 $ 1,167.0 $ — $ 967.9 $ 967.9 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2015 are as follows: Security Issuance Year Millions Fair Value 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Non-agency RMBS $ 133.2 $ 17.3 $ 8.6 $ 3.9 $ — $ 3.6 $ — $ 15.2 $ 13.5 $ 9.8 $ 14.0 $ 47.3 $ — Non-agency CMBS 140.4 — — — — — — 5.4 — 18.0 17.2 55.0 $ 44.8 Total $ 273.6 $ 17.3 $ 8.6 $ 3.9 $ — $ 3.6 $ — $ 20.6 $ 13.5 $ 27.8 $ 31.2 $ 102.3 $ 44.8
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 December 31, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 133.1 $ 65.6 $ 67.5 $ — Non-prime .1 — .1 — Sub-prime — — — — Total $ 133.2 $ 65.6 $ 67.6 $ — (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 December 31, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Fixed rate CMBS $ 101.1 $ 8.7 $ 48.6 $ 43.8 Floating rate CMBS 39.3 — — 39.3 Total $ 140.4 $ 8.7 $ 48.6 $ 83.1 (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to other “AAA” or “Aaa” bonds. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” bonds. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were junior to “AAA” or “Aaa” bonds
Other long-term investments	The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of December 31, 2015 and 2014 : December 31, 2015 December 31, 2014 Millions Fair Value Unfunded Fair Value Unfunded Hedge funds Long/short equity REIT $ 20.6 $ — $ 20.4 $ — Long/short credit & distressed — — 8.4 — Long/short banks and financial 12.8 — 29.9 — Other 3.6 — 15.7 — Total hedge funds 37.0 — 74.4 — Private equity funds Energy infrastructure & services 20.7 3.4 33.1 4.8 Manufacturing/Industrial 24.9 2.5 23.2 7.3 Multi-sector 14.8 2.1 14.5 2.2 Aerospace/Defense/Government 19.8 30.3 20.7 5.1 Healthcare 3.8 .4 3.1 1.4 Private equity secondaries 4.4 2.1 5.7 2.1 Insurance 2.0 41.3 2.1 41.3 Real estate .4 .1 1.7 0.1 Total private equity funds 90.8 82.2 104.1 64.3 Total hedge and private equity funds included in other long-term investments $ 127.8 $ 82.2 $ 178.5 $ 64.3
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following summarizes the December 31, 2015 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds: Redemption frequency Notice Period Millions 30-59 days notice 60-89 days notice 90-119 days notice 120+ days notice Total Monthly $ — $ — $ — $ — $ — Quarterly 13.8 — — — 13.8 Semi-annual — 20.6 — — 20.6 Annual — — 2.6 — 2.6 Total $ 13.8 $ 20.6 $ 2.6 $ — $ 37.0
Fair Value of private equity funds subject to lock-up periods	As of December 31, 2015 , 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 $ 24.3 $ 10.9 $ 55.6 $ — $ 90.8
Rollforward of fair value investments by level	The following tables summarize the changes in White Mountains’s fair value measurements by level for the years ended December 31, 2015 and 2014 : Level 3 Investments Millions Level 1 Investments Level 2 Investments Fixed maturity investments Common equity securities Other long-term investments Total Balance at January 1, 2015 $ 550.6 $ 2,372.9 $ 76.4 $ 39.5 $ 304.2 (1) $ 3,343.6 (1)(2) Total realized and unrealized gains (losses) 263.0 (13.7 ) (1.1 ) 7.8 (29.2 ) 226.8 (3)(4) Amortization/Accretion — (19.6 ) — — — (19.6 ) Purchases 814.5 1,436.0 35.3 — 91.3 2,377.1 Sales (825.7 ) (1,286.0 ) — (43.7 ) (54.7 ) (2,210.1 ) Symetra transfer 394.5 — — — — 394.5 Effect of redemption of Prospector hedge funds (43.5 ) — — (3.6 ) (14.3 ) (61.4 ) Transfers in — 41.8 — — — 41.8 Transfers out (1.2 ) — (40.6 ) — — (41.8 ) Balance at December 31, 2015 $ 1,152.2 $ 2,531.4 $ 70.0 $ — $ 297.3 (1) $ 4,050.9 (1)(2) (1) Excludes carrying value of $3.8 and $5.2 as of December 31, 2015 and January 1, 2015 associated with other long-term investments accounted for using the equity method. (2) Excludes carrying value of $211.3 and $376.8 as of December 31, 2015 and January 1, 2015 classified as short-term investments. (3) Excludes $ 0.8 of realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities. (4) Includes unrealized gains of $258.8 associated with the Symetra transfer from investments in unconsolidated affiliates to common equity securities. Level 3 Investments Millions Level 1 Investments Level 2 Investments Fixed Common equity securities Other long-term investments Total Balance at January 1, 2014 $ 997.2 $ 2,575.4 $ 81.8 $ 45.4 $ 189.2 (1) $ 3,889.0 (1)(2)(3) Total realized and unrealized gains 50.1 8.9 1.8 6.2 16.3 83.3 Amortization/Accretion .1 (18.7 ) .1 — — (18.5 ) Purchases 886.9 1,805.3 39.6 5.0 131.7 2,868.5 Sales (1,384.2 ) (2,065.9 ) — (17.1 ) (33.0 ) (3,500.2 ) Net change in investments related to purchases and sales of consolidated (2.7 ) 24.2 — — — 21.5 Exchange 3.4 (9.3 ) 5.9 — — — Transfers in — 53.0 — — — 53.0 Transfers out (.2 ) — (52.8 ) — — (53.0 ) Balance at December 31, 2014 $ 550.6 $ 2,372.9 $ 76.4 $ 39.5 $ 304.2 (1) $ 3,343.6 (1)(2)(3) (1) Excludes carrying value of $5.2 and $6.8 as of December 31, 2014 and January 1, 2014 associated with other long-term investment limited partnerships accounted for using the equity method. (2) Carrying value includes $236.3 as of January 1, 2014 that is classified as assets held for sale relating to discontinued operations. (3) Excludes carrying value of $376.8 and $310.4 as of December 31, 2014 and January 1, 2014 classified as short-term investments. (4) Excludes $4.0 of realized and unrealized losses associated with the Prospector Funds and Prospector Turtle Fund consolidation of investment-related liabilities.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds. Description December 31, 2015 $ in millions, except share price Rating (2) Valuation Technique(s) Fair (3) Unobservable Input Preferred Stock (1) NR Par value (8) $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 (7) : 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 & 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) For estimated value purposes, the assumption has been made that interest payouts begin in year five and that principal repayments being on a graduated basis in year ten for the seller priority notes and year fifteen for the pari passu note. (7) The decrease in the fair value of the surplus notes during the twelve months ended December 31, 2015 was primarily due to widening of non-investment grade credit spreads. (8) Valuation based on the issuer’s intent as of December 31, 2015 to call the security in the near term. Description December 31, 2014 $ in millions, except share price Rating (2) Valuation Technique(s) Fair Value (7) Unobservable Input Preferred Stock (1) NR Discounted cash flow $71.1 Discount yield - 7.1% Private equity security (1) NR Multiple of GAAP book value $39.5 Multiple of GAAP book value - 1.10 Private equity security (1) NR Share price of most recent transaction $20.1 Share price - $1.06 Convertible preferred security (1) NR Multiple of EBITDA $3.8 EBITDA multiple - 6.00 Convertible preferred security (1) NR Share price of most recent transaction $4.5 Share price - $0.71 Debt security issued by corporation (1) NR Discounted cash flow $5.3 Illiquidity discount (3) - 10.0% Private equity security (1) NR Share price of recent transaction $10.4 Share price - $290.96 Surplus notes: NR - Seller priority Discounted cash flow $44.0 Discount rate (4) - 9.3% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 10 years - Pari passu Discounted cash flow $21.1 Discount rate (5) - 13.5% Timing of interest payments (6) - 5 years Timing of principal payments (6) - 15 years (1) As of December 31, 2014 each asset type consists of one security. (2) Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor’s and 2) Moody’s. (3) Judgmentally determined based on the Company’s limited trading ability of the issuer. (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) For estimated value purposes, the assumption has been made that interest payouts begin in year five and that principal repayments being on a graduated basis in year ten for the seller priority notes and year fifteen for the pari passu note. (7) Includes the unrealized gains and losses associated with foreign currency; foreign currency effects based on observable inputs.
Other Investments Not Readily Marketable [Table Text Block]	Other long-term investments consist of the following as of December 31, 2015 and 2014: Carrying Value at Millions December 31, 2015 December 31, 2014 Hedge funds and private equity funds, at fair value (1) $ 127.8 $ 178.5 Private equity securities and limited liability companies, at fair value (1) 82.1 45.3 Surplus notes investments, at fair value (1) 51.5 65.1 Convertible preferred securities (1) 32.7 8.3 Tax advantaged federal affordable housing development fund (2) 14.7 16.8 Partnership investments accounted for under the equity method 3.8 5.2 Convertible fixed maturity investments (1) — 5.6 Other (1) 3.2 7.1 Total other-long term investments $ 315.8 $ 331.9 (1) See Fair Value Measurements by Level table. (2) Fund accounted for using the proportional amortization method.

Debt and Standby Letter of Cr43

Debt and Standby Letter of Credit Facilities (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Schedule of debt outstanding	White Mountains’s debt outstanding as of December 31, 2015 and 2014 consisted of the following: December 31, Millions 2015 2014 WTM Bank Facility $ 50.0 $ — OBH Senior Notes, at face value 275.0 275.0 Unamortized original issue discount (.2 ) (.3 ) OBH Senior Notes, carrying value 274.8 274.7 OneBeacon Bank Facility — — Tranzact Bank Facility 104.7 68.7 Unamortized issuance cost (1.8 ) (1.3 ) Tranzact Bank Facility, carrying value 102.9 67.4 MediaAlpha Bank Facility 15.0 — Unamortized issuance cost (.3 ) — MediaAlpha Bank Facility, carrying value 14.7 — Other debt — 1.0 Total debt $ 442.4 $ 343.1
Schedule of contractual repayments of debt	A schedule of contractual repayments of White Mountains’s debt as of December 31, 2015 , follows: Millions December 31, Due in one year or less $ 74.9 Due in two to three years 43.3 Due in four to five years 51.5 Due after five years 275.0 Total $ 444.7

Goodwill and Other Intangible44

Goodwill and Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	The following table shows the economic lives, acquisition date values, accumulated amortization and net carrying values for other intangible assets and goodwill, for each company acquired: $ in millions Weighted Average Economic life (in years) December 31, 2015 December 31, 2014 Acquisition date fair value Accumulated amortization Net carrying value Acquisition date fair value Accumulated amortization Net carrying value Tranzact: Domain and trade names 7 $ 30.5 $ 5.4 $ 25.1 $ 25.6 $ .9 $ 24.7 Customer relationships 11 132.3 13.1 119.2 107.5 2.1 105.4 Information technology 6 12.7 2.8 9.9 12.7 .5 12.2 Other 8 2.0 .1 1.9 .5 — .5 Subtotal 177.5 21.4 156.1 146.3 3.5 142.8 MediaAlpha: Customer relationships 4 6.5 2.9 3.6 6.5 1.2 5.3 Information technology 5 32.0 11.2 20.8 32.0 4.8 27.2 Subtotal 38.5 14.1 24.4 38.5 6.0 32.5 Wobi: Trademark 8 2.1 .5 1.6 2.1 .2 1.9 Information technology 7 3.6 .7 2.9 .8 .2 .6 Subtotal 5.7 1.2 4.5 2.9 .4 2.5 Star & Shield: Customer relationships 3 1.2 .8 .4 1.2 .4 .8 OneBeacon (EBI) 10 9.4 7.0 2.4 9.4 5.7 3.7 Total other intangible assets 232.3 44.5 187.8 198.3 16.0 182.3 Goodwill: Tranzact N/A 163.8 — 163.8 145.1 — 145.1 Quote Lab N/A 18.3 — 18.3 18.3 — 18.3 Wobi N/A 5.8 — 5.8 5.5 — 5.5 Total goodwill 187.9 — 187.9 168.9 — 168.9 Total goodwill and other intangible assets $ 420.2 $ 44.5 $ 375.7 $ 367.2 $ 16.0 $ 351.2
Schedule of Goodwill and Intangible Assets Rollforward	The following table shows the change in goodwill and other intangible assets: December 31, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 168.9 $ 182.3 $ — $ 5.1 Acquisitions of businesses 19.0 34.0 168.9 188.9 Dispositions of businesses — — — — Amortization, including foreign currency translation — (28.5 ) — (11.7 ) Ending balance $ 187.9 $ 187.8 $ 168.9 $ 182.3
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	White Mountains expects to recognize amortization expense in each of the next five years as follows: Millions Amortization expense 2016 $ 30.4 2017 29.6 2018 27.2 2019 19.4 2020 16.1 Total $ 122.7

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of total income tax benefit (expense)	The total income tax (expense) benefit for the years ended December 31, 2015, 2014 and 2013 consisted of the following: Year Ended December 31, Millions 2015 2014 2013 Current tax (expense) benefit: U.S. federal $ 9.1 $ (4.3 ) $ (20.3 ) State (1.8 ) (2.0 ) (1.0 ) Non-U.S. (1.5 ) (1.4 ) (1.2 ) Total current tax expense 5.8 (7.7 ) (22.5 ) Deferred tax (expense) benefit: U.S. federal (5.1 ) 22.5 (10.1 ) Total deferred tax benefit (expense) (5.1 ) 22.5 (10.1 ) Total income tax (expense) benefit $ .7 $ 14.8 $ (32.6 )
Reconciliation of the U.S. federal statutory income tax rate and actual effective tax rate on pre-tax income	A reconciliation of taxes calculated using the 35% U.S. statutory rate (the tax rate at which the majority of White Mountains’s worldwide operations are taxed) to the income tax (expense) benefit on pre-tax income follows: Year Ended December 31, Millions 2015 2014 2013 Tax (expense) benefit at the U.S. statutory rate $ (54.2 ) $ 10.2 $ (55.5 ) Differences in taxes resulting from: Non-U.S. earnings, net of foreign taxes 76.7 37.8 75.1 Change in valuation allowance (18.7 ) (36.0 ) (49.9 ) Tax exempt interest and dividends 2.6 2.5 2.4 Tax reserve adjustments (1.7 ) 5.2 (1.2 ) Withholding tax (1.2 ) (2.4 ) (1.0 ) Other, net (2.8 ) (2.5 ) (2.5 ) Total income tax (expense) benefit on pre-tax income $ .7 $ 14.8 $ (32.6 )
Schedule of components of deferred income tax assets and liabilities	An outline of the significant components of White Mountains’s deferred tax assets and liabilities follows: December 31, Millions 2015 2014 Deferred income tax assets related to: U.S. federal net operating and capital loss carryforwards $ 186.6 $ 152.3 Incentive compensation 45.4 40.2 Unearned premiums 37.5 38.3 Non-U.S. net operating loss carryforwards 33.8 33.1 Loss reserve discount 26.8 41.4 Tax credit carryforwards 16.9 13.4 Runoff Transaction 12.6 12.6 Deferred compensation 6.2 6.7 Fixed assets 1.7 2.3 Accrued interest 1.3 5.0 Other items 3.0 2.8 Total gross deferred income tax assets 371.8 348.1 Less: valuation allowances (156.8 ) (140.5 ) Total net deferred income tax assets 215.0 207.6 Deferred income tax liabilities related to: Deferred acquisition costs 34.9 32.0 Net unrealized investment gains 28.2 31.1 Members surplus contributions 19.0 10.3 Investment basis difference 13.8 17.0 Purchase accounting 9.3 — Pension and benefit accruals 3.5 2.5 Other items .5 .1 Total deferred income tax liabilities 109.2 93.0 Net deferred tax asset $ 105.8 $ 114.6
Schedule of net operating and capital loss carryforwards by expiration dates and the deferred tax assets thereon	Net operating loss and capital loss carryforwards as of December 31, 2015 , the expiration dates and the deferred tax assets thereon are as follows: December 31, 2015 Millions United States Luxembourg Netherlands Israel Total 2016 - 2020 $ .8 $ — $ — $ — $ .8 2021 - 2025 2.0 — .3 — 2.3 2026 - 2035 540.6 — — — 540.6 No expiration date — 96.8 — 21.8 118.6 Total $ 543.4 $ 96.8 $ .3 $ 21.8 $ 662.3 Gross deferred tax asset $ 186.6 $ 28.3 $ .1 $ 5.5 $ 220.5 Valuation allowance (121.2 ) (28.3 ) (.1 ) (5.5 ) (155.1 ) Net deferred tax asset $ 65.4 $ — $ — $ — $ 65.4
Reconciliation of changes in the amount of unrecognized tax benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: Millions Permanent Differences (1) Temporary Differences (2) Interest and Penalties (3) Total Balance at January 1, 2013 $ 8.4 $ 23.6 $ 8.0 $ 40.0 Changes in prior year tax positions — — 1.0 1.0 Tax positions taken during the current year — (7.0 ) — (7.0 ) Lapse in statute of limitations — — — — Settlements with tax authorities — — (.3 ) (.3 ) Balance at December 31, 2013 8.4 16.6 8.7 33.7 Changes in prior year tax positions (2.2 ) (.8 ) (1.9 ) (4.9 ) Tax positions taken during the current year — 7.3 — 7.3 Lapse in statute of limitations (.8 ) — (.3 ) (1.1 ) Balance at December 31, 2014 5.4 23.1 6.5 35.0 Changes in prior year tax positions — (12.4 ) 1.7 (10.7 ) Tax positions taken during the current year — — — — Lapse in statute of limitations — — — — Settlements with tax authorities — — — — Balance at December 31, 2015 $ 5.4 $ 10.7 $ 8.2 $ 24.3 (1) Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate. (2) Represents the amount of unrecognized tax benefits that, if recognized would create a temporary difference between the reported amount of an item in the Company’s Consolidated Balance Sheet and its tax basis. (3) Net of tax benefit.

Derivatives (Tables)

Derivatives (Tables) - Variable Annuity Reinsurance	12 Months Ended
	Dec. 31, 2015
Derivative [Line Items]
Pre-tax operating results	The following table summarizes the pre-tax operating results of WM Life Re for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, Millions 2015 2014 2013 Fees, included in other revenue $ 9.3 $ 18.6 $ 25.0 Change in fair value of variable annuity liability, included in other revenue (.4 ) 52.9 378.5 Change in fair value of derivatives, included in other revenue (8.8 ) (72.4 ) (402.0 ) Foreign exchange, included in other revenue (1.3 ) (3.2 ) (14.5 ) Other investment income and (losses) gains (.4 ) (1.4 ) (7.1 ) Total revenues (1.6 ) (5.5 ) (20.1 ) Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses — .6 10.2 Death benefit claims paid, included in general and administrative expenses (.1 ) (.1 ) (1.9 ) General and administrative expenses (4.0 ) (4.3 ) (4.9 ) Pre-tax loss $ (5.7 ) $ (9.3 ) $ (16.7 )
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following table summarizes the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the year ended December 31, 2015, 2014 and 2013 : Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2015 $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized (losses) gains (.4 ) (5) (9.7 ) (7.5 ) 8.4 (8.8 ) Transfers in (out) — — — — — Sales/settlements — (6.5 ) (9.8 ) (11.2 ) (27.5 ) Balance at December 31, 2015 $ .3 $ 2.7 $ 16.5 $ 0.9 $ 20.1 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2014 $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 Purchases — — — — — Realized and unrealized gains (losses) 53.5 (5) (38.6 ) (71.0 ) 37.2 (72.4 ) Transfers in (out) — — — — — Sales/settlements — (5.9 ) 100.1 (34.6 ) 59.6 Balance at December 31, 2014 $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1)(6) Level 2 (1)(2) Level 1 (3) Total (4) Balance at January 1, 2013 $ (441.5 ) $ 140.5 $ (20.5 ) $ (21.7 ) $ 98.3 Purchases — 59.4 — — 59.4 Realized and unrealized gains (losses) 388.7 (5) (136.5 ) (196.1 ) (69.4 ) (402.0 ) Transfers in (out) — — — — — Sales/settlements — — 221.3 92.2 313.5 Balance at December 31, 2013 $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 (1) Consists of over-the-counter instruments. (2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index. (3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded. (4) In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $5.8 , $33.2 and $81.3 as of December 31, 2015, 2014 and 2013 posted as collateral to its reinsurance counterparties. (5) There was no changes in the fair value of variable annuity death benefit liabilities for the year ended December 31, 2015. In 2014 and 2013, $0.6 and $10.2 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses. (6) Consists of foreign currency options and equity options.
Schedule of collateral	In addition, WM Life Re held cash and short-term investments posted as collateral to its variable annuity reinsurance and derivatives counterparties. The total collateral includes the following: December 31, Millions 2015 2014 Cash $ 5.8 $ 23.7 Fixed maturity investments — 9.5 Total $ 5.8 $ 33.2
Schedule of significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments	The following summarizes quantitative information about significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments that have been classified as Level 3 measurements: Description December 31, 2015 $ in millions Fair Value Valuation Technique(s) Unobservable Input Range Weighted Average Variable annuity benefit guarantee asset $ (.3 ) Discounted cash flows Surrenders 0.1 % - 40.0 % 40.0 % Mortality 0.0 % - 6.4% 1.0 % Foreign exchange volatilities 0-1 year 11.5 % - 18.7% 13.1 % Index volatilities 0-1 year 25.0 % - 27.5% 26.4 % Foreign Exchange Options $ 1.9 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations (0.5 )% - (7.3)% (5.9 )% Equity Index Options $ .8 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations 0.7 % - (6.7)% (2.5 )%
Offestting Assets and Liabilities	The following summarizes amounts offset under master netting agreements: December 31, 2015 December 31, 2014 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets 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 ) $ 0.3 $ 1.0 $ (5.4 ) $ (4.4 ) Exchange traded .1 (.1 ) — 2.8 (.1 ) 2.7 Foreign exchange contracts OTC 15.0 — 15.0 45.5 — 45.5 Exchange traded .1 (.3 ) (0.2 ) — (1.4 ) (1.4 ) Equity contracts OTC 4.4 (.6 ) 3.8 11.7 (.2 ) 11.5 Exchange traded 1.2 — 1.2 3.4 (.9 ) 2.5 Total (2) $ 23.2 $ (3.1 ) $ 20.1 $ 64.4 $ (8.0 ) $ 56.4 (1) Amount equal to fair value of instrument as recognized in other assets. (2) All derivative instruments held by WM Life Re are subject to master netting arrangements.
Offsetting Assets	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: December 31, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re - Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 8.5 $ — $ — $ 8.5 $ — $ — $ 5.5 $ 3.0 A + Bank of America .7 — — .7 — — — .7 A Citigroup - OTC 9.9 — — 9.9 — — 1.4 8.5 A Citigroup - Exchange Traded 1.0 — — 1.0 5.8 — — 6.8 A Total $ 20.1 $ — $ — $ 20.1 $ 5.8 $ — $ 6.9 $ 19.0 December 31, 2014 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WM Life Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) JP Morgan $ 24.3 $ — $ — $ 24.3 $ — $ — $ 8.8 $ 15.5 A + Bank of America 5.6 — — 5.6 — — — 5.6 A Nomura (3.5 ) 3.5 — — 1.7 9.5 — 11.2 BBB + Citigroup - OTC 22.2 — — 22.2 — — 1.1 21.1 A Citigroup - Exchange Traded 3.7 — — 3.7 16.0 — — 19.7 A Royal Bank of Scotland 4.0 — — 4.0 — — — 4.0 A - Barclays .1 — — .1 — — — .1 A Total $ 56.4 $ 3.5 $ — $ 59.9 $ 17.7 $ 9.5 $ 9.9 $ 77.2 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-one creditworthiness ratings),“A” (which is the sixth highest of twenty-one creditworthiness ratings), “A-” (which is the seventh highest of twenty-one creditworthiness ratings), and BBB+ (which is the eighth highest of twenty-one creditworthiness ratings).
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument	The following summarizes realized and unrealized gains (losses) recognized in other revenues for the years ended December 31, 2015, 2014 and 2013 and the carrying values as of December 31, 2015 and 2014 by type of derivative instrument: Carrying Value Year Ended December 31, December 31, Millions 2015 2014 2013 2015 2014 Fixed income/interest rate $ 6.4 $ (33.7 ) $ (108.7 ) $ .5 $ (1.7 ) Foreign exchange (7.3 ) (1.3 ) (96.7 ) 14.8 44.1 Equity (7.9 ) (37.4 ) (196.6 ) 4.8 14.0 Total $ (8.8 ) $ (72.4 ) $ (402.0 ) $ 20.1 $ 56.4

Municipal Bond Guarantee Insu47

Municipal Bond Guarantee Insurance (Tables)	12 Months Ended
	Dec. 31, 2015
Insurance [Abstract]
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 December 31, 2015 : Millions December 31, 2015 January 1, 2016 - March 31, 2016 $ 1.1 April 1, 2016 - June 30, 2016 1.1 July 1, 2016 - September 30, 2016 1.1 October 1, 2016 - December 31, 2016 1.1 4.4 2017 4.3 2018 4.1 2019 3.8 2020 3.6 2020 and thereafter 30.0 Total $ 50.2
Schedule of Insured Obligations	The following table provides a schedule of BAM’s insured obligations: December 31, 2015 December 31, 2014 Contracts outstanding 3,103 1,750 Remaining weighted average contract period (in years) 12.8 12.8 Contractual debt service outstanding (in millions): Principal $ 22,556.0 $ 12,362.5 Interest $ 11,984.4 $ 7,086.9 Gross unearned insurance premiums $ 50.2 $ 27.6

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Computation of earnings per share	The following table outlines the Company’s computation of earnings per share from continuing operations for the years ended December 31, 2015, 2014 and 2013 (see Note 22 - Discontinued Operations” for earnings per share amounts for discontinued operations): Year Ended December 31, 2015 2014 2013 Basic and diluted earnings per share numerators (in millions): Net income from continuing operations attributable to White Mountains’s common shareholders $ 198.8 $ 53.6 $ 175.1 Allocation of income for unvested restricted common shares (1) (2.3 ) (.7 ) (2.6 ) Dividends declared on participating restricted common shares (1) (.1 ) (.1 ) (.1 ) Total allocation to restricted common shares (2.4 ) (.8 ) (2.7 ) Net income attributable to White Mountains’s common shareholders, $ 196.4 $ 52.8 $ 172.4 Undistributed net earnings (in millions): Net income attributable to White Mountains’s common shareholders, $ 196.4 $ 52.8 $ 172.4 Dividends declared net of restricted common share amounts (1) (5.9 ) (6.1 ) (6.1 ) Total undistributed net earnings, net of restricted common share amounts $ 190.5 $ 46.7 $ 166.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (2) (68.0 ) (78.9 ) (91.4 ) Basic earnings per share denominator 5,811.2 6,026.0 6,109.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (2) (68.0 ) (78.9 ) (91.4 ) Average outstanding dilutive options to acquire common shares (3) — — — Diluted earnings per share denominator 5,811.2 6,026.0 6,109.0 Basic earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ 33.80 $ 8.77 $ 28.22 Dividends declared and paid (1.00 ) (1.00 ) (1.00 ) Undistributed earnings $ 32.80 $ 7.77 $ 27.22 Diluted earnings per share (in dollars) Net income attributable to White Mountains’s common shareholders $ 33.80 $ 8.77 $ 28.22 Dividends declared and paid (1.00 ) (1.00 ) (1.00 ) Undistributed earnings $ 32.80 $ 7.77 $ 27.22 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13 - “Employee Share-Based Incentive Compensation Plans” ). (3) The diluted earnings per share denominator for the years ended December 31, 2015, 2014 and 2013 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Retirement and Postretirement49

