Proceeds from the sale of Bamboo Group Net cash proceeds Proceeds from Divestiture of Businesses Other intangible assets, finite-lived, accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Assets and goodwill acquired Business Combination, Recognized Identifiable Assets Acquired and Goodwill Business Combination, Recognized Identifiable Assets Acquired and Goodwill Debt and Equity Securities, FV-NI [Line Items] Debt and Equity Securities, FV-NI [Line Items] Net realized investment gains (losses) Realized Investment Gains (Losses) Entity Listings, Exchange [Axis] Entity Listings, Exchange [Axis] Cost or Amortized Cost Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date [Abstract] Award Timing Predetermined Award Timing Predetermined [Flag] Indefinite-Lived Intangible Assets [Axis] Indefinite-Lived Intangible Assets [Axis] Entity File Number Entity File Number Debt covenant, loan to value percent Debt Instrument, Covenant, Loan to Value Ratio, Maximum Debt Instrument, Covenant, Loan to Value Ratio, Maximum Tabular List, Table Tabular List [Table Text Block] Other Significant Transactions [Table] Other Significant Transactions [Table] Represents the disclosure of other significant transactions. 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Ark 2021 Subordinated Notes Tranche 2 Ark 2021 Subordinated Notes, Tranche 2 [Member] Ark 2021 Subordinated Notes, Tranche 2 Dividends declared on common shares Dividends, Common Stock, Cash Total White Mountains’s common shareholders’ equity Total White Mountains’s common shareholders’ equity Equity, Attributable to Parent Measurement Frequency [Axis] Measurement Frequency [Axis] Debt Securities, Available-for-Sale [Table] Debt Securities, Available-for-Sale [Table] Insider Trading Policies and Procedures [Line Items] Equity interest percent of ownership basis Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest, Look-through Basis Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest, Look-through Basis Common shares, authorized shares (in shares) Common Stock, Shares Authorized Net change in short-term investments Payments For (Proceeds From) Other Short-Term Investments Payments For (Proceeds From) Other Short-Term Investments Noncontrolling Interest [Line Items] Noncontrolling Interest [Line Items] Fair value measured on a recurring basis Fair Value, Recurring [Member] Ark Ark Subsidiary - Ark Insurance Holdings Limited [Member] Subsidiary - Ark Insurance Holdings Limited Assets held-in-trust Funds Held under Reinsurance Agreements, Asset Insurance premiums receivable Premiums Receivable, Net Cover [Abstract] Cover [Abstract] Held for Sale Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Maximum Maximum [Member] Net (income) loss attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Other intangible assets, finite-lived, net carrying value Finite-Lived Intangible Assets, Net Pledged collateral Debt Instrument, Collateral Amount Restricted Stock Restricted Stock [Member] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Debt Instrument Debt Instrument [Line Items] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table] Assumed forfeitures (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Forfeited in Period Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Forfeited in Period Cash, restricted cash held for sale Restricted Cash, Held For Sale Restricted Cash, Held For Sale Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Direct Direct Direct Premiums Earned External Credit Rating, Standard & Poor's [Domain] External Credit Rating, Standard & Poor's [Domain] Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Weighted average portfolio period, excluding short-term investment Debt Securities, Weighted Average Portfolio Period, Excluding Short-Term Investments Debt Securities, Weighted Average Portfolio Period, Excluding Short-Term Investments Pre-tax income (loss) Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Entity Shell Company Entity Shell Company WTM Credit Facility WTM Credit Facility [Member] WTM Credit Facility Terminal Cash Flow Exit Multiple Measurement Input, Terminal Cash Flow Exit Multiple [Member] Measurement Input, Terminal Cash Flow Exit Multiple Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Net change in loss and loss adjustment expense reserves Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Solvency capital requirement Debt Instrument, Covenant, Solvency Capital, Minimum Debt Instrument, Covenant, Solvency Capital, Minimum Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Operating Segments Operating Segments [Member] Other comprehensive (income) loss attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes Letter of Credit Letter of Credit [Member] Distributions to other noncontrolling interests Payments to Noncontrolling Interests A- to A AM Best, A- Rating [Member] Schedule 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. 