Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] OneBeacon OneBeacon [Member] Includes the information about OneBeacon segment consisting of OneBeacon Insurance Group, Ltd., an exempted Bermuda limited liability company that owns a family of U.S. based property and casualty insurance companies. Sirius Group Sirius Group [Member] Sirius Group [Member] HG Global HG Global [Member] HG Global [Member] BAM BAM [Member] BAM [Member] Other Noncontrolling Interest [Member] Other Noncontrolling Interest [Member] Other Noncontrolling Interest [Member] Statement Statement [Line Items] Assets Assets [Abstract] Fixed maturity investments, at fair value Trading Securities, Debt Short-term investments, at amortized cost (which approximates fair value) Short-term Investments Common equity securities, at fair value Trading Securities, Equity Convertible fixed maturity investments, at fair value Trading Securities, Convertible Fixed Maturity Investments Represents the fair value of fixed maturity investments that are convertible to equity securities at a future date. Other long-term investments Other Long-term Investments Total investments Investments Cash (restricted: $27.8 and $56.1) Cash Including Restricted Cash This item includes unrestricted cash available for day-to-day operating needs and cash that is restricted as to withdrawal or usage. Reinsurance recoverable on unpaid losses Reinsurance Recoverables on Unpaid Losses Reinsurance recoverable on paid losses Reinsurance Recoverables on Paid Losses Insurance and reinsurance premiums receivable Premiums Receivable, Net Funds held by ceding companies Funds Held under Reinsurance Agreements, Asset Investments in unconsolidated affiliates Investments in Unconsolidated Affiliates. Total investments in (A) companies that are accounted for using the equity method and (B) warrants to acquire additional shares of an unconsolidated affiliate that are carried at fair value. Deferred acquisition costs Deferred Policy Acquisition Costs Deferred tax asset Deferred Tax Assets, Net Ceded unearned insurance and reinsurance premiums Prepaid Reinsurance Premiums Accrued investment income Accrued Investment Income Receivable Accounts receivable on unsettled investment sales Receivables from Brokers-Dealers and Clearing Organizations Other assets Other Assets Assets held for sale Assets Held-for-sale, at Carrying Value Total assets Assets Liabilities Liabilities [Abstract] Loss and loss adjustment expense reserves Liability for Claims and Claims Adjustment Expense Unearned insurance and reinsurance premiums Unearned Premiums Variable annuity benefit guarantee Liability for Future Policy Benefits, Individual and Group Annuities and Supplementary Contracts Debt Long-term Debt Deferred tax liability Deferred Tax Liabilities, Net Accrued incentive compensation Incentive Compensation Payable Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. Ceded reinsurance payable Reinsurance Payable Funds held under insurance and reinsurance contracts Funds Held under Reinsurance Agreements, Liability Accounts payable on unsettled investment purchases Payables to Broker-Dealers and Clearing Organizations Other liabilities Other Liabilities Liabilities held for sale Liabilities Held-for-sale Liabilities related to a disposition that have been classified as Held for Sale. Total liabilities Liabilities Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] White Mountains’s common shareholders’ equity Stockholders' Equity Attributable to Parent [Abstract] White Mountains’ common shares Common Stock, Value, Issued Paid-in surplus Additional Paid in Capital, Common Stock Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income (loss), after tax: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Equity in net unrealized gains (losses) from investments in Symetra common shares Accumulated Other Comprehensive Income (Loss) Cumulative Equity in Unrealized Gains (Losses) from Unconsolidated Affiliates Accumulated equity in unrealized gains or losses, net of tax, from investments in unconsolidated affiliates at the end of an accounting period. Net unrealized foreign currency translation gains Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Pension liability and other Accumulated Other Comprehensive Income (Loss), Net of Tax, Other Includes the net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. Also includes accumulated change, net of tax, in accumulated gains and losses from derivative instruments (interest rate swaps) designated and qualifying as the effective portion of cash flow hedges. Total White Mountains’s common shareholders’ equity Stockholders' Equity Attributable to Parent Non-controlling interests Stockholders Equity Attributable to Noncontrolling Interest [Abstract] Total non-controlling interests Stockholders' Equity Attributable to Noncontrolling Interest Total equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total liabilities and equity Liabilities and Equity Discontinued Operations and Disposal Groups [Abstract] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Disposal Group Name [Axis] Disposal Group Name [Axis] Disposal Groups, Including Discontinued Operations, Name [Domain] Disposal Groups, Including Discontinued Operations, Name [Domain] Esurance Esurance [Member] Represents the Esurance business. Statement, Operating Activities Segment [Axis] Operating Activities [Axis] Segment, Operating Activities [Domain] Operating Activities [Domain] Segment, Discontinued Operations Discontinued Operations [Member] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Revenues Revenues [Abstract] Earned insurance premiums Premiums Earned, Net Other revenue Other Income Total revenues Revenues Expenses Benefits, Losses and Expenses [Abstract] Loss and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Insurance and reinsurance acquisition expenses Deferred Policy Acquisition Cost, Amortization Expense Other underwriting expenses Other Underwriting Expense Total expenses Benefits, Losses and Expenses Pre-tax (loss) income Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax Income tax benefit Discontinued Operation, Tax Effect of Discontinued Operation Net (loss) income from discontinued operations Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Gain (Loss) on Disposal of Discontinued Operation Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Net income from discontinued operations Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Significant Transactions [Abstract] Significant Transactions [Abstract] Significant Transactions Significant Transactions [Text Block] Description of various transactions that are significant to White Mountains financial position, results of operations and/or cash flows. Fair Value Disclosures [Abstract] Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Equity Method Investments and Joint Ventures [Abstract] Investments in and Advances to Affiliates [Table] Investments in and Advances to Affiliates [Table] Extraordinary Item [Axis] Extraordinary Item [Axis] Extraordinary Item [Domain] Extraordinary Item [Domain] Amortization of common share basis difference Amortization of common share basis difference [Member] Amortization of common share basis difference [Member] impairment of net unrealized gains (losses) impairment of net unrealized gains (losses) [Member] impairment of net unrealized gains (losses) [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] equity in earnings of unconsolidated affiliates equity in earnings of unconsolidated affiliates [Member] equity in earnings of unconsolidated affiliates [Member] Net unrealized gains (losses) of unconsolidated affiliates Net unrealized gains (losses) of unconsolidated affiliates [Member] Net unrealized gains (losses) of unconsolidated affiliates. Investments in and Advances to Affiliates Categorization [Axis] Investments in and Advances to Affiliates Categorization [Axis] Investments in and Advances to Affiliates Categorization [Domain] Investments in and Advances to Affiliates Categorization [Domain] Symetra Symetra [Member] Represents the investment in Symetra by the entity. White Mountains White Mountains [Member] White Mountains [Member] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Common shareholders' equity. Common Stock [Member] Warrants Warrant [Member] Investments in unconsolidated affiliates Investments in and Advances to Affiliates [Line Items] Changes in carrying value of investment in Symetra Investments in Unconsolidated Affiliates [Roll forward] Carrying value at the beginning of the period Equity in earnings Income (Loss) from Equity Method Investments Equity in net unrealized losses from Symetra's fixed maturity portfolio Equity Method Investment, Investee Unrealized Gains (Losses) Portfolio The net unrealized gains (losses) from Symetra's fixed maturity portfolio. Dividends received Proceeds from Equity Method Investment, Dividends or Distributions Increase (decrease) in value of warrants Gain (Loss) on Warrants The increase (decrease) in the fair value of warrants included in other revenue. Carrying value at the end of the period Income Tax Expense (Benefit) Income Tax Expense (Benefit) Aggregate value of White Mountains common shares of Symetra at quoted market price Investments in Unconsolidated Affiliates at Quoted Market Price Represents the aggregate value of common shares at quoted market price. Share Price Share Price Exercise of warrants Exercise of warrants The value of the exercise of warrants. Investments, Debt and Equity Securities [Abstract] Schedule of Trading Securities and Other Trading Assets Trading Securities [Table] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Trading Securities and Assets [Domain] Major Types of Debt and Equity Securities [Domain] Fixed maturity investments. Fixed Maturities [Member] Short-term investments Short-term Investments [Member] Common equity securities Equity Securities [Member] Convertible fixed maturity investments Convertible Debt Securities [Member] Other long-term investments Other Long-term Investments [Member] Forward Contracts Forward Contracts [Member] Schedule of Trading Securities and Other Trading Assets [Line Items] Schedule of Trading Securities and Other Trading Assets [Line Items] Trading Securities, Change in Unrealized Holding Gain (Loss) Trading Securities, Change in Unrealized Holding Gain (Loss) Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments Represents the income taxes attributable to unrealized gains (losses) on trading securities. Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax Represents the net unrealized investment gains (losses) after income taxes. Foreign Currency Transaction Gain (Loss), Unrealized Foreign Currency Transaction Gain (Loss), Unrealized Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax The tax expense (benefit) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax The after-tax gain (loss) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax The after-tax aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. Derivative Instruments and Hedging Activities Disclosure [Abstract] Offsetting Liabilities [Table] Offsetting Liabilities [Table] Transaction Type [Axis] Transaction Type [Axis] Transaction [Domain] Transaction [Domain] Over The Counter Over the Counter [Member] Exchange Traded Exchange Traded [Member] Derivative Instrument Risk [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Fixed income/interest rate Interest Rate Contract [Member] Foreign exchange Foreign Exchange Contract [Member] Equity Equity Contract [Member] White Mountains Life Re White Mountains Life Re [Member] Represents the entity's variable annuity reinsurance business, which is in run-off. Offsetting Liabilities [Line Items] Offsetting Liabilities [Line Items] Derivative Assets Derivative Asset Derivative Liabilities Derivative Liability Net amounts recognized in Other assets Derivative, Fair Value, Amount Offset Against Collateral, Net Accounting Policies [Abstract] Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Table] Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Table] Interest Rate Cap Interest Rate Cap [Member] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] BAM Management BAM Management [Member] BAM Management [Member] Common shares and paid-in surplus Common Stock Including Additional Paid in Capital [Member] Non-controlling interest Noncontrolling Interest [Member] Preferred Stock Preferred Stock [Member] Common Stock Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] OneBeacon Ltd. One Beacon Insurance Group Ltd [Member] Represents OneBeacon Insurance Group, Ltd. ("OneBeacon Ltd."). Build America Mutual Build America Mutual [Member] Build America Mutual [Member] Star & Shield Insurance Exchange Star & Shield Insurance Exchange [Member] Star & Shield Insurance Exchange [Member] Sirius International Group Ltd Sirius Group Ltd [Member] Represents Sirius Group, Ltd., an intermediate holding company of Sirius. White Mountains Consolidated Limited Partnership White Mountains Consolidated Limited Partnership [Member] Represents the White Mountains consolidated limited partnerships. A W G Dewar Inc A W G Dewar Inc [Member] Represents A.W.G.Dewar Inc, a subsidiary of entity's subsidiary One Beacon. WOBI WOBI [Member] WOBI [Member] QuoteLab QuoteLab [Member] QuoteLab Basis of Presentation Basis of Presentation [Line Items] Ownership interest (as a percent) Equity Method Investment, Ownership Percentage Surplus Notes Surplus Notes Partners Capital Contribution Partners' Capital Account, Contributions Percentage of par value of policy reinsured Percentage of par value of policy reinsured Percentage of par value of policy reinsured. Noncontrolling equity percentage Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Fair Value of Net Assets Acquired Fair Value of Net Assets Acquired Fair value of net assets allocated to noncontrolling interests. Income (Loss) from Continuing Operations before Income Taxes, Domestic Income (Loss) from Continuing Operations before Income Taxes, Domestic Non-cumulative perpetual preference shares issued Sale of Stock, Number of Shares Issued in Transaction Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share) Preferred Stock, Liquidation Preference Per Share Proceeds from non-cumulative perpetual preference shares Proceeds from Preference Shares Proceeds from the issuance of preferred shares. Non-cumulative perpetual preference shares issuance cost and commission Payments of Stock Issuance Costs Preferred Stock, Dividend Rate, Percentage Preferred Stock, Dividend Rate, Percentage basis points basis points basis points years of interest rate cap years of interest rate cap Years of interest rate cap Derivative, Cap Interest Rate Derivative, Cap Interest Rate Noncontrolling interest - other Noncontrolling Interest in Limited Partnerships Non-controlling equity interest Term of LIBOR loan Term of LIBOR loan Term of LIBOR loan Summary of Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Private Equity Funds Remaining Lock-up Period [Axis] Private Equity Funds Remaining Lock-up Period [Axis] The remaining period for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Fair Value, Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period [Domain] Fair Value, Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period [Domain] The identification of the remaining period for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. 1-3 years Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period 1 To 3 Years [Member] The period of from 1 to 3 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. 3-5 years Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period 3 To 5 Years [Member] The period of from 3 to 5 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. 5-10 years Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period 5 To 10 Years [Member] The period of from 5 to 10 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Greater than 10 years Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period Over 10 Years [Member] The period greater than 10 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Private equity funds Private Equity Funds [Member] Other long-term investments Trading Securities, Other Basic and diluted earnings per share numerators (in millions): Earnings Per Share, Basic and Diluted [Abstract] Income (Loss) Allocated to Participating Unvested Restricted Common Shares Income (Loss) Allocated to Participating Unvested Restricted Common Shares Represents the portion of income (loss) allocated to participating unvested restricted common shares. Net income attributable to White Mountains' common shareholders, net of restricted common share amounts Net Income (Loss) Available to Common Stockholders, Basic Basic earnings per share denominators (in thousands): Weighted Average Number of Shares Outstanding, Basic [Abstract] Total average common shares outstanding during the period Weighted Average Number of Shares Issued, Basic Average unvested restricted common shares Weighted Average Number of Shares, Restricted Stock Basic earnings per share denominator Weighted Average Number of Shares Outstanding, Basic Diluted earnings per share denominator (in thousands): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Average outstanding dilutive options to acquire common shares Weighted Average Number Diluted Shares Outstanding Adjustment Diluted earnings per share denominator Weighted Average Number of Shares Outstanding, Diluted Basic and diluted earnings per share (in dollars): Earnings Per Share, Basic and Diluted Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Discontinued Operations Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Statement of Cash Flows [Abstract] Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Cash flows from operations: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Charges (credits) to reconcile net income to net cash (used for) provided from operations: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Net realized and unrealized investment gains Gain (Loss) on Investments Deferred income tax expense Deferred Income Tax Expense (Benefit) Net income from discontinued operations Gain on sale of subsidiary - Citation and Essentia Gain (Loss) on Disposition of Stock in Subsidiary Excess of fair value of acquired net assets over cost - Ashmere Business Combination, Bargain Purchase, Gain Recognized, Amount Undistributed equity in earnings from unconsolidated affiliates, net of tax Other operating items: Increase (Decrease) in Operating Capital [Abstract] Net change in loss and loss adjustment expense reserves Increase (Decrease) in Loss and Loss Adjustment Expense Reserve Net change in reinsurance recoverable on paid and unpaid losses Increase (Decrease) in Reinsurance Recoverable Net change in unearned insurance and reinsurance premiums Increase (Decrease) in Unearned Premiums Net change in variable annuity benefit guarantee liabilities Increase (Decrease) in Variable Annuity Benefit Guarantee Liabilities Represents the increase (decrease) during the period in variable annuity benefit guarantee liabilities. Net change in variable annuity benefit guarantee derivative instruments Increase (Decrease) in Variable Annuity Benefit Derivative Instruments Increase (Decrease) in Variable Annuity Benefit Derivative Instruments Net change in deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Net change in funds held by ceding companies Increase (Decrease) in Funds Held under Reinsurance Agreements Net change in ceded unearned premiums Increase (Decrease) in Prepaid Reinsurance Premiums Net change in funds held under reinsurance treaties Increase (Decrease) in Client Funds Held Net change in insurance and reinsurance premiums receivable Increase (Decrease) in Premiums Receivable Net change in ceded reinsurance payable Increase (Decrease) in Reinsurance Payables Net change in restricted cash Increase (Decrease) in Restricted Cash for Operating Activities Net change in other assets and liabilities, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Net cash used for operations - continuing operations Net Cash Provided by (Used in) Operating Activities, Continuing Operations Net cash used for operations - discontinued operations Cash Provided by (Used in) Operating Activities, Discontinued Operations Net cash used for operations Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Net change in short-term investments Payments for (Proceeds from) Short-term Investments Sales of fixed maturity and convertible fixed maturity investments Proceeds from Sale of Trading Fixed Maturity and Convertible Fixed Maturity Investments The cash inflow associated with the sale of fixed maturity and convertible fixed maturity investments classified as trading. Maturities, calls and paydowns of fixed maturity and convertible fixed maturity investments Proceeds from Maturities of Trading Fixed Maturity and Convertible Fixed Maturity Investments The cash inflow associated with maturities of fixed maturity and convertible fixed maturity investments classified as trading. Sales of common equity securities Proceeds from Sale of Trading Common Equity Securities The cash inflow associated with the sale of equity securities classified as trading. Distributions and redemptions of other long-term investments Proceeds from Sale and Maturity of Other Investments Sales of consolidated and unconsolidated affiliates, net of cash sold Proceeds from Divestiture of Interest in Subsidiaries and Affiliates Funding of operational cash flows for discontinued operations Payments to Acquire Additional Interest in Subsidiaries Purchases of other long-term investments Payments to Acquire Other Investments Purchases of common equity securities Payments to Acquire Trading Common Equity Securities The cash outflow to acquire common equity securities classified as trading. Purchases of fixed maturity and convertible fixed maturity investments Payments to Acquire Trading Fixed Maturity and Convertible Fixed Maturity Investments The cash outflow to acquire fixed maturity and convertible fixed maturity investments classified as trading. Purchases of consolidated and unconsolidated affiliates, net of cash acquired Payments to Acquire Businesses and Interest in Affiliates Net change in unsettled investment purchases and sales Increase (Decrease) in Unsettled Accounts Receivable and Accounts Payable The net change during the reporting period in accounts receivable on unsettled investment sales and accounts payable on unsettled investment purchases. Net acquisitions of property and equipment Net Acquisitions of Property and Equipment Net cash (outflow) inflow from purchases, sales and disposals of software and property, plant and equipment. Net cash provided from investing activities - continuing operations Net Cash Provided by (Used in) Investing Activities, Continuing Operations Net cash provided from investing activities - discontinued operations Cash Provided by (Used in) Investing Activities, Discontinued Operations Net cash provided from investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Draw down of revolving line of credit Proceeds from Lines of Credit Repayment of revolving line of credit Repayments of Debt Payments on capital lease obligation Repayments of Long-term Capital Lease Obligations Cash dividends paid to the Company’s common shareholders Payments of Ordinary Dividends, Common Stock Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders Payments of Ordinary Dividends, Noncontrolling Interest Cash dividends paid on SIG Preference Shares Payments of Dividends for Preference Shares to Minority Interest Payments of Dividends for Preference Shares to Minority Interest Common shares repurchased Payments for Repurchase of Common Stock Capital contributions from non-controlling interest of consolidated LPs Proceeds from Partnership Contribution Redemptions paid to non-controlling interest of consolidated LPs Proceeds from (Payments for) Other Financing Activities Collateral provided by interest rate cap counterparties Proceeds from Derivative Instrument, Financing Activities Capital contributions from BAM members Members capital contribution Members capital contribution to equity, gross of tax. Net cash used