Retirement and Postretirement Plans (Tables)	12 Months Ended
	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Schedule of changes in projected benefit obligations and fair value of plan assets and funded status of plan assets	The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans as of December 31, 2015 and 2014 : Pension Benefits December 31, Millions 2015 2014 Change in projected benefit obligation: Projected benefit obligation at beginning of year $ 119.7 $ 102.9 Service cost .9 .6 Interest cost 4.6 4.7 Settlement loss (1) (1.8 ) — Special termination benefits expense — .3 Assumption changes (3.5 ) 17.5 Actuarial loss .9 1.1 Benefits and expenses paid with plan assets (13.7 ) (5.2 ) Benefits paid directly by OneBeacon (2.2 ) (2.2 ) Remeasurement due to plan termination (2) 4.6 — Projected benefit obligation at end of year $ 109.5 $ 119.7 Change in plan assets: Fair value of plan assets at beginning of year $ 146.0 $ 142.8 Actual return on plan assets 7.5 8.4 Benefits and expenses paid (13.7 ) (5.2 ) Fair value of plan assets at end of year $ 139.8 $ 146.0 Funded status at end of year $ 30.3 $ 26.3 (1) During the fourth quarter of 2015, OneBeacon triggered settlement accounting for the Qualified Plan as the total lump sum payments exceeded the service plus interest costs, resulting in a $1.8 adverse effect on accumulated other comprehensive income. (2) As noted in the paragraph preceding this table, the projected benefit obligation was valued on a plan termination basis as of December 31, 2015.
Schedule of amounts recorded in financial statements	Amounts recognized in the financial statements as of December 31, 2015 and 2014 consist of: December 31, Millions 2015 2014 Net assets of the Qualified Plan recorded in other assets $ 55.8 $ 53.4 Net liabilities of the Non-qualified Plan recorded in other liabilities (25.5 ) (27.1 ) Net amount accrued in the financial statements $ 30.3 $ 26.3
Information for the Non-qualified Plan, which has accumulated benefit obligations in excess of plan assets	Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows: December 31, Millions 2015 2014 Projected benefit obligation $ 25.5 $ 27.1 Accumulated benefit obligation $ 25.5 $ 27.1 Fair value of plan assets $ — $ —
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets	Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets, was as follows: December 31, Millions 2015 2014 Projected benefit obligation $ 84.0 (1) $ 92.6 Accumulated benefit obligation $ 84.0 (1) $ 92.6 Fair value of plan net assets $ 139.8 $ 146.0 (1) Measured on a plan termination basis
Schedule of amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis and before noncontrolling interest	The amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis and before non-controlling interest for the years ended December 31, 2015 and 2014 were as follows: December 31, Millions 2015 2014 Accumulated other comprehensive (loss) income beginning balance $ (8.0 ) $ 10.5 Increase (decrease) in accumulated other comprehensive income (loss): Amortization of net actuarial gains recognized during the year 1.3 .3 Net actuarial losses occurring during the year (1) (1.3 ) (18.8 ) Accumulated other comprehensive loss ending balance $ (8.0 ) $ (8.0 ) (1) Net actuarial gains (losses) resulted from investment returns and demographic experience different than assumed. The 2015 amount reflects the valuation of the Qualified Plan on a termination basis.
Schedule of components of net periodic benefit cost (income)	The components of net periodic benefit (income) cost for the years ended December 31, 2015, 2014 and 2013 were as follows: Year Ended December 31, Millions 2015 2014 2013 Service cost $ .9 $ .6 $ .8 Interest cost 4.6 4.7 4.2 Expected return on plan assets (8.7 ) (8.5 ) (7.1 ) Amortization of unrecognized loss 1.3 .3 .9 Net periodic pension income before settlements, curtailments and special termination benefits (1.9 ) (2.9 ) (1.2 ) Special termination benefits expense (1) — .3 .3 Total net periodic benefit income $ (1.9 ) $ (2.6 ) $ (.9 ) (1) Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.
Schedule of fair value of the Plan assets and their related inputs	The fair value of the Qualified Plan’s assets and their related inputs as of December 31, 2015 and 2014 by asset category were as follows: December 31, 2015 December 31, 2014 Millions Fair Value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fair Value Level 1 Inputs Level 2 Inputs Level 3 Inputs Fixed maturity investments $ 132.4 $ 132.4 $ — $ — $ 24.2 $ — $ 24.2 $ — Common equity securities — — — — 101.0 101.0 — — Convertible fixed maturity investments — — — — 12.8 — 12.8 — Cash and short-term investments 7.0 7.0 — — 5.2 4.8 .4 — Total $ 139.4 $ 139.4 $ — $ — $ 143.2 $ 105.8 $ 37.4 $ — There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2015 and 2014 . The Qualified Plan’s asset allocations as of December 31, 2015 and 2014 , by asset category were as follows: Plan Assets at December 31, Asset Category 2015 2014 Fixed maturity investments 95.0 % 16.9 % Common equity securities — 70.6 Convertible fixed maturity investments — 8.9 Cash and short-term investments 5.0 3.6 Total 100.0 % 100.0 %
Schedule of expected future benefit payments	The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid: Millions Expected Benefit Payments 2016 $ 86.8 2017 2.1 2018 2.1 2019 2.0 2020 2.0 2021-2025 8.9 (1) Primarily reflects the anticipated payout related to annuity purchases and lump sum payments resulting from the termination of the Qualified Plan

Employee Share-Based Incentiv50

Employee Share-Based Incentive Compensation Plans (Tables)	12 Months Ended
	Dec. 31, 2015
White Mountains
Share-based compensation arrangement by share-based payment award
Summary of performance share activity	The following summarizes performance share activity for the years ended December 31, 2015, 2014 and 2013 for performance shares granted under the WTM Incentive Plan and the WTM Phantom Share Plan: Year Ended December 31, 2015 2014 2013 $ in millions Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 113,198 $ 44.4 108,605 $ 54.9 108,065 $ 26.5 Shares paid or expired (1) (42,958 ) (30.8 ) (33,730 ) (24.5 ) (43,100 ) (10.1 ) New grants 29,828 — 41,580 — 43,660 — Assumed forfeitures and cancellations (2)(3) (3,857 ) (.3 ) (3,257 ) (.1 ) (20 ) (.6 ) Expense recognized — 44.4 — 14.1 — 39.1 End of period (4) 96,211 $ 57.7 113,198 $ 44.4 108,605 $ 54.9 (1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance shares payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target. WTM performance shares payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target. (2) On November 19, 2014, the Compensation Committee canceled 3,040 WTM performance shares for the 2014-2016 performance cycle and issued the same number of WTM restricted shares. (3) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of performance shares outstanding and accrued expense for performance shares awarded under an Incentive Plan	The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2015 for each performance cycle: $ in millions Target WTM Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 29,828 $ 8.3 2014 – 2016 32,557 12.9 2013 – 2015 36,293 38.0 Sub-total 98,678 59.2 Assumed forfeitures (2,467 ) (1.5 ) Total at December 31, 2015 96,211 $ 57.7
Schedule of Nonvested Restricted Stock Activity [Table Text Block]	The following outlines the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, 2015 2014 2013 $ in millions Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 83,314 $ 14.3 94,130 $ 17.0 69,910 $ 16.8 Issued (1) 23,640 15.7 23,440 13.1 25,720 14.4 Vested (36,279 ) — (33,205 ) — (1,500 ) — Forfeited — — (1,051 ) (.5 ) — — Expense recognized — (14.3 ) — (15.3 ) — (14.2 ) End of period (2) 70,675 $ 15.7 83,314 $ 14.3 94,130 $ 17.0 (1) On November 19, 2014, the Compensation Committee canceled 3,040 WTM performance shares for the 2014-2016 performance cycle and issued the same number of WTM restricted shares.
OneBeacon
Share-based compensation arrangement by share-based payment award
Summary of performance share activity	The following summarizes activity for the years ended December 31, 2015, 2014 and 2013 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Year Ended December 31, 2015 2014 2013 $ in millions Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 517,470 $ 3.4 493,421 $ 4.0 563,190 $ 1.2 Payments and deferrals (1) (181,290 ) (1.5 ) (142,138 ) (1.0 ) (238,658 ) — New awards 154,887 — 165,800 — 179,000 — Forfeitures and cancellations (2) (41,632 ) (.1 ) 387 — (10,111 ) (.1 ) Expense recognized — (.4 ) — .4 — 2.9 End of period 449,435 $ 1.4 517,470 $ 3.4 493,421 $ 4.0 (1) OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. No payments were made in 2013 for the 2010-2012 OneBeacon performance cycle as the performance factor was zero. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of performance shares outstanding and accrued expense for performance shares awarded under an Incentive Plan	The following summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan as of December 31, 2015 for each performance cycle: $ in millions Target OneBeacon Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 146,659 $ .7 2014 – 2016 142,710 .1 2013 – 2015 167,300 .6 Sub-total 456,669 1.4 Assumed forfeitures (7,234 ) — Total at December 31, 2015 449,435 $ 1.4
Schedule of Nonvested Restricted Stock Activity [Table Text Block]	The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the years ended December 31, 2015, 2014 and 2013 . Year Ended December 31, 2015 2014 2013 $ in millions Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 612,500 $ 3.5 915,000 $ 6.5 927,000 $ 9.6 Issued 75,950 1.1 — — — — Vested (296,000 ) — (300,000 ) — (9,000 ) — Forfeited (9,728 ) (.1 ) (2,500 ) — (3,000 ) — Expense recognized — (2.0 ) — (3.0 ) — (3.1 ) End of period 382,722 $ 2.5 612,500 $ 3.5 915,000 $ 6.5

Common Shareholders_ Equity a51

Common Shareholders’ Equity and Non-controlling Interests Common Shareholders’ Equity and Non-controlling Interests (Tables)	12 Months Ended
	Dec. 31, 2015
Noncontrolling Interest [Abstract]
Noncontrolling Interest	The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of December 31, 2015 and 2014: December 31, 2015 December 31, 2014 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 24.5 % $ 245.6 24.7 % $ 258.4 SIG Preference Shares 100.0 250.0 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 17.1 3.1 17.9 MediaAlpha 40.0 14.4 40.0 22.6 Tranzact 36.8 79.4 36.8 88.2 Wobi 3.9 .6 36.7 5.4 Prospector Offshore Fund — — 23.4 31.1 Dewar 19.0 3.7 18.0 3.4 Total other, excluding mutuals and reciprocals 115.2 168.6 Mutuals and reciprocals BAM 100.0 (140.0 ) 100.0 (121.9 ) SSIE 100.0 (16.0 ) 100.0 (12.4 ) Total non-controlling interests $ 454.8 $ 542.7

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
Financial information for White Mountains' segments	Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2015 Earned insurance and reinsurance premiums $ 1,176.2 $ 2.5 $ .8 $ 8.7 $ 1,188.2 Net investment income 45.9 1.9 4.2 8.8 60.8 Net investment income (loss) - surplus note interest — 15.8 (15.8 ) — — Net realized and unrealized investment (losses) gains (35.1 ) (.3 ) .9 259.9 (2) 225.4 Other (expense) revenue (.6 ) — .7 334.1 (4) 334.2 Total revenues (3) 1,186.4 19.9 (9.2 ) 611.5 1,808.6 Losses and LAE 700.7 — — 8.2 708.9 Insurance and reinsurance acquisition expenses 213.8 .6 2.3 3.4 220.1 Other underwriting expenses 218.2 — .4 — 218.6 General and administrative expenses 14.1 1.4 35.4 408.0 (5) 458.9 Amortization of intangible assets 1.3 — — 27.3 28.6 Interest expense 13.0 — — 5.6 18.6 Total expenses 1,161.1 2.0 38.1 452.5 1,653.7 Pre-tax income (loss) $ 25.3 $ 17.9 $ (47.3 ) $ 159.0 $ 154.9 HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2014 Earned insurance and reinsurance premiums $ 1,177.1 $ 1.4 $ .4 $ 6.1 $ 1,185.0 Net investment income 43.4 1.4 5.7 9.0 59.5 Net investment income (loss) - surplus note interest — 15.7 (15.7 ) — — Net realized and unrealized investment gains 40.4 1.7 6.6 29.8 78.5 Other revenue 5.8 — .6 124.7 (4) 131.1 Total revenues (3) 1,266.7 20.2 (2.4 ) 169.6 1,454.1 Losses and LAE 815.1 — — 8.9 824.0 Insurance and reinsurance acquisition expenses 203.3 .3 1.8 .8 206.2 Other underwriting expenses 179.2 — .4 — 179.6 General and administrative expenses 12.4 1.6 35.9 196.1 246.0 Amortization of intangible assets 1.4 — — 10.3 11.7 Interest expense 13.0 — — 2.6 15.6 Total expenses 1,224.4 1.9 38.1 218.7 1,483.1 Pre-tax income (loss) $ 42.3 $ 18.3 $ (40.5 ) $ (49.1 ) $ (29.0 ) HG Global/BAM Millions OneBeacon HG Global BAM (1) Other Operations Total Year Ended December 31, 2013 Earned insurance and reinsurance premiums $ 1,120.4 $ .4 $ .1 $ — $ 1,120.9 Net investment income 43.0 1.0 4.7 11.1 59.8 Net investment income (loss) - surplus note interest — 40.2 (40.2 ) — — Net realized and unrealized investment gains (losses) 49.4 (2.0 ) (9.3 ) 95.8 133.9 Other revenue 31.2 — .4 15.4 47.0 Total revenues (3) 1,244.0 39.6 (44.3 ) 122.3 1,361.6 Losses and LAE 622.1 — — — 622.1 Insurance and reinsurance acquisition expenses 208.9 .1 1.4 — 210.4 Other underwriting expenses 204.8 — .4 — 205.2 General and administrative expenses 10.6 1.4 32.5 103.2 147.7 Amortization of intangible assets 1.4 — — — 1.4 Interest expense 13.0 — — 3.2 16.2 Total expenses 1,060.8 1.5 34.3 106.4 1,203.0 Pre-tax income (loss) $ 183.2 $ 38.1 $ (78.6 ) $ 15.9 $ 158.6 (1) BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services. (2) Net realized and unrealized investment gains (losses) in the Other Operations segment includes the unrealized gain of $258.8 related to the investment in Symetra common shares, representing the difference between the carrying value under the equity method at November 5, 2015 and the fair value at December 31, 2015. (See Note 17 - “Investments in Unconsolidated Affiliates” ). (3) Total revenues includes both revenues from customers as well as investment performance. (4) Includes $186.9 from Tranzact and $105.5 from MediaAlpha for the year ended December 31, 2015; and $43.2 from Tranzact and $65.3 from MediaAlpha for the year ended December 31, 2014. (5) Includes $167.3 from Tranzact and $99.0 from MediaAlpha for the year ended December 31, 2015; and $37.4 from Tranzact and $60.6 from MediaAlpha for the year ended December 31, 2014.
Schedule of selected balance sheet data by segment	HG Global/BAM Selected Balance Sheet Data Millions OneBeacon HG Global BAM Other Operations Discontinued Operations Total December 31, 2015: Total investments $ 2,591.4 $ 136.2 $ 460.3 $ 1,092.8 $ — $ 4,280.7 Reinsurance recoverable on paid and unpaid losses 193.5 — — .5 — 194.0 Assets held for sale — — — — 4,407.0 4,407.0 Total assets 3,602.7 739.0 (91.1 ) (1 ) 1,626.9 4,407.0 10,284.5 Loss and LAE reserves 1,389.8 — — 6.0 — 1,395.8 Liabilities held for sale — — — — 2,884.0 2,884.0 Total liabilities 2,598.2 181.2 48.9 204.2 2,884.0 5,916.5 Total White Mountains’s common shareholders’ 755.2 540.7 — 1,344.3 1,273.0 3,913.2 Non-controlling interest 249.3 17.1 (140.0 ) 78.4 — (2) 204.8 December 31, 2014: Total investments $ 2,525.8 $ 121.0 $ 454.2 $ 641.4 $ — $ 3,742.4 Reinsurance recoverable on paid and unpaid losses 173.8 — — .1 — 173.9 Assets held for sale 58.1 — — — 4,572.5 4,630.6 Total assets 3,559.8 704.4 (89.4 ) (1 ) 1,708.4 4,572.5 10,455.7 Loss and LAE reserves 1,342.2 — — 7.8 — 1,350.0 Liabilities held for sale — — — — 3,105.3 3,105.3 Total liabilities 2,510.5 121.0 32.5 148.0 3,105.3 5,917.3 Total White Mountains’s common shareholders’ 787.5 565.5 — 1,425.5 1,217.2 3,995.7 Non-controlling interest 261.8 17.9 (121.9 ) 134.9 — (2) 292.7 (1) BAM total assets reflect the elimination of $503.0 in surplus notes issued to HG Global and its subsidiaries, and $90.2 and $74.4 in accrued interest related to those surplus notes as of December 31, 2015 and 2014. (2) Excludes $250.0 related to SIG Preference Shares (See Note 14 - “Common Shareholders’ Equity and Non-controlling Interests” )

Investments in Unconsolidated53

Investments in Unconsolidated Affiliates (Tables)	12 Months Ended
	Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items]
Schedule of 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. November 5, December 31, Millions 2015 2014 Symetra common shares $ 394.4 $ 373.8 Unrealized (losses) gains from Symetra’s fixed maturity portfolio (1.6 ) 37.6 Carrying value of Symetra common shares 392.8 411.4 Hamer — 3.0 Total investments in unconsolidated affiliates $ 392.8 $ 414.4
Schedule of changes in unconsolidated affiliates	The following table summarizes amounts recorded by White Mountains under the equity method relating to its investment in Symetra through November 5, 2015: Millions Common shares Warrants Total Equity method carrying value of investment in Symetra as of December 31, 2012 (2) $ 351.2 $ 30.3 $ 381.5 Equity in earnings (1)(3)(6) 37.8 — 37.8 Equity in net unrealized gains from Symetra’s fixed maturity portfolio (4)(5) (106.4 ) — (106.4 ) Dividends received (6.4 ) — (6.4 ) Increase in value of warrants — 10.8 10.8 Exercise of warrants 41.1 (41.1 ) — Equity method carrying value of investment in Symetra as of December 31, 2013 (2) 317.3 — 317.3 Equity in earnings (1)(3)(6) 47.0 — 47.0 Equity in net unrealized losses from Symetra’s fixed maturity portfolio (4)(5) 81.2 — 81.2 Dividends received (34.1 ) — (34.1 ) Equity method carrying value of investment in Symetra as of December 31, 2014 (2) 411.4 — 411.4 Equity in earnings (1)(3)(6) 25.1 — 25.1 Equity in net unrealized gains from Symetra’s fixed maturity portfolio (4)(5) (39.2 ) — (39.2 ) Distribution from Prospector Offshore Fund 12.4 — 12.4 Dividends received (16.9 ) — (16.9 ) Equity method carrying value of investment in Symetra as of November 5, 2015 (2)(7) $ 392.8 $ — $ 392.8 (1) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 excludes tax expense of $1.6 , $3.3 , and $2.8 (2) Includes White Mountains’s equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $(1.6) , $37.6 , and $(43.6) as of November 5, 2015 and December 31, 2014 and 2013, which excludes tax benefit (expense) of $0.2 , $(2.7) and $3.2 (3) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $2.3 , $3.0 and $3.0 increases relating to the pre-tax amortization of the Symetra common share basis difference. (4) Net unrealized gains for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $9.4 , $12.7 and $11.8 increases relating to the pre-tax amortization of the Symetra common share basis difference. (5) Net unrealized gains (losses) from Symetra’s fixed maturity portfolio excludes tax benefit (expense) of $2.9 , $(5.9) and $8.3 for the period ended November 5, 2015 and years ended December 31, 2014 and 2013. (6) Equity in earnings for the period ended November 5, 2015 and years ended December 31, 2014 and 2013 includes $(0.1) , $(0.1) , and $0.2 (gain) loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra (7) The aggregate value of White Mountains’s investment in common shares of Symetra was $651.2 based upon the quoted market price of $31.67 per share as of November 5, 2015.
Symetra
Investments in and Advances to Affiliates [Line Items]
Summarized statement of financial position of Symetra	The following table summarizes financial information for Symetra as of September 30, 2015 and December 31, 2014: September 30, December 31, Millions 2015 2014 Symetra balance sheet data: Total investments $ 32,409.2 $ 30,634.3 Separate account assets 885.9 949.8 Total assets 34,962.8 33,001.7 Policyholder liabilities 29,492.0 27,276.0 Long-term debt 697.5 697.2 Separate account liabilities 885.9 949.8 Total liabilities 31,836.7 29,641.1 Common shareholders’ equity 3,126.1 3,360.6 The following table summarizes financial information for Symetra for the nine months ended September 30, 2015 and years ended December 31, 2014 and 2013: Nine months ended September 30, Year ended December 31, Millions 2015 2014 2013 Symetra income statement data: Net premiums earned $ 539.3 $ 629.1 $ 627.2 Net investment income 994.3 1,320.5 1,285.0 Total revenues 1,605.9 2,182.4 2,139.5 Policy benefits 1,143.7 1,399.7 1,394.9 Total expenses 1,543.6 1,882.5 1,865.4 Net income 89.6 254.4 220.7 Comprehensive net (loss) income (234.1 ) 397.0 (777.6 )

Fair Value of Financial Instr54

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table summarizes the fair values and book values as of December 31, 2015 and 2014 : December 31, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 276.4 $ 274.8 $ 286.0 $ 274.7 Tranzact Bank Facility 102.8 102.9 68.7 67.4 MediaAlpha Bank Facility 15.0 14.7 — —

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Summary of the fair value and carrying value of financial instruments	The following table summarizes the fair values and book values as of December 31, 2015 and 2014 : December 31, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value OBH Senior Notes $ 276.4 $ 274.8 $ 286.0 $ 274.7 Tranzact Bank Facility 102.8 102.9 68.7 67.4 MediaAlpha Bank Facility 15.0 14.7 — —
Schedule illustrating the valuation adjustments between par and fair value [Table Text Block]	Below is a table illustrating the valuation adjustments taken to arrive at the estimated fair value of the OneBeacon Surplus Notes as of December 23, 2014: Type of Surplus Note Millions Seller Priority Pari Passu Total Par Value $ 57.9 $ 43.1 $ 101.0 Fair value adjustments to reflect: Current market rates on public debt and contract-based repayments (1) 1.6 (8.4 ) (6.8 ) Regulatory approval (2) (4.6 ) (8.0 ) (12.6 ) Liquidity adjustment (3) (11.0 ) (5.7 ) (16.7 ) Total adjustments (14.0 ) (22.1 ) (36.1 ) Fair value $ 43.9 $ 21.0 $ 64.9 (1) Represents the value of the OneBeacon Surplus Notes, at current market yields on 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 delay in securing regulatory approvals of interest and principal payments to reflect graduated changes in Issuer’s statutory surplus. T he monetary impact of the anticipated delay is measured based on credit spreads of public securities 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.
Schedule of discontinued operations balance sheet, income statement, and cash flows	The following summarizes the assets and liabilities associated with the business classified as held for sale, which all relate to Sirius Group with the exception of the building held by OneBeacon at December 31, 2014: December 31, Millions 2015 2014 Assets held for sale Fixed maturity investments, at fair value $ 2,374.0 $ 2,362.3 Short term 352.0 494.9 Common Equities 174.4 189.9 Other 72.2 95.6 Total Investments 2,972.6 3,142.7 Cash 143.9 111.5 Reinsurance recoverable on unpaid losses 283.1 322.2 Reinsurance recoverable on paid losses 10.2 11.4 Insurance premiums receivable 323.6 306.6 Deferred acquisition costs 74.6 69.9 Deferred tax asset 303.1 341.5 Ceded unearned insurance and reinsurance premiums 87.7 76.2 Accounts receivable on unsettled investment sales 29.0 18.7 Intangible assets 10.2 15.2 Other assets 169.0 156.6 Other assets - OneBeacon building — 58.1 Total assets held for sale $ 4,407.0 $ 4,630.6 Liabilities held for sale Loss and loss adjustment expense reserves $ 1,644.4 $ 1,809.8 Unearned insurance premiums 342.2 338.6 Debt 403.5 403.5 Deferred tax liability 263.6 282.8 Accrued incentive compensation 63.2 76.5 Ceded reinsurance payable 67.1 71.5 Funds held under reinsurance treaties 52.9 57.9 Accounts payable on unsettled investment purchases — 2.1 Other liabilities 47.1 62.6 Total liabilities held for sale 2,884.0 3,105.3 Net assets held for sale $ 1,523.0 $ 1,525.3 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. The results of Sirius Group through the closing date of the transaction inures to White Mountains. Year Ended December 31, 2015 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 847.0 $ — $ 847.0 Net investment income 40.7 — 40.7 Net realized and unrealized investment gains 15.1 — 15.1 Other revenue (20.6 ) (.7 ) (21.3 ) Total revenues 882.2 (.7 ) 881.5 Expenses Loss and loss adjustment expenses 422.7 — 422.7 Insurance and reinsurance acquisition expenses 189.8 — 189.8 Other underwriting expenses 107.9 — 107.9 Interest expense on debt 26.6 — 26.6 General and administrative expenses 27.0 — 27.0 Total expenses 774.0 — 774.0 Pre-tax income (loss) 108.2 (.7 ) 107.5 Income tax expense (27.1 ) .2 (26.9 ) Net income (loss) from discontinued operations 81.1 (.5 ) 80.6 Net gain from sales of discontinued operations - OneBeacon — .3 .3 Net gain from sales of discontinued operations - Esurance — 17.9 17.9 Total income from discontinued operations 81.1 17.7 98.8 Change in foreign currency translation and other from discontinued operations (65.0 ) — (65.0 ) Comprehensive income from discontinued operations $ 16.1 $ 17.7 $ 33.8 Year Ended December 31, 2014 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 873.9 $ .1 $ 874.0 Net investment income 40.9 — 40.9 Net realized and unrealized investment gains 205.4 — 205.4 Other revenue (62.4 ) — (62.4 ) Total revenues 1,057.8 .1 1,057.9 Expenses Loss and loss adjustment expenses 345.3 (.7 ) 344.6 Insurance and reinsurance acquisition expenses 193.6 .1 193.7 Other underwriting expenses 129.7 3.5 133.2 Interest expense on debt 26.3 — 26.3 General and administrative expenses 30.5 — 30.5 Total expenses 725.4 2.9 728.3 Pre-tax income (loss) 332.4 (2.8 ) 329.6 Income tax (expense) benefit (70.4 ) 1.0 (69.4 ) Net income (loss) from discontinued operations 262.0 (1.8 ) 260.2 Net gain from sales of discontinued operations - FFIC — 14.0 14.0 Net losses from sales of discontinued operations - OneBeacon — (18.8 ) (18.8 ) Net gain from sales of discontinued operations - Esurance — 3.2 3.2 Total income from discontinued operations 262.0 (3.4 ) 258.6 Change in foreign currency translation and other from discontinued operations (169.5 ) — (169.5 ) Comprehensive income (loss) from discontinued operations $ 92.5 $ (3.4 ) $ 89.1 Year Ended December 31, 2013 Millions Sirius Group Other Disc Ops Total Revenues Earned insurance premiums $ 866.4 $ .8 $ 867.2 Net investment income 46.7 — 46.7 Net realized and unrealized investment gains 27.8 — 27.8 Other revenue 16.8 10.8 27.6 Total revenues 957.7 11.6 969.3 Expenses Loss and loss adjustment expenses 418.4 78.9 497.3 Insurance and reinsurance acquisition expenses 166.5 — 166.5 Other underwriting expenses 126.1 (.2 ) 125.9 Interest expense on debt 26.3 — 26.3 General and administrative expenses 32.2 — 32.2 Total expenses 769.5 78.7 848.2 Pre-tax income (loss) 188.2 (67.1 ) 121.1 Income tax (expense) benefit (46.2 ) 25.0 (21.2 ) Net income (loss) from discontinued operations 142.0 (42.1 ) 99.9 Net gain from sales of discontinued operations — 46.6 46.6 Total income from discontinued operations 142.0 4.5 146.5 Change in foreign currency translation and other from discontinued operations 6.1 — 6.1 Comprehensive income from discontinued operations $ 148.1 $ 4.5 $ 152.6 Net Change in Cash from Discontinued Operations The following summarizes the net change in cash associated with the businesses classified as discontinued operations: Year ended December 31, Millions 2015 2014 2013 Net cash provided from (used for) operations $ 14.7 $ 60.0 $ (61.5 ) Net cash provided from investing activities 31.3 35.8 148.7 Net cash used for financing activities (9.1 ) (63.2 ) (87.5 ) Effect of exchange rate changes on cash (4.5 ) (14.3 ) (.2 ) Net change in cash during the period 32.4 18.3 (.5 ) Cash balances at beginning of period 111.5 93.2 93.7 Cash balances at end of period $ 143.9 $ 111.5 $ 93.2
Discontinued operations, computation of earnings per share	The following table outlines the computation of earnings per share for discontinued operations for the years ended December 31, 2015, 2014 and 2013 : Year Ended December 31, 2015 2014 2013 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ 98.8 $ 258.6 $ 146.5 Allocation of income for participating unvested restricted common shares (1) (1.1 ) (3.3 ) (2.2 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ 97.7 $ 255.3 $ 144.3 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (3) (68.0 ) (78.9 ) (91.4 ) Basic earnings per share denominator 5,811.2 6,026.0 6,109.0 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,879.2 6,104.9 6,200.4 Average unvested restricted common shares (3) (68.0 ) (78.9 ) (91.4 ) Average outstanding dilutive options to acquire common shares (4) — — — Diluted earnings per share denominator 5,811.2 6,026.0 6,109.0 Basic and diluted earnings per share (in dollars): $ 16.80 $ 42.36 $ 23.63 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net income (loss) attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the years ended December 31, 2015, 2014 and 2013 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13 - “Employee Share-Based Incentive Compensation Plans” ). (4) The diluted earnings (loss) per share denominator for the years ended December 31, 2015, 2014 and 2013 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.
Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Summary of the fair value and carrying value of financial instruments	The following table summarizes the fair value and carrying value of the Sirius Group financials instruments, the SIG Senior Notes and the SIG Preference Shares, as of December 31, 2015 and 2014: December 31, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value SIG Senior Notes $ 410.0 $ 399.8 $ 437.8 $ 399.7 SIG Preference Shares 255.0 250.0 260.0 250.0
Interest Rate Cap | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following table summarizes the change in the fair value of the Interest Rate Cap for the year ended December 31, 2015 : December 31, Millions 2015 2014 Beginning of period $ 4.1 $ 11.1 Net realized and unrealized losses (2.2 ) (7.0 ) End of period $ 1.9 $ 4.1
Schedule of collateral	The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty: December 31, 2015 Millions Collateral Balances Held Standard & Poor’s Rating (1) Barclays Bank Plc $ 1.3 A- Nordea Bank Finland Plc .6 AA- Total $ 1.9 (1) Standard & Poor’s ratings as detailed above are: “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings) and “AA-” (Very Strong, which is the fourth highest of twenty-one creditworthiness ratings).

