White Mountains Insurance (WTM) 10-Q 4 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	4-May-15
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	776867
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		5,991,584
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Assets
Fixed maturity investments, at fair value	$4,771.90	$4,784.30
Short-term investments, at amortized cost (which approximates fair value)	929.2	871.7
Common equity securities, at fair value	786.9	801.6
Convertible fixed maturity and preferred investments, at fair value	15.8	20.5
Other long-term investments	402.6	407
Total investments	6,906.40	6,885.10
Cash (restricted: $29.0 and $23.7)	335.8	373.2
Reinsurance recoverable on unpaid losses	436.9	483.9
Reinsurance recoverable on paid losses	29.8	23.6
Insurance and reinsurance premiums receivable	706.9	547.7
Funds held by ceding entities	99.6	129
Investments in unconsolidated affiliates	450.8	414.4
Deferred acquisition costs	192	177.1
Deferred tax asset	417.9	456.1
Ceded unearned insurance and reinsurance premiums	145.1	94
Accrued investment income	30.4	34.5
Accounts receivable on unsettled investment sales	12	56.5
Goodwill and intangible assets	362.6	366.4
Other assets	323.6	356.1
Assets held for sale	58	58.1
Total assets	10,507.80	10,455.70
Liabilities
Loss and loss adjustment expense reserves	3,035	3,159.80
Unearned insurance and reinsurance premiums	1,102.50	955.3
Debt	749.8	746.6
Deferred tax liability	254	282.8
Accrued incentive compensation	100.5	184.6
Ceded reinsurance payable	151.8	105.7
Funds held under insurance and reinsurance contracts	143.3	138.9
Accounts payable on unsettled investment purchases	92.6	2.6
Other liabilities	338.5	341
Total liabilities	5,968	5,917.30
White Mountains’s common shareholders’ equity
White Mountains’ common shares	6	6
Paid-in surplus	1,032.70	1,028.70
Retained earnings	3,080.80	3,010.50
Accumulated other comprehensive income (loss), after tax:
Equity in net unrealized gains from investment in Symetra common shares	63.8	34.9
Net unrealized foreign currency translation losses	-167.6	-79.8
Pension liability and other	-4.4	-4.6
Total White Mountains’s common shareholders’ equity	4,011.30	3,995.70
Non-controlling interests
Total non-controlling interests	528.5	542.7
Total equity	4,539.80	4,538.40
Total liabilities and equity	10,507.80	10,455.70
OneBeacon
Non-controlling interests
Total non-controlling interests	258.8	258.4
Sirius Group
Non-controlling interests
Total non-controlling interests	250	250
HG Global
Non-controlling interests
Total non-controlling interests	17.7	17.9
Mutuals and reciprocals
Non-controlling interests
Total non-controlling interests	-140.5	-134.3
Other Noncontrolling Interest
Non-controlling interests
Total non-controlling interests	$160.20	$168.60

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Statement of Financial Position [Abstract]
Cash	$29	$23.70
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	5,991,584	5,986,214
Common shares, outstanding shares	5,991,584	5,986,214

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues:
Earned insurance and reinsurance premiums	$494.30	$493.60
Net investment income	23.3	24.7
Net realized and unrealized investment gains	103.4	63.8
Other revenue	35.8	-3.4
Total revenues	656.8	578.7
Expenses:
Loss and loss adjustment expenses	265.4	229.3
Insurance and Reinsurance Acquisition Expenses	95.6	95.1
Other underwriting expenses	81.5	81.4
General and administrative expenses	116.4	50.3
Interest expense	11.1	10.1
Total expenses	570	466.2
Pre-tax income from continuing operations	86.8	112.5
Income tax expense	-25	-30.9
Net income from continuing operations	61.8	81.6
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	8	0
Net loss from discontinued operations, net of tax	-7.9	-0.5
Net loss from discontinued operations, net of tax	-0.1	-0.5
Income before equity in earnings of unconsolidated affiliates	69.7	81.1
Equity in earnings of unconsolidated affiliates, net of tax	7.3	13.8
Net income	77	94.9
Net loss attributable to non-controlling interests	7.3	0.6
Net income attributable to White Mountains’s common shareholders	84.3	95.5
Other comprehensive (loss) income, net of tax:
Change in equity in net unrealized gains from investments in Symetra common shares, net of tax	28.9	36.5
Change in foreign currency translation, pension liability and other	-87.6	-7.8
Comprehensive income	25.6	124.2
Comprehensive income attributable to non-controlling interests	0	0
Comprehensive income attributable to White Mountains’s common shareholders	$25.60	$124.20
Basic income per share
Continuing operations	$12.77	$15.56
Discontinued operations	$1.32	($0.08)
Total consolidated operations	$14.09	$15.48
Diluted income per share
Continuing operations	$12.77	$15.56
Discontinued operations	$1.32	($0.08)
Total consolidated operations	$14.09	$15.48
Dividends declared per White Mountains’s common share	$1	$1

CONSOLIDATED_STATEMENTS_OF_CHA

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $)	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) [Member]	Foreign Currency Gain (Loss) [Member]	Foreign Currency Gain (Loss) [Member]	Foreign Currency Gain (Loss) [Member]	Foreign Currency Gain (Loss) [Member]	Foreign Currency Gain (Loss) [Member]	Pension liability and other accumulated comprehensive items [Member]	Pension liability and other accumulated comprehensive items [Member]	Pension liability and other accumulated comprehensive items [Member]	Pension liability and other accumulated comprehensive items [Member]	Pension liability and other accumulated comprehensive items [Member]	Pension liability and other accumulated comprehensive items [Member]
In Millions, unless otherwise specified								Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity
Balances at Dec. 31, 2013		$1,051.10	$2,801.80	$52.10	$3,905	$491.80	$4,396.80
Increase (Decrease) in Shareholders' Equity
Net income	94.9	0	95.5	0	95.5	-0.6	94.9
Net change in unrealized gains from investments in unconsolidated affiliates	36.5	0	0	36.5	36.5	0	36.5
Other Comprehensive Income (Loss), Net of Tax								0	0	-7.9	-7.9	0	-7.9	0	0	0.1	0.1	0	0.1
Total comprehensive income (loss)	124.2	0	95.5	28.7	124.2	-0.6	123.6
Dividends declared on common shares		0	-6.2	0	-6.2	0	-6.2
Dividends to non-controlling interests	0	0	0	0	0	-5.2	-5.2
Repurchases and retirements of common shares		-4.5	-10.9	0	-15.4	0	-15.4
Issuances of common shares		2.1	0	0	2.1	0	2.1
Net contributions from non-controlling interests		0	0	0	0	27	27
Amortization of restricted share awards		4.2	0	0	4.2	0.2	4.4
Balances at Mar. 31, 2014		1,052.90	2,880.20	80.8	4,013.90	513.2	4,527.10
Balances at Dec. 31, 2014	4,538.40	1,034.70	3,010.50	-49.5	3,995.70	542.7	4,538.40
Increase (Decrease) in Shareholders' Equity
Net income	77	0	84.3	0	84.3	-7.3	77
Net change in unrealized gains from investments in unconsolidated affiliates	28.9	0	0	28.9	28.9	0	28.9
Other Comprehensive Income (Loss), Net of Tax								0	0	-87.8	-87.8	0	-87.8	0	0	0.2	0.2	0	0.2
Total comprehensive income (loss)	25.6	0	84.3	-58.7	25.6	-7.3	18.3
Dividends declared on common shares		0	-6	0	-6	0	-6
Dividends to non-controlling interests	0.7	0	0	0	0	-7	-7
Repurchases and retirements of common shares		-3	-8	0	-11	0	-11
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Changes, Additional Interest Issued to Parent		4.6	0	0	4.6	-4.6	0
Net contributions from non-controlling interests		0	0	0	0	5.2	5.2
Amortization of restricted share awards		2.4	0	0	2.4	-0.5	1.9
Balances at Mar. 31, 2015	$4,539.80	$1,038.70	$3,080.80	($108.20)	$4,011.30	$528.50	$4,539.80

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Cash flows from operations:
Net income	$77	$94.90
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	-103.4	-63.8	-63.8
Deferred income tax expense	12.2	21.7
Net income from discontinued operations	7.9	0.5
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	8	0
Gain on sale of subsidiary - Citation	0	-0.7
Amortization and depreciation	22.2	15.4
Undistributed equity in earnings from unconsolidated affiliates, net of tax	-7.3	-13.8
Other operating items:
Net change in loss and loss adjustment expense reserves	-39.3	-44.6
Net change in reinsurance recoverable on paid and unpaid losses	20	13.9
Net change in unearned insurance and reinsurance premiums	181.3	217.7
Net change in variable annuity benefit guarantee liabilities	-0.4	1.3
Net change in variable annuity benefit guarantee derivative instruments	9.4	-20.4
Net change in deferred acquisition costs	-20.2	-15.3
Net change in funds held by ceding entities	21.4	12.6
Net change in ceded unearned premiums	-61.5	-60.6
Net change in funds held under reinsurance treaties	9.8	0.7
Net change in insurance and reinsurance premiums receivable	-190.5	-212.4
Net change in ceded reinsurance payable	56.4	78.6
Net change in restricted cash	-5.3	33.9
Net change in other assets and liabilities, net	-55.3	-92.2
Net cash used for operations - continuing operations	-65.6	-32.6
Net cash provided from (used for) operations - discontinued operations	7.9	-14.6
Net cash used for operations	-57.7	-47.2
Cash flows from investing activities:
Net change in short-term investments	-86.1	-61.5
Sales of fixed maturity and convertible fixed maturity investments	1,110.80	1,026.30
Maturities, calls and paydowns of fixed maturity and convertible fixed maturity investments	138.2	189.7
Sales of common equity securities	80	63.4
Distributions and redemptions of other long-term investments	6.4	23.9
Sales of consolidated and unconsolidated affiliates, net of cash sold	0	12.8
Funding from (of) operational cash flows for discontinued operations	7.9	-14.6
Purchases of other long-term investments	-9.5	-13
Purchases of common equity securities	-49.5	-65
Purchases of fixed maturity and convertible fixed maturity investments	-1,282.30	-1,138.30
Purchases of consolidated and unconsolidated affiliates, net of cash acquired	-2.4	-32.2
Net change in unsettled investment purchases and sales	135.2	52.6
Net acquisitions of property and equipment	-5	-0.5
Net cash provided from investing activities - continuing operations	43.7	43.6
Net cash (used for) provided from investing activities - discontinued operations	-7.9	14.6
Net cash provided from investing activities	35.8	58.2
Cash flows from financing activities:
Draw down of debt and revolving line of credit	4.5	0
Repayment of debt and revolving line of credit	-1.3	0
Payments on capital lease obligation	-1.3	-1.3
Cash dividends paid to the Company’s common shareholders	-6	-6.2
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders	-4.9	-4.9
Common shares repurchased	-4.3	-10.9
OneBeacon Ltd. common shares repurchased and retired	1.7	0
Distribution to non-controlling interest shareholders	-0.7	0
Collateral provided by interest rate cap counterparties	-1.2	-2.6
Capital contributions from BAM members	4.6	4.8
Net cash used for financing activities - continuing operations	-12.3	-21.1
Net cash (used for) provided from financing activities - discontinued operations	0	0
Net cash used for financing activities	-12.3	-21.1
Effect of exchange rate changes on cash	-8.5	-0.2
Net change in cash during the period	-42.7	-10.3
Cash balances at beginning of period (excludes restricted cash balances of $23.7 and $56.1)	349.5	326.7	326.7
Cash balances at end of period (excludes restricted cash balances of $29.0 and $22.2)	306.8	316.4	349.5
Supplemental cash flows information:
Interest paid	-12.8	-12.8
Net income tax (payment) refund to national governments	($15)	$3.80

CONSOLIDATED_STATEMENTS_OF_CAS1

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Statement of Cash Flows [Abstract]
Restricted cash balances, beginning of period	$29	$23.70	$22.20	$56.10
Restricted cash balances, end of period	$29	$23.70	$22.20	$56.10

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies
	Basis of Presentation
	These interim consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively, with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance and reinsurance subsidiaries and affiliates. The Company’s headquarters is located at 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.
	The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. During the third quarter of 2013, OneBeacon formed Split Rock Insurance, Ltd. (“Split Rock”), a Bermuda-based reinsurance company. As of March 31, 2015 and December 31, 2014, White Mountains owned 75.3% of OneBeacon Ltd.’s outstanding common shares for both periods.
	In December 2014, OneBeacon completed the sale of its runoff business (the “Runoff Transaction”). Accordingly, OneBeacon’s runoff business is presented as discontinued operations. (See Note 17 for discontinued operations.) OneBeacon currently owns property in Canton, MA, for which it has entered into an agreement to sell and which is presented as held for sale. As of March 31, 2015 and December 31, 2014, the carrying value of the property was $58.0 million and $58.1 million.
	The Sirius Group segment consists of Sirius International Insurance Group, Ltd., an exempted Bermuda limited liability company, and its subsidiaries (collectively, “Sirius Group”). Sirius Group provides insurance and reinsurance products for property, accident and health, aviation and space, trade credit, marine, agriculture and certain other exposures on a worldwide basis through its primary subsidiaries, Sirius International Insurance Corporation (“Sirius International”), Sirius America Insurance Company (“Sirius America”) and Lloyds Syndicate 1945 (“Syndicate 1945”). Sirius Group also specializes in the acquisition and management of runoff insurance and reinsurance companies both in the United States and internationally through its White Mountains Solutions division (“WM Solutions”).
	The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is a municipal bond insurer domiciled in New York that was established in 2012 to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). HG Global, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), also provides 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. As of both March 31, 2015 and December 31, 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 its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s ownership positions in Tranzact Holdings, LLC, Wobi Insurance Agency Ltd. (“Wobi”) and MediaAlpha Ventures, LLC (“MediaAlpha”, formerly known as “QuoteLab”). The Other Operations segment also includes Star & Shield Services LLC, Star & Shield Risk Management LLC (“SSRM”), and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. As of March 31, 2015, White Mountains holds $19.0 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 SSRM’s role as the attorney-in-fact to SSIE and the investment in SSIE’s surplus notes, White Mountains is required to consolidate SSIE in 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. These interim financial statements include all adjustments considered necessary by management to fairly present the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2014 Annual Report on Form 10-K.
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.  Refer to the Company’s 2014 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies.
	Recently Adopted Changes in Accounting Principles
	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 March 31, 2015 increased by $1.2 million. As a result of the retrospective adoption, White Mountains's common shareholders equity has been reduced by $0.5 million as of January 1, 2014. In addition, the retrospective adoption resulted in an increase of $0.4 million to net investment income and a net increase of $0.6 million to income tax expense for the three months ended March 31, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above.
	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 represent a strategic shift and 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. White Mountains has not had any transactions that occurred since ASU 2014-08 became effective on December 15, 2014.
	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 since AUS 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.
	Recently Issued Accounting Pronouncements
	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. ASU 2015-03 is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 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 should represent the compensation cost 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, such as third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $4.4 million for the three months ended March 31, 2015 and 2014, respectively. 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. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016.

Significant_Transactions

Significant Transactions	3 Months Ended
	Mar. 31, 2015
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions
	MediaAlpha
	On March 14, 2014, White Mountains acquired 60% of the outstanding Class A common units of MediaAlpha. MediaAlpha is a California-based advertising technology company offering a transparent online exchange and sophisticated analytical tools that facilitate transactions between buyers (advertisers) and sellers (publishers) of insurance media (clicks and calls), including its own media inventory generated by owned and operated websites. Its exchange operates in four verticles: auto, home, health and life. 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. As of March 31, 2015 and December 31, 2014, White Mountains has recognized a $7.9 million liability for the contingent purchase price adjustment. After adjustment for the estimated contingent purchase price adjustment, White Mountains recognized total assets acquired related to MediaAlpha of $70.1 million, including $18.3 million of goodwill and $38.5 million of other intangible assets, and total liabilities assumed of $10.0 million, reflecting acquisition date fair values.
	Wobi
	On February 19, 2014, White Mountains acquired 54% of the outstanding common shares of Wobi for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition).  During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $9.0 million based upon the foreign exchange spot rate at the date of acquisition) of newly-issued convertible preferred shares of Wobi.  Wobi is the only price comparison/aggregation business in Israel, with an insurance carrier panel that represents 85% of the premiums written in the Israeli insurance market. Wobi sells four lines of business, primarily personal auto, and operates as an agency, charging upfront commissions on all policy sales. As of March 31, 2015, White Mountains’s ownership share was 63.3% on a fully converted basis. 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 fair values.
	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 based upon the foreign exchange spot rate at the date of acquisition), representing a controlling financial interest. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million, including $0.3 million of goodwill and $2.8 million of other intangible assets; and total liabilities assumed of $1.2 million at their estimated fair values.
	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.
	White Mountains owns $19.0 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 variable interest entity (“VIE”). As a result of SSRM’s role as the attorney-in-fact to SSIE and the investment in SSIE’s surplus notes, White Mountains is required to consolidate SSIE. At March 31, 2015, consolidated amounts included total assets of $16.0 million and total liabilities of $29.5 million of SSIE. The surplus notes purchased by White Mountains and the corresponding liability included in SSIE's liabilities are eliminated in consolidation. For the three months ended March 31, 2015 and 2014, SSIE had pre-tax losses of $1.1 million and $4.6 million that were recorded in net loss attributable to non-controlling interests.

Loss_and_Loss_Adjustment_Expen

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2015
Insurance Loss Reserves [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves
	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Gross beginning balance	$	3,159.80		$	3,079.30
	Less beginning reinsurance recoverable on unpaid losses	(483.9		)	(428.1		)
	Net loss and LAE reserves	2,675.90			2,651.20
	Loss and LAE reserves consolidated — SSIE	—			13.6
	Loss and LAE incurred relating to:
	Current year losses	266.8			240.5
	Prior year losses	(1.4		)	(11.2		)
	Total incurred losses and LAE	265.4			229.3
	Foreign currency translation adjustment to loss and LAE reserves	(22.4		)	1.8
	Loss and LAE paid relating to:
	Current year losses	(40.6		)	(35.0		)
	Prior year losses	(280.2		)	(237.7		)
	Total loss and LAE payments	(320.8		)	(272.7		)
	Net ending balance	2,598.10			2,623.20
	Plus ending reinsurance recoverable on unpaid losses	436.9			425.2
	Gross ending balance	$	3,035.00		$	3,048.40
	Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2015
	For the three months ended March 31, 2015, White Mountains experienced net favorable loss reserve development of $1.4 million.
	For the three months ended March 31, 2015, OneBeacon had net favorable loss reserve development of $1.8 million, Sirius Group had net unfavorable loss reserve development of $0.5 million and SSIE had net favorable loss reserve development of $0.1 million.
	Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2014
	For the three months ended March 31, 2014, White Mountains experienced $11.2 million of net favorable loss reserve development.
	For the three months ended March 31, 2014, OneBeacon had net favorable loss reserve development of $1.4 million primarily related to its ocean marine business and its technology business. For the three months ended March 31, 2014, Sirius Group had net favorable loss reserve development of $9.8 million primarily related to its casualty and accident and health lines of business.

Third_Party_Reinsurance

Third Party Reinsurance	3 Months Ended
	Mar. 31, 2015
Reinsurance Disclosures [Abstract]
Third Party Reinsurance	Third Party Reinsurance
	In the normal course of business, White Mountains’s insurance and reinsurance subsidiaries may seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts.
	OneBeacon
	At March 31, 2015, OneBeacon had $18.2 million and $141.6 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2014, OneBeacon had $12.2 million and $161.6 million of reinsurance recoverables on paid and unpaid losses. The reinsurance balances associated with the Runoff Business are included in discontinued operations (see Note 17). Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant.
	Except as discussed below, there have been no material changes to OneBeacon's reinsurance coverage as discussed in Note 4 —“Reinsurance” in White Mountains’s 2014 Annual Report on Form 10-K.
	Effective January 1, 2015, OneBeacon purchased an aggregate stop loss on its multiple period crop insurance ("MPCI") portfolio, providing 52.0% of coverage in excess of a 98.0% loss ratio on premiums covered by the contract and a separate aggregate stop loss providing 80% of coverage in excess of a 100% loss ratio on its crop-hail portfolio. OneBeacon also purchased a new quota share contract on 30% of its MPCI portfolio.
	Sirius Group
	At March 31, 2015, Sirius Group had $11.6 million and $295.3 million of reinsurance recoverables on paid and unpaid losses that will become recoverable if claims are paid in accordance with current reserve estimates. At December 31, 2014, Sirius Group had $11.4 million and $322.2 million of reinsurance recoverables on paid and unpaid losses. Because retrocessional reinsurance contracts do not relieve Sirius Group of its obligation to its insureds, the collectability of balances due from Sirius Group’s reinsurers is important to its financial strength. Sirius Group monitors the financial strength and ratings of retrocessionaires on an ongoing basis. Uncollectible amounts historically have not been significant.
	There have been no material changes to Sirius Group's reinsurance coverage as discussed in Note 4 -“Reinsurance” in White Mountains’s 2014 Annual Report on Form 10-K.

