White Mountains Insurance (WTM) 10-Q 28 Apr 14 2014 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2014	Apr. 28, 2014
Document and Entity Information [Abstract]	'	'
Entity Registrant Name	'WHITE MOUNTAINS INSURANCE GROUP LTD	'
Entity Central Index Key	'0000776867	'
Document Type	'10-Q	'
Document Period End Date	31-Mar-14	'
Amendment Flag	'false	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	6,174,386
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q1	'

CONSOLIDATED_BALANCE_SHEET

CONSOLIDATED BALANCE SHEET (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets	'	'
Fixed maturity investments, at fair value	$4,988.70	$5,030.50
Short-term investments, at amortized cost (which approximates fair value)	674	635.9
Common equity securities, at fair value	1,174.60	1,156.80
Convertible fixed maturity investments, at fair value	73.8	80.5
Other long-term investments	302.3	288.9
Total investments	7,213.40	7,192.60
Cash (restricted: $22.2 and $56.1)	338.6	382.8
Reinsurance recoverable on unpaid losses	425.2	428.1
Reinsurance recoverable on paid losses	15.2	25.4
Insurance and reinsurance premiums receivable	734.8	518.9
Funds held by ceding companies	93.8	106.3
Investments in unconsolidated affiliates	373.3	321.4
Deferred acquisition costs	190	174.7
Deferred tax asset	484.9	512.1
Ceded unearned insurance and reinsurance premiums	149.5	92.4
Accrued investment income	37	39.3
Accounts receivable on unsettled investment sales	19.9	12.1
Other assets	537.7	458.1
Assets held for sale	1,803.10	1,880.10
Total assets	12,416.40	12,144.30
Liabilities	'	'
Loss and loss adjustment expense reserves	3,048.40	3,079.30
Unearned insurance and reinsurance premiums	1,116.60	901.4
Variable annuity benefit guarantee	54.1	52.8
Debt	677.6	676.4
Deferred tax liability	353.9	356.2
Accrued incentive compensation	124.9	218.3
Ceded reinsurance payable	157.9	71.9
Funds held under insurance and reinsurance contracts	127.8	127.1
Accounts payable on unsettled investment purchases	80.9	20.5
Other liabilities	343.3	362.9
Liabilities held for sale	1,803.10	1,880.10
Total liabilities	7,888.50	7,746.90
White Mountains’s common shareholders’ equity	'	'
White Mountains’ common shares	6.2	6.2
Paid-in surplus	1,046.70	1,044.90
Retained earnings	2,880.80	2,802.30
Accumulated other comprehensive income (loss), after tax:	'	'
Equity in net unrealized losses from investments in Symetra common shares	-3.9	-40.4
Net unrealized foreign currency translation gains	80.5	88.4
Pension liability and other	4.2	4.1
Total White Mountains’s common shareholders’ equity	4,014.50	3,905.50
Non-controlling interests	'	'
Total non-controlling interests	513.4	491.9
Total equity	4,527.90	4,397.40
Total liabilities and equity	12,416.40	12,144.30
OneBeacon	'	'
Non-controlling interests	'	'
Total non-controlling interests	279.2	273.7
Sirius Group	'	'
Non-controlling interests	'	'
Total non-controlling interests	250	250
HG Global	'	'
Non-controlling interests	'	'
Total non-controlling interests	18.4	16.6
BAM	'	'
Non-controlling interests	'	'
Total non-controlling interests	-102.7	-97.6
Other Noncontrolling Interest [Member]	'	'
Non-controlling interests	'	'
Total non-controlling interests	$68.50	$49.20

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Cash (in dollars)	$22.20	$56.10
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	6,174,386	6,176,739
Common shares, outstanding shares	6,174,386	6,176,739

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, 2014	Mar. 31, 2013
Revenues:	'	'
Earned insurance and reinsurance premiums	$493.60	$495.40
Net investment income	24.3	28.5
Net realized and unrealized investment gains	63.8	75.2
Other revenue	-3.4	28.3
Total revenues	578.3	627.4
Expenses:	'	'
Loss and loss adjustment expenses	229.3	244.3
Insurance and reinsurance acquisition expenses	95.1	98.2
Other underwriting expenses	81.4	79.1
General and administrative expenses	50.3	43.8
Interest expense	10.1	10.2
Total expenses	466.2	475.6
Pre-tax income from continuing operations	112.1	151.8
Income tax expense	-30.3	-41.6
Net income from continuing operations	81.8	110.2
Net (loss) income from discontinued operations, net of tax	-0.5	0.5
Income before equity in earnings of unconsolidated affiliates	81.3	110.7
Equity in earnings of unconsolidated affiliates, net of tax	13.8	9.2
Net income	95.1	119.9
Net loss attributable to non-controlling interests	0.5	0.5
Net income attributable to White Mountains’s common shareholders	95.6	120.4
Other comprehensive income, net of tax:	'	'
Change in equity in net unrealized gains (losses) from investments in Symetra common shares, net of tax	36.5	-8
Change in foreign currency translation, pension liability and other	-7.8	-3.7
Comprehensive income	124.3	108.7
Comprehensive income attributable to non-controlling interests	0	0
Comprehensive income attributable to White Mountains’s common shareholders	$124.30	$108.70
Basic income (loss) per share	'	'
Continuing operations	$15.56	$19.10
Discontinued operations	($0.08)	$0.07
Total consolidated operations	$15.48	$19.17
Diluted income (loss) per share	'	'
Continuing operations	$15.56	$19.10
Discontinued operations	($0.08)	$0.07
Total consolidated operations	$15.48	$19.17
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	Noncontrolling interest	Total Equity
In Millions, unless otherwise specified
Balances at Dec. 31, 2012	'	$1,057.20	$2,542.70	$131.90	$3,731.80	$526.40	$4,258.20
Increase (Decrease) in Shareholders' Equity	'	'	'	'	'	'	'
Net income	119.9	0	120.4	0	120.4	-0.5	119.9
Net change in unrealized gains from investments in unconsolidated affiliates	-8	0	0	-8	-8	0	-8
Net change in foreign currency translation	-3.7	0	0	-3.7	-3.7	0	-3.7
Total comprehensive income (loss)	108.7	0	120.4	-11.7	108.7	-0.5	108.2
Dividends declared on common shares	'	0	-6.2	0	-6.2	0	-6.2
Dividends to noncontrolling interests	'	0	0	0	0	-5.2	-5.2
Repurchases and retirements of common shares	-79	-23.5	-55.5	0	-79	0	-79
Net contribution from (distributions to) noncontrolling interests	'	0	0	0	0	2.2	2.2
Amortization of restricted share awards	'	3.6	0	0	3.6	0.2	3.8
Balances at Mar. 31, 2013	'	1,037.30	2,601.40	120.2	3,758.90	523.1	4,282
Balances at Dec. 31, 2013	4,397.40	1,051.10	2,802.30	52.1	3,905.50	491.9	4,397.40
Increase (Decrease) in Shareholders' Equity	'	'	'	'	'	'	'
Net income	95.1	0	95.6	0	95.6	-0.5	95.1
Net change in unrealized gains from investments in unconsolidated affiliates	36.5	0	0	36.5	36.5	0	36.5
Net change in foreign currency translation	-7.8	0	0	-7.9	-7.9	0	-7.9
Net change in pension liability and other accumulated comprehensive items	'	0	0	0.1	0.1	0	0.1
Total comprehensive income (loss)	124.3	0	95.6	28.7	124.3	-0.5	123.8
Dividends declared on common shares	'	0	-6.2	0	-6.2	0	-6.2
Dividends to noncontrolling interests	'	0	0	0	0	-5.2	-5.2
Repurchases and retirements of common shares	-15.4	-4.5	-10.9	0	-15.4	0	-15.4
Issuances of common shares	'	2.1	0	0	2.1	0	2.1
Net contribution from (distributions to) noncontrolling 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	$4,527.90	$1,052.90	$2,880.80	$80.80	$4,014.50	$513.40	$4,527.90

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operations:	'	'
Net income	$95.10	$119.90
Charges (credits) to reconcile net income to net cash (used for) provided from operations:	'	'
Net realized and unrealized investment gains	-63.8	-75.2
Deferred income tax expense	21.7	25.8
Net loss (income) from discontinued operations	0.5	-0.5
Gain on sale of subsidiary - Citation and Essentia	-0.7	-23
Excess of fair value of acquired net assets over cost - American Fuji	0	-6.9
Undistributed equity in earnings from unconsolidated affiliates, net of tax	-13.8	-9.2
Other operating items:	'	'
Net change in loss and loss adjustment expense reserves	-44.6	-86.2
Net change in reinsurance recoverable on paid and unpaid losses	13.9	20.1
Net change in unearned insurance and reinsurance premiums	217.7	146.1
Net change in variable annuity benefit guarantee liabilities	1.3	-189.9
Net change in variable annuity benefit guarantee derivative instruments	-20.4	22.5
Net change in deferred acquisition costs	-15.3	4.5
Net change in funds held by ceding companies	12.6	25.2
Net change in ceded unearned premiums	-60.6	-52.4
Net change in funds held under reinsurance treaties	0.7	23.7
Net change in insurance and reinsurance premiums receivable	-212.4	-179.7
Net change in ceded reinsurance payable	78.6	75.7
Net change in restricted cash	33.9	61.3
Net change in other assets and liabilities, net	-77	-86
Net cash used for operations - continuing operations	-32.6	-184.2
Net cash used for operations - discontinued operations	-14.6	-40.7
Net cash used for operations	-47.2	-224.9
Cash flows from investing activities:	'	'
Net change in short-term investments	-61.5	26.3
Sales of fixed maturity and convertible fixed maturity investments	1,026.30	1,962.90
Maturities, calls and paydowns of fixed maturity and convertible fixed maturity investments	189.7	176.3
Sales of common equity securities	63.4	181.9
Distributions and redemptions of other long-term investments	23.9	20.9
Sales of consolidated and unconsolidated affiliates, net of cash sold	12.8	31.3
Funding of operational cash flows for discontinued operations	-14.6	-40.7
Purchases of other long-term investments	-13	-17.4
Purchases of common equity securities	-65	-143.3
Purchases of fixed maturity and convertible fixed maturity investments	-1,138.30	-1,742.50
Purchases of consolidated and unconsolidated affiliates, net of cash acquired	-32.2	-9.2
Net change in unsettled investment purchases and sales	52.6	-80.8
Net acquisitions of property and equipment	-0.5	-3.5
Net cash provided from investing activities - continuing operations	43.6	362.2
Net cash provided from investing activities - discontinued operations	14.6	40.7
Net cash provided from investing activities	58.2	402.9
Cash flows from financing activities:	'	'
Draw down of revolving line of credit	0	80
Repayment of revolving line of credit	0	-155
Payments on capital lease obligation	-1.3	-1.8
Cash dividends paid to the Company’s common shareholders	-6.2	-6.2
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders	-4.9	-5
Common shares repurchased	-10.9	-79
Collateral provided by interest rate cap counterparties	-2.6	0
Capital contributions from BAM members	4.8	0
Net cash used for financing activities - continuing operations	-21.1	-167
Net cash (used for) provided from financing activities - discontinued operations	0	0
Net cash used for financing activities	-21.1	-167
Effect of exchange rate changes on cash	-0.2	-0.6
Net change in cash during the period	-10.3	10.4
Cash balances at beginning of period (excludes restricted cash balances of $56.1 and $249.8)	326.7	212.6
Cash balances at end of period (excludes restricted cash balances of $22.2 and $188.5)	316.4	223
Supplemental cash flows information:	'	'
Interest paid	-12.8	-12.9
Net income tax refund (payment) to national governments	$3.80	($2.20)

CONSOLIDATED_STATEMENTS_OF_CAS1

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Restricted cash balances, beginning of period	$22.20	$56.10	$188.50	$249.80
Restricted cash balances, end of period	$22.20	$56.10	$188.50	$249.80

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2014
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, 2014 and December 31, 2013, White Mountains owned 75.3% and 75.2% of OneBeacon Ltd.’s outstanding common shares.
	As discussed further in Note 2, OneBeacon entered into a definitive agreement to sell its runoff business (the “Runoff Business”) in October 2012 (the “Runoff Transaction”).  Accordingly, the Runoff Business is presented as discontinued operations. The OneBeacon Runoff Business includes assets and liabilities that are principally related to non-specialty commercial lines and certain other runoff business that it no longer writes, including nearly all of its asbestos and environmental reserves. Assets and liabilities associated with the Runoff Business as of March 31, 2014 and December 31, 2013 have been presented as held for sale in the financial statements (See Note 15 for discontinued operations).
	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”). In 2012, White Mountains capitalized HG Global with $594.5 million to fund the start-up of BAM. BAM is a municipal bond insurer domiciled in New York that was established to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $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 March 31, 2014 and December 31, 2013, White Mountains owned 96.9% and 97.3% of HG Global's preferred equity and 88.4% and 88.7% 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, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s investments in Wobi Insurance Agency Ltd. (“Wobi”), QuoteLab Holdings LLC (“QuoteLab”) and Star & Shield Risk Management LLC (“SSRM”). At March 31, 2014, White Mountains holds $12.0 million of Star & Shield Insurance Exchange’s (“SSIE”) surplus notes but does not have an ownership interest in SSIE, which is a reciprocal and is owned by its policyholders. 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. 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 2013 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.  Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. Refer to the Company’s 2013 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies.
	Non-controlling Interests
	Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries and are presented separately on the balance sheet.
	The percentage of the non-controlling shareholders’ ownership interest in OneBeacon Ltd. at March 31, 2014 and December 31, 2013 was 24.7% and 24.8%.
	In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from certain management members of BAM, the latter of which is included in non-controlling interest. Upon closing, certain BAM management members also received additional common and preferred shares of HG Global that resulted in a $2.2 million allocation of the carrying value of White Mountains’s investment in HG Global to the non-controlling interest, which was recorded as an adjustment to paid-in surplus in White Mountains’s consolidated statement of changes in equity.
	White Mountains is required to consolidate BAM in its GAAP financial statements. However, since BAM is a mutual insurance company that is owned by its members, BAM’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. For the three months ended March 31, 2014 and 2013, White Mountains reported $8.6 million and $18.4 million in pre-tax losses from BAM that have been allocated to non-controlling interest.
	In May 2007, Sirius International Group, Ltd. (“SIG”), an intermediate holding company of Sirius Group, issued $250.0 million non-cumulative perpetual preference shares, with a $1,000 per share liquidation preference (the “SIG Preference Shares”), and received $245.7 million of proceeds, net of $4.3 million of issuance costs and commissions. SIG Preference Shares and dividends thereon are included in non-controlling interest on the balance sheet and on the statement of income and comprehensive income. The SIG Preference Shares have 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 bps. In July 2013, SIG executed a 5-year forward LIBOR cap (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 Interest Rate Cap is recorded in other assets at fair value. Changes in fair value are recorded in other revenue.
	At March 31, 2014 and December 31, 2013, the non-controlling equity interest in White Mountains’s consolidated limited partnerships was $46.7 million and $46.1 million.  At March 31, 2014, the non-controlling equity interest in Wobi was $5.0 million. At March 31, 2014, the non-controlling equity interest in QuoteLab was $18.8 million. For the three months ended March 31, 2014, SSIE reported $4.7 million in pre-tax losses that have been allocated to non-controlling interest. At March 31, 2014 and December 31, 2013, the non-controlling equity interest in A.W.G. Dewar Inc, a subsidiary of OneBeacon, was $2.7 million and $3.1 million.
	Recently Adopted Changes in Accounting Principles
	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 becomes effective for fiscal periods beginning on or after December 15, 2013 and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective adoption is not allowed. Adoption did not have any impact on White Mountains's financial statements.
	Recently Issued Accounting Pronouncements
	Qualified Affordable Housing Projects
	On January 15, 2014, the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323), which permits companies to make an accounting policy election to account for its investment in a QAHP using the proportional amortization method, if certain conditions are met. Under this method, the initial cost of the investment is amortized in proportion to the tax credits and other tax benefits received, with the net investment performance recognized in the income statement as a component of income tax expense. The new guidance also requires certain new disclosures for all QAHP investments. ASU 2014-01 is effective for annual and interim reporting periods beginning after December 15, 2014 and must be applied retrospectively to all periods presented upon adoption. White Mountains currently holds an investment in a QAHP that is accounted for under the equity method and does not expect the adoption to have a material impact on its financial position, results of operations or cash flows.

Significant_Transactions

Significant Transactions	3 Months Ended
	Mar. 31, 2014
Significant Transactions [Abstract]	'
Significant Transactions	'
	Significant Transactions
	QuoteLab
	On March 14, 2014, White Mountains acquired 60% of the outstanding Class A common units of QuoteLab. QuoteLab is an advertising technology company that operates a transparent online advertising exchange that facilitates transactions between buyers and sellers of insurance media, including advertising inventory on QuoteLab’s owned and operated websites. On a fully converted basis, White Mountains owns 54.3% of QuoteLab. White Mountains paid an initial purchase price of $28.1 million and will pay additional consideration to the sellers equal to 62.5% of the 2015 gross profit in excess of the 2013 gross profit. At acquisition, QuoteLab had total assets of $56.9 million, including $43.7 million of intangible assets, and total liabilities of $10.0 million.
	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).  In addition to the common shares, White Mountains also purchased NIS 12.7 million (approximately $3.6 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. On a fully converted basis, White Mountains owns 60.7% of Wobi. At acquisition, Wobi had total assets of $15.3 million, including $15.0 million of intangible assets, and total liabilities of $2.6 million.
	Star & Shield
	On January 31, 2014, White Mountains acquired certain assets and liabilities of Star & Shield Holdings LLC, including SSRM, the attorney-in-fact for SSIE, for a purchase price of $1.8 million.
	White Mountains also purchased $12.0 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, 2014, consolidated amounts included total assets of $27.1 million and total liabilities of $31.7 million of SSIE. For the quarter ended March 31, 2014, SSIE had a pre-tax loss of $4.7 million that was recorded in net loss attributable to non-controlling interests.
	WM Solutions
	In the first quarter of 2014, WM Solutions completed the shell sale of Citation Insurance Company, which resulted in a gain of $0.7 million recorded in other revenue.
	In the first quarter of 2013, WM Solutions acquired Ashmere Insurance Company (“Ashmere”, formerly known as American Fuji Fire and Marine Insurance Company), an American International Group, Inc. (“AIG”) runoff subsidiary. The transaction resulted in a gain of $6.9 million recorded in other revenue.
	Sale of Essentia Insurance Company
	Effective January 1, 2013, OneBeacon completed the sale of Essentia Insurance Company (“Essentia”), an indirect wholly-owned subsidiary which wrote the collector car and boat business, to Markel Corporation. Concurrently, OneBeacon and Hagerty Insurance Agency (“Hagerty”) terminated their underwriting arrangement with respect to the collector car and boat business. OneBeacon recognized a pre-tax gain on sale of $23.0 million ($15.0 million after tax) in the first quarter of 2013.
	Sale of OneBeacon Runoff Business
	On October 17, 2012, one of OneBeacon’s indirect wholly-owned subsidiaries, OneBeacon Insurance Group LLC, entered into a definitive agreement (as amended, the “Runoff SPA”) with Trebuchet US Holdings, Inc. (“Trebuchet”), a wholly-owned subsidiary of Armour Group Holdings Limited (together with Trebuchet, “Armour”), to sell the Runoff Business. Pursuant to the terms of the agreement, at closing OneBeacon will transfer to Trebuchet all of the issued and outstanding shares of common stock of certain legal entities that will contain the assets, liabilities (including gross and ceded loss reserves) and capital supporting the Runoff Business as well as certain elements of the Runoff Business infrastructure, including staff and office space. The transaction is subject to regulatory approval and is expected to close in the second half of 2014. As a result of the agreement, the OneBeacon Runoff Business is reported as discontinued operations (see Note 15).
	Common Shares Repurchased and Retired
	During the past several years, White Mountains's board of directors has authorized the Company to repurchase its common shares, from time to time, subject to market conditions. The repurchase authorizations do not have a stated expiration date. As of March 31, 2014, White Mountains may repurchase an additional 529,648 shares under these board authorizations. In addition, from time to time White Mountains has also repurchased its common shares through tender offers that were separately approved by its board of directors.
	During the three months ended March 31, 2014, the Company repurchased 26,323 common shares for $15.4 million at an average share price of $586, which was comprised of 15,848 common shares repurchased under the board authorization and 10,475 common shares repurchased pursuant to employee benefit plans. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.
	During the three months ended March 31, 2013, the Company repurchased 140,224 common shares for $79.0 million at an average share price of $564, which was comprised of 140,000 common shares repurchased under the board authorization and 224 common shares repurchased pursuant to employee benefit plans.

Loss_and_Loss_Adjustment_Expen

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013:
		Three Months Ended
		March 31,
	Millions	2014			2013
	Gross beginning balance	$	3,079.30		$	3,168.90
	Less beginning reinsurance recoverable on unpaid losses	(428.1		)	(429.1		)
	Net loss and LAE reserves	2,651.20			2,739.80
	Loss and LAE reserves acquired (1)	—			21.3
	Loss and LAE reserves consolidated — SSIE	13.6			—
	Loss and LAE incurred relating to:
	Current year losses	240.5			242.9
	Prior year losses	(11.2		)	1.4
	Total incurred losses and LAE	229.3			244.3
	Accretion of fair value adjustment to loss and LAE reserves	0.2			1.2
	Foreign currency translation adjustment to loss and LAE reserves	1.8			(9.6		)
	Loss and LAE paid relating to:
	Current year losses	(35.0		)	(36.6		)
	Prior year losses	(237.9		)	(270.6		)
	Total loss and LAE payments	(272.9		)	(307.2		)
	Net ending balance	2,623.20			2,689.80
	Plus ending reinsurance recoverable on unpaid losses	425.2			410.8
	Gross ending balance	$	3,048.40		$	3,100.60
	(1) Loss and LAE reserves acquired relate to WM Solutions purchase of Ashmere in the first quarter of 2013.
	Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2014
	During 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.
	Loss and LAE incurred relating to prior year losses for the three months ended March 31, 2013
	During the three months ended March 31, 2013, White Mountains experienced $1.4 million of net unfavorable loss reserve development. OneBeacon had net favorable loss reserve development of $2.9 million primarily driven by medical facilities and managed care errors and omissions lines in its healthcare business, which is included in OneBeacon's Professional Insurance underwriting unit. Sirius Group had net unfavorable loss reserve development of $4.3 million primarily due to a late reported agricultural claim related to the 2012 U.S. drought.
	Fair value adjustment to loss and LAE reserves
	In connection with purchase accounting for acquisitions, White Mountains is required to adjust loss and LAE reserves and the related reinsurance recoverables to fair value on their respective acquired balance sheets.  The net reduction to loss and LAE reserves is being recognized through an income statement charge ratably with and over the period the claims are settled.
	White Mountains recognized $0.2 million of such charges, recorded as loss and LAE for the three months ended March 31, 2014, and $1.2 million for the three months ended March 31, 2013.  As of March 31, 2014, the remaining pre-tax un-accreted adjustment was $4.5 million.

Third_Party_Reinsurance

Third Party Reinsurance	3 Months Ended
	Mar. 31, 2014
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, 2014, OneBeacon had $3.0 million and $80.1 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2013, OneBeacon had $9.7 million and $80.2 million of reinsurance recoverables on paid and unpaid losses. The reinsurance balances associated with the Runoff Business are included in discontinued operations (see Note 15). Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant.
	Effective January 1, 2014, OneBeacon entered into reinsurance treaties to provide coverage for the 2014 crop year. OneBeacon purchased an aggregate stop loss on its multiple peril crop insurance portfolio, providing 48.5% of coverage in excess of a 101.5% loss ratio on premiums covered by the contract and a separate aggregate stop loss providing 80.0% of coverage in excess of a 100.0% loss ratio on its crop-hail portfolio.
	Sirius Group
	At March 31, 2014, Sirius Group had $12.2 million and $344.8 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, 2013, Sirius Group had $15.7 million and $347.9 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.