Selected Quarterly Financial 56

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	Selected quarterly financial data for 2015 and 2014 is shown in the following table. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. As a result of the Sirius sale, Esurance sale and the Runoff Business sale, the results of operations for Sirius, Esurance and the Runoff Business have been classified as discontinued operations and are now presented, net of related income taxes, as such in the statement of comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation (see Note 22 - “Discontinued Operations” ). 2015 Three Months Ended 2014 Three Months Ended Millions, except per share amounts Dec. 31 Sept. 30 June 30 Mar. 31 Dec. 31 Sept. 30 June 30 Mar. 31 Revenues $ 678.5 $ 334.7 $ 403.6 $ 391.8 $ 405.6 $ 323.2 $ 393.5 $ 331.8 Expenses 420.4 429.3 412.7 391.3 471.3 358.7 359.9 293.2 Pre-tax income 258.1 (94.6 ) (9.1 ) .5 (65.7 ) (35.5 ) 33.6 38.6 Tax benefit (expense) 1.5 1.6 2.2 (4.6 ) 26.2 4.7 (7.1 ) (9.0 ) Income (loss) from continuing operations 259.6 (93.0 ) (6.9 ) (4.1 ) (39.5 ) (30.8 ) 26.5 29.6 Income from discontinued operations, net of tax 7.3 14.2 3.5 73.8 82.1 63.9 61.1 51.5 Non-controlling interest in consolidated subsidiaries (6.1 ) 16.0 .9 7.3 15.0 11.2 (4.6 ) .6 Equity in earnings of unconsolidated affiliates 7.1 3.9 6.8 7.3 12.3 7.0 12.5 13.8 Income (loss) attributable to White Mountains’s common shareholders $ 267.9 $ (58.9 ) $ 4.3 $ 84.3 $ 69.9 $ 51.3 $ 95.5 $ 95.5 Income (loss) attributable to White Mountains’s common shareholders per share: Basic Continuing operations $ 45.99 $ (12.42 ) $ .13 $ 1.76 $ (2.03 ) $ (2.07 ) $ 5.58 $ 7.13 Discontinued operations 1.29 2.41 .59 12.33 13.68 10.49 9.92 8.35 Total consolidated operations $ 47.28 $ (10.01 ) $ .72 $ 14.09 $ 11.65 $ 8.42 $ 15.50 $ 15.48 Diluted Continuing operations $ 45.96 $ (12.42 ) $ .13 $ 1.76 $ (2.03 ) $ (2.07 ) $ 5.58 $ 7.13 Discontinued operations 1.29 2.41 .59 12.33 13.68 10.49 9.92 8.35 Total consolidated operations $ 47.25 $ (10.01 ) $ .72 $ 14.09 $ 11.65 $ 8.42 $ 15.50 $ 15.48

Summary of Significant Accoun57

Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2012
Basis of Presentation
Assets held for sale	$ 4,407		$ 4,630.6
Percentage of par value of policy reinsured	15.00%
OneBeacon Ltd.
Basis of Presentation
Ownership interest (as a percent)	75.50%		75.30%
HG Global
Basis of Presentation
Surplus notes				$ 503
Percentage of par value of policy reinsured	15.00%
Star & Shield Insurance Exchange [Member]
Basis of Presentation
Surplus notes	$ 21		$ 17
Redeemable preferred stocks | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%		88.40%
Wholly Owned Properties [Member] | OneBeacon Ltd.
Basis of Presentation
Assets held for sale		$ 58

Summary of Significant Accoun58

Summary of Significant Accounting Policies (Investment Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)security	Dec. 31, 2014
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	91.00%	88.00%
Minimum percentage of variation expected	5.00%
Minimum variation expected | $	$ 1
Securities outside variance threshold | security	8

Summary of Significant Accoun59

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Warrants) (Details) - shares shares in Thousands	Dec. 31, 2013	Jun. 20, 2013	Dec. 31, 2012
Symetra
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments in unconsolidated affiliates on conversion of warrants	9,490	2,650	9,490

Summary of Significant Accoun60

Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Workers compensation discount percent	2.50%	2.50%
Workers compensation discount amount	$ 1.1	$ 1

Summary of Significant Accoun61

Summary of Significant Accounting Policies (Deferred Softward Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred software costs	$ 18.4	$ 15.4
Capitalized Computer Software, Amortization	$ 6	$ 4	$ 1.3
Maximum | Software Development [Member]
Useful lives	36 months
Maximum | Software and Software Development Costs [Member]
Useful lives	5 years

Summary of Significant Accoun62

Summary of Significant Accounting Policies (Foreign Currency Exchange) (Details) $ in Millions	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013
Significant Transactions
Net unrealized foreign currency translation losses	$ (145.6)	$ (79.8)
SEK
Significant Transactions
Foreign currency exchange rate	8.4247	7.7737	6.4339
GBP
Significant Transactions
Foreign currency exchange rate	0.6757	0.6426	0.6044
Euro Member Countries, Euro
Significant Transactions
Foreign currency exchange rate	0.9189	0.8245	0.7259

Summary of Significant Accoun63

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Item Effected [Line Items]
Net investment income									$ 60.8	$ 59.5	$ 59.8
Income Tax Expense (Benefit)	$ (1.5)	$ (1.6)	$ (2.2)	$ 4.6	$ (26.2)	$ (4.7)	$ 7.1	$ 9	$ (0.7)	(14.8)	32.6
OneBeacon Ltd.
Item Effected [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax										0.9	0.4
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd.
Item Effected [Line Items]
Net investment income										1.7	1.9
Income Tax Expense (Benefit)										$ 2.3	$ 2.2

Significant Transactions Signif

Significant Transactions Significant Transactions (Acquisitions) (Details) ₪ in Millions, $ in Millions	Nov. 06, 2015USD ($)	Oct. 10, 2014USD ($)	Sep. 30, 2015USD ($)	Jan. 31, 2014USD ($)	Feb. 23, 2015ILS (₪)	Feb. 23, 2015USD ($)	Feb. 19, 2014ILS (₪)	Feb. 19, 2014USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2015ILS (₪)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Sep. 01, 2015USD ($)	Mar. 14, 2014USD ($)	Dec. 31, 2012USD ($)
Unusual or Infrequent Item [Line Items]
Assets									$ 10,284.5				$ 10,455.7					$ 10,284.5	$ 10,455.7
Liabilities									5,916.5				5,917.3					5,916.5	5,917.3
White Mountains’s common shareholders’ equity, including portion attributable to NCI									4,368				4,538.4					4,368	4,538.4
Other assets									318.7				348.7					318.7	348.7
Goodwill									187.9				168.9					187.9	168.9	$ 0
Intangible Assets, Net (Excluding Goodwill)									187.8				182.3					187.8	182.3	5.1
Payments to Acquire Businesses and Interest in Affiliates																		37.1	190.1	0
Non-controlling interest in consolidated subsidiaries									6.1	$ (16)	$ (0.9)	$ (7.3)	(15)	$ (11.2)	$ 4.6	$ (0.6)		(18.1)	(22.2)	$ (12.5)
Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Liabilities						$ 1.2
Ownership interest (as a percent)						56.20%
Goodwill						$ 0.3
Cancerinsurance.com [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 3.1
Cash	1.1
Liabilities	$ 2
Business Combination, Contingent Consideration, Liability									6.8									6.8
Tranzact [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 177.7
Assets		332.8
Liabilities		$ 108.7
Ownership interest (as a percent)		63.20%
Distributed Earnings		$ 44.2
White Mountains’s common shareholders’ equity, including portion attributable to NCI		281.2
Goodwill		145.1							163.8				145.1					163.8	145.1
Intangible Assets, Net (Excluding Goodwill)		146.3							156.1				142.8					156.1	142.8
Business Combination, Contingent Consideration, Liability		7.4
Tranzutary [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates												$ 12
Assets		29.6
Liabilities													$ 4.1						$ 4.1
Ownership interest (as a percent)													36.00%						36.00%
Intangible Assets, Net (Excluding Goodwill)													$ 28.9						$ 28.9
TruBridge [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates			$ 31
Assets																					$ 54.5
Liabilities																					$ 4.3
Ownership interest (as a percent)																					100.00%
Goodwill																					$ 18.7
Intangible Assets, Net (Excluding Goodwill)																					$ 28.1
Business Combination, Contingent Consideration, Liability									9.7									9.7
MediaAlpha
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates																$ 28.1
Assets																						$ 70.1
Liabilities																						$ 10
Ownership interest (as a percent)																						60.00%
Goodwill									18.3				18.3					18.3	18.3			$ 18.3
Intangible Assets, Net (Excluding Goodwill)									24.4				32.5					$ 24.4	32.5			$ 38.5
Percent of gross profit																	62.50%	62.50%
Business Combination, Contingent Consideration, Liability									$ 7.8				$ 7.9					$ 7.8	$ 7.9
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Liabilities								$ 0.7
Ownership interest (as a percent)									96.10%				63.30%					96.10%	63.30%
Goodwill								$ 5.5	$ 5.8				$ 5.5					$ 5.8	$ 5.5
Intangible Assets, Net (Excluding Goodwill)									4.5				2.5					4.5	2.5
Star & Shield LLC [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				$ 1.8
Star & Shield Insurance Exchange [Member]
Unusual or Infrequent Item [Line Items]
Purchase of Surplus Notes									4				17					4	17
Surplus notes									$ 21				$ 17					21	$ 17
HG Global
Unusual or Infrequent Item [Line Items]
Surplus notes																							$ 503
Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)								54.00%
Israel, New Shekels | Preferred stocks | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪							₪ 31.5										₪ 79.6
Israel, New Shekels | Total | Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪					₪ 9.5												26.4
Israel, New Shekels | Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪							₪ 14.4										₪ 11.8
United States of America, Dollars | Preferred stocks | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								$ 9										20.7
United States of America, Dollars | Total | Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						2.4												6.5
United States of America, Dollars | Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								4.1										$ 3.1
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Assets						5.5		13.4
Intangible Assets, Net (Excluding Goodwill)						$ 2.8		$ 2.9
Tangible Assets [Member] | Tranzact [Member]
Unusual or Infrequent Item [Line Items]
Other assets		$ 41.4

Significant Transactions (Dispo

Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2016	Dec. 31, 2014	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jul. 24, 2015	Dec. 23, 2014
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax				$ 18.2	$ (1.6)	$ 46.6
Gain (Loss) on Disposition of Stock in Subsidiary				16.1	0	21.3
Premiums Written, Net				$ 1,172.6	$ 1,239	$ 1,102.2
Dividends declared and paid per White Mountains’s common share				$ 1	$ 1	$ 1
Total investments		$ 3,742.4		$ 4,280.7	$ 3,742.4
Symetra
Significant Transactions
Sale of Stock, Price Per Share				$ 32
Dividends declared and paid per White Mountains’s common share				$ 0.50
Sirius Group
Significant Transactions
Percentage of common shareholder's equity							127.30%
Additional cash that will be paid as part of the purchase price							$ 10
Total investments				$ 686.2
OneBeacon Runoff
Significant Transactions
Surplus notes								$ 101
Essentia | OneBeacon
Significant Transactions
Gain (Loss) on Disposition of Stock in Subsidiary			$ 23
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax			$ 15
Gain (Loss) from Adjustments to Prior Period Extraordinary Items				3.7
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Significant Transactions
Surplus notes								$ 64.9
Discontinued Operations
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax						$ 46.6
Discontinued Operations | Sirius Group
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax						0
Total investments		3,142.7		2,972.6	3,142.7
Discontinued Operations | OneBeacon Runoff
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax				0.3	$ (18.8)	$ 46.6
Surplus Note | Discontinued Operations | OneBeacon Runoff
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax		$ 36.1
Crop
Significant Transactions
Proceeds related from termination of agreement				$ 3
Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	$ 45
Industry Loss Trigger on Industry Loss Warranty	15,000
Sale of Sirius | JAPAN
Significant Transactions
Industry Loss Warranty Limit on Contract	25
Industry Loss Trigger on Industry Loss Warranty	12,500
Combination One [Member] | Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	75
Industry Loss Trigger on Industry Loss Warranty	40,000
Combination Two [Member] | Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	22.5
Industry Loss Trigger on Industry Loss Warranty	$ 50,000

Reserves for Unpaid Losses an66

Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ (4)	$ 92	$ 0
Increase (decrease) in reserves				708.9	824	622.1
Current Accident Year				712.9	732	$ 622.1
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	$ 75.5	$ 75.5	$ 14.3		75.5
Increase (decrease) in reserves		109.2	109.2	1.8	89.8
Current Accident Year		33.7	$ 33.7
Star & Shield Insurance Exchange [Member]
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 2.2	2.2
Professional Insurance | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		46.4
Increase (decrease) in reserves		69.3			59.1
Current Accident Year		22.9
Crop | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		0
Increase (decrease) in reserves		3.8			0
Current Accident Year		3.8
Specialty Products | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		51.7
Increase (decrease) in reserves		80.1			$ 61.8
Current Accident Year		28.4
Management's estimate [Member] | Specialty Products | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Current Accident Year		$ 29.9

Reserves for Unpaid Losses an67

Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 1,350	$ 1,054.3	$ 1,000
Less beginning reinsurance recoverable on unpaid losses	(161.7)	(80.2)	(107.3)
Net loss and LAE reserves	1,188.3	974.1	892.7
Current Accident Year	712.9	732	622.1
Prior Accident Year	(4)	92	0
Total incurred losses and LAE	708.9	824	622.1
Loss and LAE paid relating to current year losses	(208.8)	(202.6)	(188.6)
Loss and LAE paid relating to prior year losses	(479.1)	(420.8)	(352.1)
Total loss and LAE payments	(687.9)	(623.4)	(540.7)
Net loss and LAE reserves	1,209.3	1,188.3	974.1
Plus ending reinsurance recoverable on unpaid losses	186.5	161.7	80.2
Gross ending balance	1,395.8	1,350	1,054.3
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Loss and LAE reserves consolidated — SSIE	$ 0	$ 13.6	$ 0

Reserves for Unpaid Losses an68

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) - USD ($) $ in Millions	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year				$ 712.9	$ 732	$ 622.1
Prior Accident Year				(4)	92	0
Total incurred losses and LAE				708.9	824	$ 622.1
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		$ 33.7	$ 33.7
Prior Accident Year	$ 75.5	75.5	14.3		75.5
Total incurred losses and LAE		109.2	$ 109.2	$ 1.8	89.8
Professional Insurance | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		22.9
Prior Accident Year		46.4
Total incurred losses and LAE		69.3			59.1
Specialty Property | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		(1.1)
Prior Accident Year		5.7
Total incurred losses and LAE		4.6			1.1
Crop | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		3.8
Prior Accident Year		0
Total incurred losses and LAE		3.8			0
Other | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.8
Prior Accident Year		(0.4)
Total incurred losses and LAE		2.4			1.6
Specialty Products | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		28.4
Prior Accident Year		51.7
Total incurred losses and LAE		80.1			61.8
Entertainment | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.5
Prior Accident Year		11.6
Total incurred losses and LAE		13.1			13.5
Accident | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		0
Prior Accident Year		3.5
Total incurred losses and LAE		3.5			6
Government Risks | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.2
Prior Accident Year		7.1
Total incurred losses and LAE		8.3			8.5
Other | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.6
Prior Accident Year		1.6
Total incurred losses and LAE		4.2			0
Specialty Industries | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		5.3
Prior Accident Year		23.8
Total incurred losses and LAE		29.1			$ 28
Management's estimate [Member] | Specialty Products | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		$ 29.9

Third Party Reinsurance (Writte

Third Party Reinsurance (Written and Earned Premiums) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Written premiums:
Direct	$ 1,325,700,000	$ 1,296,300,000	$ 1,116,700,000
Assumed	36,000,000	65,900,000	59,800,000
Gross written premiums	1,361,700,000	1,362,200,000	1,176,500,000
Ceded	(189,100,000)	(123,200,000)	(74,300,000)
Net written premiums	1,172,600,000	1,239,000,000	1,102,200,000
Earned premiums:
Gross amount	1,322,000,000	1,233,500,000	1,043,800,000
Assumed from other companies	45,900,000	70,900,000	148,500,000
Gross earned premiums	1,367,900,000	1,304,400,000	1,192,300,000
Ceded	(179,700,000)	(119,400,000)	(71,400,000)
Net earned premiums	1,188,200,000	1,185,000,000	1,120,900,000
Losses and LAE:
Direct	802,500,000	802,800,000	584,900,000
Policyholder Benefits and Claims Incurred, Assumed	55,700,000	115,700,000	76,300,000
Gross losses and LAE	858,200,000	918,500,000	661,200,000
Ceded	(149,300,000)	(94,500,000)	(39,100,000)
Total incurred losses and LAE	708,900,000	824,000,000	622,100,000
OneBeacon
Written premiums:
Direct	1,279,900,000	1,257,500,000	1,103,100,000
Assumed	36,000,000	65,900,000	59,800,000
Gross written premiums	1,315,900,000	1,323,400,000	1,162,900,000
Ceded	(179,300,000)	(106,500,000)	(74,300,000)
Net written premiums	1,136,600,000	1,216,900,000	1,088,600,000
Earned premiums:
Gross amount	1,298,000,000	1,209,100,000	1,043,300,000
Assumed from other companies	45,900,000	70,900,000	148,500,000
Gross earned premiums	1,343,900,000	1,280,000,000	1,191,800,000
Ceded	(167,700,000)	(102,900,000)	(71,400,000)
Net earned premiums	1,176,200,000	1,177,100,000	1,120,400,000
Losses and LAE:
Direct	783,000,000	778,700,000	584,900,000
Policyholder Benefits and Claims Incurred, Assumed	55,700,000	115,700,000	76,300,000
Gross losses and LAE	838,700,000	894,400,000	661,200,000
Ceded	(138,000,000)	(79,300,000)	(39,100,000)
Total incurred losses and LAE	700,700,000	815,100,000	622,100,000
HG/BAM
Written premiums:
Direct	25,900,000	16,200,000	13,600,000
Assumed	0	0	0
Gross written premiums	25,900,000	16,200,000	13,600,000
Ceded	0	0	0
Net written premiums	25,900,000	16,200,000	13,600,000
Earned premiums:
Gross amount	3,300,000	1,800,000	500,000
Assumed from other companies	0	0	0
Gross earned premiums	3,300,000	1,800,000	500,000
Ceded	0	0	0
Net earned premiums	3,300,000	1,800,000	500,000
Losses and LAE:
Direct	0	0	0
Policyholder Benefits and Claims Incurred, Assumed	0	0	0
Gross losses and LAE	0	0	0
Ceded	0	0	0
Total incurred losses and LAE	0	0	0
HG/BAM [Member]
Written premiums:
Ceded	(19,300,000)	(12,300,000)	(10,600,000)
Earned premiums:
Ceded	(16,000,000)	(1,400,000)	(10,200,000)
Star & Shield Insurance Exchange [Member]
Written premiums:
Ceded		(16,000,000)
Earned premiums:
Ceded		(15,700,000)
Other Segments
Written premiums:
Direct	19,900,000	22,600,000
Assumed	0	0
Gross written premiums	19,900,000	22,600,000
Ceded	(9,800,000)	(16,700,000)
Net written premiums	10,100,000	5,900,000
Earned premiums:
Gross amount	20,700,000	22,600,000
Assumed from other companies	0	0
Gross earned premiums	20,700,000	22,600,000
Ceded	(12,000,000)	(16,500,000)
Net earned premiums	8,700,000	6,100,000	0
Losses and LAE:
Direct	19,500,000	24,100,000
Policyholder Benefits and Claims Incurred, Assumed	0	0
Gross losses and LAE	19,500,000	24,100,000
Ceded	(11,300,000)	(15,200,000)
Total incurred losses and LAE	8,200,000	8,900,000	$ 0
AMTrust [Member]
Written premiums:
Ceded	(33,300,000)
Earned premiums:
Ceded	(33,300,000)
Losses and LAE:
Ceded	$ 33,400,000
Percentage of Quota Share Reinsurance Agreement	100.00%
Star & Shield Insurance Exchange [Member]
Losses and LAE:
Ceded		$ (16,900,000)

Third-Party Reinsurance (OneBea

Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions	3 Months Ended	5 Months Ended	6 Months Ended	7 Months Ended	12 Months Ended				60 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2020	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2020	Dec. 31, 2013	Dec. 31, 2012
Effects of Reinsurance [Line Items]
Reinsurance recoverable on unpaid losses	$ 186.5	$ 186.5		$ 186.5			$ 186.5	$ 161.7		$ 80.2	$ 107.3
OneBeacon
Effects of Reinsurance [Line Items]
Catastrophe Event Ratio			0.40%
OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses	7.5	7.5		7.5			7.5
Reinsurance recoverable on unpaid losses	186	$ 186		186			$ 186
Property Catastrophe Program [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							95.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation							$ 10
Property Catastrophe Program [Member] | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							130
Property Catastrophe Program [Member] | Minimum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							$ 20
Second Casualty Excess of Loss [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Property Per-Risk, Acts of Terrorism | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 100,000
Property Per-Risk, Acts of Terrorism | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100
Property Per-Risk, Acts of Terrorism | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3
Property Per Risk Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100
Property Per Risk Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3	$ 5
Individual Risk Facultative [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 34
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							$ 6
Inland and Ocean Marine | Marine Treaty Per Occurrence | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							2.5
Inland and Ocean Marine | Marine Treaty | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							60
Inland and Ocean Marine | Layer One | Property Per Risk Treaty | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							20
Inland and Ocean Marine | Layer One | Catastrophic Losses [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							2.5
Annual Aggregate Deductible to Reinsurance Retention Policy							5
Reinsurance retention policy, excess retention, amount reinsured							7.5
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							10
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3
Health Insurance Product Line | HMO Provider Excess Reinsurance [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							5
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							11
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 3
Workers compensation | Casualty Reinsurance Program | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Workers compensation | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 10
Workers compensation | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							2
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							10
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							60
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							10
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							40
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							20
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							10
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							21
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							11
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 100
Reinsurance Retention Policy, Reinsured Risk, Percentage	100.00%
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 50
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5
Crop | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage		100.00%
Film Completion Bond Business [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				35
Film Completion Bond Business [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				2
Film Completion Bond Business [Member] | Facultative Treaty Layer [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				60
OneBeacon Runoff | BBB | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	20.4	$ 20.4		20.4			20.4
Scenario, Forecast [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act					80.00%	84.00%
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act									1.00%
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act					20.00%	16.00%
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act						1.00%
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained						$ 120
Reinsurance Retention Policy, Reinsured Risk, Percentage						84.00%
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained						$ 100,000
Department of Treasury [Member] | Scenario, Forecast [Member] | Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured					$ 200	$ 120			$ 20
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	193.5	193.5		193.5			$ 193.5
Reinsurance recoverables, percentage of total							100.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AA | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	41.7	41.7		41.7			$ 41.7
Reinsurance recoverables, percentage of total							22.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | A | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	126.2	126.2		126.2			$ 126.2
Reinsurance recoverables, percentage of total							65.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | BBB | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	$ 25.6	$ 25.6		$ 25.6			$ 25.6
Reinsurance recoverables, percentage of total							13.00%
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			$ 7,500
Reinsurance retention policy, excess retention, amount reinsured			5
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000
Reinsurance retention policy, excess retention, amount reinsured			5
Expiration December 2015 [Member] | All Natural Perils [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			15,000
Reinsurance retention policy, excess retention, amount reinsured			15
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured			30
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			10,000
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured			7.5
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			7,500
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000
Expiration June 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			20,000
Reinsurance retention policy, excess retention, amount reinsured			$ 5

Third Party Reinsurance (Recove

Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid and unpaid losses	$ 194	$ 173.9

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment income:
Total investment income	$ 66.2	$ 72.4	$ 72.2
Third-party investment expenses	(5.4)	(12.9)	(12.4)
Net investment income, pre-tax	60.8	59.5	59.8
Fixed maturity investments
Investment income:
Total investment income	52.6	51.3	48.1
Short-term investments
Investment income:
Total investment income	0.2	0.1	0.8
Common Stock
Investment income:
Total investment income	10.1	16.6	16.5
Other long-term investments
Investment income:
Total investment income	$ 3.3	$ 4.4	$ 6.8

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 77.3	$ 166.8	$ 95.5
Net unrealized investment gains (losses), pre-tax	148.1	(88.3)	38.4
Net realized and unrealized investment gains, pre-tax	225.4	78.5	133.9
Income tax expense attributable to net realized and unrealized investment gains	(35.2)	(18)	(12.8)
Net realized and unrealized investment gains, after tax	$ 190.2	$ 60.5	$ 121.1

Investment Securities Investmen

Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Trading Securities, Realized Gain (Loss)	$ 76.9	$ 166.8	$ 95.7
Foreign Currency Transaction Gain (Loss), Realized	0.4	0	(0.2)
Net realized investment gains, pre-tax	77.3	166.8	95.5
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(22.8)	(28.4)	(16.2)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0	0	0.1
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(22.8)	(28.4)	(16.1)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	54.1	138.4	79.5
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	0.4	0	(0.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	54.5	138.4	79.4
Fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	1.9	5.8	(0.2)
Foreign Currency Transaction Gain (Loss), Realized	0	0	(0.2)
Net realized investment gains, pre-tax	1.9	5.8	(0.4)
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)			0.1
Foreign Currency Transaction Gain (Loss), Realized			0
Net realized investment gains, pre-tax			0.1
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	64.4	138	89.4
Foreign Currency Transaction Gain (Loss), Realized	0.4	0	0
Net realized investment gains, pre-tax	64.8	138	89.4
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	10.6	23	6.4
Foreign Currency Transaction Gain (Loss), Realized	0	0	0
Net realized investment gains, pre-tax	$ 10.6	$ 23	$ 6.4

Investment Securities Investm75

Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 152.9	$ (79.3)	$ 40.3
Foreign Currency Transaction Gain (Loss), Unrealized	(4.8)	(9)	(1.9)
Net unrealized investment gains (losses), pre-tax	148.1	(88.3)	38.4
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(12.3)	9.9	3.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	(0.1)	0.5	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(12.4)	10.4	3.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	140.6	(69.4)	43.6
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(4.9)	(8.5)	(1.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	135.7	(77.9)	41.7
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(15.6)	11.3	(49.9)
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0
Net unrealized investment gains (losses), pre-tax	(15.6)	11.3	(49.9)
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	207.6	(83)	86.1
Foreign Currency Transaction Gain (Loss), Unrealized	(3.7)	(7.7)	(1.9)
Net unrealized investment gains (losses), pre-tax	203.9	(90.7)	84.2
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(39.1)	(7.6)	4.1
Foreign Currency Transaction Gain (Loss), Unrealized	(1.1)	(1.3)	0
Net unrealized investment gains (losses), pre-tax	$ (40.2)	$ (8.9)	$ 4.1

Investment Securities (Investme

Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		$ 152.9	$ (79.3)	$ 40.3
Realized gains		112.9	194	152.8
Realized losses		35.6	27.2	57.3
Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		(50.2)	7.7	1.4
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		(15.6)	11.3	(49.9)
Fixed maturity investments | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		(1.1)	1.9	(1.6)
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		207.6	(83)	86.1
Common equity securities | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		(9)	5.8	0.9
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		(39.1)	(7.6)	4.1
Other long-term investments | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)		$ (40.1)	$ 0	$ 2.1
Symetra
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 258.8

Investment Securities (Net Re77

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions	10 Months Ended	12 Months Ended
	Nov. 05, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates		$ (39.2)	$ 81.2	$ (106.4)
Income tax benefit (expense)		2.9	(5.9)	8.3
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax		(36.3)	75.3	(98.1)
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax		(34.9)	75.3	(98.1)
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax		190.2	60.5	121.1
Total investment gains recorded during the period, after-tax		155.3	135.8	23
Symetra
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates	$ (39.2)		81.2	(106.4)
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax		(34.9)	75.3	(98.1)
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra		$ 1.4	$ 0	$ 0

Investment Securities (Invest78

Investment Securities (Investment Holdings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fixed maturity investments, at fair value	$ 2,639.7	$ 2,422
Including Short Term Fixed
Investment securities
Duration of Fixed Maturities	2 years 2 months 12 days
US Government and agency obligations
Investment securities
Trading Securities, Cost	$ 160.4	105.4
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	160	105.2
Trading Securities, Unrealized Holding Gain	0	0.1
Trading Securities, Unrealized Holding Loss	(0.4)	(0.3)
Debt securities issued by corporations
Investment securities
Trading Securities, Cost	1,001	1,162
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1,000	1,171.7
Trading Securities, Unrealized Holding Gain	4.3	13.1
Trading Securities, Unrealized Holding Loss	(5.3)	(3.4)
Municipal obligations
Investment securities
Trading Securities, Cost	227.8	81
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	228.8	82.2
Trading Securities, Unrealized Holding Gain	2.2	1.4
Trading Securities, Unrealized Holding Loss	(1.2)	(0.2)
Mortgage-backed and asset-backed securities
Investment securities
Trading Securities, Cost	1,170.6	967.5
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1,167	967.9
Trading Securities, Unrealized Holding Gain	2	2.8
Trading Securities, Unrealized Holding Loss	(5.6)	(2.4)
Foreign government, agency and provincial obligations
Investment securities
Trading Securities, Cost	1	11.5
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	1.2	10.8
Trading Securities, Unrealized Holding Gain	0.2	0.3
Trading Securities, Unrealized Holding Loss	0	(1)
Preferred stocks
Investment securities
Trading Securities, Cost	78.3	78.3
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	82.7	84.2
Trading Securities, Unrealized Holding Gain	4.4	5.9
Trading Securities, Unrealized Holding Loss	0	0
Fixed Income Investments [Member]
Investment securities
Trading Securities, Cost	2,639.1	2,405.7
Net foreign currency gains (losses)	0	0
Total fixed maturity investments	2,639.7	2,422
Trading Securities, Unrealized Holding Gain	13.1	23.6
Trading Securities, Unrealized Holding Loss	$ (12.5)	$ (7.3)
Excluding Short Term Investments
Investment securities
Duration of Fixed Maturities	2 years 4 months 24 days

Investment Securities (Cost and

Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) $ in Millions	Dec. 31, 2015USD ($)
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	$ 2,639.1
Trading Securities by Maturity, Fair Value
Trading Securities	2,639.7
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	241.6
Due after one year through five years	982.3
Due after five years through ten years	96.4
Due after ten years	69.9
Trading Securities by Maturity, Fair Value
Due in one year or less	242
Due after one year through five years	981.5
Due after five years through ten years	96.1
Due after ten years	70.4
Mortgage-backed and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	1,170.6
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	1,167
Preferred stocks
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	78.3
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	$ 82.7