Investment_Securities

Investment Securities	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investment Securities
	White Mountains’s invested assets consist of securities and other long-term investments held for general investment purposes.  The portfolio of investment securities includes fixed maturity investments, short-term investments, common equity securities, and convertible fixed maturity and preferred investments which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date.  Realized and unrealized investment gains and losses on trading securities are reported in pre-tax revenues. White Mountains’s investments in fixed maturity investments are generally valued using industry standard 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.
	Realized investment gains and losses resulting from sales of investment securities are accounted for using the specific identification method.  Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment.  Short-term investments consist of money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year.  Short-term investments are carried at amortized or accreted cost, which approximated fair value as of March 31, 2015 and December 31, 2014.
	Other long-term investments consist primarily of hedge funds, private equity funds, surplus note investments, private equity securities and limited liability companies and other investments in limited partnerships.
	Net Investment Income
	Pre-tax net investment income for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2015				2014
	Investment income:
	Fixed maturity investments		$	23.1			$	22.4
	Short-term investments		0.6				0.6
	Common equity securities		3.4				5.4
	Convertible fixed maturity and preferred investments		0.2				0.4
	Other long-term investments		(.1		)		0.7
	Interest on funds held under reinsurance treaties		(.1		)		—
	Total investment income		27.1				29.5
	Third-party investment expenses		(3.8		)		(4.8		)
	Net investment income, pre-tax		$	23.3			$	24.7
	Net Realized and Unrealized Investment Gains and Losses
	Net realized and unrealized investment gains and losses for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2015				2014
	Net realized investment gains, pre-tax		$	52.8			$	21.7
	Net unrealized investment gains, pre-tax		50.6				42.1
	Net realized and unrealized investment gains, pre-tax		103.4				63.8
	Income tax expense attributable to net realized and		(26.3		)		(15.7		)
	unrealized investment gains
	Net realized and unrealized investment gains, after tax		$	77.1			$	48.1
	Net realized investment gains (losses)
	Net realized investment gains (losses) for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended												Three Months Ended
			March 31, 2015												March 31, 2014
	Millions		Net				Net				Total net realized				Net				Net				Total net realized
			realized				foreign				gains (losses)				realized				foreign				gains (losses)
			gains				currency gains				reflected in				gains				currency				reflected in
			(losses)								earnings				(losses)				losses				earnings
	Fixed maturity investments		$	9.8			$	21.2			$	31			$	4.4			$	(3.5	)		$	0.9
	Short-term investments		—				3.5				3.5				—				—				—
	Common equity securities		24.4				1.1				25.5				18.9				(.1		)		18.8
	Convertible fixed maturity and preferred investments		(4.2		)		—				(4.2		)		2.4				—				2.4
	Other long-term investments		(4.1		)		1.1				(3.0		)		(.3		)		—				(.3		)
	Forward contracts		—				—				—				(.1		)		—				(.1		)
	Net realized investment gains		25.9				26.9				52.8				25.3				(3.6		)		21.7
	(losses), pre-tax
	Income tax (expense) benefit		(6.5		)		(6.7		)		(13.2		)		(4.6		)		0.9				(3.7		)
	attributable to net realized
	investment gains (losses)
	Net realized investment		$	19.4			$	20.2			$	39.6			$	20.7			$	(2.7	)		$	18
	gains (losses), after tax
	Net unrealized investment gains (losses)
	The following table summarizes net unrealized investment gains (losses) for the three months ended March 31, 2015 and 2014:
			Three Months Ended												Three Months Ended
			March 31, 2015												March 31, 2014
	Millions		Net				Net				Total net unrealized				Net				Net				Total net unrealized
			unrealized				foreign				gains (losses)				unrealized				foreign currency				gains
			gains				currency gains				reflected in				gains				gains				reflected in
			(losses)								earnings												earnings
	Fixed maturity investments		$	12.1			$	40.2			$	52.3			$	19.4			$	12.2			$	31.6
	Short-term investments		—				—				—				—				—				—
	Common equity securities		(4.2		)		0.8				(3.4		)		4				0.5				4.5
	Convertible fixed maturity and preferred investments		0.7				0.1				0.8				0.7				—				0.7
	Other long-term investments		0.9				—				0.9				4.9				0.4				5.3
Forward contracts		—				—				—				—				—				—
Net unrealized investment		9.5				41.1				50.6				29				13.1				42.1
gains, pre-tax
Income tax expense		(2.8		)		(10.3		)		(13.1		)		(9.1		)		(2.9		)		(12.0		)
attributable to net unrealized
investment gains
Net unrealized investment		$	6.7			$	30.8			$	37.5			$	19.9			$	10.2			$	30.1
gains, after tax
The following table summarizes the amount of total pre-tax unrealized investment (losses) gains included in earnings for Level 3 investments for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
Millions		2015				2014
Fixed maturity investments		$	(.7	)		$	0.2
Common equity securities		(1.8		)		0.8
Convertible fixed maturity and preferred investments		0.1				—
Other long-term investments		(2.8		)		6.1
Total unrealized investment (losses) gains, pre-tax - Level 3 investments		$	(5.2	)		$	7.1
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 March 31, 2015 and December 31, 2014 were as follows:
		March 31, 2015
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains
U.S. Government and agency obligations		$	165.3			$	0.3			$	(.1	)		$	0.2			$	165.7
Debt securities issued by corporations		2,103.80				46.7				(1.0		)		66.2				2,215.70
Municipal obligations		96				1.8				(.2		)		—				97.6
Mortgage-backed and asset-backed securities		2,049.30				11.9				(2.6		)		39.5				2,098.10
Foreign government, agency and provincial obligations		106.9				1.2				(.2		)		0.9				108.8
Preferred stocks		79.5				6.3				—				0.2				86
Total fixed maturity investments		$	4,600.80			$	68.2			$	(4.1	)		$	107			$	4,771.90
		December 31, 2014
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains
U.S. Government and agency obligations		$	184.7			$	0.1			$	(.3	)		$	3.6			$	188.1
Debt securities issued by corporations		2,221.30				45.2				(5.1		)		49.8				2,311.20
Municipal obligations		82				1.4				(.2		)		—				83.2
Mortgage-backed and asset-backed securities		1,811.10				7.6				(3.5		)		25.7				1,840.90
Foreign government, agency and provincial obligations		274.6				4.2				(1.0		)		(2.7		)		275.1
Preferred stocks		79.6				6.1				—				0.1				85.8
Total fixed maturity investments		$	4,653.30			$	64.6			$	(10.1	)		$	76.5			$	4,784.30
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, convertible fixed maturity and preferred investments and other long-term investments as of March 31, 2015 and December 31, 2014 were as follows:
		March 31, 2015
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains
Common equity securities		$	623.3			$	163.4			$	(8.8	)		$	9			$	786.9
Convertible fixed maturity and		$	13.1			$	1.8			$	(.3	)		$	1.2			$	15.8
preferred investments
Other long-term investments		$	339.8			$	74.7			$	(13.1	)		$	1.2			$	402.6
		December 31, 2014
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				(losses) gains
Common equity securities		$	633.6			$	175.1			$	(5.2	)		$	(1.9	)		$	801.6
Convertible fixed maturity and		$	19.1			$	0.9			$	(.2	)		$	0.7			$	20.5
preferred investments
Other long-term investments		$	343.3			$	73			$	(10.9	)		$	1.6			$	407
Other Long-term Investments
Other long-term investments consist of the following as of March 31, 2015 and December 31, 2014:
		Carrying Value at
Millions		31-Mar-15				31-Dec-14
Hedge funds and private equity funds, at fair value(1)		$	242.5			$	242.9
Surplus notes investments, at fair value(1)		68				65.1
Private equity securities and limited liability companies, at fair value(1)		66.6				69.7
Tax advantaged federal affordable housing development fund(2)		16.2				16.8
Partnership investments accounted for under the equity method		3.8				5.2
Other(1)		5.5				7.3
Total other-long term investments		$	402.6			$	407
(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 March 31, 2015, White Mountains held investments in 12 hedge funds and 31 private equity funds.  The largest investment in a single fund was $29.4 million as of March 31, 2015. The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of March 31, 2015 and December 31, 2014:
		March 31, 2015								December 31, 2014
Millions		Fair Value				Unfunded				Fair Value				Unfunded
						Commitments								Commitments
Hedge funds
Long/short equity		$	44.3			$	—			$	43.9			$	—
Long/short equity REIT		21				—				20.3				—
Long/short credit & distressed		20				—				21.4				—
Long/short equity activist		6.6				—				6.2				—
Long bank loan		0.2				—				0.2				—
Total hedge funds		92.1				—				92				—
Private equity funds
Energy infrastructure & services		51.3				10				59.6				11
Manufacturing/Industrial		30.5				2.9				23.2				7.3
Multi-sector		24.3				4.4				24.2				5.3
Aerospace/Defense/Government		20.9				5.1				20.7				5.1
Healthcare		8.7				1.3				6.1				2.8
Private equity secondaries		8				3.2				8.5				3.1
Insurance		2.1				41.3				2.1				41.2
Real estate		2				0.1				3.6				3.3
Venture capital		1.4				0.3				1.4				0.3
International multi-sector, Europe		1.2				2.1				1.5				2.3
Total private equity funds		150.4				70.7				150.9				81.7
Total hedge funds and private equity funds		$	242.5			$	70.7			$	242.9			$	81.7
included in other long-term investments
Redemption of investments in certain hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions.  Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period.
The following summarizes the March 31, 2015 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
		Notice Period
Millions		30-59 days				60-89 days				90-119 days				120+ days				Total
Redemption frequency		notice				notice				notice				notice
Monthly		$	4.5			$	—			$	—			$	—			$	4.5
Quarterly		31.8				20.1				—				7.9				59.8
Semi-annual		—				22.9				—				—				22.9
Annual		—				—				4.7				0.2				4.9
Total		$	36.3			$	43			$	4.7			$	8.1			$	92.1
Certain of the hedge fund investments in which White Mountains is invested are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund’s underlying investments are liquidated.  As of March 31, 2015, distributions of $2.1 million were outstanding from these investments. The actual amount of the final distribution remittances remain subject to market fluctuations. The date at which such remittances will be received is not determinable as of March 31, 2015.
White Mountains has also submitted redemption requests for certain of its investments in active hedge funds.  As of March 31, 2015, redemptions of $6.9 million are outstanding and are subject to market fluctuations. The date at which such redemptions will be received is not determinable as of March 31, 2015. Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations.
Investments in private equity funds are generally subject to a “lock-up” period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds provide an option to extend the lock-up period at either the sole discretion of the fund manager or upon agreement between the fund and the investors.
As of March 31, 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		$29.70		$32.40		$84.10		$4.20		$150.40
White Mountains also owns 68% of the limited partnership interests in the Prospector Offshore Fund. As of March 31, 2015 and December 31, 2014, White Mountains’s investment in the Prospector Offshore Fund was $64.7 million and $64.9 million. The Prospector Offshore Fund is a hedge fund that pursues investment opportunities in a variety of equity and equity-related instruments, with a principle focus on the financial services sector and a special emphasis on the insurance industry. White Mountains has determined that the Prospector Offshore Fund is a VIE and is required to consolidate. As of March 31, 2015 and December 31, 2014, White Mountains consolidated total assets of $136.1 million and $135.8 million and total liabilities of $40.4 million and $39.8 million of the Prospector Offshore Fund. As of March 31, 2015 and December 31, 2014, White Mountains also recorded non-controlling interest of $31.0 million and $31.1 million in the Prospector Offshore Fund.
Fair value measurements as of March 31, 2015
White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common equity securities, convertible fixed maturity and preferred investments and other long-term investments, such as interests in hedge funds and private equity funds. Fair value measurements reflect management’s best estimate of the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurements fall into a hierarchy with three levels based on the nature of the inputs. Fair value measurements based on quoted prices in active markets for identical assets are at the top of the hierarchy (“Level 1”), followed by fair value measurements based on observable inputs that do not meet the criteria for Level 1, including quoted prices in inactive markets and quoted prices in active markets for similar, but not identical instruments (“Level 2”). Measurements based on unobservable inputs, including a reporting entity’s estimates of the assumptions that market participants would use are at the bottom of the hierarchy (“Level 3”).
White Mountains uses quoted market prices or other observable inputs to determine fair value for the vast majority 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 consist of fixed maturity investments including corporate debt, state and other governmental debt, convertible fixed maturity and preferred securities 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 and equity securities where quoted market prices are unavailable. 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, matrix pricing, 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 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. 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. 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 fixed maturity investments 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 or accreted prospectively over the remaining economic life.
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 each hedge fund and private equity fund 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.
In addition to the investments described above, White Mountains has $38.8 million and $38.0 million of investment-related liabilities recorded at fair value and included in other liabilities as of March 31, 2015 and December 31, 2014.  These liabilities relate to securities that have been sold short by limited partnerships in which White Mountains has investments and is 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 March 31, 2015 and December 31, 2014, by level:
		March 31, 2015
Millions		Fair Value				Level 1				Level 2				Level 3
Fixed maturity investments:
U.S. Government and agency obligations		$	165.7			$	110.8			$	54.9			$	—
Debt securities issued by corporations:
Consumer		538.6				—				538.6				—
Financials		398.5				—				398.5				—
Health Care		267.3								267.3
Industrial		252				—				252				—
Communications		203.9				—				203.9				—
Energy		193.7				—				193.7				—
Utilities		159.9				—				159.9				—
Technology		106.7				—				106.7				—
Materials		95.1				—				95.1				—
Other		—				—				—				—
Total debt securities issued by corporations:		2,215.70				—				2,215.70				—
Municipal obligations		97.6				—				97.6				—
Mortgage-backed and asset-backed securities		2,098.10				—				2,030.40				67.7
Foreign government, agency and provincial obligations		108.8				18.5				90.3				—
Preferred stocks		86				—				14.6				71.4
Total fixed maturity investments		4,771.90				129.3				4,503.50				139.1
Short-term investments		929.2				926.8				2.4				—
Common equity securities:
Financials		226.8				194.2				—				32.6
Consumer		170				170				—				—
Health Care		89.7				89.7
Industrial		70				70				—				—
Technology		54.9				54.9				—				—
Communications		28.4				28.4				—				—
Energy		27.7				27.7				—				—
Materials		21.7				21.7				—				—
Utilities		9.4				9.4				—				—
Other		88.3				9.8				78.5				—
Total common equity securities		786.9				675.8				78.5				32.6
Convertible fixed maturity and preferred investments		15.8				—				11.9				3.9
Other long-term investments (1)		382.6				—				—				382.6
Total investments		$	6,886.40			$	1,731.90			$	4,596.30			$	558.2
(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 $16.2 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				Level 2				Level 3
Fixed maturity investments:
U.S. Government and agency obligations		$	188.1			$	134.1			$	54			$	—
Debt securities issued by corporations:
Consumer		575.3				—				575.3				—
Financials		431.4				—				431.4				—
Health Care		284.2				—				284.2
Industrial		225.4								219.8				5.6
Communications		214.3				—				214.3				—
Energy		177.9				—				177.9				—
Utilities		173.6				—				173.6				—
Technology		118				—				118				—
Materials		103				—				103
Other		8.1				—				8.1				—
Total debt securities issued by corporations:		2,311.20				—				2,305.60				5.6
Municipal obligations		83.2				—				83.2				—
Mortgage-backed and asset-backed securities		1,840.90				—				1,840.90				—
Foreign government, agency and provincial obligations		275.1				21.3				253.8				—
Preferred stocks		85.8				—				14.7				71.1
Total fixed maturity investments		4,784.30				155.4				4,552.20				76.7
Short-term investments		871.7				868.8				2.9				—
Common equity securities:
Financials		237.5				197.3				—				40.2
Consumer		161.2				161.1				0.1				—
Health Care		101.9				101.9				—				—
Industrial		67.5				67.5
Technology		54.3				54.3				—				—
Communications		32.9				32.9				—				—
Energy		32.6				32.6				—				—
Materials		21.2				21.2				—				—
Utilities		9.5				9.4				0.1				—
Other		83				9.7				73.3				—
Total common equity securities		801.6				687.9				73.5				40.2
Convertible fixed maturity and preferred investments		20.5				—				12.3				8.2
Other long-term investments (1)		385				—				—				385
Total investments		$	6,863.10			$	1,712.10			$	4,640.90			$	510.1
(1) Excludes carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes 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 March 31, 2015 and December 31, 2014:
		Fair Value at
Millions		March 31, 2015				December 31, 2014
AAA		$	—			$	—
AA		194.1				236.9
A		818.2				957.8
BBB		1,188.80				1,105.90
BB		6.2				—
Other		8.4				10.6
Debt securities issued by corporations(1)		$	2,215.70			$	2,311.20
(1) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC ("Standard & Poor’s") and 2) Moody’s Investor Service ("Moody's").
Mortgage-backed, Asset-backed Securities
White Mountains purchases commercial and residential mortgage-backed securities with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains’s investments in hedge funds and private equity funds contain negligible amounts of sub-prime mortgage-backed securities as of March 31, 2015. 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.
The following table summarizes mortgage and asset-backed securities as of March 31, 2015 and December 31, 2014:
		March 31, 2015												December 31, 2014
Millions		Fair Value				Level 2				Level 3				Fair Value				Level 2				Level 3
Mortgage-backed securities:
Agency:
GNMA		$	373			$	373			$	—			$	411.2			$	411.2			$	—
FNMA		35.2				35.2				—				36.6				36.6				—
FHLMC		38.9				38.9				—				49.6				49.6				—
Total Agency(1)		447.1				447.1				—				497.4				497.4				—
Non-agency:
Residential		194.3				147.2				47.1				131.2				131.2				—
Commercial		292.2				271.6				20.6				236.9				236.9				—
Total Non-agency		486.5				418.8				67.7				368.1				368.1				—
Total mortgage-backed securities		933.6				865.9				67.7				865.5				865.5				—
Other asset-backed securities:
Credit card receivables		472.9				472.9				—				522.2				522.2				—
Vehicle receivables		537.2				537.2				—				289.4				289.4				—
Other		154.4				154.4				—				163.8				163.8				—
Total other asset-backed securities		1,164.50				1,164.50				—				975.4				975.4				—
Total mortgage and asset-backed securities		$	2,098.10			$	2,030.40			$	67.7			$	1,840.90			$	1,840.90			$	—
(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 residential mortgage-backed securities ("RMBS") and non-agency commercial mortgage-backed securities (“CMBS”) as of March 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		$	194.3			$	35			$	13.7			$	9.3			$	—			$	15.8		$	—	$	57.7	$	20.1	$	4.6	$	28.2	$	9.9	$	—
RMBS
Non-agency		292.2				—				—				8.4				—				—			—		11.2		—		26.1		88.6		122.3		35.6
CMBS
Total		$	486.5			$	35			$	13.7			$	17.7			$	—			$	15.8		$	—	$	68.9	$	20.1	$	30.7	$	116.8	$	132.2	$	35.6
Non-agency Residential Mortgage-backed Securities
White Mountains’s non-agency RMBS portfolio is generally of 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 $85.4 million of non-agency RMBS resecuritization tranches, each a senior tranche in its own right and each collateralized by a single earlier vintage Super Senior or Senior non-agency residential mortgage backed security.
The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of March 31, 2015:
Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
Prime		$	186.4			$	48.3			$	138.1			$	—
Non-prime		6.9				—				6.9				—
Sub-prime		1				1				—				—
Total		$	194.3			$	49.3			$	145			$	—
(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch Ratings ("Fitch") and were senior to other "AAA" or "Aaa" securities.
(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" securities.
(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" securities.
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 March 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 March 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 March 31, 2015:
Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
Fixed rate CMBS		$	165.6			$	18.6			$	76.2			$	70.8
Floating rate CMBS		126.6				—				—				126.6
Total		$	292.2			$	18.6			$	76.2			$	197.4
(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" securities.
(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" securities.
(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" securities.
Rollforward of Fair Value Measurements by Level
White Mountains uses quoted market prices where available as the inputs to estimate fair value for its investments in active markets. Such measurements are considered to be either Level 1 or Level 2 measurements, depending on whether the quoted market price inputs are for identical securities (Level 1) or similar securities (Level 2). Level 3 measurements for fixed maturity investments, common equity securities, convertible fixed maturity and preferred investments and other long-term investments as of March 31, 2015 and 2014 consist of securities for which the estimated fair value has not been determined based upon quoted market price inputs for identical or similar securities.
The following tables summarize the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2015 and 2014:
							Level 3 Investments
Millions	Level 1 investments			Level 2			Fixed			Common			Convertible fixed maturity and			Other long-term				Total
				investments			maturity investments			equity			preferred investments			investments
										securities
Balance at January 1, 2015	$	843.3		$	4,638.00		$	76.7		$	40.2		$	8.2		$	385			$	5,991.40		(1)(2)
Total realized and	18.3			87.3			0.3			1.8			(4.3		)	(2.0		)		101.4			(3)
unrealized gains (losses)
Foreign currency losses	(9.6		)	(115.6		)	(.9		)	—			—			(3.5		)		(129.6		)
through OCI
Amortization/Accretion	(.1		)	(10.0		)	—			—			—			—				(10.1		)
Purchases	222.9			1,047.70			68.6			—			—			9.3				1,348.50
Sales	(269.8		)	(1,059.0		)	—			(9.4		)	—			(6.2		)		(1,344.4		)
Transfers in	—			5.6			—			—			—			—				5.6
Transfers out	—			—			(5.6		)	—			—			—				(5.6		)
Balance at	$	805		$	4,594.00		$	139.1		$	32.6		$	3.9		$	382.6			$	5,957.20		(1)(2)
March 31, 2015
(1)  Excludes carrying value of $5.2 and $3.8 at January 1, 2015 and March 31, 2015 associated with other long-term investments accounted for using the equity method. Excludes carrying value of $16.8 and $16.2 at January 1, 2015 and March 31, 2015 million associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
(2)  Excludes carrying value of $871.7 and $929.2 at January 1, 2015 and March 31, 2015 associated with short-term investments.
(3) Excludes $1.0 realized and unrealized losses associated with the Prospector Offshore Fund consolidation of investment-related liabilities and realized and unrealized loss for the period of $3.5 associated with short-term investments.
							Level 3 Investments
Millions	Level 1 investments			Level 2			Fixed			Common			Convertible fixed maturity and			Other long-term				Total
				investments			maturity investments			equity			preferred investments			investments
										securities
Balance at January 1, 2014	$	1,376.70		$	4,982.20		$	93		$	46.1		$	6.1		$	262.4			$	6,766.50		(1)(2)(3)
Total realized and	22.2			34.7			0.4			0.8			—			6.3				64.4			(4)
unrealized gains
Foreign currency gains	(2.9		)	(10.5		)	(.1		)	—			—			(.3		)		(13.8		)
through OCI
Amortization/Accretion	(.2		)	(11.1		)	—			—			—			—				(11.3		)
Purchases	352.6			840.4			19.6			0.3			—			12.7				1,225.60
Sales	(180.1		)	(1,115.5		)	—			—			—			(3.7		)		(1,299.3		)
Net change in investments	(2.7		)	7.3			—			—			—			—				4.6
related to purchases of
consolidated affiliates
Transfers in	—			—			—			—			—			—				—
Transfers out	—			—			—			—			—			—				—
Balance at	$	1,565.60		$	4,727.50		$	112.9		$	47.2		$	6.1		$	277.4			$	6,736.70		(1)(2)(3)
March 31, 2014
(1) Excludes carrying value of $6.8 and $5.7 at January 1, 2014 and March 31, 2014 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) and $(0.1) at January 1, 2014 and March 31, 2014 related to forward contracts. Excludes carrying value of $19.1 and $18.5 at January 1, 2014 and March 31, 2014 million associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
(2)  Carrying value includes $236.3 and 222.2 at January 1, 2014 and March 31, 2014 that is classified as assets held for sale relating to discontinued operations.
(3) Excludes carrying value of $635.9 and $674.0 at January 1, 2014 and March 31, 2014 associated with short-term investments.
(4) Excludes $0.7 realized and unrealized losses associated with the Prospector Offshore Fund and Prospector Turtle Fund consolidation of investment-related liabilities. The Prospector Turtle Fund was liquidated as of December 31, 2014.
Fair Value Measurements — transfers between levels - Three-month period ended March 31, 2015 and 2014
During the first three months of 2015, one fixed maturity investment classified as a Level 3 measurement in the prior period was recategorized as a Level 2 measurement because quoted market prices for similar securities that were considered reliable and could be validated against an alternative source were available as of March 31, 2015. These measurements comprise “Transfers out” of Level 3 and “Transfers in” to Level 2 of $5.6 million for the period March 31, 2015.
During the first three months of 2014, no fixed maturity investments classified as Level 3 measurements in the prior period were recategorized as Level 2 measurements.
Significant Unobservable Inputs
The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 other than hedge funds and private equity funds as of March 31, 2015 and December 31, 2014. The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds.
($ in millions)						31-Mar-15						31-Dec-14
Description		Rating(2)		Valuation		Fair		Unobservable Input				Fair		Unobservable Input
				Technique(s)		Value(7)						Value(7)
Preferred Stock(1)		NR		Discounted cash flow		$71.40		Discount yield	-	7.10%		$71.10		Discount yield	-	7.10%
Private equity security(1)		NR		Multiple of GAAP book value		$32.60		Book value multiple	-	1.1		$40.20		Book value multiple	-	1.1
Private equity security(1)		NR		Share price of recent transaction		$20.10		Share price	-	$1.06		$20.10		Share price	-	$1.06
Private equity security(1)		NR		Share price of recent transaction		$9.20		Share price	-	$290.96		$10.40		Share price	-	$290.96
Private equity security(1)		NR		Share price of recent transaction		$14.20		Share price	-	$0.13		$15.80		Share price	-	$0.13
Convertible preferred		NR		Multiple of EBITDA		$3.90		EBITDA multiple	-	6		$3.80		EBITDA multiple	-	6
security(1)
Convertible preferred		NR		Share price of recent transaction		N/A		N/A		N/A		$4.50		Share price	-	$0.71
security(1)
Debt security issued		NR		Discounted cash flow		N/A		N/A		N/A		$5.60		Illiquidity discount(3)	-	10%
by corporation(1)
Non-agency commercial		A-		Broker pricing		$20.60		Broker quote		N/A		N/A		N/A		N/A
mortgage backed security(1)
Non-agency residential		NR		Discounted cash flow		$47.10		Prepayment		12 CPR(8)		N/A		N/A		N/A
mortgage backed securities
						Default Rate				2.5-8.75 CDR(9)		N/A		N/A		N/A
						Discount spread/swap				45-180 bps		N/A		N/A		N/A
						Loss severity(10)				50.00%		N/A		N/A		N/A
Surplus notes:		NR				Discount yield				11.30%		Discount yield				11.00%
- Seller priority				Discounted cash flow		$46.50		Discount	-	9.40%		$44.00		Discount	-	9.30%
								rate (4)						rate (4)
						Timing of interest			-	5 years		Timing of interest			-	5 years
						payments(5)						payments(5)
						Timing of principal			-	10 years		Timing of principal			-	10 years
						payments(5)						payments(5)
- Pari passu				Discounted cash flow		$21.60		Discount	-	14.40%		$21.10		Discount	-	13.50%
								rate(5)						rate(5)
						Timing of interest			-	5 years		Timing of interest			-	5 years
						payments(6)						payments(6)
						Timing of principal			-	15 years		Timing of principal			-	10 years
						payments(6)						payments(6)
(1) As of March 31, 2015, consists of 1 security.
(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and 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 principal repayments begin on a graduated basis in year ten for the seller priority note 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.
(8) Conditional Prepayment Rate (CPR) is an annualized percentage of the existing mortgage pool that is expected to be prepaid in a year.
(9) Constant Default Rate (CDR) is an annualized rate of default of on a pool of mortgage loans, expressed in percentage terms.
(10) Loss severity represents the expected total lifetime losses (both interest and principal losses) as a percentage of the loan principal balance.
The assumed prepayment rate is a significant unobservable input used to estimate the fair value of investments in mortgage backed securities. Generally for bonds priced at a premium, increased in prepayment speeds will result in a lower fair value, while decreases in prepayment speed may result in a higher fair value.