Investment_Securities

Investment Securities	3 Months Ended
	Mar. 31, 2014
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 short-term investments, fixed maturity investments, convertible fixed maturity investments and equity securities 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 debt securities, including mortgage-backed and asset-backed securities, 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, 2014 and December 31, 2013.
	Other long-term investments primarily comprise White Mountains’s investments in hedge funds and private equity funds.
	Net Investment Income
	Pre-tax net investment income for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2014				2013
	Investment income:
	Fixed maturity investments		$	22.4			$	26
	Short-term investments		0.6				0.8
	Common equity securities		5.4				4.7
	Convertible fixed maturity investments		0.4				0.8
	Other long-term investments		0.3				0.7
	Total investment income		29.1				33
	Less third-party investment expenses		(4.8		)		(4.5		)
	Net investment income, pre-tax		$	24.3			$	28.5
	Net Realized and Unrealized Investment Gains and Losses
	Net realized and unrealized investment gains and losses for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2014				2013
	Net realized investment gains, pre-tax		$	21.7			$	36.5
	Net unrealized investment gains, pre-tax		42.1				38.7
	Net realized and unrealized investment gains, pre-tax		63.8				75.2
	Income tax expense attributable to net realized and		(15.7		)		(13.7		)
	unrealized investment gains
	Net realized and unrealized investment gains, after tax		$	48.1			$	61.5
	Net realized investment gains (losses)
	Net realized investment gains (losses) for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended												Three Months Ended
			March 31, 2014												March 31, 2013
	Millions		Net				Net				Total net realized				Net				Net				Total net realized
			realized				foreign				gains (losses)				realized				foreign				gains (losses)
			gains				exchange				reflected in				gains				currency				reflected in
			(losses)				gains				earnings				(losses)				gains				earnings
							(losses)												(losses)
	Fixed maturity investments		$	4.4			$	(3.5	)		$	0.9			$	22.6			$	(14.6	)		$	8
	Short-term investments		—				—				—				—				0.1				0.1
	Common equity securities		18.9				(.1		)		18.8				22.6				(.2		)		22.4
	Convertible fixed maturity investments		2.4				—				2.4				2.7				—				2.7
	Other long-term investments		(.3		)		—				(.3		)		2.9				—				2.9
	Forward contracts		(.1		)		—				(.1		)		0.4				—				0.4
	Net realized investment gains (losses),		25.3				(3.6		)		21.7				51.2				(14.7		)		36.5
	pre-tax
	Income tax expense attributable to		(4.6		)		0.9				(3.7		)		(11.8		)		3.3				(8.5		)
	net realized investment
	(losses) gains
	Net realized investment gains (losses),		$	20.7			$	(2.7	)		$	18			$	39.4			$	(11.4	)		$	28
	after tax
	Net unrealized investment gains (losses)
	The following table summarizes net unrealized investment gains (losses) for the three months ended March 31, 2014 and 2013:
			Three Months Ended												Three Months Ended
			March 31, 2014												March 31, 2013
	Millions		Net				Net				Total net unrealized				Net				Net				Total net unrealized
			unrealized				foreign				gains (losses)				unrealized				foreign				gains (losses)
			gains				exchange				reflected in				gains				currency				reflected in
			(losses)				gains				earnings				(losses)				gains				earnings
							(losses)												(losses)
	Fixed maturity investments		$	19.4			$	12.2			$	31.6			$	(30.6	)		$	4.6			$	(26.0	)
	Short-term investments		—				—				—				—				—				—
	Common equity securities		4				0.5				4.5				61.9				—				61.9
Convertible fixed maturity investments		0.7				—				0.7				(1.1		)		(.2		)		(1.3		)
Other long-term investments		4.9				0.4				5.3				3.6				0.5				4.1
Forward contracts		—				—				—				—				—				—
Net unrealized investment gains		29				13.1				42.1				33.8				4.9				38.7
(losses), pre-tax
Income tax expense attributable to		(9.1		)		(2.9		)		(12.0		)		(4.1		)		(1.1		)		(5.2		)
net unrealized investment
(losses) gains
Net unrealized investment gains		$	19.9			$	10.2			$	30.1			$	29.7			$	3.8			$	33.5
(losses), after tax
The following table summarizes the amount of total pre-tax gains included in earnings attributable to unrealized investment gains for Level 3 investments for the three months ended March 31, 2014 and 2013:
		Three Months Ended
		March 31,
Millions		2014				2013
Fixed maturity investments		$	0.2			$	0.2
Common equity securities		0.8				(.1		)
Other long-term investments		6.1				6.4
Total unrealized investment gains, pre-tax - Level 3 investments		$	7.1			$	6.5
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, 2014 and December 31, 2013 were as follows:
		March 31, 2014
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains (losses)
U.S. Government and agency obligations		$	539.2			$	0.6			$	(.8	)		$	(.6	)		$	538.4
Debt securities issued by corporations		2,311.60				48.3				(7.1		)		(6.3		)		2,346.50
Municipal obligations		33.5				0.2				(.2		)		—				33.5
Mortgage-backed and asset-backed securities		1,847.90				4.1				(6.3		)		(3.5		)		1,842.20
Foreign government, agency and provincial obligations		366.4				3.4				(1.1		)		(4.3		)		364.4
Preferred stocks		79.9				6.1				—				(.1		)		85.9
Total fixed maturity investments including assets		$	5,178.50			$	62.7			$	(15.5	)		$	(14.8	)		$	5,210.90
held for sale
Fixed maturity investments reclassified to assets																		(222.2		)
held for sale related to the Runoff Transaction
Total fixed maturity investments																		$	4,988.70
		December 31, 2013
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains (losses)
U.S. Government and agency obligations		$	365.5			$	0.5			$	(1.0	)		$	(2.5	)		$	362.5
Debt securities issued by corporations		2,330.70				44				(13.2		)		(14.3		)		2,347.20
Municipal obligations		18.3				—				(.4		)		—				17.9
Mortgage-backed and asset-backed securities		2,027.30				2.4				(9.9		)		(5.3		)		2,014.50
Foreign government, agency and provincial obligations		444.2				3.7				(3.2		)		(4.8		)		439.9
Preferred stocks		79.9				5.1				—				(.2		)		84.8
Total fixed maturity investments including assets		$	5,265.90			$	55.7			$	(27.7	)		$	(27.1	)		$	5,266.80
held for sale
Fixed maturity investments reclassified to assets																		(236.3		)
held for sale related to the Runoff Transaction
Total fixed maturity investments																		$	5,030.50
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 maturities and other long-term investments as of March 31, 2014 and December 31, 2013 were as follows:
		March 31, 2014
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains (losses)
Common equity securities		$	905.6			$	273.9			$	(4.5	)		$	(.4	)		$	1,174.60
Convertible fixed maturity investments		$	64.3			$	10.6			$	(1.0	)		$	(.1	)		$	73.8
Other long-term investments		$	246.4			$	83.4			$	(25.5	)		$	(2.0	)		$	302.3
		December 31, 2013
Millions		Cost or				Gross				Gross				Net foreign				Carrying
		amortized				unrealized				unrealized				currency				value
		cost				gains				losses				gains (losses)
Common equity securities		$	890.2			$	271			$	(3.6	)		$	(.8	)		$	1,156.80
Convertible fixed maturity investments		$	71.7			$	9.9			$	(.9	)		$	(.2	)		$	80.5
Other long-term investments		$	238.3			$	79.6			$	(26.6	)		$	(2.4	)		$	288.9
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. At March 31, 2014, White Mountains held investments in 16 hedge funds and 39 private equity funds.  The largest investment in a single fund was $18.8 million at March 31, 2014. The following table summarizes investments in hedge funds and private equity interests by investment objective and sector at March 31, 2014 and December 31, 2013:
		March 31, 2014								December 31, 2013
Millions		Fair Value				Unfunded				Fair Value				Unfunded
						Commitments								Commitments
Hedge funds
Long/short equity		$	67.6			$	—			$	62.6			$	—
Long/short credit & distressed		23.3				—				22.8				—
Long/short equity REIT		18.8				—				18.3				—
Long/short equity activist		16.3				—				16.8				—
Long bank loan		0.2				—				0.2				—
Long diversified strategies		—				—				0.1				—
Total hedge funds		126.2				—				120.8				—
Private equity funds
Energy infrastructure & services		46.8				12.7				45.9				13.1
Multi-sector		24.1				6.4				23.8				6.5
Manufacturing/Industrial		11.5				15.2				11.2				15.5
Aerospace/Defense/Government		10.6				15.5				5.8				19.2
Private equity secondaries		9.2				3.1				9.5				3.1
Healthcare		7.8				2.8				5.6				2.8
Real estate		7.1				3.3				8.2				3.3
Insurance		2.4				41.3				2.3				41.3
Venture capital		1.6				0.3				1.6				0.3
International multi-sector, Asia		—				2.7				—				2.7
International multi-sector, Europe		3.8				2.8				3.9				2.8
Total private equity funds		124.9				106.1				118.2				110.6
Total hedge and private equity funds included in		$	251.1			$	106.1			$	239			$	110.6
other long-term investments(1)
(1)  Excludes carrying value of $25.0 and $26.6 at March 31, 2014 and December 31, 2013 associated with hedge funds and private equity funds accounted for using the equity method.
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, 2014 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		$	4.9			$	—			$	—			$	5.5			$	10.4
Quarterly		29.4				30.4				11.9				8.8				80.5
Semi-annual		—				25.8				—				—				25.8
Annual		—				—				9.3				0.2				9.5
Total		$	34.3			$	56.2			$	21.2			$	14.5			$	126.2
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.  At March 31, 2014, 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 at March 31, 2014.
White Mountains has also submitted redemption requests for certain of its investments in active hedge funds.  At March 31, 2014, redemptions of $2.2 million are outstanding and are subject to market fluctuations. The date at which such redemptions will be received is not determinable at March 31, 2014. 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.
At March 31, 2014, 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		$6.50		$29.50		$75.70		$13.20		$124.90
Fair value measurements at March 31, 2014
White Mountains’s invested assets that are measured at fair value include fixed maturity investments, common and preferred equity securities, convertible fixed maturity securities 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 equities 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 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 investments in debt securities are generally valued using matrix and other pricing models. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds.  Income on mortgage-backed and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized 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 $84.6 million and $86.3 million of investment-related liabilities recorded at fair value and included in other liabilities as of March 31, 2014 and December 31, 2013.  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.  All of the liabilities included have a Level 1 designation.
Fair Value Measurements by Level
The following tables summarize White Mountains’s fair value measurements for investments at March 31, 2014 and December 31, 2013, by level:
		March 31, 2014
Millions		Fair value				Level 1				Level 2				Level 3
Fixed maturity investments:
U.S. Government and agency obligations		$	538.4			$	471.7			$	66.7			$	—
Debt securities issued by corporations:
Consumer		741				—				741				—
Financials		446				—				446				—
Communications		267.5				—				267.5				—
Industrial		251.5				—				251.5				—
Energy		194.2				—				194.2				—
Utilities		132.7				—				132.7				—
Basic Materials		140.1				—				140.1				—
Technology		91.5				—				91.5				—
Other		82				—				82				—
Total debt securities issued by corporations:		2,346.50				—				2,346.50				—
Mortgage-backed and asset-backed securities		1,842.20				—				1,800.60				41.6
Foreign government, agency and provincial obligations		364.4				42.8				321.6				—
Preferred stocks		85.9				—				14.6				71.3
Municipal obligations		33.5				—				33.5				—
Total fixed maturity investments (1)		5,210.90				514.5				4,583.50				112.9
Short-term investments		674				666.6				7.4				—
Common equity securities:
Financials		359.8				312.9				—				46.9
Consumer		313.8				313.8				—				—
Industrial		106.5				106.5				—				—
Energy		81.1				81.1				—				—
Communications		56.5				56.5				—				—
Basic materials		56				56				—				—
Technology		44.9				44.9				—				—
Utilities		36.7				36.7				—				—
Other		119.3				42.7				76.3				0.3
Total common equity securities		1,174.60				1,051.10				76.3				47.2
Convertible fixed maturity investments		73.8				—				67.7				6.1
Other long-term investments (2)		277.4				—				—				277.4
Total investments		$	7,410.70			$	2,232.20			$	4,734.90			$	443.6
(1) Carrying value includes $222.2 that is classified as assets held for sale relating to discontinued operations.
(2) Excludes carrying value of $25.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts.
		December 31, 2013
Millions		Fair value				Level 1				Level 2				Level 3
Fixed maturity investments:
U.S. Government and agency obligations		$	362.5			$	295.8			$	66.7			$	—
Debt securities issued by corporations:
Consumer		754.4				—				754.4				—
Industrial		434.4				—				434.4				—
Communications		281.1				—				281.1				—
Financials		265				—				265				—
Basic materials		173.6				—				173.6				—
Energy		159.7				—				159.7				—
Utilities		149.1				—				149.1				—
Technology		91.2				—				91.2				—
Other		38.7				—				38.7				—
Total debt securities issued by corporations:		2,347.20				—				2,347.20				—
Mortgage-backed and asset-backed securities		2,014.50				—				1,992.50				22
Foreign government, agency and provincial obligations		439.9				44.5				395.4				—
Preferred stocks		84.8				—				13.8				71
Municipal obligations		17.9				—				17.9				—
Total fixed maturity investments (1)		5,266.80				340.3				4,833.50				93
Short-term investments		635.9				621.5				14.4				—
Common equity securities:
Financials		360.4				314.3				—				46.1
Consumer		308.2				308.2				—				—
Industrial		105.4				105.4				—				—
Energy		78.6				78.6				—				—
Utilities		60.6				60.6				—				—
Communications		57.1				57.1				—				—
Basic materials		53.4				53.4				—				—
Technology		34.3				34.3				—				—
Other		98.8				24.5				74.3				—
Total common equity securities		1,156.80				1,036.40				74.3				46.1
Convertible fixed maturity investments		80.5				—				74.4				6.1
Other long-term investments (2)		262.4				—				—				262.4
Total investments		$	7,402.40			$	1,998.20			$	4,996.60			$	407.6
(1) Carrying value includes $236.3 that is classified as assets held for sale relating to discontinued operations.
(2) Excludes carrying value of $26.6 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.
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, 2014 and December 31, 2013:
		Fair Value at
Millions		March 31, 2014				December 31, 2013
AAA		$	—			$	—
AA		221.3				228.8
A		1,028.90				1,039.50
BBB		1,089.90				1,075.50
BB		—				—
Other		6.4				3.4
Debt securities issued by corporations(1)		$	2,346.50			$	2,347.20
(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 non-agency commercial mortgage-backed portfolio (“CMBS”) is generally short-term and structurally senior, with more than 25 points of subordination on average for both fixed rate CMBS and floating rate CMBS as of March 31, 2014.  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. White Mountains believes these levels of protection will mitigate the risk of loss tied to the refinancing challenges facing the commercial real estate market.  As of March 31, 2014, on average less than 1% of the underlying loans were reported as non-performing for all non-agency CMBS held by White Mountains. White Mountains is not an originator of residential mortgage loans. White Mountains’s investments in hedge funds and private equity funds contain negligible amounts of sub-prime mortgage-backed securities at March 31, 2014. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics).
White Mountains categorizes mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. White Mountains’s non-agency residential mortgage-backed portfolio is generally moderate-term and structurally senior. White Mountains does not own any collateralized loan obligations. White Mountains does not own any collateralized debt obligations, with the exception of $37.8 million of non-agency residential mortgage resecuritization tranches, each a senior tranche in its own right and each collateralized by a single earlier vintage Super Senior or Senior non-agency residential mortgage backed security.
The following table summarizes mortgage and asset-backed securities as of March 31, 2014 and December 31, 2013:
		March 31, 2014												December 31, 2013
Millions		Fair Value				Level 2				Level 3				Fair Value				Level 2				Level 3
Mortgage-backed securities:
Agency:
GNMA		$	464.8			$	464.8			$	—			$	512.3			$	512.3			$	—
FNMA		167.3				167.3				—				81.2				81.2				—
FHLMC		85.6				85.6				—				91.3				91.3				—
Total Agency(1)		717.7				717.7				—				684.8				684.8				—
Non-agency:
Residential		132.3				132.3				—				125.7				125.7				—
Commercial		329.7				310.1				19.6				282.3				282.3				—
Total Non-agency		462				442.4				19.6				408				408				—
Total mortgage-backed securities		1,179.70				1,160.10				19.6				1,092.80				1,092.80				—
Other asset-backed securities:
Credit card receivables		229.7				207.7				22				311.4				289.4				22
Vehicle receivables		231.5				231.5				—				365				365				—
Other		201.3				201.3				—				245.3				245.3				—
Total other asset-backed securities		662.5				640.5				22				921.7				899.7				22
Total mortgage and asset-backed securities		$	1,842.20			$	1,800.60			$	41.6			$	2,014.50			$	1,992.50			$	22
(1)  Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Non-agency Mortgage-backed Securities
The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of March 31, 2014 are as follows:
										Security Issuance Year
Millions		Fair Value				2004				2005				2006				2007				2008			2009		2010		2011		2012		2013		2014
Non-agency RMBS		$	132.3			$	11.3			$	20.2			$	11			$	—			$	19.6		$	—	$	26.4	$	—	$	—	$	32.8	$	11
Non-agency CMBS		329.7				—				—				8.5				5				26.4			—		13		27		131.8		94.3		23.7
Total		$	462			$	11.3			$	20.2			$	19.5			$	5			$	46		$	—	$	39.4	$	27	$	131.8	$	127.1	$	34.7
Non-agency Residential Mortgage-backed Securities
The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of March 31, 2014:
Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
Prime		$	105.9			$	32.4			$	73.5			$	—
Non-prime		19.2				—				19.2				—
Sub-prime		7.2				7.2				—				—
Total		$	132.3			$	39.6			$	92.7			$	—
(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to other "AAA" or "Aaa" bonds.
(2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to non-"AAA" or non-"Aaa" bonds.
(3) At issuance, Subordinate were not rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were junior to "AAA" or "Aaa" bonds.
Non-agency Commercial Mortgage-backed Securities
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, 2014:
Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
Fixed rate CMBS		$	225.1			$	123.5			$	76			$	25.6
Floating rate CMBS		104.6				5				26.4				73.2
Total		$	329.7			$	128.5			$	102.4			$	98.8
(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to other "AAA" or "Aaa" bonds.
(2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to non-"AAA" or non-"Aaa" bonds.
(3) At issuance, Subordinate were not rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were junior to "AAA" or "Aaa" bonds.
Rollforward of Fair Value 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 investments and other long-term investments at March 31, 2014 and 2013 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, 2014 and 2013:
							Level 3 Investments
Millions	Level 1 Investments			Level 2			Fixed			Common			Convertible			Other long-term				Total
				Investments			Maturities			equity			fixed			investments
										securities			maturities
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 unrealized	22.2			34.7			0.4			0.8			—			6.3				64.4			(4)
gains (losses)
Foreign currency losses	(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 (sales) 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)(4)
March 31, 2014
(1)  Excludes carrying value of $26.6 and $25.0 at January 1, 2014 and March 31, 2014 associated with other long-term investments accounted for using the equity method and $(0.1) and $(0.1) at January 1, 2014 and March 31, 2014 related to forward contracts.
(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 gains associated with the Prospector Funds consolidation of investment-related liabilities and $1.2 realized and unrealized losses associated with other long-term investments accounted for using the equity method.
							Level 3 Investments
Millions	Level 1 Investments			Level 2			Fixed			Common			Convertible			Other long-term				Total
				Investments			Maturities			equity			fixed			investments
										securities			maturities
Balance at January 1, 2013	$	1,355.10		$	5,206.10		$	92.9		$	37.3		$	—		$	259.3			$	6,950.70		(1)(2)(3)
Total realized and	80.1			(10.8		)	0.1			(.1		)	—			5.8				75.1			(3)
unrealized gains (losses)
Foreign currency losses	(.4		)	(4.8		)	—			—			—			(.4		)		(5.6		)
through OCI
Amortization/Accretion	(.4		)	(14.0		)	—			—			—			—				(14.4		)
Purchases	318.6			1,445.80			8			—			—			18.9				1,791.30
Sales	(382.3		)	(1,828.3		)	—			—			—			(23.1		)		(2,233.7		)
Net change in investments	14.5			2.7			—			—			—			—				17.2
related to purchases (sales)
of consolidated affiliates
Transfers in	1.2			—			—			—			—			—				1.2
Transfers out	—			(.8		)	—			(.4		)	—			—				(1.2		)
Balance at	1,386.40			4,795.90			101			36.8			—			260.5				6,580.60			(1)(2)(3)
March 31, 2013
(1) Excludes carrying value of $35.0 and $34.9 at January 1, 2013 and March 31, 2013 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) and $(0.2) at January 1, 2013 and March 31, 2013 related to forward contracts.
(2)  Carrying value includes $338.1 and 296.9 at January 1, 2013 and March 31, 2013 that is classified as assets held for sale relating to discontinued operations.
(3) Excludes carrying value of $630.6 and $654.4 at January 1, 2013 and March 31, 2013 and realized and unrealized loss for the period of $0.1 associated with short-term investments.
Fair Value Measurements — transfers between levels - Three-month period ended March 31, 2014 and 2013
During the first three months of 2014 and 2013, no fixed income securities classified as Level 3 measurements in the prior
period were recategorized as Level 2 measurements.
Significant Unobservable Inputs
The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 other than hedge funds and private equity funds. The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds.
($ in Millions)		March 31, 2014
Description		Fair Value		Rating (2)		Valuation Technique(s)		Unobservable Input
Non-agency commercial mortgage backed securities(1)		$13.30		AAA		Discounted cash flow		Discount Spread over Swap		0.80%
								Prepayment rate		0.00%	CPY
								Default rate		0.00%	CDR
Asset-backed securities(1)		$22.00		AA+		Broker pricing		Broker quote
Non-agency commercial mortgage backed securities(1)		$6.30		BBB-		Broker pricing		Broker quote
Preferred stock(1)		$71.30		NR		Discounted cash flow		Discount yield		6.90%
Private equity security(1)		$36.40		NR		Multiple of GAAP book value		Book value multiple		1
Private equity security(1)		$10.50		NR		Share price of recent transaction		Average share price		$1.10
Convertible preferred securities		$6.10		NR		Share price of recent transaction		Recent market transaction		$6.10
(1) As of March 31, 2014, consists of one security.
(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.
($ in Millions)		December 31, 2013
Description		Fair Value		Rating (2)		Valuation Technique(s)		Unobservable Input
Asset-backed securities(1)		$22.00		AA+		Broker pricing		Broker quote
Preferred stock(1)		$71.00		NR		Discounted cash flow		Discount yield		7.40%
Private equity security(1)		$35.60		NR		Multiple of GAAP book value		Book value multiple		1
Private equity security(1)		$10.50		NR		Share price of recent transaction		Average share price		$1.10
Convertible preferred securities		$6.10		NR		Share price of recent transaction		Recent market transaction		$6.10
(1) As of December 31, 2013, consists of one security.
(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.
The assumed prepayment rate is a significant unobservable input used to estimate the fair value of investments in non-agency CMBS. Generally for bonds priced at a premium, increases in prepayment speeds will result in a lower fair value, while decreases in prepayment speed may result in a higher fair value, with the inverse for bonds priced at a discount.

Debt

Debt	3 Months Ended
	Mar. 31, 2014
Debt Disclosure [Abstract]	'
Debt	'
	Debt
	White Mountains’s debt outstanding as of March 31, 2014 and December 31, 2013 consisted of the following:
	Millions	March 31,			December 31,
		2014			2013
	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	(.4		)	(.4		)
	SIG Senior Notes, carrying value	399.6			399.6
	WTM Bank Facility	—			—
	Old Lyme Note	2.1			2.1
	Other	1.2			—
	Total debt	$	677.6		$	676.4
	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, 2014, 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. Failure to meet one or more of these covenants could result in an event of default, which ultimately could eliminate availability under these facilities and result in acceleration of principal repayment on any amounts outstanding.
	Debt Covenants
	At March 31, 2014, White Mountains was in compliance with all debt covenants.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate.  The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Australia, Belgium, Canada, Germany, Gibraltar, Israel, Luxembourg, the Netherlands, 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, 2014 and 2013 represented net effective tax rates of 27.0% and 27.4%. The effective tax rates for the three months ended March 31, 2014 and 2013 were lower than the U.S. statutory rate of 35% due primarily to income generated in jurisdictions other than the United States.
	In arriving at the effective tax rate for the three months ended March 31, 2014 and 2013, 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, 2014 and 2013.
	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.  During the next twelve months, it is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense.
	With few exceptions, White Mountains is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2005.
	On February 14, 2014, OneBeacon received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Overassessment) from the IRS Appeals Office relating to the examination of tax years 2005 and 2006. All disputed items have now been agreed and resolved with the Joint Committee. OneBeacon recorded a tax benefit of $5.0 million in the first quarter of 2014 relating to the settlement of the IRS examination for tax years 2005 and 2006.
	On July 28, 2011, the IRS commenced an examination of the income tax returns for 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon.  On July 17, 2013, 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 $69.3 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 2, 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.

Derivatives

Derivatives	3 Months Ended
	Mar. 31, 2014
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, 2014 and December 31, 2013, the total guarantee value was approximately ¥187.9 billion (approximately $1.8 billion at exchange rates on that date) and ¥203.6 billion (approximately $1.9 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 103% and 104% of the guarantee value at March 31, 2014 and December 31, 2013, respectively.  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, 2014 and 2013.
		Three Months Ended
		March 31,
	Millions	2014				2013
	Fees, included in other revenue	$	5.4			$	6.8
	Change in fair value of variable annuity liability,	(1.3		)		184.3
	included in other revenue
	Change in fair value of derivatives, included in other revenue	(6.6		)		(181.8		)
	Foreign exchange, included in other revenue	0.2				(14.6		)
	Other investment income and gains (losses)	0.4				(4.0		)
	Total revenue	(1.9		)		(9.3		)
	Change in fair value of variable annuity death benefit liabilities,	—				5.6
	included in other general and administrative expenses
	Death benefit claims paid, included in general and	—				(1.1		)
	administrative expenses
	General and administrative expenses	(1.3		)		(1.7		)
	Pre-tax loss	$	(3.2	)		$	(6.5	)
	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three months ended March 31, 2014 and 2013 and the carrying values, included in other assets, at March 31, 2014 and December 31, 2013 by type of instrument:
		Gains (losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	Fixed income/interest rate	$	(6.4	)		$	(30.9	)	$	(5.3	)		$	(9.7	)
	Foreign exchange	(6.5		)		(53.6		)	63.9				58
	Equity	6.3				(97.3		)	31				20.9
	Total	$	(6.6	)		$	(181.8	)	$	89.6			$	69.2
	The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2014 and 2013:
		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 (losses) gains	(1.3		)	(4)	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
		Three Months Ended March 31, 2013
		Variable Annuity				Derivative Instruments
		(Liabilities)
	Millions	Level 3				Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	(441.5	)		$	140.5		$	(20.5	)		$	(21.7	)		$	98.3
	Purchases	—				36.1			—				—				36.1
	Realized and unrealized gains (losses)	189.9			(4)	(49.2		)	(85.5		)		(47.1		)		(181.8		)
	Transfers in	—				—			—				—				—
	Sales/settlements	—				—			63.1				60.1				123.2
	End of period	$	(251.6	)		$	127.4		$	(42.9	)		$	(8.7	)		$	75.8
	(1) Consists of over-the-counter instruments.
	(2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available.  Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
	(3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
	(4) Includes $0.0 and $5.6 for the three months ended March 31, 2014 and 2013 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.
	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments posted as collateral to its variable annuity reinsurance hedging counterparties.  The total collateral comprises the following:
	Millions	March 31, 2014				December 31, 2013			March 31, 2013
	Cash	$	22.2			$	56.1		$	188.5
	Short-term investments	—				2			15.6
	Fixed maturity investments	14.3				23.2			25.7
	Total	$	36.5			$	81.3		$	229.8
	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, 2014 and 2013. 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, 2014
Description	Fair				Valuation Technique(s)		Unobservable Input	Range					Weighted
	Value												Average
Variable annuity benefit guarantee liabilities	$	54.1			Discounted cash flows		Surrenders
							1 year	0.3	%	-	30.00%		18	%
							2 year	0.2	%	-	32.00%		13.6	%
							Mortality	0	%	-	5.90%		1	%
							Foreign exchange volatilities
							1 year	10.5	%	-	13.50%		11.4	%
							2 year	11.7	%	-	14.60%		12.6	%
							Index volatilities
							1 year	16.1	%	-	19.70%		17.8	%
							2 year	17	%	-	19.40%		18.3	%
Foreign exchange options	$	31.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	(1.2	)%	-	8.00%		2.4	%
Equity index options	$	32.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	0.5	%	-	9.70%		3.5	%
WM Life Re enters into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee.  In the case of OTC derivatives, WM Life Re has exposure to credit risk for amounts that are uncollateralized by counterparties. WM Life Re’s internal risk management guidelines establish net counterparty exposure thresholds that take into account OTC counterparties’ credit ratings. The OTC derivative contracts are subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements.
In the case of exchange traded instruments, WM Life Re has exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re has entered into master netting agreements with certain of its counterparties whereby the collateral provided (held) is calculated on a net basis. The following summarizes amounts offset under master netting agreements:
	March 31, 2014											December 31, 2013
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	$	1.2			$	(6.4	)	$	(5.2	)		$	2.4			$	(11.7	)	$	(9.3	)
Exchange traded	0.5				(.6		)	(.1		)		1				(1.6		)	(.6		)
Foreign exchange contracts
OTC	64.4				—			64.4				67.8				(12.0		)	55.8
Exchange traded	0.1				(.6		)	(.5		)		2.3				—			2.3
Equity contracts
OTC	32.6				(2.5		)	30.1				30.7				(9.2		)	21.5
Exchange traded	0.9				—			0.9				1.8				(2.3		)	(.5		)
Total(2)	$	99.7			$	(10.1	)	$	89.6			$	106			$	(36.8	)	$	69.2
(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-14
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	$	29.1			$	—		$	—			$	29.1			$	—		$	—		$	—	$	29.1	A
Barclays	1.2				—			—				1.2				—			—			—		1.2		A
JP Morgan	22.6				—			—				22.6				4.7			—			—		27.3		A	+
Royal Bank	11.4				—			—				11.4				—			—			—		11.4		A	-
of Scotland
Nomura	0.7				—			—				0.7				4.8			14.3			—		19.8		BBB	+
Citigroup - OTC	24.2				—			—				24.2				—			—			3.8		20.4		A
Citigroup -	0.4				—			—				0.4				10.9			—			—		11.3		A
Exchange Traded
Total	$	89.6			$	—		$	—			$	89.6			$	20.4		$	14.3		$	3.8	$	120.5
	31-Dec-13
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	$	27.2			$	—		—				$	27.2			$	—		$	—		$	—	$	27.2	A
Barclays	1.4				—			—				1.4				—			—			—		1.4		A	+
JP Morgan	9.1				—			—				9.1				22			—			—		31.1		A	+
Royal Bank	11.3				—			—				11.3				—			—			—		11.3		A
of Scotland
Nomura	(.4		)		—			0.4				—				—			22.8			0.8		22		BBB	+
Citigroup - OTC	19.4				—			—				19.4				2.3			—			—		21.7		A
Citigroup -	1.2				—			—				1.2				19.8			—			—		21		A
Exchange Traded
Total	$	69.2			$	—		$	0.4			$	69.6			$	44.1		$	22.8		$	0.8	$	135.7
(1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-one creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and
“BBB+” (Adequate, which is the eighth highest of twenty-one creditworthiness ratings).
Forward Contracts	'
Derivatives	'
	Forward Contracts
	Beginning in September 2012, White Mountains 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, 2014, White Mountains held approximately $23.2 million (SEK 150.5 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, 2014.
	The following tables summarize the changes in White Mountains’s forward contracts for the three months ended March 31, 2014:
		Three Months Ended
		March 31,
	Millions	2014				2013
	Beginning of period	$	(.1	)		$	(.1	)
	Purchases	—				—
	Net realized and unrealized (losses) gains	(.1		)		0.4
	Sales/settlements	0.1				(.5		)
	End of period	$	(.1	)		$	(.2	)
	The following summarizes realized and unrealized derivative gains (losses) recognized in net realized and unrealized investment gains for the three months ended March 31, 2014 and the carrying values, included in other long-term investments, at March 31, 2014 and December 31, 2012 by type of currency:
		Gains (Losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	USD	$	—			$	(.3	)	$	—			$	—
	SEK	—				—			—				—
	EUR	(.1		)					(.1		)		(.1		)
	GBP	—				0.7			—				—
	Total	$	(.1	)		$	0.4		$	(.1	)		$	(.1	)
	All of White Mountains’s forward contracts are subject to master netting agreements. The following summarizes amounts offset under master netting agreements:
		March 31, 2014											December 31, 2013
	Millions	Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets				Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets
		amounts before											amounts before
		offsets(1)											offsets(1)
	USD	$	—			$	—		$	—			$	—			$	—		$	—
	EUR	—				(.1		)	(.1		)		—				(.1		)	(.1		)
	GBP	—				—			—				—				—			—
	Total	$	—			$	(.1	)	$	(.1	)		$	—			$	(.1	)	$	(.1	)
	(1) Amount equal to fair value of instrument as recognized in other assets.
	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-14									31-Dec-13
	Millions	Notional Amount				Carrying Value			Standard & Poor's		Notional Amount				Carrying Value
									Rating(1)
	Barclays Bank Plc	$	8.1			$	(.1	)	A		$	5.8			$	—
	Deutsche Bank	7.1							A		7.7				—
	Goldman Sachs	4.4				—			AA-		2.1				—
	HSBC Bank Plc	3.6				—			AA-		3.3				(.1		)
	JP Morgan	—				—			A+		1.3				—
	Royal Bank of Canada	—				—			AA-		0.2				—
	Total	$	23.2			$	(.1	)			$	20.4			$	(.1	)
	(1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).
Interest Rate Cap	'
Derivatives	'
	Interest Rate Cap
	In May 2007, SIG issued the SIG Preference Shares, with an initial fixed annual dividend rate of 7.506%. In June 2017, the fixed rate will move to a floating rate equal to the greater of (i) 7.506% and (ii) 3-month LIBOR plus 320 bps. 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 bps of the $250.0 million notional value, or approximately $9.9 million for the full notional amount.
	The Interest Rate Cap does not qualify for hedge accounting. It is recorded in other assets at fair value. Changes in fair value are recognized 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, 2014.
	The following tables summarize the changes in the fair value of the Interest Rate Cap for the three months ended March 31, 2014:
		Three Months Ended
		March 31,
	Millions	2014
	Beginning of period	$	11.1
	Purchases	—
	Net realized and unrealized losses	(2.9		)
	Sales/settlements	—
	End of period	$	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-14
	Millions	Collateral Balances Held				Standard & Poor’s
						Rating(1)
	Barclays Bank Plc	$	5.7			A
	Nordea Bank Finland Plc	2.5				AA-
	Total	$	8.2
	(1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-one creditworthiness ratings) and “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings).
Weather derivatives	'
Derivatives	'
	Weather Derivatives
	For the three months ended March 31, 2014, Sirius Group recognized $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.