Investment Securities (Cost or

Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fixed maturity investments, at fair value	$ 2,639.7	$ 2,422
Sales and maturities of investment securities	2,162.8	3,337.5	$ 2,672.8
Common Stock
Investment securities
Trading Securities, Cost	822.5	495.3
Net foreign currency gains (losses)	0	(7.4)
Fixed maturity investments, at fair value	1,113.9	611.7
Trading Securities, Unrealized Holding Gain	302.8	127.4
Trading Securities, Unrealized Holding Loss	(11.4)	(3.6)
Other long-term investments
Investment securities
Trading Securities, Cost	304.5	286.7
Net foreign currency gains (losses)	(2.3)	(1.3)
Fixed maturity investments, at fair value	315.8	331.9
Trading Securities, Unrealized Holding Gain	32	53.3
Trading Securities, Unrealized Holding Loss	$ (18.4)	$ (6.8)

Investment Securities (Invest81

Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Investments held in trusts	$ 104.8	$ 103.4
Assets held by insurance regulators	88	68.9
Surety Product Line [Member] | OneBeacon
Investment securities
Assets Held in Collateral	137.7	81
Fixed maturity investments | White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	$ 0	$ 9.5

Investment Securities (Fair Val

Investment Securities (Fair Value Measurement by Level) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Percentage of investments recorded at fair value	91.00%	88.00%
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
Fair value, investment-related liabilities		31.6
Debt securities issued by corporations
Investment securities
Fair value investments	1,000	1,171.7
Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	1,167	967.9
Discontinued Operations
Investment securities
Investments reclassified to assets held for sale			236.3
Level 1
Investment securities
Fair value investments	1,152.2	550.6	997.2
Level 2
Investment securities
Fair value investments	2,531.4	2,372.9	2,575.4
Level 2 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	1,167	967.9
Level 3 Inputs | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	0	0
Level 3 Inputs | Fixed maturity investments
Investment securities
Fair value investments	70	76.4	81.8
Level 3 Inputs | Common equity securities
Investment securities
Fair value investments	0	39.5	45.4
Level 3 Inputs | Other long-term investments
Investment securities
Fair value investments	297.3	304.2	189.2
Fair value measured on a recurring basis
Investment securities
Fair value investments	4,262.2	3,720.4
Fair value measured on a recurring basis | US Government and agency obligations
Investment securities
Fair value investments	160	105.2
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	1,000	1,171.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Investment securities
Fair value investments	175.9	254.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments	253.3	259.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments	135.6	107.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Investment securities
Fair value investments	49.2	71.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Investment securities
Fair value investments	61.5	84.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments	82	65.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Investment securities
Fair value investments	31.2	51.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Investment securities
Fair value investments	60	88.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Other
Investment securities
Fair value investments	0	7.5
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	1,167	967.9
Fair value measured on a recurring basis | Preferred stocks
Investment securities
Fair value investments	82.7	84.2
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fair value investments	228.8	82.2
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Investment securities
Fair value investments	1.2	10.8
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fair value investments	2,639.7	2,422
Fair value measured on a recurring basis | Short-term investments
Investment securities
Fair value investments	211.3	376.8	310.4
Fair value measured on a recurring basis | Common equity securities | Financials
Investment securities
Fair value investments	694.7	180.7
Fair value measured on a recurring basis | Common equity securities | Consumer
Investment securities
Fair value investments	70	116
Fair value measured on a recurring basis | Common equity securities | Industrial
Investment securities
Fair value investments	26.6	57.1
Fair value measured on a recurring basis | Common equity securities | Communications
Investment securities
Fair value investments	43.7	28.5
Fair value measured on a recurring basis | Common equity securities | Energy
Investment securities
Fair value investments		4.9
Fair value measured on a recurring basis | Common equity securities | Utilities
Investment securities
Fair value investments		18.1
Fair value measured on a recurring basis | Common equity securities | Materials
Investment securities
Fair value investments		12
Fair value measured on a recurring basis | Common equity securities | Technology
Investment securities
Fair value investments	27	44.8
Fair value measured on a recurring basis | Common equity securities | Other
Investment securities
Fair value investments	74.4	75.2
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member]
Investment securities
Fair value investments	141.8
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Investment securities
Fair value investments	1,113.9	611.7
Fair value measured on a recurring basis | Other long-term investments
Investment securities
Fair value investments	297.3	309.9
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	1,363.5	926.9
Fair value measured on a recurring basis | Level 1 | US Government and agency obligations
Investment securities
Fair value investments	133.4	51.2
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Other
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Preferred stocks
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Municipal obligations
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 1 | Foreign government, agency and provincial obligations
Investment securities
Fair value investments	0.6	0.6
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments
Investment securities
Fair value investments	134	51.8
Fair value measured on a recurring basis | Level 1 | Short-term investments
Investment securities
Fair value investments	211.3	376.3
Fair value measured on a recurring basis | Level 1 | Common equity securities | Financials
Investment securities
Fair value investments	694.7	141.2
Fair value measured on a recurring basis | Level 1 | Common equity securities | Consumer
Investment securities
Fair value investments	70	115.9
Fair value measured on a recurring basis | Level 1 | Common equity securities | Industrial
Investment securities
Fair value investments	26.6	57.1
Fair value measured on a recurring basis | Level 1 | Common equity securities | Communications
Investment securities
Fair value investments	43.7	28.5
Fair value measured on a recurring basis | Level 1 | Common equity securities | Energy
Investment securities
Fair value investments		4.9
Fair value measured on a recurring basis | Level 1 | Common equity securities | Utilities
Investment securities
Fair value investments		18.1
Fair value measured on a recurring basis | Level 1 | Common equity securities | Materials
Investment securities
Fair value investments		12
Fair value measured on a recurring basis | Level 1 | Common equity securities | Technology
Investment securities
Fair value investments	27	44.8
Fair value measured on a recurring basis | Level 1 | Common equity securities | Other
Investment securities
Fair value investments	0	1.9
Fair value measured on a recurring basis | Level 1 | Common equity securities | Exchange Traded Funds [Member]
Investment securities
Fair value investments	120.5
Fair value measured on a recurring basis | Level 1 | Common equity securities | Common equity securities
Investment securities
Fair value investments	1,018.2	498.8
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments	2,531.4	2,373.4
Fair value measured on a recurring basis | Level 2 | US Government and agency obligations
Investment securities
Fair value investments	26.6	54
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	1,000	1,166.4
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials
Investment securities
Fair value investments	175.9	254.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments	253.3	259.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments	135.6	102
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications
Investment securities
Fair value investments	49.2	71.5
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy
Investment securities
Fair value investments	61.5	84.1
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments	82	65.6
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials
Investment securities
Fair value investments	31.2	51.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology
Investment securities
Fair value investments	60	88.3
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Other
Investment securities
Fair value investments	0	7.5
Fair value measured on a recurring basis | Level 2 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	1,167	967.9
Fair value measured on a recurring basis | Level 2 | Preferred stocks
Investment securities
Fair value investments	12.7	13.1
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Investment securities
Fair value investments	228.8	82.2
Fair value measured on a recurring basis | Level 2 | Foreign government, agency and provincial obligations
Investment securities
Fair value investments	0.6	10.2
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Investment securities
Fair value investments	2,435.7	2,293.8
Fair value measured on a recurring basis | Level 2 | Short-term investments
Investment securities
Fair value investments	0	0.5
Fair value measured on a recurring basis | Level 2 | Common equity securities | Financials
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Consumer
Investment securities
Fair value investments	0	0.1
Fair value measured on a recurring basis | Level 2 | Common equity securities | Industrial
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Communications
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Energy
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Utilities
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Materials
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Technology
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other
Investment securities
Fair value investments	74.4	73.3
Fair value measured on a recurring basis | Level 2 | Common equity securities | Exchange Traded Funds [Member]
Investment securities
Fair value investments	21.3
Fair value measured on a recurring basis | Level 2 | Common equity securities | Common equity securities
Investment securities
Fair value investments	95.7	73.4
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Investment securities
Fair value investments	0	5.7
Fair value measured on a recurring basis | Level 3 Inputs
Investment securities
Fair value investments	367.3	420.1
Fair value measured on a recurring basis | Level 3 Inputs | US Government and agency obligations
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0	5.3
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Financials
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments	0	5.3
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Communications
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Energy
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Materials
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Technology
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Other
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Preferred stocks
Investment securities
Fair value investments	70	71.1
Fair value measured on a recurring basis | Level 3 Inputs | Municipal obligations
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Foreign government, agency and provincial obligations
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Fixed maturity investments
Investment securities
Fair value investments	70	76.4
Fair value measured on a recurring basis | Level 3 Inputs | Short-term investments
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Financials
Investment securities
Fair value investments	0	39.5
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Consumer
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Industrial
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Communications
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Energy
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Utilities
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Materials
Investment securities
Fair value investments		0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Technology
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Other
Investment securities
Fair value investments	0	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Exchange Traded Funds [Member]
Investment securities
Fair value investments	0
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Common equity securities
Investment securities
Fair value investments	0	39.5
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments
Investment securities
Fair value investments	297.3	304.2
Carrying value of investment accounted for using the equity method	3.8	5.2	$ 6.8
Affordable Housing Development Fund [Member] | Other long-term investments
Investment securities
Fair value investments	$ 14.7	$ 16.8

Investment Securities (Debt Sec

Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
Debt securities issued by corporations
Investment securities
Fair value investments	1,000	1,171.7
Other Standard Poors Rating [Member] | Debt securities issued by corporations
Investment securities
Fair value investments	0	5.4
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	507.1	426.5
A | Debt securities issued by corporations
Investment securities
Fair value investments	397.7	594.9
AA | Debt securities issued by corporations
Investment securities
Fair value investments	$ 95.2	$ 144.9

Investment Securities (Mortgage

Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
GNMA
Investment securities
Fair value investments	265.5	308.7
FNMA
Investment securities
Fair value investments	42.2	33.4
FHLMC
Investment securities
Fair value investments	22.8	28.9
Total Agency
Investment securities
Fair value investments	330.5	371
Residential
Investment securities
Fair value investments	133.2	71
Commercial
Investment securities
Fair value investments	140.4	109.3
Total Non-agency
Investment securities
Fair value investments	273.6	180.3
Total mortgage-backed securities
Investment securities
Fair value investments	604.1	551.3
Credit card receivables
Investment securities
Fair value investments	217.7	218.1
Vehicle receivables
Investment securities
Fair value investments	269.7	152.8
Other
Investment securities
Fair value investments	75.5	45.7
Total asset-backed securities
Investment securities
Fair value investments	562.9	416.6
Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,167	967.9
Non-Agency Residential Mortgage
Investment securities
Fair value investments	39.6
Level 2 Inputs
Investment securities
Fair value investments	2,531.4	2,372.9	$ 2,575.4
Level 2 Inputs | GNMA
Investment securities
Fair value investments	265.5	308.7
Level 2 Inputs | FNMA
Investment securities
Fair value investments	42.2	33.4
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	22.8	28.9
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	330.5	371
Level 2 Inputs | Residential
Investment securities
Fair value investments	133.2	71
Level 2 Inputs | Commercial
Investment securities
Fair value investments	140.4	109.3
Level 2 Inputs | Total Non-agency
Investment securities
Fair value investments	273.6	180.3
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	604.1	551.3
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	217.7	218.1
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	269.7	152.8
Level 2 Inputs | Other
Investment securities
Fair value investments	75.5	45.7
Level 2 Inputs | Total asset-backed securities
Investment securities
Fair value investments	562.9	416.6
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,167	967.9
Level 3 Inputs | GNMA
Investment securities
Fair value investments	0	0
Level 3 Inputs | FNMA
Investment securities
Fair value investments	0	0
Level 3 Inputs | FHLMC
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total Agency
Investment securities
Fair value investments	0	0
Level 3 Inputs | Residential
Investment securities
Fair value investments	0	0
Level 3 Inputs | Commercial
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total Non-agency
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	0	0
Level 3 Inputs | Credit card receivables
Investment securities
Fair value investments	0	0
Level 3 Inputs | Vehicle receivables
Investment securities
Fair value investments	0	0
Level 3 Inputs | Other
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total asset-backed securities
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0	$ 0

Investment Securities (Non-agen

Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
Residential
Investment securities
Fair value investments	133.2	71
Commercial
Investment securities
Fair value investments	140.4	109.3
Total Non-agency
Investment securities
Fair value investments	273.6	$ 180.3
Securities Issued in 2004 [Member] | Residential
Investment securities
Fair value investments	17.3
Securities Issued in 2004 [Member] | Commercial
Investment securities
Fair value investments	0
Securities Issued in 2004 [Member] | Total Non-agency
Investment securities
Fair value investments	17.3
2005 | Residential
Investment securities
Fair value investments	8.6
2005 | Commercial
Investment securities
Fair value investments	0
2005 | Total Non-agency
Investment securities
Fair value investments	8.6
2006 | Residential
Investment securities
Fair value investments	3.9
2006 | Commercial
Investment securities
Fair value investments	0
2006 | Total Non-agency
Investment securities
Fair value investments	3.9
2007 | Residential
Investment securities
Fair value investments	0
2007 | Commercial
Investment securities
Fair value investments	0
2007 | Total Non-agency
Investment securities
Fair value investments	0
2008 | Residential
Investment securities
Fair value investments	3.6
2008 | Commercial
Investment securities
Fair value investments	0
2008 | Total Non-agency
Investment securities
Fair value investments	3.6
2009 | Residential
Investment securities
Fair value investments	0
2009 | Commercial
Investment securities
Fair value investments	0
2009 | Total Non-agency
Investment securities
Fair value investments	0
2010 | Residential
Investment securities
Fair value investments	15.2
2010 | Commercial
Investment securities
Fair value investments	5.4
2010 | Total Non-agency
Investment securities
Fair value investments	20.6
2011 | Residential
Investment securities
Fair value investments	13.5
2011 | Commercial
Investment securities
Fair value investments	0
2011 | Total Non-agency
Investment securities
Fair value investments	13.5
2012 | Residential
Investment securities
Fair value investments	9.8
2012 | Commercial
Investment securities
Fair value investments	18
2012 | Total Non-agency
Investment securities
Fair value investments	27.8
2013 | Residential
Investment securities
Fair value investments	14
2013 | Commercial
Investment securities
Fair value investments	17.2
2013 | Total Non-agency
Investment securities
Fair value investments	31.2
2014 | Residential
Investment securities
Fair value investments	47.3
2014 | Commercial
Investment securities
Fair value investments	55
2014 | Total Non-agency
Investment securities
Fair value investments	102.3
Securities Issued in 2015 [Member] | Residential
Investment securities
Fair value investments	0
Securities Issued in 2015 [Member] | Commercial
Investment securities
Fair value investments	44.8
Securities Issued in 2015 [Member] | Total Non-agency
Investment securities
Fair value investments	$ 44.8

Investment Securities (Non-ag86

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
Residential
Investment securities
Fair value investments	133.2	$ 71
Residential | Prime [Member]
Investment securities
Fair value investments	133.1
Residential | Non-prime
Investment securities
Fair value investments	0.1
Residential | Subprime [Member]
Investment securities
Fair value investments	0
Residential | Super Senior
Investment securities
Fair value investments	65.6
Residential | Super Senior | Prime [Member]
Investment securities
Fair value investments	65.6
Residential | Super Senior | Non-prime
Investment securities
Fair value investments	0
Residential | Super Senior | Subprime [Member]
Investment securities
Fair value investments	0
Residential | Senior
Investment securities
Fair value investments	67.6
Residential | Senior | Prime [Member]
Investment securities
Fair value investments	67.5
Residential | Senior | Non-prime
Investment securities
Fair value investments	0.1
Residential | Senior | Subprime [Member]
Investment securities
Fair value investments	0
Residential | Subordinate
Investment securities
Fair value investments	0
Residential | Subordinate | Prime [Member]
Investment securities
Fair value investments	0
Residential | Subordinate | Non-prime
Investment securities
Fair value investments	0
Residential | Subordinate | Subprime [Member]
Investment securities
Fair value investments	$ 0

Investment Securities (Other Lo

Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Cost	$ 3,958,800
Other Investments	315,800	$ 331,900
Private equity funds
Investment securities
Other Investments	82,100	45,300
Surplus Note
Investment securities
Other Investments	51,500	65,100
Convertible fixed maturity investments
Investment securities
Other Investments	0	5,600
Private equity securities
Investment securities
Other Investments	32,700	8,300
Affordable Housing Development Fund [Member]
Investment securities
Other Investments	14,700	16,800
Partnership investments [Member]
Investment securities
Other Investments	3,800	5,200
Total hedge and private equity funds included in other long-term investments
Investment securities
Other Investments	127,800	178,500
Other Debt Obligations [Member]
Investment securities
Other Investments	3,200	7,100
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments
Investment securities
Carrying value of investment accounted for using the equity method	$ 3,800	$ 5,200	$ 6,800

Investment Securities (Non-ag88

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889
Fixed rate CMBS
Investment securities
Average Basis Points of Subordination	0.25
Fair value investments	$ 101.1
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	8.7
Fixed rate CMBS | Senior
Investment securities
Fair value investments	48.6
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	43.8
Floating rate CMBS
Investment securities
Fair value investments	39.3
Floating rate CMBS | Super Senior
Investment securities
Fair value investments	0
Floating rate CMBS | Senior
Investment securities
Fair value investments	0
Floating rate CMBS | Subordinate
Investment securities
Fair value investments	39.3
Commercial
Investment securities
Fair value investments	140.4	$ 109.3
Commercial | Super Senior
Investment securities
Fair value investments	8.7
Commercial | Senior
Investment securities
Fair value investments	48.6
Commercial | Subordinate
Investment securities
Fair value investments	$ 83.1
Maximum | Commercial
Investment securities
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans	1.00%

Investment Securities (Fair V89

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2015USD ($)fund	Dec. 31, 2014USD ($)
Private equity funds
Investment securities
Number of Investments Held | fund	22
Trading Securities, Other	$ 90.8
Hedge funds
Investment securities
Number of Investments Held | fund	5
Trading Securities, Other	$ 37
Total hedge and private equity funds included in other long-term investments
Investment securities
Trading Securities, Other	127.8	$ 178.5
Fair value unfunded commitments	82.2	64.3
Other Long-term Investments Largest Single Investment	22.7	22
Private equity funds | Private equity funds
Investment securities
Trading Securities, Other	90.8	104.1
Fair value unfunded commitments	82.2	64.3
Private equity funds | Multi-sector
Investment securities
Trading Securities, Other	14.8	14.5
Fair value unfunded commitments	2.1	2.2
Private equity funds | Energy infrastructure & services
Investment securities
Trading Securities, Other	20.7	33.1
Fair value unfunded commitments	3.4	4.8
Private equity funds | Real estate
Investment securities
Trading Securities, Other	0.4	1.7
Fair value unfunded commitments	0.1	0.1
Private equity funds | Private equity secondaries
Investment securities
Trading Securities, Other	4.4	5.7
Fair value unfunded commitments	2.1	2.1
Private equity funds | Manufacturing/Industrial
Investment securities
Trading Securities, Other	24.9	23.2
Fair value unfunded commitments	2.5	7.3
Private equity funds | Healthcare
Investment securities
Trading Securities, Other	3.8	3.1
Fair value unfunded commitments	0.4	1.4
Private equity funds | Insurance
Investment securities
Trading Securities, Other	2	2.1
Fair value unfunded commitments	41.3	41.3
Private equity funds | Aerospace/Defense/Government
Investment securities
Trading Securities, Other	19.8	20.7
Fair value unfunded commitments	30.3	5.1
Hedge funds | Hedge funds
Investment securities
Trading Securities, Other	37	74.4
Fair value unfunded commitments	0	0
Hedge funds | Long/short credit & distressed
Investment securities
Trading Securities, Other	0	8.4
Fair value unfunded commitments	0	0
Hedge funds | Long/short equity REIT
Investment securities
Trading Securities, Other	20.6	20.4
Fair value unfunded commitments	0	0
Hedge funds | Other hedge funds
Investment securities
Trading Securities, Other	3.6	15.7
Fair value unfunded commitments	0	0
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Investment securities
Trading Securities, Other	12.8	29.9
Fair value unfunded commitments	$ 0	$ 0

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Dec. 31, 2015USD ($)
Investment securities
Trading Securities, Other	$ 37
Distributions from inactive hedge funds	1
Outstanding redemptions	2.5
30-59 days notice
Investment securities
Trading Securities, Other	13.8
60-89 days notice
Investment securities
Trading Securities, Other	20.6
90-119 days notice
Investment securities
Trading Securities, Other	2.6
120 days notice
Investment securities
Trading Securities, Other	0
Monthly
Investment securities
Trading Securities, Other	0
Monthly | 30-59 days notice
Investment securities
Trading Securities, Other	0
Monthly | 60-89 days notice
Investment securities
Trading Securities, Other	0
Monthly | 90-119 days notice
Investment securities
Trading Securities, Other	0
Monthly | 120 days notice
Investment securities
Trading Securities, Other	0
Quarterly
Investment securities
Trading Securities, Other	13.8
Quarterly | 30-59 days notice
Investment securities
Trading Securities, Other	13.8
Quarterly | 60-89 days notice
Investment securities
Trading Securities, Other	0
Quarterly | 90-119 days notice
Investment securities
Trading Securities, Other	0
Quarterly | 120 days notice
Investment securities
Trading Securities, Other	0
Semi-annual
Investment securities
Trading Securities, Other	20.6
Semi-annual | 30-59 days notice
Investment securities
Trading Securities, Other	0
Semi-annual | 60-89 days notice
Investment securities
Trading Securities, Other	20.6
Semi-annual | 90-119 days notice
Investment securities
Trading Securities, Other	0
Semi-annual | 120 days notice
Investment securities
Trading Securities, Other	0
Annual
Investment securities
Trading Securities, Other	2.6
Annual | 30-59 days notice
Investment securities
Trading Securities, Other	0
Annual | 60-89 days notice
Investment securities
Trading Securities, Other	0
Annual | 90-119 days notice
Investment securities
Trading Securities, Other	2.6
Annual | 120 days notice
Investment securities
Trading Securities, Other	$ 0

Investment Securities (Invest91

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity funds $ in Millions	Dec. 31, 2015USD ($)
Investment securities
Trading Securities, Other	$ 90.8
1-3 years
Investment securities
Trading Securities, Other	24.3
3 – 5 years
Investment securities
Trading Securities, Other	10.9
5 – 10 years
Investment securities
Trading Securities, Other	55.6
10 years
Investment securities
Trading Securities, Other	$ 0

Investment Securities (Rollforw

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	12 Months Ended
	Dec. 31, 2015USD ($)Investment	Dec. 31, 2014USD ($)Investment	Dec. 31, 2013USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 3,343,600,000	$ 3,889,000,000
Total realized and unrealized gains (losses)	(226,800,000)	83,300,000
Amortization/Accretion	(19,600,000)	(18,500,000)
Purchases	(2,377,100,000)	2,868,500,000
Sales	(2,210,100,000)	3,500,200,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(394,500,000)	21,500,000
Fair Value, Assets Measured on Recurring Basis, Other	(61,400,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	41,800,000	53,000,000
Transfers out	(41,800,000)	(53,000,000)
Fair value investments	4,050,900,000	3,343,600,000	$ 3,889,000,000
Net realized and unrealized investment gains	225,400,000	78,500,000	133,900,000
Discontinued Operations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments reclassified to assets held for sale (1)			(236,300,000)
Net realized and unrealized investment gains	15,100,000	205,400,000	27,800,000
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	3,720,400,000
Fair value investments	4,262,200,000	3,720,400,000
US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	105,200,000
Fair value investments	160,000,000	105,200,000
Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,422,000,000
Fair value investments	2,639,700,000	2,422,000,000
Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	309,900,000
Fair value investments	297,300,000	309,900,000
Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	376,800,000	310,400,000
Fair value investments	211,300,000	376,800,000	310,400,000
Debt securities issued by corporations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,171,700,000
Fair value investments	1,000,000,000	1,171,700,000
Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,171,700,000
Fair value investments	1,000,000,000	1,171,700,000
Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	82,200,000
Fair value investments	228,800,000	82,200,000
Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	10,800,000
Fair value investments	1,200,000	10,800,000
Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,200,000
Fair value investments	82,700,000	84,200,000
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	550,600,000	997,200,000
Total realized and unrealized gains (losses)	(263,000,000)	50,100,000
Amortization/Accretion	0	100,000
Purchases	(814,500,000)	886,900,000
Sales	(825,700,000)	1,384,200,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	(394,500,000)	(2,700,000)
Fair Value, Assets Measured on Recurring Basis, Other	(43,500,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		3,400,000
Transfers in	0	0
Transfers out	(1,200,000)	(200,000)
Fair value investments	1,152,200,000	550,600,000	997,200,000
Level 1 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	926,900,000
Fair value investments	1,363,500,000	926,900,000
Level 1 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,200,000
Fair value investments	133,400,000	51,200,000
Level 1 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,800,000
Fair value investments	134,000,000	51,800,000
Level 1 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	376,300,000
Fair value investments	211,300,000	376,300,000
Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 1 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	600,000
Fair value investments	600,000	600,000
Level 1 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,372,900,000	2,575,400,000
Total realized and unrealized gains (losses)	13,700,000	8,900,000
Amortization/Accretion	(19,600,000)	(18,700,000)
Purchases	(1,436,000,000)	1,805,300,000
Sales	(1,286,000,000)	2,065,900,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	24,200,000
Fair Value, Assets Measured on Recurring Basis, Other	0
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		(9,300,000)
Transfers in	41,800,000	53,000,000
Transfers out	0	0
Fair value investments	2,531,400,000	2,372,900,000	2,575,400,000
Level 2 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,373,400,000
Fair value investments	2,531,400,000	2,373,400,000
Level 2 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	54,000,000
Fair value investments	26,600,000	54,000,000
Level 2 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,293,800,000
Fair value investments	2,435,700,000	2,293,800,000
Level 2 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,700,000
Fair value investments	0	5,700,000
Level 2 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	500,000
Fair value investments	0	500,000
Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,166,400,000
Fair value investments	1,000,000,000	1,166,400,000
Level 2 Inputs | Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Level 2 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	967,900,000
Fair value investments	1,167,000,000	967,900,000
Level 2 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	82,200,000
Fair value investments	228,800,000	82,200,000
Level 2 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	10,200,000
Fair value investments	600,000	10,200,000
Level 2 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	13,100,000
Fair value investments	12,700,000	13,100,000
Level 3 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	420,100,000
Fair value investments	367,300,000	420,100,000
Level 3 Inputs | US Government and agency obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	76,400,000	81,800,000
Total realized and unrealized gains (losses)	1,100,000	1,800,000
Amortization/Accretion	0	100,000
Purchases	(35,300,000)	39,600,000
Sales	0	0
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	0
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		5,900,000
Transfers in	0	0
Transfers out	(40,600,000)	(52,800,000)
Fair value investments	$ 70,000,000	$ 76,400,000	81,800,000
Number of investments | Investment	6	6
Level 3 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 76,400,000
Fair value investments	70,000,000	$ 76,400,000
Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000	45,400,000
Total realized and unrealized gains (losses)	(7,800,000)	6,200,000
Amortization/Accretion	0	0
Purchases	0	5,000,000
Sales	(43,700,000)	17,100,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	(3,600,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	0	0
Transfers out	0	0
Fair value investments	0	39,500,000	45,400,000
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	304,200,000	189,200,000
Total realized and unrealized gains (losses)	29,200,000	16,300,000
Amortization/Accretion	0	0
Purchases	(91,300,000)	131,700,000
Sales	(54,700,000)	33,000,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0	0
Fair Value, Assets Measured on Recurring Basis, Other	(14,300,000)
Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation, Assets, Exchange		0
Transfers in	0	0
Transfers out	0	0
Fair value investments	297,300,000	304,200,000	189,200,000
Level 3 Inputs | Other long-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	304,200,000
Fair value investments	297,300,000	304,200,000
Carrying value of investment accounted for using the equity method	3,800,000	5,200,000	$ 6,800,000
Level 3 Inputs | Short-term investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,300,000
Fair value investments	0	5,300,000
Level 3 Inputs | Mortgage-backed and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Municipal obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Foreign government, agency and provincial obligations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Level 3 Inputs | Preferred stocks | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,100,000
Fair value investments	70,000,000	71,100,000
Prospector
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains	800,000	4,000,000
Consumer | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	116,000,000
Fair value investments	70,000,000	116,000,000
Consumer | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259,200,000
Fair value investments	253,300,000	259,200,000
Consumer | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	115,900,000
Fair value investments	70,000,000	115,900,000
Consumer | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Consumer | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	100,000
Fair value investments	0	100,000
Consumer | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259,200,000
Fair value investments	253,300,000	259,200,000
Consumer | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Consumer | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	180,700,000
Fair value investments	694,700,000	180,700,000
Financials | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	254,700,000
Fair value investments	175,900,000	254,700,000
Financials | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	141,200,000
Fair value investments	694,700,000	141,200,000
Financials | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Financials | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	254,700,000
Fair value investments	175,900,000	254,700,000
Financials | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000
Fair value investments	0	39,500,000
Financials | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	74,400,000
Fair value investments	35,700,000	74,400,000
Health Care [Member] | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	181,800,000
Fair value investments	151,300,000	181,800,000
Health Care [Member] | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	74,400,000
Fair value investments	35,700,000	74,400,000
Health Care [Member] | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	181,800,000
Fair value investments	151,300,000	181,800,000
Health Care [Member] | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Health Care [Member] | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	57,100,000
Fair value investments	26,600,000	57,100,000
Industrial | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	107,300,000
Fair value investments	135,600,000	107,300,000
Industrial | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	57,100,000
Fair value investments	26,600,000	57,100,000
Industrial | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	102,000,000
Fair value investments	135,600,000	102,000,000
Industrial | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Industrial | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,300,000
Fair value investments	0	5,300,000
Utilities | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	18,100,000
Fair value investments		18,100,000
Utilities | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	65,600,000
Fair value investments	82,000,000	65,600,000
Utilities | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	18,100,000
Fair value investments		18,100,000
Utilities | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Utilities | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Utilities | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	65,600,000
Fair value investments	82,000,000	65,600,000
Utilities | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Utilities | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Energy | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,900,000
Fair value investments		4,900,000
Energy | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,100,000
Fair value investments	61,500,000	84,100,000
Energy | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,900,000
Fair value investments		4,900,000
Energy | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Energy | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Energy | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	84,100,000
Fair value investments	61,500,000	84,100,000
Energy | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Energy | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	44,800,000
Fair value investments	27,000,000	44,800,000
Technology | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	88,300,000
Fair value investments	60,000,000	88,300,000
Technology | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	44,800,000
Fair value investments	27,000,000	44,800,000
Technology | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	88,300,000
Fair value investments	60,000,000	88,300,000
Technology | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Technology | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	28,500,000
Fair value investments	43,700,000	28,500,000
Communications | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,500,000
Fair value investments	49,200,000	71,500,000
Communications | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	28,500,000
Fair value investments	43,700,000	28,500,000
Communications | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	71,500,000
Fair value investments	49,200,000	71,500,000
Communications | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Communications | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Materials | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	12,000,000
Fair value investments		12,000,000
Materials | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,700,000
Fair value investments	31,200,000	51,700,000
Materials | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	12,000,000
Fair value investments		12,000,000
Materials | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Materials | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Materials | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	51,700,000
Fair value investments	31,200,000	51,700,000
Materials | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments		0
Materials | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	75,200,000
Fair value investments	74,400,000	75,200,000
Other | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	7,500,000
Fair value investments	0	7,500,000
Other | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,900,000
Fair value investments	0	1,900,000
Other | Level 1 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	73,300,000
Fair value investments	74,400,000	73,300,000
Other | Level 2 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	7,500,000
Fair value investments	0	7,500,000
Other | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Other | Level 3 Inputs | Debt securities issued by corporations | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Fair value investments	0	0
Exchange Traded Funds [Member] | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	141,800,000
Exchange Traded Funds [Member] | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	120,500,000
Exchange Traded Funds [Member] | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	21,300,000
Exchange Traded Funds [Member] | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0
Common equity securities | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	611,700,000
Fair value investments	1,113,900,000	611,700,000
Common equity securities | Level 1 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	498,800,000
Fair value investments	1,018,200,000	498,800,000
Common equity securities | Level 2 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	73,400,000
Fair value investments	95,700,000	73,400,000
Common equity securities | Level 3 Inputs | Common equity securities | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	39,500,000
Fair value investments	0	39,500,000
Symetra
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total realized and unrealized gains (losses)	(258,800,000)
Affordable Housing Development Fund [Member] | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	16,800,000
Fair value investments	$ 14,700,000	$ 16,800,000