Goodwill_and_Other_Intangible_

Goodwill and Other Intangible Assets	3 Months Ended
	Mar. 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.
	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 based upon the foreign exchange spot rate at the date of acquisition), representing a controlling financial interest. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million, including $0.3 million of goodwill and $2.8 million of other intangible assets; and total liabilities assumed of $1.2 million at their estimated fair values.
	The following table shows the change in goodwill and other intangible assets:
		Three Months Ended March 31,
	Millions	2015					2014
		Goodwill		Other intangible assets			Goodwill		Other intangible assets
	Beginning balance	$	168.9	$	197.5		$	—	$	20.7
	Acquisitions of businesses	0.3		2.8			23.8		42.4
	Dispositions of businesses	—		—			—		(.4		)
	Amortization, including foreign currency translation	—		(6.9		)	—		(.3		)
	Ending balance	$	169.2	$	193.4		$	23.8	$	62.4

Debt

Debt	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Debt	Debt and Standby Letter of Credit Facilities
	White Mountains’s debt outstanding as of March 31, 2015 and December 31, 2014 consisted of the following:
	Millions	March 31,			December 31,
		2015			2014
	2012 OBH Senior Notes, at face value	$	275		$	275
	Unamortized original issue discount	(.3		)	(.3		)
	2012 OBH Senior Notes, carrying value	274.7			274.7
	SIG Senior Notes, at face value	400			400
	Unamortized original issue discount	(.3		)	(.3		)
	SIG Senior Notes, carrying value	399.7			399.7
	WTM Bank Facility	—			—
	Tranzact Bank Facility	71.9			68.7
	Unamortized issuance cost	(1.3		)	(1.3		)
	Tranzact Bank Facility, carrying value	70.6			67.4
	Other debt	4.8			4.8
	Total debt	$	749.8		$	746.6
	WTM Bank Facility
	On August 14, 2013, White Mountains entered into a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A., which has a total commitment of $425.0 million and has a maturity date of August 14, 2018 (the “WTM Bank Facility”). As of March 31, 2015, the WTM Bank Facility was undrawn.
	The WTM Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards.
	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. The Tranzact Bank Facility has a total commitment of $74.5 million, which consists of a $70.0 million term loan facility and a $4.5 million revolving facility. During the first quarter of 2015, Tranzact repaid a total of $1.3 million under the term loan portion and borrowed $4.5 million under the revolving portion of the Tranzact Bank Facility. As of March 31, 2015, the total amount outstanding under the Tranzact Bank Facility was $71.9 million.
	The Tranzact Bank Facility, which is secured by intellectual property and the common stock of Tranzact and its 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.
	Stand By Letter of Credit Facilities
	On November 25, 2014, Sirius International entered into two stand by letter of credit facility agreements totaling $200.0 million to provide capital support for its Lloyds Syndicate 1945. One letter of credit is a $125.0 million facility from Nordea Bank Finland plc (the “Nordea facility”), $100.0 million of which is issued on an unsecured basis. The second letter of credit is a $75.0 million facility with Lloyds Bank plc (the “Lloyds Bank facility”), $25.0 million of which is issued on an unsecured basis. The Nordea facility and the Lloyds Bank facility are renewable annually.
	The unsecured portions of the Nordea facility and the Lloyds Bank facility are subject to 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.
	Sirius International has other secured letter of credit and trust arrangements with various financial institutions to support its insurance operations.
	Debt and Standby Letter of Credit Facility Covenants
	At March 31, 2015, White Mountains was in compliance with all of the covenants under the WTM Bank Facility, the OneBeacon U.S. Holdings, Inc. (“OBH”) Senior Notes, the Sirius International Group, Ltd. (“SIG”), Senior Notes, the Tranzact Bank Facility, the Nordea facility and the Lloyd’s Bank facility.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 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 consolidated subsidiaries and branches are subject to tax are Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Israel, Luxembourg, Malaysia, the Netherlands, Peru, Singapore, Sweden, Switzerland, the United Kingdom and the United States.
	White Mountains’s income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2015 and 2014 were 28.8% and 27.5%. The effective tax rates for the three months ended March 31, 2015 and 2014 were lower than the U.S. statutory rate of 35% due to income generated in jurisdictions with lower tax rates than the United States.
	In arriving at the effective tax rate for the three months ended March 31, 2015 and 2014, White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2015 and 2014.
	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.
	White Mountains is no longer subject to U.S. federal or state tax examinations by tax authorities for years before 2007. With few exceptions, White Mountains is no longer subject to non-U.S. income tax examinations by tax authorities for years before 2005.
	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, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. The estimated total assessment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $73.8 million. However, $60.2 million of the proposed 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 these deductions in the exam period would not affect the effective tax rate, but would accelerate the payment of cash to the taxing authority. White Mountains disagrees with the adjustments proposed by the IRS and is defending its position. Although the timing of the resolution of these issues is uncertain, it is reasonably possible that the resolution could occur within the next twelve months. An estimate of the range of potential outcomes cannot be made at this time. White Mountains does not expect the resolution of this examination to result in a material change to its financial position.
	On September 5, 2013, the IRS commenced an examination of the income tax returns for 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. White Mountains does not expect the resolution of this examination to result in a material change to its financial position.
	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	3 Months Ended
	Mar. 31, 2015
Derivatives	Derivatives
Variable Annuity
Derivatives	Variable Annuity Reinsurance
	White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. At March 31, 2015 and December 31, 2014, the total guarantee value was approximately ¥117.8 billion (approximately $1.0 billion at exchange rates on that date) and ¥134.2 billion (approximately $1.1 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 113% of the guarantee value at both March 31, 2015 and December 31, 2014.  WM Life Re is in runoff, and all of its contracts will mature by June 30, 2016.
	The following table summarizes the pre-tax operating results of WM Life Re for the three months ended March 31, 2015 and 2014.
		Three Months Ended
		March 31,
	Millions	2015			2014
	Fees, included in other revenue	$	3		$	5.4
	Change in fair value of variable annuity liability,	0.4			(1.3		)
	included in other revenue
	Change in fair value of derivatives, included in other revenue	(5.5		)	(6.6		)
	Foreign exchange, included in other revenue	(1.1		)	0.2
	Other investment income and (losses) gains	—			0.4
	Total revenue	(3.2		)	(1.9		)
	Change in fair value of variable annuity death benefit liabilities,	—			—
	included in other general and administrative expenses
	Death benefit claims paid, included in general and	—			—
	administrative expenses
	General and administrative expenses	(1.4		)	(1.3		)
	Pre-tax loss	$	(4.6	)	$	(3.2	)
	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three months ended March 31, 2015 and 2014 and the carrying values, included in other assets, at March 31, 2015 and December 31, 2014 by type of instrument:
		Gains (losses)						Carrying Value
		Three Months Ended						As of
		March 31,						March 31,				31-Dec-14
	Millions	2015			2014			2015
	Fixed income/interest rate	$	9.3		$	(6.4	)	$	3.7			$	(1.7	)
	Foreign exchange	(12.1		)	(6.5		)	35.3				44.1
	Equity	(2.7		)	6.3			8				14
	Total	$	(5.5	)	$	(6.6	)	$	47			$	56.4
	The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015
		Variable Annuity			Derivative Instruments
		Assets
	Millions	Level 3			Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	0.7		$	18.9		$	33.8			$	3.7			$	56.4
	Purchases	—			—			—				—				—
	Realized and unrealized gains (losses)	0.4			(.6		)	(6.5		)		1.6				(5.5		)
	Transfers in	—			—			—				—				—
	Sales/settlements	—			(1.3		)	3.9				(6.5		)		(3.9		)
	End of period	$	1.1		$	17		$	31.2			$	(1.2	)		$	47
		Three Months Ended March 31, 2014
		Variable Annuity			Derivative Instruments
		(Liabilities)
	Millions	Level 3			Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	(52.8	)	$	63.4		$	4.7			$	1.1			$	69.2
	Purchases	—			—			—				—				—
	Realized and unrealized gains (losses)	(1.3		)	0.7			(7.1		)		(.2		)		(6.6		)
	Transfers in	—			—			—				—				—
	Sales/settlements	—			—			27.5				(.5		)		27
	End of period	$	(54.1	)	$	64.1		$	25.1			$	0.4			$	89.6
	(1) Consists of over-the-counter instruments.
	(2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available.  Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
	(3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
	In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties.  The total collateral includes the following:
	Millions	March 31, 2015			December 31, 2014			March 31, 2014
	Cash	$	29		$	23.7		$	22.2
	Fixed maturity investments	4.5			9.5			14.3
	Total	$	33.5		$	33.2		$	36.5
	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 at March 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. Actuarial 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:
	($ in Millions)	March 31, 2015
	Description	Fair			Valuation Technique(s)		Unobservable Input	Range					Weighted
		Value											Average
	Variable annuity benefit guarantee liabilities	$	1.1		Discounted cash flows		Surrenders
					0-1 year	0.1	%	-	40.00%		40	%
					1-2 years	0.2	%	-	40.00%		32.2	%
					Mortality	0	%	-	6.40%		1	%
					Foreign exchange volatilities
						0-1 year	10.9	%	-	16.20%		13.1	%
						1-2 years	10.9	%	-	14.10%		12.5	%
						Index volatilities
						0-1 year	24.8	%	-	27.20%		26.7	%
						1-2 years	21.2	%	-	23.90%		22.9	%
Foreign exchange options	$	11.1		Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	0.5	%	-	11.60%		5.9	%
Equity index options	$	5.9		Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	(0.7	)%	-	6.50%		2.1	%
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:
	March 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	$	3.4		$	(.9	)	$	2.5			$	1			$	(5.4	)	$	(4.4	)
Exchange traded	1.1			(.3		)	0.8				2.8				(.1		)	2.7
Foreign exchange contracts
OTC	38.9			—			38.9				45.5				—			45.5
Exchange traded	—			(3.6		)	(3.6		)		—				(1.4		)	(1.4		)
Equity contracts
OTC	7.3			(.4		)	6.9				11.7				(.2		)	11.5
Exchange traded	1.6			(.1		)	1.5				3.4				(.9		)	2.5
Total(2)	$	52.3		$	(5.3	)	$	47			$	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:
	31-Mar-15
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 WMLife Re - Cash		Net amount of exposure to counter-party		Standard
																									& Poor's
																									Rating(1)
Bank of America	$	4.9		$	—		$	—			$	4.9			$	—		$	—		$	—	$	4.9	A
Barclays	—			—			—				—				—			—			—		—		A
JP Morgan	21.3			—			—				21.3				—			—			7.4		13.9		A	+
Royal Bank	3.8			—			—				3.8				—			—			—		3.8		A	-
of Scotland
Nomura	(.4		)	0.4			—				—				5.6			4.5			—		10.1		BBB	+
Citigroup - OTC	18.6			—			—				18.6				0.7			—			—		19.3		A
Citigroup -	(1.2		)	1.2			—				—				21.1			—			—		21.1		A
Exchange Traded
Total	$	47		$	1.6		$	—			$	48.6			$	27.4		$	4.5		$	7.4	$	73.1
	31-Dec-14
Millions	Net amount of assets reflected in Balance Sheet			Collateral provided to counter-party - Cash			Collateral provided to counter-party - Financial Instruments				Net amount of exposure after effect of collateral provided				Excess collateral provided to counter-party- Cash			Excess collateral provided - Financial Instruments			Counter-party collateral held by WMLife Re- Cash		Net amount of exposure to counter-party		Standard
																									& Poor's
																									Rating(1)
Bank of America	$	5.6		$	—		$	—			$	5.6			$	—		$	—		$	—	$	5.6	A
Barclays	0.1			—			—				0.1				—			—			—		0.1		A
JP Morgan	24.3			—			—				24.3				—			—			8.8		15.5		A	+
Royal Bank	4			—			—				4				—			—			—		4		A
of Scotland
Nomura	(3.5		)	3.5			—				—				1.7			9.5			—		11.2		BBB	+
Citigroup - OTC	22.2			—			—				22.2				—			—			1.1		21.1		A
Citigroup -	3.7			—			—				3.7				16			—			—		19.7		A
Exchange Traded
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-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and
“BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).
Forward Contracts
Derivatives	Forward Contracts
	White Mountains has entered into currency forward contracts at Sirius Group. White Mountains monitors its exposure to foreign currency and adjusts its forward positions within the risk guidelines and ranges established by senior management for each currency, as necessary. While White Mountains actively manages its forward positions, mismatches between movements in foreign currency rates and its forward contracts may result in currency positions being outside the pre-defined ranges and/or foreign currency losses. At March 31, 2015, White Mountains held approximately $56.5 million (SEK 489.3 million) total gross notional value of foreign currency forward contracts.
	All of White Mountains’s forward contracts are traded over-the-counter. The fair value of the contracts has been estimated using OTC quotes for similar instruments and accordingly, the measurements have been classified as Level 2 measurements at March 31, 2015.
	The net realized and unrealized derivative gains (losses) recognized in net realized and unrealized investment gains (losses) for the three months ended March 31, 2015 and 2014 were $0.0 million and $(0.1) million.
	All of White Mountains’s forward contracts are subject to master netting agreements. As of March 31, 2015 and December 31, 2014, the gross liability amount offset under master netting arrangements and the net amount recognized in other investments approximately offset each other.
	White Mountains does not hold or provide any collateral for the forward contracts.  The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts:
		31-Mar-15								31-Dec-14
	Millions	Notional Amount			Carrying Value			Standard & Poor's		Notional Amount				Carrying Value
								Rating(1)
	Barclays Bank Plc	$	6.3		$	—		A		$	2.1			$	—
	Deutsche Bank	4.4			0.1			A		—				—
	Goldman Sachs	10.9			—			A		8.7				—
	HSBC Bank Plc	21.7			—			AA-		11.2				—
	JP Morgan	7.3			(.1		)	A+		5.7				—
	Royal Bank of Canada	5.9			—			AA-		5.4				—
	Total	$	56.5		$	—				$	33.1			$	—
	(1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the eighth highest of twenty-three creditworthiness ratings).
Interest Rate Cap
Derivatives	Interest Rate Cap
	In May 2007, Sirius International Group, Ltd. (“SIG”), an intermediate holding company of Sirius Group, 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% and (ii) 3-month LIBOR plus 320 basis points. In July 2013, SIG executed the 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 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 basis points 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 within other revenue. 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 at March 31, 2015.
	The following tables summarize the changes in the fair value of the Interest Rate Cap for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Beginning of period	$	4.1		$	11.1
	Net realized and unrealized losses	(1.0		)	(2.9		)
	End of period	$	3.1		$	8.2
	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 counterparty and investment earnings thereon. The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty:
		31-Mar-15
	Millions	Collateral Balances Held			Standard & Poor’s
					Rating(1)
	Barclays Bank Plc	$	2.1		A
	Nordea Bank Finland Plc	1			AA-
	Total	$	3.1
	(1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings).
Weather derivatives
Derivatives	Weather Derivatives
	For the three months ended March 31, 2015 and 2014, Sirius Group recognized $0.8 million and $0.4 million of net gains on its weather and weather contingent derivatives portfolio. The fair values of the assumed contracts are subject to change in the near-term and reflect management’s best estimate based on various factors including, but not limited to, observed and forecasted weather conditions, changes in interest or foreign currency exchange rates and other market factors. Estimating the fair value of derivative instruments that do not have quoted market prices requires management’s judgment in determining amounts that could reasonably be expected to be received from or paid to a third party to settle the contracts. Such amounts could be materially different from the amounts that might be realized in an actual transaction to settle the contract with a third party. Because of the significance of the unobservable inputs used to estimate the fair value of Sirius Group’s weather risk contracts, the fair value measurements of the contracts are deemed to be Level 3 measurements in the fair value hierarchy.
Tranzact Interest Rate Swap
Derivatives	Tranzact Interest Rate Swap
	Tranzact has a $70.0 million term loan facility that carries a variable rate equal to the U.S. dollar LIBOR rate, plus an applicable margin. At March 31, 2015, the variable interest rate on the term loan was 4.172%, including a margin over LIBOR of 4.0%. Effective October 10, 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 at inception, which decreases over the term of the swap by amounts equivalent to scheduled principal repayments made on Tranzact’s term loan. As of March 31, 2015, the notional amount was $67.4 million. 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 0.1518% at inception and 0.178% at March 31, 2015. The total current effective rate on Tranzact's debt was 5.3% at March 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 March 31, 2015, the estimated fair value of the swap was $(0.5) million. There are no collateral arrangements associated with the swap.

Municipal_Bond_Guarantee

Municipal Bond Guarantee	3 Months Ended
	Mar. 31, 2015
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance
	In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of March 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. Through HG Re, which had statutory capital of $453.0 million at March 31, 2015, HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time.
	For the three months ended March 31, 2015 and 2014, HG Global had pre-tax income of $5.4 million and $4.6 million, which included $4.0 million and $3.9 million of interest income on the BAM surplus notes.
	For the three months ended March 31, 2015 and 2014, White Mountains reported pre-tax losses of $8.7 million and $8.6 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.0 million and $3.9 million of interest expense on the BAM surplus notes.
	Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM surplus notes for the five years ending December 31, 2018 from a fixed rate of 8% to a variable rate equal to the one-year U.S. treasury rate plus 300 basis points, set annually, which is 3.15% and 3.13% for 2015 and 2014. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years.  At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8%. BAM is required to seek regulatory approval to pay interest and principal on its surplus notes only when adequate capital resources have accumulated beyond BAM’s initial capitalization and a level that continues to support its outstanding obligations, business plan and ratings.
	All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are received upfront and an unearned premium revenue liability, equal to the amount of the cash received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods.
	The following table provides a schedule of BAM’s insured obligations:
		31-Mar-15		31-Dec-14
	Contracts outstanding	2,029		1,750
	Remaining weighted average contract period outstanding (in years)	12.8		12.8
	Contractual debt service outstanding (in millions):
	Par	$	14,421.60	$	12,362.50
	Interest	8,044.70		7,086.90
	Total debt service outstanding	$	22,466.30	$	19,449.40
	Gross unearned insurance premiums	$	31.2	$	27.6

Earnings_Per_Share

Earnings Per Share	3 Months Ended
	Mar. 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 three months ended March 31, 2015 and 2014 (see Note 17 for earnings per share amounts for discontinued operations):
		Three Months Ended
		March 31,
		2015			2014
	Basic and diluted earnings per share numerators (in millions):
	Net income from continuing operations attributable to	$	76.4		$	96.1
	White Mountains’s common shareholders
	Allocation of income for unvested restricted common shares	(.7		)	(1.1		)
	Dividends declared on participating restricted common shares(1)	(.1		)	(.1		)
	Total allocation to restricted common shares	(.8		)	(1.2		)
	Net income attributable to White Mountains’s common shareholders,	$	75.6		$	94.9
	net of restricted common share amounts
	Undistributed net earnings (in millions):
	Net income attributable to White Mountains’s common shareholders,	$	75.6		$	94.9
	net of restricted common share amounts
	Dividends declared net of restricted common share amounts(1)	(5.9		)	(6.1		)
	Total undistributed net earnings, net of restricted common share amounts	$	69.7		$	88.8
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted shares(2)	(58.8		)	(70.4		)
	Basic earnings per share denominator	5,919.60			6,099.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(2)	(58.8		)	(70.4		)
	Average outstanding dilutive options to acquire common shares(3)	—			—
	Diluted earnings per share denominator	5,919.60			6,099.50
	Basic earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	12.77		$	15.56
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	11.77		$	14.56
	Diluted earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	12.77		$	15.56
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	11.77		$	14.56
	(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.
	(2) Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 15).
	(3) The diluted earnings per share denominator for the three months ended March 31, 2015 and 2014 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.