for financing activities - continuing operations Net Cash Provided by (Used in) Financing Activities, Continuing Operations Net cash (used for) provided from financing activities - discontinued operations Cash Provided by (Used in) Financing Activities, Discontinued Operations Net cash used for financing activities Net Cash Provided by (Used in) Financing Activities Effect of exchange rate changes on cash Effect of Exchange Rate on Cash and Cash Equivalents Net change in cash during the period Cash and Cash Equivalents, Period Increase (Decrease) Cash balances at beginning of period (excludes restricted cash balances of $56.1 and $249.8) Cash and Cash Equivalents, at Carrying Value Cash balances at end of period (excludes restricted cash balances of $27.8 and $61.2) Supplemental cash flows information: Supplemental Cash Flow Information [Abstract] Interest paid Interest Paid Net income tax refund (payment) to national governments Income Taxes Paid, Net Insurance Loss Reserves [Abstract] Loss and loss adjustment expense ("LAE") reserve activities Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Investments in and Advances to Affiliates Investments in and Advances to Affiliates [Table Text Block] Schedule of Changes in Investments in Unconsolidated Affiliates Schedule of Changes in Investments in Unconsolidated Affiliates [Table Text Block] Disclosure of changes in the investment in unconsolidated affiliates. Investments in Unconsolidated Affiliates Equity Method Investments and Joint Ventures Disclosure [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Employee Share-Based Incentive Compensation Plans Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Fair Value Inputs, Liabilities, Quantitative Information [Table] Fair Value Inputs, Liabilities, Quantitative Information [Table] Volatility Year [Axis] Volatility Year [Axis] Volatility Year [Axis] Volatility Year [Domain] Volatility Year [Domain] [Domain] for Volatility Year [Axis] Year One Year One [Member] Year One [Member] Year Two Year Two [Member] Year Two [Member] Fair Value by Liability Class [Axis] Liability Class [Axis] Fair Value, Liabilities, Measured on Recurring Basis, Unobservable Input Reconciliation, by Liability Class [Domain] Fair Value by Liability Class [Domain] Discounted cash flows Discounted Cash Flow Valuation Technique [Member] Represents the discounted cash flow technique used to measure fair value. Variable annuity benefit guarantee liabilities Variable Annuity [Member] Foreign Exchange Option [Member] Foreign Exchange Option [Member] Counterparty valuations Counterparty valuations [Member] Counterparty valuations [Member] Foreign Exchange Options Foreign Exchange Options [Member] Foreign Exchange Options [Member] Equity Index Options Equity Index Options [Member] Equity Index Options [Member] Valuation Technique [Axis] Valuation Technique [Axis] Valuation Technique [Domain] Valuation Technique [Domain] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Weighted Average Weighted Average [Member] Quantitative information for Level 3 Fair Value Measurements Liabilities Fair Value Inputs, Liabilities, Quantitative Information [Line Items] Fair value Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Surrenders (as a percent) Fair Value Inputs, Surrenders Represents the surrenders, used as an input to measure fair value. Mortality (as a percent) Fair Value Inputs, Mortality Represents the mortality, used as an input to measure fair value. Foreign exchange volatilities (as a percent) Fair Value Inputs, Foreign Exchange Volatilities Represents the foreign exchange volatilities, used as an input to measure fair value. Expected equity dividends (as a percent) Fair Value Inputs, Significant unobservable inputs for equity index options Significant unobservable inputs for equity index options Index volatilities (as a percent) Fair Value Inputs, Index Volatilities Represents the index volatilities, used as an input to measure fair value. Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair value measured on a recurring basis Fair Value, Measurements, Recurring [Member] U.S. Government and agency obligations US Treasury and Government [Member] Debt securities issued by corporations Corporate Debt Securities [Member] Municipal obligations Municipal Bonds [Member] Mortgage-backed and asset-backed securities Asset-backed Securities [Member] Foreign government, agency and provincial obligations Foreign Government Excluding United States Debt Securities [Member] This category includes information about debt securities (such as bonds) issued by a national (excluding the United States), local, or municipal government not within the nation of domicile of the reporting entity. Investment Sector [Axis] Investment Sector [Axis] Investment Sector [Domain] Investment Sector [Domain] Consumer Consumer [Member] Represents the consumer sector. Industrial Industrial [Member] Represents the industrial sector. Financials Financials [Member] Represents the financial sector. Communications Communications [Member] Represents the communications sector. Basic materials Basic Materials [Member] Represents the basic materials sector. Energy Energy [Member] Represents the energy sector. Technology Technology [Member] Represents the technology sector. Utilities Utilities [Member] Represents the utilities sector. Other: Other Sector [Member] Represents all other sectors that are not otherwise covered by the taxonomy. Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Fair Value Estimate of Fair Value Measurement [Member] Level 1 Inputs Fair Value, Inputs, Level 1 [Member] Level 2 Inputs Fair Value, Inputs, Level 2 [Member] Level 3 Inputs Fair Value, Inputs, Level 3 [Member] Fair value investments Investments, Fair Value Disclosure Carrying value of investment accounted for using the equity method Equity Method Investments, Fair Value Disclosure Realized and unrealized gains (losses) Derivative, Gain (Loss) on Derivative, Net Fixed maturity investments reclassified to assets held for sale Trading Securities Debt Reclassified to Assets Held-for-sale Represents the fair value of trading debt securities re-classified to assets held for sale. Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Table] Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Notional amount Derivative, Notional Amount Derivative, fair value, net Derivative, Fair Value, Net Carrying Value Uncollaterailized Balance of Forward Currency Derivatives Uncollaterailized Balance of Forward Currency Derivatives Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Tax Assesessment Year [Axis] Tax Assessment Year [Axis] [Axis] Tax Assessment Year [Axis] [Axis] Tax Assessment Year [Axis] [Domain] Tax Assessment Year [Axis] [Domain] [Domain] for Tax Assessment Year [Axis] [Axis] Tax Assessment Year 2007 to 2009 Tax Assessment Year 2007 to 2009 [Member] Tax Assessment Year 2007 to 2009 [Member] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Internal Revenue Service (IRS) Internal Revenue Service (IRS) [Member] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Effective tax rates (as a percent) Effective Income Tax Rate Reconciliation, Percent U.S. statutory rate (as a percent) Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Total tax assessment Total Tax from Assessment Total Tax from Assessment Tax assessments related to disallowed deductions Tax Assessments Related to Timing of Deductibility Tax Assessments Related to Timing of Deductibility. Credit Rating, Standard & Poor's [Axis] Credit Rating, Standard & Poor's [Axis] External Credit Rating, Standard & Poor's [Domain] External Credit Rating, Standard & Poor's [Domain] AAA Standard & Poor's, AAA Rating [Member] AA Standard & Poor's, AA Rating [Member] A Standard & Poor's, A Rating [Member] BBB Standard & Poor's, BBB Rating [Member] BB Standard & Poor's, BB Rating [Member] Other Other Standard Poors Rating [Member] Remainder of ratings not separately disclosed. Government National Mortgage Association Certificates and Obligations (GNMA) Government National Mortgage Association Certificates and Obligations (GNMA) [Member] Federal National Mortgage Association Certificates and Obligations (FNMA) Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] Mortgage-backed Securities, Issued by US Government Sponsored Enterprises Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] Residential Mortgage Backed Securities Residential Mortgage Backed Securities [Member] Commercial Mortgage Backed Securities Commercial Mortgage Backed Securities [Member] Mortgage-backed Securities, Issued by Private Enterprises Mortgage-backed Securities, Issued by Private Enterprises [Member] Collateralized Mortgage Backed Securities Collateralized Mortgage Backed Securities [Member] Collateralized Credit Card Securities Collateralized Credit Card Securities [Member] Collateralized Auto Loans Collateralized Auto Loans [Member] Collateralized Securities, Other Collateralized Securities, Other [Member] Asset-backed Securities, Securitized Loans and Receivables Asset-backed Securities, Securitized Loans and Receivables [Member] Fair Value Inputs Assets Quantitative Information [Table] Fair Value Inputs, Assets, Quantitative Information [Table] Credit Rating Source [Axis] Credit Rating Source [Axis] Credit Rating Source [Domain] Credit Rating Source [Domain] NR Standard Poors NR Rating [Member] NR credit rating as defined by the external credit rating agency, Standard & Poor's. AA Standard & Poor's, AA+ Rating [Member] Fair Value by Asset Class [Axis] Asset Class [Axis] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class [Domain] Asset Class [Domain] Non-Agency Residential Mortgage Resecuritization Non-Agency Residential Mortgage [Member] Non-Agency Residential Mortgage [Member] Non-Agency Commercial Mortgage Backed Securities [Member] [Member] Non-Agency Commercial Mortgage Backed Securities [Member] [Member] Non-Agency Commercial Mortgage Backed Securities [Member] [Member] Convertible Preferred Stock [Member] Convertible Preferred Stock [Member] Broker Indication [Member] Broker Indication [Member] Broker Indication [Member] Discount Yield [Member] Discount Yield [Member] Discount Yield [Member] Broker Pricing Broker Pricing [Member] Broker Pricing [Member] Multiple of GAAP book value Multiple of GAAP book value [Member] Multiple of GAAP book value [Member] Average Share Price [Member] Average Share Price [Member] Average Share Price [Member] Multiple of EBITA [Member] Multiple of EBITDA [Member] Multiple of EBITDA Recent transaction [Member] Recent transaction [Member] Recent transaction [Member] Quantitative information for Level 3 Fair Value Measurements Assets Fair Value Inputs, Assets, Quantitative Information [Line Items] Unobservable Inputs Unobservable Inputs Unobservable Inputs Fair value Assets Assets, Fair Value Disclosure Average Share Price Used As Unobservable Input Average Share Price Used As Unobservable Input Average Share Price Used As Unobservable Input Fair value input, swap spread rate Fair value input, swap spread rate Represents the swap spread input used to measure fair value. Discount Yield (as a percent) Fair Value Inputs, Discount Rate Book value multiple (as a percent) Book Value Multiple to Calculate Fair Value Book Value Multiple to Calculate Fair Value Share Price of Recent Transaction Used As Unobservable Input Share Price of Recent Transaction Used As Unobservable Input Share Price of Recent Transaction Used As Unobservable Input Class of Financing Receivable [Axis] Class of Financing Receivable [Axis] Class of Financing Receivable [Domain] Class of Financing Receivable [Domain] Prime Residential, Prime, Financing Receivable [Member] Non-prime Residential Nonprime Financing Receivable [Member] Class of financing receivables related to nonprime residential financing receivables. Sub-prime Residential, Subprime, Financing Receivable [Member] Investment by Tranche Levels [Axis] Investment by Tranche Levels [Axis] Information by tranche level of non-agency RMBS securities and non-agency CMBS securities. Investment Tranche Levels [Domain] Investment Tranche Levels [Domain] Tranche level