Municipal_Bond_Guarantee

Municipal Bond Guarantee	3 Months Ended
	Mar. 31, 2014
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, 2014, 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 $439.9 million at March 31, 2014, 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, 2014 and 2013, HG Global had pre-tax income of $4.6 million and $9.9 million, which included $3.9 million and $10.1 million of interest income on the BAM surplus notes. For the three months ended March 31, 2014 and 2013, White Mountains reported pre-tax losses of $8.6 million and $18.4 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $3.9 million and $10.1 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 3.13% for 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-14		31-Dec-13
	Contracts outstanding	907		701
	Remaining weighted average contract period outstanding (in years)	13.6		13.8
	Contractual debt service outstanding (in millions):
	Par	$	6,015.00	$	4,703.70
	Interest	4,140.90		3,264.40
	Total debt service outstanding	$	10,155.90	$	7,968.10
	Gross unearned insurance premiums (in millions)	$	17.7	$	13.2

Earnings_Per_Share

Earnings Per Share	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013 (see Note 15 for earnings per share amounts for discontinued operations):
		Three Months Ended
		March 31,
		2014			2013
	Basic and diluted earnings per share numerators (in millions):
	Net income from continuing operations attributable to	$	96.1		$	119.9
	White Mountains’s common shareholders
	Allocation of income for unvested restricted common shares	(1.1		)	(1.5		)
	Dividends declared on participating restricted common shares(1)	(.1		)	(.1		)
	Total allocation to restricted common shares	(1.2		)	(1.6		)
	Net income attributable to White Mountains’s common shareholders,	$	94.9		$	118.3
	net of restricted common share amounts
	Undistributed net earnings (in millions):
	Net income attributable to White Mountains’s common shareholders,	$	94.9		$	118.3
	net of restricted common share amounts
	Dividends declared net of restricted common share amounts(1)	(6.1		)	(6.1		)
	Total undistributed net earnings, net of restricted common share amounts	$	88.8		$	112.2
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted shares(2)	(70.4		)	(79.0		)
	Basic earnings per share denominator	6,099.50			6,194.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares(2)	(70.4		)	(79.0		)
	Average outstanding dilutive options to acquire common shares(3)	—			—
	Diluted earnings per share denominator	6,099.50			6,194.50
	Basic earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	15.56		$	19.1
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	14.56		$	18.1
	Diluted earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	15.56		$	19.1
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	14.56		$	18.1
	(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.
	(2) Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).
	(3) The diluted earnings per share denominator for the three months ended March 31, 2014 and 2013 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2014
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, 2014
	Earned insurance and reinsurance premiums	$	276.5	$	215.5		$	0.2		$	0.1		$	1.3		$	493.6
	Net investment income	10		8.7			0.3			1.4			3.9			24.3
	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	1		(5.3		)	—			0.2			0.7			(3.4		)
	Total revenues	306.4		247			5.1			0.8			19			578.3
	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)	$	54.4	$	74		$	4.6		$	(8.6	)	$	(12.3	)	$	112.1
							HG Global/BAM
	Millions	OneBeacon		Sirius			HG			BAM			Other Operations			Total
				Group
	Three Months Ended March 31, 2013
	Earned insurance and reinsurance premiums	$	286.5	$	208.9		$	—		$	—		$	—		$	495.4
	Net investment income	9.4		13.1			0.3			1.1			4.6			28.5
	Net investment income (loss) - surplus note	—		—			10.1			(10.1		)	—			—
	interest
	Net realized and unrealized investment	28.4		13.1			(.2		)	(1.1		)	35			75.2
	gains (losses)
	Other revenue	24		10.5			—			0.1			(6.3		)	28.3
	Total revenues	348.3		245.6			10.2			(10.0		)	33.3			627.4
	Losses and LAE	148.9		95.4			—			—			—			244.3
	Insurance and reinsurance acquisition expenses	54.8		43.3			—			0.1			—			98.2
	Other underwriting expenses	49.3		29.7			—			0.1			—			79.1
	General and administrative expenses	4		9.5			0.3			8.2			21.8			43.8
	Interest expense	3.2		6.6			—			—			0.4			10.2
	Total expenses	260.2		184.5			0.3			8.4			22.2			475.6
	Pre-tax income (loss)	$	88.1	$	61.1		$	9.9		$	(18.4	)	$	11.1		$	151.8

Investments_in_Unconsolidated_

Investments in Unconsolidated Affiliates	3 Months Ended
	Mar. 31, 2014
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,
		2014			2013
	Symetra common shares	$	373.2		$	360.9
	Unrealized losses from Symetra’s fixed maturity portfolio	(4.2		)	(43.6		)
	Carrying value of Symetra common shares	369			317.3
	Hamer	4.3			4.1
	Total investments in unconsolidated affiliates	$	373.3		$	321.4
	Symetra
	At March 31, 2014 and December 31, 2013, White Mountains owned 20.05 million common shares of Symetra Financial Corporation (“Symetra”), about a 17% common share ownership. White Mountains accounts for its investment in common shares of Symetra using the equity method.
	On June 20, 2013, both White Mountains and Berkshire Hathaway Inc., which each owned warrants to purchase 9.49 million common shares of Symetra, exercised their warrants in a cashless transaction and each received 2.65 million common shares of Symetra in exchange for the warrants. During 2013, Symetra also repurchased 6.6 million of its common shares at an average price of $13.44. The net effect of Symetra’s share repurchases and the warrant exercises 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. This basis difference totaled $19.3 million, of which $0.4 million was attributable to equity in earnings of unconsolidated affiliates and $18.9 million is attributable to equity in net unrealized gains of unconsolidated affiliates.
	At December 31, 2011, due to the prolonged low interest rate environment in which life insurance companies currently operate, 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 at December 31, 2011.  This impairment also 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. White Mountains recorded $45.9 million of after-tax equity in losses of unconsolidated affiliates and $136.6 million of after-tax equity in net unrealized losses of unconsolidated affiliates.
	As a result of the various basis adjustments, White Mountains’s carrying value of its investment in Symetra differs from the carrying value by applying its ownership share against Symetra’s GAAP equity as normally done under the equity method. The pre-tax basis difference is being amortized over a 30-year period with a weighted average of 29-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, 2014, White Mountains recognized after-tax amortization of $0.7 million through equity in earnings of unconsolidated affiliates and $2.9 million through equity in net unrealized gains from investments in unconsolidated affiliates. At March 31, 2014, the pre-tax unamortized basis difference was $183.3 million, of which $39.0 million is attributable to equity in earnings of unconsolidated affiliates and $144.3 million is attributable to equity in net unrealized gains of unconsolidated affiliates.
	Prior to the exercise of the warrants, White Mountains accounted for its Symetra warrants as derivatives with changes in fair value recognized through the income statement as a gain or loss recognized through other revenue in the income statement.  White Mountains used a Black Scholes valuation model to determine the fair value of the Symetra warrants. The major assumptions used in valuing the Symetra warrants at March 31, 2013 were a risk free rate of a 0.17%, volatility of 29.1%, an expected life of 1.33 years, a strike price of $11.49 per share and a share price of $13.41 per share.
	The following table summarizes amounts recorded by White Mountains relating to its investment in Symetra for the three months ended March 31, 2014 and 2013:
		Three Months Ended									Three Months Ended
		March 31, 2014									March 31, 2013
	Millions	Common Shares			Warrants			Total			Common Shares			Warrants			Total
	Carrying value of investment in Symetra at	$	317.3		$	—		$	317.3		$	351.2		$	30.3		$	381.5
	beginning of period
	Equity in earnings (1)(2)	14.4			—			14.4			9.8			—			9.8
	Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio(3)	39.3			—			39.3			(8.7		)	—			(8.7		)
	Dividends received	(2.0		)	—			(2.0		)	(1.4		)	—			(1.4		)
	Decrease in value of warrants	—			—			—			—			(3.7		)	(3.7		)
	Carrying value of investment in Symetra at	$	369		$	—		$	369		$	350.9		$	26.6		$	377.5
	end of period(4)(5)
	(1) Equity in earnings excludes tax expense of $1.0 and $0.8.
	(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 $3.1 and $2.7 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(4) Includes White Mountains’s equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $39.3 and $(8.7) as of March 31, 2014 and 2013, which excludes tax (expense) benefit of $(2.8) and $0.7.
	(5) The aggregate value of White Mountains’s investment in common shares of Symetra was $397.4 based upon the quoted market price of $19.82 per share at March 31, 2014.
	During the three months ended March 31, 2014 and 2013, White Mountains received cash dividends from Symetra of $2.0 million and $1.4 million on its common share investment that were recorded as a reduction of White Mountains’s investment in Symetra. During the three months ended March 31, 2013, White Mountains also received dividends of $0.8 million from Symetra on its investment in Symetra warrants that was recorded in net investment income.
	Hamer
	White Mountains received an equity interest in Hamer, a small manufacturing company distributed to White Mountains in connection with the dissolution of the Tuckerman Capital, LP fund. Effective October 1, 2012, this investment has been accounted for under the equity method. For the three months ended March 31, 2014 and 2013, White Mountains recorded equity in earnings of $0.4 million and $0.3 million. For March 31, 2014, White Mountains also received $0.2 million of cash dividends.

Employee_ShareBased_Incentive_

Employee Share-Based Incentive Compensation Plans	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013 for performance shares granted under the WTM Incentive Plan:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	119,220		$	60.2		119,357		$	29.4
	Shares paid or expired(1)	(37,130	)	(26.7		)	(47,310	)	(11.0		)
	New grants	45,660		—			46,920		—
	Assumed forfeitures and cancellations(2)	(213	)	0.4			9		0.1
	Expense recognized	—		8.4			—		10.1
	Ending March 31,	127,537		$	42.3		118,976		$	28.6
	(1) WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.  WTM performance share payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target.
	(2) Amounts include changes in assumed forfeitures, as required under GAAP.
	For the 2011-2013 performance cycle, the Company issued common shares for 3,570 performance shares earned and all other performance shares earned were settled in cash. For the 2010-2012 performance cycle, the Company settled all performance shares earned in cash.
	If the outstanding WTM performance shares had vested on March 31, 2014, the total additional compensation cost to be recognized would have been $54.1 million, based on accrual factors at March 31, 2014 (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, 2014 for each performance cycle:
	Millions, except share amounts	Target		Accrued
		Performance		Expense
		Shares
		Outstanding
	Performance cycle:
	2014 – 2016	45,660		$	2.5
	2013 – 2015	47,170		15.9
	2012 – 2014	37,977		25
	Sub-total	130,807		43.4
	Assumed forfeitures	(3,270	)	(1.1		)
	Total at March 31, 2014	127,537		$	42.3
	Restricted Shares
	The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three months ended March 31, 2014 and 2013:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	94,130		$	17		69,910		$	16.8
	Issued	20,400		11.8			25,470		14.3
	Vested	(33,205	)	—			—		—
	Forfeited	—		—			—		—
	Expense recognized	—		(4.0		)	—		(2.9		)
	Non-vested at March 31,	81,325		$	24.8		95,380		$	28.2
	During the first quarter of 2014, White Mountains issued 20,400 restricted shares that vest on January 1, 2017. During the first quarter of 2013, White Mountains issued 25,470 restricted shares that vest on January 1, 2016. The unrecognized compensation cost at March 31, 2014 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, 2014 and 2013 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	493,421		$	4		563,190		$	1.2
	Shares paid or expired(1)	(142,138	)	(1.0		)	(238,658	)	—
	New grants	165,800		—			179,000		—
	Assumed forfeitures and cancellations(2)	(4,145	)	—			(13,665	)	—
Expense recognized	—		0.6			—		0.4
Ending March 31,	512,938		$	3.6		489,867		$	1.6
(1) OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. No payments were made in 2013 for the 2010-2012 OneBeacon performance cycle as the performance factor was zero. Amounts include deposits into OneBeacon’s deferred compensation plan.
(2) Amounts include changes in assumed forfeitures, as required under GAAP.
If the outstanding OneBeacon performance shares had been vested on March 31, 2014, the total additional compensation cost to be recognized would have been $4.7 million, based on accrual factors at March 31, 2014 (common share price and payout assumptions).
The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at March 31, 2014 for each performance cycle:
Millions, except share amounts	Target OneBeacon Performance Shares Outstanding		Accrued Expense
Performance cycle:
2014 – 2016	165,800		$	0.2
2013 – 2015	179,000		1.2
2012 – 2014	181,290		2.3
Sub-total	526,090		3.7
Assumed forfeitures	(13,152	)	(.1		)
Total at March 31, 2014	512,938		$	3.6
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, 2014 and 2013:
	Three Months Ended March 31,
	2014					2013
Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
	Shares		Issue Date			Shares		Issue Date
			Fair Value					Fair Value
Non-vested,
Beginning of period	915,000		$	6.5		927,000		$	9.6
Issued	—		—			—		—
Vested	(300,000	)	—			(9,000	)	—
Forfeited	(2,500	)	—			—		—
Expense recognized	—		(.8		)	—		(.8		)
Non-vested at March 31,	612,500		$	5.7		918,000		$	8.8
On March 1, 2012, OneBeacon issued 300,000 restricted shares that vest in two equal annual installments beginning on February 28, 2014.
On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments beginning on February 22, 2014.  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, 2014 is expected to be recognized ratably over the remaining vesting periods.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2014
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 liabilities 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 book value of financial instruments as of March 31, 2014 and December 31, 2013:
		March 31, 2014				December 31, 2013
	Millions	Fair		Carrying		Fair		Carrying
		Value		Value		Value		Value
	2012 OBH Senior Notes	$	275.1	$	274.7	$	269.8	$	274.7
	SIG Senior Notes	438		399.6		438.1		399.6
	SIG Preference Shares	262.5		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.

Discontinued_Operations

Discontinued Operations	3 Months Ended
	Mar. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]	'
Discontinued Operations	'
	Discontinued Operations
	Runoff Transaction
	As described in Note 1 and Note 2, in October 2012, OneBeacon entered into an agreement to sell the Runoff Business to Armour. During three months ended March 31, 2014 and 2013, the results of operations for the Runoff Business have been classified as discontinued operations and are presented, net of related income taxes, in the statement of comprehensive income. The assets and liabilities associated with the Runoff Business as of March 31, 2014 and December 31, 2013 have been presented in the balance sheet as held for sale. The amounts classified as discontinued operations exclude investing and financing activities that are conducted on an overall consolidated level and, accordingly, there were no separately identifiable investments associated with the Runoff Business.
	The Pennsylvania Insurance Department is currently conducting a required examination of the Runoff Business as part of its regulatory review of the Runoff Transaction. OneBeacon expects the Runoff Transaction to close in the second half of 2014.
	Results of Discontinued Operations
	For the three months ended March 31, 2014, White Mountains recorded a loss from discontinued operations of $.5 million substantially all a result of non-claims expenses related to the Runoff Business, including dedicated staff. For the three months ended March 31, 2013, White Mountains recorded income from discontinued operations of $.5 million primarily related to earned premiums from involuntary pools in the Runoff Business.
	Reinsurance
	Included in the assets held for sale are reinsurance recoverables from two reinsurance contracts with subsidiaries of Berkshire Hathaway Inc. that OneBeacon was required to purchase in connection with White Mountains’s acquisition of OneBeacon in 2001 (the “OneBeacon Acquisition”): a reinsurance contract with National Indemnity Company (“NICO”) for up to $2.5 billion in old asbestos and environmental (“A&E”) claims and certain other exposures (the “NICO Cover”) and an adverse loss reserve development cover from General Reinsurance Corporation (“GRC”) for up to $570.0 million, comprised of $400.0 million of adverse loss reserve development occurring in years 2000 and prior in addition to $170.0 million of reserves ceded as of the date of the OneBeacon Acquisition (the “GRC Cover”). The NICO Cover and GRC Cover, which were contingent on and occurred contemporaneously with the OneBeacon Acquisition, were put in place in lieu of a seller guarantee of loss and LAE reserves and are therefore accounted for under GAAP as a seller guarantee. As of March 31, 2014 and December 31, 2013, the total reinsurance recoverables on paid and unpaid losses of $1,188.8 million and $1,243.7 million related to both the NICO cover and the GRC cover have been included in assets held for sale. Both NICO and GRC have an A.M Best rating of A++, Superior, which is the highest of sixteen ratings.
	The total reinsurance recoverables on paid and unpaid losses in assets held for sale were $9.5 million and $1,673.6 million as of March 31, 2014. The reinsurance recoverable on unpaid amount is gross of $133.6 million in purchase accounting adjustments that will become recoverable if claims are paid in accordance with current reserve estimates.
	Net Assets Held for Sale
	The following summarizes the assets and liabilities associated with the business classified as held for sale:
	Millions	March 31,			December 31,
		2014			2013
	Assets held for sale
	Fixed maturity investments, at fair value	$	222.2		$	236.3
	Reinsurance recoverable on unpaid losses	1,540.00			1,604.70
	Reinsurance recoverable on paid losses	9.5			10.7
	Insurance premiums receivable	12.8			9.1
	Deferred tax asset	3.2			3.3
	Other assets	15.4			16
	Total assets held for sale	$	1,803.10		$	1,880.10
	Liabilities held for sale
	Loss and loss adjustment expense reserves	$	1,717.30		$	1,793.10
	Unearned insurance premiums	0.2			0.2
	Ceded reinsurance payable	12.1			12.3
	Other liabilities	73.5			74.5
	Total liabilities held for sale	1,803.10			1,880.10
	Net assets held for sale	$	—		$	—
	Net (Loss) Income from Discontinued Operations
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
		Three Months Ended
		March 31,
	Millions	2014			2013
	Revenues
	Earned insurance premiums	$	(.1	)	$	0.9
	Other revenue	—			—
	Total revenues	(.1		)	0.9
	Expenses
	Loss and loss adjustment expenses	—			—
	Insurance and reinsurance acquisition expenses	—			0.1
	Other underwriting expenses	0.7			0.1
	Total expenses	0.7			0.2
	Pre-tax (loss) income	(.8		)	0.7
	Income tax benefit (expense)	0.3			(.2		)
	Net (loss) income from discontinued operations	$	(.5	)	$	0.5
	(Loss) Earnings Per Share
	Basic (loss) 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 (loss) 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 (loss) earnings per share for discontinued operations for the three months ended March 31, 2014 and 2013:
		Three Months Ended
		March 31,
		2014			2013
	Basic and diluted (loss) earnings per share numerators (in millions):
	Net (loss) income attributable to White Mountains’s common shareholders	$	(.5	)	$	0.5
	Allocation of income for participating unvested restricted common shares (1)	—			—
	Net (loss) income attributable to White Mountains’s common shareholders,	$	(.5	)	$	0.5
	net of restricted common share amounts (2)
	Basic (loss) earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares (3)	(70.4		)	(79.0		)
	Basic (loss) earnings per share denominator	6,099.50			6,194.50
	Diluted (loss) earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares (3)	(70.4		)	(79.0		)
	Average outstanding dilutive options to acquire common shares (4)	—			—
Diluted (loss) earnings per share denominator	6,099.50			6,194.50
Basic and diluted (loss) earnings per share (in dollars):	$	(.08	)	$	0.07
(1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.
(2) Net (loss) earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed (loss) earnings for the three months ended March 31, 2014 and 2013.
(3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).
(4) The diluted (loss) earnings per share denominator for the three months ended March 31, 2014 and 2013 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Contingencies

Contingencies	3 Months Ended
	Mar. 31, 2014
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, 2014:
	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).
	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 have filed a notice of appeal.
	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. The Committee Action will proceed upon the lifting of the stay and a scheduling order from the court.

Loss_and_Loss_Adjustment_Expen1

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013:
		Three Months Ended
		March 31,
	Millions	2014			2013
	Gross beginning balance	$	3,079.30		$	3,168.90
	Less beginning reinsurance recoverable on unpaid losses	(428.1		)	(429.1		)
	Net loss and LAE reserves	2,651.20			2,739.80
	Loss and LAE reserves acquired (1)	—			21.3
	Loss and LAE reserves consolidated — SSIE	13.6			—
	Loss and LAE incurred relating to:
	Current year losses	240.5			242.9
	Prior year losses	(11.2		)	1.4
	Total incurred losses and LAE	229.3			244.3
	Accretion of fair value adjustment to loss and LAE reserves	0.2			1.2
	Foreign currency translation adjustment to loss and LAE reserves	1.8			(9.6		)
	Loss and LAE paid relating to:
	Current year losses	(35.0		)	(36.6		)
	Prior year losses	(237.9		)	(270.6		)
	Total loss and LAE payments	(272.9		)	(307.2		)
	Net ending balance	2,623.20			2,689.80
	Plus ending reinsurance recoverable on unpaid losses	425.2			410.8
	Gross ending balance	$	3,048.40		$	3,100.60
	(1) Loss and LAE reserves acquired relate to WM Solutions purchase of Ashmere in the first quarter of 2013.