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)Investment	Dec. 31, 2014USD ($)Investment
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	$ 41.8	$ 53
Number of Investments with Significant Unobseravble Inputs | Investment	1	1
Standard Poors NR Rating [Member] | Redeemable preferred stocks | Par Value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 70
Standard Poors NR Rating [Member] | Redeemable preferred stocks | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 71.1
Standard Poors NR Rating [Member] | Redeemable preferred stocks | Discount Yield
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	70	.07065
Standard Poors NR Rating [Member] | Private equity funds | Multiple of GAAP book value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 39.5
Standard Poors NR Rating [Member] | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 21
Standard Poors NR Rating [Member] | Private equity securities | Multiple of GAAP book value
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		290.96
Standard Poors NR Rating [Member] | Private equity securities | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 5.7
Standard Poors NR Rating [Member] | Private equity securities | Multiple of EBITDA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	6	6
Assets, Fair Value Disclosure		$ 3.8
Standard Poors NR Rating [Member] | Fixed maturity investments | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 5.3
Standard Poors NR Rating [Member] | Fixed maturity investments | Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		.10
Standard Poors NR Rating [Member] | Seller Priority Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Timing of Interest Payments	5 years	5 years
Unobservable Inputs	0.13
Assets, Fair Value Disclosure		$ 44
Timing of principal payments	10 years	10 years
Standard Poors NR Rating [Member] | Seller Priority Surplus Note [Member] | Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		0.093
Standard Poors NR Rating [Member] | Pari Passu Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Timing of Interest Payments	5 years
Unobservable Inputs	0.224
Assets, Fair Value Disclosure	$ 13.5	$ 21.1
Timing of principal payments	10 years	10 years
Standard Poors NR Rating [Member] | Pari Passu Surplus Note [Member] | Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		0.135
Level 2
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	$ 0	$ 0
Level 3 Inputs | Fixed maturity investments
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value, Assets Measured on Recurring Basis, Transfers out	$ 40.6	$ 52.8
Ironshore [Member] | Standard Poors NR Rating [Member] | Private equity funds | Multiple of GAAP book value
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		1.10
PassportCard [Member] | Standard Poors NR Rating [Member] | Private equity funds | Multiple of GAAP book value
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	1
Surplus Note
Quantitative information for Level 3 Fair Value Measurements Assets
RBC Score	250.00%
durchblicker.at [Member] | Standard Poors NR Rating [Member] | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 20.1
durchblicker.at [Member] | Standard Poors NR Rating [Member] | Private equity funds | Recent Transaction Two [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	1.03	1.06
Assets, Fair Value Disclosure	$ 33.8
Captricity [Member] | Standard Poors NR Rating [Member] | Private equity securities | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs	3.83
Assets, Fair Value Disclosure	$ 27
durchblicker.at [Member] | Standard Poors NR Rating [Member] | Private equity funds | Option pricing method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 9.6
durchblicker.at [Member] | Standard Poors NR Rating [Member] | Private equity funds | Recent Transaction Three [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 10.4
TaClaro [Member] | Standard Poors NR Rating [Member] | Private equity securities | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs		.71
Assets, Fair Value Disclosure		$ 4.5
BE Reinsurance [Member] | Standard Poors NR Rating [Member] | Private equity funds
Quantitative information for Level 3 Fair Value Measurements Assets
Average Share Price Used As Unobservable Input	4 years
Volatility Rate	60.00%
Risk Free Rate	1.15%
Pari Passu Surplus Note [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
basis points	2.50%	2.50%
Seller Priority Surplus Note [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
basis points	2.50%	2.50%

Investment Securities Investm94

Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details) - USD ($)	Dec. 23, 2014	Dec. 31, 2015	Dec. 31, 2014
Total realized and unrealized gains (losses)		$ 226,800,000	$ (83,300,000)
OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	$ (36,100,000)	(49,500,000)	(35,900,000)
Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(6,800,000)	(15,100,000)	(6,600,000)
Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(12,600,000)	(24,200,000)	(12,600,000)
Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(16,700,000)	(10,200,000)	(16,700,000)
Fair Value [Member] | OneBeacon
Surplus notes	64,900,000	51,500,000	65,100,000
Par Value [Member] | OneBeacon
Surplus notes	101,000,000	101,000,000	$ 101,000,000
Seller Priority Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(14,000,000)	(19,900,000)
Seller Priority Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	1,600,000	(400,000)
Seller Priority Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(4,600,000)	(11,700,000)
Seller Priority Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(11,000,000)	(7,800,000)
Seller Priority Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	43,900,000	38,000,000
Seller Priority Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes		57,900,000
Pari Passu Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(22,100,000)	(29,600,000)
Pari Passu Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(8,400,000)	(14,700,000)
Pari Passu Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(8,000,000)	(12,500,000)
Pari Passu Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(5,700,000)	(2,400,000)
Pari Passu Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	21,000,000	13,500,000
Pari Passu Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes	$ 43,100,000	43,100,000
Symetra
Total realized and unrealized gains (losses)		$ 258,800,000
Surplus Note
RBC Score		250.00%

Debt and Standby Letter of Cr95

Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 10, 2014	Aug. 14, 2013
Debt Instrument
Debt	$ 442.4	$ 343.1
Interest expense on debt	18.6	15.6	$ 16.2
Draw down of debt and revolving line of credit	195.5	133.6	200
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility					$ 425
Line of Credit Facility, Fair Value of Amount Outstanding	50
Interest expense on debt	0.1	0.3	$ 0.3
Draw down of debt and revolving line of credit	125	65
Repayments of lines of credit	$ 75	$ 65
Effective yield (as a percent)	3.74%	3.65%
Prior W T M Bank Facility
Debt Instrument
Total commitment under revolving credit facility					$ 375
Other Debt [Member]
Debt Instrument
Debt	$ 0	$ 1
OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	275	275
Unamortized original issue discount	(0.2)	(0.3)
Debt	274.8	274.7
White Mountains | WTM Bank Facility
Debt Instrument
Debt	50	0
Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Total commitment under revolving credit facility	116
Debt		67.4
Line of Credit Facility, Fair Value of Amount Outstanding	104.7			$ 70
Interest expense on debt	4	0.7
Line of Credit Facility, Amount Outstanding	104.7	68.7
Unamortized Debt Issuance Expense	(1.8)	(1.3)
Draw down of debt and revolving line of credit	24
Repayments of lines of credit	13
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility	20
Line of Credit Facility, Fair Value of Amount Outstanding		0
Line of Credit Facility, Amount Outstanding	15
Unamortized Debt Issuance Expense	(0.3)	0
Carrying Value | OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Debt	274.8	274.7
Carrying Value | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Debt	102.9	$ 67.4
Carrying Value | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Debt	$ 14.7

Goodwill and Other Intangible96

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Oct. 10, 2014	Mar. 14, 2014	Feb. 19, 2014	Dec. 31, 2013
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 232.3	$ 198.3
Amortization of Intangible Assets	44.5	16
Intangible Assets, Net (Excluding Goodwill)	187.8	182.3				$ 5.1
Goodwill	187.9	168.9				$ 0
Intangible Assets, Gross (Including Goodwill)	420.2	367.2
Goodwill and intangible assets	375.7	351.2
Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	177.5	146.3
Amortization of Intangible Assets	21.4	3.5
Intangible Assets, Net (Excluding Goodwill)	156.1	142.8	$ 146.3
Goodwill	163.8	145.1	$ 145.1
MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	38.5	38.5
Amortization of Intangible Assets	14.1	6
Intangible Assets, Net (Excluding Goodwill)	24.4	32.5		$ 38.5
Goodwill	18.3	18.3		$ 18.3
WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	5.7	2.9
Amortization of Intangible Assets	1.2	0.4
Intangible Assets, Net (Excluding Goodwill)	4.5	2.5
Goodwill	5.8	5.5			$ 5.5
Developed Technology Rights [Member] | Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	12.7	12.7
Amortization of Intangible Assets	2.8	0.5
Intangible Assets, Net (Excluding Goodwill)	$ 9.9	12.2
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	6 years
Developed Technology Rights [Member] | MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 32	32
Amortization of Intangible Assets	11.2	4.8
Intangible Assets, Net (Excluding Goodwill)	$ 20.8	27.2
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Developed Technology Rights [Member] | WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 3.6	0.8
Amortization of Intangible Assets	0.7	0.2
Intangible Assets, Net (Excluding Goodwill)	$ 2.9	0.6
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Customer Relationships [Member] | Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 132.3	107.5
Amortization of Intangible Assets	13.1	2.1
Intangible Assets, Net (Excluding Goodwill)	$ 119.2	105.4
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	11 years
Customer Relationships [Member] | MediaAlpha
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 6.5	6.5
Amortization of Intangible Assets	2.9	1.2
Intangible Assets, Net (Excluding Goodwill)	$ 3.6	5.3
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	4 years
Customer Relationships [Member] | Star & Shield LLC [Member]
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Customer Relationships [Member] | Star & Shield Insurance Exchange [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 1.2	1.2
Amortization of Intangible Assets	0.8	0.4
Intangible Assets, Net (Excluding Goodwill)	0.4	0.8
Trademarks [Member] | WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	2.1	2.1
Amortization of Intangible Assets	0.5	0.2
Intangible Assets, Net (Excluding Goodwill)	$ 1.6	1.9
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Other Intangible Assets [Member] | Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 2	0.5
Amortization of Intangible Assets	0.1	0
Intangible Assets, Net (Excluding Goodwill)	$ 1.9	0.5
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Other Intangible Assets [Member] | OneBeacon
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 9.4	9.4
Amortization of Intangible Assets	7	5.7
Intangible Assets, Net (Excluding Goodwill)	$ 2.4	3.7
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Trade Names [Member] | Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 30.5	25.6
Amortization of Intangible Assets	5.4	0.9
Intangible Assets, Net (Excluding Goodwill)	$ 25.1	$ 24.7
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years

Debt and Standby Letter of Cr97

Debt and Standby Letter of Credit Facilities (Schedule of Contractual Repayments) (Details) $ in Millions	Dec. 31, 2015USD ($)
Debt Disclosure [Abstract]
Due in one year or less	$ 74.9
Due in two to three years	43.3
Due in four to five years	51.5
Due after five years	275
Total	$ 444.7

Goodwill and Other Intangible98

Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Roll Forward]
Beginning balance	$ 168.9	$ 0
Acquisitions of businesses	19	168.9
Ending balance	187.9	168.9	$ 0
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	182.3	5.1
Finite-Lived and Indefinite-Lived Intangibles Acquired	34	188.9
Goodwill, Other Changes	0	0
Amortization, including foreign currency translation	(28.5)	(11.7)	(1.4)
Ending balance	$ 187.8	$ 182.3	$ 5.1

Debt and Standby Letter of Cr99

Debt and Standby Letter of Credit Facilities (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 30, 2012	Dec. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 29, 2015	Oct. 10, 2014	Aug. 14, 2013	May. 31, 2003
Debt Instrument
Interest expense on debt			$ 18.6	$ 15.6	$ 16.2
Draw down of debt and revolving line of credit			195.5	133.6	200
Repayments of debt, principal			$ 95.6	66.5	275
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate			6.07%
Interest Paid			$ 17.6	15.6	16.2
Debt			$ 442.4	$ 343.1
OneBeacon Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility						$ 65
WTM Bank Facility
Debt Instrument
Effective yield (as a percent)			3.74%	3.65%
Interest expense on debt			$ 0.1	$ 0.3	$ 0.3
Total commitment under revolving credit facility								$ 425
Draw down of debt and revolving line of credit			125	65
Line of Credit Facility, Fair Value of Amount Outstanding			50
Repayments of lines of credit			75	65
Prior W T M Bank Facility
Debt Instrument
Total commitment under revolving credit facility								$ 375
One Beacon US Holdings Inc. | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	$ 275
Percentage of par value at which debt was issued	99.90%
Issuance of debt, net of debt issuance costs	$ 272.9
Interest rate (as a percent)	4.60%
Debt issue costs	$ 2.8
Underwriting discount	$ 1.8
Effective yield (as a percent)	4.70%
Interest Expense			13
Expenses related to issuance, including underwriting fees	$ 2.8
One Beacon US Holdings Inc. | 2003 OBH Senior Notes
Debt Instrument
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent)									2.50%
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes
Debt Instrument
Interest rate (as a percent)		5.875%
Debt Instrument, Repurchase Amount		$ 269.8
Aggregate purchase price of senior notes purchased and retired		275.9
Loss on retirement of debt		(6.3)
Write off of deferred issuance costs		$ 0.2
Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Interest expense on debt			4	0.7
Total commitment under revolving credit facility			116
Draw down of debt and revolving line of credit			24
Line of Credit Facility, Fair Value of Amount Outstanding			104.7				$ 70
Line of Credit Facility, Amount Outstanding			$ 104.7	68.7
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate			4.73275%
Repayments of lines of credit			$ 13
Debt				67.4
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			20
Line of Credit Facility, Fair Value of Amount Outstanding				$ 0
Line of Credit Facility, Amount Outstanding			$ 15
Minimum | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
LIBOR rate			2.50%
Maximum | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
LIBOR rate			4.50%
Term Loan [Member] | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Total commitment under revolving credit facility			$ 101
Debt Instrument, Increase (Decrease), Other, Net			$ 31
Line of Credit Facility, Collateral			70
Repayments of lines of credit			$ 6.5
Term Loan [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			15
Revolving Credit Facility [Member] | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Total commitment under revolving credit facility			15
Revolving Credit Facility [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			$ 5

Goodwill and Other Intangibl100

Goodwill and Other Intangible Assets (Amortization Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of Intangible Assets	$ 28.5	$ 11.7	$ 1.4
Finite-lived intangible assets, next twelve months	30.4
Finite-lived intangible assets, year two	29.6
Finite-lived intangible assets, year three	27.2
Finite-lived intangible assets, year four	19.4
Finite-lived intangible assets, year five	16.1
Total	$ 122.7

Income Taxes (Income Tax (Benef

Income Taxes (Income Tax (Benefit) Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current tax (expense) benefit:
U.S. federal									$ (9.1)	$ 4.3	$ 20.3
State									1.8	2	1
Non-U.S.									1.5	1.4	1.2
Total current tax expense									(5.8)	7.7	22.5
Deferred tax (expense) benefit:
U.S. federal									5.1	(22.5)	10.1
Total deferred tax benefit (expense)									5.1	(22.5)	10.1
Total income tax (expense) benefit	$ (1.5)	$ (1.6)	$ (2.2)	$ 4.6	$ (26.2)	$ (4.7)	$ 7.1	$ 9	$ (0.7)	$ (14.8)	$ 32.6

Income Taxes (Effective Rate Re

Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)									35.00%
Tax (expense) benefit at the U.S. statutory rate									$ (54.2)	$ 10.2	$ (55.5)
Differences in taxes resulting from:
Non-U.S. earnings, net of foreign taxes									(76.7)	(37.8)	(75.1)
Change in valuation allowance									(18.7)	(36)	(49.9)
Withholding tax									(1.2)	(2.4)	(1)
Tax exempt interest and dividends									2.6	2.5	2.4
Tax reserve adjustments									(1.7)	5.2	(1.2)
Other, net									(2.8)	(2.5)	(2.5)
Income tax (expense) benefit	$ 1.5	$ 1.6	$ 2.2	$ (4.6)	$ 26.2	$ 4.7	$ (7.1)	$ (9)	0.7	14.8	(32.6)
Pre-tax loss	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	154.9	(29)	158.6
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total									(6.5)	2.7	3.6
Non-US
Differences in taxes resulting from:
Pre-tax loss									$ 209.5	$ 60.4	$ 163.7

Income Taxes (Deferred Tax Inve

Income Taxes (Deferred Tax Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$ 186.6	$ 152.3
Unearned premiums	37.5	38.3
Non-U.S. net operating loss carryforwards	33.8	33.1
Loss reserve discount	26.8	41.4
Tax credit carryforwards	16.9	13.4
Fixed assets	1.7	2.3
Accrued interest	1.3	5
Other items	3	2.8
Total gross deferred income tax assets	371.8	348.1
Less: valuation allowances	(156.8)	(140.5)
Total net deferred income tax assets	215	207.6
Deferred income tax liabilities related to:
Deferred acquisition costs	34.9	32
Net unrealized investment gains	28.2	31.1
Noncurrent deferred tax liabilities	3.5	2.5
Investment basis difference	13.8	17
Other items	0.5	0.1
Total deferred income tax liabilities	109.2	93
Net deferred tax asset	105.8	114.6
OneBeacon Runoff
Deferred income tax assets related to:
Runoff Transaction	12.6	12.6
Purchase Accounting [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	9.3	0
Members Surplus Contributions [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	19	10.3
Incentive Compensation [Member]
Deferred income tax assets related to:
Compensation	45.4	40.2
Deferred Compensation, Share-based Payments [Member]
Deferred income tax assets related to:
Compensation	6.2	$ 6.7
UNITED STATES
Deferred income tax assets related to:
Tax credit carryforwards	$ 14.6

Income Taxes (Valuation Allowan

Income Taxes (Valuation Allowance) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation Allowance [Line Items]
Valuation allowance	$ 156,800,000				$ 140,500,000				$ 156,800,000	$ 140,500,000
Change in valuation allowance									18,700,000	36,000,000	$ 49,900,000
Income Tax Expense (Benefit)	(1,500,000)	$ (1,600,000)	$ (2,200,000)	$ 4,600,000	(26,200,000)	$ (4,700,000)	$ 7,100,000	$ 9,000,000	(700,000)	(14,800,000)	$ 32,600,000
Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	28,300,000								28,300,000
U.S. losses and other federal deferred tax benefits
Valuation Allowance [Line Items]
Valuation allowance	122,900,000				107,500,000				122,900,000	107,500,000
Net Operating Losses in Israel Subsidiaries [Member]
Valuation Allowance [Line Items]
Valuation allowance	5,500,000				1,900,000				5,500,000	1,900,000
Net Operating Losses in Netherlands Subsidiaries [Member] [Member]
Valuation Allowance [Line Items]
Valuation allowance	$ 100,000								100,000
Non-US | Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Change in valuation allowance									300,000	15,700,000
Non-US | Net Operating Losses in Israel Subsidiaries [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									3,000,000	1,200,000
Subsidiaries | Luxembourg | Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					31,000,000					31,000,000
Subsidiaries | NETHERLANDS | Net operating losses in Luxembourg subsidiaries
Valuation Allowance [Line Items]
Valuation allowance					$ 100,000					100,000
Guilford Holdings, Inc. | UNITED STATES
Valuation Allowance [Line Items]
Change in valuation allowance									8,200,000	7,200,000
BAM
Valuation Allowance [Line Items]
Change in valuation allowance									6,300,000	8,500,000
Income Tax Expense (Benefit)									(8,700,000)	4,900,000
Houston General Insurance [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									(500,000)	0.1
Star & Shield Insurance Exchange [Member]
Valuation Allowance [Line Items]
Change in valuation allowance									$ (1,200,000)	$ (3,200,000)

Income Taxes (Net Operating and

Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net deferred tax assets
Total net deferred income tax assets	$ 215	$ 207.6
Change in valuation allowance	18.7	36	$ 49.9
Alternative minimum tax credit carryforwards, not subject to expiration	1.9
Tax credit carryforwards	16.9	13.4
UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	22.6
Net deferred tax assets
Foreign tax credit carryforwards, subject to expiration	0.4
Tax credit carryforwards	14.6
Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	662.3
Net deferred tax assets
Gross deferred tax asset	220.5
Valuation allowance	(155.1)
Total net deferred income tax assets	65.4
Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	543.4
Net deferred tax assets
Gross deferred tax asset	186.6
Valuation allowance	(121.2)
Total net deferred income tax assets	65.4
Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	96.8
Net deferred tax assets
Gross deferred tax asset	28.3
Valuation allowance	(28.3)
Total net deferred income tax assets	0
Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	21.8
Net deferred tax assets
Gross deferred tax asset	5.5
Valuation allowance	(5.5)
Total net deferred income tax assets	0
Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.3
Net deferred tax assets
Gross deferred tax asset	0.1
Valuation allowance	(0.1)
Total net deferred income tax assets	0
No expiration date [Member] | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	118.6
No expiration date [Member] | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
No expiration date [Member] | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	96.8
No expiration date [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	21.8
No expiration date [Member] | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2026-2035 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	540.6
2026-2035 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	540.6
2026-2035 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2026-2035 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2026-2035 | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2021-2025 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	2.3
2021-2025 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	2
2021-2025 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2021-2025 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2021-2025 | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.3
2016-2020 | Operating and Capital Loss Carryforward
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2016-2020 | Operating and Capital Loss Carryforward | UNITED STATES
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0.8
2016-2020 | Operating and Capital Loss Carryforward | Luxembourg
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2016-2020 | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member]
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
2016-2020 | Operating and Capital Loss Carryforward | NETHERLANDS
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Operating Loss Carryforwards	0
Star & Shield Insurance Exchange [Member]
Net deferred tax assets
Change in valuation allowance	$ (1.2)	$ (3.2)

Income Taxes (ASC 740-10) (Deta

Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
Income Tax Contingency [Line Items]
Percentage of likelihood for realization of benefit upon ultimate settlement		50.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	$ 35	$ 35	$ 33.7	$ 40
Changes in prior year tax positions	(10.7)		(4.9)	1
Tax positions taken during the current year	0
Lapse in statute of limitations	0		1.1	0
Settlements with tax authorities	0			(0.3)
Tax positions taken during the current year			7.3	7
Balance at year end		24.3	35	33.7
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense		13.6			$ 5
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		1.7	(2.2)	0.7
Permanent Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	5.4	5.4	8.4	8.4
Changes in prior year tax positions	0		(2.2)	0
Tax positions taken during the current year	0
Lapse in statute of limitations	0		0.8	0
Settlements with tax authorities	0			0
Tax positions taken during the current year			0	0
Balance at year end		5.4	5.4	8.4
Temporary Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	23.1	23.1	16.6	23.6
Changes in prior year tax positions	(12.4)		(0.8)	0
Tax positions taken during the current year		0
Lapse in statute of limitations	0		0	0
Settlements with tax authorities	0			0
Tax positions taken during the current year			7.3	7
Balance at year end		10.7	23.1	16.6
Interest and Penalties Net of Tax Benefit [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Balance at beginning	6.5	6.5	8.7	8
Changes in prior year tax positions	1.7		(1.9)	1
Tax positions taken during the current year	0
Lapse in statute of limitations	0		0.3	0
Settlements with tax authorities	$ 0			(0.3)
Tax positions taken during the current year			0	0
Balance at year end		$ 8.2	$ 6.5	$ 8.7

Income Taxes (Tax Examinations)

Income Taxes (Tax Examinations) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2014
Income Tax Examination [Line Items]
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense		$ 13.6	$ 5
Tax Assessment Year 2005 to 2006 [Member] | IRS
Income Tax Examination [Line Items]
Tax Examination		3.3
Tax Assessment Year 2007 to 2009 [Member] | IRS
Income Tax Examination [Line Items]
Tax Examination		4
Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Dissallowed		2.7
OneBeacon
Income Tax Examination [Line Items]
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority		$ 1.1
Scenario, Forecast [Member]
Income Tax Examination [Line Items]
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense	$ 12

Derivatives (Details)

Derivatives (Details) - White Mountains Life Re ¥ in Billions, $ in Billions	Dec. 31, 2015USD ($)	Dec. 31, 2015JPY (¥)
Derivative [Line Items]
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

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pre-tax operating results of WM Life Re
Total revenues	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	$ 1,808.6	$ 1,454.1	$ 1,361.6
General and administrative expenses									(487.5)	(257.7)	(149.1)
Pre-tax income (loss)	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	154.9	(29)	158.6
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue									9.3	18.6	25
Change in Fair Value of Variable Annuity Liability, included in other revenues									(0.4)	52.9	378.5
Change in fair value of derivatives, included in other revenue									(8.8)	(72.4)	(402)
Foreign exchange, included in other revenue									(1.3)	(3.2)	(14.5)
Other investment income and (losses) gains									(0.4)	(1.4)	(7.1)
Total revenues									(1.6)	(5.5)	(20.1)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses									0	0.6	10.2
Death benefit claims paid, included in general and administrative expenses									(0.1)	(0.1)	(1.9)
General and administrative expenses									(4)	(4.3)	(4.9)
Pre-tax income (loss)									$ (5.7)	(9.3)	$ (16.7)
Level 3 Inputs | Projected Surrender Assumption [Member] | White Mountains Life Re
Pre-tax operating results of WM Life Re
Change in fair value of derivatives, included in other revenue										$ 0.2

Derivatives (Realized and Unrea

Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (8.8)	$ (72.4)	$ (402)
Derivative, Fair Value, Net	20.1	56.4	69.2	$ 98.3
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	6.4	(33.7)	(108.7)
Derivative, Fair Value, Net	0.5	(1.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(7.3)	(1.3)	(96.7)
Derivative, Fair Value, Net	14.8	44.1
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(7.9)	(37.4)	$ (196.6)
Derivative, Fair Value, Net	$ 4.8	14
Level 3 | Projected Surrender Assumption [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue		$ 0.2

Derivatives (Variable Annuity R

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - White Mountains Life Re - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		$ 0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	$ 0
Changes in derivative instruments
Beginning of period	56.4	69.2	$ 98.3
Purchases			59.4
Realized and unrealized gains (losses)	8.8	72.4	402
Transfers in (out)	0	0	0
Sales/settlements	27.5	(59.6)	313.5
Ending balance	20.1	56.4	69.2
Collateral Cash and Investments	5.8	33.2	81.3
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	0	0.6	10.2
Level 3
Changes in variable annuity (liabilities)
Purchases			0
Level 2
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0
Changes in derivative instruments
Beginning of period	33.8	4.7	(20.5)
Purchases			0
Realized and unrealized gains (losses)	7.5	71	196.1
Transfers in (out)	0	0	0
Sales/settlements	9.8	(100.1)	221.3
Ending balance	16.5	33.8	4.7
Level 1
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0
Changes in derivative instruments
Beginning of period	3.7	1.1	(21.7)
Purchases			0
Realized and unrealized gains (losses)	(8.4)	(37.2)	69.4
Transfers in (out)	0	0	0
Sales/settlements	11.2	34.6	92.2
Ending balance	0.9	3.7	1.1
Variable Annuity [Member] | Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0
Changes in variable annuity (liabilities)
Beginning balance	(0.7)	(52.8)	(441.5)
Realized and unrealized (losses) gains	0.4	(53.5)	(388.7)
Transfers in (out)	0	0	0
Sales/settlements	0	0	0
Ending balance	(0.3)	(0.7)	(52.8)
Derivative [Member] | Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0
Changes in derivative instruments
Beginning of period	18.9	63.4	140.5
Purchases			59.4
Realized and unrealized gains (losses)	9.7	38.6	136.5
Transfers in (out)	0	0	0
Sales/settlements	6.5	5.9	0
Ending balance	$ 2.7	$ 18.9	$ 63.4

Derivatives (Additional Collate

Derivatives (Additional Collateral) (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Credit risk on derivative instruments
Collateral Cash and Investments	$ 5.8	$ 33.2	$ 81.3
Cash
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	5.8	23.7
Fixed maturity investments
Credit risk on derivative instruments
Derivative Collateral Right to Reclaim	$ 0	$ 9.5