Noncontrolling_Interest

Non-controlling Interest	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Noncontrolling Interest Disclosure [Text Block]	The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of March 31, 2015 and December 31, 2014:
		31-Mar-15					31-Dec-14
	$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
	OneBeacon Ltd.	24.7	%	$	258.8		24.7	%	$	258.4
	SIG Preference Shares	100		250			100		250
	Other, excluding mutuals and reciprocals
	HG Global	3.1		17.7			3.1		17.9
	MediaAlpha	40		22			40		22.6
	Tranzact	36.8		81.8			36.8		88.2
	Wobi	36.7		4.7			36.7		5.4
	Prospector Offshore Fund	32.4		31			23.4		31.1
	Dewar	20.3		3			18		3.4
	Total other, excluding mutuals and reciprocals			160.2					168.6
	Mutuals and reciprocals
	BAM	100		(127.0		)	100		(121.9		)
	SSIE	100		(13.5		)	100		(12.4		)
	Total non-controlling interests			$	528.5				$	542.7

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Segment Informartion	Segment Information
	White Mountains has determined that its reportable segments are OneBeacon, Sirius Group, HG Global/BAM and Other Operations.
	White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors.
	Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows:
								HG Global/BAM
	Millions	OneBeacon			Sirius			HG		BAM			Other Operations			Total
					Group
	Three Months Ended March 31, 2015
	Earned insurance and reinsurance premiums	$	286.6		$	205		$	0.5	$	0.2		$	2		$	494.3
	Net investment income	10.5			8			0.4		1			3.4			23.3
	Net investment income (loss) - surplus note	—			—			4		(4.0		)	—			—
	interest
	Net realized and unrealized investment gains	14.2			79.9			1.1		3			5.2			103.4
	Other (loss) revenue	(3.1		)	(27.4		)	—		0.1			66.2			35.8
	Total revenues	308.2			265.5			6		0.3			76.8			656.8
	Losses and LAE	165.9			97.5			—		—			2			265.4
	Insurance and reinsurance acquisition expenses	51			42.6			0.1		0.8			1.1			95.6
	Other underwriting expenses	55.9			25.5			—		0.1			—			81.5
	General and administrative expenses	4.1			6.2			0.4		8.2			97.5			116.4
	Interest expense	3.2			6.8			—		—			1.1			11.1
	Total expenses	280.1			178.6			0.5		9.1			101.7			570
	Pre-tax income (loss)	$	28.1		$	86.9		$	5.5	$	(8.8	)	$	(24.9	)	$	86.8
								HG Global/BAM
	Millions	OneBeacon			Sirius			HG		BAM			Other Operations			Total
					Group
	Three Months Ended March 31, 2014
	Earned insurance and reinsurance premiums	$	276.5		$	215.5		$	0.2	$	0.1		$	1.3		$	493.6
	Net investment income	10.4			8.7			0.3		1.4			3.9			24.7
	Net investment income (loss) - surplus note	—			—			3.9		(3.9		)	—			—
	interest
	Net realized and unrealized investment gains	18.9			28.1			0.7		3			13.1			63.8
	Other revenue (loss)	1			(5.3		)	—		0.2			0.7			(3.4		)
	Total revenues	306.8			247			5.1		0.8			19			578.7
	Losses and LAE	149.4			78.8			—		—			1.1			229.3
	Insurance and reinsurance acquisition expenses	46.7			47.4			0.1		0.4			0.5			95.1
	Other underwriting expenses	49.4			31.8			—		0.1			0.1			81.4
	General and administrative expenses	3.3			8.4			0.4		8.9			29.3			50.3
	Interest expense	3.2			6.6			—		—			0.3			10.1
	Total expenses	252			173			0.5		9.4			31.3			466.2
	Pre-tax income (loss)	$	54.8		$	74		$	4.6	$	(8.6	)	$	(12.3	)	$	112.5

Investments_in_Unconsolidated_

Investments in Unconsolidated Affiliates	3 Months Ended
	Mar. 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.
	Millions	March 31,			December 31,
		2015			2014
	Symetra common shares	$	378.9		$	373.8
	Unrealized gains from Symetra’s fixed maturity portfolio	68.4			37.6
	Carrying value of Symetra common shares	447.3			411.4
	Hamer	3.5			3
	Total investments in unconsolidated affiliates	$	450.8		$	414.4
	Symetra
	At March 31, 2015 and December 31, 2014, White Mountains owned 20.05 million common shares of Symetra Financial Corporation (“Symetra”), a 17.27% and 17.31% common share ownership. White Mountains accounts for its investment in common shares of Symetra using the equity method. During the three months ended March 31, 2015 and 2014, White Mountains received cash dividends from Symetra of $2.2 million and $2.0 million on its common share investment that were recorded as a reduction of White Mountains’s investment in Symetra.
	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 March 31, 2015, the pre-tax unamortized basis difference was $165.7 million, of which $35.1 million is attributable to equity in earnings of unconsolidated affiliates and $130.6 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 difference is being amortized over a 30-year period with a weighted average of 28-years remaining. The amortization is based on estimated future cash flows associated with Symetra’s underlying assets and liabilities to which the basis differences have been attributed. White Mountains continues to record its equity in Symetra's earnings and net unrealized gains (losses). In addition, White Mountains recognizes the amortization of the basis difference through equity in earnings of unconsolidated affiliates and equity in net unrealized gains (losses) from investments in unconsolidated affiliates consistent with the original attribution of the basis differences between equity in earnings and equity in net unrealized gains (losses). For the three months ended March 31, 2015 and 2014, White Mountains recognized after-tax amortization of $0.7 million and $0.7 million through equity in earnings of unconsolidated affiliates and $2.8 million and $2.9 million through equity in net unrealized gains from investments in unconsolidated affiliates.
	The following table summarizes amounts recorded by White Mountains relating to its investment in Symetra for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Carrying value of investment in Symetra at beginning of period	$	411.4		$	317.3
	Equity in earnings (1)(2)	7.2			14.4
	Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio(3)	30.9			39.3
	Dividends received	(2.2		)	(2.0		)
	Carrying value of investment in Symetra at end of period(4)(5)	$	447.3		$	369
	(1) Equity in earnings excludes tax expense of $0.4 and $1.0.
	(2) Equity in earnings includes $0.7 and $0.7 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(3) Net unrealized gains includes $2.9 and $3.1 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $68.4 and $39.3 as of March 31, 2015 and 2014, which excludes tax expense of $4.6 and $2.8.
	(5) The aggregate value of White Mountains’s investment in common shares of Symetra was $470.3 based upon the quoted market price of $23.46 per share at March 31, 2015.

Employee_ShareBased_Incentive_

Employee Share-Based Incentive Compensation Plans	3 Months Ended
	Mar. 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 Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non share-based incentive awards to key employees and service providers of White Mountains. White Mountains’s share-based compensation incentive awards consist of performance shares, restricted shares and stock options.
	Share-Based Compensation Based on White Mountains Common Shares
	WTM Performance Shares
	Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three-year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid.  The following table summarizes performance share activity for the three months ended March 31, 2015 and 2014 for performance shares granted under the WTM Incentive Plan:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	123,549		$	48.1		119,220		$	60.2
	Shares paid or expired(1)	(37,977	)	(27.3		)	(37,130	)	(26.7		)
	New grants	30,640		—			45,660		—
	Assumed forfeitures and cancellations(2)	184		0.6			(213	)	0.4
	Expense recognized	—		6.2			—		8.4
	Ending of period	116,396		$	27.6		127,537		$	42.3
	(1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target.  WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.
	(2) Amounts include changes in assumed forfeitures, as required under GAAP.
	For the 2012-2014 performance cycle, all performance shares earned were settled in cash. 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
	If the outstanding WTM performance shares had vested on March 31, 2015, the total additional compensation cost to be recognized would have been $33.1 million, based on accrual factors at March 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 at March 31, 2015 for each performance cycle:
	Millions, except share amounts	Target		Accrued
		Performance		Expense
		Shares
		Outstanding
	Performance cycle:
	2015 – 2017	30,640		$	1.8
	2014 – 2016	41,937		5.6
	2013 – 2015	46,803		20.9
	Sub-total	119,380		28.3
	Assumed forfeitures	(2,984	)	(.7		)
	Total at March 31, 2015	116,396		$	27.6
	Restricted Shares
	The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	83,314		$	14.3		94,130		$	17
	Issued	22,890		15.2			20,400		11.8
	Vested	(35,079	)	—			(33,205	)	—
	Forfeited	—		—			—		—
	Expense recognized	—		(3.0		)	—		(4.0		)
	Non-vested at March 31,	71,125		$	26.5		81,325		$	24.8
	During the first three months of 2015, White Mountains issued 22,890 restricted shares that vest on January 1, 2018. During the first three months of 2014, White Mountains issued 20,400 restricted shares that vest on January 1, 2017. The unrecognized compensation cost at March 31, 2015 is expected to be recognized ratably over the remaining vesting periods.
	Share-Based Compensation Based on OneBeacon Ltd. Common Shares
	The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based and non share-based incentive awards.  OneBeacon’s share-based incentive awards include OneBeacon performance shares and restricted shares.
	OneBeacon Performance Shares
	OneBeacon performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash.  OneBeacon performance share awards generally vest at the end of a three-year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of OneBeacon Ltd. common shares at the time awards are paid.  The following table summarizes performance share activity for the three months ended March 31, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	517,470		$	3.4		493,421		$	4
	Shares paid or expired(1)	(181,290	)	(1.5		)	(142,138	)	(1.0		)
	New grants	154,887		—			165,800		—
	Assumed forfeitures and cancellations(2)	(28,920	)	—			(4,145	)	—
	Expense recognized	—		0.2			—		0.6
Ending of period	462,147		$	2.1		512,938		$	3.6
(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.
(2) Amounts include changes in assumed forfeitures, as required under GAAP.
If the outstanding OneBeacon performance shares had been vested on March 31, 2015, the total additional compensation cost to be recognized would have been $3.3 million, based on accrual factors at March 31, 2015 (common share price, accumulated dividends and payout assumptions).
The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at March 31, 2015 for each performance cycle:
Millions, except share amounts	Target		Accrued Expense
	Performance
	Shares
	Outstanding
Performance cycle:
2015 – 2017	154,887		$	0.2
2014 – 2016	151,810		0.5
2013 – 2015	167,300		1.4
Sub-total	473,997		2.1
Assumed forfeitures	(11,850	)	—
Total at March 31, 2015	462,147		$	2.1
OneBeacon Restricted Shares
The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three months ended March 31, 2015 and 2014:
	Three Months Ended March 31,
	2015					2014
Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
	Shares		Issue Date			Shares		Issue Date
			Fair Value					Fair Value
Non-vested,
Beginning of period	612,500		$	3.5		915,000		$	6.5
Issued	75,950		1.1			—		—
Vested	(296,000	)	—			(300,000	)	—
Forfeited	(1,500	)	—			(2,500	)	—
Expense recognized	—		(.6		)	—		(.8		)
Non-vested at March 31,	390,950		$	4		612,500		$	5.7
On February 24, 2015, OneBeacon issued 75,950 restricted shares that fully vest on January 1, 2018.
On March 1, 2012, OneBeacon issued 300,000 restricted shares that vest in two equal annual installments. The first installment vested on February 28, 2014.
On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments. The first installment vested on February 22, 2014.  Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent five years have been or will also be reduced by 35,000 shares.
The unrecognized compensation cost at March 31, 2015 is expected to be recognized ratably over the remaining vesting periods.
OneBeacon Restricted Stock Units
On February 24, 2015, the OneBeacon Compensation Committee awarded to certain employees 207,848 restricted stock units (“RSUs”), of which 205,319 remained outstanding as of March 31, 2015. The RSUs are scheduled to vest on January 1, 2018 and will be paid out in cash or shares at the discretion of the OneBeacon Compensation Committee. For the three months ended March 31, 2015, the expense associated with the RSUs was $0.1 million.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 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 and the SIG Senior Notes, which are recorded as debt at face value less unamortized original issue discount, and the SIG Preference Shares, which are recorded as non-controlling interest at face value.
	The following table summarizes the fair value and carrying value of financial instruments as of March 31, 2015 and December 31, 2014:
		March 31, 2015				December 31, 2014
	Millions	Fair		Carrying		Fair		Carrying
		Value		Value		Value		Value
	2012 OBH Senior Notes	$	291.5	$	274.7	$	286	$	274.7
	SIG Senior Notes	438.2		399.7		437.8		399.7
	SIG Preference Shares	261		250		260		250
	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 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.
	The Tranzact Bank Facility is a recently issued variable rate instrument. As a result it is considered a Level 2 measurement. As of March 31, 2015, the carrying value of the amount drawn on the Tranzact Debt Facility approximates fair value.

Discontinued_Operations

Discontinued Operations	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations
	For the three months ended March 31, 2015, White Mountains recorded a net gain on sale of discontinued operations of $8.0 million, which is primarily related to the favorable loss reserve development on loss reserves transferred in the sale of Esurance and Answer Financial. See “Esurance” in Note 18.
	For the three months ended March 31, 2015 and 2014, White Mountains recorded a net loss on discontinued operation of $0.1 million and $0.5 million. The loss from discontinued operations, net of tax, of $0.1 million for the three months ended March 31, 2015, was the result of post-closing expenses incurred in connection with the Runoff Business. The loss from discontinued operations, net of tax, of $0.5 million for the three months ended March 31, 2014, was primarily due to non-claims expenses related to the Runoff Business, including dedicated staff.
	The loss from sale of discontinued operations related to the Runoff Business is subject to post-closing adjustments related to the true-up of the final closing balance sheet. Armour has presented its proposed final closing deliverables pursuant to the stock purchase agreement, and OneBeacon is in the process of reviewing and responding.
	Earnings Per Share
	Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities.  Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
		2015			2014
	Basic and diluted earnings per share numerators (in millions):
	Net income attributable to White Mountains’s common shareholders	$	7.9		$	(.5	)
	Allocation of income for participating unvested restricted common shares(1)	(.1		)	—
	Net income attributable to White Mountains’s common shareholders,	$	7.8		$	(.5	)
	net of restricted common share amounts (2)
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(3)	(58.8		)	(70.4		)
	Basic earnings per share denominator	5,919.60			6,099.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(3)	(58.8		)	(70.4		)
	Average outstanding dilutive options to acquire common shares(4)	—			—
	Diluted earnings per share denominator	5,919.60			6,099.50
	Basic and diluted earnings per share (in dollars):	$	1.32		$	(.08	)
	(1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.
	(2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three months ended March 31, 2015 and 2014.
	(3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 15).
	(4) The diluted earnings per share denominator for the three months ended March 31, 2015 and 2014 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.

Contingencies

Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies
	Legal Contingencies
	White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3.
	White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred.
	Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows.
	The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of March 31, 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 (filed as an exhibit to the Company’s current report on Form 8-K on May 18, 2011, the “Agreement”).  The Company has certain contingencies under the Agreement as follows:  (i) subject to specified thresholds and limits, the Company generally indemnifies Allstate for breaches of its representations and warranties in the Agreement for a period of eighteen months (although longer for specified representations and warranties) from the closing, (ii) the Company indemnifies Allstate for breaches of certain covenants in the Agreement, including certain agreements by the Company not to solicit certain employees of the Transferred Subsidiaries for three years after the closing, and (iii) subject to specified thresholds and limits, the Company indemnifies 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 retains 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. During the first quarter of 2015, Allstate informed the Company that they owe $8.0 million to the Company for favorable development in the transferred loss reserves, which the Company has recorded as gain from sale of discontinued operations. The amount is not final and is being reviewed by the Company.
	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 filed a Petition for Review with the Commonwealth Court of Pennsylvania (“Commonwealth Court”) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (“PID”) orders approving the Runoff Transaction and denying their right to intervene in the PID’s regulatory review of the Runoff Transaction. White Mountains believes the claims made by the petitioners are without merit and has intervened in the proceedings before the Commonwealth Court to vigorously defend the propriety of the PID’s orders in their entirety. White Mountains believes that the possibility is remote that these proceedings could result in an adverse outcome or have a material financial impact on the Company’s results of operations or financial position in the future.
	Tribune Company
	In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. Certain subsidiaries of White Mountains received approximately $39.0 million for Tribune common stock tendered in connection with the LBO.
	The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs 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 March 31, 2015, as the amount of loss, if any, cannot be reasonably estimated.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles
	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 March 31, 2015 increased by $1.2 million. As a result of the retrospective adoption, White Mountains's common shareholders equity has been reduced by $0.5 million as of January 1, 2014. In addition, the retrospective adoption resulted in an increase of $0.4 million to net investment income and a net increase of $0.6 million to income tax expense for the three months ended March 31, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above.
	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 represent a strategic shift and 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. White Mountains has not had any transactions that occurred since ASU 2014-08 became effective on December 15, 2014.
	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 since AUS 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.
	Recently Issued Accounting Pronouncements
	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. ASU 2015-03 is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 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 should represent the compensation cost 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, such as third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $4.4 million for the three months ended March 31, 2015 and 2014, respectively. 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. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016.

Loss_and_Loss_Adjustment_Expen1

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2015
Insurance Loss Reserves [Abstract]
Loss and loss adjustment expense ("LAE") reserve activities	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Gross beginning balance	$	3,159.80		$	3,079.30
	Less beginning reinsurance recoverable on unpaid losses	(483.9		)	(428.1		)
	Net loss and LAE reserves	2,675.90			2,651.20
	Loss and LAE reserves consolidated — SSIE	—			13.6
	Loss and LAE incurred relating to:
	Current year losses	266.8			240.5
	Prior year losses	(1.4		)	(11.2		)
	Total incurred losses and LAE	265.4			229.3
	Foreign currency translation adjustment to loss and LAE reserves	(22.4		)	1.8
	Loss and LAE paid relating to:
	Current year losses	(40.6		)	(35.0		)
	Prior year losses	(280.2		)	(237.7		)
	Total loss and LAE payments	(320.8		)	(272.7		)
	Net ending balance	2,598.10			2,623.20
	Plus ending reinsurance recoverable on unpaid losses	436.9			425.2
	Gross ending balance	$	3,035.00		$	3,048.40