Investment_Securities_Tables

Investment Securities (Tables)	3 Months Ended
	Mar. 31, 2014
Investments, Debt and Equity Securities [Abstract]	'
Pre-tax net investment income	'
	Pre-tax net investment income for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2014				2013
	Investment income:
	Fixed maturity investments		$	22.4			$	26
	Short-term investments		0.6				0.8
	Common equity securities		5.4				4.7
	Convertible fixed maturity investments		0.4				0.8
	Other long-term investments		0.3				0.7
	Total investment income		29.1				33
	Less third-party investment expenses		(4.8		)		(4.5		)
	Net investment income, pre-tax		$	24.3			$	28.5
Schedule of Realized and Unrealized Gain (Loss) on Investments [Table Text Block]	'
	Net realized and unrealized investment gains and losses for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended
			March 31,
	Millions		2014				2013
	Net realized investment gains, pre-tax		$	21.7			$	36.5
	Net unrealized investment gains, pre-tax		42.1				38.7
	Net realized and unrealized investment gains, pre-tax		63.8				75.2
	Income tax expense attributable to net realized and		(15.7		)		(13.7		)
	unrealized investment gains
	Net realized and unrealized investment gains, after tax		$	48.1			$	61.5
Realized Gain (Loss) on Investments [Table Text Block]	'
	Net realized investment gains (losses) for the three months ended March 31, 2014 and 2013 consisted of the following:
			Three Months Ended												Three Months Ended
			March 31, 2014												March 31, 2013
	Millions		Net				Net				Total net realized				Net				Net				Total net realized
			realized				foreign				gains (losses)				realized				foreign				gains (losses)
			gains				exchange				reflected in				gains				currency				reflected in
			(losses)				gains				earnings				(losses)				gains				earnings
							(losses)												(losses)
	Fixed maturity investments		$	4.4			$	(3.5	)		$	0.9			$	22.6			$	(14.6	)		$	8
	Short-term investments		—				—				—				—				0.1				0.1
	Common equity securities		18.9				(.1		)		18.8				22.6				(.2		)		22.4
	Convertible fixed maturity investments		2.4				—				2.4				2.7				—				2.7
	Other long-term investments		(.3		)		—				(.3		)		2.9				—				2.9
	Forward contracts		(.1		)		—				(.1		)		0.4				—				0.4
	Net realized investment gains (losses),		25.3				(3.6		)		21.7				51.2				(14.7		)		36.5
	pre-tax
	Income tax expense attributable to		(4.6		)		0.9				(3.7		)		(11.8		)		3.3				(8.5		)
	net realized investment
	(losses) gains
	Net realized investment gains (losses),		$	20.7			$	(2.7	)		$	18			$	39.4			$	(11.4	)		$	28
	after tax
Schedule of unrealized gain (loss) on investments	'
	he following table summarizes net unrealized investment gains (losses) for the three months ended March 31, 2014 and 2013:
			Three Months Ended												Three Months Ended
			March 31, 2014												March 31, 2013
	Millions		Net				Net				Total net unrealized				Net				Net				Total net unrealized
			unrealized				foreign				gains (losses)				unrealized				foreign				gains (losses)
			gains				exchange				reflected in				gains				currency				reflected in
			(losses)				gains				earnings				(losses)				gains				earnings
							(losses)												(losses)
	Fixed maturity investments		$	19.4			$	12.2			$	31.6			$	(30.6	)		$	4.6			$	(26.0	)
	Short-term investments		—				—				—				—				—				—
	Common equity securities		4				0.5				4.5				61.9				—				61.9
	Convertible fixed maturity investments		0.7				—				0.7				(1.1		)		(.2		)		(1.3		)
	Other long-term investments		4.9				0.4				5.3				3.6				0.5				4.1
	Forward contracts		—				—				—				—				—				—
	Net unrealized investment gains		29				13.1				42.1				33.8				4.9				38.7
	(losses), pre-tax
	Income tax expense attributable to		(9.1		)		(2.9		)		(12.0		)		(4.1		)		(1.1		)		(5.2		)
	net unrealized investment
	(losses) gains
	Net unrealized investment gains		$	19.9			$	10.2			$	30.1			$	29.7			$	3.8			$	33.5
	(losses), after tax
Net unrealized investment gains (losses) for Level 3 investments	'
	The following table summarizes the amount of total pre-tax gains included in earnings attributable to unrealized investment gains for Level 3 investments for the three months ended March 31, 2014 and 2013:
			Three Months Ended
			March 31,
	Millions		2014				2013
	Fixed maturity investments		$	0.2			$	0.2
	Common equity securities		0.8				(.1		)
	Other long-term investments		6.1				6.4
	Total unrealized investment gains, pre-tax - Level 3 investments		$	7.1			$	6.5
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, 2014 and December 31, 2013 were as follows:
			March 31, 2014
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains (losses)
	U.S. Government and agency obligations		$	539.2			$	0.6			$	(.8	)		$	(.6	)		$	538.4
	Debt securities issued by corporations		2,311.60				48.3				(7.1		)		(6.3		)		2,346.50
	Municipal obligations		33.5				0.2				(.2		)		—				33.5
	Mortgage-backed and asset-backed securities		1,847.90				4.1				(6.3		)		(3.5		)		1,842.20
	Foreign government, agency and provincial obligations		366.4				3.4				(1.1		)		(4.3		)		364.4
	Preferred stocks		79.9				6.1				—				(.1		)		85.9
	Total fixed maturity investments including assets		$	5,178.50			$	62.7			$	(15.5	)		$	(14.8	)		$	5,210.90
	held for sale
	Fixed maturity investments reclassified to assets																		(222.2		)
	held for sale related to the Runoff Transaction
	Total fixed maturity investments																		$	4,988.70
			December 31, 2013
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains (losses)
	U.S. Government and agency obligations		$	365.5			$	0.5			$	(1.0	)		$	(2.5	)		$	362.5
	Debt securities issued by corporations		2,330.70				44				(13.2		)		(14.3		)		2,347.20
	Municipal obligations		18.3				—				(.4		)		—				17.9
	Mortgage-backed and asset-backed securities		2,027.30				2.4				(9.9		)		(5.3		)		2,014.50
	Foreign government, agency and provincial obligations		444.2				3.7				(3.2		)		(4.8		)		439.9
	Preferred stocks		79.9				5.1				—				(.2		)		84.8
	Total fixed maturity investments including assets		$	5,265.90			$	55.7			$	(27.7	)		$	(27.1	)		$	5,266.80
	held for sale
	Fixed maturity investments reclassified to assets																		(236.3		)
	held for sale related to the Runoff Transaction
	Total fixed maturity investments																		$	5,030.50
	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 maturities and other long-term investments as of March 31, 2014 and December 31, 2013 were as follows:
			March 31, 2014
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains (losses)
	Common equity securities		$	905.6			$	273.9			$	(4.5	)		$	(.4	)		$	1,174.60
	Convertible fixed maturity investments		$	64.3			$	10.6			$	(1.0	)		$	(.1	)		$	73.8
	Other long-term investments		$	246.4			$	83.4			$	(25.5	)		$	(2.0	)		$	302.3
			December 31, 2013
	Millions		Cost or				Gross				Gross				Net foreign				Carrying
			amortized				unrealized				unrealized				currency				value
			cost				gains				losses				gains (losses)
	Common equity securities		$	890.2			$	271			$	(3.6	)		$	(.8	)		$	1,156.80
	Convertible fixed maturity investments		$	71.7			$	9.9			$	(.9	)		$	(.2	)		$	80.5
	Other long-term investments		$	238.3			$	79.6			$	(26.6	)		$	(2.4	)		$	288.9
Other long-term investments	'
	The following table summarizes investments in hedge funds and private equity interests by investment objective and sector at March 31, 2014 and December 31, 2013:
			March 31, 2014								December 31, 2013
	Millions		Fair Value				Unfunded				Fair Value				Unfunded
							Commitments								Commitments
	Hedge funds
	Long/short equity		$	67.6			$	—			$	62.6			$	—
	Long/short credit & distressed		23.3				—				22.8				—
	Long/short equity REIT		18.8				—				18.3				—
	Long/short equity activist		16.3				—				16.8				—
	Long bank loan		0.2				—				0.2				—
	Long diversified strategies		—				—				0.1				—
	Total hedge funds		126.2				—				120.8				—
	Private equity funds
	Energy infrastructure & services		46.8				12.7				45.9				13.1
	Multi-sector		24.1				6.4				23.8				6.5
	Manufacturing/Industrial		11.5				15.2				11.2				15.5
	Aerospace/Defense/Government		10.6				15.5				5.8				19.2
	Private equity secondaries		9.2				3.1				9.5				3.1
	Healthcare		7.8				2.8				5.6				2.8
	Real estate		7.1				3.3				8.2				3.3
	Insurance		2.4				41.3				2.3				41.3
	Venture capital		1.6				0.3				1.6				0.3
	International multi-sector, Asia		—				2.7				—				2.7
	International multi-sector, Europe		3.8				2.8				3.9				2.8
	Total private equity funds		124.9				106.1				118.2				110.6
	Total hedge and private equity funds included in		$	251.1			$	106.1			$	239			$	110.6
	other long-term investments(1)
	(1)  Excludes carrying value of $25.0 and $26.6 at March 31, 2014 and December 31, 2013 associated with hedge funds and private equity funds accounted for using the equity method.
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, 2014 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		$	4.9			$	—			$	—			$	5.5			$	10.4
	Quarterly		29.4				30.4				11.9				8.8				80.5
	Semi-annual		—				25.8				—				—				25.8
	Annual		—				—				9.3				0.2				9.5
	Total		$	34.3			$	56.2			$	21.2			$	14.5			$	126.2
Fair Value of private equity funds subject to lock-up periods	'
	At March 31, 2014, 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		$6.50		$29.50		$75.70		$13.20		$124.90
Fair value measurements by level, investment securities	'
	The following tables summarize White Mountains’s fair value measurements for investments at March 31, 2014 and December 31, 2013, by level:
			March 31, 2014
	Millions		Fair value				Level 1				Level 2				Level 3
	Fixed maturity investments:
	U.S. Government and agency obligations		$	538.4			$	471.7			$	66.7			$	—
	Debt securities issued by corporations:
	Consumer		741				—				741				—
	Financials		446				—				446				—
	Communications		267.5				—				267.5				—
	Industrial		251.5				—				251.5				—
	Energy		194.2				—				194.2				—
	Utilities		132.7				—				132.7				—
	Basic Materials		140.1				—				140.1				—
	Technology		91.5				—				91.5				—
	Other		82				—				82				—
	Total debt securities issued by corporations:		2,346.50				—				2,346.50				—
	Mortgage-backed and asset-backed securities		1,842.20				—				1,800.60				41.6
	Foreign government, agency and provincial obligations		364.4				42.8				321.6				—
	Preferred stocks		85.9				—				14.6				71.3
	Municipal obligations		33.5				—				33.5				—
	Total fixed maturity investments (1)		5,210.90				514.5				4,583.50				112.9
	Short-term investments		674				666.6				7.4				—
	Common equity securities:
	Financials		359.8				312.9				—				46.9
	Consumer		313.8				313.8				—				—
	Industrial		106.5				106.5				—				—
	Energy		81.1				81.1				—				—
	Communications		56.5				56.5				—				—
	Basic materials		56				56				—				—
	Technology		44.9				44.9				—				—
	Utilities		36.7				36.7				—				—
	Other		119.3				42.7				76.3				0.3
	Total common equity securities		1,174.60				1,051.10				76.3				47.2
	Convertible fixed maturity investments		73.8				—				67.7				6.1
	Other long-term investments (2)		277.4				—				—				277.4
	Total investments		$	7,410.70			$	2,232.20			$	4,734.90			$	443.6
	(1) Carrying value includes $222.2 that is classified as assets held for sale relating to discontinued operations.
	(2) Excludes carrying value of $25.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts.
			December 31, 2013
	Millions		Fair value				Level 1				Level 2				Level 3
	Fixed maturity investments:
	U.S. Government and agency obligations		$	362.5			$	295.8			$	66.7			$	—
	Debt securities issued by corporations:
	Consumer		754.4				—				754.4				—
	Industrial		434.4				—				434.4				—
	Communications		281.1				—				281.1				—
	Financials		265				—				265				—
	Basic materials		173.6				—				173.6				—
	Energy		159.7				—				159.7				—
	Utilities		149.1				—				149.1				—
	Technology		91.2				—				91.2				—
	Other		38.7				—				38.7				—
	Total debt securities issued by corporations:		2,347.20				—				2,347.20				—
	Mortgage-backed and asset-backed securities		2,014.50				—				1,992.50				22
	Foreign government, agency and provincial obligations		439.9				44.5				395.4				—
	Preferred stocks		84.8				—				13.8				71
	Municipal obligations		17.9				—				17.9				—
	Total fixed maturity investments (1)		5,266.80				340.3				4,833.50				93
	Short-term investments		635.9				621.5				14.4				—
	Common equity securities:
	Financials		360.4				314.3				—				46.1
	Consumer		308.2				308.2				—				—
	Industrial		105.4				105.4				—				—
Energy		78.6				78.6				—				—
Utilities		60.6				60.6				—				—
Communications		57.1				57.1				—				—
Basic materials		53.4				53.4				—				—
Technology		34.3				34.3				—				—
Other		98.8				24.5				74.3				—
Total common equity securities		1,156.80				1,036.40				74.3				46.1
Convertible fixed maturity investments		80.5				—				74.4				6.1
Other long-term investments (2)		262.4				—				—				262.4
Total investments		$	7,402.40			$	1,998.20			$	4,996.60			$	407.6
(1) Carrying value includes $236.3 that is classified as assets held for sale relating to discontinued operations.
(2) Excludes carrying value of $26.6 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.
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, 2014 and December 31, 2013:
			Fair Value at
	Millions		March 31, 2014				December 31, 2013
	AAA		$	—			$	—
	AA		221.3				228.8
	A		1,028.90				1,039.50
	BBB		1,089.90				1,075.50
	BB		—				—
	Other		6.4				3.4
	Debt securities issued by corporations(1)		$	2,346.50			$	2,347.20
	(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, 2014 and December 31, 2013:
			March 31, 2014												December 31, 2013
	Millions		Fair Value				Level 2				Level 3				Fair Value				Level 2				Level 3
	Mortgage-backed securities:
	Agency:
	GNMA		$	464.8			$	464.8			$	—			$	512.3			$	512.3			$	—
	FNMA		167.3				167.3				—				81.2				81.2				—
	FHLMC		85.6				85.6				—				91.3				91.3				—
	Total Agency(1)		717.7				717.7				—				684.8				684.8				—
	Non-agency:
	Residential		132.3				132.3				—				125.7				125.7				—
	Commercial		329.7				310.1				19.6				282.3				282.3				—
	Total Non-agency		462				442.4				19.6				408				408				—
	Total mortgage-backed securities		1,179.70				1,160.10				19.6				1,092.80				1,092.80				—
	Other asset-backed securities:
	Credit card receivables		229.7				207.7				22				311.4				289.4				22
	Vehicle receivables		231.5				231.5				—				365				365				—
	Other		201.3				201.3				—				245.3				245.3				—
	Total other asset-backed securities		662.5				640.5				22				921.7				899.7				22
	Total mortgage and asset-backed securities		$	1,842.20			$	1,800.60			$	41.6			$	2,014.50			$	1,992.50			$	22
	(1)  Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	'
	The security issuance years of White Mountains’s investments in non-agency RMBS and non-agency CMBS securities as of March 31, 2014 are as follows:
											Security Issuance Year
	Millions		Fair Value				2004				2005				2006				2007				2008			2009		2010		2011		2012		2013		2014
	Non-agency RMBS		$	132.3			$	11.3			$	20.2			$	11			$	—			$	19.6		$	—	$	26.4	$	—	$	—	$	32.8	$	11
	Non-agency CMBS		329.7				—				—				8.5				5				26.4			—		13		27		131.8		94.3		23.7
	Total		$	462			$	11.3			$	20.2			$	19.5			$	5			$	46		$	—	$	39.4	$	27	$	131.8	$	127.1	$	34.7
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, 2014:
	Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
	Prime		$	105.9			$	32.4			$	73.5			$	—
	Non-prime		19.2				—				19.2				—
	Sub-prime		7.2				7.2				—				—
	Total		$	132.3			$	39.6			$	92.7			$	—
	(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to other "AAA" or "Aaa" bonds.
	(2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to non-"AAA" or non-"Aaa" bonds.
	(3) At issuance, Subordinate were not rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were junior to "AAA" or "Aaa" bonds.
Non-agency commercial mortgage securities, type of interest rate and tranche levels	'
	The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of March 31, 2014:
	Millions		Fair Value				Super Senior (1)				Senior (2)				Subordinate (3)
	Fixed rate CMBS		$	225.1			$	123.5			$	76			$	25.6
	Floating rate CMBS		104.6				5				26.4				73.2
	Total		$	329.7			$	128.5			$	102.4			$	98.8
	(1)  At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to other "AAA" or "Aaa" bonds.
	(2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were senior to non-"AAA" or non-"Aaa" bonds.
	(3) At issuance, Subordinate were not rated "AAA" by Standard & Poor’s, "Aaa" by Moody’s or "AAA" by Fitch and were junior to "AAA" or "Aaa" bonds.
Rollforward of fair value investments by level	'
	The following tables summarize the changes in White Mountains’s fair value measurements by level for the three months ended March 31, 2014 and 2013:
								Level 3 Investments
	Millions	Level 1 Investments			Level 2			Fixed			Common			Convertible			Other long-term				Total
					Investments			Maturities			equity			fixed			investments
											securities			maturities
	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 unrealized	22.2			34.7			0.4			0.8			—			6.3				64.4			(4)
	gains (losses)
	Foreign currency losses	(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 (sales) 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)(4)
	March 31, 2014
	(1)  Excludes carrying value of $26.6 and $25.0 at January 1, 2014 and March 31, 2014 associated with other long-term investments accounted for using the equity method and $(0.1) and $(0.1) at January 1, 2014 and March 31, 2014 related to forward contracts.
	(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 gains associated with the Prospector Funds consolidation of investment-related liabilities and $1.2 realized and unrealized losses associated with other long-term investments accounted for using the equity method.
								Level 3 Investments
	Millions	Level 1 Investments			Level 2			Fixed			Common			Convertible			Other long-term				Total
					Investments			Maturities			equity			fixed			investments
											securities			maturities
	Balance at January 1, 2013	$	1,355.10		$	5,206.10		$	92.9		$	37.3		$	—		$	259.3			$	6,950.70		(1)(2)(3)
	Total realized and	80.1			(10.8		)	0.1			(.1		)	—			5.8				75.1			(3)
	unrealized gains (losses)
	Foreign currency losses	(.4		)	(4.8		)	—			—			—			(.4		)		(5.6		)
	through OCI
	Amortization/Accretion	(.4		)	(14.0		)	—			—			—			—				(14.4		)
	Purchases	318.6			1,445.80			8			—			—			18.9				1,791.30
	Sales	(382.3		)	(1,828.3		)	—			—			—			(23.1		)		(2,233.7		)
	Net change in investments	14.5			2.7			—			—			—			—				17.2
	related to purchases (sales)
	of consolidated affiliates
	Transfers in	1.2			—			—			—			—			—				1.2
	Transfers out	—			(.8		)	—			(.4		)	—			—				(1.2		)
	Balance at	1,386.40			4,795.90			101			36.8			—			260.5				6,580.60			(1)(2)(3)
	March 31, 2013
	(1) Excludes carrying value of $35.0 and $34.9 at January 1, 2013 and March 31, 2013 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) and $(0.2) at January 1, 2013 and March 31, 2013 related to forward contracts.
	(2)  Carrying value includes $338.1 and 296.9 at January 1, 2013 and March 31, 2013 that is classified as assets held for sale relating to discontinued operations.
	(3) Excludes carrying value of $630.6 and $654.4 at January 1, 2013 and March 31, 2013 and realized and unrealized loss for the period of $0.1 associated with short-term investments.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	'
	The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 other than hedge funds and private equity funds. The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds.
	($ in Millions)		March 31, 2014
	Description		Fair Value		Rating (2)		Valuation Technique(s)		Unobservable Input
	Non-agency commercial mortgage backed securities(1)		$13.30		AAA		Discounted cash flow		Discount Spread over Swap		0.80%
									Prepayment rate		0.00%	CPY
									Default rate		0.00%	CDR
	Asset-backed securities(1)		$22.00		AA+		Broker pricing		Broker quote
	Non-agency commercial mortgage backed securities(1)		$6.30		BBB-		Broker pricing		Broker quote
	Preferred stock(1)		$71.30		NR		Discounted cash flow		Discount yield		6.90%
	Private equity security(1)		$36.40		NR		Multiple of GAAP book value		Book value multiple		1
	Private equity security(1)		$10.50		NR		Share price of recent transaction		Average share price		$1.10
	Convertible preferred securities		$6.10		NR		Share price of recent transaction		Recent market transaction		$6.10
	(1) As of March 31, 2014, consists of one security.
	(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.
	($ in Millions)		December 31, 2013
	Description		Fair Value		Rating (2)		Valuation Technique(s)		Unobservable Input
	Asset-backed securities(1)		$22.00		AA+		Broker pricing		Broker quote
	Preferred stock(1)		$71.00		NR		Discounted cash flow		Discount yield		7.40%
	Private equity security(1)		$35.60		NR		Multiple of GAAP book value		Book value multiple		1
	Private equity security(1)		$10.50		NR		Share price of recent transaction		Average share price		$1.10
	Convertible preferred securities		$6.10		NR		Share price of recent transaction		Recent market transaction		$6.10
	(1) As of December 31, 2013, consists of one security.
	(2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s.

Debt_Tables

Debt (Tables)	3 Months Ended
	Mar. 31, 2014
Debt Disclosure [Abstract]	'
Schedule of debt outstanding	'
	White Mountains’s debt outstanding as of March 31, 2014 and December 31, 2013 consisted of the following:
	Millions	March 31,			December 31,
		2014			2013
	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	(.4		)	(.4		)
	SIG Senior Notes, carrying value	399.6			399.6
	WTM Bank Facility	—			—
	Old Lyme Note	2.1			2.1
	Other	1.2			—
	Total debt	$	677.6		$	676.4