Derivatives (Level 3 Derivative

Derivatives (Level 3 Derivative Instruments) (Details) - White Mountains Life Re $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ (0.3)
Equity Index Options | Counterparty valuations [Member]
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 0.8
Equity Index Options | Counterparty valuations [Member] | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	0.70%
Equity Index Options | Counterparty valuations [Member] | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	(6.70%)
Equity Index Options | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	(2.50%)
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality	0.00%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality	6.40%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality	1.00%
Foreign Exchange Option [Member] | Counterparty valuations [Member]
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 1.9
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	(0.50%)
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	(7.30%)
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	(5.90%)
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	0.10%
Foreign exchange volatilities	11.50%
Index volatilities	25.00%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	40.00%
Foreign exchange volatilities	18.70%
Index volatilities	27.50%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	40.00%
Foreign exchange volatilities	13.10%
Index volatilities	26.40%

Derivatives Derivatives (Gross

Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details) - White Mountains Life Re - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Asset	$ 23.2	$ 64.4
Derivative Liability	(3.1)	(8)
Net amounts recognized in Other Assets	20.1	56.4
Interest Rate Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	2.4	1
Derivative Liability	(2.1)	(5.4)
Net amounts recognized in Other Assets	0.3	(4.4)
Interest Rate Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	0.1	2.8
Derivative Liability	(0.1)	(0.1)
Net amounts recognized in Other Assets	0	2.7
Foreign Exchange Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	15	45.5
Derivative Liability	0	0
Net amounts recognized in Other Assets	15	45.5
Foreign Exchange Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	0.1	0
Derivative Liability	(0.3)	(1.4)
Net amounts recognized in Other Assets	(0.2)	(1.4)
Equity Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	4.4	11.7
Derivative Liability	(0.6)	(0.2)
Net amounts recognized in Other Assets	3.8	11.5
Equity Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	1.2	3.4
Derivative Liability	0	(0.9)
Net amounts recognized in Other Assets	$ 1.2	$ 2.5

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 ($)Rating	Dec. 31, 2014USD ($)
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 20,100	$ 56,400
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 19,000	77,200
S&P Credit Ratings | Rating	21
Derivative, Collateral, Right to Reclaim Cash	$ 0	3,500
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	20,100	59,900
Excess collateral provided to counterparty in cash	5,800	17,700
Excess collateral providedto counterparty in financial instruments	0	9,500
Derivative, Collateral, Obligation to Return Cash	6,900	9,900
A | Citigroup OTC [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	9,900	22,200
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	8,500	21,100
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	9,900	22,200
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	1,400	1,100
A | Royal Bank of Scotland [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		4,000
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		4,000
Derivative, Collateral, Right to Reclaim Cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		4,000
Excess collateral provided to counterparty in cash		0
Excess collateral providedto counterparty in financial instruments		0
Derivative, Collateral, Obligation to Return Cash		0
A | Barclays [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		100
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		100
Derivative, Collateral, Right to Reclaim Cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		100
Excess collateral provided to counterparty in cash		0
Excess collateral providedto counterparty in financial instruments		0
Derivative, Collateral, Obligation to Return Cash		0
A | Bank of America [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	700	5,600
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	700	5,600
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	700	5,600
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
A | Citigroup Exchange Traded [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1,000	3,700
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	6,800	19,700
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1,000	3,700
Excess collateral provided to counterparty in cash	5,800	16,000
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
BBB plus | Nomura [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		(3,500)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		11,200
Derivative, Collateral, Right to Reclaim Cash		3,500
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0
Excess collateral provided to counterparty in cash		1,700
Excess collateral providedto counterparty in financial instruments		9,500
Derivative, Collateral, Obligation to Return Cash		0
Standard & Poor's, A+ Rating [Member] | J P Morgan Chase [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	8,500	24,300
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	3,000	15,500
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	8,500	24,300
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	$ 5,500	$ 8,800

Derivatives Derivative (Tranzac

Derivatives Derivative (Tranzact Interest Rate Swap) (Details) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Oct. 10, 2014
Derivative [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	6.07%
Tranzact [Member] | Tranzact Bank Facility
Derivative [Line Items]
Total commitment under revolving credit facility	$ 116
Line of Credit Facility, Fair Value of Amount Outstanding	104.7		$ 70
Line of Credit Facility, Amount Outstanding	$ 104.7	$ 68.7
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.73275%
Maximum | Tranzact [Member] | Tranzact Bank Facility
Derivative [Line Items]
LIBOR rate	4.50%
Minimum | Tranzact [Member] | Tranzact Bank Facility
Derivative [Line Items]
LIBOR rate	2.50%
Interest Rate Swap [Member] | Tranzact [Member]
Derivative [Line Items]
Derivative, Fixed Interest Rate	1.34%
Derivative, Variable Interest Rate	0.23275%	0.1518%
Derivative, Fair Value, Net	$ (0.2)
Term Loan [Member] | Tranzact [Member] | Tranzact Bank Facility
Derivative [Line Items]
Total commitment under revolving credit facility	$ 101

Municipal Bond Guarantee Ins117

Municipal Bond Guarantee Insurance (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015USD ($)Contract	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)Contract	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2015USD ($)Contract	Dec. 31, 2014USD ($)Contract	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax loss	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	$ 154.9	$ (29)	$ 158.6
Net investment income (loss) - surplus note interest									0	$ 0	0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Quarter	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months	4.4								4.4
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	4.3								4.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three	4.1								4.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four	3.8								3.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five	3.6								3.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten	30								30
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Third Quarter	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Fourth Quarter	$ 1.1								$ 1.1
Municipal bond contract | Contract	3,103				1,750				3,103	1,750
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									12 years 10 months	12 years 10 months
Municipal bond outstanding principal	$ 22,556				$ 12,362.5				$ 22,556	$ 12,362.5
Municipal bond outstanding interest	11,984.4				7,086.9				11,984.4	7,086.9
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	$ 50.2				$ 27.6				$ 50.2	$ 27.6
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes												503
Percentage of par value of policy reinsured	15.00%								15.00%
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 14.5
Preferred stocks | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	96.90%								96.90%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.40%				88.40%				88.40%	88.40%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 503								$ 503
Pre-tax loss									(47.3)	$ (40.5)	(78.6)
Net investment income (loss) - surplus note interest									(15.8)	(15.7)	(40.2)
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	$ 460.8				$ 449.1				$ 460.8	$ 449.1
Collateral Held in Supplement Trust	136,200,000				120,000,000				136,200,000	120,000,000
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax loss									$ 17.9	$ 18.3	38.1
Net investment income (loss) - surplus note interest									15.8	$ 15.7	$ 40.2
Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest Receivable	$ 90.2								$ 90.2

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’ common shareholders									$ 198.8	$ 53.6	$ 175.1
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares(1)									(2.3)	(0.7)	(2.6)
Dividends declared on participating restricted common shares(1)									(0.1)	(0.1)	(0.1)
Total allocation to restricted common shares									(2.4)	(0.8)	(2.7)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									196.4	52.8	172.4
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									196.4	52.8	172.4
Dividends declared net of restricted common share amounts(1)									(5.9)	(6.1)	(6.1)
Total undistributed net earnings, net of restricted common share amounts									$ 190.5	$ 46.7	$ 166.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Basic earnings per share denominator									5,811,200	6,026,000	6,109,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings (loss) per share denominator									5,811,200	6,026,000	6,109,000
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$ 45.99	$ (12.42)	$ 0.13	$ 1.76	$ (2.03)	$ (2.07)	$ 5.58	$ 7.13	$ 33.80	$ 8.77	$ 28.22
Dividends declared and paid									(1)	(1)	(1)
Undistributed earnings									32.80	7.77	27.22
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$ 45.96	$ (12.42)	$ 0.13	$ 1.76	$ (2.03)	$ (2.07)	$ 5.58	$ 7.13	33.80	8.77	28.22
Dividends declared and paid									(1)	(1)	(1)
Undistributed earnings									$ 32.80	$ 7.77	$ 27.22
Incremental common shares attributable to share-based payment arrangements									125,000	125,000	125,000

Retirement and Postretiremen119

Retirement and Postretirement Plans (Benefit Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation	$ (1.8)	$ 0
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	119.7	102.9
Service cost	0.9	0.6	$ 0.8
Interest cost	4.6	4.7	4.2
Special termination benefits expense	0	0.3	0.3
Assumption changes	(3.5)	17.5
Actuarial loss	0.9	1.1
Benefits and expenses paid with plan assets	(13.7)	(5.2)
Benefits paid directly by OneBeacon	(2.2)	(2.2)
Projected benefit obligation at end of year	109.5	119.7	102.9
Change in plan assets:
Fair value of plan assets at beginning of year	146	142.8
Actual return on plan assets	7.5	8.4
Benefits and expenses paid with plan assets	(13.7)	(5.2)
Fair value of plan assets at end of year	139.8	146	$ 142.8
Funded status at end of year	30.3	26.3
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	55.8	53.4
Net liabilities of the Non-qualified Plan recorded in other liabilities	(25.5)	(27.1)
Defined Benefit Plan, Amounts Recognized in Balance Sheet	30.3	26.3
Defined Benefit Plan, Other Costs	(4.6)	0
Qualified Plan [Member]
Change in plan assets:
Fair value of plan assets at beginning of year	143.2
Fair value of plan assets at end of year	139.4	$ 143.2
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	55.8
Other Postretirement Benefit Plan, Defined Benefit [Member]
Amounts recognized in the financial statements
Net liabilities of the Non-qualified Plan recorded in other liabilities	(25.5)
Other Postretirement Benefit Plan, Defined Benefit [Member] | OneBeacon Ltd.
Change in plan assets:
Amount set aside in irrevocable rabbi trust	$ 18.6

Retirement and Postretiremen120

Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest
Accumulated other comprehensive (loss) income beginning balance	$ (8)	$ (8)	$ 10.5
Increase (decrease) in accumulated other comprehensive income (loss):
Amortization of net actuarial gains recognized during the year		1.3	0.3
Net actuarial (losses) gains occurring during the year		(1.3)	(18.8)
Accumulated other comprehensive loss ending balance		(8)	(8)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets
Projected benefit obligation		25.5	27.1
Accumulated benefit obligation		25.5	27.1
Fair value of plan assets		0	0
Qualified Plan [Member]
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets
Projected benefit obligation		84	92.6
Accumulated benefit obligation		84	92.6
Fair value of plan net assets		$ 139.8	$ 146
Scenario, Forecast [Member] | OneBeacon Ltd.
Increase (decrease) in accumulated other comprehensive income (loss):
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012	$ 1.1

Retirement and Postretiremen121

Retirement and Postretirement Plans (Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net periodic benefit cost (income)
Service cost	$ 0.9	$ 0.6	$ 0.8
Interest cost	4.6	4.7	4.2
Expected return on plan assets	(8.7)	(8.5)	(7.1)
Amortization of unrecognized loss	1.3	0.3	0.9
Net periodic pension income before settlements, curtailments and special termination benefits	(1.9)	(2.9)	(1.2)
Special termination benefits expense	0	(0.3)	(0.3)
Total net periodic benefit income	$ (1.9)	$ (2.6)	$ (0.9)
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount Yield (as a percent)	3.22%	3.91%
Discount rate (as a percent)	3.91%
Pension Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (as a percent)		4.66%
Long-Duration Contracts, Assumptions by Product and Guarantee, Estimated Average Investment Yield	6.00%	6.00%

Retirement and Postretiremen122

Retirement and Postretirement Plans (Qualified Plan's Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 139.8	$ 146	$ 142.8
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 139.4	$ 143.2
Asset allocations (as a percent)	100.00%	100.00%
Qualified Plan [Member] | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 139.4	$ 105.8
Qualified Plan [Member] | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	37.4
Qualified Plan [Member] | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0
Qualified Plan [Member] | Fixed maturity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 132.4	$ 24.2
Asset allocations (as a percent)	95.00%	16.90%
Qualified Plan [Member] | Fixed maturity investments | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 132.4	$ 0
Qualified Plan [Member] | Fixed maturity investments | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	24.2
Qualified Plan [Member] | Fixed maturity investments | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0
Qualified Plan [Member] | Common equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0	$ 101
Asset allocations (as a percent)	0.00%	70.60%
Qualified Plan [Member] | Common equity securities | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0	$ 101
Qualified Plan [Member] | Common equity securities | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0
Qualified Plan [Member] | Common equity securities | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0
Qualified Plan [Member] | Convertible fixed maturity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0	$ 12.8
Asset allocations (as a percent)	0.00%	8.90%
Qualified Plan [Member] | Convertible fixed maturity investments | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0	$ 0
Qualified Plan [Member] | Convertible fixed maturity investments | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	12.8
Qualified Plan [Member] | Convertible fixed maturity investments | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0
Qualified Plan [Member] | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 7	$ 5.2
Asset allocations (as a percent)	5.00%	3.60%
Qualified Plan [Member] | Cash | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 7	$ 4.8
Qualified Plan [Member] | Cash | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0	0.4
Qualified Plan [Member] | Cash | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0	$ 0

Retirement and Postretiremen123

Retirement and Postretirement Plans Retirement and Postretirment Plans (Assumptions) (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Qualified Plan [Member]
Discount Yield (as a percent)	3.22%	3.91%
Discount rate (as a percent)	3.91%
Pension Costs [Member]
Discount rate (as a percent)		4.66%
Long-Duration Contracts, Assumptions by Product and Guarantee, Estimated Average Investment Yield	6.00%	6.00%

Retirement and Postretiremen124

Retirement and Postretirement Plans (Cash Flow) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee savings plan
Defined Benefit Plan, Benefit Obligation	$ 109,500,000	$ 119,700,000	$ 102,900,000
Funded status at end of year	30,300,000	26,300,000
OneBeacon Ltd.
Expected Benefit Payments
Expected future benefit payments, next twelve months	86,800,000
Expected future benefit payments, year two	2,100,000
Expected future benefit payments, year three	2,100,000
Expected future benefit payments, year four	2,000,000
Expected future benefit payments, year five	2,000,000
Expected future benefit payments, five fiscal years thereafter	8,900,000
Employee savings plan
Anticipated contribution to plan	2,100,000
OneBeacon
Employee savings plan
Assets Held-in-trust, Noncurrent	33,900,000	33,200,000
Disability and Health Benefits [Member] | OneBeacon
Employee savings plan
Post-employment benefit liability	4,100,000	4,800,000
Death benefits [Member] | OneBeacon
Employee savings plan
Postemployment Benefits Liability	12,500,000	12,900,000
One Beacon Employee Stock Ownership Plan
Employee savings plan
Defined Contribution Plan, Cost Recognized	$ 5,700,000	4,400,000	6,300,000
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 118,500
Variable contribution provided to participants (as percentage)	6.00%
One Beacon Employee Stock Ownership Plan | OneBeacon
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Cost Recognized	$ 3,000,000	$ 2,700,000	$ 2,300,000
IRS | OneBeacon
Employee savings plan
Defined Benefit Plan Compensation LImits	$ 265,000

Employee Share-Based Incenti125

Employee Share-Based Incentive Compensation Plan (Details)	12 Months Ended
	Dec. 31, 2015USD ($)
Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 118,500
Variable contribution provided to participants (as percentage)	6.00%
OneBeacon | IRS
Share-based compensation arrangement by share-based payment award
Defined Benefit Plan Compensation LImits	$ 265,000

Employee Share-Based Incenti126

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)multipliershares	Dec. 31, 2014USD ($)shares	Dec. 31, 2013USD ($)shares
Unamortized grant date fair value
Beginning of period	$ 108.1
End of period (4)	$ 141.7	$ 108.1
Performance Shares | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Vested (in shares) | shares		3,570
Performance Shares | White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range | multiplier	0
Multiplier for determining payout, high end of range | multiplier	2
Performance cycle period	3 years
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range Of Performance Cycle From 2014-2016 [Member]
Target Performance Share activity
Assumed forfeitures and cancellations(2)(3) | shares	(3,040)
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout	88.00%
Payout for maximum growth target percentage	131.50%
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2010 to 2012 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout		33.00%
Payout for maximum growth target percentage		98.00%
Performance Shares | W T M Incentive Plan | Range Of Performance Cycle From 2014-2016 [Member]
Unamortized grant date fair value
Payout for maximum growth target percentage	200.00%
Performance Shares | W T M Incentive Plan | White Mountains
Target Performance Share activity
Beginning of period | shares	113,198	108,605	108,065
Shares paid or expired(1) | shares	(42,958)	(33,730)	(43,100)
New grants | shares	29,828	41,580	43,660
Assumed forfeitures and cancellations(2)(3) | shares	(3,857)	(3,257)	(20)
End of period (4) | shares	96,211	113,198	108,605
Unamortized grant date fair value
Beginning of period	$ 44.4	$ 54.9	$ 26.5
Shares paid or expired(1)	(30.8)	(24.5)	(10.1)
Assumed forfeitures and cancellations(2)(3)	(0.3)	(0.1)	(0.6)
Expense recognized	44.4	14.1	39.1
End of period (4)	57.7	$ 44.4	$ 54.9
Unamortized grant date fair value	$ 22.5

Employee Share-Based Incenti127

Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 141.7	$ 108.1
Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	9.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2012 to 2014 | W T M Incentive and Phantom Plan [Member] | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	91.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	145.50%
White Mountains | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	98,678
Incentive compensation payable before forfeitures	$ 59.2
forfeitures	(2,467)
Assumed forfeitures in period amount	(1.5)
Shares outstanding (in shares)	96,211	113,198	108,605	108,065
Accrued incentive compensation	$ 57.7	$ 44.4	$ 54.9	$ 26.5
White Mountains | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	29,828
Incentive compensation payable before forfeitures	$ 8.3
White Mountains | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	32,557
Incentive compensation payable before forfeitures	$ 12.9
White Mountains | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	36,293
Incentive compensation payable before forfeitures	$ 38
Sirius Group | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	7,997	10,351	10,615
Minimum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Minimum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Maximum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Maximum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	15.00%
Maximum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%

Employee Share-Based Incenti128

Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restricted Share Cliff Vesting in January 2016 [Member]
Target Performance Share activity
Issued (in shares)			25,720
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2017 [Member]
Target Performance Share activity
Issued (in shares)		23,440
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2018 [Member]
Target Performance Share activity
Issued (in shares)	23,640
White Mountains | W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period	83,314	94,130	69,910
Issued (in shares)	23,640	23,440	25,720
Vested (in shares)	(36,279)	(33,205)	(1,500)
Assumed forfeitures and cancellations (in shares)	0	(1,051)	0
End of period (4)	70,675	83,314	94,130
Unamortized grant date fair value
Beginning balance	$ 14.3	$ 17	$ 16.8
Issued	15.7	13.1	14.4
Assumed forfeitures and cancellations	0	(0.5)	0
Expense recognized	(14.3)	(15.3)	(14.2)
Ending balance	$ 15.7	$ 14.3	$ 17
White Mountains | W T M Incentive Plan | Performance Shares
Target Performance Share activity
Beginning of period	113,198	108,605	108,065
Assumed forfeitures and cancellations (in shares)	(3,857)	(3,257)	(20)
End of period (4)	96,211	113,198	108,605
Unamortized grant date fair value
Assumed forfeitures and cancellations	$ (0.3)	$ (0.1)	$ (0.6)
Expense recognized	(44.4)	$ (14.1)	$ (39.1)
Ending balance	$ 22.5
Range of Performance Cycle from 2011 to 2013 [Member] | Performance Shares
Target Performance Share activity
Vested (in shares)		(3,570)
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage		37.10%
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive and Phantom Plan [Member] | Restricted Stock
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)		(3,040)
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive and Phantom Plan [Member] | Performance Shares
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)	(3,040)

Employee Share-Based Incenti129

Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	125,000
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding Amended, Exercise Price	$ 742
Qualified Plan [Member]
Share-based compensation arrangement by share-based payment award
Percentage of common shares outstanding	3.00%	3.00%
Eligible contribution percentage	3.00%	2.00%	4.00%

Employee Share-Based Incenti130

Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Expense Amortized
Beginning of period	$ 108.1
End of period (4)	$ 141.7	$ 108.1
OneBeacon | Performance Shares | Long Term Incentive Plan
Target options outstanding
Beginning of period	517,470	493,421	563,190
Shares paid or expired (in shares)	(181,290)	(142,138)	(238,658)
New grants	154,887	165,800	179,000
Assumed forfeitures and cancellations (in shares)	(41,632)	(387)	(10,111)
End of period (4)	449,435	517,470	493,421
Expense Amortized
Beginning of period	$ 3.4	$ 4	$ 1.2
Payments and deferrals	(1.5)	(1)	0
Assumed forfeitures and cancellations	(0.1)	0	(0.1)
Expense recognized	(0.4)	(0.4)	(2.9)
End of period (4)	$ 1.4	$ 3.4	$ 4
Performance Cycle From 2012 to 2014 | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	45.70%
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage		37.10%

Employee Share-Based Incenti131

Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 141.7	$ 108.1
OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	456,669
Incentive compensation payable before forfeitures	$ 1.4
forfeitures	(7,234)
Assumed forfeitures in period amount	0
Shares outstanding (in shares)	449,435	517,470	493,421	563,190
Accrued incentive compensation	$ 1.4	$ 3.4	$ 4	$ 1.2
Unamortized grant date fair value	$ 1.3
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	13.00%	14.00%	13.00%
Range Of Performance Cycle From 2015 to 2017 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	146,659
Incentive compensation payable before forfeitures	$ 0.7
Range Of Performance Cycle From 2014-2016 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	142,710
Incentive compensation payable before forfeitures	$ 0.1
Performance Cycle From 2013 to 2015 | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	167,300
Incentive compensation payable before forfeitures	$ 0.6
Maximum | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	20.00%	21.00%	20.00%
Percentage of performance shares earned	200.00%	200.00%	200.00%
Minimum | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	6.00%	7.00%	6.00%

Employee Share-Based Incenti132

Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions	Mar. 01, 2012Installmentshares	May. 31, 2011shares	Dec. 31, 2015USD ($)shares	Dec. 31, 2014USD ($)shares	Dec. 31, 2013USD ($)shares	May. 25, 2011Installment
Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)			226,778
End of period (4)			200,194
Unamortized grant date fair value
Expense recognized | $			$ (0.8)
Restricted Share Cliff Vesting in January 2018 [Member] | One Beacon Insurance Company | Restricted Stock
Target Performance Share activity
Issued (in shares)			75,950
Unamortized grant date fair value
Issued | $			$ 1.1
Restricted Stock | Sirius Group | W T M Incentive Plan
Target Performance Share activity
Beginning of period			3,265	3,789
End of period (4)			3,571	3,265	3,789
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan
Unamortized grant date fair value
Forfeited shares		35,000
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan | Chief Executive Officer
Target Performance Share activity
Issued (in shares)	300,000	630,000
Unamortized grant date fair value
Number of installments in which the shares vest | Installment	2					4
Forfeited shares		35,000
Years performance shares will be forfeited		5 years
Restricted Stock | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
Beginning of period			612,500	915,000	927,000
Issued (in shares)			75,950	0	0
Vested (in shares)			(296,000)	(300,000)	(9,000)
Assumed forfeitures and cancellations (in shares)			(9,728)	(2,500)	(3,000)
End of period (4)			382,722	612,500	915,000
Unamortized grant date fair value
Beginning balance | $			$ 3.5	$ 6.5	$ 9.6
Issued | $			1.1	0	0
Vested | $			0		0
Assumed forfeitures and cancellations | $			(0.1)	0	0
Expense recognized | $			(2)	(3)	(3.1)
Ending balance | $			$ 2.5	$ 3.5	$ 6.5

Employee Share-Based Incenti133

Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Defined Contribution Plan, Cost Recognized	$ 5.7	$ 4.4	$ 6.3
Qualified Plan [Member]
Share-based compensation arrangement by share-based payment award
Eligible contribution percentage	3.00%	2.00%	4.00%
Percentage of common shares outstanding	3.00%	3.00%

Common Shareholders_ Equity 134

Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	May. 31, 2007	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		$ 284.2	$ 134.5	$ 79.8
Value of each common share repurchased and retired during the period (in dollars per share)		$ 733	$ 617	$ 564
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		25,016	28,405	27,310
Shares issued to directors (in shares)		1,376	1,395	1,590
Dividends, Common Stock [Abstract]
Cash dividends declared and paid		$ 6	$ 6.2	$ 6.2
Dividends delared and paid (in dollars per share)		$ 1	$ 1	$ 1
Non-controlling interests		$ 454.8	$ 542.7
Non-controlling Interests
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		0	0	$ 0
Total Non-Controlling Excluding Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 115.2	$ 168.6
Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		23,640	23,440	25,720
Performance Shares
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues			3,570
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		387,495	217,879	141,535
General Board Authorization [Member]
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares		476,253
Common shares repurchased and retired during the period		361,839	207,404	140,000
Value of common shares repurchased and retired during the period		$ 267.4	$ 128.2	$ 78.9
Value of each common share repurchased and retired during the period (in dollars per share)		$ 739	$ 618	$ 564
Repurchased for Employee Benefit Plans [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		10,802		1,535
Repurchased for Employee Benefit Plans [Member] | Combination Three [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period			10,475
White Mountains
Dividends, Common Stock [Abstract]
Capital contributions					$ 594.5
Sirius Internation Group Ltd | Preferred stocks
Dividends, Common Stock [Abstract]
Preference shares floating rate of dividend amount in excess of libor	3.20%
Preferred Stock Redemption Basis Spread on Treasury Issue Rate	45.00%
OneBeacon Ltd.
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		24.50%	24.70%
Non-controlling interests		$ 245.6	$ 258.4
HG/BAM [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (140)	$ (121.9)
Star & Shield Insurance Exchange [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (16)	$ (12.4)
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.10%	3.10%
Non-controlling interests		$ 17.1	$ 17.9
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		40.00%	40.00%
Non-controlling interests		$ 14.4	$ 22.6
Tranzact [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		36.80%	36.80%
Non-controlling interests		$ 79.4	$ 88.2
WOBI [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.90%	36.70%
Non-controlling interests		$ 0.6	$ 5.4
Prospector Offshore Fund Ltd [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		0.00%	23.40%
Non-controlling interests		$ 0	$ 31.1
A W G Dewar Inc [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		19.00%	18.00%
Non-controlling interests		$ 3.7	$ 3.4
SIG Preference Shares
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ 250	$ 250
BAM Management
Dividends, Common Stock [Abstract]
Capital contributions					$ 14.5
OneBeacon
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 249.3	$ 261.8

Statutory Capital and Surplus (

Statutory Capital and Surplus (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2014USD ($)	Dec. 31, 2015USD ($)yr	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016	Dec. 31, 2012USD ($)
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	$ 3,995,700,000	$ 3,913,200,000	$ 3,995,700,000
Amount available for dividend distributions without prior approval from regulatory authorities		$ 48,000,000
Equity held in insurance and reinsurance subsidiaries		505,000,000
Distributions made		$ 6,000,000
Payments for Repurchase of Common Stock		268,600,000	128,200,000	$ 79,800,000
Fair value investments	3,343,600,000	4,050,900,000	3,343,600,000	3,889,000,000
Trading Securities		2,639,700,000
Dividend Capacity
Other long-term investments	331,900,000	315,800,000	331,900,000
OneBeacon
Statutory Accounting Practices [Line Items]
Statutory net income (loss)		44,500,000	(14,200,000)	100,100,000
Stockholders' Equity Attributable to Parent	787,500,000	755,200,000	787,500,000
Distributions made		80,000,000		80,200,000
Unrestricted cash		65,300,000
Other Operations
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	1,425,500,000	1,344,300,000	1,425,500,000
Unrestricted cash		73,700,000
Dividend Capacity
Other long-term investments		17,600,000
Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		5,300,000	4,800,000
Surplus notes		101,000,000
Unrestricted cash		104,400,000
Lone Tree Insurance Group Ltd
Statutory Accounting Practices [Line Items]
Noncash distribution received				27,900,000
Proceeds from Contributions from Affiliates		15
Star & Shield Insurance Exchange [Member]
Statutory Accounting Practices [Line Items]
Surplus notes	17,000,000	21,000,000	17,000,000
Star & Shield Insurance Exchange [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	6,500,000	5,000,000	6,500,000
Statutory net income (loss)		3,500,000	9,500,000
Surplus required		1,500,000
OneBeacon
Statutory Accounting Practices [Line Items]
Dividends available, net of noncontrolling interest		180,000,000
HG Global
Statutory Accounting Practices [Line Items]
Surplus notes						$ 503,000,000
HG Global | HG Global-BAM
Dividend Capacity
Preferred stock, value		$ 613,000,000
Ownership interest (as a percent)		96.90%
Preferred stock dividend rate		6.00%
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 460,800,000
Collateral Held in Supplement Trust		401,900,000
Dividend Capacity
Accrued interest on surplus notes		$ 53,800,000
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		437,300,000
Statutory net income (loss)		32,000,000	31,800,000	29,300,000
Surplus required		66,000,000
Subsidiaries
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		554,000,000
One Beacon Insurance Company | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made	700,500,000
Noncash distribution received	151,200,000
Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	122,600,000	249,500,000	122,600,000
Statutory net income (loss)		45,500,000	46,200,000	$ (38,700,000)
Amount available for dividend distributions without prior approval from regulatory authorities		37,400,000
Payments of Distributions to Affiliates		85,000,000
White Mountains | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		60,300,000
White Mountains | Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash		14,400,000
Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Surplus required		117,700,000
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		622,300,000
Amount available for dividend distributions without prior approval from regulatory authorities		26,600,000
Distributions made		44,900,000	10,000,000
Payments for Repurchase of Common Stock		66,000,000
Payments of Capital Distribution			10,000,000
Statutory accounting practices earned surplus		68,300,000
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulatory Approval [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities		62,200,000
White Mountains Life Re | Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 76,000,000	77,500,000	$ 76,000,000
Surplus required		$ 500,000
Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)		8.00%
basis points		3.00%
Preferred stocks | HG Global | HG Global-BAM
Dividend Capacity
Dividends Payable		$ 139,700,000
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Dividend Capacity
Dividends Payable		$ 135,300,000
SEK
Dividend Capacity
Exchange rate	7.7737	8.4247	7.7737	6.4339
Common Stock [Member] | OneBeacon
Dividend Capacity
Other long-term investments		$ 51,500,000
Common Equities [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Trading Securities		54,600,000
Common Equities [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Trading Securities		292,300,000
Retained earnings
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	$ 3,010,500,000	3,084,900,000	$ 3,010,500,000	$ 2,801,900,000		$ 2,542,500,000
Amount available for dividend distributions without prior approval from regulatory authorities		$ 2,600,000,000
Retained earnings | Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		15.00%
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		10.00%
U.S and Bermuda [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 721,500,000	$ 622,300,000	$ 721,500,000
Surplus Note | Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Fair value investments		$ 51,500,000
Combination One [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Number of years of change in interest rate		5
Combination Two [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Number of years of change in interest rate | yr		3
US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)		3.15%
Fixed interest rate [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)		8.00%
Scenario, Forecast [Member] | US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)					3.54%