indicators related to investments. Super Senior Super Senior [Member] This element represent the tranche level of Super Senior, which were rated AAA and were senior to other AAA bonds. Senior Senior [Member] This element represent the tranche level of Senior, which were rated AAA and were senior to non-AAA bonds. Subordinate Subordinate [Member] This element represent the tranche level of Subordinate, which were not rated AAA and were junior to AAA bonds. Funds Held under Reinsurance Agreements [Member] Funds Held under Reinsurance Agreements [Member] Funds held by the ceding insurer on behalf of the reinsurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of obligations there under, which represent an asset for the reinsurer. Investment income Investment Income, Net [Abstract] Total investment income Interest and Dividend Income, Operating Less third-party investment expenses Investment Income, Investment Expense Net investment income, pre-tax Net Investment Income Derivative [Table] Derivative [Table] Standard & Poor's, A Rating Standard & Poor's, A- Rating Standard & Poor's, A- Rating [Member] Standard & Poor's, A plus Standard & Poor's, A+ Rating [Member] Standard & Poor's, AA- Rating Standard & Poor's, AA- Rating [Member] Derivative Counterparty Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Barclays Barclays [Member] Represents Barclays, a counterparty of the entity. Deutsche Bank Deutsche Bank [Member] Deutsche Bank [Member] Goldman Sachs Goldman Sachs [Member] Represents Goldman Sachs, a counterparty of the entity. HSBC HSBC Bank plc [Member] HSBC Bank plc [Member] JP Morgan J P Morgan Chase [Member] Represents JP Morgan Chase, which is a counterparty of the entity. Royal Bank of Canada Royal Bank of Canada [Member] Royal Bank of Canada [Member] Derivative [Line Items] Derivative [Line Items] S and P Credit Ratings S and P Credit Ratings Represents the S&P creditworthiness heirarchy ratings used by the entity. Trading Securities, Realized Gain (Loss) Trading Securities, Realized Gain (Loss) Trading securities, tax on realized holding gain (loss) on investments Trading securities, Tax on Realized Holding Gain (Loss) on Investments Trading securities, Tax on Realized Holding Gain (Loss) on Investments Trading securities realized holding gain (loss) on investments after tax Trading Securities Realized Holding Gain (Loss) on Investments after Tax Trading Securities Realized Holding Gain (Loss) on Investments after Tax Foreign Currency Transaction Gain (Loss), Realized Foreign Currency Transaction Gain (Loss), Realized Trading securities, realized foreign currency transaction gain (loss) t Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax Trading securities, realized foreign currency gain (loss) net of tax Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax Trading Securities, Realized Gain (Loss), Total Trading Securities, Realized Gain (Loss), Total Trading Securities, Realized Gain (Loss), Total Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax Reinsurance Disclosures [Abstract] Reinsurance Retention Policy [Table] Reinsurance Retention Policy [Table] Sirius Sirius Group Segment [Member] Represents the segment of the entity that offers reinsurance capacity for property, accident & health, aviation and space, trade credit, agriculture, casualty and certain other exposures on a worldwide basis. Reinsurance Retention Policy [Line Items] Reinsurance Retention Policy [Line Items] Summary of performance share activity Schedule of Nonvested Performance Share Activity [Table Text Block] Tabular disclosure of the changes in nonvested performance shares outstanding and the amount accrued for performance shares. Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan Schedule of Performance Shares Outstanding and Accrued Expense by Performance Cycle under Incentive Plan [Table Text Block] Tabular disclosure of performance shares outstanding and the amount accrued by performance cycle, under the entity's Incentive Plan. Summary of restricted share activity Schedule of Nonvested Restricted Stock Activity [Table Text Block] Tabular disclosure of the changes in nonvested restricted shares and the unamortized grant date fair value. Summary of performance share activity for OneBeacon performance shares granted under OneBeacon Incentive Plan Schedule of Nonvested One Beacon Performance Share Activity [Table Text Block] A disclosure of the changes in nonvested OneBeacon performance shares outstanding and the amount accrued for OneBeacon performance shares. Summary of OneBeacon performance shares outstanding and accrued expense for OneBeacon performance shares awarded under the OneBeacon Incentive Plan Schedule of One Beacon Performance Shares Outstanding and Accrued Expense by Performance Cycle under One Beacon Incentive Plan [Table Text Block] Tabular disclosure of OneBeacon performance shares outstanding and the amount accrued by performance cycle, under the OneBeacon Incentive Plan. Summary of OneBeacon restricted shares activity Schedule of Nonvested One Beacon Restricted Share Activity [Table Text Block] Tabular disclosure of the changes in nonvested OneBeaco restricted shares and the unamortized grant date fair value. Hamer, LLC Hamer, LLC [Member] Hamer, LLC [Member] Symetra Common Shares excluding unrealized gains (losses) Shares excluding unrealized gains (losses) Shares excluding unrealized gains (losses) from fixed maturities. Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. Total investments in unconsolidated affiliates Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet And Additional Disclosures Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet and Additional Disclosures [Table Text Block] Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Earnings Per Share [Abstract] Earnings Per Share Earnings Per Share [Text Block] Guarantees [Abstract] Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] Combination One Combination One [Member] Combination One [Member] Combination Two Combination Two [Member] Combination Two [Member] Fixed interest rate Fixed interest rate [Member] Fixed interest rate [Member] Variable Rate Variable Rate [Domain] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Surplus Note Surplus Note [Member] Surplus Note [Member] HG Re HG Re [Member] HG Re [Member] Preferred Stock Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Municipal bond contract Municipal bond contract Municipal bond contracts issued and outstanding. Insured Financial Obligations, Remaining Weighted Average Contract Period Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period Municipal bond outstanding principal Municipal bond outstanding principal Municipal bond outstanding principal Municipal bond outstanding interest Municipal bond outstanding interest Municipal bond outstanding interest Municipal bond debt service outstanding Municipal bond debt service outstanding Municipal bond debt service outstanding Statutory Accounting Practices, Statutory Capital and Surplus, Balance Statutory Accounting Practices, Statutory Capital and Surplus, Balance Interest income (expense), surplus notes Interest income (expense), surplus notes Interest income (expense) earned from surplus notes. Number of years of change in interest rate Number of years of change in interest rate Number of years of change in interest rate Financial Guarantee Insurance Contracts, Unearned Premium Revenue Financial Guarantee Insurance Contracts, Unearned Premium Revenue Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Year of U.S. Treasury Rate Year of U.S. Treasury Rate Year of U.S. Treasury Rate Nordea Bank Finland Nordea Bank Finland [Member] Nordea Bank Finland Cash Cash and Cash Equivalents [Member] Derivative collateral right to reclaim Derivative Collateral Right to Reclaim The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions. Derivative, Cost of Hedge Derivative, Cost of Hedge Realized and unrealized gains (losses) Sales/settlements Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements Sales and settlements that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis. Income Statement [Abstract] Scenario Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Revenues: Earned insurance and reinsurance premiums Net investment income Net realized and unrealized investment gains (losses) Total revenues Expenses: General and administrative expenses General and Administrative Expense Interest expense Interest Expense, Debt Pre-tax income from continuing operations Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax (expense) benefit Net income from continuing operations Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest Net income from discontinued operations, net of tax Income before equity in earnings of unconsolidated affiliates Income (Loss) before Equity Earnings of Unconsolidated Affiliates Extraordinary Items and Cumulative Effect of Change in Accounting Principle Income (loss) before equity in earnings of unconsolidated affiliates, extraordinary items and cumulative effect of changes in accounting principle. Equity in earnings of unconsolidated affiliates, net of tax Net (income) loss attributable to non-controlling interests Net Income (Loss) Attributable to Noncontrolling Interest Net income attributable to White Mountains’s common shareholders Other comprehensive income, net of tax: Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Change in equity in net unrealized gains (losses) from investments in Symetra common shares, net of tax Other Comprehensive Income (Loss) Equity in Unrealized Gains Losses from Investments in Unconsolidated Affiliates Other Comprehensive Income (Loss) Equity in Unrealized Gains Losses from Investments in Unconsolidated Affiliates Change in foreign currency translation, pension liability and other Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income (loss) attributable to White Mountains’s common shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Basic income per share Earnings Per Share, Basic [Abstract] Continuing operations Income (Loss) from Continuing Operations, Per Basic Share Discontinued operations Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share Total consolidated operations Earnings Per Share, Basic Diluted income per share Earnings Per Share, Diluted [Abstract] Continuing operations Income (Loss) from Continuing Operations, Per Diluted Share Discontinued operations Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share Total consolidated operations Earnings Per Share, Diluted Dividends declared per White Mountains’s common share Common Stock, Dividends, Per Share, Declared Variable Annuity Reinsurance and Weather Derivatives [Table] Variable Annuity Reinsurance and Weather Derivatives [Table] Represents disclosure pertaining to variable annuity reinsurance and weather derivative contracts of the entity. Variable Annuity Derivative [Member] Derivative [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Variable Annuity Reinsurance [Line Items] Variable Annuity Reinsurance [Line Items] -- None. No documentation exists for this element. -- Changes in derivative instruments Derivative Instruments [Roll Forward] Beginning balance Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Realized and unrealized gains (losses) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers Sales/settlements Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements Sales or settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. Ending balance Beginning balance Purchases Fair Value Derivatives Measured on Recurring Basis, Asset Purchases Purchases that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis. Transfers in Derivative Transfers In (Out), Net Derivative Transfers In (Out), Net Ending balance Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses Gain (Loss) on Liability for Death Benefit Net increase or decrease in the fair value of the variable annuity death benefit liabilities included in earnings. Computation of earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Restricted cash balances, beginning of period Restricted Cash and Cash Equivalents, Current Restricted cash balances, end of period Loss and Loss Adjustment Expense Reserves Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Subsequent Event [Abstract] Subsequent Event [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Performance Cycle 2011 to 2013 Range of Performance Cycle from 2011 to 2013 [Member] This element represents period of performance cycle from 2011 to 2013. Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Performance Share Performance Shares [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] W T M Incentive Plan W T M Incentive Plan [Member] Represents WTM incentive plan of the entity. Share-based Compensation Arrangement by Performance Shares Award, Performance Cycle [Axis] Share-based Compensation Arrangement by Performance Shares Award, Performance Cycle [Axis] Disclosure of minimum performance period for right to receive performance share from the entity. Share-based Compensation Arrangement by Performance Shares Award, Performance Cycle [Domain] Share-based Compensation Arrangement by Performance Shares Award, Performance Cycle [Domain] This element represents minimum performance period for right to receive performance share from the entity. Performance cycle 2010-2012 Range of Performance Cycle from 2010 to 2012 [Member] This element represents period of performance cycle from 2010 to 2012. Share-based compensation arrangement by share-based payment award Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Target Performance Share Activity Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Shares paid or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Target Shares Paid in Period The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. New grants (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Assumed forfeitures and cancellations Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Ending of period (in shares) Accrued Expense Accrued Expense [Roll Forward] Accrued expense at the beginning of period Payments and deferrals Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Amounts Paid in Period The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. Assumed forfeitures and cancellations Employee Service Share-based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not yet Recognized The total reduction to unamortized compensation cost for performance shares forfeited during the period. Expense recognized Allocated Share-based Compensation Expense Accrued expense at the end of the period Vesting Period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Percentage of target payout - minimum actual Share-based Compensation Arrangement by Share-based Payment Award, Payout for Minimum Growth Target Percentage Represents the lpercentage of target achieved, low end of range. Percentage of target payout - maximum actual Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage Represents the lpercentage of target achieved, high end of range. Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Additional compensation cost that would be recognized if all outstanding performance shares vested Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Commitments and Contingencies Disclosure [Abstract] Contingencies Contingencies Disclosure [Text Block] Net Realized and Unrealized Gain (Loss) on Trading Securities Net Realized and Unrealized Gain (Loss) on Trading Securities Tax on net realized and unrealized gains (loss), trading securities Tax on net realized and unrealized gains (loss), trading securities Tax on net realized and unrealized gains (loss), trading securities Realized and unrealized gain Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax Business Combinations [Abstract] Other Significant Transactions [Table] Other Significant Transactions [Table] Represents the disclosure of other significant transactions. Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] General Board Authorization [Member] General Board Authorization [Member] General Board Authorization [Member] Shares Repurchased For Employee Benefit Plan [Member] Shares Repurchased For Employee Benefit Plan [Member] Shares Repurchased For Employee Benefit Plan Business Combination, Separately Recognized Transactions [Axis] Business Combination, Separately Recognized Transactions [Axis] Business Combination, Separately Recognized Transactions [Domain] Business Combination, Separately Recognized Transactions [Domain] Other Significant Transactions by Unique Description [Axis] Other Significant Transactions by Unique Description [Axis] Represents the information pertaining to other significant transactions. Other Significant Transaction Name [Domain] Other Significant Transaction Name [Domain] Represents other significant transactions. Significant Transactions Significant Transactions [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Additional authorized repurchase of common shares Stock Repurchase Program, Number of Additional Shares Authorized to be Repurchased The number of additional shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan. Stock Repurchased and Retired During Period, Shares Stock Repurchased and Retired During Period, Shares Value of common shares repurchased and retired during the period Stock Repurchased and Retired During Period, Value Average price per share Treasury Stock Acquired, Average Cost Per Share Subsequent Event [Table] Subsequent Event [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] durchblicker durchblicker [Member] durchblicker Currency Types [Axis] Currency [Axis] All Currencies [Domain] All Currencies [Domain] Euro Member Countries, Euro Euro Member Countries, Euro United States of America, Dollars United States of America, Dollars Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Noncontrolling Interest, Ownership Percentage by Parent Noncontrolling Interest, Ownership Percentage by Parent Payments to Acquire Interest in Subsidiaries and Affiliates Payments to Acquire Interest in Subsidiaries and Affiliates Pre-tax operating results of WM Life Re Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest [Abstract] Fees, included in other revenue Insurance Commissions and Fees Change in fair value of variable annuity liability, included in other revenue Gain (Loss) on Variable Annuity Liabilities From Changes Associated with Projected Surrender Assumptions Net increase or decrease in the fair value of the variable annuity liabilities included in earnings, from losses associated with chrages in projected surrender assumptions Foreign exchange, included in other revenue Foreign Exchange Derivatives Gain (Loss) This item represents the change in fair value of derivatives due to changes in foreign currency exchange rates, which is included in other revenues. Other investment income and gains (losses) Other Investment Income and Gain (Loss) on Investments This item represents other investment income and gains (losses) included in earnings for the period. Total revenues Death benefit claims paid, included in general and administrative expenses Liability for Future Policy Benefits, Benefits Paid General and administrative expenses OneBeacon Runoff OneBeacon Runoff [Member] OneBeacon Runoff [Member] Foreign government, agency and provincial obligations Foreign Government Debt Securities [Member] Fixed Income Investments Fixed Income Investments [Member] Cost or amortized cost Trading Securities, Cost Trading Securities, Unrealized Holding Gain Trading Securities, Unrealized Holding Gain Trading Securities, Unrealized Holding Loss Trading Securities, Unrealized Holding Loss Net foreign currency gains (losses) Trading Securities, Net Unrealized Foreign Currency Gain (Loss) The net amount of unrealized gains (losses) from foreign currency transaction adjustments related to trading securities. Total fixed maturity investments including assets held for sale Trading Securities Debt Including Assets Held-for-sale Represents the fair value of trading debt securities including assets classified as held for sale. Fixed maturity investments reclassified to assets held for sale One Beacon Long Term Incentive Plan Long Term Incentive Plan [Member] The share-based incentive plans of the entity's subsidiary Target Performance Shares Outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Shares paid or expired (in shares) Assumed forfeitures and cancellations Expense Amortized Security Issuance Year [Axis] Security Issuance Year [Axis] Represents the year of issuance for securities held by the entity. Security Issuance Year [Domain] Security Issuance Year [Domain] Represents the respective year in which the security was issued. Securities Issued in 2004 Securities Issued in 2004 [Member] Securities Issued in 2004 Securities Issued in 2005 Securities Issued in 2005 [Member] Represents securities issued in 2012. Securities Issued In 2006 Securities Issued In 2006 [Member] Represents securities issued in 2006. Securities Issued In 2007 Securities Issued In 2007 [Member] Represents securities issued in 2007. Securities Issued In 2008 Securities Issued In 2008 [Member] Securities Issued In 2008 [Member] Securities Issued In 2009 Securities Issued In 2009 [Member] Represents securities issued in 2009. Securities Issued In 2010 Securities Issued In 2010 [Member] Represents securities issued in 2010. Securities Issued In 2011 Securities Issued In 2011 [Member] Represents securities issued in 2011. Securities Issued in 2012 Securities Issued in 2012 [Member] Represents securities issued in 2012. Securities Issued in 2013 Securities Issued in 2013 [Member] Securities Issued in 2013 [Member] Securities Issued in 2014 Securities Issued in 2014 [Member] Securities Issued in 2014 [Member] Segment Reporting [Abstract] Financial information for White Mountains' segments Schedule of Segment Reporting Information, by Segment [Table Text Block] Convertible Debt Securities Unrealized gains (losses) on investments Unrealized Gains (Losses) on Investments The aggregate increase (decrease) in the market value of unsold investments whose gains (losses) are included in earnings. Loss and Loss Adjustment, Expense Reserves [Table] Loss and Loss Adjustment, Expense Reserves [Table] Schedule of loss and loss adjustment expense ("LAE") reserve activities. Loss and Loss Adjustment, Expense Reserves [Line Items] Loss and Loss Adjustment, Expense Reserves [Line Items] Net (favorable) and unfavorable loss reserve development Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years Accretion of fair value adjustment to net loss and LAE reserves Accreted Fair Value Adjustment to Loss and Loss Adjustment, Expense Reserves Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. Adjustment to loss and loss adjustment expense reserves, before tax Unaccreted Fair Value, Adjustment to Loss and Loss Adjustment Expense Reserves, Before Tax Represents the amount of pre-tax unaccreted fair value adjustment to loss and LAE reserves related to purchase accounting. Loss Contingencies [Table] Loss Contingencies [Table] Litigation Case [Axis] Litigation Case [Axis] Litigation Case [Domain] Litigation Case [Domain] Tribune Company Litigation Tribune Company Litigation [Member] Represents the litigation filed by Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company against certain of entity subsidiaries in their capacity as former shareholders of tribune. Litigation Status [Axis] Litigation Status [Axis] Litigation Status [Domain] Litigation Status [Domain] Pending Litigation Pending Litigation [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Warranty agreement period Warranty Agreement Period Warranty Agreement Period Non-solicitation agreement period Non-Soliciation Agreement Period Non-Soliciation Agreement Period Percentage of fluctuation in loss reserves of transferred subsidiaries retained Percentage of Fluctuation in Loss Reserves of Transferred Subsidiaries Retained Percentage of favorable or unfavorable loss reserve development in the Transferred Subsidiaries retained by the entity. Proceeds from Sale, Maturity and Collection of Investments Proceeds From Tribune Common Stock Proceeds from Tribune common stock tendered in connecttion with Tribune's leveraged buyout. Statement