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Debt Covenant, Period 1 Debt Covenant, Period 1 [Member] Debt Covenant, Period 1 Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Sales and distributions Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] PEO Total Compensation Amount PEO Total Compensation Amount Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Other Operations debt Other Operations - Debt [Member] Other Operations - Debt Basic earnings per share denominators (in thousands): Weighted Average Number of Shares Outstanding, Basic [Abstract] Equity Components [Axis] Equity Components [Axis] P&C Insurance Distribution (Bamboo), net Bamboo Insurance Services, LLC - Segment, Net of Reconciling Items [Member] Bamboo Insurance Services, LLC - Segment, Net of Reconciling Items Schedule of undistributed earnings Schedule of Undistributed Earnings [Table Text Block] Schedule of Undistributed Earnings Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Number of reportable segments Number of Reportable Segments Business combination, pro forma information, pro forma revenue Significant Transactions [Abstract] Significant Transactions [Abstract] 2024 Interest Rate Cap 2024 Interest Rate Cap [Member] 2024 Interest Rate Cap Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Entity Address, State or Province Entity Address, State or Province Amortization of restricted share awards APIC, Share-Based Payment Arrangement, Restricted Stock Unit, Increase for Cost Recognition Delayed Draw Term Loan Delayed Draw Term Loan (DDTL) [Member] Effects of Reinsurance [Line Items] Effects of Reinsurance [Line Items] Acquisition Date Fair Value Indefinite-Lived Intangible Assets (Excluding Goodwill), Acquisition Date, Fair Value Indefinite-Lived Intangible Assets (Excluding Goodwill), Acquisition Date, Fair Value Industrial Commercial and Industrial Sector [Member] Outrigger Re Outrigger Re [Member] Outrigger Re Individual: Individual [Axis] Cost or Amortized Cost Equity Method Investment, Aggregate Cost Other revenues Other Income, Adjusted Other Income, Adjusted Income Tax Jurisdiction [Domain] Income Tax Jurisdiction [Domain] Cash dividends paid to common shareholders Payments of Ordinary Dividends, Common Stock Limited Partnerships Limited Partnerships [Member] Limited Partnerships Loss and Loss Adjustment Expense Reserves Insurance Disclosure [Text Block] Contributions from other noncontrolling interests Proceeds from Noncontrolling Interests Partnership interest, carrying value Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Net deferred tax benefit Deferred Other Tax Expense (Benefit) Nonredeemable noncontrolling percentage Subsidiary, Ownership Percentage, Noncontrolling Owner Repayment of debt and revolving lines of credit Repayments Repayments of Long-Term Debt Entity Interactive Data Current Entity Interactive Data Current Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company [Member] Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company Issued Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Issued The total fair value (on the date of issue) for restricted share awards issued during the period. 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Segment [Member] HG Global/BAM - Segment Additional compensation cost that would be recognized if all outstanding performance shares vested Beginning of period End of period Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Asset Management (Kudu) Kudu Investment Management, LLC - Segment [Member] Kudu Investment Management, LLC - Segment Target performance shares outstanding (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Nonvested, Number, Before Assumed Forfeitures Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Nonvested, Number, Before Assumed Forfeitures Other intangible assets, finite-lived, acquisition date fair value Finite-Lived Intangible Assets, Gross Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Total expenses Total expenses Benefits, Losses and Expenses Beginning fair value discount Ending fair value discount Notes Receivable, Interest Rate Fair Value Discount Notes Receivable, Interest Rate Fair Value Discount Debt Covenant, Period 2 Debt Covenant, Period 2 [Member] Debt Covenant, Period 2 Gross written premiums Premiums written, gross Premiums Written, Gross Entity Address, Address Line One Entity Address, Address Line One Percentage of borrowing base Debt