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 1,188.2	$ 1,185	$ 1,120.9
Net investment income									60.8	59.5	59.8
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									225.4	78.5	133.9
Other revenue									334.2	131.1	47
Total revenues	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	1,808.6	1,454.1	1,361.6
Loss and loss adjustment expenses									708.9	824	622.1
Insurance and reinsurance acquisition expenses									220.1	206.2	210.4
Other underwriting expenses									218.6	179.6	205.2
General and administrative expenses									487.5	257.7	149.1
Amortization of Intangible Assets									28.5	11.7	1.4
Interest expense on debt									18.6	15.6	16.2
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203
Pre-tax loss	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	154.9	(29)	158.6
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									458.9	246	147.7
OneBeacon
Segment reporting information
Earned insurance premiums									1,176.2	1,177.1	1,120.4
Net investment income									45.9	43.4	43
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									(35.1)	40.4	49.4
Other revenue									(0.6)	5.8	31.2
Total revenues									1,186.4	1,266.7	1,244
Loss and loss adjustment expenses									700.7	815.1	622.1
Insurance and reinsurance acquisition expenses									213.8	203.3	208.9
Other underwriting expenses									218.2	179.2	204.8
Interest expense on debt									13	13	13
Total expenses									1,161.1	1,224.4	1,060.8
Pre-tax loss									25.3	42.3	183.2
OneBeacon | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									14.1	12.4	10.6
BAM
Segment reporting information
Earned insurance premiums									0.8	0.4	0.1
Net investment income									4.2	5.7	4.7
Net investment income (loss) - surplus note interest									(15.8)	(15.7)	(40.2)
Net realized and unrealized investment gains									0.9	6.6	(9.3)
Other revenue									0.7	0.6	0.4
Total revenues									(9.2)	(2.4)	(44.3)
Loss and loss adjustment expenses									0	0	0
Insurance and reinsurance acquisition expenses									2.3	1.8	1.4
Other underwriting expenses									0.4	0.4	0.4
Interest expense on debt									0	0	0
Total expenses									38.1	38.1	34.3
Pre-tax loss									(47.3)	(40.5)	(78.6)
BAM | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									$ 35.4	35.9	32.5
HG Global
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 2.5	1.4	0.4
Net investment income									1.9	1.4	1
Net investment income (loss) - surplus note interest									15.8	15.7	40.2
Net realized and unrealized investment gains									(0.3)	1.7	(2)
Other revenue									0	0	0
Total revenues									19.9	20.2	39.6
Loss and loss adjustment expenses									0	0	0
Insurance and reinsurance acquisition expenses									0.6	0.3	0.1
Other underwriting expenses									0	0	0
Interest expense on debt									0	0	0
Total expenses									2	1.9	1.5
Pre-tax loss									17.9	18.3	38.1
HG Global | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									1.4	1.6	1.4
Other Segments
Segment reporting information
Earned insurance premiums									8.7	6.1	0
Net investment income									8.8	9	11.1
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									259.9	29.8	95.8
Other revenue									334.1	124.7	15.4
Total revenues									611.5	169.6	122.3
Loss and loss adjustment expenses									8.2	8.9	0
Insurance and reinsurance acquisition expenses									3.4	0.8	0
Other underwriting expenses									0	0	0
Interest expense on debt									5.6	2.6	3.2
Total expenses									452.5	218.7	106.4
Pre-tax loss									159	(49.1)	15.9
Other Segments | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									408	196.1	103.2
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets									28.6	11.7	1.4
General and Administrative Expense [Member] | OneBeacon
Segment reporting information
Amortization of Intangible Assets									1.3	1.4	1.4
General and Administrative Expense [Member] | BAM
Segment reporting information
Amortization of Intangible Assets									0	0	0
General and Administrative Expense [Member] | HG Global
Segment reporting information
Amortization of Intangible Assets									0	0	0
General and Administrative Expense [Member] | Other Segments
Segment reporting information
Amortization of Intangible Assets									27.3	10.3	$ 0
Symetra | Other Segments
Segment reporting information
Net realized and unrealized investment gains									258.8
Tranzact [Member] | Other Segments
Segment reporting information
Other revenue									186.9	43.2
General and administrative expenses									167.3	37.4
MediaAlpha [Member] | Other Segments
Segment reporting information
Other revenue									105.5	65.3
General and administrative expenses									$ 99	$ 60.6

Segment Information (Assets) (D

Segment Information (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Total investments	$ 4,280.7	$ 3,742.4
Reinsurance recoverable on paid and unpaid losses	194	173.9
Assets held for sale	4,407	4,630.6
Assets	10,284.5	10,455.7
Loss and loss adjustment expense reserves	1,395.8	1,350	$ 1,054.3	$ 1,000
Liabilities held for sale	2,884	3,105.3
Liabilities	5,916.5	5,917.3
Non-controlling interests	454.8	542.7
White Mountains’ common shareholders’ equity	3,913.2	3,995.7
OneBeacon
Segment reporting information
Total investments	2,591.4	2,525.8
Reinsurance recoverable on paid and unpaid losses	193.5	173.8
Assets held for sale	0	58.1
Assets	3,602.7	3,559.8
Loss and loss adjustment expense reserves	1,389.8	1,342.2
Liabilities held for sale	0	0
Liabilities	2,598.2	2,510.5
Non-controlling interests	249.3	261.8
White Mountains’ common shareholders’ equity	755.2	787.5
HG Global
Segment reporting information
Total investments	136.2	121
Reinsurance recoverable on paid and unpaid losses	0	0
Assets held for sale	0	0
Assets	739	704.4
Loss and loss adjustment expense reserves	0	0
Liabilities held for sale	0	0
Liabilities	181.2	121
Non-controlling interests	17.1	17.9
White Mountains’ common shareholders’ equity	540.7	565.5
BAM
Segment reporting information
Total investments	460.3	454.2
Reinsurance recoverable on paid and unpaid losses	0	0
Assets held for sale	0	0
Assets	(91.1)	(89.4)
Loss and loss adjustment expense reserves	0	0
Liabilities held for sale	0	0
Liabilities	48.9	32.5
Non-controlling interests	(140)	(121.9)
White Mountains’ common shareholders’ equity	0	0
Surplus notes	503
Accrued interest on surplus notes	90.2	74.4
Other Operations
Segment reporting information
Total investments	1,092.8	641.4
Reinsurance recoverable on paid and unpaid losses	0.5	0.1
Assets held for sale	0	0
Assets	1,626.9	1,708.4
Loss and loss adjustment expense reserves	6	7.8
Liabilities held for sale	0	0
Liabilities	204.2	148
Non-controlling interests	78.4	134.9
White Mountains’ common shareholders’ equity	1,344.3	1,425.5
Discontinued Operations, Held-for-sale [Member]
Segment reporting information
Total investments	0	0
Reinsurance recoverable on paid and unpaid losses	0	0
Assets held for sale	4,407	4,572.5
Assets	4,407	4,572.5
Loss and loss adjustment expense reserves	0	0
Liabilities held for sale	2,884	3,105.3
Liabilities	2,884	3,105.3
Non-controlling interests	0	0
White Mountains’ common shareholders’ equity	1,273	1,217.2
Continuing Operations [Member]
Segment reporting information
Non-controlling interests	$ 204.8	292.7
SIG Preference Shares
Segment reporting information
Non-controlling interests		$ 250

Investments in Unconsolidate138

Investments in Unconsolidated Affiliates (Details) - USD ($) $ in Millions	Dec. 31, 2015	Nov. 05, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments in and Advances to Affiliates [Line Items]
Investments in unconsolidated affiliates	$ 0	$ 392.8	$ 414.4
Symetra
Investments in and Advances to Affiliates [Line Items]
Unrealized (losses) gains from Symetra’s fixed maturity portfolio		(1.6)	37.6
Investments in unconsolidated affiliates		392.8	411.4	$ 317.3	$ 381.5
Symetra | Common Stock
Investments in and Advances to Affiliates [Line Items]
Symetra common shares		394.4	373.8
Investments in unconsolidated affiliates		392.8	411.4	317.3	351.2
Symetra | Warrants
Investments in and Advances to Affiliates [Line Items]
Investments in unconsolidated affiliates		0	0	$ 0	$ 30.3
Hamer, LLC
Investments in and Advances to Affiliates [Line Items]
Investments in unconsolidated affiliates		$ 0	$ 3

Investments in Unconsolidate139

Investments in Unconsolidated Affiliates (Symetra Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 01, 2016	Dec. 31, 2015	Jun. 30, 2013	Nov. 05, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 20, 2013	Dec. 31, 2012	Dec. 31, 2011
Investments in unconsolidated affiliates
Trading Securities, Change in Unrealized Holding Gain (Loss)					$ 152.9	$ (79.3)	$ 40.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax					140.6	(69.4)	43.6
After tax amortization through equity in net unrealized losses					(36.3)	75.3	(98.1)
Sales of common equity securities					$ 461.4	$ 609.8	435
Common shares, par value per share (in dollars per share)		$ 1			$ 1	$ 1
Symetra
Investments in unconsolidated affiliates
Trading Securities, Change in Unrealized Holding Gain (Loss)		$ 258.8
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax		$ 241.1
Stock Dividends, Shares					513,500
Shares of symetra financial corporation owned (in shares)				20,562,379		20,048,879
Ownership interest (as a percent)				17.70%		17.31%
Dividends received				$ (16.9)		$ (34.1)	$ (6.4)
Investments in unconsolidated affiliates on conversion of warrants							9,490,000	2,650,000	9,490,000
Gain (Loss) on Warrants							$ 10.8
Fair value of investment										$ 261
Estimated fair value of investment per share										$ 15
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity				159.9		170.4
Term of amortization (in years)					30 years
Common Stock | Symetra
Investments in unconsolidated affiliates
Dividends received				$ (16.9)		(34.1)	(6.4)
Per share price, class of warrant (in dollars per share)				$ 31.67
Gain (Loss) on Warrants							0
Warrants
Investments in unconsolidated affiliates
Gain (Loss) on Warrants			$ 10.8
Warrants | Symetra
Investments in unconsolidated affiliates
Dividends received				$ 0		0	$ 0
Per share price, class of warrant (in dollars per share)							$ 11.49
Gain (Loss) on Warrants							$ 10.8
Cash dividends received on warrants			$ 1.5
equity in earnings of unconsolidated affiliates [Member] | Symetra
Investments in unconsolidated affiliates
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity				32.6		36.4
After tax amortization through equity in earnings				2.2		2.8	2.7
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Symetra
Investments in unconsolidated affiliates
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity				127.3		134
After tax amortization through equity in net unrealized losses				$ 8.9		$ 11.7	$ 10.8
Subsequent Event [Member] | Symetra
Investments in unconsolidated affiliates
Sales of common equity securities	$ 658
Common shares, par value per share (in dollars per share)	$ 32

Investments in Unconsolidate140

Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								6 Months Ended	10 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Nov. 05, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax	$ 7.1	$ 3.9	$ 6.8	$ 7.3	$ 12.3	$ 7	$ 12.5	$ 13.8			$ 25.1	$ 45.6	$ 36.6
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates											(39.2)	81.2	(106.4)
Carrying value at the end of the period	0				414.4					$ 392.8	0	414.4
Tax benefit (expense)	$ 1.5	$ 1.6	$ 2.2	$ (4.6)	26.2	$ 4.7	$ (7.1)	$ (9)			$ 0.7	14.8	(32.6)
Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										25.1		47	37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										(39.2)		81.2	(106.4)
Dividends received										(16.9)		(34.1)	(6.4)
Increase (decrease) in value of warrants													10.8
Carrying value at the end of the period					411.4					392.8		411.4	317.3
Unrealized (losses) gains from Symetra’s fixed maturity portfolio					37.6					(1.6)		37.6
Exercise of warrants													0
Common Stock | Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										25.1		47	37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										(39.2)		81.2	(106.4)
Carrying value of a distribution										12.4
Dividends received										(16.9)		(34.1)	(6.4)
Increase (decrease) in value of warrants													0
Carrying value at the end of the period					411.4					392.8		411.4	317.3
Loss from dilutive effect resulting from dividend and issuance of restricted shares										(0.1)		(0.1)	0.2
Aggregate value of White Mountains common shares of Symetra at quoted market price										$ 651.2
Share Price										$ 31.67
Exercise of warrants													41.1
Common Stock | White Mountains
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										$ (1.6)		37.6	(43.6)
Warrants
Changes in carrying value of investment in Symetra
Increase (decrease) in value of warrants									$ 10.8
Warrants | Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										0		0	0
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										0		0	0
Dividends received										0		0	0
Increase (decrease) in value of warrants													10.8
Carrying value at the end of the period					$ 0					0		0	$ 0
Share Price													$ 11.49
Exercise of warrants													$ (41.1)
equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra
Changes in carrying value of investment in Symetra
Tax benefit (expense)										(1.6)		(3.3)	(2.8)
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra
Changes in carrying value of investment in Symetra
Tax benefit (expense)										2.9		(5.9)	8.3
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | White Mountains
Changes in carrying value of investment in Symetra
Tax benefit (expense)										0.2		(2.7)	3.2
impairment of equity in earnings [Member] | equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax													3
impairment of net unrealized gains (losses) [Member] | Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										9.4		12.7	$ 11.8
Amortization of common share basis difference [Member] | Common Stock | Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										$ 2.3		$ 3

Investments in Unconsolidate141

Investments in Unconsolidated Affiliates (Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments in and Advances to Affiliates
Total investments	$ 4,280.7				$ 3,742.4					$ 4,280.7	$ 3,742.4
Total assets	10,284.5				10,455.7					10,284.5	10,455.7
Long-term debt	442.4				343.1					442.4	343.1
Total liabilities	5,916.5				5,917.3					5,916.5	5,917.3
Common shareholders’ equity	4,368				4,538.4					4,368	4,538.4
Earned insurance premiums										1,188.2	1,185	$ 1,120.9
Net investment income										60.8	59.5	59.8
Revenues (including realized gains and losses)	678.5	$ 334.7	$ 403.6	$ 391.8	405.6	$ 323.2	$ 393.5	$ 331.8		1,808.6	1,454.1	1,361.6
Expenses	$ 420.4	429.3	$ 412.7	$ 391.3	471.3	$ 358.7	$ 359.9	$ 293.2		1,653.7	1,483.1	1,203
Net income										279.5	290	309.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										$ 197.2	207.3	247
Symetra
Investments in and Advances to Affiliates
Total investments		32,409.2			30,634.3				$ 32,409.2		30,634.3
Separate account assets		885.9			949.8				885.9		949.8
Total assets		34,962.8			33,001.7				34,962.8		33,001.7
Policyholder liabilities		29,492			27,276				29,492		27,276
Long-term debt		697.5			697.2				697.5		697.2
Separate account liabilities		885.9			949.8				885.9		949.8
Total liabilities		31,836.7			29,641.1				31,836.7		29,641.1
Common shareholders’ equity		$ 3,126.1			$ 3,360.6				3,126.1		3,360.6
Earned insurance premiums									539.3		629.1	627.2
Net investment income									994.3		1,320.5	1,285
Revenues (including realized gains and losses)									1,605.9		2,182.4	2,139.5
Liability for Future Policy Benefits, Period Expense (Income)									1,143.7		1,399.7	1,394.9
Expenses									1,543.6		1,882.5	1,865.4
Net income									89.6		254.4	220.7
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ (234.1)		$ 397	$ (777.6)

Investments in Unconsolidate142

Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (Hamer and Bri-Mar) (Details) - USD ($) $ in Millions

3 Months Ended

6 Months Ended

9 Months Ended

10 Months Ended

12 Months Ended

Dec. 31, 2015

Sep. 30, 2015

Jun. 30, 2015

Mar. 31, 2015

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Jun. 30, 2015

Sep. 30, 2013

Nov. 05, 2015

Dec. 31, 2015

Dec. 31, 2014

Dec. 31, 2013

Schedule of Equity Method Investments [Line Items]

Equity in earnings of unconsolidated affiliates, net of tax

$ 7.1

$ 3.9

$ 6.8

$ 7.3

$ 12.3

$ 7

$ 12.5

$ 13.8

$ 25.1

$ 45.6

$ 36.6

Symetra

Schedule of Equity Method Investments [Line Items]

Equity in earnings of unconsolidated affiliates, net of tax

$ 25.1

47

37.8

Hamer, LLC

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

$ 1.6

$ 1.9

0.9

Bri-Mar Manufacturing, LLC [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Realized Gain (Loss) on Disposal

(1.7)

Equity in earnings of unconsolidated affiliates, net of tax

$ 0.9

Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital

$ 1.1

Variable Interest Entities (Det

Variable Interest Entities (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015USD ($)trust	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2015USD ($)trustdirector	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Accumulated losses	$ 3,084.9				$ 3,010.5				$ 3,084.9	$ 3,010.5
Noncontrolling interest in VIE	454.8				542.7				454.8	542.7
Non-controlling interest in consolidated subsidiaries	$ 6.1	$ (16)	$ (0.9)	$ (7.3)	(15)	$ (11.2)	$ 4.6	$ (0.6)	(18.1)	(22.2)	$ (12.5)
Payments to Acquire Other Investments									$ 78	114.7	$ 22
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Noncontrolling interest in VIE	$ 17.1				17.9				$ 17.1	17.9
Star & Shield Insurance Exchange [Member]
Variable Interest Entity [Line Items]
Surplus notes	21								21
Total assets related to consolidated VIE's	14.2				13.5				14.2	13.5
Total liabilities related to consolidated VIE's	30.3				25.9				30.3	25.9
Accumulated losses	16								$ 16
Prospector
Variable Interest Entity [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage									67.60%
Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	5								$ 5
Total assets related to consolidated VIE's	5.1				2.4				5.1	2.4
Total liabilities related to consolidated VIE's	5				4				5	4
Accumulated losses	0.2								0.2
Accrued interest on surplus notes	$ 0.3								$ 0.3
Common Stock | BAM
Variable Interest Entity [Line Items]
Issuance of notes												$ 503
BAM
Variable Interest Entity [Line Items]
Right to elect directors on board number | director									2
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Surplus notes												$ 503
Noncontrolling interest in VIE	$ 17.1				$ 17.9				$ 17.1	$ 17.9
First Loss Reinsurance Treaty [Member] | HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
First Loss Reinsurance Treaty [Member] | BAM
Variable Interest Entity [Line Items]
Percentage of Premiums	60.00%								60.00%
First Loss Reinsurance Treaty [Member] | HG Global
Variable Interest Entity [Line Items]
Number of Trust | trust	2								2
Amendment [Member] | Houston General Insurance [Member]
Variable Interest Entity [Line Items]
Surplus notes	$ 5								$ 5

Fair Value of Financial Inst144

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 442.4	$ 343.1
Tranzact [Member] | Tranzact Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt		67.4
Tranzact [Member] | Fair value | Tranzact Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	102.8	68.7
Tranzact [Member] | Carrying Value | Tranzact Bank Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	102.9	67.4
OneBeacon | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	274.8	274.7
OneBeacon | Fair value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	276.4	286
OneBeacon | Carrying Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	274.8	$ 274.7
MediaAlpha [Member] | Fair value | MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	15
MediaAlpha [Member] | Carrying Value | MediaAlpha Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 14.7

Transactions with Related Pe145

Transactions with Related Persons (Details) - USD ($)	Jul. 16, 2012	Jun. 30, 2015	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Assets				$ 10,284,500,000	$ 10,455,700,000
Purchases				2,377,100,000	(2,868,500,000)
Sales of common equity securities				$ 461,400,000	$ 609,800,000	$ 435,000,000
Treasury Stock Acquired, Average Cost Per Share				$ 733	$ 617	$ 564
OneBeacon
Related Party Transaction [Line Items]
Assets				$ 3,602,700,000	$ 3,559,800,000
Prospector
Related Party Transaction [Line Items]
Assets under management					82,400,000
Asset Management Fees				2,100,000	6,500,000	$ 6,500,000
Shares paid or expired(1)				$ 5,500,000
Percentage of fees in excess of $500,000 revenue chargeable				6.00%
Amount of revenue over which 6% of fees chargeable				$ 500,000
Management Fees, Base Revenue				400,000	1,100,000	1,300,000
Incentive fees incurred				100,000	700,000	3,200,000
Franklin Mutual Advisers [Member]
Related Party Transaction [Line Items]
Treasury Stock Acquired, Average Cost Per Share		$ 650.03	$ 761.50
OneBeacon | Prospector
Related Party Transaction [Line Items]
Assets under management					15,800,000
White Mountains Advisors LLC | Symetra
Related Party Transaction [Line Items]
Assets under management				22,100,000,000	30,900,000,000
Asset Management Fees				8,000,000	18,400,000	16,500,000
Oakum Bay Capital
Related Party Transaction [Line Items]
Ownerhsip percentage	7.50%
Sales of common equity securities						$ 500,000
Number of years to receive portion of revenue						10 years
Oakum Bay Capital | Director
Related Party Transaction [Line Items]
Related party ownerhship percentage	67.50%
Combination One [Member] | Prospector
Related Party Transaction [Line Items]
Assets				200,000,000	195,700,000
Combination Two [Member] | Prospector
Related Party Transaction [Line Items]
Assets				200,000,000	431,200,000
Combination Three [Member] | Prospector
Related Party Transaction [Line Items]
Assets				400,000,000
Preferred Equity Securities | Oakum Bay Capital
Related Party Transaction [Line Items]
Purchases	$ 2,000,000
revenue sharing agreement | Prospector
Related Party Transaction [Line Items]
Fees from Revenue Sharing Agreement with Related Party					$ 400,000	$ 900,000
Trimarc Capital Fund | Hedge funds | Oakum Bay Capital
Related Party Transaction [Line Items]
Trading Securities, Other				$ 8,900,000			$ 9,500,000
First Threshold Limit | White Mountains Advisors LLC | Prospector
Related Party Transaction [Line Items]
basis points				1.00%
Second Threshold Limit | White Mountains Advisors LLC | Prospector
Related Party Transaction [Line Items]
basis points				0.50%
Third Threshold Limit | White Mountains Advisors LLC | Prospector
Related Party Transaction [Line Items]
basis points				0.25%
W T M Incentive Plan | Performance Shares | White Mountains
Related Party Transaction [Line Items]
Number of performance shares granted				29,828	41,580	43,660
Shares paid or expired(1)				$ (30,800,000)	$ (24,500,000)	$ (10,100,000)
W T M Incentive Plan | Performance Shares | White Mountains | Prospector
Related Party Transaction [Line Items]
Number of performance shares granted				7,000
ERISA assets [Member] | Combination Two [Member] | Prospector
Related Party Transaction [Line Items]
Assets					$ 202,900,000
Total
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period				387,495	217,879	141,535
Total | Franklin Mutual Advisers [Member]
Related Party Transaction [Line Items]
Common shares repurchased and retired during the period		19,688	26,300
2014-2016 Cycle | W T M Incentive Plan | Performance Shares | White Mountains | Prospector
Related Party Transaction [Line Items]
Number of performance shares granted				6,250

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011
Loss Contingencies [Line Items]
Rent expense	$ 15.5	$ 11.8	$ 12.1
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	14.3
Capital lease payments due in two years	13.4
Capital lease payments due in three years	11.4
Capital Lease payments due in four years and thereafter	50.6
Commitments to fund other-long term investments	82.2
Reserve for probable liability from mandatory shared market mechanisms	8.5
OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	1.7
Cash proceeds received a result of entering into the sale-leaseback transactions				$ 23.1
Capital lease obligation	1.7	7.1
Capital lease asset	2.4	$ 7
Furniture and equipment | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease term (in years)				5 years
Capitalized software | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease term (in years)				4 years
Furniture, Equipment and Capitalized Software [Member] | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease asset	$ 1.3

Commitments and Contingencie147

Commitments and Contingencies (Legal Contingencies) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2007	Dec. 31, 2015	Dec. 31, 2014
Esurance [Member]
Loss Contingencies [Line Items]
Percentage of loss reserve development retained			90.00%
OneBeacon | Pending Litigation [Member] | Tribune Company Litigation [Member]
Legal Contingency [Abstract]
Proceeds From Tribune Common Stock	$ 32
OneTitle [Member]
Legal Contingency [Abstract]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		20.00%
Surplus notes		$ 10
Sirius Group | Pending Litigation [Member] | Tribune Company Litigation [Member]
Legal Contingency [Abstract]
Proceeds From Tribune Common Stock	$ 6.1

Discontinued Operations (Detail

Discontinued Operations (Details) - USD ($) $ in Millions	Dec. 23, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										$ 18.2	$ (1.6)	$ 46.6
Net realized and unrealized investment gains										225.4	78.5	133.9
Net investment income										60.8	59.5	59.8
Loss and loss adjustment expense reserves		$ 1,395.8				$ 1,350				1,395.8	1,350	1,054.3	$ 1,395.8	$ 1,000
Loss and loss adjustment expenses										708.9	824	622.1
Other revenue										334.2	131.1	47
Net income from discontinued operations, net of tax		7.3	$ 14.2	$ 3.5	$ 73.8	82.1	$ 63.9	$ 61.1	$ 51.5	80.6	260.2	99.9
Reinsurance recoverable on paid losses		7.5				12.2				7.5	12.2		7.5
Prior Accident Year										(4)	92	0
Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax												46.6
Net realized and unrealized investment gains										15.1	205.4	27.8
Net investment income										40.7	40.9	46.7
Total income from discontinued operations										98.8	258.6	146.5
Loss and loss adjustment expenses										422.7	344.6	497.3
Other revenue										(21.3)	(62.4)	27.6
Net income from discontinued operations, net of tax										80.6	260.2	99.9
Esurance [Member] | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										17.9	3.2		28.3
Net income from discontinued operations, net of tax												4.5
OneBeacon Runoff
Discontinued Operations
Surplus notes	$ 101
OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										0.3	(18.8)	46.6
Sirius Group
Discontinued Operations
Net realized and unrealized investment gains										225	4	1
Net investment income										225	4	1
Sirius Group | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax												0
Net realized and unrealized investment gains										15.1	205.4	27.8
Net investment income										40.7	40.9	46.7
Loss and loss adjustment expense reserves		1,644.4				1,809.8				1,644.4	1,809.8		1,644.4
Total income from discontinued operations										81.1	262	142
Loss and loss adjustment expenses										422.7	345.3	418.4
Other revenue										(20.6)	(62.4)	16.8
Net income from discontinued operations, net of tax										81.1	262	$ 142
Reinsurance recoverable on paid losses		$ 10.2				11.4				$ 10.2	11.4		$ 10.2
Fireman's Fund Insurance Company [Member] | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax											14
Surplus Note
Discontinued Operations
RBC Score										250.00%
Surplus Note | OneBeacon Runoff
Discontinued Operations
RBC Score	250.00%
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Discontinued Operations
Surplus notes	$ 64.9
Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax						36.1
Gain (loss) on sale of discontinued operations, net of tax	36.1
Assets, Fair Value Adjustment, Net of Tax	23.5
Interest rate (as a percent)		6.00%								6.00%			6.00%
RBC Score										200.00%
Symetra | Sirius Group
Discontinued Operations
Net realized and unrealized investment gains										$ 218.5
OneBeacon
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax	(36.1)									(49.5)	(35.9)
Par Value [Member] | Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Surplus notes	101
Par Value [Member] | OneBeacon
Discontinued Operations
Surplus notes	101	$ 101				101				101	101		$ 101
Fair Value [Member] | Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Surplus notes	64.9
Fair Value [Member] | OneBeacon
Discontinued Operations
Surplus notes	$ 64.9	$ 51.5				$ 65.1				$ 51.5	$ 65.1		$ 51.5

Discontinued Operations (Assets

Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred Income Tax Assets, Net	$ 112.8	$ 114.6
Assets held for sale
Fixed maturity investments, at fair value	2,639.7	2,422
Short-term investments, at amortized cost (which approximates fair value)	211.3	376.8
Common equity securities, at fair value	1,113.9	611.7
Other long-term investments	315.8	331.9
Total investments	4,280.7	3,742.4
Insurance premiums receivable	223.3	241.1
Ceded unearned insurance premiums	29.5	17.8
Accounts receivable on unsettled investment sales	41.9	37.8
Goodwill and intangible assets	375.7	351.2
Other assets	318.7	348.7
Assets held for sale	4,407	4,630.6
Deferred acquisition costs	107.6	107.2
Cash Including Restricted Cash	179.3	261.7
Liabilities held for sale
Loss and loss adjustment expense reserves	1,395.8	1,350	$ 1,054.3	$ 1,000
Unearned insurance premiums	612.6	616.7
Debt	442.4	343.1
Deferred tax liability	7	0
Accrued incentive compensation	141.7	108.1
Ceded reinsurance payable	30.5	34.2
Funds held under insurance contracts	137.8	81
Accounts payable on unsettled investment purchases	0	0.5
Other liabilities	264.7	278.4
Liabilities held for sale	2,884	3,105.3
Reinsurance recoverable on unpaid losses	186.5	161.7
Reinsurance recoverable on paid losses	7.5	12.2
Sirius Group
Assets held for sale
Total investments	686.2
Cash Including Restricted Cash	143.9	111.5	$ 93.2	$ 93.7
Sirius Group | Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred Income Tax Assets, Net	303.1	341.5
Assets held for sale
Fixed maturity investments, at fair value	2,374	2,362.3
Short-term investments, at amortized cost (which approximates fair value)	352	494.9
Common equity securities, at fair value	174.4	189.9
Other long-term investments	72.2	95.6
Total investments	2,972.6	3,142.7
Insurance premiums receivable	323.6	306.6
Ceded unearned insurance premiums	87.7	76.2
Accounts receivable on unsettled investment sales	29	18.7
Goodwill and intangible assets	10.2	15.2
Other assets	169	156.6
Assets held for sale	4,407	4,630.6
Deferred acquisition costs	74.6	69.9
Cash Including Restricted Cash	143.9	111.5
Liabilities held for sale
Loss and loss adjustment expense reserves	1,644.4	1,809.8
Unearned insurance premiums	342.2	338.6
Debt	403.5	403.5
Deferred tax liability	263.6	282.8
Accrued incentive compensation	63.2	76.5
Ceded reinsurance payable	67.1	71.5
Funds held under insurance contracts	52.9	57.9
Accounts payable on unsettled investment purchases	0	2.1
Other liabilities	47.1	62.6
Liabilities held for sale	2,884	3,105.3
Net assets held for sale	1,523	1,525.3
Reinsurance recoverable on unpaid losses	283.1	322.2
Reinsurance recoverable on paid losses	10.2	11.4
OneBeacon Runoff | Discontinued Operations
Assets held for sale
Other assets	$ 0	$ 58.1