Derivatives_Tables

Derivatives (Tables)	3 Months Ended
	Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]	'
Derivatives	'
	Derivatives
Interest Rate Cap	'
Derivative Instruments and Hedging Activities Disclosures [Line Items]	'
Derivatives	'
	Interest Rate Cap
	In May 2007, SIG issued the SIG Preference Shares, with an initial fixed annual dividend rate of 7.506%. In June 2017, the fixed rate will move to a floating rate equal to the greater of (i) 7.506% and (ii) 3-month LIBOR plus 320 bps. 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 bps of the $250.0 million notional value, or approximately $9.9 million for the full notional amount.
	The Interest Rate Cap does not qualify for hedge accounting. It is recorded in other assets at fair value. Changes in fair value are recognized 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, 2014.
	The following tables summarize the changes in the fair value of the Interest Rate Cap for the three months ended March 31, 2014:
		Three Months Ended
		March 31,
	Millions	2014
	Beginning of period	$	11.1
	Purchases	—
	Net realized and unrealized losses	(2.9		)
	Sales/settlements	—
	End of period	$	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-14
	Millions	Collateral Balances Held				Standard & Poor’s
						Rating(1)
	Barclays Bank Plc	$	5.7			A
	Nordea Bank Finland Plc	2.5				AA-
	Total	$	8.2
	(1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-one creditworthiness ratings) and “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings).
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, 2014:
		Three Months Ended
		March 31,
	Millions	2014
	Beginning of period	$	11.1
	Purchases	—
	Net realized and unrealized losses	(2.9		)
	Sales/settlements	—
	End of period	$	8.2
Summary of additional collateral posted for variable reinsurance counterparties	'
	The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty:
		31-Mar-14
	Millions	Collateral Balances Held				Standard & Poor’s
						Rating(1)
	Barclays Bank Plc	$	5.7			A
	Nordea Bank Finland Plc	2.5				AA-
	Total	$	8.2
	(1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-one creditworthiness ratings) and “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings).
Weather derivatives	'
Derivative Instruments and Hedging Activities Disclosures [Line Items]	'
Derivatives	'
	Weather Derivatives
	For the three months ended March 31, 2014, Sirius Group recognized $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.
Variable Annuity	'
Derivative Instruments and Hedging Activities Disclosures [Line Items]	'
Derivatives	'
	Variable Annuity Reinsurance
	White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. At March 31, 2014 and December 31, 2013, the total guarantee value was approximately ¥187.9 billion (approximately $1.8 billion at exchange rates on that date) and ¥203.6 billion (approximately $1.9 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 103% and 104% of the guarantee value at March 31, 2014 and December 31, 2013, respectively.  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, 2014 and 2013.
		Three Months Ended
		March 31,
	Millions	2014				2013
	Fees, included in other revenue	$	5.4			$	6.8
	Change in fair value of variable annuity liability,	(1.3		)		184.3
	included in other revenue
	Change in fair value of derivatives, included in other revenue	(6.6		)		(181.8		)
	Foreign exchange, included in other revenue	0.2				(14.6		)
	Other investment income and gains (losses)	0.4				(4.0		)
	Total revenue	(1.9		)		(9.3		)
	Change in fair value of variable annuity death benefit liabilities,	—				5.6
	included in other general and administrative expenses
	Death benefit claims paid, included in general and	—				(1.1		)
	administrative expenses
	General and administrative expenses	(1.3		)		(1.7		)
	Pre-tax loss	$	(3.2	)		$	(6.5	)
	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three months ended March 31, 2014 and 2013 and the carrying values, included in other assets, at March 31, 2014 and December 31, 2013 by type of instrument:
		Gains (losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	Fixed income/interest rate	$	(6.4	)		$	(30.9	)	$	(5.3	)		$	(9.7	)
	Foreign exchange	(6.5		)		(53.6		)	63.9				58
	Equity	6.3				(97.3		)	31				20.9
	Total	$	(6.6	)		$	(181.8	)	$	89.6			$	69.2
	The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three months ended March 31, 2014 and 2013:
		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 (losses) gains	(1.3		)	(4)	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
		Three Months Ended March 31, 2013
		Variable Annuity				Derivative Instruments
		(Liabilities)
	Millions	Level 3				Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	(441.5	)		$	140.5		$	(20.5	)		$	(21.7	)		$	98.3
	Purchases	—				36.1			—				—				36.1
	Realized and unrealized gains (losses)	189.9			(4)	(49.2		)	(85.5		)		(47.1		)		(181.8		)
	Transfers in	—				—			—				—				—
	Sales/settlements	—				—			63.1				60.1				123.2
	End of period	$	(251.6	)		$	127.4		$	(42.9	)		$	(8.7	)		$	75.8
	(1) Consists of over-the-counter instruments.
	(2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available.  Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
	(3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
	(4) Includes $0.0 and $5.6 for the three months ended March 31, 2014 and 2013 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.
	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments posted as collateral to its variable annuity reinsurance hedging counterparties.  The total collateral comprises the following:
	Millions	March 31, 2014				December 31, 2013			March 31, 2013
	Cash	$	22.2			$	56.1		$	188.5
	Short-term investments	—				2			15.6
	Fixed maturity investments	14.3				23.2			25.7
	Total	$	36.5			$	81.3		$	229.8
	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, 2014 and 2013. 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, 2014
Description	Fair				Valuation Technique(s)		Unobservable Input	Range					Weighted
	Value												Average
Variable annuity benefit guarantee liabilities	$	54.1			Discounted cash flows		Surrenders
							1 year	0.3	%	-	30.00%		18	%
							2 year	0.2	%	-	32.00%		13.6	%
							Mortality	0	%	-	5.90%		1	%
							Foreign exchange volatilities
							1 year	10.5	%	-	13.50%		11.4	%
							2 year	11.7	%	-	14.60%		12.6	%
							Index volatilities
							1 year	16.1	%	-	19.70%		17.8	%
							2 year	17	%	-	19.40%		18.3	%
Foreign exchange options	$	31.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	(1.2	)%	-	8.00%		2.4	%
Equity index options	$	32.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	0.5	%	-	9.70%		3.5	%
WM Life Re enters into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee.  In the case of OTC derivatives, WM Life Re has exposure to credit risk for amounts that are uncollateralized by counterparties. WM Life Re’s internal risk management guidelines establish net counterparty exposure thresholds that take into account OTC counterparties’ credit ratings. The OTC derivative contracts are subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements.
In the case of exchange traded instruments, WM Life Re has exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re has entered into master netting agreements with certain of its counterparties whereby the collateral provided (held) is calculated on a net basis. The following summarizes amounts offset under master netting agreements:
	March 31, 2014											December 31, 2013
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	$	1.2			$	(6.4	)	$	(5.2	)		$	2.4			$	(11.7	)	$	(9.3	)
Exchange traded	0.5				(.6		)	(.1		)		1				(1.6		)	(.6		)
Foreign exchange contracts
OTC	64.4				—			64.4				67.8				(12.0		)	55.8
Exchange traded	0.1				(.6		)	(.5		)		2.3				—			2.3
Equity contracts
OTC	32.6				(2.5		)	30.1				30.7				(9.2		)	21.5
Exchange traded	0.9				—			0.9				1.8				(2.3		)	(.5		)
Total(2)	$	99.7			$	(10.1	)	$	89.6			$	106			$	(36.8	)	$	69.2
(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-14
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	$	29.1			$	—		$	—			$	29.1			$	—		$	—		$	—	$	29.1	A
Barclays	1.2				—			—				1.2				—			—			—		1.2		A
JP Morgan	22.6				—			—				22.6				4.7			—			—		27.3		A	+
Royal Bank	11.4				—			—				11.4				—			—			—		11.4		A	-
of Scotland
Nomura	0.7				—			—				0.7				4.8			14.3			—		19.8		BBB	+
Citigroup - OTC	24.2				—			—				24.2				—			—			3.8		20.4		A
Citigroup -	0.4				—			—				0.4				10.9			—			—		11.3		A
Exchange Traded
Total	$	89.6			$	—		$	—			$	89.6			$	20.4		$	14.3		$	3.8	$	120.5
	31-Dec-13
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	$	27.2			$	—		—				$	27.2			$	—		$	—		$	—	$	27.2	A
Barclays	1.4				—			—				1.4				—			—			—		1.4		A	+
JP Morgan	9.1				—			—				9.1				22			—			—		31.1		A	+
Royal Bank	11.3				—			—				11.3				—			—			—		11.3		A
of Scotland
Nomura	(.4		)		—			0.4				—				—			22.8			0.8		22		BBB	+
Citigroup - OTC	19.4				—			—				19.4				2.3			—			—		21.7		A
Citigroup -	1.2				—			—				1.2				19.8			—			—		21		A
Exchange Traded
Total	$	69.2			$	—		$	0.4			$	69.6			$	44.1		$	22.8		$	0.8	$	135.7
(1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-one creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and
“BBB+” (Adequate, which is the eighth highest of twenty-one creditworthiness ratings).
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, 2014 and 2013.
		Three Months Ended
		March 31,
	Millions	2014				2013
	Fees, included in other revenue	$	5.4			$	6.8
	Change in fair value of variable annuity liability,	(1.3		)		184.3
	included in other revenue
	Change in fair value of derivatives, included in other revenue	(6.6		)		(181.8		)
	Foreign exchange, included in other revenue	0.2				(14.6		)
	Other investment income and gains (losses)	0.4				(4.0		)
	Total revenue	(1.9		)		(9.3		)
	Change in fair value of variable annuity death benefit liabilities,	—				5.6
	included in other general and administrative expenses
	Death benefit claims paid, included in general and	—				(1.1		)
	administrative expenses
	General and administrative expenses	(1.3		)		(1.7		)
	Pre-tax loss	$	(3.2	)		$	(6.5	)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument	'
	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three months ended March 31, 2014 and 2013 and the carrying values, included in other assets, at March 31, 2014 and December 31, 2013 by type of instrument:
		Gains (losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	Fixed income/interest rate	$	(6.4	)		$	(30.9	)	$	(5.3	)		$	(9.7	)
	Foreign exchange	(6.5		)		(53.6		)	63.9				58
	Equity	6.3				(97.3		)	31				20.9
	Total	$	(6.6	)		$	(181.8	)	$	89.6			$	69.2
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, 2014 and 2013:
		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 (losses) gains	(1.3		)	(4)	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
		Three Months Ended March 31, 2013
		Variable Annuity				Derivative Instruments
		(Liabilities)
	Millions	Level 3				Level 3 (1)			Level 2 (1)(2)				Level 1 (3)				Total
	Beginning of period	$	(441.5	)		$	140.5		$	(20.5	)		$	(21.7	)		$	98.3
	Purchases	—				36.1			—				—				36.1
	Realized and unrealized gains (losses)	189.9			(4)	(49.2		)	(85.5		)		(47.1		)		(181.8		)
	Transfers in	—				—			—				—				—
	Sales/settlements	—				—			63.1				60.1				123.2
	End of period	$	(251.6	)		$	127.4		$	(42.9	)		$	(8.7	)		$	75.8
	(1) Consists of over-the-counter instruments.
	(2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available.  Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.
	(3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
	(4) Includes $0.0 and $5.6 for the three months ended March 31, 2014 and 2013 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses
Summary of additional collateral posted for variable reinsurance counterparties	'
	In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments posted as collateral to its variable annuity reinsurance hedging counterparties.  The total collateral comprises the following:
	Millions	March 31, 2014				December 31, 2013			March 31, 2013
	Cash	$	22.2			$	56.1		$	188.5
	Short-term investments	—				2			15.6
	Fixed maturity investments	14.3				23.2			25.7
	Total	$	36.5			$	81.3		$	229.8
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, 2014
	Description	Fair				Valuation Technique(s)		Unobservable Input	Range					Weighted
		Value												Average
	Variable annuity benefit guarantee liabilities	$	54.1			Discounted cash flows		Surrenders
								1 year	0.3	%	-	30.00%		18	%
								2 year	0.2	%	-	32.00%		13.6	%
								Mortality	0	%	-	5.90%		1	%
								Foreign exchange volatilities
								1 year	10.5	%	-	13.50%		11.4	%
								2 year	11.7	%	-	14.60%		12.6	%
								Index volatilities
								1 year	16.1	%	-	19.70%		17.8	%
								2 year	17	%	-	19.40%		18.3	%
	Foreign exchange options	$	31.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	(1.2	)%	-	8.00%		2.4	%
	Equity index options	$	32.5			Counterparty valuations, adjusted for unwind quote discount		Adjustment to counterparty valuations	0.5	%	-	9.70%		3.5	%
Summary of value, collateral provided (held) by WM Life Re and net exposure on OTC derivative instruments	'
	The following summarizes amounts offset under master netting agreements:
		March 31, 2014											December 31, 2013
	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	$	1.2			$	(6.4	)	$	(5.2	)		$	2.4			$	(11.7	)	$	(9.3	)
	Exchange traded	0.5				(.6		)	(.1		)		1				(1.6		)	(.6		)
	Foreign exchange contracts
	OTC	64.4				—			64.4				67.8				(12.0		)	55.8
	Exchange traded	0.1				(.6		)	(.5		)		2.3				—			2.3
	Equity contracts
	OTC	32.6				(2.5		)	30.1				30.7				(9.2		)	21.5
	Exchange traded	0.9				—			0.9				1.8				(2.3		)	(.5		)
	Total(2)	$	99.7			$	(10.1	)	$	89.6			$	106			$	(36.8	)	$	69.2
	(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.
Schedule of Uncollateralized Amounts Due	'
	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-14
	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	$	29.1			$	—		$	—			$	29.1			$	—		$	—		$	—	$	29.1	A
	Barclays	1.2				—			—				1.2				—			—			—		1.2		A
	JP Morgan	22.6				—			—				22.6				4.7			—			—		27.3		A	+
	Royal Bank	11.4				—			—				11.4				—			—			—		11.4		A	-
	of Scotland
	Nomura	0.7				—			—				0.7				4.8			14.3			—		19.8		BBB	+
	Citigroup - OTC	24.2				—			—				24.2				—			—			3.8		20.4		A
	Citigroup -	0.4				—			—				0.4				10.9			—			—		11.3		A
	Exchange Traded
	Total	$	89.6			$	—		$	—			$	89.6			$	20.4		$	14.3		$	3.8	$	120.5
		31-Dec-13
	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	$	27.2			$	—		—				$	27.2			$	—		$	—		$	—	$	27.2	A
	Barclays	1.4				—			—				1.4				—			—			—		1.4		A	+
	JP Morgan	9.1				—			—				9.1				22			—			—		31.1		A	+
	Royal Bank	11.3				—			—				11.3				—			—			—		11.3		A
	of Scotland
	Nomura	(.4		)		—			0.4				—				—			22.8			0.8		22		BBB	+
	Citigroup - OTC	19.4				—			—				19.4				2.3			—			—		21.7		A
	Citigroup -	1.2				—			—				1.2				19.8			—			—		21		A
	Exchange Traded
	Total	$	69.2			$	—		$	0.4			$	69.6			$	44.1		$	22.8		$	0.8	$	135.7
	(1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-one creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and
	“BBB+” (Adequate, which is the eighth highest of twenty-one creditworthiness ratings).
Forward Contracts	'
Derivative Instruments and Hedging Activities Disclosures [Line Items]	'
Derivatives	'
	Forward Contracts
	Beginning in September 2012, White Mountains 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, 2014, White Mountains held approximately $23.2 million (SEK 150.5 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, 2014.
	The following tables summarize the changes in White Mountains’s forward contracts for the three months ended March 31, 2014:
		Three Months Ended
		March 31,
	Millions	2014				2013
	Beginning of period	$	(.1	)		$	(.1	)
	Purchases	—				—
	Net realized and unrealized (losses) gains	(.1		)		0.4
	Sales/settlements	0.1				(.5		)
	End of period	$	(.1	)		$	(.2	)
	The following summarizes realized and unrealized derivative gains (losses) recognized in net realized and unrealized investment gains for the three months ended March 31, 2014 and the carrying values, included in other long-term investments, at March 31, 2014 and December 31, 2012 by type of currency:
		Gains (Losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	USD	$	—			$	(.3	)	$	—			$	—
	SEK	—				—			—				—
	EUR	(.1		)					(.1		)		(.1		)
	GBP	—				0.7			—				—
	Total	$	(.1	)		$	0.4		$	(.1	)		$	(.1	)
	All of White Mountains’s forward contracts are subject to master netting agreements. The following summarizes amounts offset under master netting agreements:
		March 31, 2014											December 31, 2013
	Millions	Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets				Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets
		amounts before											amounts before
		offsets(1)											offsets(1)
	USD	$	—			$	—		$	—			$	—			$	—		$	—
	EUR	—				(.1		)	(.1		)		—				(.1		)	(.1		)
	GBP	—				—			—				—				—			—
	Total	$	—			$	(.1	)	$	(.1	)		$	—			$	(.1	)	$	(.1	)
	(1) Amount equal to fair value of instrument as recognized in other assets.
	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-14									31-Dec-13
	Millions	Notional Amount				Carrying Value			Standard & Poor's		Notional Amount				Carrying Value
									Rating(1)
	Barclays Bank Plc	$	8.1			$	(.1	)	A		$	5.8			$	—
	Deutsche Bank	7.1							A		7.7				—
	Goldman Sachs	4.4				—			AA-		2.1				—
	HSBC Bank Plc	3.6				—			AA-		3.3				(.1		)
	JP Morgan	—				—			A+		1.3				—
	Royal Bank of Canada	—				—			AA-		0.2				—
	Total	$	23.2			$	(.1	)			$	20.4			$	(.1	)
	(1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	'
	The following tables summarize the changes in White Mountains’s forward contracts for the three months ended March 31, 2014:
		Three Months Ended
		March 31,
	Millions	2014				2013
	Beginning of period	$	(.1	)		$	(.1	)
	Purchases	—				—
	Net realized and unrealized (losses) gains	(.1		)		0.4
	Sales/settlements	0.1				(.5		)
	End of period	$	(.1	)		$	(.2	)
Schedule of Uncollateralized Amounts Due	'
	The following summarizes amounts offset under master netting agreements:
		March 31, 2014											December 31, 2013
	Millions	Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets				Gross asset				Gross liability amounts offset under master netting arrangements			Net amounts recognized in Other assets
		amounts before											amounts before
		offsets(1)											offsets(1)
	USD	$	—			$	—		$	—			$	—			$	—		$	—
	EUR	—				(.1		)	(.1		)		—				(.1		)	(.1		)
	GBP	—				—			—				—				—			—
	Total	$	—			$	(.1	)	$	(.1	)		$	—			$	(.1	)	$	(.1	)
	(1) Amount equal to fair value of instrument as recognized in other assets.
Unrealized derivative gains and losses	'
	The following summarizes realized and unrealized derivative gains (losses) recognized in net realized and unrealized investment gains for the three months ended March 31, 2014 and the carrying values, included in other long-term investments, at March 31, 2014 and December 31, 2012 by type of currency:
		Gains (Losses)							Carrying Value
		Three Months Ended							As of
		March 31,							March 31,				31-Dec-13
	Millions	2014				2013			2014
	USD	$	—			$	(.3	)	$	—			$	—
	SEK	—				—			—				—
	EUR	(.1		)					(.1		)		(.1		)
	GBP	—				0.7			—				—
	Total	$	(.1	)		$	0.4		$	(.1	)		$	(.1	)
Schedule of uncollateralized amounts due	'
	The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts:
		31-Mar-14									31-Dec-13
	Millions	Notional Amount				Carrying Value			Standard & Poor's		Notional Amount				Carrying Value
									Rating(1)
	Barclays Bank Plc	$	8.1			$	(.1	)	A		$	5.8			$	—
	Deutsche Bank	7.1							A		7.7				—
	Goldman Sachs	4.4				—			AA-		2.1				—
	HSBC Bank Plc	3.6				—			AA-		3.3				(.1		)
	JP Morgan	—				—			A+		1.3				—
	Royal Bank of Canada	—				—			AA-		0.2				—
	Total	$	23.2			$	(.1	)			$	20.4			$	(.1	)
	(1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-one creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-one creditworthiness ratings) and “A-” (Strong, which is the ninth highest of twenty-one creditworthiness ratings).

Municipal_Bond_Guarantee_Table

Municipal Bond Guarantee (Tables)	3 Months Ended
	Mar. 31, 2014
Guarantees [Abstract]	'
Schedule of Municipal Guarantee Insured Obligations [Table Text Block]	'
	The following table provides a schedule of BAM’s insured obligations:
		31-Mar-14		31-Dec-13
	Contracts outstanding	907		701
	Remaining weighted average contract period outstanding (in years)	13.6		13.8
	Contractual debt service outstanding (in millions):
	Par	$	6,015.00	$	4,703.70
	Interest	4,140.90		3,264.40
	Total debt service outstanding	$	10,155.90	$	7,968.10
	Gross unearned insurance premiums (in millions)	$	17.7	$	13.2

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013 (see Note 15 for earnings per share amounts for discontinued operations):
		Three Months Ended
		March 31,
		2014			2013
	Basic and diluted earnings per share numerators (in millions):
	Net income from continuing operations attributable to	$	96.1		$	119.9
	White Mountains’s common shareholders
	Allocation of income for unvested restricted common shares	(1.1		)	(1.5		)
	Dividends declared on participating restricted common shares(1)	(.1		)	(.1		)
	Total allocation to restricted common shares	(1.2		)	(1.6		)
	Net income attributable to White Mountains’s common shareholders,	$	94.9		$	118.3
	net of restricted common share amounts
	Undistributed net earnings (in millions):
	Net income attributable to White Mountains’s common shareholders,	$	94.9		$	118.3
	net of restricted common share amounts
	Dividends declared net of restricted common share amounts(1)	(6.1		)	(6.1		)
	Total undistributed net earnings, net of restricted common share amounts	$	88.8		$	112.2
	Basic earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted shares(2)	(70.4		)	(79.0		)
	Basic earnings per share denominator	6,099.50			6,194.50
	Diluted earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares(2)	(70.4		)	(79.0		)
	Average outstanding dilutive options to acquire common shares(3)	—			—
	Diluted earnings per share denominator	6,099.50			6,194.50
	Basic earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	15.56		$	19.1
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	14.56		$	18.1
	Diluted earnings per share (in dollars):
	Net income attributable to White Mountains’s common shareholders	$	15.56		$	19.1
	Dividends declared and paid	(1.00		)	(1.00		)
	Undistributed earnings	$	14.56		$	18.1
	(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.
	(2) Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).
	(3) The diluted earnings per share denominator for the three months ended March 31, 2014 and 2013 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2014
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, 2014
	Earned insurance and reinsurance premiums	$	276.5	$	215.5		$	0.2		$	0.1		$	1.3		$	493.6
	Net investment income	10		8.7			0.3			1.4			3.9			24.3
	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	1		(5.3		)	—			0.2			0.7			(3.4		)
	Total revenues	306.4		247			5.1			0.8			19			578.3
	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)	$	54.4	$	74		$	4.6		$	(8.6	)	$	(12.3	)	$	112.1
							HG Global/BAM
	Millions	OneBeacon		Sirius			HG			BAM			Other Operations			Total
				Group
	Three Months Ended March 31, 2013
	Earned insurance and reinsurance premiums	$	286.5	$	208.9		$	—		$	—		$	—		$	495.4
	Net investment income	9.4		13.1			0.3			1.1			4.6			28.5
	Net investment income (loss) - surplus note	—		—			10.1			(10.1		)	—			—
	interest
	Net realized and unrealized investment	28.4		13.1			(.2		)	(1.1		)	35			75.2
	gains (losses)
	Other revenue	24		10.5			—			0.1			(6.3		)	28.3
	Total revenues	348.3		245.6			10.2			(10.0		)	33.3			627.4
	Losses and LAE	148.9		95.4			—			—			—			244.3
	Insurance and reinsurance acquisition expenses	54.8		43.3			—			0.1			—			98.2
	Other underwriting expenses	49.3		29.7			—			0.1			—			79.1
	General and administrative expenses	4		9.5			0.3			8.2			21.8			43.8
	Interest expense	3.2		6.6			—			—			0.4			10.2
	Total expenses	260.2		184.5			0.3			8.4			22.2			475.6
	Pre-tax income (loss)	$	88.1	$	61.1		$	9.9		$	(18.4	)	$	11.1		$	151.8

Investments_in_Unconsolidated_1

Investments in Unconsolidated Affiliates (Tables)	3 Months Ended
	Mar. 31, 2014
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,
		2014			2013
	Symetra common shares	$	373.2		$	360.9
	Unrealized losses from Symetra’s fixed maturity portfolio	(4.2		)	(43.6		)
	Carrying value of Symetra common shares	369			317.3
	Hamer	4.3			4.1
	Total investments in unconsolidated affiliates	$	373.3		$	321.4
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, 2014 and 2013:
		Three Months Ended									Three Months Ended
		March 31, 2014									March 31, 2013
	Millions	Common Shares			Warrants			Total			Common Shares			Warrants			Total
	Carrying value of investment in Symetra at	$	317.3		$	—		$	317.3		$	351.2		$	30.3		$	381.5
	beginning of period
	Equity in earnings (1)(2)	14.4			—			14.4			9.8			—			9.8
	Equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio(3)	39.3			—			39.3			(8.7		)	—			(8.7		)
	Dividends received	(2.0		)	—			(2.0		)	(1.4		)	—			(1.4		)
	Decrease in value of warrants	—			—			—			—			(3.7		)	(3.7		)
	Carrying value of investment in Symetra at	$	369		$	—		$	369		$	350.9		$	26.6		$	377.5
	end of period(4)(5)
	(1) Equity in earnings excludes tax expense of $1.0 and $0.8.
	(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 $3.1 and $2.7 increase relating to the pre-tax amortization of the Symetra common share basis difference.
	(4) Includes White Mountains’s equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $39.3 and $(8.7) as of March 31, 2014 and 2013, which excludes tax (expense) benefit of $(2.8) and $0.7.
	(5) The aggregate value of White Mountains’s investment in common shares of Symetra was $397.4 based upon the quoted market price of $19.82 per share at March 31, 2014.

Employee_ShareBased_Incentive_1

Employee Share-Based Incentive Compensation Plans (Tables)	3 Months Ended
	Mar. 31, 2014
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, 2014 and 2013 for performance shares granted under the WTM Incentive Plan:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	119,220		$	60.2		119,357		$	29.4
	Shares paid or expired(1)	(37,130	)	(26.7		)	(47,310	)	(11.0		)
	New grants	45,660		—			46,920		—
	Assumed forfeitures and cancellations(2)	(213	)	0.4			9		0.1
	Expense recognized	—		8.4			—		10.1
	Ending March 31,	127,537		$	42.3		118,976		$	28.6
	(1) WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target.  WTM performance share payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% 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, 2014 for each performance cycle:
	Millions, except share amounts	Target		Accrued
		Performance		Expense
		Shares
		Outstanding
	Performance cycle:
	2014 – 2016	45,660		$	2.5
	2013 – 2015	47,170		15.9
	2012 – 2014	37,977		25
	Sub-total	130,807		43.4
	Assumed forfeitures	(3,270	)	(1.1		)
	Total at March 31, 2014	127,537		$	42.3
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, 2014 and 2013:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	94,130		$	17		69,910		$	16.8
	Issued	20,400		11.8			25,470		14.3
	Vested	(33,205	)	—			—		—
	Forfeited	—		—			—		—
	Expense recognized	—		(4.0		)	—		(2.9		)
	Non-vested at March 31,	81,325		$	24.8		95,380		$	28.2
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, 2014 and 2013 for OneBeacon performance shares granted under the OneBeacon Incentive Plan:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Target		Accrued			Target		Accrued
		Performance		Expense			Performance		Expense
		Shares					Shares
		Outstanding					Outstanding
	Beginning of period	493,421		$	4		563,190		$	1.2
	Shares paid or expired(1)	(142,138	)	(1.0		)	(238,658	)	—
	New grants	165,800		—			179,000		—
	Assumed forfeitures and cancellations(2)	(4,145	)	—			(13,665	)	—
	Expense recognized	—		0.6			—		0.4
	Ending March 31,	512,938		$	3.6		489,867		$	1.6
	(1) OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. No payments were made in 2013 for the 2010-2012 OneBeacon performance cycle as the performance factor was zero. Amounts include deposits into OneBeacon’s deferred compensation plan.
	(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, 2014 for each performance cycle:
	Millions, except share amounts	Target OneBeacon Performance Shares Outstanding		Accrued Expense
	Performance cycle:
	2014 – 2016	165,800		$	0.2
	2013 – 2015	179,000		1.2
	2012 – 2014	181,290		2.3
	Sub-total	526,090		3.7
	Assumed forfeitures	(13,152	)	(.1		)
	Total at March 31, 2014	512,938		$	3.6
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, 2014 and 2013:
		Three Months Ended March 31,
		2014					2013
	Millions, except share amounts	Restricted		Unamortized			Restricted		Unamortized
		Shares		Issue Date			Shares		Issue Date
				Fair Value					Fair Value
	Non-vested,
	Beginning of period	915,000		$	6.5		927,000		$	9.6
	Issued	—		—			—		—
	Vested	(300,000	)	—			(9,000	)	—
	Forfeited	(2,500	)	—			—		—
	Expense recognized	—		(.8		)	—		(.8		)
	Non-vested at March 31,	612,500		$	5.7		918,000		$	8.8

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2014
Fair Value Disclosures [Abstract]	'
Summary of the fair value and carrying value of financial instruments	'
	The following table summarizes the fair value and book value of financial instruments as of March 31, 2014 and December 31, 2013:
		March 31, 2014				December 31, 2013
	Millions	Fair		Carrying		Fair		Carrying
		Value		Value		Value		Value
	2012 OBH Senior Notes	$	275.1	$	274.7	$	269.8	$	274.7
	SIG Senior Notes	438		399.6		438.1		399.6
	SIG Preference Shares	262.5		250		260		250

Discontinued_Operations_Tables

Discontinued Operations (Tables)	3 Months Ended
	Mar. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]	'
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet And Additional Disclosures	'
	The following summarizes the assets and liabilities associated with the business classified as held for sale:
	Millions	March 31,			December 31,
		2014			2013
	Assets held for sale
	Fixed maturity investments, at fair value	$	222.2		$	236.3
	Reinsurance recoverable on unpaid losses	1,540.00			1,604.70
	Reinsurance recoverable on paid losses	9.5			10.7
	Insurance premiums receivable	12.8			9.1
	Deferred tax asset	3.2			3.3
	Other assets	15.4			16
	Total assets held for sale	$	1,803.10		$	1,880.10
	Liabilities held for sale
	Loss and loss adjustment expense reserves	$	1,717.30		$	1,793.10
	Unearned insurance premiums	0.2			0.2
	Ceded reinsurance payable	12.1			12.3
	Other liabilities	73.5			74.5
	Total liabilities held for sale	1,803.10			1,880.10
	Net assets held for sale	$	—		$	—
	Net (Loss) Income from Discontinued Operations
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
		Three Months Ended
		March 31,
	Millions	2014			2013
	Revenues
	Earned insurance premiums	$	(.1	)	$	0.9
	Other revenue	—			—
	Total revenues	(.1		)	0.9
	Expenses
	Loss and loss adjustment expenses	—			—
	Insurance and reinsurance acquisition expenses	—			0.1
	Other underwriting expenses	0.7			0.1
	Total expenses	0.7			0.2
	Pre-tax (loss) income	(.8		)	0.7
	Income tax benefit (expense)	0.3			(.2		)
	Net (loss) income from discontinued operations	$	(.5	)	$	0.5
	(Loss) Earnings Per Share
	Basic (loss) 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 (loss) 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 (loss) earnings per share for discontinued operations for the three months ended March 31, 2014 and 2013:
		Three Months Ended
		March 31,
		2014			2013
	Basic and diluted (loss) earnings per share numerators (in millions):
	Net (loss) income attributable to White Mountains’s common shareholders	$	(.5	)	$	0.5
	Allocation of income for participating unvested restricted common shares (1)	—			—
	Net (loss) income attributable to White Mountains’s common shareholders,	$	(.5	)	$	0.5
	net of restricted common share amounts (2)
	Basic (loss) earnings per share denominators (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares (3)	(70.4		)	(79.0		)
	Basic (loss) earnings per share denominator	6,099.50			6,194.50
	Diluted (loss) earnings per share denominator (in thousands):
	Total average common shares outstanding during the period	6,169.90			6,273.50
	Average unvested restricted common shares (3)	(70.4		)	(79.0		)
	Average outstanding dilutive options to acquire common shares (4)	—			—
	Diluted (loss) earnings per share denominator	6,099.50			6,194.50
	Basic and diluted (loss) earnings per share (in dollars):	$	(.08	)	$	0.07
	(1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.
	(2) Net (loss) earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed (loss) earnings for the three months ended March 31, 2014 and 2013.
	(3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).
	(4) The diluted (loss) earnings per share denominator for the three months ended March 31, 2014 and 2013 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $)

3 Months Ended

3 Months Ended

1 Months Ended

3 Months Ended

1 Months Ended

3 Months Ended

1 Months Ended

3 Months Ended

In Millions, except Per Share data, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Jul. 31, 2012

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Feb. 19, 2014

Mar. 31, 2014

Mar. 14, 2014

31-May-07

24-May-07

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Mar. 31, 2014

Jul. 31, 2012

Mar. 31, 2014

OneBeacon Ltd.

OneBeacon Ltd.

Build America Mutual

Build America Mutual

HG Global

HG Global

HG Global

HG Global

Star & Shield Insurance Exchange [Member]

White Mountains Consolidated Limited Partnership

White Mountains Consolidated Limited Partnership

A W G Dewar Inc

A W G Dewar Inc

WOBI [Member]

WOBI [Member]

QuoteLab [Member]

QuoteLab [Member]

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Common Stock

Common Stock

Common shares and paid-in surplus

Noncontrolling interest

BAM Management

Interest Rate Cap

Sirius International Group Ltd

Sirius International Group Ltd

HG Global

HG Global

HG Global

HG Global

HG Global

Star & Shield Insurance Exchange [Member]

Sirius International Group Ltd

Basis of Presentation

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ownership interest (as a percent)

'

'

'

75.30%

75.20%

'

'

'

'

'

'

'

'

'

'

'

'

60.70%

'

54.30%

'

'

96.90%

'

88.40%

'

'

'

'

'

Surplus Notes

'

'

'

'

'

'

'

$503

$503

'

'

$12

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of par value of policy reinsured

15.00%

'

'

'

'

'

'

'

15.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Noncontrolling equity percentage

'

'

'

24.70%

24.80%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

96.90%

97.30%

88.40%

88.70%

'

'

'

'

Partners Capital Contribution

'

'

'

'

'

'

'

594.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

14.5

'

Fair Value of Net Assets Acquired

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2.2

'

'

'

Income (Loss) from Continuing Operations before Income Taxes, Domestic

'

'

'

'

'

-8.6

-18.4

'

4.6

9.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net loss attributable to non-controlling interests

0.5

0.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4.7

'

'

Non-cumulative perpetual preference shares issued

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

250

'

'

'

'

'

'

'

'

'

Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$1,000

'

'

'

'

'

'

'

'

Proceeds from non-cumulative perpetual preference shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

245.7

'

'

'

'

'

'

'

'

'

Non-cumulative perpetual preference shares issuance cost and commission

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4.3

'

'

'

'

'

'

'

'

'

Preferred Stock, Dividend Rate, Percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

7.51%

'

'

'

'

'

'

'

'

'

basis points

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3.20%

'

'

'

'

'

'

'

'

3.95%

years of interest rate cap

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'5 years

Derivative, Cap Interest Rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

8.30%

Noncontrolling interest - other

'

'

'

'

'

'

'

'

'

'

'

'

46.7

46.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Non-controlling equity interest

$513.40

'

$491.90

'

'

'

'

'

$18.40

'

$16.60

'

'

'

$2.70

$3.10

$5

'

$18.80

'

'

'

'

'

'

'

'

'

'

'

Term of LIBOR loan

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 months

'

'

'

'

'

'

'

'

'

Significant_Transactions_Detai

Significant Transactions (Details)

3 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

1 Months Ended

3 Months Ended

2 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

Jan. 31, 2013

Mar. 31, 2014

Mar. 14, 2014

Dec. 31, 2013

Mar. 31, 2013

Mar. 31, 2013

Mar. 31, 2014

Feb. 19, 2014

Jan. 31, 2014

Mar. 14, 2014

Feb. 19, 2014

Mar. 31, 2014

Feb. 19, 2014

Feb. 19, 2014

Feb. 19, 2014

Feb. 19, 2014

USD ($)

USD ($)

USD ($)

Star & Shield Insurance Exchange [Member]

Star & Shield Insurance Exchange [Member]

QuoteLab [Member]

QuoteLab [Member]

OneBeacon

WM Solutions [Member]

WM Solutions [Member]

WOBI [Member]

WOBI [Member]

Star & Shield LLC [Member]

Common Class A [Member]

Common Stock

Noncontrolling interest

Israel, New Shekels

Israel, New Shekels

United States of America, Dollars

United States of America, Dollars

USD ($)

USD ($)

USD ($)

USD ($)

Essentia

Citation [Member]

American Fuji [Member]

USD ($)

USD ($)

QuoteLab [Member]

WOBI [Member]

Star & Shield Insurance Exchange [Member]

Preferred Stock

Common Stock

Preferred Stock

Common Stock

USD ($)

USD ($)

USD ($)

USD ($)

WOBI [Member]

WOBI [Member]

WOBI [Member]

WOBI [Member]

ILS

ILS

USD ($)

USD ($)

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ownership interest (as a percent)

'

'

'

'

'

'

54.30%

'

'

'

'

60.70%

'

60.00%

54.00%

'

'

'

'

'

Payments to Acquire Businesses and Interest in Affiliates

$32.20

$9.20

'

'

'

$28.10

'

'

'

'

'

'

$1.80

'

'

'