Investment_Securities_Tables

Investment Securities (Tables)	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	Pre-tax net investment income for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2015				2014
	Investment income:
	Fixed maturity investments		$	23.1			$	22.4
	Short-term investments		0.6				0.6
	Common equity securities		3.4				5.4
	Convertible fixed maturity and preferred investments		0.2				0.4
	Other long-term investments		(.1		)		0.7
	Interest on funds held under reinsurance treaties		(.1		)		—
	Total investment income		27.1				29.5
	Third-party investment expenses		(3.8		)		(4.8		)
	Net investment income, pre-tax		$	23.3			$	24.7
Schedule of Realized and Unrealized Gain (Loss) on Investments	Net realized and unrealized investment gains and losses for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2015				2014
	Net realized investment gains, pre-tax		$	52.8			$	21.7
	Net unrealized investment gains, pre-tax		50.6				42.1
	Net realized and unrealized investment gains, pre-tax		103.4				63.8
	Income tax expense attributable to net realized and		(26.3		)		(15.7		)
	unrealized investment gains
	Net realized and unrealized investment gains, after tax		$	77.1			$	48.1
Realized Gain (Loss) on Investments	Net realized investment gains (losses) for the three months ended March 31, 2015 and 2014 consisted of the following:
			Three Months Ended												Three Months Ended
			March 31, 2015												March 31, 2014
	Millions		Net				Net				Total net realized				Net				Net				Total net realized
			realized				foreign				gains (losses)				realized				foreign				gains (losses)
			gains				currency gains				reflected in				gains				currency				reflected in
			(losses)								earnings				(losses)				losses				earnings
	Fixed maturity investments		$	9.8			$	21.2			$	31			$	4.4			$	(3.5	)		$	0.9
	Short-term investments		—				3.5				3.5				—				—				—
	Common equity securities		24.4				1.1				25.5				18.9				(.1		)		18.8
	Convertible fixed maturity and preferred investments		(4.2		)		—				(4.2		)		2.4				—				2.4
	Other long-term investments		(4.1		)		1.1				(3.0		)		(.3		)		—				(.3		)
	Forward contracts		—				—				—				(.1		)		—				(.1		)
	Net realized investment gains		25.9				26.9				52.8				25.3				(3.6		)		21.7
	(losses), pre-tax
	Income tax (expense) benefit		(6.5		)		(6.7		)		(13.2		)		(4.6		)		0.9				(3.7		)
	attributable to net realized
	investment gains (losses)
	Net realized investment		$	19.4			$	20.2			$	39.6			$	20.7			$	(2.7	)		$	18
	gains (losses), after tax
Schedule of unrealized gain (loss) on investments	The following table summarizes net unrealized investment gains (losses) for the three months ended March 31, 2015 and 2014:
			Three Months Ended												Three Months Ended
			March 31, 2015												March 31, 2014
	Millions		Net				Net				Total net unrealized				Net				Net				Total net unrealized
			unrealized				foreign				gains (losses)				unrealized				foreign currency				gains
			gains				currency gains				reflected in				gains				gains				reflected in
			(losses)								earnings												earnings
	Fixed maturity investments		$	12.1			$	40.2			$	52.3			$	19.4			$	12.2			$	31.6
	Short-term investments		—				—				—				—				—				—
	Common equity securities		(4.2		)		0.8				(3.4		)		4				0.5				4.5
	Convertible fixed maturity and preferred investments		0.7				0.1				0.8				0.7				—				0.7
	Other long-term investments		0.9				—				0.9				4.9				0.4				5.3
	Forward contracts		—				—				—				—				—				—
	Net unrealized investment		9.5				41.1				50.6				29				13.1				42.1
	gains, pre-tax
	Income tax expense		(2.8		)		(10.3		)		(13.1		)		(9.1		)		(2.9		)		(12.0		)
	attributable to net unrealized
	investment gains
	Net unrealized investment		$	6.7			$	30.8			$	37.5			$	19.9			$	10.2			$	30.1
	gains, after tax
Net unrealized investment gains (losses) for Level 3 investments	The following table summarizes the amount of total pre-tax unrealized investment (losses) gains included in earnings for Level 3 investments for the three months ended March 31, 2015 and 2014:
			Three Months Ended
			March 31,
	Millions		2015				2014
	Fixed maturity investments		$	(.7	)		$	0.2
	Common equity securities		(1.8		)		0.8
	Convertible fixed maturity and preferred investments		0.1				—
	Other long-term investments		(2.8		)		6.1
	Total unrealized investment (losses) gains, pre-tax - Level 3 investments		$	(5.2	)		$	7.1
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 fixed maturity investments as of March 31, 2015 and December 31, 2014 were as follows:
			March 31, 2015
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains
	U.S. Government and agency obligations		$	165.3			$	0.3			$	(.1	)		$	0.2			$	165.7
	Debt securities issued by corporations		2,103.80				46.7				(1.0		)		66.2				2,215.70
	Municipal obligations		96				1.8				(.2		)		—				97.6
	Mortgage-backed and asset-backed securities		2,049.30				11.9				(2.6		)		39.5				2,098.10
	Foreign government, agency and provincial obligations		106.9				1.2				(.2		)		0.9				108.8
	Preferred stocks		79.5				6.3				—				0.2				86
	Total fixed maturity investments		$	4,600.80			$	68.2			$	(4.1	)		$	107			$	4,771.90
			December 31, 2014
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains
	U.S. Government and agency obligations		$	184.7			$	0.1			$	(.3	)		$	3.6			$	188.1
	Debt securities issued by corporations		2,221.30				45.2				(5.1		)		49.8				2,311.20
	Municipal obligations		82				1.4				(.2		)		—				83.2
	Mortgage-backed and asset-backed securities		1,811.10				7.6				(3.5		)		25.7				1,840.90
	Foreign government, agency and provincial obligations		274.6				4.2				(1.0		)		(2.7		)		275.1
	Preferred stocks		79.6				6.1				—				0.1				85.8
	Total fixed maturity investments		$	4,653.30			$	64.6			$	(10.1	)		$	76.5			$	4,784.30
	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, convertible fixed maturity and preferred investments and other long-term investments as of March 31, 2015 and December 31, 2014 were as follows:
			March 31, 2015
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains
	Common equity securities		$	623.3			$	163.4			$	(8.8	)		$	9			$	786.9
	Convertible fixed maturity and		$	13.1			$	1.8			$	(.3	)		$	1.2			$	15.8
	preferred investments
	Other long-term investments		$	339.8			$	74.7			$	(13.1	)		$	1.2			$	402.6
			December 31, 2014
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				(losses) gains
	Common equity securities		$	633.6			$	175.1			$	(5.2	)		$	(1.9	)		$	801.6
	Convertible fixed maturity and		$	19.1			$	0.9			$	(.2	)		$	0.7			$	20.5
	preferred investments
	Other long-term investments		$	343.3			$	73			$	(10.9	)		$	1.6			$	407
	Other Long-term Investments
	Other long-term investments consist of the following as of March 31, 2015 and December 31, 2014:
			Carrying Value at
	Millions		31-Mar-15				31-Dec-14
	Hedge funds and private equity funds, at fair value(1)		$	242.5			$	242.9
	Surplus notes investments, at fair value(1)		68				65.1
	Private equity securities and limited liability companies, at fair value(1)		66.6				69.7
	Tax advantaged federal affordable housing development fund(2)		16.2				16.8
	Partnership investments accounted for under the equity method		3.8				5.2
	Other(1)		5.5				7.3
	Total other-long term investments		$	402.6			$	407
	(1) See Fair Value Measurements by Level table.
	(2) Fund accounted for using the proportional amortization method.
Other long-term investments	The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of March 31, 2015 and December 31, 2014:
			March 31, 2015								December 31, 2014
	Millions		Fair Value				Unfunded				Fair Value				Unfunded
							Commitments								Commitments
	Hedge funds
	Long/short equity		$	44.3			$	—			$	43.9			$	—
	Long/short equity REIT		21				—				20.3				—
	Long/short credit & distressed		20				—				21.4				—
	Long/short equity activist		6.6				—				6.2				—
	Long bank loan		0.2				—				0.2				—
	Total hedge funds		92.1				—				92				—
	Private equity funds
	Energy infrastructure & services		51.3				10				59.6				11
	Manufacturing/Industrial		30.5				2.9				23.2				7.3
	Multi-sector		24.3				4.4				24.2				5.3
	Aerospace/Defense/Government		20.9				5.1				20.7				5.1
	Healthcare		8.7				1.3				6.1				2.8
	Private equity secondaries		8				3.2				8.5				3.1
	Insurance		2.1				41.3				2.1				41.2
	Real estate		2				0.1				3.6				3.3
	Venture capital		1.4				0.3				1.4				0.3
	International multi-sector, Europe		1.2				2.1				1.5				2.3
	Total private equity funds		150.4				70.7				150.9				81.7
	Total hedge funds and private equity funds		$	242.5			$	70.7			$	242.9			$	81.7
	included in other long-term investments
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following summarizes the March 31, 2015 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
			Notice Period
	Millions		30-59 days				60-89 days				90-119 days				120+ days				Total
	Redemption frequency		notice				notice				notice				notice
	Monthly		$	4.5			$	—			$	—			$	—			$	4.5
	Quarterly		31.8				20.1				—				7.9				59.8
	Semi-annual		—				22.9				—				—				22.9
	Annual		—				—				4.7				0.2				4.9
	Total		$	36.3			$	43			$	4.7			$	8.1			$	92.1
Fair Value of private equity funds subject to lock-up periods	As of March 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		$29.70		$32.40		$84.10		$4.20		$150.40
Fair value measurements by level, investment securities	The following tables summarize White Mountains’s fair value measurements for investments as of March 31, 2015 and December 31, 2014, by level:
			March 31, 2015
	Millions		Fair Value				Level 1				Level 2				Level 3
	Fixed maturity investments:
	U.S. Government and agency obligations		$	165.7			$	110.8			$	54.9			$	—
	Debt securities issued by corporations:
	Consumer		538.6				—				538.6				—
	Financials		398.5				—				398.5				—
	Health Care		267.3								267.3
	Industrial		252				—				252				—
	Communications		203.9				—				203.9				—
	Energy		193.7				—				193.7				—
	Utilities		159.9				—				159.9				—
	Technology		106.7				—				106.7				—
	Materials		95.1				—				95.1				—
	Other		—				—				—				—
	Total debt securities issued by corporations:		2,215.70				—				2,215.70				—
	Municipal obligations		97.6				—				97.6				—
	Mortgage-backed and asset-backed securities		2,098.10				—				2,030.40				67.7
	Foreign government, agency and provincial obligations		108.8				18.5				90.3				—
	Preferred stocks		86				—				14.6				71.4
	Total fixed maturity investments		4,771.90				129.3				4,503.50				139.1
	Short-term investments		929.2				926.8				2.4				—
	Common equity securities:
	Financials		226.8				194.2				—				32.6
	Consumer		170				170				—				—
	Health Care		89.7				89.7
	Industrial		70				70				—				—
	Technology		54.9				54.9				—				—
	Communications		28.4				28.4				—				—
	Energy		27.7				27.7				—				—
	Materials		21.7				21.7				—				—
	Utilities		9.4				9.4				—				—
	Other		88.3				9.8				78.5				—
	Total common equity securities		786.9				675.8				78.5				32.6
	Convertible fixed maturity and preferred investments		15.8				—				11.9				3.9
	Other long-term investments (1)		382.6				—				—				382.6
	Total investments		$	6,886.40			$	1,731.90			$	4,596.30			$	558.2
	(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 $16.2 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				Level 2				Level 3
	Fixed maturity investments:
	U.S. Government and agency obligations		$	188.1			$	134.1			$	54			$	—
	Debt securities issued by corporations:
	Consumer		575.3				—				575.3				—
	Financials		431.4				—				431.4				—
	Health Care		284.2				—				284.2
	Industrial		225.4								219.8				5.6
	Communications		214.3				—				214.3				—
	Energy		177.9				—				177.9				—
	Utilities		173.6				—				173.6				—
	Technology		118				—				118				—
	Materials		103				—				103
	Other		8.1				—				8.1				—
	Total debt securities issued by corporations:		2,311.20				—				2,305.60				5.6
	Municipal obligations		83.2				—				83.2				—
	Mortgage-backed and asset-backed securities		1,840.90				—				1,840.90				—
	Foreign government, agency and provincial obligations		275.1				21.3				253.8				—
	Preferred stocks		85.8				—				14.7				71.1
	Total fixed maturity investments		4,784.30				155.4				4,552.20				76.7
	Short-term investments		871.7				868.8				2.9				—
	Common equity securities:
	Financials		237.5				197.3				—				40.2
	Consumer		161.2				161.1				0.1				—
Health Care		101.9				101.9				—				—
Industrial		67.5				67.5
Technology		54.3				54.3				—				—
Communications		32.9				32.9				—				—
Energy		32.6				32.6				—				—
Materials		21.2				21.2				—				—
Utilities		9.5				9.4				0.1				—
Other		83				9.7				73.3				—
Total common equity securities		801.6				687.9				73.5				40.2
Convertible fixed maturity and preferred investments		20.5				—				12.3				8.2
Other long-term investments (1)		385				—				—				385
Total investments		$	6,863.10			$	1,712.10			$	4,640.90			$	510.1
(1) Excludes carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes 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 March 31, 2015 and December 31, 2014:
			Fair Value at
	Millions		March 31, 2015				December 31, 2014
	AAA		$	—			$	—
	AA		194.1				236.9
	A		818.2				957.8
	BBB		1,188.80				1,105.90
	BB		6.2				—
	Other		8.4				10.6
	Debt securities issued by corporations(1)		$	2,215.70			$	2,311.20
	(1) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC ("Standard & Poor’s") and 2) Moody’s Investor Service ("Moody's").
Mortgage-backed, asset-backed securities	The following table summarizes mortgage and asset-backed securities as of March 31, 2015 and December 31, 2014:
			March 31, 2015												December 31, 2014
	Millions		Fair Value				Level 2				Level 3				Fair Value				Level 2				Level 3
	Mortgage-backed securities:
	Agency:
	GNMA		$	373			$	373			$	—			$	411.2			$	411.2			$	—
	FNMA		35.2				35.2				—				36.6				36.6				—
	FHLMC		38.9				38.9				—				49.6				49.6				—
	Total Agency(1)		447.1				447.1				—				497.4				497.4				—
	Non-agency:
	Residential		194.3				147.2				47.1				131.2				131.2				—
	Commercial		292.2				271.6				20.6				236.9				236.9				—
	Total Non-agency		486.5				418.8				67.7				368.1				368.1				—
	Total mortgage-backed securities		933.6				865.9				67.7				865.5				865.5				—
	Other asset-backed securities:
	Credit card receivables		472.9				472.9				—				522.2				522.2				—
	Vehicle receivables		537.2				537.2				—				289.4				289.4				—
	Other		154.4				154.4				—				163.8				163.8				—
	Total other asset-backed securities		1,164.50				1,164.50				—				975.4				975.4				—
	Total mortgage and asset-backed securities		$	2,098.10			$	2,030.40			$	67.7			$	1,840.90			$	1,840.90			$	—
	(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 residential mortgage-backed securities ("RMBS") and non-agency commercial mortgage-backed securities (“CMBS”) as of March 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		$	194.3			$	35			$	13.7			$	9.3			$	—			$	15.8		$	—	$	57.7	$	20.1	$	4.6	$	28.2	$	9.9	$	—
	RMBS
	Non-agency		292.2				—				—				8.4				—				—			—		11.2		—		26.1		88.6		122.3		35.6
	CMBS
	Total		$	486.5			$	35			$	13.7			$	17.7			$	—			$	15.8		$	—	$	68.9	$	20.1	$	30.7	$	116.8	$	132.2	$	35.6
Non-agency residential mortgage securities, collateral quality and tranche levels	The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of March 31, 2015:
	Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
	Prime		$	186.4			$	48.3			$	138.1			$	—
	Non-prime		6.9				—				6.9				—
	Sub-prime		1				1				—				—
	Total		$	194.3			$	49.3			$	145			$	—
	(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch Ratings ("Fitch") and were senior to other "AAA" or "Aaa" securities.
	(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" securities.
	(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" securities.
Non-agency commercial mortgage securities, type of interest rate and tranche levels	The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of March 31, 2015:
	Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
	Fixed rate CMBS		$	165.6			$	18.6			$	76.2			$	70.8
	Floating rate CMBS		126.6				—				—				126.6
	Total		$	292.2			$	18.6			$	76.2			$	197.4
	(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" securities.
	(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" securities.
	(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" securities.
Rollforward of fair value investments by level	The following tables summarize the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2015 and 2014:
								Level 3 Investments
	Millions	Level 1 investments			Level 2			Fixed			Common			Convertible fixed maturity and			Other long-term				Total
					investments			maturity investments			equity			preferred investments			investments
											securities
	Balance at January 1, 2015	$	843.3		$	4,638.00		$	76.7		$	40.2		$	8.2		$	385			$	5,991.40		(1)(2)
	Total realized and	18.3			87.3			0.3			1.8			(4.3		)	(2.0		)		101.4			(3)
	unrealized gains (losses)
	Foreign currency losses	(9.6		)	(115.6		)	(.9		)	—			—			(3.5		)		(129.6		)
	through OCI
	Amortization/Accretion	(.1		)	(10.0		)	—			—			—			—				(10.1		)
	Purchases	222.9			1,047.70			68.6			—			—			9.3				1,348.50
	Sales	(269.8		)	(1,059.0		)	—			(9.4		)	—			(6.2		)		(1,344.4		)
	Transfers in	—			5.6			—			—			—			—				5.6
	Transfers out	—			—			(5.6		)	—			—			—				(5.6		)
	Balance at	$	805		$	4,594.00		$	139.1		$	32.6		$	3.9		$	382.6			$	5,957.20		(1)(2)
	March 31, 2015
	(1)  Excludes carrying value of $5.2 and $3.8 at January 1, 2015 and March 31, 2015 associated with other long-term investments accounted for using the equity method. Excludes carrying value of $16.8 and $16.2 at January 1, 2015 and March 31, 2015 million associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
	(2)  Excludes carrying value of $871.7 and $929.2 at January 1, 2015 and March 31, 2015 associated with short-term investments.
	(3) Excludes $1.0 realized and unrealized losses associated with the Prospector Offshore Fund consolidation of investment-related liabilities and realized and unrealized loss for the period of $3.5 associated with short-term investments.
								Level 3 Investments
	Millions	Level 1 investments			Level 2			Fixed			Common			Convertible fixed maturity and			Other long-term				Total
					investments			maturity investments			equity			preferred investments			investments
											securities
	Balance at January 1, 2014	$	1,376.70		$	4,982.20		$	93		$	46.1		$	6.1		$	262.4			$	6,766.50		(1)(2)(3)
	Total realized and	22.2			34.7			0.4			0.8			—			6.3				64.4			(4)
	unrealized gains
	Foreign currency gains	(2.9		)	(10.5		)	(.1		)	—			—			(.3		)		(13.8		)
	through OCI
	Amortization/Accretion	(.2		)	(11.1		)	—			—			—			—				(11.3		)
	Purchases	352.6			840.4			19.6			0.3			—			12.7				1,225.60
	Sales	(180.1		)	(1,115.5		)	—			—			—			(3.7		)		(1,299.3		)
	Net change in investments	(2.7		)	7.3			—			—			—			—				4.6
	related to purchases of
	consolidated affiliates
	Transfers in	—			—			—			—			—			—				—
	Transfers out	—			—			—			—			—			—				—
	Balance at	$	1,565.60		$	4,727.50		$	112.9		$	47.2		$	6.1		$	277.4			$	6,736.70		(1)(2)(3)
	March 31, 2014
	(1) Excludes carrying value of $6.8 and $5.7 at January 1, 2014 and March 31, 2014 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) and $(0.1) at January 1, 2014 and March 31, 2014 related to forward contracts. Excludes carrying value of $19.1 and $18.5 at January 1, 2014 and March 31, 2014 million associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
	(2)  Carrying value includes $236.3 and 222.2 at January 1, 2014 and March 31, 2014 that is classified as assets held for sale relating to discontinued operations.
	(3) Excludes carrying value of $635.9 and $674.0 at January 1, 2014 and March 31, 2014 associated with short-term investments.
	(4) Excludes $0.7 realized and unrealized losses associated with the Prospector Offshore Fund and Prospector Turtle Fund consolidation of investment-related liabilities. The Prospector Turtle Fund was liquidated as of December 31, 2014.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 other than hedge funds and private equity funds as of March 31, 2015 and December 31, 2014. The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds.
	($ in millions)						31-Mar-15						31-Dec-14
	Description		Rating(2)		Valuation		Fair		Unobservable Input				Fair		Unobservable Input
					Technique(s)		Value(7)						Value(7)
	Preferred Stock(1)		NR		Discounted cash flow		$71.40		Discount yield	-	7.10%		$71.10		Discount yield	-	7.10%
	Private equity security(1)		NR		Multiple of GAAP book value		$32.60		Book value multiple	-	1.1		$40.20		Book value multiple	-	1.1
	Private equity security(1)		NR		Share price of recent transaction		$20.10		Share price	-	$1.06		$20.10		Share price	-	$1.06
	Private equity security(1)		NR		Share price of recent transaction		$9.20		Share price	-	$290.96		$10.40		Share price	-	$290.96
	Private equity security(1)		NR		Share price of recent transaction		$14.20		Share price	-	$0.13		$15.80		Share price	-	$0.13
	Convertible preferred		NR		Multiple of EBITDA		$3.90		EBITDA multiple	-	6		$3.80		EBITDA multiple	-	6
	security(1)
	Convertible preferred		NR		Share price of recent transaction		N/A		N/A		N/A		$4.50		Share price	-	$0.71
	security(1)
	Debt security issued		NR		Discounted cash flow		N/A		N/A		N/A		$5.60		Illiquidity discount(3)	-	10%
	by corporation(1)
	Non-agency commercial		A-		Broker pricing		$20.60		Broker quote		N/A		N/A		N/A		N/A
	mortgage backed security(1)
	Non-agency residential		NR		Discounted cash flow		$47.10		Prepayment		12 CPR(8)		N/A		N/A		N/A
	mortgage backed securities
							Default Rate				2.5-8.75 CDR(9)		N/A		N/A		N/A
							Discount spread/swap				45-180 bps		N/A		N/A		N/A
							Loss severity(10)				50.00%		N/A		N/A		N/A
	Surplus notes:		NR				Discount yield				11.30%		Discount yield				11.00%
	- Seller priority				Discounted cash flow		$46.50		Discount	-	9.40%		$44.00		Discount	-	9.30%
									rate (4)						rate (4)
							Timing of interest			-	5 years		Timing of interest			-	5 years
							payments(5)						payments(5)
							Timing of principal			-	10 years		Timing of principal			-	10 years
							payments(5)						payments(5)
	- Pari passu				Discounted cash flow		$21.60		Discount	-	14.40%		$21.10		Discount	-	13.50%
									rate(5)						rate(5)
							Timing of interest			-	5 years		Timing of interest			-	5 years
							payments(6)						payments(6)
							Timing of principal			-	15 years		Timing of principal			-	10 years
							payments(6)						payments(6)
	(1) As of March 31, 2015, consists of 1 security.
	(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and 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 principal repayments begin on a graduated basis in year ten for the seller priority note 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.
	(8) Conditional Prepayment Rate (CPR) is an annualized percentage of the existing mortgage pool that is expected to be prepaid in a year.
	(9) Constant Default Rate (CDR) is an annualized rate of default of on a pool of mortgage loans, expressed in percentage terms.
	(10) Loss severity represents the expected total lifetime losses (both interest and principal losses) as a percentage of the loan principal balance.

Goodwill_and_Other_Intangible_1

Goodwill and Other Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	The following table shows the change in goodwill and other intangible assets:
		Three Months Ended March 31,
	Millions	2015					2014
		Goodwill		Other intangible assets			Goodwill		Other intangible assets
	Beginning balance	$	168.9	$	197.5		$	—	$	20.7
	Acquisitions of businesses	0.3		2.8			23.8		42.4
	Dispositions of businesses	—		—			—		(.4		)
	Amortization, including foreign currency translation	—		(6.9		)	—		(.3		)
	Ending balance	$	169.2	$	193.4		$	23.8	$	62.4

Debt_Tables

Debt (Tables)	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Schedule of debt outstanding	White Mountains’s debt outstanding as of March 31, 2015 and December 31, 2014 consisted of the following:
	Millions	March 31,			December 31,
		2015			2014
	2012 OBH Senior Notes, at face value	$	275		$	275
	Unamortized original issue discount	(.3		)	(.3		)
	2012 OBH Senior Notes, carrying value	274.7			274.7
	SIG Senior Notes, at face value	400			400
	Unamortized original issue discount	(.3		)	(.3		)
	SIG Senior Notes, carrying value	399.7			399.7
	WTM Bank Facility	—			—
	Tranzact Bank Facility	71.9			68.7
	Unamortized issuance cost	(1.3		)	(1.3		)
	Tranzact Bank Facility, carrying value	70.6			67.4
	Other debt	4.8			4.8
	Total debt	$	749.8		$	746.6

Derivatives_Tables

Derivatives (Tables)	3 Months Ended
	Mar. 31, 2015
Interest Rate Cap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following tables summarize the changes in the fair value of the Interest Rate Cap for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Beginning of period	$	4.1		$	11.1
	Net realized and unrealized losses	(1.0		)	(2.9		)
	End of period	$	3.1		$	8.2
Variable Annuity
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Pre-tax operating results of WM Life Re	The following table summarizes the pre-tax operating results of WM Life Re for the three months ended March 31, 2015 and 2014.
		Three Months Ended
		March 31,
	Millions	2015			2014
	Fees, included in other revenue	$	3		$	5.4
	Change in fair value of variable annuity liability,	0.4			(1.3		)
	included in other revenue
	Change in fair value of derivatives, included in other revenue	(5.5		)	(6.6		)
	Foreign exchange, included in other revenue	(1.1		)	0.2
	Other investment income and (losses) gains	—			0.4
	Total revenue	(3.2		)	(1.9		)
	Change in fair value of variable annuity death benefit liabilities,	—			—
	included in other general and administrative expenses
	Death benefit claims paid, included in general and	—			—
	administrative expenses
	General and administrative expenses	(1.4		)	(1.3		)
	Pre-tax loss	$	(4.6	)	$	(3.2	)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three months ended March 31, 2015 and 2014 and the carrying values, included in other assets, at March 31, 2015 and December 31, 2014 by type of instrument:
		Gains (losses)						Carrying Value
		Three Months Ended						As of
		March 31,						March 31,				31-Dec-14
	Millions	2015			2014			2015
	Fixed income/interest rate	$	9.3		$	(6.4	)	$	3.7			$	(1.7	)
	Foreign exchange	(12.1		)	(6.5		)	35.3				44.1
	Equity	(2.7		)	6.3			8				14
	Total	$	(5.5	)	$	(6.6	)	$	47			$	56.4
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015
		Variable Annuity			Derivative Instruments
		Assets
	Millions	Level 3			Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	0.7		$	18.9		$	33.8			$	3.7			$	56.4
	Purchases	—			—			—				—				—
	Realized and unrealized gains (losses)	0.4			(.6		)	(6.5		)		1.6				(5.5		)
	Transfers in	—			—			—				—				—
	Sales/settlements	—			(1.3		)	3.9				(6.5		)		(3.9		)
	End of period	$	1.1		$	17		$	31.2			$	(1.2	)		$	47
		Three Months Ended March 31, 2014
		Variable Annuity			Derivative Instruments
		(Liabilities)
	Millions	Level 3			Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	(52.8	)	$	63.4		$	4.7			$	1.1			$	69.2
	Purchases	—			—			—				—				—
	Realized and unrealized gains (losses)	(1.3		)	0.7			(7.1		)		(.2		)		(6.6		)
	Transfers in	—			—			—				—				—
	Sales/settlements	—			—			27.5				(.5		)		27
	End of period	$	(54.1	)	$	64.1		$	25.1			$	0.4			$	89.6
	(1) Consists of over-the-counter instruments.
	(2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available.  Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
	(3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
Assets Held under Reinsurance Counterparty Trust Agreement	In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties.  The total collateral includes the following:
	Millions	March 31, 2015			December 31, 2014			March 31, 2014
	Cash	$	29		$	23.7		$	22.2
	Fixed maturity investments	4.5			9.5			14.3
	Total	$	33.5		$	33.2		$	36.5
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:
	($ in Millions)	March 31, 2015
	Description	Fair			Valuation Technique(s)		Unobservable Input	Range					Weighted
		Value											Average
	Variable annuity benefit guarantee liabilities	$	1.1		Discounted cash flows		Surrenders
							0-1 year	0.1	%	-	40.00%		40	%
							1-2 years	0.2	%	-	40.00%		32.2	%
							Mortality	0	%	-	6.40%		1	%
							Foreign exchange volatilities
							0-1 year	10.9	%	-	16.20%		13.1	%
							1-2 years	10.9	%	-	14.10%		12.5	%
							Index volatilities
							0-1 year	24.8	%	-	27.20%		26.7	%
							1-2 years	21.2	%	-	23.90%		22.9	%
	Foreign exchange options	$	11.1		Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	0.5	%	-	11.60%		5.9	%
	Equity index options	$	5.9		Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	(0.7	)%	-	6.50%		2.1	%
Exchange traded and OTC derivative instruments	The following summarizes amounts offset under master netting agreements:
		March 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	$	3.4		$	(.9	)	$	2.5			$	1			$	(5.4	)	$	(4.4	)
	Exchange traded	1.1			(.3		)	0.8				2.8				(.1		)	2.7
	Foreign exchange contracts
	OTC	38.9			—			38.9				45.5				—			45.5
	Exchange traded	—			(3.6		)	(3.6		)		—				(1.4		)	(1.4		)
	Equity contracts
	OTC	7.3			(.4		)	6.9				11.7				(.2		)	11.5
	Exchange traded	1.6			(.1		)	1.5				3.4				(.9		)	2.5
	Total(2)	$	52.3		$	(5.3	)	$	47			$	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.
Net exposure to collateral	The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets:
		31-Mar-15
	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 WMLife Re - Cash		Net amount of exposure to counter-party		Standard
																										& Poor's
																										Rating(1)
	Bank of America	$	4.9		$	—		$	—			$	4.9			$	—		$	—		$	—	$	4.9	A
	Barclays	—			—			—				—				—			—			—		—		A
	JP Morgan	21.3			—			—				21.3				—			—			7.4		13.9		A	+
	Royal Bank	3.8			—			—				3.8				—			—			—		3.8		A	-
	of Scotland
	Nomura	(.4		)	0.4			—				—				5.6			4.5			—		10.1		BBB	+
	Citigroup - OTC	18.6			—			—				18.6				0.7			—			—		19.3		A
	Citigroup -	(1.2		)	1.2			—				—				21.1			—			—		21.1		A
	Exchange Traded
	Total	$	47		$	1.6		$	—			$	48.6			$	27.4		$	4.5		$	7.4	$	73.1
		31-Dec-14
	Millions	Net amount of assets reflected in Balance Sheet			Collateral provided to counter-party - Cash			Collateral provided to counter-party - Financial Instruments				Net amount of exposure after effect of collateral provided				Excess collateral provided to counter-party- Cash			Excess collateral provided - Financial Instruments			Counter-party collateral held by WMLife Re- Cash		Net amount of exposure to counter-party		Standard
																										& Poor's
																										Rating(1)
	Bank of America	$	5.6		$	—		$	—			$	5.6			$	—		$	—		$	—	$	5.6	A
	Barclays	0.1			—			—				0.1				—			—			—		0.1		A
	JP Morgan	24.3			—			—				24.3				—			—			8.8		15.5		A	+
	Royal Bank	4			—			—				4				—			—			—		4		A
	of Scotland
	Nomura	(3.5		)	3.5			—				—				1.7			9.5			—		11.2		BBB	+
	Citigroup - OTC	22.2			—			—				22.2				—			—			1.1		21.1		A
	Citigroup -	3.7			—			—				3.7				16			—			—		19.7		A
	Exchange Traded
	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-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and
	“BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Schedule of uncollateralized amounts due	The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts:
		31-Mar-15								31-Dec-14
	Millions	Notional Amount			Carrying Value			Standard & Poor's		Notional Amount				Carrying Value
								Rating(1)
	Barclays Bank Plc	$	6.3		$	—		A		$	2.1			$	—
	Deutsche Bank	4.4			0.1			A		—				—
	Goldman Sachs	10.9			—			A		8.7				—
	HSBC Bank Plc	21.7			—			AA-		11.2				—
	JP Morgan	7.3			(.1		)	A+		5.7				—
	Royal Bank of Canada	5.9			—			AA-		5.4				—
	Total	$	56.5		$	—				$	33.1			$	—
	(1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the eighth highest of twenty-three creditworthiness ratings).