Cash (in dollars) Common shares, par value per share (in dollars per share) Common Stock, Par or Stated Value Per Share Common shares, authorized shares Common Stock, Shares Authorized Common shares, issued shares Common Stock, Shares, Issued Common shares, outstanding shares Common Stock, Shares, Outstanding Summary of the fair value and carrying value of financial instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Performance Cycle 2012 To 2014 Range Of Performance Cycle From 2012 To 2014 [Member] This element represents period of performance cycle from 2012 to 2014. Performance Cycle 2014 to 2016 Range Of Performance Cycle From 2014-2016 [Member] Range Of Performance Cycle From 2014-2016 [Member] Performance Cycle 2013 to 2015 Range Of Performance Cycle From 2013 to 2015 [Member] This element represents period of performance cycle from 2013-2015. Target Performance Shares Outstanding Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures This element represents the number of non-vested performance shares before forfeiture assumptions. Incentive Compensation Payable before Forfeitures Incentive Compensation Payable before Forfeitures Carrying value as of the balance sheet date for performance shares before forfeitures. Target Performance Shares Outstanding, Assumed forfeitures Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options, Assumed Forfeitures This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. Accrued Expense, Assumed forfeitures Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Assumed Forfeitures Amount This element represents the reduction to accrued expense for assumed forfeitures of performance shares. Total Target Performance Shares Outstanding Debt Security [Axis] Debt Security [Axis] Major Types of Debt Securities [Domain] Major Types of Debt Securities [Domain] Partnership investments [Member] Partnership investments [Member] Partnership investments Hedge and private equity funds included in other long-term investments Hedge Funds and Private Equity Funds [Member] Investments in hedge funds and private equity funds. Limited Liability Company [Member] Limited Liability Company [Member] Other Other Debt Obligations [Member] Other Investments Other Investments Maximum percentage of price difference provided by pricing services Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers Represents the maximum percentage of price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. Maximum price difference provided by pricing services Maximum Differences Between Prices Provided by Pricing Services Considered Outliers Represents the maximum price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. Fair value, investment-related liabilities Financial Instruments Sold, Not yet Purchased, at Fair Value Monthly Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Monthly [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment monthly. Quarterly Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Quarterly [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment quarterly. Semi-annual Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Semi Annual [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment semi-annually. Annual Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Annual [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment annually. Fair Value, Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency [Axis] Fair Value, Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency [Axis] The frequency with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated. Advance Notice Period [Domain] Fair Value Investments Entities That Calculate Net Asset Value Per Share Investment Redemption Frequency [Domain] The type of frequency with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated. 30-59 days notice Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period30 To59 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver 30 to 59 days notice before it can redeem its investment. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver 60 to 89 days notice before it can redeem its investment. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver 90 to 119 days notice before it can redeem its investment. 120+ days notice Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period Over120 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver over 120 days notice before it can redeem its investment. Hedge funds Hedge Funds [Member] Distributions from inactive hedge funds Distributions from Inactive Hedge Funds Distributions pending from hedge funds that are no longer active and are in process of disposing their underlying investments. Redemption requests Fair Value, Investments Entities that Calculate Net Asset Value Per Share Redemption Request The amount of redemption requests submitted for investments for hedge funds. Reinsurance Policy, Type [Axis] Reinsurance Policy, Type [Axis] Reinsurance Policy [Domain] Reinsurance Policy [Domain] Property Catastrophe Reinsurance [Member] Property Catastrophe Reinsurance [Member] Property Catastrophe Reinsurance [Member] Property Per Risk Reinsurance Program [Member] Property Per Risk Reinsurance Program [Member] Represents the Property-per-risk reinsurance program which provides coverage for certain risks to reduce large loss volatility. Insurance Product Line [Axis] Insurance Product Line [Axis] Insurance Product Line [Domain] Insurance Product Line [Domain] Property [Member] Property [Member] Property [Member] Crop insurance Crop insurance [Member] Crop insurance Crop-hail insurance Crop-hail insurance [Member] Crop-hail insurance [Member] Reinsurance Retention Policy, Retained Percentage Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage Reinsurance Retention Policy, Amount Retained Reinsurance Retention Policy, Amount Retained Reinsurance Retention Policy, Reinsured Risk, Percentage Reinsurance Retention Policy, Reinsured Risk, Percentage Reinsurance Retention Policy, Excess Retention, Amount Reinsured Reinsurance Retention Policy, Excess Retention, Amount Reinsured Hedge Funds, Equity Long (Short) [Member] Hedge Funds, Equity Long (Short) [Member] Hedge Funds Credit and Distressed Hedge Funds Credit and Distressed [Member] Investments in registered hedge funds that invest in credit & distressed securities, taking both long and short positions. Hedge Funds Diversified Strategies Hedge Funds Diversified Strategies [Member] Investments in registered hedge funds that invest in diversified securities, taking both long and short positions. Hedge Funds Equity R E I T Hedge Funds Equity R E I T [Member] Investments in registered hedge funds that invest in REIT (Real Estate Investment Trust) equity securities, taking both long and short positions. Hedge Funds Equity Activist Hedge Funds Equity Activist [Member] Investments in registered hedge funds that invest in equity securities companies and use the ownership to be active in the management, taking both long and short positions. Hedge Funds Bank Loan Hedge Funds Bank Loan [Member] Investments in registered hedge funds that invest in bank loans, taking both long and short positions. Private Equity Multi Sector Private Equity Multi Sector [Member] Investments in private equity funds of more than one industry. Private Equity Energy Infrastructure and Services Private Equity Energy Infrastructure and Services [Member] Investments in private equity funds related to energy infrastructure and services. Real Estate Funds Real Estate Funds [Member] Private Equity Secondaries Private Equity Secondaries [Member] Investments in private equity funds of the private equity secondary market. Private Equity Funds Multi Sector Europe Private Equity Funds Multi Sector Europe [Member] Investments in private equity funds that invest in multiple sectors in the European region. Private Equity Distressed Residential Real Estate [Member] Private Equity Distressed Residential Real Estate [Member] Investments in private equity funds of distressed residential real estate. Private Equity Funds Manufacturing and Industrial Private Equity Funds Manufacturing and Industrial [Member] Investments in private equity funds of the manufacturing and industrial sectors. Private Equity Healthcare Private Equity Healthcare [Member] Represents investments in private equity funds of the healthcare industry. Private Equity Funds Multi Sector Asia Private Equity Funds Multi Sector Asia [Member] Investments in private equity funds that invest in multiple sectors in the Asian region. Private Equity Insurance Private Equity Insurance [Member] Represents investments in private equity funds of the insurance industry. Aerospace Defense Government Aerospace Defense Government [Member] Aerospace Defense Government [Member] Venture Capital Funds Venture Capital Funds [Member] Unfunded Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Israel, New Shekels Israel, New Shekels Common Class A Common Class A [Member] DavidShield [Member] DavidShield [Member] DavidShield WM Solutions [Member] WM Solutions [Member] WM Solutions Star & Shield LLC [Member] Star & Shield LLC [Member] Star & Shield LLC [Member] Citation [Member] Citation [Member] Citation [Member] Essentia Essentia [Member] Essentia [Member] Ashmere American Fuji [Member] American Fuji [Member] Other Commitment Other Commitment Payments to Acquire Businesses and Interest in Affiliates Premiums Written, Percentage of Total Premiums Written, Percentage of Total Premiums Written, Percentage of Total Number of lines of business Number of lines of business Number of lines of business Purchase of Surplus Notes Purchase of Surplus Notes Purchase of Surplus Notes Percentage of gross profit Percentage of gross profit Percentage of gross profit Assets Intangible Assets, Net (Including Goodwill) Intangible Assets, Net (Including Goodwill) Intangible Assets, Net Intangible Assets, Net (Excluding Goodwill) Liabilities Net Income (Loss) Attributable to Noncontrolling Interest Gain on sale of subsidiary Gain on sale of subsidiary, after tax Gain (Loss) on Disposition of Stock in Subsidiary, After Tax Gain (Loss) on Disposition of Stock in Subsidiary, After Tax Weather derivatives weather derivatives [Member] weather derivatives [Member] Energy [Axis] Energy [Axis] Energy [Domain] Energy [Domain] Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivative, Name [Domain] Position [Axis] Position [Axis] Position [Domain] Position [Domain] Debt Disclosure [Abstract] Schedule of debt outstanding Schedule of Long-term Debt Instruments [Table Text Block] Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments Derivative Instruments Gain (Loss) Recognized in Income, Net and Carrying Value [Abstract] Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Investments in unconsolidated affiliates Subsidiary or Equity Method Investee [Line Items] Shares of Symetra Financial Corporation owned (in shares) Investments in and Advances to Affiliates, Balance, Shares Investments In Unconsolidated Affiliates On Conversion Of Warrants Investments In Unconsolidated Affiliates On Conversion Of Warrants Represents additional shares of an unconsolidated affiliate to be obtained on conversion of warrants. Common shares received from exercised warrants Number of common shares received from exercised warrants Number of common shares received from exercised warrants Unamortized basis difference Unamortized basis difference Unamortized basis difference. Fair value of investment Term of amortization (in years) Amortization Period of Basis Difference Represents the period over which basis difference amortized. Weight Average Years Remaining Weight Average Years Remaining Weight Average Years Remaining After tax amortization through equity in earnings Investments in Unconsolidated Affiliates Amortization Through Equity in Earnings After Tax Represents recognized after tax amortization through equity in earnings of unconsolidated affiliates. After tax amortization through equity in net unrealized losses Investments in Unconsolidated Affiliates Amortization Through Equity in Net Unrealized Losses After Tax Represents recognized after tax amortization