12.7

14.4

$3.60

$4.10

Premiums Written, Percentage of Total

'

'

'

'

'

'

'

'

'

'

85.00%

'

'

'

'

'

'

'

'

'

Number of lines of business

'

'

'

'

'

'

'

'

'

'

4

'

'

'

'

'

'

'

'

'

Purchase of Surplus Notes

'

'

'

'

12

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of gross profit

'

'

'

'

'

'

62.50%

'

'

'

'

'

'

'

'

'

'

'

'

'

Assets

12,416.40

'

12,144.30

27.1

'

'

56.9

'

'

'

'

15.3

'

'

'

'

'

'

'

'

Intangible Assets, Net (Including Goodwill)

'

'

'

'

'

'

43.7

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible Assets, Net

'

'

'

'

'

'

'

'

'

'

'

15

'

'

'

'

'

'

'

'

Liabilities

7,888.50

'

7,746.90

31.7

'

'

10

'

'

'

'

2.6

'

'

'

'

'

'

'

'

Net Income (Loss) Attributable to Noncontrolling Interest

0.5

0.5

'

'

'

'

'

'

'

'

'

'

'

'

'

4.7

'

'

'

'

Gain on sale of subsidiary

0.7

23

'

'

'

'

'

23

0.7

6.9

'

'

'

'

'

'

'

'

'

'

Gain on sale of subsidiary, after tax

'

'

'

'

'

'

'

$15

'

'

'

'

'

'

'

'

'

'

'

'

Significant_Transactions_Share

Significant Transactions (Share Repurchases) (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Significant Transactions	'	'
Additional authorized repurchase of common shares	529,648	'
Value of common shares repurchased and retired during the period	$15.40	$79
Average price per share	$586	$564
Common Stock	'	'
Significant Transactions	'	'
Stock Repurchased and Retired During Period, Shares	26,323	140,224
Combination One | Common Stock	'	'
Significant Transactions	'	'
Stock Repurchased and Retired During Period, Shares	15,848	140,000
Combination Two | Common Stock	'	'
Significant Transactions	'	'
Stock Repurchased and Retired During Period, Shares	10,475	224

Loss_and_Loss_Adjustment_Expen2

Loss and Loss Adjustment Expense Reserves Details (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Loss and Loss Adjustment, Expense Reserves [Line Items]	'	'
Net favorable and unfavorable loss reserve development	($11.20)	($1.40)
Accretion of fair value adjustment to net loss and LAE reserves	0.2	1.2
Adjustment to loss and loss adjustment expense reserves, before tax	4.5	'
Sirius Group	'	'
Loss and Loss Adjustment, Expense Reserves [Line Items]	'	'
Net favorable and unfavorable loss reserve development	-9.8	-4.3
OneBeacon	'	'
Loss and Loss Adjustment, Expense Reserves [Line Items]	'	'
Net favorable and unfavorable loss reserve development	($1.40)	($2.90)

Loss_and_Loss_Adjustment_Expen3

Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Loss and Loss Adjustment Expense Reserve	'	'
Gross beginning balance	$3,079.30	$3,168.90
Less beginning reinsurance recoverable on unpaid losses	-428.1	-429.1
Net loss and LAE reserves	2,651.20	2,739.80
Loss and LAE reserves acquired	0	21.3
Loss and LAE reserves consolidated — SSIE	13.6	0
Loss and LAE incurred relating to current year losses	240.5	242.9
Net favorable and unfavorable loss reserve development	11.2	1.4
Total incurred losses and LAE	229.3	244.3
Accretion of fair value adjustment to net loss and LAE reserves	0.2	1.2
Foreign currency translation adjustment to net loss and LAE reserves	1.8	-9.6
Current year losses	-35	-36.6
Prior year losses	-237.9	-270.6
Total loss and LAE payments	-272.9	-307.2
Net loss and LAE reserves	2,623.20	2,689.80
Reinsurance recoverable on unpaid losses	425.2	410.8
Gross ending balance	$3,048.40	$3,100.60

Third_Party_Reinsurance_One_Be

Third Party Reinsurance (One Beacon) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Mar. 31, 2014
In Millions, unless otherwise specified			OneBeacon	OneBeacon	Crop insurance [Member]	Crop-hail insurance [Member]
					OneBeacon	OneBeacon
Reinsurance Retention Policy [Line Items]	'	'	'	'	'	'
Reinsurance Retention Policy, Retained Percentage	'	'	'	'	101.50%	100.00%
Reinsurance recoverable on paid losses	$15.20	$25.40	$3	$9.70	'	'
Reinsurance recoverable on unpaid losses	$425.20	$428.10	$80.10	$80.20	'	'
Reinsurance Retention Policy, Reinsured Risk, Percentage	'	'	'	'	48.50%	80.00%

Third_Party_Reinsurance_Sirius

Third Party Reinsurance (Sirius Group) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Reinsurance Retention Policy [Line Items]	'	'
Reinsurance recoverable on paid losses	$15.20	$25.40
Reinsurance recoverable on unpaid losses	425.2	428.1
Sirius	'	'
Reinsurance Retention Policy [Line Items]	'	'
Reinsurance recoverable on paid losses	12.2	15.7
Reinsurance recoverable on unpaid losses	$344.80	$347.90

Investment_Securities_Net_Inve

Investment Securities (Net Investment Income) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Investment income	'	'
Total investment income	$29.10	$33
Less third-party investment expenses	-4.8	-4.5
Net investment income, pre-tax	24.3	28.5
Fixed maturity investments.	'	'
Investment income	'	'
Total investment income	22.4	26
Short-term investments	'	'
Investment income	'	'
Total investment income	0.6	0.8
Common Stock	'	'
Investment income	'	'
Total investment income	5.4	4.7
Convertible fixed maturity investments	'	'
Investment income	'	'
Total investment income	0.4	0.8
Other long-term investments	'	'
Investment income	'	'
Total investment income	$0.30	$0.70

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, 2014	Mar. 31, 2013
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	$21.70	$36.50
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	42.1	38.7
Net Realized and Unrealized Gain (Loss) on Trading Securities	63.8	75.2
Tax on net realized and unrealized gains (loss), trading securities	15.7	13.7
Realized and unrealized gain	48.1	61.5
Fixed maturity investments.	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	0.9	8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	31.6	-26
Short-term investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	0	0.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	18.8	22.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	4.5	61.9
Convertible fixed maturity investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	2.4	2.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.7	-1.3
Other long-term investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	-0.3	2.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	5.3	4.1
Forward Contracts	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss), Total	-0.1	0.4
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, 2014	Mar. 31, 2013
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	$25.30	$51.20
Trading securities, tax on realized holding gain (loss) on investments	-4.6	-11.8
Trading securities realized holding gain (loss) on investments after tax	20.7	39.4
Foreign Currency Transaction Gain (Loss), Realized	-3.6	-14.7
Trading securities, realized foreign currency transaction gain (loss) t	0.9	3.3
Trading securities, realized foreign currency gain (loss) net of tax	-2.7	-11.4
Trading Securities, Realized Gain (Loss), Total	21.7	36.5
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	-3.7	-8.5
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	18	28
Fixed maturity investments.	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	4.4	22.6
Foreign Currency Transaction Gain (Loss), Realized	-3.5	-14.6
Trading Securities, Realized Gain (Loss), Total	0.9	8
Short-term investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Realized	0	0.1
Trading Securities, Realized Gain (Loss), Total	0	0.1
Common equity securities	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	18.9	22.6
Foreign Currency Transaction Gain (Loss), Realized	-0.1	-0.2
Trading Securities, Realized Gain (Loss), Total	18.8	22.4
Convertible fixed maturity investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	2.4	2.7
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	2.4	2.7
Other long-term investments	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	-0.3	2.9
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	-0.3	2.9
Forward Contracts	'	'
Investment securities	'	'
Trading Securities, Realized Gain (Loss)	-0.1	0.4
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	($0.10)	$0.40

Investment_Securities_Net_Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Investment securities	'	'
Trading Securities, Change in Unrealized Holding Gain (Loss)	$29	$33.80
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	-9.1	-4.1
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	19.9	29.7
Foreign Currency Transaction Gain (Loss), Unrealized	13.1	4.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	-2.9	-1.1
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	10.2	3.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	42.1	38.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	-12	-5.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	30.1	33.5
Fixed maturity investments.	'	'
Investment securities	'	'
Trading Securities, Change in Unrealized Holding Gain (Loss)	19.4	-30.6
Foreign Currency Transaction Gain (Loss), Unrealized	12.2	4.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	31.6	-26
Short-term investments	'	'
Investment securities	'	'
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	'	'
Investment securities	'	'
Trading Securities, Change in Unrealized Holding Gain (Loss)	4	61.9
Foreign Currency Transaction Gain (Loss), Unrealized	0.5	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	4.5	61.9
Convertible fixed maturity investments	'	'
Investment securities	'	'
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.7	-1.1
Foreign Currency Transaction Gain (Loss), Unrealized	0	-0.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.7	-1.3
Other long-term investments	'	'
Investment securities	'	'
Trading Securities, Change in Unrealized Holding Gain (Loss)	4.9	3.6
Foreign Currency Transaction Gain (Loss), Unrealized	0.4	0.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	5.3	4.1
Forward Contracts	'	'
Investment securities	'	'
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, 2014	Mar. 31, 2013
Investment securities	'	'
Unrealized gains (losses) on investments	$7.10	$6.50
Fixed maturity investments.	'	'
Investment securities	'	'
Unrealized gains (losses) on investments	0.2	0.2
Common equity securities	'	'
Investment securities	'	'
Unrealized gains (losses) on investments	0.8	-0.1
Other long-term investments	'	'
Investment securities	'	'
Unrealized gains (losses) on investments	$6.10	$6.40

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, 2014	Dec. 31, 2013
Investment securities	'	'
Fixed maturity investments, at fair value	$4,988.70	$5,030.50
Fixed maturity investments reclassified to assets held for sale	'	-236.3
U.S. Government and agency obligations	'	'
Investment securities	'	'
Cost or amortized cost	539.2	365.5
Trading Securities, Unrealized Holding Gain	0.6	0.5
Trading Securities, Unrealized Holding Loss	-0.8	-1
Net foreign currency gains (losses)	-0.6	-2.5
Fixed maturity investments, at fair value	538.4	362.5
Debt securities issued by corporations	'	'
Investment securities	'	'
Cost or amortized cost	2,311.60	2,330.70
Trading Securities, Unrealized Holding Gain	48.3	44
Trading Securities, Unrealized Holding Loss	-7.1	-13.2
Net foreign currency gains (losses)	-6.3	-14.3
Fixed maturity investments, at fair value	2,346.50	2,347.20
Municipal obligations	'	'
Investment securities	'	'
Cost or amortized cost	33.5	18.3
Trading Securities, Unrealized Holding Gain	0.2	0
Trading Securities, Unrealized Holding Loss	-0.2	-0.4
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	33.5	17.9
Mortgage-backed and asset-backed securities	'	'
Investment securities	'	'
Cost or amortized cost	1,847.90	2,027.30
Trading Securities, Unrealized Holding Gain	4.1	2.4
Trading Securities, Unrealized Holding Loss	-6.3	-9.9
Net foreign currency gains (losses)	-3.5	-5.3
Fixed maturity investments, at fair value	1,842.20	2,014.50
Foreign government, agency and provincial obligations	'	'
Investment securities	'	'
Cost or amortized cost	366.4	444.2
Trading Securities, Unrealized Holding Gain	3.4	3.7
Trading Securities, Unrealized Holding Loss	-1.1	-3.2
Net foreign currency gains (losses)	-4.3	-4.8
Fixed maturity investments, at fair value	364.4	439.9
Preferred Stock	'	'
Investment securities	'	'
Cost or amortized cost	79.9	79.9
Trading Securities, Unrealized Holding Gain	6.1	5.1
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	-0.1	-0.2
Fixed maturity investments, at fair value	85.9	84.8
Fixed Income Investments	'	'
Investment securities	'	'
Cost or amortized cost	5,178.50	5,265.90
Trading Securities, Unrealized Holding Gain	62.7	55.7
Trading Securities, Unrealized Holding Loss	-15.5	-27.7
Net foreign currency gains (losses)	-14.8	-27.1
Total fixed maturity investments including assets held for sale	5,210.90	5,266.80
OneBeacon Runoff	'	'
Investment securities	'	'
Fixed maturity investments reclassified to assets held for sale	($222.20)	($236.30)

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, 2014	Dec. 31, 2013
Investment securities	'	'
Fixed maturity investments, at fair value	$4,988.70	$5,030.50
Common shareholders' equity.	'	'
Investment securities	'	'
Cost or amortized cost	905.6	890.2
Trading Securities, Unrealized Holding Gain	273.9	271
Trading Securities, Unrealized Holding Loss	-4.5	-3.6
Net foreign currency gains (losses)	-0.4	-0.8
Fixed maturity investments, at fair value	1,174.60	1,156.80
Convertible Debt Securities	'	'
Investment securities	'	'
Cost or amortized cost	64.3	71.7
Trading Securities, Unrealized Holding Gain	10.6	9.9
Trading Securities, Unrealized Holding Loss	-1	-0.9
Net foreign currency gains (losses)	-0.1	-0.2
Fixed maturity investments, at fair value	73.8	80.5
Other long-term investments.	'	'
Investment securities	'	'
Cost or amortized cost	246.4	238.3
Trading Securities, Unrealized Holding Gain	83.4	79.6
Trading Securities, Unrealized Holding Loss	-25.5	-26.6
Net foreign currency gains (losses)	-2	-2.4
Fixed maturity investments, at fair value	$302.30	$288.90

Investment_Securities_Other_lo

Investment Securities (Other long-term investments) (Details) (USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Hedge and private equity funds included in other long-term investments	'
Investment securities	'
Largest single fund investment	$18.80
Hedge funds	'
Investment securities	'
Number of investments	16
Private equity funds	'
Investment securities	'
Number of investments	39

Investment_Securities_Fair_Val

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Hedge funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	$126.20	'	'	'
Private equity funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	124.9	'	'	'
Hedge and private equity funds included in other long-term investments	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	251.1	239	'	'
Unfunded Commitments	106.1	110.6	'	'
Level 3 Inputs | Fair value measured on a recurring basis | Other long-term investments	'	'	'	'
Investment securities	'	'	'	'
Carrying value of investment accounted for using the equity method	25	26.6	34.9	35
Private equity funds | Private Equity Multi Sector	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	24.1	23.8	'	'
Unfunded Commitments	6.4	6.5	'	'
Private equity funds | Private Equity Energy Infrastructure and Services	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	46.8	45.9	'	'
Unfunded Commitments	12.7	13.1	'	'
Private equity funds | Real Estate Funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	7.1	8.2	'	'
Unfunded Commitments	3.3	3.3	'	'
Private equity funds | Private Equity Secondaries	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	9.2	9.5	'	'
Unfunded Commitments	3.1	3.1	'	'
Private equity funds | Private Equity Funds Multi Sector Europe	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	3.8	3.9	'	'
Unfunded Commitments	2.8	2.8	'	'
Private equity funds | Private Equity Funds Manufacturing and Industrial	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	11.5	11.2	'	'
Unfunded Commitments	15.2	15.5	'	'
Private equity funds | Private Equity Healthcare	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	7.8	5.6	'	'
Unfunded Commitments	2.8	2.8	'	'
Private equity funds | Private Equity Funds Multi Sector Asia	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	0	0	'	'
Unfunded Commitments	2.7	2.7	'	'
Private equity funds | Private Equity Insurance	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	2.4	2.3	'	'
Unfunded Commitments	41.3	41.3	'	'
Private equity funds | Aerospace Defense Government	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	10.6	5.8	'	'
Unfunded Commitments	15.5	19.2	'	'
Private equity funds | Venture Capital Funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	1.6	1.6	'	'
Unfunded Commitments	0.3	0.3	'	'
Private equity funds | Private equity funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	124.9	118.2	'	'
Unfunded Commitments	106.1	110.6	'	'
Hedge funds | Hedge Funds, Equity Long (Short) [Member]	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	67.6	62.6	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge Funds Credit and Distressed	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	23.3	22.8	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge Funds Diversified Strategies	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	0	0.1	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge Funds Equity R E I T	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	18.8	18.3	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge Funds Equity Activist	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	16.3	16.8	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge Funds Bank Loan	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	0.2	0.2	'	'
Unfunded Commitments	0	0	'	'
Hedge funds | Hedge funds	'	'	'	'
Investment securities	'	'	'	'
Other long-term investments	126.2	120.8	'	'
Unfunded Commitments	$0	$0	'	'

Investment_Securities_Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) (Hedge funds, USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Investment securities	'
Other long-term investments	$126.20
Distributions from inactive hedge funds	2.1
Redemption requests	2.2
30-59 days notice	'
Investment securities	'
Other long-term investments	34.3
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]	'
Investment securities	'
Other long-term investments	56.2
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]	'
Investment securities	'
Other long-term investments	21.2
120+ days notice	'
Investment securities	'
Other long-term investments	14.5
Monthly	'
Investment securities	'
Other long-term investments	10.4
Monthly | 30-59 days notice	'
Investment securities	'
Other long-term investments	4.9
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]	'
Investment securities	'
Other long-term investments	0
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]	'
Investment securities	'
Other long-term investments	0
Monthly | 120+ days notice	'
Investment securities	'
Other long-term investments	5.5
Quarterly	'
Investment securities	'
Other long-term investments	80.5
Quarterly | 30-59 days notice	'
Investment securities	'
Other long-term investments	29.4
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]	'
Investment securities	'
Other long-term investments	30.4
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]	'
Investment securities	'
Other long-term investments	11.9
Quarterly | 120+ days notice	'
Investment securities	'
Other long-term investments	8.8
Semi-annual	'
Investment securities	'
Other long-term investments	25.8
Semi-annual | 30-59 days notice	'
Investment securities	'
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]	'
Investment securities	'
Other long-term investments	25.8
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]	'
Investment securities	'
Other long-term investments	0
Semi-annual | 120+ days notice	'
Investment securities	'
Other long-term investments	0
Annual	'
Investment securities	'
Other long-term investments	9.5
Annual | 30-59 days notice	'
Investment securities	'
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]	'
Investment securities	'
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]	'
Investment securities	'
Other long-term investments	9.3
Annual | 120+ days notice	'
Investment securities	'
Other long-term investments	$0.20

Investment_Securities_Investme3

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) (Private equity funds, USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Investment securities	'
Other long-term investments	$124.90
1-3 years	'
Investment securities	'
Other long-term investments	6.5
3-5 years	'
Investment securities	'
Other long-term investments	29.5
5-10 years	'
Investment securities	'
Other long-term investments	75.7
Greater than 10 years	'
Investment securities	'
Other long-term investments	$13.20

Investment_Securities_Fair_Val1

Investment Securities (Fair Value Measurements) (Details) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
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	$84.60	$86.30

Investment_Securities_Fair_Val2

Investment Securities (Fair Value Measurement by Level) (Details) (USD $)

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

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Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

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Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

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Dec. 31, 2012

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Mar. 31, 2013

Dec. 31, 2012

In Millions, unless otherwise specified

Fair Value

Fair Value

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 2 Inputs

Level 2 Inputs

Debt securities issued by corporations

Debt securities issued by corporations

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Fair Value

Fair Value

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

U.S. Government and agency obligations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Debt securities issued by corporations

Municipal obligations

Municipal obligations

Municipal obligations

Municipal obligations

Municipal obligations

Municipal obligations

Municipal obligations

Municipal obligations

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Mortgage-backed and asset-backed securities

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Foreign government, agency and provincial obligations

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Preferred Stock

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Common equity securities

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Convertible fixed maturity investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Forward Contracts

Forward Contracts

Forward Contracts

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Financials

Financials

Financials

Financials

Financials

Financials

Financials

Financials

Communications

Communications

Communications

Communications

Communications

Communications

Communications

Communications

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Energy

Energy

Energy

Energy

Energy

Energy

Energy

Energy

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Technology

Technology

Technology

Technology

Technology

Technology

Technology

Technology

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Consumer

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Industrial

Financials

Financials

Financials

Financials

Financials

Financials

Financials

Financials

Communications

Communications

Communications

Communications

Communications

Communications

Communications

Communications

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Basic materials

Energy

Energy

Energy

Energy

Energy

Energy

Energy

Energy

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Utilities

Technology

Technology

Technology

Technology

Technology

Technology

Technology

Technology

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 2 Inputs

Level 2 Inputs

Level 2 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Investment securities

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Fair value investments

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'

$6,736.70

$6,766.50

$6,580.60

$6,950.70

$1,565.60

$1,376.70

$1,386.40

$1,355.10

$4,727.50

$4,982.20

$4,795.90

$5,206.10

$2,346.50

$2,347.20

$1,842.20

$2,014.50

$1,800.60

$1,992.50

$41.60

$22

$112.90

$93

$101

$92.90

$47.20

$46.10

$36.80

$37.30

$6.10

$6.10

$0

$0

$277.40

$262.40

$260.50

$259.30

$7,410.70

$7,402.40

$2,232.20

$1,998.20

$4,734.90

$4,996.60

$443.60

$407.60

$538.40

$362.50

$471.70

$295.80

$66.70

$66.70

$0

$0

$2,346.50

$2,347.20

$0

$0

$2,346.50

$2,347.20

$0

$0

$741

$754.40

$0

$0

$741

$754.40

$0

$0

$251.50

$434.40

$0

$0

$251.50

$434.40

$0

$0

$446

$265

$0

$0

$446

$265

$0

$0

$267.50

$281.10

$0

$0

$267.50

$281.10

$0

$0

$140.10

$173.60

$0

$0

$140.10

$173.60

$0

$0

$194.20

$159.70

$0

$0

$194.20

$159.70

$0

$0

$132.70

$149.10

$0

$0

$132.70

$149.10

$0

$0

$91.50

$91.20

$0

$0

$91.50

$91.20

$0

$0

$82

$38.70

$0

$0

$82

$38.70

$0

$0

$33.50

$17.90

$0

$0

$33.50

$17.90

$0

$0

$1,842.20

$2,014.50

$0

$0

$1,800.60

$1,992.50

$41.60

$22

$364.40

$439.90

$42.80

$44.50

$321.60

$395.40

$0

$0

$85.90

$84.80

$0

$0

$14.60

$13.80

$71.30

$71

$5,210.90

$5,266.80

$514.50

$340.30

$4,583.50

$4,833.50

$112.90

$93

$674

$635.90

$666.60

$621.50

'

'

$7.40

$14.40

$0

$0

$313.80

$308.20

$313.80

$308.20

$0

$0

$0

$0

$106.50

$105.40

$106.50

$105.40

$0

$0

$0

$0

$359.80

$360.40

$312.90

$314.30

$0

$0

$46.90

$46.10

$56.50

$57.10

$56.50

$57.10

$0

$0

$0

$0

$56

$53.40

$56

$53.40

$0

$0

$0

$0

$81.10

$78.60

$81.10

$78.60

$0

$0

$0

$0

$36.70

$60.60

$36.70

$60.60

$0

$0

$0

$0

$44.90

$34.30

$44.90

$34.30

$0

$0

$0

$0

$119.30

$98.80

$42.70

$24.50

$76.30

$74.30

$0.30

$0

$1,174.60

$1,156.80

$1,051.10

$1,036.40

$76.30

$74.30

$47.20

$46.10

$73.80

$80.50

$0

$0

$67.70

$74.40

$6.10

$6.10

$277.40

$262.40

$0

$0

$0

$0

$277.40

$262.40

'

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Carrying value of investment accounted for using the equity method

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25

26.6

34.9

35

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Realized and unrealized gains (losses)

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-0.1

-0.1

-0.1

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Fixed maturity investments reclassified to assets held for sale

'

236.3

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222.2

236.3

296.9

338.1

Short-term investments, at amortized cost (which approximates fair value)

$674

$635.90

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$635.90

$674

$654.40

$630.60

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Investment_Securities_Debt_sec

Investment Securities (Debt securities issued by corporation) (Details) (Debt securities issued by corporations, USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Investment securities	'	'
Fair value investments	$2,346.50	$2,347.20
AAA	'	'
Investment securities	'	'
Fair value investments	0	0
AA	'	'
Investment securities	'	'
Fair value investments	221.3	228.8
A	'	'
Investment securities	'	'
Fair value investments	1,028.90	1,039.50
BBB	'	'
Investment securities	'	'
Fair value investments	1,089.90	1,075.50
BB	'	'
Investment securities	'	'
Fair value investments	0	0
Other	'	'
Investment securities	'	'
Fair value investments	$6.40	$3.40

Investment_Securities_Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
	Fixed rate CMBS	Maximum	Fair Value	Fair Value	Fair Value	Fair Value	Fair Value	Fair Value	Fair Value	Fair Value	Fair Value
		Commercial Mortgage Backed Securities					Fixed rate CMBS	Floating rate CMBS	Commercial Mortgage Backed Securities	Commercial Mortgage Backed Securities	Non-Agency Residential Mortgage Resecuritization
Investment securities	'	'	'	'	'	'	'	'	'	'	'
Average basis points of subordination	0.25	'	'	'	'	'	'	'	'	'	'
Percentage of underlying loans reported as non-performing	'	1.00%	'	'	'	'	'	'	'	'	'
Fair value investments	'	'	$6,736.70	$6,766.50	$6,580.60	$6,950.70	$225.10	$104.60	$329.70	$282.30	$37.80

Investment_Securities_Mortgage1

Investment Securities (Mortgage-backed Asset Securities Table) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Level 2 Inputs	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$4,727.50	$4,982.20	$4,795.90	$5,206.10
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	464.8	512.3	'	'
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	167.3	81.2	'	'
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	85.6	91.3	'	'
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	717.7	684.8	'	'
Level 2 Inputs | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	132.3	125.7	'	'
Level 2 Inputs | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	310.1	282.3	'	'
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	442.4	408	'	'
Level 2 Inputs | Collateralized Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	1,160.10	1,092.80	'	'
Level 2 Inputs | Collateralized Credit Card Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	207.7	289.4	'	'
Level 2 Inputs | Collateralized Auto Loans	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	231.5	365	'	'
Level 2 Inputs | Collateralized Securities, Other	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	201.3	245.3	'	'
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	640.5	899.7	'	'
Level 2 Inputs | Mortgage-backed and asset-backed securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	1,800.60	1,992.50	'	'
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.6	0	'	'
Level 3 Inputs | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.6	0	'	'
Level 3 Inputs | Collateralized Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.6	0	'	'
Level 3 Inputs | Collateralized Credit Card Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	22	22	'	'
Level 3 Inputs | Collateralized Auto Loans	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Collateralized Securities, Other	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	0	'	'
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	22	22	'	'
Level 3 Inputs | Mortgage-backed and asset-backed securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	41.6	22	'	'
Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	6,736.70	6,766.50	6,580.60	6,950.70
Fair Value | Government National Mortgage Association Certificates and Obligations (GNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	464.8	512.3	'	'
Fair Value | Federal National Mortgage Association Certificates and Obligations (FNMA)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	167.3	81.2	'	'
Fair Value | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	85.6	91.3	'	'
Fair Value | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	717.7	684.8	'	'
Fair Value | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	132.3	125.7	'	'
Fair Value | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	329.7	282.3	'	'
Fair Value | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	462	408	'	'
Fair Value | Collateralized Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	1,179.70	1,092.80	'	'
Fair Value | Collateralized Credit Card Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	229.7	311.4	'	'
Fair Value | Collateralized Auto Loans	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	231.5	365	'	'
Fair Value | Collateralized Securities, Other	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	201.3	245.3	'	'
Fair Value | Asset-backed Securities, Securitized Loans and Receivables	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	662.5	921.7	'	'
Fair Value | Mortgage-backed and asset-backed securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$1,842.20	$2,014.50	'	'

Investment_Securities_Nonagenc

Investment Securities (Non-agency Mortgage-backed Securities) (Details) (Fair Value, USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investment securities	'	'	'	'
Fair value investments	$6,736.70	$6,766.50	$6,580.60	$6,950.70
Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	132.3	125.7	'	'
Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	329.7	282.3	'	'
Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	462	408	'	'
Securities Issued in 2004 [Member] | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	11.3	'	'	'
Securities Issued in 2004 [Member] | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued in 2004 [Member] | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	11.3	'	'	'
Securities Issued in 2005 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	20.2	'	'	'
Securities Issued in 2005 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	20.2	'	'	'
Securities Issued In 2006 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	11	'	'	'
Securities Issued In 2006 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	8.5	'	'	'
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.5	'	'	'
Securities Issued In 2007 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued In 2007 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	5	'	'	'
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	5	'	'	'
Securities Issued In 2008 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.6	'	'	'
Securities Issued In 2008 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	26.4	'	'	'
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	46	'	'	'
Securities Issued In 2009 [Member] | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued In 2009 [Member] | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued In 2009 [Member] | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued In 2010 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	26.4	'	'	'
Securities Issued In 2010 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	13	'	'	'
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	39.4	'	'	'
Securities Issued In 2011 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued In 2011 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	27	'	'	'
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	27	'	'	'
Securities Issued in 2012 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Securities Issued in 2012 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	131.8	'	'	'
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	131.8	'	'	'
Securities Issued in 2013 | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	32.8	'	'	'
Securities Issued in 2013 | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	94.3	'	'	'
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	127.1	'	'	'
Securities Issued in 2014 [Member] | Residential Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	11	'	'	'
Securities Issued in 2014 [Member] | Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	23.7	'	'	'
Securities Issued in 2014 [Member] | Mortgage-backed Securities, Issued by Private Enterprises	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$34.70	'	'	'

Investment_Securities_Nonagenc1

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$6,736.70	$6,766.50	$6,580.60	$6,950.70
Residential Mortgage Backed Securities | Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	132.3	125.7	'	'
Residential Mortgage Backed Securities | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	39.6	'	'	'
Residential Mortgage Backed Securities | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	92.7	'	'	'
Residential Mortgage Backed Securities | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	7.2	'	'	'
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	7.2	'	'	'
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.2	'	'	'
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	19.2	'	'	'
Residential Nonprime Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	0	'	'	'
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Fair Value	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	105.9	'	'	'
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	32.4	'	'	'
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	73.5	'	'	'
Residential, Prime, Financing Receivable [Member] | Residential Mortgage Backed Securities | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$0	'	'	'

Investment_Securities_Nonagenc2

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (Fair Value, USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investment securities	'	'	'	'
Fair value investments	$6,736.70	$6,766.50	$6,580.60	$6,950.70
Fixed rate CMBS	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	225.1	'	'	'
Fixed rate CMBS | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	123.5	'	'	'
Fixed rate CMBS | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	76	'	'	'
Fixed rate CMBS | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	25.6	'	'	'
Floating rate CMBS	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	104.6	'	'	'
Floating rate CMBS | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	5	'	'	'
Floating rate CMBS | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	26.4	'	'	'
Floating rate CMBS | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	73.2	'	'	'
Commercial Mortgage Backed Securities	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	329.7	282.3	'	'
Commercial Mortgage Backed Securities | Super Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	128.5	'	'	'
Commercial Mortgage Backed Securities | Senior	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	102.4	'	'	'
Commercial Mortgage Backed Securities | Subordinate	'	'	'	'
Investment securities	'	'	'	'
Fair value investments	$98.80	'	'	'

Rollforward_of_Fair_Value_Meas

(Rollforward of Fair Value Measurements by Level) (Details) (USD $)

3 Months Ended

3 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

3 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2012

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Fair Value

Fair Value

Fixed maturity investments.