Municipal_Bond_Guarantee_Table

Municipal Bond Guarantee (Tables)	3 Months Ended
	Mar. 31, 2015
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table provides a schedule of BAM’s insured obligations:
		31-Mar-15		31-Dec-14
	Contracts outstanding	2,029		1,750
	Remaining weighted average contract period outstanding (in years)	12.8		12.8
	Contractual debt service outstanding (in millions):
	Par	$	14,421.60	$	12,362.50
	Interest	8,044.70		7,086.90
	Total debt service outstanding	$	22,466.30	$	19,449.40
	Gross unearned insurance premiums	$	31.2	$	27.6

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	3 Months Ended
	Mar. 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 three months ended March 31, 2015 and 2014 (see Note 17 for earnings per share amounts for discontinued operations):
		Three Months Ended
		March 31,
		2015			2014
	Basic and diluted earnings per share numerators (in millions):
	Net income from continuing operations attributable to	$	76.4		$	96.1
	White Mountains’s common shareholders
	Allocation of income for unvested restricted common shares	(.7		)	(1.1		)
	Dividends declared on participating restricted common shares(1)	(.1		)	(.1		)
	Total allocation to restricted common shares	(.8		)	(1.2		)
	Net income attributable to White Mountains’s common shareholders,	$	75.6		$	94.9
	net of restricted common share amounts
	Undistributed net earnings (in millions):
	Net income attributable to White Mountains’s common shareholders,	$	75.6		$	94.9
	net of restricted common share amounts
	Dividends declared net of restricted common share amounts(1)	(5.9		)	(6.1		)
	Total undistributed net earnings, net of restricted common share amounts	$	69.7		$	88.8
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted shares(2)	(58.8		)	(70.4		)
	Basic earnings per share denominator	5,919.60			6,099.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(2)	(58.8		)	(70.4		)
	Average outstanding dilutive options to acquire common shares(3)	—			—
	Diluted earnings per share denominator	5,919.60			6,099.50
	Basic earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	12.77		$	15.56
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	11.77		$	14.56
	Diluted earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	12.77		$	15.56
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	11.77		$	14.56
	(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.
	(2) Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 15).
	(3) The diluted earnings per share denominator for the three months ended March 31, 2015 and 2014 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.

Noncontrolling_Interest_Tables

Non-controlling Interest (Tables)	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Line Items]
Noncontrolling Interest Disclosure [Text Block]	The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of March 31, 2015 and December 31, 2014:
		31-Mar-15					31-Dec-14
	$ in millions	Non-controlling Percentage		Non-controlling Equity			Non-controlling Percentage		Non-controlling Equity
	OneBeacon Ltd.	24.7	%	$	258.8		24.7	%	$	258.4
	SIG Preference Shares	100		250			100		250
	Other, excluding mutuals and reciprocals
	HG Global	3.1		17.7			3.1		17.9
	MediaAlpha	40		22			40		22.6
	Tranzact	36.8		81.8			36.8		88.2
	Wobi	36.7		4.7			36.7		5.4
	Prospector Offshore Fund	32.4		31			23.4		31.1
	Dewar	20.3		3			18		3.4
	Total other, excluding mutuals and reciprocals			160.2					168.6
	Mutuals and reciprocals
	BAM	100		(127.0		)	100		(121.9		)
	SSIE	100		(13.5		)	100		(12.4		)
	Total non-controlling interests			$	528.5				$	542.7

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Financial information for White Mountains' segments	Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows:
								HG Global/BAM
	Millions	OneBeacon			Sirius			HG		BAM			Other Operations			Total
					Group
	Three Months Ended March 31, 2015
	Earned insurance and reinsurance premiums	$	286.6		$	205		$	0.5	$	0.2		$	2		$	494.3
	Net investment income	10.5			8			0.4		1			3.4			23.3
	Net investment income (loss) - surplus note	—			—			4		(4.0		)	—			—
	interest
	Net realized and unrealized investment gains	14.2			79.9			1.1		3			5.2			103.4
	Other (loss) revenue	(3.1		)	(27.4		)	—		0.1			66.2			35.8
	Total revenues	308.2			265.5			6		0.3			76.8			656.8
	Losses and LAE	165.9			97.5			—		—			2			265.4
	Insurance and reinsurance acquisition expenses	51			42.6			0.1		0.8			1.1			95.6
	Other underwriting expenses	55.9			25.5			—		0.1			—			81.5
	General and administrative expenses	4.1			6.2			0.4		8.2			97.5			116.4
	Interest expense	3.2			6.8			—		—			1.1			11.1
	Total expenses	280.1			178.6			0.5		9.1			101.7			570
	Pre-tax income (loss)	$	28.1		$	86.9		$	5.5	$	(8.8	)	$	(24.9	)	$	86.8
								HG Global/BAM
	Millions	OneBeacon			Sirius			HG		BAM			Other Operations			Total
					Group
	Three Months Ended March 31, 2014
	Earned insurance and reinsurance premiums	$	276.5		$	215.5		$	0.2	$	0.1		$	1.3		$	493.6
	Net investment income	10.4			8.7			0.3		1.4			3.9			24.7
	Net investment income (loss) - surplus note	—			—			3.9		(3.9		)	—			—
	interest
	Net realized and unrealized investment gains	18.9			28.1			0.7		3			13.1			63.8
	Other revenue (loss)	1			(5.3		)	—		0.2			0.7			(3.4		)
	Total revenues	306.8			247			5.1		0.8			19			578.7
	Losses and LAE	149.4			78.8			—		—			1.1			229.3
	Insurance and reinsurance acquisition expenses	46.7			47.4			0.1		0.4			0.5			95.1
	Other underwriting expenses	49.4			31.8			—		0.1			0.1			81.4
	General and administrative expenses	3.3			8.4			0.4		8.9			29.3			50.3
	Interest expense	3.2			6.6			—		—			0.3			10.1
	Total expenses	252			173			0.5		9.4			31.3			466.2
	Pre-tax income (loss)	$	54.8		$	74		$	4.6	$	(8.6	)	$	(12.3	)	$	112.5

Investments_in_Unconsolidated_1

Investments in Unconsolidated Affiliates (Tables)	3 Months Ended
	Mar. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Investments in and Advances to 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.
	Millions	March 31,			December 31,
		2015			2014
	Symetra common shares	$	378.9		$	373.8
	Unrealized gains from Symetra’s fixed maturity portfolio	68.4			37.6
	Carrying value of Symetra common shares	447.3			411.4
	Hamer	3.5			3
	Total investments in unconsolidated affiliates	$	450.8		$	414.4
Schedule of Changes in Investments in Unconsolidated Affiliates	The following table summarizes amounts recorded by White Mountains relating to its investment in Symetra for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
	Millions	2015			2014
	Carrying value of investment in Symetra at beginning of period	$	411.4		$	317.3
	Equity in earnings (1)(2)	7.2			14.4
	Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio(3)	30.9			39.3
	Dividends received	(2.2		)	(2.0		)
	Carrying value of investment in Symetra at end of period(4)(5)	$	447.3		$	369
	(1) Equity in earnings excludes tax expense of $0.4 and $1.0.
	(2) Equity in earnings includes $0.7 and $0.7 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(3) Net unrealized gains includes $2.9 and $3.1 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $68.4 and $39.3 as of March 31, 2015 and 2014, which excludes tax expense of $4.6 and $2.8.
	(5) The aggregate value of White Mountains’s investment in common shares of Symetra was $470.3 based upon the quoted market price of $23.46 per share at March 31, 2015.

Employee_ShareBased_Incentive_1

Employee Share-Based Incentive Compensation Plans (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of performance share activity	The following table summarizes performance share activity for the three months ended March 31, 2015 and 2014 for performance shares granted under the WTM Incentive Plan:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	123,549		$	48.1		119,220		$	60.2
	Shares paid or expired(1)	(37,977	)	(27.3		)	(37,130	)	(26.7		)
	New grants	30,640		—			45,660		—
	Assumed forfeitures and cancellations(2)	184		0.6			(213	)	0.4
	Expense recognized	—		6.2			—		8.4
	Ending of period	116,396		$	27.6		127,537		$	42.3
	(1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target.  WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.
	(2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan	The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at March 31, 2015 for each performance cycle:
	Millions, except share amounts	Target		Accrued
		Performance		Expense
		Shares
		Outstanding
	Performance cycle:
	2015 – 2017	30,640		$	1.8
	2014 – 2016	41,937		5.6
	2013 – 2015	46,803		20.9
	Sub-total	119,380		28.3
	Assumed forfeitures	(2,984	)	(.7		)
	Total at March 31, 2015	116,396		$	27.6
Summary of restricted share activity	The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	83,314		$	14.3		94,130		$	17
	Issued	22,890		15.2			20,400		11.8
	Vested	(35,079	)	—			(33,205	)	—
	Forfeited	—		—			—		—
	Expense recognized	—		(3.0		)	—		(4.0		)
	Non-vested at March 31,	71,125		$	26.5		81,325		$	24.8
Summary of performance share activity for OneBeacon performance shares granted under OneBeacon Incentive Plan	The following table summarizes performance share activity for the three months ended March 31, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	517,470		$	3.4		493,421		$	4
	Shares paid or expired(1)	(181,290	)	(1.5		)	(142,138	)	(1.0		)
	New grants	154,887		—			165,800		—
	Assumed forfeitures and cancellations(2)	(28,920	)	—			(4,145	)	—
	Expense recognized	—		0.2			—		0.6
	Ending of period	462,147		$	2.1		512,938		$	3.6
	(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.
	(2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of OneBeacon performance shares outstanding and accrued expense for OneBeacon performance shares awarded under the OneBeacon Incentive Plan	The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at March 31, 2015 for each performance cycle:
	Millions, except share amounts	Target		Accrued Expense
		Performance
		Shares
		Outstanding
	Performance cycle:
	2015 – 2017	154,887		$	0.2
	2014 – 2016	151,810		0.5
	2013 – 2015	167,300		1.4
	Sub-total	473,997		2.1
	Assumed forfeitures	(11,850	)	—
	Total at March 31, 2015	462,147		$	2.1
Summary of OneBeacon restricted shares activity	The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31,
		2015					2014
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	612,500		$	3.5		915,000		$	6.5
	Issued	75,950		1.1			—		—
	Vested	(296,000	)	—			(300,000	)	—
	Forfeited	(1,500	)	—			(2,500	)	—
	Expense recognized	—		(.6		)	—		(.8		)
	Non-vested at March 31,	390,950		$	4		612,500		$	5.7

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table summarizes the fair value and carrying value of financial instruments as of March 31, 2015 and December 31, 2014:
		March 31, 2015				December 31, 2014
	Millions	Fair		Carrying		Fair		Carrying
		Value		Value		Value		Value
	2012 OBH Senior Notes	$	291.5	$	274.7	$	286	$	274.7
	SIG Senior Notes	438.2		399.7		437.8		399.7
	SIG Preference Shares	261		250		260		250

Discontinued_Operations_Tables

Discontinued Operations (Tables)	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet And Additional Disclosures	The following table outlines the computation of earnings per share for discontinued operations for the three months ended March 31, 2015 and 2014:
		Three Months Ended
		March 31,
		2015			2014
	Basic and diluted earnings per share numerators (in millions):
	Net income attributable to White Mountains’s common shareholders	$	7.9		$	(.5	)
	Allocation of income for participating unvested restricted common shares(1)	(.1		)	—
	Net income attributable to White Mountains’s common shareholders,	$	7.8		$	(.5	)
	net of restricted common share amounts (2)
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(3)	(58.8		)	(70.4		)
	Basic earnings per share denominator	5,919.60			6,099.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	5,978.40			6,169.90
	Average unvested restricted common shares(3)	(58.8		)	(70.4		)
	Average outstanding dilutive options to acquire common shares(4)	—			—
	Diluted earnings per share denominator	5,919.60			6,099.50
	Basic and diluted earnings per share (in dollars):	$	1.32		$	(.08	)
	(1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.
	(2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three months ended March 31, 2015 and 2014.
	(3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 15).
	(4) The diluted earnings per share denominator for the three months ended March 31, 2015 and 2014 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.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $)	3 Months Ended	12 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Jul. 31, 2012
Basis of Presentation
Assets held for sale	$58	$58.10
OneBeacon Ltd.
Basis of Presentation
Cumulative Effect on Retained Earnings, Net of Tax	1.2	0.5
Ownership interest (as a percent)	75.30%
Noncontrolling equity percentage	24.70%	24.70%
HG Global
Basis of Presentation
Surplus Notes				503
Percentage of par value of policy reinsured	15.00%
Noncontrolling equity percentage	3.10%	3.10%
Star & Shield Insurance Exchange
Basis of Presentation
Surplus Notes	19
Noncontrolling equity percentage	100.00%	100.00%
White Mountains Advisors L L C [Member]
Basis of Presentation
Investment Advisory, Management and Administrative Fees	2		4.4
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%
Noncontrolling equity percentage	96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%
Noncontrolling equity percentage	88.40%
Wholly Owned Properties [Member] | OneBeacon Ltd.
Basis of Presentation
Assets held for sale	58	58.1
Investment Income [Member] | OneBeacon Ltd.
Basis of Presentation
Cumulative-Effect Adjustment, Gross Gains	0.4
Tax Expense [Member] | OneBeacon Ltd.
Basis of Presentation
Cumulative-Effect Adjustment, Gross Gains	$0.60

Significant_Transactions_Detai

Significant Transactions (Details)

3 Months Ended

3 Months Ended

2 Months Ended

1 Months Ended

3 Months Ended

2 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 14, 2014

Feb. 19, 2014

Mar. 31, 2015

Jan. 31, 2014

Mar. 14, 2014

Feb. 23, 2015

Feb. 19, 2014

Mar. 31, 2015

Mar. 31, 2014

Feb. 19, 2014

Feb. 23, 2015

Feb. 19, 2014

Feb. 19, 2014

Feb. 23, 2015

Feb. 19, 2014

Feb. 23, 2015

Feb. 19, 2014

USD ($)

USD ($)

USD ($)

USD ($)

Star & Shield Insurance Exchange

QuoteLab

QuoteLab

QuoteLab

QuoteLab

WOBI

WOBI

Star & Shield LLC [Member]

Common Class A

Common Stock

Common Stock

Non-controlling interest

Non-controlling interest

Israel, New Shekels

Israel, New Shekels

Israel, New Shekels

United States of America, Dollars

United States of America, Dollars

United States of America, Dollars

Cashboard [Member]

WOBI

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

QuoteLab

Cashboard [Member]

WOBI

Star & Shield Insurance Exchange

Star & Shield Insurance Exchange

Preferred Stock

Common Stock

Common Stock

Preferred Stock

Common Stock

Common Stock

USD ($)

USD ($)

business

USD ($)

USD ($)

WOBI

Cashboard [Member]

WOBI

WOBI

Cashboard [Member]

WOBI

ILS

ILS

ILS

USD ($)

USD ($)

USD ($)

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

Business Combination, Contingent Consideration, Liability

$7.90

$7.90

Ownership interest (as a percent)

63.30%

60.00%

56.20%

54.00%

Payments to Acquire Businesses and Interest in Affiliates

2.4

32.2

28.1

1.8

31.5

9.5

14.4

9

2.4

4.1

Premiums Written, Percentage of Total

85.00%

Number of lines of business

4

Purchase of Surplus Notes

19

Percentage of gross profit

62.50%

Assets

10,507.80

10,455.70

16

70.1

5.5

13.4

Goodwill

169.2

23.8

168.9

0

18.3

5.5

0.3

Intangible Assets, Net (Excluding Goodwill)

193.4

62.4

197.5

20.7

2.8

2.9

Goodwill and intangible assets

362.6

366.4

38.5

Liabilities

5,968

5,917.30

29.5

10

0.7

1.2

Net Income (Loss) Attributable to Noncontrolling Interest

$7.30

$0.60

$1.10

$4.60

Loss_and_Loss_Adjustment_Expen2

Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$3,159.80	$3,079.30
Less beginning reinsurance recoverable on unpaid losses	-483.9	-428.1
Net loss and LAE reserves	2,675.90	2,651.20
Loss and LAE reserves acquired		13.6
Loss and LAE reserves consolidated — SSIE	0
Loss and LAE incurred relating to current year losses	266.8	240.5
Net (favorable) and unfavorable loss reserve development	-1.4	-11.2
Total incurred losses and LAE	265.4	229.3
Foreign currency translation adjustment to net loss and LAE reserves	-22.4	1.8
Current year losses	-40.6	-35
Prior year losses	-280.2	-237.7
Total loss and LAE payments	-320.8	-272.7
Net ending balance	2,598.10	2,623.20
Plus ending reinsurance recoverable on unpaid losses	436.9	425.2
Gross ending balance	$3,035	$3,048.40

Loss_and_Loss_Adjustment_Expen3

Loss and Loss Adjustment Expense Reserves Details (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	($1.40)	($11.20)
Sirius Group
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	0.5	-9.8
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	-0.1
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	($1.80)	($1.40)

Third_Party_Reinsurance_One_Be

Third Party Reinsurance (One Beacon) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	29.8	$23.60
Reinsurance recoverable on unpaid losses	436.9	483.9
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	18.2	12.2
Reinsurance recoverable on unpaid losses	141.6	$161.60
Crop insurance [Member] | OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	52.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	98.00%
Percentage of Quota Share Reinsurance Agreement	30.00%
Crop-hail insurance [Member] | OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	80.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	100.00%

Third_Party_Reinsurance_Sirius

Third Party Reinsurance (Sirius Group) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$29.80	$23.60
Reinsurance recoverable on unpaid losses	436.9	483.9
Sirius Group
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	11.6	11.4
Reinsurance recoverable on unpaid losses	$295.30	$322.20

Investment_Securities_Net_Inve

Investment Securities (Net Investment Income) (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Investment income
Total investment income	$27.10	$29.50
Third-party investment expenses	-3.8	-4.8
Net investment income, pre-tax	23.3	24.7	24.7
Fixed maturity investments.
Investment income
Total investment income	23.1	22.4
Short-term investments
Investment income
Total investment income	0.6	0.6
Common Stock
Investment income
Total investment income	3.4	5.4
Convertible fixed maturity investments
Investment income
Total investment income	0.2	0.4
Other long-term investments
Investment income
Total investment income	-0.1	0.7
Funds Held under Reinsurance Agreements [Member]
Investment income
Total investment income	($0.10)	$0

Investment_Securities_Net_Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$52.80	$21.70
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	50.6	42.1
Net Realized and Unrealized Gain (Loss) on Trading Securities	103.4	63.8
Income tax expense attributable to net realized and unrealized investment gains	-26.3	-15.7
Realized and unrealized gain	77.1	48.1
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	31	0.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	52.3	31.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	3.5	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	25.5	18.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	-3.4	4.5
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	-4.2	2.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.8	0.7
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	-3	-0.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.9	5.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	0	-0.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$0	$0

Investment_Securities_Net_Real1

Investment Securities (Net Realized Investment Gains (Losses)) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$25.90	$25.30
Trading securities, tax on realized holding gain (loss) on investments	-6.5	-4.6
Trading securities realized holding gain (loss) on investments after tax	19.4	20.7
Foreign Currency Transaction Gain (Loss), Realized	26.9	-3.6
Trading securities, realized foreign currency transaction gain (loss) t	-6.7	0.9
Trading securities, realized foreign currency gain (loss) net of tax	20.2	-2.7
Trading Securities, Realized Gain (Loss), Total	52.8	21.7
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	-13.2	-3.7
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	39.6	18
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	9.8	4.4
Foreign Currency Transaction Gain (Loss), Realized	21.2	-3.5
Trading Securities, Realized Gain (Loss), Total	31	0.9
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Realized	3.5	0
Trading Securities, Realized Gain (Loss), Total	3.5	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	24.4	18.9
Foreign Currency Transaction Gain (Loss), Realized	1.1	-0.1
Trading Securities, Realized Gain (Loss), Total	25.5	18.8
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	-4.2	2.4
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	-4.2	2.4
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	-4.1	-0.3
Foreign Currency Transaction Gain (Loss), Realized	1.1	0
Trading Securities, Realized Gain (Loss), Total	-3	-0.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0	-0.1
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	$0	($0.10)

Investment_Securities_Net_Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	$9.50	$29
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	-2.8	-9.1
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	6.7	19.9
Foreign Currency Transaction Gain (Loss), Unrealized	41.1	13.1
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	-10.3	-2.9
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	30.8	10.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	50.6	42.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	-13.1	-12
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	37.5	30.1
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	12.1	19.4
Foreign Currency Transaction Gain (Loss), Unrealized	40.2	12.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	52.3	31.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	-4.2	4
Foreign Currency Transaction Gain (Loss), Unrealized	0.8	0.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	-3.4	4.5
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.7	0.7
Foreign Currency Transaction Gain (Loss), Unrealized	0.1	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.8	0.7
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.9	4.9
Foreign Currency Transaction Gain (Loss), Unrealized	0	0.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.9	5.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$0	$0

Investment_Securities_Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	($5.20)	$7.10
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	-0.7	0.2
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	-1.8	0.8
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.1	0
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	($2.80)	$6.10

Investment_Securities_Investme1

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) (USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$4,771.90	$4,784.30
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	165.3	184.7
Trading Securities, Unrealized Holding Gain	0.3	0.1
Trading Securities, Unrealized Holding Loss	-0.1	-0.3
Net foreign currency gains (losses)	0.2	3.6
Fixed maturity investments, at fair value	165.7	188.1
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,103.80	2,221.30
Trading Securities, Unrealized Holding Gain	46.7	45.2
Trading Securities, Unrealized Holding Loss	-1	-5.1
Net foreign currency gains (losses)	66.2	49.8
Fixed maturity investments, at fair value	2,215.70	2,311.20
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	96	82
Trading Securities, Unrealized Holding Gain	1.8	1.4
Trading Securities, Unrealized Holding Loss	-0.2	-0.2
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	97.6	83.2
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,049.30	1,811.10
Trading Securities, Unrealized Holding Gain	11.9	7.6
Trading Securities, Unrealized Holding Loss	-2.6	-3.5
Net foreign currency gains (losses)	39.5	25.7
Fixed maturity investments, at fair value	2,098.10	1,840.90
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	106.9	274.6
Trading Securities, Unrealized Holding Gain	1.2	4.2
Trading Securities, Unrealized Holding Loss	-0.2	-1
Net foreign currency gains (losses)	0.9	-2.7
Fixed maturity investments, at fair value	108.8	275.1
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	79.5	79.6
Trading Securities, Unrealized Holding Gain	6.3	6.1
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0.2	0.1
Fixed maturity investments, at fair value	86	85.8
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,600.80	4,653.30
Trading Securities, Unrealized Holding Gain	68.2	64.6
Trading Securities, Unrealized Holding Loss	-4.1	-10.1
Net foreign currency gains (losses)	107	76.5
Total fixed maturity investments including assets held for sale	$4,771.90	$4,784.30

Investment_Securities_Investme2

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$4,771.90	$4,784.30
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	623.3	633.6
Trading Securities, Unrealized Holding Gain	163.4	175.1
Trading Securities, Unrealized Holding Loss	-8.8	-5.2
Net foreign currency gains (losses)	9	-1.9
Fixed maturity investments, at fair value	786.9	801.6
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	13.1	19.1
Trading Securities, Unrealized Holding Gain	1.8	0.9
Trading Securities, Unrealized Holding Loss	-0.3	-0.2
Net foreign currency gains (losses)	1.2	0.7
Fixed maturity investments, at fair value	15.8	20.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	339.8	343.3
Trading Securities, Unrealized Holding Gain	74.7	73
Trading Securities, Unrealized Holding Loss	-13.1	-10.9
Net foreign currency gains (losses)	1.2	1.6
Fixed maturity investments, at fair value	$402.60	$407