Discontinued Operations (Loss f

Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended			51 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from (used for) discontinued operations									$ 14.7	$ 60	$ (61.5)
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Revenues
Earned insurance premiums									1,188.2	1,185	1,120.9
Net investment income									60.8	59.5	59.8
Other revenue									334.2	131.1	47
Net realized and unrealized investment gains									225.4	78.5	133.9
Revenues (including realized gains and losses)	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	1,808.6	1,454.1	1,361.6
Expenses
Loss and loss adjustment expenses									708.9	824	622.1
Insurance and reinsurance acquisition expenses									220.1	206.2	210.4
Other underwriting expenses									218.6	179.6	205.2
Interest expense on debt									18.6	15.6	16.2
General and administrative expenses									487.5	257.7	149.1
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203
Net income from discontinued operations, net of tax	7.3	$ 14.2	$ 3.5	$ 73.8	82.1	$ 63.9	$ 61.1	$ 51.5	80.6	260.2	99.9
Gain (loss) on sale of discontinued operations, net of tax									18.2	(1.6)	46.6
Change in foreign currency translation, net of tax									(65.8)	(168.5)	2.7
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									197.2	207.3	247
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Cash and Cash Equivalents, at Carrying Value	173.5				238				173.5	238	233.5	$ 173.5	$ 118.9
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from (used for) discontinued operations									14.7	60	(61.5)
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Revenues
Earned insurance premiums									847	874	867.2
Net investment income									40.7	40.9	46.7
Other revenue									(21.3)	(62.4)	27.6
Net realized and unrealized investment gains									15.1	205.4	27.8
Revenues (including realized gains and losses)									881.5	1,057.9	969.3
Expenses
Loss and loss adjustment expenses									422.7	344.6	497.3
Insurance and reinsurance acquisition expenses									189.8	193.7	166.5
Other underwriting expenses									107.9	133.2	125.9
Interest expense on debt									26.6	26.3	26.3
General and administrative expenses									27	30.5	32.2
Total expenses									774	728.3	848.2
Net income (loss) from discontinued operations									107.5	329.6	121.1
Income tax expense									(26.9)	(69.4)	(21.2)
Net income from discontinued operations, net of tax									80.6	260.2	99.9
Gain (loss) on sale of discontinued operations, net of tax											46.6
Total income from discontinued operations									98.8	258.6	146.5
Change in foreign currency translation, net of tax										(169.5)	6.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									33.8	89.1	152.6
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Effect of Exchange Rate on Cash									(4.5)	(14.3)	(0.2)
Net Cash Provided by (Used in) Discontinued Operations									32.4	18.3	(0.5)
Cash and Cash Equivalents, at Carrying Value	$ 143.9				$ 111.5				143.9	111.5	93.2	143.9	$ 93.7
Sirius Group
Revenues
Net investment income									225	4	1
Net realized and unrealized investment gains									225	4	1
Sirius Group | Discontinued Operations
Revenues
Earned insurance premiums									847	873.9	866.4
Net investment income									40.7	40.9	46.7
Other revenue									(20.6)	(62.4)	16.8
Net realized and unrealized investment gains									15.1	205.4	27.8
Revenues (including realized gains and losses)									882.2	1,057.8	957.7
Expenses
Loss and loss adjustment expenses									422.7	345.3	418.4
Insurance and reinsurance acquisition expenses									189.8	193.6	166.5
Other underwriting expenses									107.9	129.7	126.1
Interest expense on debt									26.6	26.3	26.3
General and administrative expenses									27	30.5	32.2
Total expenses									774	725.4	769.5
Net income (loss) from discontinued operations									108.2	332.4	188.2
Income tax expense									27.1	70.4	46.2
Net income from discontinued operations, net of tax									81.1	262	142
Gain (loss) on sale of discontinued operations, net of tax											0
Total income from discontinued operations									81.1	262	142
Change in foreign currency translation, net of tax									(65)	(169.5)	6.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									16.1	92.5	148.1
Other Operations | Discontinued Operations
Revenues
Earned insurance premiums									0	0.1	0.8
Net investment income									0	0	0
Other revenue									(0.7)	0	10.8
Net realized and unrealized investment gains									0	0	0
Revenues (including realized gains and losses)									(0.7)	0.1	11.6
Expenses
Loss and loss adjustment expenses									0	(0.7)	78.9
Insurance and reinsurance acquisition expenses									0	0.1	0
Other underwriting expenses									0	3.5	(0.2)
Interest expense on debt									0	0	0
General and administrative expenses									0	0	0
Total expenses									0	2.9	78.7
Net income (loss) from discontinued operations									(0.7)	(2.8)	(67.1)
Income tax expense									(0.2)	(1)	(25)
Net income from discontinued operations, net of tax									(0.5)	(1.8)	(42.1)
Total income from discontinued operations									17.7	(3.4)	4.5
Change in foreign currency translation, net of tax									(65)	0	0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									17.7	(3.4)	4.5
Fireman's Fund Insurance Company [Member] | Discontinued Operations
Expenses
Gain (loss) on sale of discontinued operations, net of tax										14
OneBeacon Runoff | Discontinued Operations
Expenses
Gain (loss) on sale of discontinued operations, net of tax									0.3	(18.8)	46.6
Esurance [Member] | Discontinued Operations
Expenses
Net income from discontinued operations, net of tax											4.5
Gain (loss) on sale of discontinued operations, net of tax									17.9	3.2		$ 28.3
Other Segments
Revenues
Earned insurance premiums									8.7	6.1	0
Net investment income									8.8	9	11.1
Other revenue									334.1	124.7	15.4
Net realized and unrealized investment gains									259.9	29.8	95.8
Revenues (including realized gains and losses)									611.5	169.6	122.3
Expenses
Loss and loss adjustment expenses									8.2	8.9	0
Insurance and reinsurance acquisition expenses									3.4	0.8	0
Other underwriting expenses									0	0	0
Interest expense on debt									5.6	2.6	3.2
Total expenses									452.5	218.7	$ 106.4
Symetra | Sirius Group
Revenues
Net realized and unrealized investment gains									218.5
Symetra | Other Segments
Revenues
Net realized and unrealized investment gains									258.8
Tranzact [Member] | Other Segments
Revenues
Other revenue									186.9	43.2
Expenses
General and administrative expenses									167.3	37.4
MediaAlpha [Member] | Other Segments
Revenues
Other revenue									105.5	65.3
Expenses
General and administrative expenses									$ 99	$ 60.6

Discontinued Operations (Earnin

Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									$ (2.3)	$ (0.7)	$ (2.6)
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Basic earnings per share denominator									5,811,200	6,026,000	6,109,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings per share denominator									5,811,200	6,026,000	6,109,000
Incremental common shares attributable to share-based payment arrangements									125,000	125,000	125,000
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total income from discontinued operations									$ 98.8	$ 258.6	$ 146.5
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									(1.1)	3.3	(2.2)
Net income attributable to White Mountains’s common shareholders									$ 97.7	$ 255.3	$ 144.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Basic earnings per share denominator									5,811,200	6,026,000	6,109,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings per share denominator									5,811,200	6,026,000	6,109,000
Basic and diluted earnings per share (in dollars):									$ 16.80	$ 42.36	$ 23.63
Incremental common shares attributable to share-based payment arrangements									125,000	125,000	125,000

Discontinued Operations Discont

Discontinued Operations Discontinued Operations (Surplus Note Table) (Details) (Details) - USD ($) $ in Millions	Dec. 23, 2014	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax			$ 18.2	$ (1.6)	$ 46.6
Surplus Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RBC Score			250.00%
OneBeacon Runoff
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	$ 101
OneBeacon Runoff | Other long-term investments | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	$ 64.9
OneBeacon Runoff | Surplus Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RBC Score	250.00%
Discontinued Operations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax					46.6
Discontinued Operations | OneBeacon Runoff
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax			$ 0.3	(18.8)	$ 46.6
Discontinued Operations | OneBeacon Runoff | Surplus Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RBC Score			200.00%
Gain (loss) on sale of discontinued operations, net of tax		$ 36.1
Interest rate (as a percent)			6.00%
Gain (loss) on sale of discontinued operations, net of tax	$ 36.1
Par Value [Member] | Discontinued Operations | OneBeacon Runoff | Surplus Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	101
Fair Value [Member] | Discontinued Operations | OneBeacon Runoff | Surplus Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	64.9
OneBeacon
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(36.1)		$ (49.5)	(35.9)
OneBeacon | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(14)		(19.9)
OneBeacon | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(22.1)		(29.6)
OneBeacon | Par Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	101	101	101	101
OneBeacon | Par Value [Member] | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes			57.9
OneBeacon | Par Value [Member] | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	43.1		43.1
OneBeacon | Par Value [Member] | OneBeacon Runoff | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	57.9
OneBeacon | Current market rates on repayments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(6.8)		(15.1)	(6.6)
OneBeacon | Current market rates on repayments | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	1.6		(0.4)
OneBeacon | Current market rates on repayments | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(8.4)		(14.7)
OneBeacon | Regulatory Approval [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(12.6)		(24.2)	(12.6)
OneBeacon | Regulatory Approval [Member] | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(4.6)		(11.7)
OneBeacon | Regulatory Approval [Member] | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(8)		(12.5)
OneBeacon | Liquidity Adjustment [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(16.7)		(10.2)	(16.7)
OneBeacon | Liquidity Adjustment [Member] | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(11)		(7.8)
OneBeacon | Liquidity Adjustment [Member] | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gain (loss) on sale of discontinued operations, net of tax	(5.7)		(2.4)
OneBeacon | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	64.9	$ 65.1	51.5	$ 65.1
OneBeacon | Fair Value [Member] | Seller Priority Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	43.9		38
OneBeacon | Fair Value [Member] | Pari Passu Surplus Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Surplus notes	$ 21		$ 13.5

Discontinued Operations Disc153

Discontinued Operations Discontinued Operations (Interest Rate Cap) (Details) $ in Millions	1 Months Ended	12 Months Ended
	May. 31, 2007shares	Dec. 31, 2015USD ($)Rating	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Sirius Group | Interest Rate Cap
Derivative [Line Items]
basis points		3.95%
Derivative, Fair Value, Net		$ 1.9	$ 4.1	$ 11.1
Derivative Collateral Right to Reclaim		$ 1.9
S&P Credit Ratings | Rating		21
Change in fair value of derivatives, included in other revenue		$ 2.2	$ 7
years of interest rate cap		5 years
Derivative, Cap Interest Rate		8.30%
Derivative, Cost of Hedge		$ 9.9
Barclays [Member] | Standard & Poor's, A Rating [Member] | Sirius Group | Interest Rate Cap
Derivative [Line Items]
Derivative Collateral Right to Reclaim		1.3
Nordea Bank Finland [Member] | Standard & Poor's, AA- Rating [Member] | Sirius Group | Interest Rate Cap
Derivative [Line Items]
Derivative Collateral Right to Reclaim		$ 0.6
Preferred stocks | Sirius Internation Group Ltd
Derivative [Line Items]
Preference shares floating rate of dividend amount in excess of libor	3.20%
Preferred stocks | Sirius Group | Interest Rate Cap
Derivative [Line Items]
Preferred stock dividend rate	7.506%
Preference shares reference rate for dividend	3
Sale of Stock, Number of Shares Issued in Transaction | shares	250,000,000

Discontinued Operations Disc154

Discontinued Operations Discontinued Operations (Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 442.4	$ 343.1
Sirius Group | 2012 OBH Senior Notes [Member] | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	410	437.8
Sirius Group | 2012 OBH Senior Notes [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	399.8	399.7
Sirius Group | SIG Preference Shares | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	255	260
Sirius Group | SIG Preference Shares | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 250	$ 250

Subsequent Event (Details)

Subsequent Event (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Feb. 01, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsequent Event [Line Items]
Sales of common equity securities		$ 461.4	$ 609.8	$ 435
Common shares, par value per share (in dollars per share)		$ 1	$ 1
Symetra
Subsequent Event [Line Items]
Sales of common equity securities	$ 658
Common shares, par value per share (in dollars per share)	$ 32

Selected Quarterly Financial156

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues (including realized gains and losses)	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	$ 1,808.6	$ 1,454.1	$ 1,361.6
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203
Pre-tax loss	258.1	(94.6)	(9.1)	0.5	(65.7)	(35.5)	33.6	38.6	154.9	(29)	158.6
Tax benefit (expense)	1.5	1.6	2.2	(4.6)	26.2	4.7	(7.1)	(9)	0.7	14.8	(32.6)
Net income (loss) from continuing operations	259.6	(93)	(6.9)	(4.1)	(39.5)	(30.8)	26.5	29.6	155.6	(14.2)	126
Net income from discontinued operations, net of tax	7.3	14.2	3.5	73.8	82.1	63.9	61.1	51.5	80.6	260.2	99.9
Non-controlling interest in consolidated subsidiaries	(6.1)	16	0.9	7.3	15	11.2	(4.6)	0.6	18.1	22.2	12.5
Equity in earnings of unconsolidated affiliates, net of tax	7.1	3.9	6.8	7.3	12.3	7	12.5	13.8	25.1	45.6	36.6
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Basic earnings per share
Continuing operations	$ 45.99	$ (12.42)	$ 0.13	$ 1.76	$ (2.03)	$ (2.07)	$ 5.58	$ 7.13	$ 33.80	$ 8.77	$ 28.22
Discontinued operations	1.29	2.41	0.59	12.33	13.68	10.49	9.92	8.35	16.80	42.36	23.63
Total consolidated operations	47.28	(10.01)	0.72	14.09	11.65	8.42	15.50	15.48	50.60	51.13	51.85
Diluted earnings per share
Continuing operations	45.96	(12.42)	0.13	1.76	(2.03)	(2.07)	5.58	7.13	33.80	8.77	28.22
Discontinued operations	1.29	2.41	0.59	12.33	13.68	10.49	9.92	8.35	16.80	42.36	23.63
Total consolidated operations	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 11.65	$ 8.42	$ 15.50	$ 15.48	$ 50.60	$ 51.13	$ 51.85

SCHEDULE I SUMMARY OF INVEST157

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of investments other than investments in related parties
Cost	$ 3,958.8
Carrying Value	4,262.2
Fair Value	4,262.2
Fair value investments	4,050.9	$ 3,343.6	$ 3,889
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,639.1
Carrying Value	2,639.7
Fair Value	2,639.7
US Government and agency obligations
Summary of investments other than investments in related parties
Cost	160.4
Carrying Value	160
Fair Value	160
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,001
Carrying Value	1,000
Fair Value	1,000
Municipal obligations
Summary of investments other than investments in related parties
Cost	227.8
Carrying Value	228.8
Fair Value	228.8
Mortgage-backed and asset-backed securities
Summary of investments other than investments in related parties
Cost	1,170.6
Carrying Value	1,167
Fair Value	1,167
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	1
Carrying Value	1.2
Fair Value	1.2
Redeemable preferred stocks
Summary of investments other than investments in related parties
Cost	78.3
Carrying Value	82.7
Fair Value	82.7
Short-term investments
Summary of investments other than investments in related parties
Cost	211.3
Carrying Value	211.3
Fair Value	211.3
Exchange Traded Funds [Member]
Summary of investments other than investments in related parties
Cost	147.2
Carrying Value	141.8
Fair Value	141.8
Common equity securities
Summary of investments other than investments in related parties
Cost	822.5
Carrying Value	1,113.9
Fair Value	1,113.9
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	436.9
Carrying Value	694.7
Fair Value	694.7
Public utilities
Summary of investments other than investments in related parties
Cost	0
Carrying Value	0
Fair Value	0
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	238.4
Carrying Value	277.4
Fair Value	277.4
Other long-term investments
Summary of investments other than investments in related parties
Cost	285.9
Carrying Value	297.3
Fair Value	297.3
Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	4,262.2	3,720.4
Fair value measured on a recurring basis | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	297.3	309.9
Level 3 | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	297.3	304.2	189.2
Level 3 | Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	367.3	420.1
Level 3 | Fair value measured on a recurring basis | Other long-term investments
Summary of investments other than investments in related parties
Carrying value of investment accounted for using the equity method	3.8	5.2	$ 6.8
Fair value investments	297.3	304.2
Affordable Housing Development Fund [Member] | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	$ 14.7	$ 16.8

SCHEDULE II CONDENSED FINANC158

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash Including Restricted Cash	$ 179,300,000	$ 261,700,000
Fixed maturity investments, at fair value	2,639,700,000	2,422,000,000
Short-term investments, at amortized cost	211,300,000	376,800,000
Other assets	318,700,000	348,700,000
Total assets	10,284,500,000	10,455,700,000
Liabilities
Debt	442,400,000	343,100,000
Payable to subsidiary (2)	2.4
Other liabilities	264,700,000	278,400,000
Total liabilities	5,916,500,000	5,917,300,000
White Mountains’s common shareholders’ equity	4,368,000,000	4,538,400,000
Total liabilities and equity	10,284,500,000	10,455,700,000
White Mountains Insurance Group Ltd.
Assets
Cash Including Restricted Cash	300,000	200,000
Short-term investments, at amortized cost	23,400,000	31,000,000
Other assets	6,300,000	200,000
Investments in consolidated and unconsolidated affiliates	4,350,900,000	4,118,500,000
Total assets	4,380,900,000	4,149,900,000
Liabilities
Debt	50,000,000	0
Payable to subsidiary (2)	400,000,000	161,600,000
Other liabilities	17,700,000	(7,400,000)
Total liabilities	467,700,000	154,200,000
White Mountains’s common shareholders’ equity	3,913,200,000	3,995,700,000
Total liabilities and equity	$ 4,380,900,000	$ 4,149,900,000

SCHEDULE II CONDENSED FINANC159

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (including realized gains and losses)	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	$ 1,808.6	$ 1,454.1	$ 1,361.6
Expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203
Pre-tax loss	258.1	(94.6)	(9.1)	0.5	(65.7)	(35.5)	33.6	38.6	154.9	(29)	158.6
Income tax (expense) benefit	1.5	1.6	2.2	(4.6)	26.2	4.7	(7.1)	(9)	0.7	14.8	(32.6)
Net income (loss) from continuing operations	259.6	(93)	(6.9)	(4.1)	(39.5)	(30.8)	26.5	29.6	155.6	(14.2)	126
Net income from discontinued operations, net of tax	7.3	14.2	3.5	73.8	82.1	63.9	61.1	51.5	80.6	260.2	99.9
Net income attributable to White Mountains’s common shareholders	267.9	(58.9)	4.3	84.3	69.9	51.3	95.5	95.5	297.6	312.2	321.6
Comprehensive income attributable to White Mountains’ common shareholders									197.2	210.6	241.8
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	297.6	312.2	321.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (including realized gains and losses)									5	0.3	1.2
Expenses									59.9	32.3	39
Pre-tax loss									(54.9)	(32)	(37.8)
Income tax (expense) benefit									0	0.9	(0.1)
Net income (loss) from continuing operations									(54.9)	(31.1)	(37.9)
Net income from discontinued operations, net of tax									0	13.9	0
Equity in earnings (losses) from consolidated and unconsolidated affiliates									352.5	329.4	359.5
Net income attributable to White Mountains’s common shareholders									297.6	312.2	321.6
Other comprehensive income (loss), after-tax									(100.4)	(101.6)	(79.8)
Comprehensive income attributable to White Mountains’ common shareholders									197.2	210.6	241.8
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									$ 297.6	$ 312.2	$ 321.6

SCHEDULE II CONDENSED FINANC160

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(225.4)	(78.5)	(133.9)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(7.3)	$ (14.2)	$ (3.5)	(73.8)	(82.1)	$ (63.9)	$ (61.1)	(51.5)	(80.6)	(260.2)	(99.9)
Net change in other assets and liabilities, net									40.1	(127.5)	(23)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									161	58.8	(39.5)
Net cash provided from (used for) discontinued operations									14.7	60	(61.5)
Net cash provided from (used for) operations									175.7	118.8	(101)
Cash flows from investing activities:
Net decrease in short-term investments									140	(138)	64.6
Purchases of investment securities									(2,377.1)	2,868.5
Sales and maturities of investment securities									2,210.1	(3,500.2)
Receipt of cash flow from discontinued operations									17.5	(88.1)	(72.3)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									(20)	12.1	253.1
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Net cash provided from investing activities									11.3	47.9	401.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit									195.5	133.6	200
Repayment of debt and revolving line of credit									(95.6)	(66.5)	(275)
Repurchases and retirement of common shares									(268.6)	(128.2)	(79.8)
Dividends paid on common shares									(6)	(6.2)	(6.2)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(205.5)	(66.4)	(99)
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Net cash used for financing activities									(214.6)	(129.6)	(186.5)
Net change in cash during the period - continuing operations									(64.5)	4.5	114.6
Cash balance at beginning of year				238				233.5	238	233.5	118.9
Cash balance at end of year	173.5				238				173.5	238	233.5
Interest Paid									(17.6)	(15.6)	(16.2)
OneBeacon Ltd. common shares repurchased and retired									(3.7)	(1.7)	0
Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants										29.7	28
Noncash distribution received											27.9
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									297.6	312.2	321.6
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									0	(0.2)	(0.2)
Undistributed current earnings from subsidiaries									(352.5)	(329.4)	(359.5)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0	(13.9)	0
Other non-cash reconciling items including restricted share and option amortization									(0.4)	12.9	15.4
Net change in other assets and liabilities, net									21.4	(7.6)	(2.7)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(33.9)	(26)	(25.4)
Net cash provided from (used for) discontinued operations									0	13.9	0
Net cash provided from (used for) operations									(33.9)	(12.1)	(25.4)
Cash flows from investing activities:
Net decrease in short-term investments									7.6	(29.6)	7.5
Purchases of investment securities									0	(7.6)	(26.2)
Sales and maturities of investment securities									0	39.4	61.1
Issuance of debt (to) from subsidiaries									271	144.6	153.9
Repayment of debt (to) from subsidiaries									(35)	(30)	(10.3)
Receipt of cash flow from discontinued operations									0	13.9	0
Payments to subsidiaries									0	0	0
Distributions from subsidiaries									15	29.7	0.1
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									258.6	160.4	186.1
Net cash provided from investing activities — discontinued operations									0	(13.9)	0
Net cash provided from investing activities									258.6	146.5	186.1
Cash flows from financing activities:
Draw down of debt and revolving line of credit									125	65	200
Repayment of debt and revolving line of credit									(75)	(65)	(275)
Proceeds from issuances of common shares									0	0	0
Repurchases and retirement of common shares									(268.6)	(128.2)	(79.8)
Dividends paid on common shares									(6)	(6.2)	(6.2)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(224.6)	(134.4)	(161)
Net cash (used for) provided from financing activities — discontinued operations									0	0	0
Net cash used for financing activities									(224.6)	(134)	(161)
Net change in cash during the period - continuing operations									0.1	0	(0.3)
Cash balance at beginning of year				$ 0.2				$ 0.2	0.2	0.2	0.5
Cash balance at end of year	$ 0.3				$ 0.2				0.3	0.2	0.2
Interest Paid									$ 0	$ (0.3)	(2)
Fixed maturity investments | Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants											27.9
Cash | Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants											0.1
Revolving Credit Facility [Member] | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Repayment of debt and revolving line of credit											$ (75)

SCHEDULE III SUPPLEMENTARY I161

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 60.8	$ 59.5	$ 59.8
OneBeacon
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	100.7	103.2	103.7
Future policy benefits, losses, claims and loss expenses	1,389.8	1,342.2	1,054.3
Unearned premiums	560.3	588.3	544.9
Other policy claims and benefits payable	0	0	0
Premiums earned	1,176.2	1,177.1	1,120.4
Net investment income	45.9	43.4	43
Benefits, claims, losses, and settlement expenses	700.7	815.1	622.1
Amortization of deferred policy acquisition costs	213.8	203.3	208.9
Other operating expenses	218.2	179.2	204.8
Premiums written	1,136.6	1,216.9	1,088.6
Net investment income	45.9	43.4	43
HG Global-BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	6.9	4
Future policy benefits, losses, claims and loss expenses	0	0
Unearned premiums	50.2	27.6
Other policy claims and benefits payable	0	0
Premiums earned	3.3	1.8
Net investment income	6.1	7.1
Benefits, claims, losses, and settlement expenses	0	0
Amortization of deferred policy acquisition costs	2.9	2.1
Other operating expenses	0.4	0.4
Premiums written	25.9	16.2
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0	0	0
Future policy benefits, losses, claims and loss expenses	6	7.8	0
Unearned premiums	2.1	0.8	0
Other policy claims and benefits payable	0	0	0
Premiums earned	8.7	6.1	0
Net investment income	0.2	0.2	0.1
Benefits, claims, losses, and settlement expenses	8.2	8.9	0
Amortization of deferred policy acquisition costs	3.4	0.8	0
Other operating expenses	0	0	0
Premiums written	10.1	5.9	0
Non-insurance [Member] | Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 8.6	$ 8.8	$ 11

SCHEDULE IV REINSURANCE (Detail

SCHEDULE IV REINSURANCE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Premiums earned
Gross amount	$ 1,322	$ 1,233.5	$ 1,043.8
Ceded to other companies	(179.7)	(119.4)	(71.4)
Assumed from other companies	45.9	70.9	148.5
Net earned premiums	1,188.2	1,185	1,120.9
OneBeacon
Premiums earned
Gross amount	1,298	1,209.1	1,043.3
Ceded to other companies	(167.7)	(102.9)	(71.4)
Assumed from other companies	45.9	70.9	148.5
Net earned premiums	$ 1,176.2	$ 1,177.1	$ 1,120.4
Percentage of amount assumed to net	3.90%	6.00%	13.30%
HG/BAM
Premiums earned
Gross amount	$ 3.3	$ 1.8	$ 0.5
Ceded to other companies	0	0	0
Assumed from other companies	0	0	0
Net earned premiums	$ 3.3	$ 1.8	0.5
Percentage of amount assumed to net	0.00%	0.00%
Other Segments
Premiums earned
Gross amount	$ 20.7	$ 22.6
Ceded to other companies	(12)	(16.5)
Assumed from other companies	0	0
Net earned premiums	$ 8.7	$ 6.1	$ 0
Percentage of amount assumed to net	0.00%	0.00%

SCHEDULE V VALUATION AND QUA163

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance recoverable on paid losses: Allowance for reinsurance balances
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	$ 2.2	$ 14.3	$ 25.6
Charged to costs and expenses	(0.1)	(0.5)	0
Charged to other accounts	0	0	0
Deductions described	(1.4)	(11.6)	(11.3)
Balance at end of period	0.7	2.2	14.3
Property and casualty insurance and reinsurance premiums receivable: Allowance for uncollectible accounts
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	1.9	3.1	4.4
Charged to costs and expenses	0.8	(0.2)	0.3
Charged to other accounts	(0.7)	(1)	0
Deductions described	0	0	(1.6)
Balance at end of period	2	1.9	3.1
Allowance for Doubtful Accounts [Member]
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	3.3	0
Charged to costs and expenses	3.2	(3.6)
Charged to other accounts	0	0
Deductions described	(0.7)	0.3
Balance at end of period	$ 5.8	$ 3.3	$ 0

SCHEDULE VI SUPPLEMENTAL INF164

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Accident Year	$ 712.9	$ 732	$ 622.1
Prior Accident Year	$ (4)	$ 92	$ 0
Rate at which reserves are discounted (as a percent)	2.50%	2.50%	3.50%
OneBeacon
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 100.7	$ 103.2	$ 103.7
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,389.8	1,342.2	1,054.3
Discount, if any, deducted in Column C	1.1	1	3
Unearned Premiums	560.3	588.3	544.9
Earned Premiums	1,176.2	1,177.1	1,120.4
Net investment income	45.9	43.4	43
Current Accident Year	702.5	725.3	622.1
Prior Accident Year	(1.8)	89.8	0
Amortization of deferred policy acquisition	(213.8)	(203.3)	(208.9)
Paid Claims and Claims Adjustment Expenses	677.5	608.6	540.7
Premiums written	1,136.6	1,216.9	1,088.6
Other Operations
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	0	0	0
Reserves for Unpaid Claims and Claims Adjustment Expenses	6	7.8	0
Discount, if any, deducted in Column C	0	0	0
Unearned Premiums	2.1	0.8	0
Earned Premiums	8.7	6.1	0
Net investment income	0.2	0.2	0
Current Accident Year	10.4	6.7	0
Prior Accident Year	(2.2)	2.2	0
Amortization of deferred policy acquisition	0	0	0
Paid Claims and Claims Adjustment Expenses	10.4	14.8	0
Premiums written	$ 10.1	$ 5.9	$ 0