through equity in net unrealized losses from investments in unconsolidated affiliates. Equity Method Investment, Difference Between Carrying Amount and Underlying Equity Equity Method Investment, Difference Between Carrying Amount and Underlying Equity Pre-tax unamortized basis difference Unamortized Basis Difference Between Fair Value and Carrying Amount of Investment Before Tax Represents the unamortized dollar spread between the fair value and the carrying amount at the date the impariment was recognized. Expected life (in years) Fair Value Assumptions, Expected Term Class of warrant or right fair value assumptions per strike price Class of Warrant or Right, Exercise Price of Warrants or Rights Fair Value Assumptions, Risk Free Interest Rate Fair Value Assumptions, Risk Free Interest Rate Fair Value Assumptions, Expected Volatility Rate Fair Value Assumptions, Expected Volatility Rate Standard & Poor's, BBB+ Rating [Member] Standard & Poor's, BBB+ Rating [Member] Bank of America [Member] Bank of America [Member] Represents Bank of America, which is a counterparty of the entity. Royal Bank of Scotland Royal Bank of Scotland [Member] Represents Royal Bank of Scotland, which is a counterparty of the entity. Nomura [Member] Nomura [Member] Represents Nomura, which is a counterparty of the entity. Citigroup - OTC Citigroup OTC [Member] Citigroup OTC [Member] Citigroup Exchange Traded [Member] Citigroup Exchange Traded [Member] Citigroup Exchange Traded Credit risk on derivative instruments Derivative, Credit Risk Related Contingent Features [Abstract] Excess collateral provided - Financial Instruments Collertal provided to counterparty in cash Derivative, Collateral, Right to Reclaim Cash Collateral provided to counterparty in financial instruments Derivative, Collateral, Right to Reclaim Securities Net amount of exposure after effect of collateral Derivative Asset, Fair Value, Amount Offset Against Collateral Excess collateral provided to counterparty in cash Excess collateral provided to counterparty in cash Excess collateral provided to counterparty in cash. Derivative, Collateral, Right to Reclaim Securities Excess collateral providedto counterparty in financial instruments Excess collateral provided to counterparty in financial instruments Counter-party collateral held by WMLife Re - Cash Derivative, Collateral, Obligation to Return Cash Net amount of exposure to counter-party Net Exposure On Fair Value Of Over The Counter Derivative Instruments The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral. Segment Informartion Segment Reporting Disclosure [Text Block] Other long-term investments. Other Long-term Investments Excluding Equity Method Investments [Member] Investments, excluding equity method investments, which are not otherwise included in another category or item that the entity has the ability and intent to hold for more than a year from the date of the balance sheet. Statement of Stockholders' Equity [Abstract] Retained earnings Retained Earnings [Member] AOCI, after tax Accumulated Other Comprehensive Income (Loss) [Member] Total Parent [Member] Total Equity Total Equity includes Noncontrolling Interest [Member] Total Equity includes Noncontrolling Interest Increase (Decrease) in Shareholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Balances Net change in unrealized (losses) gains from investments in unconsolidated affiliates Net change in foreign currency translation Other Comprehensive Income (Loss), Net of Tax Other Comprehensive Income (Loss), Net of Tax Total comprehensive income (loss) Dividends declared on common shares Dividends, Common Stock, Cash Dividends to non-controlling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Repurchases and retirements of common shares Issuances of common shares Stock Issued During Period, Value, New Issues Net contributions from non-controlling interests Change in Noncontrolling Ownership Change in noncontrolling interest ownership, excluding net income to noncontrolling interests, dividends paid to noncontrolling interests, noncontrolling interest in comprehensive gains (losses), cumulative effect adjustments relating to changes in accounting principles related to noncontrolling interests and noncontrolling interest in the amortization of restricted share and option awards. Amortization of restricted share awards Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Balances Assets held for sale Liabilities held for sale Unearned insurance premiums Net assets held for sale Assets, Net Municipal Bond Guarantee Insurance Financial Guarantee Insurance Contracts [Text Block] Collateral cash and investments Collateral Cash and Investments Represents cash and short-term investments held and posted as collateral to reinsurance counterparties. Third Party Reinsurance Reinsurance [Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value, Disclosure Item Amounts [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Book Value Reported Value Measurement [Member] 2012 OBH Senior Notes 2012 OBH Senior Notes [Member] 2012 OBH Senior Notes [Member] SIG Senior Notes Sirius International Senior Notes [Member] Represents the Sirius International Senior Notes. SIG Preference Shares SIG Preference Shares [Member] SIG Preference Shares [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fixed rate CMBS Commercial Mortgage Backed Securities Fixed Rate [Member] Securities collateralized by commercial real estate mortgage loans which have a fixed interest rate. Floating rate CMBS Commercial Mortgage Backed Securities Floating Rate [Member] Securities collateralized by commercial real estate mortgage loans which have a floating interest rate. Loss and Loss Adjustment Expense Reserve Loss and Loss Adjustment, Expense Reserves [Roll Forward] Gross beginning balance Less beginning reinsurance recoverable on unpaid losses Reinsurance Recoverables, Incurred but Not Reported Claims Net loss and LAE reserves Liability for Unpaid Claims and Claims Adjustment Expense, Net Loss and LAE reserves acquired Liability for Unpaid Claims and Claims Adjustment Expense, Acquired Claims Current Year Liability for Unpaid Claims and Claims Adjustment Expense, Acquired Claims Current Year Loss and LAE reserves consolidated — SSIE Liability for Unpaid Claims and Claims Adjustment Expense, Consolidated Claims Current Year Liability for Unpaid Claims and Claims Adjustment Expense, Consolidated Claims Current Year Loss and LAE incurred relating to current year losses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year Total incurred losses and LAE Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) Foreign currency translation adjustment to net loss and LAE reserves Foreign Currency Translation, Adjustment to Loss and Loss Adjustment, Expense Reserves Represents the amount of foreign currency translation adjustment to loss and LAE reserves. Current year losses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior year losses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total loss and LAE payments Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Net ending balance Plus ending reinsurance recoverable on unpaid losses Gross ending balance Average basis points of subordination Average Basis Points of Subordination Basis points of subordination on average. Percentage of underlying loans reported as non-performing Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans Reinsurance [Table] Reinsurance [Table] Schedule reflecting the name of each reinsurer from whom amounts are due and other details like percentage of amounts recoverable from each reinsurer and their respective ratings. Safeco v. American International Group, Inc. [Member] Safeco v. American International Group, Inc. [Member] Safeco v. American International Group, Inc. [Member] N I C O Cover [Member] N I C O Cover [Member] Represents the full risk-transfer cover from National Indemnity Company in old asbestos and environmental ("A&E") claims and certain other exposures. G R C Cover [Member] G R C Cover [Member] Represents the adverse loss reserve development cover from General Reinsurance Corporation. GRC and NICO Cover [Member] GRC and NICO Cover [Member] GRC and NICO Cover [Member] Discontinued Operations [Line Items} Reinsurance Items [Line Items] Reinsurance Items Other Income Increase decrease cap of seller financing Increase decrease cap of seller financing Increase decrease cap of seller financing Seller financing Seller financing Seller financing Discontinued Operation, Estimated Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Discontinued Operation, Estimated Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Discontinued Operation, Estimated Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Discontinued Operation, Estimated Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Discontinued Operation, Estimated Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Discontinued Operation, Estimated Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Number of contracts seller caused one beacon to purchase prior to acquisition Number of Reinsurance Contracts Purchased Prior to Acquisition Represents the number of reinsurance contracts the seller caused subsidiary entity to purchase from subsidiaries of Berkshire Hathaway Inc. immediately prior to the entity's acquisition of subsidiary entity. Reinsurance recoverable cover under reinsurance contract Reinsurance Recoverables, Gross Adverse loss reserve development on losses occurring in years 2000 and prior Adverse Loss Reserve Development on Losses Occurring in Years 2000 and Prior Represents the amount of adverse loss reserve development on losses occurring in years 2000 and prior. Loss reserves ceded at acquisition Loss Reserves Ceded at Acquisition Represents the amount of reserves ceded at acquisition. Reinsurance recoverables Reinsurance Recoverables A M Best Rating System, Number of Debt Ratings A M Best Rating System, Number of Debt Ratings The number of ratings in the A.M. Best debt rating system. Reinsurance recoverable on paid losses Reinsurance Receivables Reinsurance recoverables, gross of purchase accounting adjustments Reinsurance Recoverables, Gross of Purchase Accounting Adjustments Reinsurance recoverables gross of purchase accounting adjustments. Purchase accounting adjustments Reinsurance Recoverables, Gross Purchase Accounting Adjustments Purchase accounting adjustments related to reinsurance recoverables. Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Reserve charge under the SPA, Gross Reserve charge under the SPA, Gross Reserve charge under the SPA, Gross Reserve charge under the SPA Reserve charge under the SPA, Net of Tax Reserve charge under the SPA, Net of Tax Transfer of assets Transfer of assets Transfer of assets Pre-tax net investment income Investment Income [Table Text Block] Schedule of Realized and Unrealized Gain (Loss) on Investments [Table Text Block] Schedule of Realized and Unrealized Gain (Loss) on Investments [Table Text Block] Tabular disclosure of realized and unrealized gains and losses on investments reported in the statement of income. Realized Gain (Loss) on Investments Realized Gain (Loss) on Investments [Table Text Block] Schedule of unrealized gain (loss) on investments Unrealized Gain (Loss) on Investments [Table Text Block] Net unrealized investment gains (losses) for Level 3 investments Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Table Text Block] Investment holdings, equity securities, convertible fixed maturities and other long-term investments Trading Securities [Table Text Block] Other long-term investments Hedge Fund and Private Equity Investment by Objective or Sector [Table Text Block] Tabular disclosure of hedge funds and private equity investments held at fair value by investment objective or sector and the amount of unfunded commitments associated with each objective or sector. Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds Hedge Fund Redemption Frequency Notice Period [Table Text Block] Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds. Fair Value of private equity funds subject to lock-up periods Private Equity Lock-Up Period [Table Text Block] Tabular disclosure of expected lock up period of private equity investments. Fair value measurements by level, investment securities Fair Value, Assets Measured on Recurring Basis [Table Text Block] Debt securities issued by corporations, credit ratings Investment Fixed Maturity Securities Credit Quality Indicators [Table Text Block] Tabular disclosure of fixed maturity securities by credit quality indicator. Mortgage-backed, asset-backed securities Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities Schedule of Security Issuance Years of Investments in Mortgage Backed Securities Issued by Private Enterprises [Table Text Block] Tabular disclosure of security issuance years of investments in non-agency RMBS and non-agency CMBS securities. Non-agency residential mortgage securities, collateral quality and tranche levels Non Agency Residential Mortgage Backed Securities Credit Quality Indicators and Seniority [Table Text Block] Tabular disclosure of non-agency residential mortgage-backed securities by credit quality indicator and seniority. Non-agency commercial mortgage securities, type of interest rate and tranche levels Non Agency Commercial Mortgage Backed Securities, Interest Rate and Seniority [Table Text Block] Tabular disclosure of non-agency commercial mortgage-backed securities by type and by seniority level. Rollforward of fair value investments by level Fair Value, Assets Measured on Recurring Basis, Measurement Input Reconciliation [Table Text Block] Tabular disclosure of the fair value measurement of assets measured at fair value on a recurring basis, including a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: total gains or losses for the period (realized and unrealized), amortization or accretion of discount, purchases, sales, issuances, transfers in and transfers out of each level of the fair value hierarchy by class of asset. Schedule of significant unobservable inputs used in estimating the fair value of investment securities Fair Value Inputs, Assets, Quantitative Information [Table Text Block] Prospector Offshore Fund Ltd [Member] Prospector Offshore Fund Ltd [Member] Represents Prospector Offshore Fund Ltd, classified as variable interest entity primary beneficiary by the reporting entity. Investment related liabilities [Member] Investment related liabilities [Member] Investment related liabilities [Member] Balance at January 1, 2014 Total realized and unrealized gains Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. Foreign currency gains (losses) through OCI Foreign Currency Transaction Gain (Loss) through Other Comprehensive Income Represents the foreign transaction gain (loss) recognized through other comprehensive income. Amortization/Accretion Accretion (Amortization) of Discounts and Premiums, Investments Purchases Payments to Acquire Investments Sales Proceeds from Sale and Maturity of Trading Securities Held-for-investment Net change in investments related to (sales) purchases of consolidated affiliates Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates Transfers in Fair Value, Assets Measured on Recurring Basis, Transfers in This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. Transfers out Fair Value, Assets Measured on Recurring Basis, Transfers out This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. Balance at June 30, 2014 Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers Collective account values of underlying variable annuities as a percentage of the guarantee value Collective Account Value as Percentage of Guaranteed, Value Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability. Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding, Assumed forfeitures Total Accrued Expense Document and Entity Information [Abstract] -- None. No documentation exists for this element. -- Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Largest single fund investment Other Long-term Investments Largest Single Investment The largest investment which the entity has made in a single fund. Number of investments Number of Investments Held Number of investments held. Restricted Stock Restricted Stock [Member] Title of Individual [Axis] Title of Individual [Axis] Title of Individual with Relationship to Entity [Domain] Relationship to Entity [Domain] Chief Executive Officer Chief Executive Officer [Member] Target Restricted Share Activity Issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Unamortized grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grant Date Fair Value [Roll Forward] Unamortized grant date fair value, beginning balance Issued Employee Service Share-based Compensation, Nonvested Awards Issued in Period, Total Compensation Cost Not Yet Recognized The total fair value (on the date of issue) for restricted share awards issued during the period. Unamortized grant date fair value, ending balance Number of Installments in which the Shares Vest Number of Installments in which the Shares Vest Represents the number of installments in which the shares vest. Annual reduction in performance shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in Grant, Total Represents the total number of performance share grants that will be reduced. Years Performance Shares will be Forfeited Years Performance Shares will be Forfeited Period Years Performance Shares will be Forfeited Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Pre-tax operating results of WM Life Re Life Re-Pretax Operating Results [Table Text Block] Tabular disclosure of Life Re's pre-tax operating results. Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position, Location [Table Text Block] Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table [Text Block] Represents the disclosure, in tabular form, related to assets and liabilities by class, that are measured at fair value on a recurring and/or nonrecurring basis in periods after initial recognition. The disclosures that may be required or desired include: (a) the fair value measurements recorded during the period and the reasons for the measurements and (b) the level within the fair value hierarchy in which the fair value measurements fall, segregating fair value measurements using Levels 1, 2, and 3. Assets Held under Reinsurance Counterparty Trust Agreement Assets Held under Reinsurance Counterparty Trust Agreement [Table Text Block] Assets posted as collateral under variable annuity reinsurance trust agreement for the benefit of the reinsurance counterparty. Assets Held under Derivative Agreement Assets Held under Derivative Agreement [Table Text Block] Assets posted as collateral under a derivative agreement. Schedule of significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments Fair Value Inputs, Quantitative Information [Table Text Block] Tabular disclosure of quantitative information about the inputs used in the fair value measurement of variable annuity liabilities and derivative instruments. This disclosure may include, but is not limited to, the fair value of the liability, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs. Exchange traded and OTC derivative instruments Offestting Assets and Liabilities [Table Text Block] Offestting Assets and Liabilities [Table Text Block] Net exposure to collateral Offsetting Liabilities [Table Text Block] Schedule of uncollateralized amounts due Schedule of Uncollateralized Amounts Due [Table Text Block] Tabular disclosure of uncollateralized amounts due under derivative contracts. Investment Securities Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Other Operations Other Segments [Member] Segment reporting information Segment Reporting Information [Line Items] Net investment income (loss) - surplus note interest Net Investment Income, Surplus Notes Net Investment Income, Surplus Notes Net realized and unrealized investment gains (losses) Losses and LAE Insurance and reinsurance acquisition expenses Insurance and Reinsurance Acquisition Expenses Amount of policy acquisition costs charged to expense in the period. Includes deferred policy acquisition costs amortized in the period, generally in proportion to related revenue earned over the policy or contract term. Total expenses Pre-tax income (loss) Schedule of Municipal Guarantee Insured Obligations [Table Text Block] Schedule of Municipal Guarantee Insured Obligations [Table Text Block] [Table Text Block] for Schedule of Municipal Guarantee Insured Obligations [Table] Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table] Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Basic and diluted earnings per share numerators (in millions): Net income (loss) attributable to White Mountains’ common shareholders Income (Loss) from Continuing Operations Attributable to Parent Allocation of income for unvested restricted common shares Dividends declared on participating restricted common shares Dividends, Share-based Compensation Total allocation to restricted common shares Aggregate Allocation to Restricted Shares Including Dividend Represents the total amount allocated to restricted shares including dividend on restricted stock. Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts Income (Loss) from Continuing Operatings, Excluding Portion Attributable to Noncontrolling Interest, Net Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. Undistributed net earnings (in millions): Undistributed Earnings, Basic [Abstract] Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts Dividends declared net of restricted common share amounts Dividends Common Stock Cash Net of Restricted Common Share Amounts Equity impact of common stock cash dividends declared by the entity during the period, net of restricted share amounts. Total undistributed net earnings, net of restricted common share amounts Undistributed Earnings, Basic Basic earnings per share denominators Total average common shares outstanding during the period Basic earnings per share denominator Diluted earnings per share denominator Average outstanding dilutive options to acquire common shares Diluted earnings (loss) per share denominator Basic earnings per share (in dollars): Net income (loss) attributable to White Mountains' common shareholders Dividends declared Undistributed earnings (loss) (in dollars per share) Earnings Per Share, Basic, Undistributed Net income (loss) attributable to White Mountains' common shareholders (in dollars per share) Dividends declared Undistributed earnings (loss) (in dollars per share) Earnings Per Share, Diluted, Undistributed Debt Debt Disclosure [Text Block] Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Restricted Share Cliff Vesting in January 2017 [Member] Restricted Share Cliff Vesting in January 2017 [Member] Restricted Share Cliff Vesting in January 2017 [Member] Restricted Share Cliff Vesting in January 2016 Restricted Share Cliff Vesting in January 2016 [Member] Restricted Share Cliff Vesting in January 2016 [Member] Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Award Type [Axis] Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Award Type [Axis] Pertinent data describing and reflecting required disclosures pertaining to an equity-based compensation arrangement, by awards name. Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Award Type [Domain] Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Award Type [Domain] Types of equity-based compensation awards of the entity. Vested (in shares) Subsidiary, Sale of Stock [Line Items] Subsidiary, Sale of Stock [Line Items] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] SIG Senior Notes Sirius Group Senior Notes [Member] Represents the Sirius Group Senior Notes. WTM Bank Facility WTM Bank Facility [Member] Represents the WTM Bank Facility. Old Lyme Old Lyme Purchase Note [Member] Represents the purchase note issued by Old Lyme at the time of acquisition. Debt Instrument Debt Instrument [Line Items] Debt instrument at face value Debt Instrument, Face Amount Unamortized original issue discount Debt Instrument, Unamortized Discount Total debt Total commitment under revolving credit facility Line of Credit Facility, Maximum Borrowing Capacity Repayments of debt