Investment_Securities_Other_lo

Investment Securities (Other long-term investments) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$402.60	$407
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.8	5.2
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	242.5	242.9
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	68	65.1
Limited Liability Companies and Private Equity Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	66.6	69.7
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16.2	16.8	18.5	19.1
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$5.50	$7.30

Investment_Securities_Hedge_Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$402.60	$407
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	12
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments	31
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Largest single fund investment	$29.40

Investment_Securities_Fair_Val

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$92.10
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	150.4
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	242.5	242.9
Unfunded Commitments	70.7	81.7
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.3	24.2
Unfunded Commitments	4.4	5.3
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	51.3	59.6
Unfunded Commitments	10	11
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2	3.6
Unfunded Commitments	0.1	3.3
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8	8.5
Unfunded Commitments	3.2	3.1
Private equity funds | Private Equity Funds Multi Sector Europe
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.2	1.5
Unfunded Commitments	2.1	2.3
Private equity funds | Private Equity Funds Manufacturing and Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	30.5	23.2
Unfunded Commitments	2.9	7.3
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8.7	6.1
Unfunded Commitments	1.3	2.8
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.1	2.1
Unfunded Commitments	41.3	41.2
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.9	20.7
Unfunded Commitments	5.1	5.1
Private equity funds | Venture Capital Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.4	1.4
Unfunded Commitments	0.3	0.3
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	150.4	150.9
Unfunded Commitments	70.7	81.7
Hedge funds | Hedge Funds, Equity Long (Short) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	44.3	43.9
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Credit and Distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20	21.4
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity R E I T
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	21	20.3
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity Activist
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.6	6.2
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Bank Loan
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.2	0.2
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	92.1	92
Unfunded Commitments	$0	$0

Investment_Securities_Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) (Hedge funds, USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Distributions from Inactive Hedge Funds	$2.10
Other long-term investments	92.1
Redemption requests	6.9
30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	36.3
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	43
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.7
120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8.1
Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.5
Monthly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.5
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	59.8
Quarterly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	31.8
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.1
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	7.9
Semi-annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.9
Semi-annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.9
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.9
Annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.7
Annual | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$0.20

Investment_Securities_Investme3

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	($7.30)	($0.60)
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	150.4
1-3 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	29.7
3-5 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	32.4
5-10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.1
Greater than 10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.2
Prospector Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage	68.00%
Equity Method Investment, Aggregate Cost	64.7		64.9
Variable Interest Entity, Consolidated, Carrying Amount, Assets	136.1		135.8
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	40.4		39.8
Net Income (Loss) Attributable to Noncontrolling Interest	$31		$31.10

Investment_Securities_Fair_Val1

Investment Securities (Fair Value Measurements) (Details) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$1
Fair value, investment-related liabilities	$38.80	$38

Investment_Securities_Fair_Val2

Investment Securities (Fair Value Measurement by Level) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Other Investments	402.6	407
Short-term investments, at amortized cost (which approximates fair value)	929.2	871.7
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	805	843.3	1,565.60	1,376.70
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,594	4,638	4,727.50	4,982.20
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,215.70	2,311.20
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,098.10	1,840.90
Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,030.40	1,840.90
Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	67.7	0
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	139.1	76.7	112.9	93
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	32.6	40.2	47.2	46.1
Convertible fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.9	8.2	6.1	6.1
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	382.6	385	277.4	262.4
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6,886.40	6,863.10
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,731.90	1,712.10
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,596.30	4,640.90
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	558.2	510.1
Fair value measured on a recurring basis | U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	165.7	188.1
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	110.8	134.1
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	54.9	54
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,215.70	2,311.20
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,215.70	2,305.60
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	5.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	538.6	575.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	538.6	575.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	252	225.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	252	219.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	5.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	398.5	431.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	398.5	431.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	203.9	214.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	203.9	214.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	267.3	284.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	267.3	284.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	193.7	177.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	193.7	177.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	106.7	118
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	106.7	118
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	159.9	173.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	159.9	173.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	95.1	103
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	95.1	103
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	8.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	8.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	97.6	83.2
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	97.6	83.2
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,098.10	1,840.90
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,030.40	1,840.90
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	67.7	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	108.8	275.1
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.5	21.3
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.3	253.8
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	86	85.8
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.6	14.7
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	71.4	71.1
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,771.90	4,784.30
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	129.3	155.4
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,503.50	4,552.20
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	139.1	76.7
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	929.2	871.7
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	926.8	868.8
Short-term investments, at amortized cost (which approximates fair value)	929.2	871.7	674	635.9
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.4	2.9
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	170	161.2
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	170	161.1
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0.1
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70	67.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70	67.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair value measured on a recurring basis | Common equity securities | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	226.8	237.5
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.2	197.3
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	32.6	40.2
Fair value measured on a recurring basis | Common equity securities | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	28.4	32.9
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	28.4	32.9
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	89.7	101.9
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	89.7	101.9
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Common equity securities | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	27.7	32.6
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	27.7	32.6
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	54.9	54.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	54.9	54.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.4	9.5
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.4	9.4
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0.1
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21.7	21.2
Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21.7	21.2
Fair value measured on a recurring basis | Common equity securities | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	88.3	83
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.8	9.7
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78.5	73.3
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	786.9	801.6
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	675.8	687.9
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78.5	73.5
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	32.6	40.2
Fair value measured on a recurring basis | Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	15.8	20.5
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.9	12.3
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.9	8.2
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	382.6	385
Carrying value of investment accounted for using the equity method	3.8
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	382.6	385
Carrying value of investment accounted for using the equity method		5.2	5.7	6.8
Segment, Discontinued Operations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale			222.2	236.3
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16.2	16.8	18.5	19.1
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$16.20	$16.80

Investment_Securities_Debt_sec

Investment Securities (Debt securities issued by corporation) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,215.70	2,311.20
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.1	236.9
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	818.2	957.8
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,188.80	1,105.90
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.2	0
Other | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$8.40	$10.60

Investment_Securities_Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$292.20	$236.90

Investment_Securities_Mortgage1

Investment Securities (Mortgage-backed Asset Securities Table) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	165.6
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	373	411.2
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.2	36.6
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.9	49.6
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	447.1	497.4
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.3	131.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	292.2	236.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	486.5	368.1
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	933.6	865.5
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	472.9	522.2
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	537.2	289.4
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	154.4	163.8
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,164.50	975.4
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,098.10	1,840.90
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,594	4,638	4,727.50	4,982.20
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	373	411.2
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.2	36.6
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.9	49.6
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	447.1	497.4
Level 2 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	147.2	131.2
Level 2 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	271.6	236.9
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	418.8	368.1
Level 2 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	865.9	865.5
Level 2 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	472.9	522.2
Level 2 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	537.2	289.4
Level 2 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	154.4	163.8
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,164.50	975.4
Level 2 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,030.40	1,840.90
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47.1	0
Level 3 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.6	0
Level 3 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	67.7	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	67.7	0
Level 3 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$67.70	$0
Maximum | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans	1.00%

Investment_Securities_Nonagenc

Investment Securities (Non-agency Mortgage-backed Securities) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	165.6
Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.6
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.3	131.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	292.2	236.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	486.5	368.1
Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35
Securities Issued in 2004 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35
Securities Issued in 2005 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13.7
Securities Issued in 2005 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	13.7
Securities Issued In 2006 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.3
Securities Issued In 2006 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.4
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17.7
Securities Issued In 2007 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	15.8
Securities Issued In 2008 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	15.8
Securities Issued In 2009 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2010 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57.7
Securities Issued In 2010 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	11.2
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	68.9
Securities Issued In 2011 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.1
Securities Issued In 2011 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	20.1
Securities Issued in 2012 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.6
Securities Issued in 2012 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26.1
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	30.7
Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	28.2
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	88.6
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	116.8
Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	9.9
Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	122.3
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	132.2
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.6
Securities Issued in 2015 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	35.6
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70.8
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	126.6
Subordinate | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Subordinate | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	197.4
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.6
Super Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Super Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	49.3
Super Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.6
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	76.2
Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	145
Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$76.20

Investment_Securities_Nonagenc1

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Non-Agency Residential Mortgage [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	85.4
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.3	131.2
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	49.3
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	145
Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	186.4
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	48.3
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	138.1
Prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.9
Non-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.9
Non-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1
Sub-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1
Sub-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$0

Investment_Securities_Nonagenc2

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$5,957.20	$5,991.40	$6,736.70	$6,766.50
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	292.2	236.9
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.6
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	76.2
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$197.40

Rollforward_of_Fair_Value_Meas

(Rollforward of Fair Value Measurements by Level) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	$5,991,400,000	$6,766,500,000	$6,766,500,000
Total realized and unrealized gains (losses)	101,400,000	64,400,000
Foreign currency gains (losses) through OCI	-129,600,000	-13,800,000
Amortization/Accretion	-10,100,000	-11,300,000
Purchases	1,348,500,000	1,225,600,000
Sales	-1,344,400,000	-1,299,300,000
Net change in investments related to (sales) purchases of consolidated affiliates		4,600,000
Transfers in	5,600,000	0
Transfers out	-5,600,000	0
Balance at March 31, 2015	5,957,200,000	6,736,700,000	5,991,400,000
Other Investments	402,600,000		407,000,000
Realized and unrealized gain	77,100,000	48,100,000
Net realized and unrealized investment gains	103,400,000	63,800,000	63,800,000
Short-term investments, at amortized cost (which approximates fair value)	929,200,000		871,700,000
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers	-5.00%
Maximum price difference provided by pricing services	1,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	843,300,000	1,376,700,000	1,376,700,000
Total realized and unrealized gains (losses)	18,300,000	22,200,000
Foreign currency gains (losses) through OCI	-9,600,000	-2,900,000
Amortization/Accretion	-100,000	-200,000
Purchases	222,900,000	352,600,000
Sales	-269,800,000	-180,100,000
Net change in investments related to (sales) purchases of consolidated affiliates		-2,700,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	805,000,000	1,565,600,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	4,638,000,000	4,982,200,000	4,982,200,000
Total realized and unrealized gains (losses)	87,300,000	34,700,000
Foreign currency gains (losses) through OCI	-115,600,000	-10,500,000
Amortization/Accretion	-10,000,000	-11,100,000
Purchases	1,047,700,000	840,400,000
Sales	-1,059,000,000	-1,115,500,000
Net change in investments related to (sales) purchases of consolidated affiliates		7,300,000
Transfers in	5,600,000	0
Transfers out	0	0
Balance at March 31, 2015	4,594,000,000	4,727,500,000
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	76,700,000	93,000,000	93,000,000
Total realized and unrealized gains (losses)	300,000	400,000
Foreign currency gains (losses) through OCI	-900,000	-100,000
Amortization/Accretion	0	0
Purchases	68,600,000	19,600,000
Sales	0	0
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	-5,600,000	0
Balance at March 31, 2015	139,100,000	112,900,000
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	40,200,000	46,100,000	46,100,000
Total realized and unrealized gains (losses)	1,800,000	800,000
Foreign currency gains (losses) through OCI	0	0
Amortization/Accretion	0	0
Purchases	0	300,000
Sales	-9,400,000	0
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	32,600,000	47,200,000
Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	8,200,000	6,100,000	6,100,000
Total realized and unrealized gains (losses)	-4,300,000	0
Foreign currency gains (losses) through OCI	0	0
Amortization/Accretion	0	0
Purchases	0	0
Sales	0	0
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	3,900,000	6,100,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	385,000,000	262,400,000	262,400,000
Total realized and unrealized gains (losses)	-2,000,000	6,300,000
Foreign currency gains (losses) through OCI	-3,500,000	-300,000
Amortization/Accretion	0	0
Purchases	9,300,000	12,700,000
Sales	-6,200,000	-3,700,000
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	382,600,000	277,400,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	6,886,400,000		6,863,100,000
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	1,731,900,000		1,712,100,000
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	4,596,300,000		4,640,900,000
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	558,200,000		510,100,000
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	4,771,900,000		4,784,300,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	129,300,000		155,400,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	4,503,500,000		4,552,200,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	139,100,000		76,700,000
Fair value measured on a recurring basis | Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	15,800,000		20,500,000
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	0		0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	11,900,000		12,300,000
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	3,900,000		8,200,000
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	382,600,000		385,000,000
Carrying value of investment accounted for using the equity method	3,800,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	382,600,000		385,000,000
Carrying value of investment accounted for using the equity method		5,700,000	5,200,000	6,800,000
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	929,200,000		871,700,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	926,800,000		868,800,000
Short-term investments, at amortized cost (which approximates fair value)	929,200,000	674,000,000	871,700,000	635,900,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	2,400,000		2,900,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	0		0
Segment, Discontinued Operations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale		222,200,000		236,300,000
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	1,000,000		700,000
Forward Contracts | Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Carrying value of investment accounted for using the equity method		-100,000		-100,000
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16,200,000	18,500,000	16,800,000	19,100,000
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16,200,000		16,800,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	$3.50

Investment_Securities_Signific

Investment Securities (Significant Unobservable Inputs) (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Quantitative information for Level 3 Fair Value Measurements Assets
Number of Investments with Significant Unobseravble Inputs	1
Conditional Prepayment Rate	12.00%
loss severity [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs, Discount Rate	50.00%
Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs, Discount Rate	11.30%	11.00%
NR | Preferred Stock | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	71,400,000	71,100,000
NR | Preferred Stock | Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs, Discount Rate	7.10%	7.10%
NR | Private equity funds | Multiple of GAAP book value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	32,600,000	40,200,000
Book Value Multiple to Calculate Fair Value	110.00%	110.00%
NR | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	20,100,000	20,100,000
NR | Private equity funds | Average Share Price [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Average Share Price Used As Unobservable Input	1.06	1.06
NR | Convertible Preferred Stock [Member] | Multiple of EBITA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	3,900,000	3,800,000
Average Share Price Used As Unobservable Input	6	6
Unobservable Inputs	0
NR | Convertible Preferred Stock [Member] | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		4,500,000
Average Share Price Used As Unobservable Input		0.71
NR | Seller Priority Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	46,500,000	44,000,000
Fair Value Inputs, Discount Rate	9.40%	9.30%
Timing of interest payments	5 years	5 years
Timing of principal payments	10 years	10 years
NR | Pari Passu Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	21,600,000	21,100,000
Fair Value Inputs, Discount Rate	14.40%	13.50%
Timing of interest payments	5 years	5 years
Timing of principal payments	15 years	10 years
Standard & Poor's, A- Rating | Non-Agency Commercial Mortgage Backed Securities [Member] [Member] | Broker Pricing [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	20,600,000
Standard & Poor's, A- Rating | Non-Agency Residential Mortgage [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	47,100,000
durchblicker.at [Member] | NR | Private equity funds | Recent Transaction Two [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	9,200,000	10,400,000
Average Share Price Used As Unobservable Input	290.96	290.96
BE Reinsurance [Member] | NR | Private equity funds | Recent Transaction Three [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	14,200,000	15,800,000
Average Share Price Used As Unobservable Input	0.13	0.13
Centrus [Member] | NR | Debt securities issued by corporations | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		5,600,000
Unobservable Inputs		0.1
Minimum | discount spread [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	0.45%
Minimum | Default Rate [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Constant Default Rate	2.50%
Maximum | discount spread [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	1.80%
Maximum | Default Rate [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Constant Default Rate	8.75%
Standard & Poor's, B Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	2.50%
Standard & Poor's, CCC Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	2.50%

Goodwill_and_Other_Intangible_2

Goodwill and Other Intangible Assets (Details)	3 Months Ended					2 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Feb. 23, 2015	Feb. 23, 2015	Feb. 23, 2015	Feb. 23, 2015
	USD ($)	USD ($)	USD ($)	USD ($)	Cashboard [Member]	Common Stock	Common Stock	Cashboard [Member]
					Common Stock	Israel, New Shekels	United States of America, Dollars	USD ($)
						Cashboard [Member]	Cashboard [Member]
						ILS	USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Ownership interest (as a percent)					56.20%
Payments to Acquire Businesses and Interest in Affiliates	$2.40	$32.20				9.5	$2.40
Assets	10,507.80		10,455.70					5.5
Goodwill	169.2	23.8	168.9	0				0.3
Intangible Assets, Net (Excluding Goodwill)	193.4	62.4	197.5	20.7				2.8
Liabilities	$5,968		$5,917.30					$1.20

Goodwill_and_Other_Intangible_3

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$169.20	$23.80	$168.90	$0
Intangible Assets, Net (Excluding Goodwill)	193.4	62.4	197.5	20.7
Goodwill, Acquired During Period	0.3	23.8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	2.8	42.4
Disposition of Goodwill through disposition of business	0	-0.4
Amortization of Intangible Assets	($6.90)	($0.30)

Debt_Details

Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Aug. 14, 2013	Oct. 10, 2014	Nov. 25, 2014
Debt Instrument
Total debt	$749,800,000		$746,600,000
Repayments of Debt	1,300,000	0
Draw down of debt and revolving line of credit	4,500,000	0
2012 OBH Senior Notes
Debt Instrument
Total debt	291,500,000		286,000,000
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility				425,000,000
Other Debt [Member]
Debt Instrument
Total debt	4,800,000		4,800,000
OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	275,000,000		275,000,000
Unamortized original issue discount	-300,000		-300,000
Total debt	274,700,000		274,700,000
Sirius Group | SIG Senior Notes
Debt Instrument
Debt instrument at face value	400,000,000		400,000,000
Unamortized original issue discount	-300,000		-300,000
Total debt	399,700,000		399,700,000
White Mountains | WTM Bank Facility
Debt Instrument
Total debt	0		0
Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
Line of Credit Facility, Amount Outstanding	71,900,000		68,700,000
Unamortized Debt Issuance Expense	-1,300,000		-1,300,000
Line of Credit Facility, Fair Value of Amount Outstanding	70,600,000		67,400,000		70,000,000
Total commitment under revolving credit facility					74,500,000
Line of Credit Facility, Remaining Borrowing Capacity					4,500,000
Repayments of Debt	1,300,000
Draw down of debt and revolving line of credit	4,500,000
Sirius International [Member] | Letter of Credit [Member]
Debt Instrument
number of Credit Lines						2
Line of Credit Facility, Fair Value of Amount Outstanding						200,000,000
Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility						125,000,000
Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility						75,000,000
Unsecured Debt [Member] | Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility						100,000,000
Unsecured Debt [Member] | Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility						$25,000,000

Income_Taxes_Details

Income Taxes (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Jul. 17, 2013
Income Tax Contingency [Line Items]
Effective tax rates (as a percent)	28.80%	27.50%
U.S. statutory rate (as a percent)	35.00%	35.00%
Income Tax Expense (Benefit)	$25	$30.90
Tax Assessment Year 2007 to 2009 | Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Total tax assessment			73.8
Tax assessments related to disallowed deductions			$60.20

Derivatives_Details

Derivatives (Details)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
In Billions, unless otherwise specified	USD ($)	JPY (¥)	USD ($)	JPY (¥)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Notional amount	$1	¥ 117.8	$1.10	¥ 134.2
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%	113.00%	113.00%	113.00%

Derivatives_PreTax_Operating_R

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Pre-tax operating results of WM Life Re
Total revenues	$656.80	$578.70	$578.70
General and administrative expenses	-116.4	-50.3	-50.3
Pre-tax income from continuing operations	86.8	112.5	112.5
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	3	5.4
Change in fair value of variable annuity liability, included in other revenue	0.4	-1.3
Realized and unrealized gains (losses)	-5.5	-6.6
Foreign exchange, included in other revenue	-1.1	0.2
Other investment income and (losses) gains	0	0.4
Total revenues	-3.2	-1.9
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	0
Death benefit claims paid, included in general and administrative expenses	0	0
General and administrative expenses	-1.4	-1.3
Pre-tax income from continuing operations	($4.60)	($3.20)

Derivatives_Realized_and_Unrea

Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	($5.50)	($6.60)
Derivative, fair value, net	47	89.6	56.4	69.2
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	9.3	-6.4
Derivative, fair value, net	3.7		-1.7
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	-12.1	-6.5
Derivative, fair value, net	35.3		44.1
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	-2.7	6.3
Derivative, fair value, net	$8		$14

Derivatives_Variable_Annuity_R

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Changes in derivative instruments
Beginning balance	$56.40	$69.20
Purchases	0	0
Realized and unrealized gains (losses)	-5.5	-6.6
Transfers in	0	0
Sales/settlements	3.9	-27
Ending balance	47	89.6
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	0
Level 2 Inputs
Changes in derivative instruments
Beginning balance	33.8	4.7
Purchases	0	0
Realized and unrealized gains (losses)	-6.5	-7.1
Transfers in	0	0
Sales/settlements	-3.9	-27.5
Ending balance	31.2	25.1
Level 1 Inputs
Changes in derivative instruments
Beginning balance	3.7	1.1
Purchases	0	0
Realized and unrealized gains (losses)	1.6	-0.2
Transfers in	0	0
Sales/settlements	6.5	0.5
Ending balance	-1.2	0.4
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance	0.7	-52.8
Realized and unrealized gains (losses)	0.4	-1.3
Transfers in	0	0
Sales/settlements	0	0
Ending balance	1.1	-54.1
Purchases	0	0
Derivative [Member] | Level 3 Inputs
Changes in derivative instruments
Beginning balance	18.9	63.4
Purchases	0	0
Realized and unrealized gains (losses)	-0.6	0.7
Transfers in	0	0
Sales/settlements	1.3	0
Ending balance	$17	$64.10

Derivatives_Additional_Collate

Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Millions, unless otherwise specified
Credit risk on derivative instruments
Collateral cash and investments	$33.50	$33.20	$36.50
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	29	23.7	22.2
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$4.50	$9.50	$14.30

Derivatives_Level_3_Derivative

Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	1.1
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	0.00%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	6.40%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	1.00%
Foreign Exchange Option [Member] | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	11.1
Foreign Exchange Option [Member] | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	0.50%
Foreign Exchange Option [Member] | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	11.60%
Foreign Exchange Option [Member] | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	5.90%
Equity Index Options | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	5.9
Equity Index Options | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	-0.70%
Equity Index Options | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	6.50%
Equity Index Options | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	2.10%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.10%
Foreign exchange volatilities (as a percent)	10.90%
Index volatilities (as a percent)	24.80%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	16.20%
Index volatilities (as a percent)	27.20%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	13.10%
Index volatilities (as a percent)	26.70%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.20%
Foreign exchange volatilities (as a percent)	10.90%
Index volatilities (as a percent)	21.20%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	14.10%
Index volatilities (as a percent)	23.90%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	32.20%
Foreign exchange volatilities (as a percent)	12.50%
Index volatilities (as a percent)	22.90%

Derivatives_Value_Collateral_h

Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) (White Mountains Life Re, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Offsetting Liabilities [Line Items]
Derivative Assets	$52.30	$64.40
Derivative Liabilities	-5.3	-8
Net amounts recognized in Other assets	47	56.4
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	3.4	1
Derivative Liabilities	-0.9	-5.4
Net amounts recognized in Other assets	2.5	-4.4
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	38.9	45.5
Derivative Liabilities	0	0
Net amounts recognized in Other assets	38.9	45.5
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	7.3	11.7
Derivative Liabilities	-0.4	-0.2
Net amounts recognized in Other assets	6.9	11.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	1.1	2.8
Derivative Liabilities	-0.3	-0.1
Net amounts recognized in Other assets	0.8	2.7
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0	0
Derivative Liabilities	-3.6	-1.4
Net amounts recognized in Other assets	-3.6	-1.4
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.6	3.4
Derivative Liabilities	-0.1	-0.9
Net amounts recognized in Other assets	$1.50	$2.50