Fixed maturity investments.

Common equity securities

Common equity securities

Convertible Debt Securities

Convertible Debt Securities

Other long-term investments

Other long-term investments

Other long-term investments

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Prospector Offshore Fund Ltd [Member]

Forward Contracts

Forward Contracts

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Fixed maturity investments.

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Convertible Debt Securities

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Other long-term investments

Forward Contracts

Forward Contracts

Forward Contracts

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Short-term investments

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Fair value measured on a recurring basis

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Level 2 Inputs

Level 2 Inputs

Level 2 Inputs

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 1 Inputs

Level 2 Inputs

Level 2 Inputs

Level 3 Inputs

Level 3 Inputs

Fair Value

Fair Value

Investment related liabilities [Member]

Level 3 Inputs

Level 3 Inputs

Investment securities

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair value investments

'

'

'

$1,376.70

$1,355.10

$4,982.20

$5,206.10

$6,766.50

$6,950.70

$93

$92.90

$46.10

$37.30

$6.10

$0

'

$262.40

$259.30

$2,232.20

$1,998.20

$4,734.90

$4,996.60

$443.60

$407.60

$7,410.70

$7,402.40

$514.50

$340.30

$4,583.50

$4,833.50

$112.90

$93

$5,210.90

$5,266.80

$0

$0

$67.70

$74.40

$6.10

$6.10

$73.80

$80.50

$0

$0

$0

$0

$277.40

$262.40

'

'

$277.40

$262.40

'

'

'

'

$666.60

$621.50

'

$7.40

$14.40

$0

$0

$674

$635.90

'

'

'

'

'

'

'

Total realized and unrealized gains (losses)

'

'

'

22.2

80.1

34.7

-10.8

64.4

75.1

0.4

0.1

0.8

-0.1

0

0

'

6.3

5.8

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Foreign currency gains (losses) through OCI

'

'

'

-2.9

-0.4

-10.5

-4.8

-13.8

-5.6

-0.1

0

0

0

0

0

'

-0.3

-0.4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amortization/Accretion

'

'

'

-0.2

-0.4

-11.1

-14

-11.3

-14.4

0

0

0

0

0

0

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchases

'

'

'

352.6

318.6

840.4

1,445.80

1,225.60

1,791.30

19.6

8

0.3

0

0

0

'

12.7

18.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Sales

'

'

'

-180.1

-382.3

-1,115.50

-1,828.30

-1,299.30

-2,233.70

0

0

0

0

0

0

'

-3.7

-23.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net change in investments related to purchases and sales of consolidated affiliates

'

'

'

-2.7

14.5

7.3

2.7

4.6

17.2

0

0

0

0

0

0

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Transfers in

'

'

'

0

1.2

0

0

0

1.2

0

0

0

0

0

0

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Transfers out

'

'

'

0

0

0

-0.8

0

1.2

0

0

0

-0.4

0

0

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair value investments

'

'

'

1,565.60

1,386.40

4,727.50

4,795.90

6,736.70

6,580.60

112.9

101

47.2

36.8

6.1

0

'

277.4

260.5

2,232.20

1,998.20

4,734.90

4,996.60

443.6

407.6

7,410.70

7,402.40

514.5

340.3

4,583.50

4,833.50

112.9

93

5,210.90

5,266.80

0

0

67.7

74.4

6.1

6.1

73.8

80.5

0

0

0

0

277.4

262.4

'

'

277.4

262.4

'

'

'

'

666.6

621.5

'

7.4

14.4

0

0

674

635.9

'

'

'

'

'

'

'

Carrying value of investment accounted for using the equity method

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25

26.6

34.9

35

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-0.2

-0.1

Realized and unrealized gain

48.1

61.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net realized and unrealized investment gains

63.8

75.2

'

'

'

'

'

'

'

'

'

'

'

'

'

1.2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.7

'

'

Short-term investments, at amortized cost (which approximates fair value)

674

'

635.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

654.4

635.9

674

630.6

'

'

'

'

'

'

'

'

'

'

'

'

'

Realized and unrealized gains (losses)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-0.1

-0.1

-0.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fixed maturity investments reclassified to assets held for sale

'

'

$236.30

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$222.20

$236.30

$296.90

$338.10

'

'

'

Investment_Securities_Signific

Investment Securities (Significant Unobservable Inputs) (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
NR | Preferred Stock | Discounted cash flows	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	71,300,000	71,000,000
NR | Preferred Stock | Discount Yield [Member]	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Discount Yield (as a percent)	6.90%	7.40%
NR | Private equity funds | Multiple of GAAP book value	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	36,400,000	35,600,000
Book value multiple (as a percent)	100.00%	100.00%
NR | Private equity funds | Average Share Price [Member]	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Average Share Price Used As Unobservable Input	1.1	1.1
NR | Private equity funds | Recent transaction [Member]	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	10,500,000	10,500,000
NR | Convertible Preferred Stock [Member] | Recent transaction [Member]	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	6,100,000	6,100,000
Share Price of Recent Transaction Used As Unobservable Input	6.1	6.1
AAA | Non-Agency Commercial Mortgage Backed Securities [Member] [Member] | Broker Indication [Member]	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	13,300,000	'
Fair value input, swap spread rate	0.80%	'
AA | Mortgage-backed and asset-backed securities | Broker Pricing	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	22,000,000	22,000,000
Standard & Poor's, BBB- Rating [Member] | Non-Agency Commercial Mortgage Backed Securities [Member] [Member] | Broker Pricing	'	'
Quantitative information for Level 3 Fair Value Measurements Assets	'	'
Fair value Assets	6,300,000	'

Debt_Details

Debt (Details) (USD $)

3 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Aug. 14, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

WTM Bank Facility

Old Lyme

Old Lyme

Other

Other

OneBeacon

OneBeacon

Sirius Group

Sirius Group

White Mountains

White Mountains

2012 OBH Senior Notes

2012 OBH Senior Notes

SIG Senior Notes

SIG Senior Notes

WTM Bank Facility

WTM Bank Facility

Debt Instrument

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument at face value

'

'

'

'

'

'

'

'

$275

$275

$400

$400

'

'

Unamortized original issue discount

'

'

'

'

'

'

'

'

-0.3

-0.3

-0.4

-0.4

'

'

Total debt

677.6

'

676.4

'

2.1

2.1

1.2

0

274.7

274.7

399.6

399.6

0

0

Total commitment under revolving credit facility

'

'

'

425

'

'

'

'

'

'

'

'

'

'

Repayments of debt

$0

$155

'

'

'

'

'

'

'

'

'

'

'

'

Income_Taxes_Details

Income Taxes (Details) (USD $)	3 Months Ended			3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Jul. 28, 2011	Mar. 31, 2014
			Tax Assessment Year 2007 to 2009	OneBeacon
			Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]	'	'	'	'
Effective tax rates (as a percent)	27.00%	27.40%	'	'
U.S. statutory rate (as a percent)	35.00%	35.00%	'	'
Income Tax Expense (Benefit)	$30.30	$41.60	'	$5
Total tax assessment	'	'	69.3	'
Tax assessments related to disallowed deductions	'	'	$60.20	'

Derivatives_Details

Derivatives (Details)	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2013
In Billions, unless otherwise specified	USD ($)	JPY (¥)	USD ($)	JPY (¥)
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'	'	'	'
Notional amount	$1.80	¥ 187.9	$1.90	¥ 203.6
Collective account values of underlying variable annuities as a percentage of the guarantee value	103.00%	103.00%	104.00%	104.00%

Derivatives_PreTax_Operating_R

Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Pre-tax operating results of WM Life Re	'	'
Total revenues	$578.30	$627.40
General and administrative expenses	-50.3	-43.8
Pre-tax income from continuing operations	112.1	151.8
White Mountains Life Re	'	'
Pre-tax operating results of WM Life Re	'	'
Fees, included in other revenue	5.4	6.8
Change in fair value of variable annuity liability, included in other revenue	-1.3	184.3
Realized and unrealized gains (losses)	-6.6	-181.8
Foreign exchange, included in other revenue	0.2	-14.6
Other investment income and gains (losses)	0.4	-4
Total revenues	-1.9	-9.3
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	5.6
Death benefit claims paid, included in general and administrative expenses	0	-1.1
General and administrative expenses	-1.3	-1.7
Pre-tax income from continuing operations	($3.20)	($6.50)

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, 2014	Mar. 31, 2013	Dec. 31, 2013
Variable Annuity Reinsurance [Line Items]	'	'	'
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$0	$5.60	'
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments	'	'	'
Realized and unrealized gains (losses)	-6.6	-181.8	'
Derivative, fair value, net	89.6	'	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)	-6.4	-30.9	'
Derivative, fair value, net	-5.3	'	-9.7
Foreign exchange	'	'	'
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments	'	'	'
Realized and unrealized gains (losses)	-6.5	-53.6	'
Derivative, fair value, net	63.9	'	58
Equity	'	'	'
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments	'	'	'
Realized and unrealized gains (losses)	6.3	-97.3	'
Derivative, fair value, net	$31	'	$20.90

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, 2014	Mar. 31, 2013
Changes in derivative instruments	'	'
Beginning balance	$69.20	'
Realized and unrealized gains (losses)	-6.6	-181.8
Ending balance	89.6	'
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	-5.6
Fair Value	'	'
Changes in derivative instruments	'	'
Purchases	0	'
Beginning balance	69.2	98.3
Purchases	'	36.1
Realized and unrealized gains (losses)	6.6	181.8
Transfers in	0	0
Sales/settlements	27	123.2
Ending balance	89.6	75.8
Level 3 Inputs	'	'
Changes in derivative instruments	'	'
Beginning balance	-52.8	-441.5
Purchases	0	0
Realized and unrealized gains (losses)	-1.3	189.9
Transfers in	0	0
Sales/settlements	0	0
Ending balance	-54.1	-251.6
Beginning balance	63.4	140.5
Purchases	'	36.1
Realized and unrealized gains (losses)	0.7	-49.2
Transfers in	0	0
Sales/settlements	0	0
Ending balance	64.1	127.4
Level 2 Inputs	'	'
Changes in derivative instruments	'	'
Purchases	0	'
Beginning balance	4.7	-20.5
Purchases	'	0
Realized and unrealized gains (losses)	7.1	85.5
Transfers in	0	0
Sales/settlements	27.5	63.1
Ending balance	25.1	-42.9
Level 1 Inputs	'	'
Changes in derivative instruments	'	'
Purchases	0	'
Beginning balance	1.1	-21.7
Purchases	'	0
Realized and unrealized gains (losses)	0.2	47.1
Transfers in	0	0
Sales/settlements	-0.5	60.1
Ending balance	$0.40	($8.70)

Derivatives_Additional_Collate

Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
In Millions, unless otherwise specified
Credit risk on derivative instruments	'	'	'
Collateral cash and investments	$36.50	$81.30	$229.80
Cash	'	'	'
Credit risk on derivative instruments	'	'	'
Derivative collateral right to reclaim	22.2	56.1	188.5
Short-term investments	'	'	'
Credit risk on derivative instruments	'	'	'
Derivative collateral right to reclaim	0	2	15.6
Fixed maturity investments.	'	'	'
Credit risk on derivative instruments	'	'	'
Derivative collateral right to reclaim	$14.30	$23.20	$25.70

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, 2014
Discounted cash flows	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Fair value Liabilities	54.1
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)	5.90%
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	31.5
Foreign Exchange Options | Counterparty valuations | Minimum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Foreign exchange volatilities (as a percent)	-1.20%
Foreign Exchange Options | Counterparty valuations | Maximum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Foreign exchange volatilities (as a percent)	8.00%
Foreign Exchange Options | Counterparty valuations | Weighted Average	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Foreign exchange volatilities (as a percent)	2.40%
Equity Index Options | Counterparty valuations	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Fair value Liabilities	32.5
Equity Index Options | Counterparty valuations | Minimum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Expected equity dividends (as a percent)	0.50%
Equity Index Options | Counterparty valuations | Maximum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Expected equity dividends (as a percent)	9.70%
Equity Index Options | Counterparty valuations | Weighted Average	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Expected equity dividends (as a percent)	3.50%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Surrenders (as a percent)	0.30%
Foreign exchange volatilities (as a percent)	10.50%
Index volatilities (as a percent)	16.10%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Surrenders (as a percent)	30.00%
Foreign exchange volatilities (as a percent)	13.50%
Index volatilities (as a percent)	19.70%
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)	18.00%
Foreign exchange volatilities (as a percent)	11.40%
Index volatilities (as a percent)	17.80%
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)	11.70%
Index volatilities (as a percent)	17.00%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum	'
Quantitative information for Level 3 Fair Value Measurements Liabilities	'
Surrenders (as a percent)	32.00%
Foreign exchange volatilities (as a percent)	14.60%
Index volatilities (as a percent)	19.40%
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)	13.60%
Foreign exchange volatilities (as a percent)	12.60%
Index volatilities (as a percent)	18.30%

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, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	$99.70	$106
Derivative Liabilities	-10.1	-36.8
Net amounts recognized in Other assets	89.6	69.2
Over The Counter | Fixed income/interest rate	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	1.2	2.4
Derivative Liabilities	-6.4	-11.7
Net amounts recognized in Other assets	-5.2	-9.3
Over The Counter | Foreign exchange	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	64.4	67.8
Derivative Liabilities	0	-12
Net amounts recognized in Other assets	64.4	55.8
Over The Counter | Equity	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	32.6	30.7
Derivative Liabilities	-2.5	-9.2
Net amounts recognized in Other assets	30.1	21.5
Exchange Traded | Fixed income/interest rate	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	0.5	1
Derivative Liabilities	-0.6	-1.6
Net amounts recognized in Other assets	-0.1	-0.6
Exchange Traded | Foreign exchange	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	0.1	2.3
Derivative Liabilities	-0.6	0
Net amounts recognized in Other assets	-0.5	2.3
Exchange Traded | Equity	'	'
Offsetting Liabilities [Line Items]	'	'
Derivative Assets	0.9	1.8
Derivative Liabilities	0	-2.3
Net amounts recognized in Other assets	$0.90	($0.50)

Derivatives_Uncollateralized_A

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified	Rating
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	$89.60	$69.20
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0.4
Net amount of exposure after effect of collateral	89.6	69.6
Excess collateral provided to counterparty in cash	20.4	44.1
Derivative, Collateral, Right to Reclaim Securities	14.3	22.8
Counter-party collateral held by WMLife Re - Cash	3.8	0.8
Net amount of exposure to counter-party	120.5	135.7
S and P Credit Ratings	21	'
Standard & Poor's, A Rating | Bank of America [Member]	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	29.1	27.2
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	29.1	27.2
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	29.1	27.2
Standard & Poor's, A Rating | Barclays	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	1.2	'
Collertal provided to counterparty in cash	0	'
Collateral provided to counterparty in financial instruments	0	'
Net amount of exposure after effect of collateral	1.2	'
Excess collateral provided to counterparty in cash	0	'
Derivative, Collateral, Right to Reclaim Securities	0	'
Counter-party collateral held by WMLife Re - Cash	0	'
Net amount of exposure to counter-party	1.2	'
Standard & Poor's, A Rating | Royal Bank of Scotland	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	'	11.3
Collertal provided to counterparty in cash	'	0
Collateral provided to counterparty in financial instruments	'	0
Net amount of exposure after effect of collateral	'	11.3
Excess collateral provided to counterparty in cash	'	0
Derivative, Collateral, Right to Reclaim Securities	'	0
Counter-party collateral held by WMLife Re - Cash	'	0
Net amount of exposure to counter-party	'	11.3
Standard & Poor's, A Rating | Citigroup - OTC	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	24.2	19.4
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	24.2	19.4
Excess collateral provided to counterparty in cash	0	2.3
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	3.8	0
Net amount of exposure to counter-party	20.4	21.7
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	0.4	1.2
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	0.4	1.2
Excess collateral provided to counterparty in cash	10.9	19.8
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	11.3	21
Standard & Poor's, A plus | Barclays	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	'	1.4
Collertal provided to counterparty in cash	'	0
Collateral provided to counterparty in financial instruments	'	0
Net amount of exposure after effect of collateral	'	1.4
Excess collateral provided to counterparty in cash	'	0
Derivative, Collateral, Right to Reclaim Securities	'	0
Counter-party collateral held by WMLife Re - Cash	'	0
Net amount of exposure to counter-party	'	1.4
Standard & Poor's, A plus | JP Morgan	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	22.6	9.1
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	22.6	9.1
Excess collateral provided to counterparty in cash	4.7	22
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	27.3	31.1
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member]	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	0.7	-0.4
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0.4
Net amount of exposure after effect of collateral	0.7	0
Excess collateral provided to counterparty in cash	4.8	0
Derivative, Collateral, Right to Reclaim Securities	14.3	22.8
Counter-party collateral held by WMLife Re - Cash	0	0.8
Net amount of exposure to counter-party	19.8	22
Standard & Poor's, A- Rating | Royal Bank of Scotland	'	'
Credit risk on derivative instruments	'	'
Excess collateral provided - Financial Instruments	11.4	'
Collertal provided to counterparty in cash	0	'
Collateral provided to counterparty in financial instruments	0	'
Net amount of exposure after effect of collateral	11.4	'
Excess collateral provided to counterparty in cash	0	'
Derivative, Collateral, Right to Reclaim Securities	0	'
Counter-party collateral held by WMLife Re - Cash	0	'
Net amount of exposure to counter-party	$11.40	'

Derivatives_Forward_Contracts_

Derivatives Forward Contracts (Details)	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013
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	$1,800	¥ 187,900	$1,900	¥ 203,600	$23.20	'	150.5	$20.40
Changes in derivative instruments	'	'	'	'	'	'	'	'
Beginning balance	'	'	'	'	-0.1	-0.1	'	'
Purchases	'	'	'	'	0	0	'	'
Realized and unrealized gains (losses)	'	'	'	'	-0.1	0.4	'	'
Sales/settlements	'	'	'	'	0.1	-0.5	'	'
Ending balance	'	'	'	'	($0.10)	($0.20)	'	'

Derivatives_Unrealized_and_Rea

Derivatives (Unrealized and Realized Derivative Gains (Losses) Forward Contracts) (Details) (Forward Contracts, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]	'	'	'	'
Realized and unrealized gains (losses)	($0.10)	$0.40	'	'
Sales/settlements	0.1	-0.5	'	'
Derivative, fair value, net	-0.1	-0.2	-0.1	-0.1
United States of America, Dollars	'	'	'	'
Derivative [Line Items]	'	'	'	'
Realized and unrealized gains (losses)	0	-0.3	'	'
Derivative, fair value, net	0	'	0	'
Sweden, Kronor	'	'	'	'
Derivative [Line Items]	'	'	'	'
Realized and unrealized gains (losses)	0	0	'	'
Derivative, fair value, net	0	'	0	'
Euro Member Countries, Euro	'	'	'	'
Derivative [Line Items]	'	'	'	'
Realized and unrealized gains (losses)	-0.1	'	'	'
Derivative, fair value, net	-0.1	'	-0.1	'
United Kingdom, Pounds	'	'	'	'
Derivative [Line Items]	'	'	'	'
Realized and unrealized gains (losses)	0	0.7	'	'
Derivative, fair value, net	$0	'	$0	'

Derivatives_Forward_Contracts_1

Derivatives (Forward Contracts Netting) (Details) (Forward Contracts, USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'	'
Derivative Assets	$0	$0
Derivative Liabilities	-0.1	-0.1
Net amounts recognized in Other assets	-0.1	-0.1
United States of America, Dollars	'	'
Derivative [Line Items]	'	'
Derivative Assets	0	0
Derivative Liabilities	0	0
Net amounts recognized in Other assets	0	0
Euro Member Countries, Euro	'	'
Derivative [Line Items]	'	'
Derivative Assets	0	0
Derivative Liabilities	-0.1	-0.1
Net amounts recognized in Other assets	-0.1	-0.1
United Kingdom, Pounds	'	'
Derivative [Line Items]	'	'
Derivative Assets	0	0
Derivative Liabilities	0	0
Net amounts recognized in Other assets	$0	$0

Derivatives_Forward_Contract_C

Derivatives (Forward Contract Collateral) (Details)

Mar. 31, 2014

Mar. 31, 2014

Dec. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2014

Dec. 31, 2013

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 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

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

JP Morgan

JP Morgan

Goldman Sachs

Goldman Sachs

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

$1,800

¥ 187,900

$1,900

¥ 203,600

$23.20

150.5

$20.40

'

$8.10

$5.80

$7.10

$7.70

$0

$1.30

$4.40

$2.10

$3.60

$3.30

$0

$0.20

Carrying Value

'

'

'

'

($0.10)

'

($0.10)

'

($0.10)

$0

'

$0

$0

$0

$0

$0

$0

($0.10)

$0

$0

S and P Credit Ratings

'

'

'

'

21

21

'

21

'

'

'

'

'

'

'

'

'

'

'

'

Derivatives_Interest_rate_cap_

Derivatives (Interest rate cap) (Details) (USD $)	3 Months Ended		1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	31-May-07	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014
	Interest Rate Cap	Interest Rate Cap	Sirius International Group Ltd	Sirius International Group Ltd	Standard & Poor's, A Rating	Standard & Poor's, AA- Rating
	Rating		Preferred Stock	Interest Rate Cap	Barclays	Nordea Bank Finland [Member]
					Interest Rate Cap	Interest Rate Cap
Derivative [Line Items]	'	'	'	'	'	'
Derivative Collateral Right to Reclaim	($8.20)	'	'	'	$5.70	$2.50
Preferred Stock, Dividend Rate, Percentage	'	'	7.51%	'	'	'
Term of LIBOR loan	'	'	'3 months	'	'	'
Derivative, Cap Interest Rate	'	'	'	8.30%	'	'
Derivative, Cost of Hedge	'	'	'	9.9	'	'
basis points	'	'	3.20%	3.95%	'	'
Non-cumulative perpetual preference shares issued	'	'	250	'	'	'
years of interest rate cap	'	'	'	'5 years	'	'
Derivative, fair value, net	8.2	11.1	'	'	'	'
Purchases	0	'	'	'	'	'
Realized and unrealized gains (losses)	-2.9	'	'	'	'	'
Sales/settlements	$0	'	'	'	'	'
S and P Credit Ratings	21	'	'	'	'	'

Derivatives_Weather_Derivative

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

Municipal_Bond_Guarantee_Detai

Municipal Bond Guarantee (Details) (USD $)

Mar. 31, 2014

Mar. 31, 2014

Dec. 31, 2013

Jul. 31, 2012

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Jul. 31, 2012

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2014

Mar. 31, 2014

In Millions, unless otherwise specified

BAM

BAM

HG Global

HG Global

HG Global

HG Global

HG Global

Build America Mutual

Build America Mutual

HG Re [Member]

BAM Management

Surplus Note [Member]

Fixed interest rate [Member]

Variable Rate [Domain]

Combination One

Combination Two

Contract

Contract

Preferred Stock [Member]

Common Stock

BAM

Surplus Note [Member]

Surplus Note [Member]

Surplus Note [Member]

Surplus Note [Member]

BAM

BAM

BAM

BAM

Guarantor Obligations [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

basis points

'

'

'

'

'

'

'

'

'

'

'

'

3.00%

'

'

'

'

Municipal bond contract

'

907

701

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Insured Financial Obligations, Remaining Weighted Average Contract Period

'

'13 years 7 months 10 days

'13 years 9 months 21 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Municipal bond outstanding principal

'

$6,015

$4,703.70

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Municipal bond outstanding interest

'

4,140.90

3,264.40

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Municipal bond debt service outstanding

'

10,155.90

7,968.10

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Partners Capital Contribution

'

'

'

594.5

'

'

'

'

'

'

'

14.5

'

'

'

'

'

Ownership interest (as a percent)

'

'

'

'

'

'

96.90%

88.40%

'

'

'

'

'

'

'

'

'

Surplus Notes

'

'

'

503

503

'

'

'

'

'

'

'

'

'

'

'

'

Statutory Accounting Practices, Statutory Capital and Surplus, Balance

'

'

'

'

'

'

'

'

'

'

439.9

'

'

'

'

'

'

Percentage of par value of policy reinsured

15.00%

'

'

'

15.00%

'

'

'

'

'

'

'

'

'

'

'

'

Income (Loss) from Continuing Operations before Income Taxes, Domestic

'

'

'

'

4.6

9.9

'

'

-8.6

-18.4

'

'

'

'

'

'

'

Interest income (expense), surplus notes

'

'

'

'

3.9

10.1

'

'

-3.9

-10.1

'

'

'

'

'

'

'

Number of years of change in interest rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5

3

Financial Guarantee Insurance Contracts, Unearned Premium Revenue

'

$17.70

$13.20

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt Instrument, Interest Rate, Stated Percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