Derivatives_Uncollateralized_A

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Rating
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$47	$56.40
Collertal provided to counterparty in cash	1.6	3.5
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	48.6	59.9
Excess collateral provided to counterparty in cash	27.4	17.7
Derivative, Collateral, Right to Reclaim Securities	4.5	9.5
Counter-party collateral held by WMLife Re - Cash	7.4	9.9
Net amount of exposure to counter-party	73.1	77.2
S and P Credit Ratings	23
Standard & Poor's, A Rating | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	4.9	5.6
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	4.9	5.6
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	4.9	5.6
Standard & Poor's, A Rating | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0	0.1
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0.1
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	0	0.1
Standard & Poor's, A Rating | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	18.6	22.2
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	18.6	22.2
Excess collateral provided to counterparty in cash	0.7	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	1.1
Net amount of exposure to counter-party	19.3	21.1
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	-1.2	3.7
Collertal provided to counterparty in cash	1.2	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	3.7
Excess collateral provided to counterparty in cash	21.1	16
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	21.1	19.7
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	21.3	24.3
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	21.3	24.3
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	7.4	8.8
Net amount of exposure to counter-party	13.9	15.5
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	-0.4	-3.5
Collertal provided to counterparty in cash	0.4	3.5
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0
Excess collateral provided to counterparty in cash	5.6	1.7
Derivative, Collateral, Right to Reclaim Securities	4.5	9.5
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	10.1	11.2
Standard & Poor's, A- Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	3.8	4
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	3.8	4
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	$3.80	$4

Derivatives_Forward_Contracts_

Derivatives Forward Contracts (Details)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	USD ($)	JPY (¥)	USD ($)	JPY (¥)	Forward Contracts	Forward Contracts	Forward Contracts	Forward Contracts
					USD ($)	USD ($)	SEK	USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$1,000	¥ 117,800	$1,100	¥ 134,200	$56.50		489.3	$33.10
Realized and unrealized gains (losses)					0	-0.1
Carrying Value					$0			$0

Derivatives_Forward_Contract_C

Derivatives (Forward Contract Collateral) (Details)

Mar. 31, 2015

Mar. 31, 2015

Dec. 31, 2014

Dec. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

In Millions, unless otherwise specified

USD ($)

JPY (¥)

USD ($)

JPY (¥)

Forward Contracts

Forward Contracts

Forward Contracts

Interest Rate Cap

Standard & Poor's, A Rating

Standard & Poor's, A Rating

Standard & Poor's, A Rating

Standard & Poor's, A Rating

Standard & Poor's, A Rating

Standard & Poor's, A Rating

Standard & Poor's, A plus

Standard & Poor's, A plus

Standard & Poor's, AA- Rating

Standard & Poor's, AA- Rating

Standard & Poor's, AA- Rating

Standard & Poor's, AA- Rating

USD ($)

SEK

USD ($)

Rating

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Forward Contracts

Rating

Barclays

Barclays

Deutsche Bank

Deutsche Bank

Goldman Sachs

Goldman Sachs

JP Morgan

JP Morgan

HSBC

HSBC

Royal Bank of Canada

Royal Bank of Canada

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Derivative [Line Items]

Notional amount

$1,000

¥ 117,800

$1,100

¥ 134,200

$56.50

489.3

$33.10

$6.30

$2.10

$4.40

$0

$10.90

$8.70

$7.30

$5.70

$21.70

$11.20

$5.90

$5.40

Carrying Value

$0

$0

$0

$0

$0.10

$0

$0

$0

($0.10)

$0

$0

$0

$0

$0

S and P Credit Ratings

23

23

23

Derivatives_Interest_rate_cap_

Derivatives (Interest rate cap) (Details) (USD $)	3 Months Ended		1 Months Ended		3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	31-May-07	Jul. 31, 2013	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013
	Rating
Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim	$3.10
Derivative, fair value, net	3.1	8.2				4.1	11.1
Realized and unrealized gains (losses)	-1	-2.9
S and P Credit Ratings	23
Sirius International Group Ltd | Preferred Stock
Derivative [Line Items]
Preferred Stock, Dividend Rate, Percentage			7.51%
Term of LIBOR loan			3 months
Basis points			3.20%
Sirius International Group Ltd | Interest Rate Cap
Derivative [Line Items]
Derivative, Cap Interest Rate							8.30%
Derivative, Cost of Hedge				9.9
Basis points	3.95%
years of interest rate cap					5 years
Sirius International Group Ltd | Interest Rate Cap | Preferred Stock
Derivative [Line Items]
Non-cumulative perpetual preference shares issued			250,000,000
Standard & Poor's, A Rating | Barclays | Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim	2.1
Standard & Poor's, AA- Rating | Nordea Bank Finland | Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim	$1

Derivatives_Weather_Derivative

Derivatives (Weather Derivatives) (Details) (Weather derivatives, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Weather derivatives
Derivative [Line Items]
Realized and unrealized gains (losses)	$0.80	$0.40

Derivatives_Tranzact_Interest_

Derivatives (Tranzact Interest Rate Swap) (Details)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Mar. 31, 2015	Dec. 31, 2014	Oct. 10, 2014	Mar. 31, 2015	Mar. 31, 2015	Oct. 10, 2014
In Millions, unless otherwise specified	USD ($)	JPY (¥)	USD ($)	JPY (¥)	Tranzact [Member]	Tranzact [Member]	Tranzact [Member]	Minimum	Interest Rate Swap [Member]	Interest Rate Swap [Member]
					Tranzact Bank Facility [Member]	Tranzact Bank Facility [Member]	Tranzact Bank Facility [Member]	Tranzact [Member]	Tranzact [Member]	Tranzact [Member]
					USD ($)	USD ($)	USD ($)	Tranzact Bank Facility [Member]	USD ($)
Derivative [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding					$70.60	$67.40	$70
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate					4.17%
LIBOR rate								4.00%
Notional amount	1,000	117,800	1,100	134,200	67.4
Derivative, Fixed Interest Rate									1.34%
Derivative, Variable Interest Rate									0.18%	0.15%
Debt Instrument, Interest Rate, Effective Percentage									5.30%
Derivative, fair value, net									($0.50)

Municipal_Bond_Guarantee_Detai

Municipal Bond Guarantee (Details) (USD $)	3 Months Ended		12 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Jul. 31, 2012
	Y		Contract
	Contract
BAM
Guarantor Obligations [Line Items]
Municipal bond contract	2,029		1,750
Insured Financial Obligations, Remaining Weighted Average Contract Period	12 years 10 months 0 days		12 years 10 months 0 days
Municipal bond outstanding principal	$14,421.60		$12,362.50
Municipal bond outstanding interest	8,044.70		7,086.90
Municipal bond debt service outstanding	22,466.30		19,449.40
Interest income (expense), surplus notes	4	3.9
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	31.2		27.6
Year of U.S. Treasury Rate	1
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution				594.5
Surplus Notes				503
Income (Loss) from Continuing Operations before Income Taxes, Domestic	5.4	4.6
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	-8.7	-8.6
Interest income (expense), surplus notes	-4	-3.9
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	453
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution				$14.50
Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points	3.00%
Derivative, Fixed Interest Rate	3.15%		3.13%
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate	5
Combination Two | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate	3
Financial Guarantee [Member]
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%
Fixed interest rate [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate	8.00%

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$76.40	$96.10
Allocation of income for unvested restricted common shares	-0.7	-1.1
Dividends declared on participating restricted common shares	-0.1	-0.1
Total allocation to restricted common shares	-0.8	-1.2
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	75.6	94.9
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	75.6	94.9
Dividends declared net of restricted common share amounts	-5.9	-6.1
Total undistributed net earnings, net of restricted common share amounts	69.7	88.8
Basic earnings per share denominators
Total average common shares outstanding during the period	5,978,400	6,169,900
Average unvested restricted common shares	-58,800	-70,400
Basic earnings per share denominator	5,919,600	6,099,500
Diluted earnings per share denominator
Total average common shares outstanding during the period	5,978,400	6,169,900
Average unvested restricted common shares	-58,800	-70,400
Average outstanding dilutive options to acquire common shares	0	0
Diluted earnings (loss) per share denominator	5,919,600	6,099,500
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains' common shareholders	$12.77	$15.56
Dividends declared	($1)	($1)
Undistributed earnings (loss) (in dollars per share)	$11.77	$14.56
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$12.77	$15.56
Dividends declared	($1)	($1)
Undistributed earnings (loss) (in dollars per share)	$11.77	$14.56
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000
Segment, Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	($0.10)	$0
Basic earnings per share denominators
Total average common shares outstanding during the period	5,978,400	6,169,900
Average unvested restricted common shares	-58,800	-70,400
Basic earnings per share denominator	5,919,600	6,099,500
Diluted earnings per share denominator
Total average common shares outstanding during the period	5,978,400	6,169,900
Average unvested restricted common shares	-58,800	-70,400
Average outstanding dilutive options to acquire common shares	0	0
Diluted earnings (loss) per share denominator	5,919,600	6,099,500
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		125,000

Noncontrolling_Interest_Detail

Non-controlling Interest (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Noncontrolling Interest [Line Items]
Total non-controlling interests	$528.50	$542.70
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	160.2	168.6
OneBeacon Ltd.
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.70%	24.70%
Total non-controlling interests	258.8
OneBeacon
Noncontrolling Interest [Line Items]
Total non-controlling interests	258.8	258.4
SIG Preference Shares
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	250	250
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	17.7	17.9
QuoteLab
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	22	22.6
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.80%	36.80%
Total non-controlling interests	81.8	88.2
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.70%	36.70%
Total non-controlling interests	4.7	5.4
Prospector Offshore Fund Ltd [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	32.40%	23.40%
Total non-controlling interests	31	31.1
A W G Dewar Inc [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	20.30%	18.00%
Total non-controlling interests	3	3.4
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	-127	-121.9
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	($13.50)	($12.40)

Segment_Information_Details

Segment Information (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment reporting information
Net investment income	$23,300,000	$24,700,000	$24,700,000
Earned insurance and reinsurance premiums	494,300,000	493,600,000	493,600,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	103,400,000	63,800,000	63,800,000
Other revenue	35,800,000	-3,400,000	-3,400,000
Total revenues	656,800,000	578,700,000	578,700,000
Losses and LAE	265,400,000	229,300,000	229,300,000
Insurance and Reinsurance Acquisition Expenses	95,600,000	95,100,000	95,100,000
Other underwriting expenses	81,500,000	81,400,000	81,400,000
General and administrative expenses	116,400,000	50,300,000	50,300,000
Interest expense	11,100,000	10,100,000	10,100,000
Total expenses	570,000,000	466,200,000	466,200,000
Pre-tax income (loss)	86,800,000	112,500,000	112,500,000
OneBeacon
Segment reporting information
Net investment income	10,500,000		10,400,000
Earned insurance and reinsurance premiums	286,600,000		276,500,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	14,200,000		18,900,000
Other revenue	-3,100,000		1,000,000
Total revenues	308,200,000		306,800,000
Losses and LAE	165,900,000		149,400,000
Insurance and Reinsurance Acquisition Expenses	51,000,000		46,700,000
Other underwriting expenses	55,900,000		49,400,000
General and administrative expenses	4,100,000		3,300,000
Interest expense	3,200,000		3,200,000
Total expenses	280,100,000		252,000,000
Pre-tax income (loss)	28,100,000		54,800,000
Sirius Group
Segment reporting information
Net investment income	8,000,000		8,700,000
Earned insurance and reinsurance premiums	205,000,000		215,500,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	79,900,000		28,100,000
Other revenue	-27,400,000		-5,300,000
Total revenues	265,500,000		247,000,000
Losses and LAE	97,500,000		78,800,000
Insurance and Reinsurance Acquisition Expenses	42,600,000		47,400,000
Other underwriting expenses	25,500,000		31,800,000
General and administrative expenses	6,200,000		8,400,000
Interest expense	6,800,000		6,600,000
Total expenses	178,600,000		173,000,000
Pre-tax income (loss)	86,900,000		74,000,000
HG Global
Segment reporting information
Net investment income	400,000		300,000
Earned insurance and reinsurance premiums	500,000		200,000
Net investment income (loss) - surplus note interest	4,000,000		3,900,000
Net realized and unrealized investment gains (losses)	1,100,000		700,000
Other revenue	0		0
Total revenues	6,000,000		5,100,000
Losses and LAE	0		0
Insurance and Reinsurance Acquisition Expenses	100,000		100,000
Other underwriting expenses	0		0
General and administrative expenses	400,000		400,000
Interest expense	0		0
Total expenses	500,000		500,000
Pre-tax income (loss)	5,500,000		4,600,000
BAM
Segment reporting information
Net investment income	1,000,000		1,400,000
Earned insurance and reinsurance premiums	200,000		100,000
Net investment income (loss) - surplus note interest	-4,000,000		-3,900,000
Net realized and unrealized investment gains (losses)	3,000,000		3,000,000
Other revenue	100,000		200,000
Total revenues	300,000		800,000
Losses and LAE	0		0
Insurance and Reinsurance Acquisition Expenses	800,000		400,000
Other underwriting expenses	100,000		100,000
General and administrative expenses	8,200,000		8,900,000
Interest expense	0		0
Total expenses	9,100,000		9,400,000
Pre-tax income (loss)	-8,800,000		-8,600,000
Other Operations
Segment reporting information
Net investment income	3,400,000		3,900,000
Earned insurance and reinsurance premiums	2,000,000		1,300,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	5,200,000		13,100,000
Other revenue	66,200,000		700,000
Total revenues	76,800,000		19,000,000
Losses and LAE	2,000,000		1,100,000
Insurance and Reinsurance Acquisition Expenses	1,100,000		500,000
Other underwriting expenses	0		100,000
General and administrative expenses	97,500,000		29,300,000
Interest expense	1,100,000		300,000
Total expenses	101,700,000		31,300,000
Pre-tax income (loss)	($24,900,000)		($12,300,000)

Investments_in_Unconsolidated_2

Investments in Unconsolidated Affiliates (Investment in Unconsolidated Affiliates Table) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$450.80	$414.40
Symetra
Investments in unconsolidated affiliates
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates	68.4	37.6
Symetra | Common Stock
Investments in unconsolidated affiliates
Symetra Common Shares excluding unrealized gains (losses)	378.9	373.8
Total investments in unconsolidated affiliates	447.3	411.4	369	317.3
Hamer, LLC
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$3.50	$3

Investments_in_Unconsolidated_3

Investments in Unconsolidated Affiliates (Symetra) (Details) (Symetra, USD $)	3 Months Ended	12 Months Ended	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2013	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2011	Dec. 31, 2012
Investments in unconsolidated affiliates
Shares of Symetra Financial Corporation owned (in shares)	20.05			20.05
Ownership interest (as a percent)	17.27%			17.31%
Equity Method Investments Fair Value Per Share					$15
Term of amortization (in years)		30 years
Weight Average Years Remaining	28 years
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$165.70			$170.40
Common Stock
Investments in unconsolidated affiliates
Fair value of investment						261
Share Price	$23.46
equity in earnings of unconsolidated affiliates
Investments in unconsolidated affiliates
After tax amortization through equity in earnings	0.7		0.7
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	35.1			36.4
Net unrealized gains (losses) of unconsolidated affiliates
Investments in unconsolidated affiliates
After tax amortization through equity in net unrealized losses	2.8		2.9
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$130.60			$134

Investments_in_Unconsolidated_4

Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	$414.40
Equity in earnings	7.3	13.8
Carrying value at the end of the period	450.8
Income Tax Expense (Benefit)	25	30.9
Symetra | Common Stock
Investments in unconsolidated affiliates
Share Price	$23.46
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	411.4	317.3
Equity in earnings	7.2	14.4
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	30.9	39.3
Dividends received	-2.2	-2
Carrying value at the end of the period	447.3	369
Aggregate value of White Mountains common shares of Symetra at quoted market price	470.3
White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	68.4	39.3
equity in earnings of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	0.4	1
Net unrealized gains (losses) of unconsolidated affiliates | White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	4.6	2.8
Amortization of common share basis difference | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings	0.7	0.7
impairment of net unrealized gains (losses) | Net unrealized gains (losses) of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	$2.90	$3.10

Employee_ShareBased_Incentive_2

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $)	12 Months Ended	3 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Accrued Expense
Accrued expense at the end of the period	184.6	100.5
Performance Share | Performance Cycle 2011 to 2013
Accrued Expense
Vested (in shares)	3,570
Performance Share | W T M Incentive Plan | Performance Cycle 2011 to 2013
Accrued Expense
Percentage of target payout - minimum actual		91.00%
Percentage of target payout - maximum actual		145.50%
Performance Share | W T M Incentive Plan | Performance cycle 2010-2012
Accrued Expense
Percentage of target payout - minimum actual	88.00%
Percentage of target payout - maximum actual	131.50%
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	119,220	123,549	119,220
Shares paid or expired (in shares)		-37,977	-37,130
New grants (in shares)		30,640	45,660
Assumed forfeitures and cancellations		184	-213
Ending of period (in shares)		116,396	127,537
Accrued Expense
Accrued expense at the beginning of period	60.2	48.1	60.2
Payments and deferrals		-27.3	-26.7
Assumed forfeitures and cancellations		0.6	0.4
Expense recognized		6.2	8.4
Accrued expense at the end of the period		27.6	42.3
Vesting Period		3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested		33.1

Employee_ShareBased_Incentive_3

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$100.50	$184.60
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	119,380
Accrued Expense	28.3
Target Performance Shares Outstanding, Assumed forfeitures	-2,984
Accrued Expense, Assumed forfeitures	-0.7
Total Target Performance Shares Outstanding	116,396	123,549	127,537	119,220
Total Accrued Expense	27.6	48.1	42.3	60.2
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	30,640
Accrued Expense	1.8
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,937
Accrued Expense	5.6
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	46,803
Accrued Expense	$20.90

Employee_ShareBased_Incentive_4

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (White Mountains, W T M Incentive Plan, USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)	22,890
Restricted Share Cliff Vesting in January 2017
Target Restricted Share Activity
Issued (in shares)	20,400
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	83,314	94,130
Issued (in shares)	22,890	20,400
Vested (in shares)	-35,079	-33,205
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	71,125	81,325
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$14.30	$17
Issued	15.2	11.8
Assumed forfeitures and cancellations	0	0
Expense recognized	-3	-4
Unamortized grant date fair value, ending balance	$26.50	$24.80

Employee_ShareBased_Incentive_5

Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Expense Amortized
Accrued expense at the beginning of period			$184.60
Accrued expense at the end of the period	100.5		184.6
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
OneBeacon | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual	45.70%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Cycle 2011 to 2013
Expense Amortized
Percentage of target payout - minimum actual	37.10%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	517,470	493,421
Shares paid or expired (in shares)	-181,290	-142,138
New grants (in shares)	154,887	165,800
Assumed forfeitures and cancellations	-28,920	-4,145
Ending of period (in shares)	462,147	512,938
Expense Amortized
Accrued expense at the beginning of period	3.4	4
Payments and deferrals	-1.5	-1
Assumed forfeitures and cancellations	0	0
Expense recognized	0.2	0.6
Accrued expense at the end of the period	2.1	3.6
Additional compensation cost that would be recognized if all outstanding performance shares vested	3.3

Employee_ShareBased_Incentive_6

Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$100.50	$184.60
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	473,997
Incentive Compensation Payable before Forfeitures	2.1
Target Performance Shares Outstanding, Assumed forfeitures	-11,850
Accrued Expense, Assumed forfeitures	0
Total Target Performance Shares Outstanding	462,147	517,470	512,938	493,421
Accrued incentive compensation	2.1	3.4	3.6	4
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	154,887
Incentive Compensation Payable before Forfeitures	0.2
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	151,810
Incentive Compensation Payable before Forfeitures	0.5
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	167,300
Incentive Compensation Payable before Forfeitures	$1.40

Employee_ShareBased_Incentive_7

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) (OneBeacon, One Beacon Long Term Incentive Plan, USD $)	0 Months Ended		3 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 24, 2015	Mar. 01, 2012	Mar. 31, 2015	Mar. 31, 2014	31-May-11	Feb. 28, 2012	25-May-11
						Installment	Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)			612,500	915,000
Issued (in shares)	75,950	300,000	75,950	0
Vested (in shares)			-296,000	-300,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value			$0
Assumed forfeitures and cancellations			-1,500	-2,500
Ending of period (in shares)			390,950	612,500
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance			3.5	6.5
Issued			1.1	0
Assumed forfeitures and cancellations			0	0
Expense recognized			-0.6	-0.8
Unamortized grant date fair value, ending balance			4	5.7
Number of Installments in which the Shares Vest						2
Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)					630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest							4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years					35,000
Performance Share
Target Restricted Share Activity
Beginning of period (in shares)			517,470	493,421
Assumed forfeitures and cancellations			-28,920	-4,145
Ending of period (in shares)			462,147	512,938
Unamortized grant date fair value
Assumed forfeitures and cancellations			0	0
Expense recognized			0.2	0.6
Unamortized grant date fair value, ending balance			$3.30
Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares					35,000
Years Performance Shares will be Forfeited					5 years

Employee_ShareBased_Incentive_8

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) (Restricted Stock Units (RSUs) [Member], OneBeacon, One Beacon Long Term Incentive Plan, USD $)	0 Months Ended	3 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 24, 2015	Mar. 31, 2015
Restricted Stock Units (RSUs) [Member] | OneBeacon | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	207,848
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number		205,319
Expense recognized		$0.10

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$749.80	$746.60
2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	291.5	286
SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	438.2	437.8
SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	261	260
Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
Book Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	399.7	399.7
Book Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$250	$250

Discontinued_Operations_Detail

Discontinued Operations (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Discontinued Operations [Line Items}
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	$8	$0
Net loss from discontinued operations, net of tax	-7.9	-0.5
Other revenue	35.8	-3.4	-3.4
Segment, Discontinued Operations
Discontinued Operations [Line Items}
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	8
Net loss from discontinued operations, net of tax	-0.1	-0.5
OneBeacon Runoff [Member] | Segment, Discontinued Operations
Discontinued Operations [Line Items}
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	($0.10)	($0.50)

Discontinued_Operations_Earnin

Discontinued Operations - Earnings Per Share (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	($0.10)	($0.50)
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	-0.7	-1.1
Net Income (Loss) Available to Common Stockholders, Basic	84.3	95.5
Total average common shares outstanding during the period	5,978,400	6,169,900
Weighted Average Number of Shares, Restricted Stock	-58,800	-70,400
Basic earnings per share denominator	5,919,600	6,099,500
Average outstanding dilutive options to acquire common shares	0	0
Diluted earnings (loss) per share denominator	5,919,600	6,099,500
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000
Segment, Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	7.9	-0.5
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	-0.1	0
Net Income (Loss) Available to Common Stockholders, Basic	$7.80	($0.50)
Total average common shares outstanding during the period	5,978,400	6,169,900
Weighted Average Number of Shares, Restricted Stock	-58,800	-70,400
Basic earnings per share denominator	5,919,600	6,099,500
Average outstanding dilutive options to acquire common shares	0	0
Diluted earnings (loss) per share denominator	5,919,600	6,099,500
Earnings Per Share, Basic and Diluted	$1.32	($0.08)
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		125,000

Contingencies_Details

Contingencies (Details) (USD $)	3 Months Ended		1 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Oct. 31, 2011	Jun. 30, 2007	Oct. 07, 2011	Jan. 22, 2015
Loss Contingencies [Line Items]
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	$8	$0
Esurance
Loss Contingencies [Line Items]
Warranty agreement period			18 months
Non-solicitation agreement period			3 years
Percentage of fluctuation in loss reserves of transferred subsidiaries retained					90.00%
Tribune Company Litigation | Pending Litigation
Loss Contingencies [Line Items]
Proceeds from Sale, Maturity and Collection of Investments				39
Segment, Discontinued Operations
Loss Contingencies [Line Items]
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	8
OneBeacon Runoff [Member]
Loss Contingencies [Line Items]
Number of insurance holders						3
OneBeacon Runoff [Member] | Segment, Discontinued Operations
Loss Contingencies [Line Items]
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	($0.10)	($0.50)