8.00%

3.13%

'

'

Year of U.S. Treasury Rate

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]	'	'
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000
Basic and diluted earnings per share numerators (in millions):	'	'
Net income (loss) attributable to White Mountains’ common shareholders	$96.10	$119.90
Allocation of income for unvested restricted common shares	-1.1	-1.5
Dividends declared on participating restricted common shares	-0.1	-0.1
Total allocation to restricted common shares	-1.2	-1.6
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	94.9	118.3
Undistributed net earnings (in millions):	'	'
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	94.9	118.3
Dividends declared net of restricted common share amounts	-6.1	-6.1
Total undistributed net earnings, net of restricted common share amounts	$88.80	$112.20
Basic earnings per share denominators	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Basic earnings per share denominator	6,099,500	6,194,500
Diluted earnings per share denominator	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Average outstanding dilutive options to acquire common shares	0	0
Diluted earnings (loss) per share denominator	6,099,500	6,194,500
Basic earnings per share (in dollars):	'	'
Net income (loss) attributable to White Mountains' common shareholders	$15.56	$19.10
Dividends declared	($1)	($1)
Undistributed earnings (loss) (in dollars per share)	$14.56	$18.10
Earnings Per Share, Diluted [Abstract]	'	'
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	$15.56	$19.10
Dividends declared	($1)	($1)
Undistributed earnings (loss) (in dollars per share)	$14.56	$18.10

Segment_Information_Details

Segment Information (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Segment reporting information	'	'
Earned insurance premiums	$493.60	$495.40
Net investment income	24.3	28.5
Net investment income (loss) - surplus note interest	0	0
Net realized and unrealized investment gains (losses)	63.8	75.2
Other revenue	-3.4	28.3
Total revenues	578.3	627.4
Losses and LAE	229.3	244.3
Insurance and reinsurance acquisition expenses	95.1	98.2
Other underwriting expenses	81.4	79.1
General and administrative expenses	50.3	43.8
Interest expense	10.1	10.2
Total expenses	466.2	475.6
Pre-tax income (loss)	112.1	151.8
OneBeacon	'	'
Segment reporting information	'	'
Earned insurance premiums	276.5	286.5
Net investment income	10	9.4
Net investment income (loss) - surplus note interest	0	0
Net realized and unrealized investment gains (losses)	18.9	28.4
Other revenue	1	24
Total revenues	306.4	348.3
Losses and LAE	149.4	148.9
Insurance and reinsurance acquisition expenses	46.7	54.8
Other underwriting expenses	49.4	49.3
General and administrative expenses	3.3	4
Interest expense	3.2	3.2
Total expenses	252	260.2
Pre-tax income (loss)	54.4	88.1
Sirius Group	'	'
Segment reporting information	'	'
Earned insurance premiums	215.5	208.9
Net investment income	8.7	13.1
Net investment income (loss) - surplus note interest	0	0
Net realized and unrealized investment gains (losses)	28.1	13.1
Other revenue	-5.3	10.5
Total revenues	247	245.6
Losses and LAE	78.8	95.4
Insurance and reinsurance acquisition expenses	47.4	43.3
Other underwriting expenses	31.8	29.7
General and administrative expenses	8.4	9.5
Interest expense	6.6	6.6
Total expenses	173	184.5
Pre-tax income (loss)	74	61.1
HG Global	'	'
Segment reporting information	'	'
Earned insurance premiums	0.2	0
Net investment income	0.3	0.3
Net investment income (loss) - surplus note interest	3.9	10.1
Net realized and unrealized investment gains (losses)	0.7	-0.2
Other revenue	0	0
Total revenues	5.1	10.2
Losses and LAE	0	0
Insurance and reinsurance acquisition expenses	0.1	0
Other underwriting expenses	0	0
General and administrative expenses	0.4	0.3
Interest expense	0	0
Total expenses	0.5	0.3
Pre-tax income (loss)	4.6	9.9
BAM	'	'
Segment reporting information	'	'
Earned insurance premiums	0.1	0
Net investment income	1.4	1.1
Net investment income (loss) - surplus note interest	-3.9	-10.1
Net realized and unrealized investment gains (losses)	3	-1.1
Other revenue	0.2	0.1
Total revenues	0.8	-10
Losses and LAE	0	0
Insurance and reinsurance acquisition expenses	0.4	0.1
Other underwriting expenses	0.1	0.1
General and administrative expenses	8.9	8.2
Interest expense	0	0
Total expenses	9.4	8.4
Pre-tax income (loss)	-8.6	-18.4
Other Operations	'	'
Segment reporting information	'	'
Earned insurance premiums	1.3	0
Net investment income	3.9	4.6
Net investment income (loss) - surplus note interest	0	0
Net realized and unrealized investment gains (losses)	13.1	35
Other revenue	0.7	-6.3
Total revenues	19	33.3
Losses and LAE	1.1	0
Insurance and reinsurance acquisition expenses	0.5	0
Other underwriting expenses	0.1	0
General and administrative expenses	29.3	21.8
Interest expense	0.3	0.4
Total expenses	31.3	22.2
Pre-tax income (loss)	($12.30)	$11.10

Investments_in_Unconsolidated_2

Investments in Unconsolidated Affiliates (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investments in unconsolidated affiliates	'	'	'	'
Total investments in unconsolidated affiliates	$373.30	$321.40	'	'
Symetra	'	'	'	'
Investments in unconsolidated affiliates	'	'	'	'
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates	-4.2	-43.6	'	'
Total investments in unconsolidated affiliates	369	317.3	377.5	381.5
Symetra | Common shareholders' equity.	'	'	'	'
Investments in unconsolidated affiliates	'	'	'	'
Symetra Common Shares excluding unrealized gains (losses)	373.2	360.9	'	'
Total investments in unconsolidated affiliates	369	317.3	350.9	351.2
Symetra | Warrants	'	'	'	'
Investments in unconsolidated affiliates	'	'	'	'
Total investments in unconsolidated affiliates	0	0	26.6	30.3
Hamer, LLC	'	'	'	'
Investments in unconsolidated affiliates	'	'	'	'
Total investments in unconsolidated affiliates	$4.30	$4.10	'	'

Investments_in_Unconsolidated_3

Investments in Unconsolidated Affiliates (Symetra) (Details) (USD $)

3 Months Ended

12 Months Ended

3 Months Ended

3 Months Ended

6 Months Ended

12 Months Ended

12 Months Ended

In Millions, except Share data, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2011

Jun. 20, 2013

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2011

Mar. 31, 2014

Mar. 31, 2013

Jun. 20, 2013

Mar. 31, 2014

Jun. 20, 2013

Dec. 31, 2011

Mar. 31, 2014

Jun. 20, 2013

Dec. 31, 2011

Symetra

Symetra

Symetra

Symetra

Common Stock

Common Stock

Common Stock

Warrants

Warrants

Symetra

equity in earnings of unconsolidated affiliates [Member]

equity in earnings of unconsolidated affiliates [Member]

equity in earnings of unconsolidated affiliates [Member]

Net unrealized gains (losses) of unconsolidated affiliates [Member]

Net unrealized gains (losses) of unconsolidated affiliates [Member]

Net unrealized gains (losses) of unconsolidated affiliates [Member]

Symetra

Symetra

Symetra

Symetra

Symetra

Common Stock

Symetra

Symetra

Common Stock

Symetra

Symetra

Common Stock

Symetra

Symetra

Investments in unconsolidated affiliates

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares of Symetra Financial Corporation owned (in shares)

'

'

20,050,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ownership interest (as a percent)

'

'

17.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Investments In Unconsolidated Affiliates On Conversion Of Warrants

'

'

'

'

'

9,490,000

'

'

'

'

'

'

'

'

'

'

'

'

Common shares received from exercised warrants

'

'

'

'

'

2,650,000

'

'

'

'

'

'

'

'

'

'

'

'

Stock Repurchased and Retired During Period, Shares

'

'

'

'

'

'

'

'

'

'

'

6,600,000

'

'

'

'

'

'

Average price per share

$586

$564

'

'

'

'

'

'

'

'

'

$13.44

'

'

'

'

'

'

Unamortized basis difference

'

'

'

'

'

$19.30

'

'

'

'

'

'

'

$0.40

'

'

$18.90

'

Fair value of investment

'

'

'

'

'

'

'

'

261

'

'

'

'

'

'

'

'

'

Estimated fair value of investment per share

'

'

'

'

'

'

'

'

$15

'

'

'

'

'

'

'

'

'

Other than Temporary Impairment Losses, Investments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

45.9

'

'

136.6

Term of amortization (in years)

'

'

'

'

'30 years

'

'

'

'

'

'

'

'

'

'

'

'

'

Weight Average Years Remaining

'

'

'29 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

After tax amortization through equity in earnings

'

'

0.7

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

After tax amortization through equity in net unrealized losses

'

'

2.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Equity Method Investment, Difference Between Carrying Amount and Underlying Equity

'

'

183.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Pre-tax unamortized basis difference

'

'

'

'

'

'

'

'

'

'

'

'

39

'

'

144.3

'

'

Expected life (in years)

'

'

'

'

'

'

'

'

'

'

'1 year 1 month 90 days

'

'

'

'

'

'

'

Class of warrant or right fair value assumptions per strike price

'

'

'

'

'

'

'

'

'

'

11.49

'

'

'

'

'

'

'

Proceeds from Equity Method Investment, Dividends or Distributions

'

'

2

1.4

'

'

2

1.4

'

0

0

'

'

'

'

'

'

'

Proceeds from Dividends Received

'

'

'

'

'

'

'

'

'

'

$0.80

'

'

'

'

'

'

'

Fair Value Assumptions, Risk Free Interest Rate

'

'

'

'

'

'

'

'

'

'

0.17%

'

'

'

'

'

'

'

Fair Value Assumptions, Expected Volatility Rate

'

'

'

'

'

'

'

'

'

'

29.10%

'

'

'

'

'

'

'

Share Price

'

'

'

'

'

'

$19.82

'

'

'

$13.41

'

'

'

'

'

'

'

Investments_in_Unconsolidated_4

Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Investments in unconsolidated affiliates	'	'
Income Tax Expense (Benefit)	$30.30	$41.60
Changes in carrying value of investment in Symetra	'	'
Carrying value at the beginning of the period	321.4	'
Equity in earnings	13.8	9.2
Carrying value at the end of the period	373.3	'
Symetra	'	'
Changes in carrying value of investment in Symetra	'	'
Carrying value at the beginning of the period	317.3	381.5
Equity in earnings	14.4	9.8
Equity in net unrealized losses from Symetra's fixed maturity portfolio	39.3	-8.7
Dividends received	-2	-1.4
Increase (decrease) in value of warrants	0	-3.7
Carrying value at the end of the period	369	377.5
Symetra | Common shareholders' equity.	'	'
Changes in carrying value of investment in Symetra	'	'
Carrying value at the beginning of the period	317.3	351.2
Equity in earnings	14.4	9.8
Equity in net unrealized losses from Symetra's fixed maturity portfolio	39.3	-8.7
Dividends received	-2	-1.4
Increase (decrease) in value of warrants	0	0
Carrying value at the end of the period	369	350.9
Aggregate value of White Mountains common shares of Symetra at quoted market price	397.4	'
Share Price	$19.82	'
Symetra | Warrants	'	'
Changes in carrying value of investment in Symetra	'	'
Carrying value at the beginning of the period	0	30.3
Equity in earnings	0	0
Equity in net unrealized losses from Symetra's fixed maturity portfolio	0	0
Dividends received	0	0
Increase (decrease) in value of warrants	0	-3.7
Carrying value at the end of the period	0	26.6
Share Price	'	$13.41
White Mountains | Common shareholders' equity.	'	'
Changes in carrying value of investment in Symetra	'	'
Equity in net unrealized losses from Symetra's fixed maturity portfolio	39.3	-8.7
equity in earnings of unconsolidated affiliates [Member] | Symetra | Common shareholders' equity.	'	'
Investments in unconsolidated affiliates	'	'
Income Tax Expense (Benefit)	1	0.8
Net unrealized gains (losses) of unconsolidated affiliates [Member] | White Mountains | Common shareholders' equity.	'	'
Investments in unconsolidated affiliates	'	'
Income Tax Expense (Benefit)	-2.8	0.7
Amortization of common share basis difference [Member] | Symetra | Common shareholders' equity.	'	'
Changes in carrying value of investment in Symetra	'	'
Equity in earnings	0.7	0.7
impairment of net unrealized gains (losses) [Member] | Net unrealized gains (losses) of unconsolidated affiliates [Member] | Symetra | Common shareholders' equity.	'	'
Changes in carrying value of investment in Symetra	'	'
Equity in net unrealized losses from Symetra's fixed maturity portfolio	$3.10	$2.70

Investments_in_Unconsolidated_5

Investments in Unconsolidated Affiliates (Hamer) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Subsidiary, Sale of Stock [Line Items]	'	'	'
Equity in earnings of unconsolidated affiliates, net of tax	$13.80	$9.20	'
Investments in unconsolidated affiliates	373.3	'	321.4
Hamer, LLC	'	'	'
Subsidiary, Sale of Stock [Line Items]	'	'	'
Equity in earnings of unconsolidated affiliates, net of tax	0.4	0.3	'
Investments in unconsolidated affiliates	4.3	'	4.1
Proceeds from Dividends Received	$0.20	'	'

Employee_ShareBased_Incentive_2

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
In Millions, except Share data, unless otherwise specified			Performance Share	Performance Share	White Mountains	White Mountains	White Mountains
			W T M Incentive Plan [Member]	W T M Incentive Plan [Member]	Performance Share	Performance Share	Performance Cycle 2011 to 2013
			Performance Cycle 2011 to 2013	Performance cycle 2010-2012	W T M Incentive Plan [Member]	W T M Incentive Plan [Member]	Performance Share
Target Performance Share Activity	'	'	'	'	'	'	'
Beginning of period (in shares)	'	'	'	'	119,220	119,357	'
Shares paid or expired (in shares)	'	'	'	'	-37,130	-47,310	'
New grants (in shares)	'	'	'	'	45,660	46,920	'
Assumed forfeitures and cancellations	'	'	'	'	-213	9	'
Ending of period (in shares)	'	'	'	'	127,537	118,976	'
Accrued Expense	'	'	'	'	'	'	'
Accrued expense at the beginning of period	$124.90	$218.30	'	'	$60.20	$29.40	'
Payments and deferrals	'	'	'	'	-26.7	-11	'
Assumed forfeitures and cancellations	'	'	'	'	0.4	0.1	'
Expense recognized	'	'	'	'	8.4	10.1	'
Accrued expense at the end of the period	124.9	218.3	'	'	42.3	28.6	'
Vesting Period	'	'	'	'	'3 years	'	'
Percentage of target payout - minimum actual	'	'	88.00%	33.00%	'	'	'
Percentage of target payout - maximum actual	'	'	131.50%	98.00%	'	'	'
Vested (in shares)	'	'	'	'	'	'	3,570
Additional compensation cost that would be recognized if all outstanding performance shares vested	'	'	'	'	$54.10	'	'

Employee_ShareBased_Incentive_3

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Share-based compensation arrangement by share-based payment award	'	'	'	'
Total Accrued Expense	$124.90	$218.30	'	'
White Mountains | Performance Share | W T M Incentive Plan [Member]	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	130,807	'	'	'
Accrued Expense	43.4	'	'	'
Target Performance Shares Outstanding, Assumed forfeitures	-3,270	'	'	'
Accrued Expense, Assumed forfeitures	-1.1	'	'	'
Total Target Performance Shares Outstanding	127,537	119,220	118,976	119,357
Total Accrued Expense	42.3	60.2	28.6	29.4
White Mountains | Performance Cycle 2012 To 2014 | Performance Share | W T M Incentive Plan [Member]	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	37,977	'	'	'
Accrued Expense	25	'	'	'
White Mountains | Performance Cycle 2014 to 2016 | Performance Share | W T M Incentive Plan [Member]	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	45,660	'	'	'
Accrued Expense	2.5	'	'	'
White Mountains | Performance Cycle 2013 to 2015 | Performance Share | W T M Incentive Plan [Member]	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	47,170	'	'	'
Accrued Expense	$15.90	'	'	'

Employee_ShareBased_Incentive_4

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (White Mountains, W T M Incentive Plan [Member], USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Restricted Share Cliff Vesting in January 2017 [Member]	'	'
Target Restricted Share Activity	'	'
Issued (in shares)	20,400	'
Restricted Share Cliff Vesting in January 2016 [Member]	'	'
Target Restricted Share Activity	'	'
Issued (in shares)	25,470	'
Restricted Stock	'	'
Target Restricted Share Activity	'	'
Beginning of period (in shares)	94,130	69,910
Issued (in shares)	20,400	25,470
Vested (in shares)	-33,205	0
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	81,325	95,380
Unamortized grant date fair value	'	'
Unamortized grant date fair value, beginning balance	$17	$16.80
Issued	11.8	14.3
Assumed forfeitures and cancellations	0	0
Expense recognized	-4	-2.9
Unamortized grant date fair value, ending balance	$24.80	$28.20

Employee_ShareBased_Incentive_5

Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
In Millions, except Share data, unless otherwise specified			OneBeacon	OneBeacon	OneBeacon	Performance Cycle 2011 to 2013
			Performance Share	One Beacon Long Term Incentive Plan	One Beacon Long Term Incentive Plan	OneBeacon
			One Beacon Long Term Incentive Plan	Performance Share	Performance Share	One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award	'	'	'	'	'	'
Vesting Period	'	'	'3 years	'	'	'
Target Performance Shares Outstanding	'	'	'	'	'	'
Beginning of period (in shares)	'	'	'	493,421	563,190	'
Shares paid or expired (in shares)	'	'	'	-142,138	-238,658	'
New grants (in shares)	'	'	'	165,800	179,000	'
Assumed forfeitures and cancellations	'	'	'	-4,145	-13,665	'
Ending of period (in shares)	'	'	'	512,938	489,867	'
Expense Amortized	'	'	'	'	'	'
Accrued expense at the beginning of period	$124.90	$218.30	'	$4	$1.20	'
Payments and deferrals	'	'	'	-1	0	'
Assumed forfeitures and cancellations	'	'	'	0	0	'
Expense recognized	'	'	'	0.6	0.4	'
Accrued expense at the end of the period	124.9	218.3	'	3.6	1.6	'
Percentage of target payout - minimum actual	'	'	'	'	'	37.10%
Additional compensation cost that would be recognized if all outstanding performance shares vested	'	'	'	$4.70	'	'

Employee_ShareBased_Incentive_6

Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Share-based compensation arrangement by share-based payment award	'	'	'	'
Accrued incentive compensation	$124.90	$218.30	'	'
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	526,090	'	'	'
Incentive Compensation Payable before Forfeitures	3.7	'	'	'
Target Performance Shares Outstanding, Assumed forfeitures	-13,152	'	'	'
Accrued Expense, Assumed forfeitures	-0.1	'	'	'
Total Target Performance Shares Outstanding	512,938	493,421	489,867	563,190
Accrued incentive compensation	3.6	4	1.6	1.2
Performance Cycle 2012 To 2014 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	181,290	'	'	'
Incentive Compensation Payable before Forfeitures	2.3	'	'	'
Performance Cycle 2014 to 2016 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	165,800	'	'	'
Incentive Compensation Payable before Forfeitures	0.2	'	'	'
Performance Cycle 2013 to 2015 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan	'	'	'	'
Share-based compensation arrangement by share-based payment award	'	'	'	'
Target Performance Shares Outstanding	179,000	'	'	'
Incentive Compensation Payable before Forfeitures	$1.20	'	'	'

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		3 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 01, 2012	Mar. 31, 2014	Mar. 31, 2013	Feb. 28, 2012	31-May-11	25-May-11	Mar. 31, 2014	Mar. 31, 2013	31-May-11	31-May-11
	Restricted Stock	Restricted Stock	Restricted Stock	Restricted Stock	Restricted Stock	Restricted Stock	Performance Share	Performance Share	Performance Share	Performance Cycle 2011 to 2013
				Installment	Chief Executive Officer	Chief Executive Officer			Chief Executive Officer	Performance Share
						Installment				Chief Executive Officer
Target Restricted Share Activity	'	'	'	'	'	'	'	'	'	'
Beginning of period (in shares)	'	915,000	927,000	'	'	'	493,421	563,190	'	'
Issued (in shares)	300,000	0	0	'	630,000	'	'	'	'	'
Vested (in shares)	'	300,000	9,000	'	'	'	'	'	'	'
Assumed forfeitures and cancellations	'	-2,500	0	'	'	'	-4,145	-13,665	'	'
Ending of period (in shares)	'	612,500	918,000	'	'	'	512,938	489,867	'	'
Unamortized grant date fair value	'	'	'	'	'	'	'	'	'	'
Unamortized grant date fair value, beginning balance	'	$6.50	$9.60	'	'	'	'	'	'	'
Issued	'	0	0	'	'	'	'	'	'	'
Assumed forfeitures and cancellations	'	0	0	'	'	'	0	0	'	'
Expense recognized	'	-0.8	-0.8	'	'	'	-0.6	-0.4	'	'
Unamortized grant date fair value, ending balance	'	$5.70	$8.80	'	'	'	$4.70	'	'	'
Number of Installments in which the Shares Vest	'	'	'	2	'	4	'	'	'	'
Annual reduction in performance shares	'	'	'	'	'	'	'	'	'	35,000
Years Performance Shares will be Forfeited	'	'	'	'	'	'	'	'	'5 years	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years	'	'	'	'	35,000	'	'	'	'	'

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions	'	'
Debt	$677.60	$676.40
Fair Value | 2012 OBH Senior Notes	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions	'	'
Debt	275.1	269.8
Fair Value | SIG Senior Notes	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions	'	'
Debt	438	438.1
Fair Value | SIG Preference Shares [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions	'	'
Debt	262.5	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	399.6	399.6
Book Value | SIG Preference Shares [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions	'	'
Debt	$250	$250

Discontinued_Operations_Detail

Discontinued Operations (Details) (USD $)

3 Months Ended

3 Months Ended

3 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Mar. 31, 2014

Mar. 31, 2013

Dec. 31, 2013

Mar. 31, 2014

31-May-01

31-May-01

Mar. 31, 2014

Dec. 31, 2013

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

N I C O Cover [Member]

G R C Cover [Member]

GRC and NICO Cover [Member]

GRC and NICO Cover [Member]

OneBeacon Runoff

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Contract

OneBeacon Runoff

OneBeacon Runoff

OneBeacon Runoff

OneBeacon Runoff

Discontinued Operations [Line Items}

'

'

'

'

'

'

'

'

'

'

'

'

Loss and loss adjustment expense reserves

$3,048.40

$3,100.60

$3,079.30

$3,168.90

$1,717.30

'

$1,793.10

'

'

'

'

'

Net (loss) income from discontinued operations, net of tax

-0.5

0.5

'

'

-0.5

0.5

'

'

'

'

'

'

Number of contracts seller caused one beacon to purchase prior to acquisition

'

'

'

'

'

'

'

2

'

'

'

'

Reinsurance recoverable cover under reinsurance contract

'

'

'

'

'

'

'

'

2,500

570

'

'

Adverse loss reserve development on losses occurring in years 2000 and prior

'

'

'

'

'

'

'

'

'

400

'

'

Loss reserves ceded at acquisition

'

'

'

'

'

'

'

'

'

170

'

'

Reinsurance recoverables

'

'

'

'

'

'

'

'

'

'

1,188.80

1,243.70

A M Best Rating System, Number of Debt Ratings

'

'

'

'

'

'

'

16

'

'

'

'

Reinsurance recoverable on paid losses

'

'

'

'

'

'

'

9.5

'

'

'

'

Reinsurance recoverables, gross of purchase accounting adjustments

'

'

'

'

'

'

'

1,673.60

'

'

'

'

Purchase accounting adjustments

'

'

'

'

'

'

'

$133.60

'

'

'

'

Discontinued_Operations_Net_As

Discontinued Operations (Net Assets Held for Sale) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets held for sale	'	'	'	'
Fixed maturity investments, at fair value	$4,988.70	$5,030.50	'	'
Reinsurance recoverable on unpaid losses	425.2	428.1	'	'
Reinsurance recoverable on paid losses	15.2	25.4	'	'
Insurance and reinsurance premiums receivable	734.8	518.9	'	'
Other assets	537.7	458.1	'	'
Total assets	12,416.40	12,144.30	'	'
Liabilities held for sale	'	'	'	'
Loss and loss adjustment expense reserves	3,048.40	3,079.30	3,100.60	3,168.90
Unearned insurance premiums	1,116.60	901.4	'	'
Ceded reinsurance payable	157.9	71.9	'	'
Other liabilities	343.3	362.9	'	'
Total liabilities	7,888.50	7,746.90	'	'
Segment, Discontinued Operations	'	'	'	'
Assets held for sale	'	'	'	'
Fixed maturity investments, at fair value	222.2	236.3	'	'
Reinsurance recoverable on unpaid losses	1,540	1,604.70	'	'
Reinsurance recoverable on paid losses	9.5	10.7	'	'
Insurance and reinsurance premiums receivable	12.8	9.1	'	'
Deferred tax asset	3.2	3.3	'	'
Other assets	15.4	16	'	'
Total assets	1,803.10	1,880.10	'	'
Liabilities held for sale	'	'	'	'
Loss and loss adjustment expense reserves	1,717.30	1,793.10	'	'
Unearned insurance premiums	0.2	0.2	'	'
Ceded reinsurance payable	12.1	12.3	'	'
Other liabilities	73.5	74.5	'	'
Total liabilities	1,803.10	1,880.10	'	'
Net assets held for sale	$0	$0	'	'

Income_Loss_from_Discontinued_

Income (Loss) from Discontinued Operations (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Revenues	'	'
Earned insurance premiums	$493.60	$495.40
Other revenue	-3.4	28.3
Total revenues	578.3	627.4
Expenses	'	'
Loss and loss adjustment expenses	229.3	244.3
Insurance and reinsurance acquisition expenses	95.1	98.2
Other underwriting expenses	81.4	79.1
Total expenses	466.2	475.6
Net (loss) income from discontinued operations	-0.5	0.5
Segment, Discontinued Operations	'	'
Revenues	'	'
Earned insurance premiums	-0.1	0.9
Other revenue	0	0
Total revenues	-0.1	0.9
Expenses	'	'
Loss and loss adjustment expenses	0	0
Insurance and reinsurance acquisition expenses	0	0.1
Other underwriting expenses	0.7	0.1
Total expenses	0.7	0.2
Pre-tax (loss) income	-0.8	0.7
Income tax benefit (expense)	0.3	-0.2
Net (loss) income from discontinued operations	($0.50)	$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, 2014	Mar. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000
Basic and diluted (loss) earnings per share numerators (in millions):	'	'
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	($1.10)	($1.50)
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	95.6	120.4
Basic (loss) earnings per share denominators (in thousands):	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Basic (loss) earnings per share denominator	6,099,500	6,194,500
Diluted (loss) earnings per share denominator (in thousands):	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Average outstanding dilutive options to acquire common shares	0	0
Diluted (loss) earnings per share denominator	6,099,500	6,194,500
Segment, Discontinued Operations	'	'
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000
Basic and diluted (loss) earnings per share numerators (in millions):	'	'
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	-0.5	0.5
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	0	0
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	($0.50)	$0.50
Basic (loss) earnings per share denominators (in thousands):	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Basic (loss) earnings per share denominator	6,099,500	6,194,500
Diluted (loss) earnings per share denominator (in thousands):	'	'
Total average common shares outstanding during the period	6,169,900	6,273,500
Average unvested restricted common shares	-70,400	-79,000
Average outstanding dilutive options to acquire common shares	0	0
Diluted (loss) earnings per share denominator	6,099,500	6,194,500
Basic and diluted (loss) earnings per share (in dollars):	($0.08)	$0.07

Contingencies_Details

Contingencies (Details) (USD $)	3 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Jun. 30, 2007
	Esurance	Tribune Company Litigation [Member]
		Pending Litigation [Member]
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%	'
Proceeds from Sale, Maturity and Collection of Investments	'	$39