Instrument, Borrowing Base, Percentage Debt Instrument, Borrowing Base, Percentage Temporary equity, beginning balance Temporary equity, ending balance Temporary Equity, Including Noncontrolling Interest AOCI, after tax AOCI Attributable to Parent [Member] Investments, Excluding MediaAlpha Investments, Excluding MediaAlpha [Member] Investments, Excluding MediaAlpha Investments [Domain] Investments [Domain] Less: beginning reinsurance recoverable on unpaid losses Plus: ending reinsurance recoverable on unpaid losses Reinsurance recoverables on unpaid losses Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Cash distribution received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received Redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Carrying Amount Ownership [Axis] Ownership [Axis] Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Distributions and redemptions of other long-term investments Proceeds from Sale and Maturity of Other Investments Industry Sector [Axis] Industry Sector [Axis] 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] Fair Value as of Grant Date Award Grant Date Fair Value Cost or Amortized Cost Equity Securities, FV-NI, Cost Goodwill Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Including Goodwill Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Including Goodwill Performance Period [Domain] Performance Period [Domain] This element represents minimum performance period for right to receive performance share from the entity. 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[Member] Deferred income tax expense (benefit) Deferred Income Taxes and Tax Credits Fair Value Long-Term Debt, Fair Value Schedule of significant unobservable inputs used in estimating the fair value of investment securities Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Net Foreign Currency Gains (Losses) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Accrued investment income Accrued Investment Income Receivable Schedule of investments and unfunded commitments by investment objective and sector Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Other underwriting expenses General and Administrative Expense, Other Underwriting Expenses General and Administrative Expense, Other Underwriting Expenses Investment Type [Axis] Investment Type [Axis] B++ or lower and not rated AM Best, B++ Rating [Member] Total investments Total investments Investments Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Private preferred securities Mandatorily Redeemable Preferred Securities [Member] Mandatorily Redeemable Preferred Securities Non-PEO NEO Non-PEO NEO [Member] Letters of credit outstanding Letters of Credit Outstanding, Amount Gross Unrealized Gains Debt Securities, Trading, Unrealized Gain Net change in premiums and commissions payable Increase (Decrease) in Premiums and Commissions Payable Increase (Decrease) in Premiums and Commissions Payable Other Other Intangible Assets [Member] Award Type [Domain] Award Type [Domain] Other assets Business Combination, Recognized Assets Acquired, Other Assets Business Combination, Recognized Assets Acquired, Other Assets Measurement period adjustments Goodwill and Intangible Assets, Measurement Period Adjustments Goodwill and Intangible Assets, Measurement Period Adjustments Nonredeemable noncontrolling interests Nonredeemable Noncontrolling Equity Equity, Attributable to Noncontrolling Interest Equity Awards Adjustments Equity Awards Adjustments [Member] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Other revenues Other Income Canadian CANADA Compensation Amount Outstanding Recovery Compensation Amount Subsegments [Domain] Subsegments [Domain] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Summary of loss and LAE reserve activity Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Recovery of Erroneously Awarded Compensation Disclosure [Line Items] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Quantitative information for Level 3 Fair Value Measurements Assets Fair Value, Option, Quantitative Disclosures [Line Items] Purchases of fixed maturity investments Payments to Acquire Trading Fixed Maturities Investments The cash outflow to acquire fixed maturity and convertible fixed maturity investments classified as trading. Euro Interbank Offered Rate (EURIBOR) Euro Interbank Offered Rate (EURIBOR) [Member] Euro Interbank Offered Rate (EURIBOR) Change in fair value of contingent consideration Change in fair value of Ark’s contingent consideration Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease) Net gain Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Utilities Utilities Sector [Member] Utilities Sector Revenues: Revenues [Abstract] PassportCard/DavidShield DavidShield & PassportCard [Member] DavidShield & PassportCard [Member] Dividends declared, net of restricted common share amounts Dividends, Common Stock, Cash, Net of Restricted Stock Dividends, Common Stock, Cash, Net of Restricted Stock Financial Instruments [Domain] Financial Instruments [Domain] Financial Instruments [Domain] Purchases and contributions Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Performance Share Performance Shares [Member] Bermuda BERMUDA Accumulated other comprehensive income (loss), after-tax: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Net Investment Income [Line Items] Net Investment Income [Line Items] Developed technology Developed Technology Rights [Member] Net change in accrued incentive compensation from Other Operations Increase (Decrease) in Accrued Bonuses Increase (Decrease) in Accrued Bonuses Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Minimum Minimum [Member] Debt Covenant Period [Domain] Debt Covenant Period [Domain] Debt Covenant Period [Domain] Measurement period adjustments Goodwill, Measurement Period Adjustment Company Selected Measure Name Company Selected Measure Name Mortgage and asset-backed securities and collateralized loan obligations Asset-Backed Securities And Collateralized Loan Obligations [Member] Asset-Backed Securities And Collateralized Loan Obligations Geographical presence Limited Liability Company or Limited Partnership, Geographical Presence Limited Liability Company or Limited Partnership, Geographical Presence Outstanding par value of policies assumed (in millions) Financial Guarantee Insurance Contracts, Principal Balance Financial Guarantee Insurance Contracts, Principal Balance Less: net realized and unrealized gains (losses) on investment securities sold during the period Gain (Loss) on Sale of Investments Debt Debt Disclosure [Text Block] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Citibank LOC Facility Citibank LOC Facility [Member] Citibank LOC Facility Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Dividends payable Dividends Payable Diluted earnings (losses) per share numerators, Total Net Income (Loss) Available to Common Stockholders, Diluted Subsidiary - Distinguished Managing General Agent Subsidiary - Distinguished Managing [Member] Subsidiary - Distinguished Managing General Agent Terminal Revenue Growth Rate Measurement Input, Long-Term Revenue Growth Rate [Member] Goodwill [Roll Forward] Goodwill [Roll Forward] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] BAM Surplus Notes BAM Surplus Notes [Member] BAM Surplus Note [Member] Diluted earnings (losses) per share denominator (in shares) Weighted Average Number of Shares Outstanding, Diluted Reportable Segments Segment Reporting, Policy [Policy Text Block] Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Derivative [Table] Derivative [Table] Total Equity Total Equity Including Noncontrolling Interest [Member] Total Equity Including Noncontrolling Interest Entity Small Business Entity Small Business Payout range for growth target percentage Share-based Compensation Arrangement by Share-based Payment Award, Average Annual Growth Rate, Percentage Represents the percentage growth in intrinsic business value per share to achieve payment. Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Aerospace/Defense/Government Aerospace, Defense Government Sector [Member] Aerospace Defense Government [Member] Document Transition Report Document Transition Report Expense recognized Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount of Expense Recognized Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount of Expense Recognized Recognition of equity-based compensation expense of subsidiaries APIC, Share-Based Payment Arrangement, Other, Increase for Cost Recognition Schedule of nominal and fair value of notes receivable Schedule of Nominal and Fair Value of Notes Receivable [Table Text Block] Schedule of Nominal and Fair Value of Notes Receivable Net distributions Proceeds From Return Of Capital On Subsidiaries Proceeds From Return Of Capital On Subsidiaries Underlying Securities Award Underlying Securities Amount Equity Component [Domain] Equity Component [Domain] Document Period End Date Document Period End Date Income Taxes Income Tax Disclosure [Text Block] Ceded Credit Risk, Collateralization [Domain] Ceded Credit Risk, Collateralization [Domain] Adjustments of redeemable noncontrolling interest Stockholders' Equity, Adjustments To Temporary Equity Stockholders' Equity, Adjustments To Temporary Equity Average unvested restricted common shares (in shares) Weighted Average Number of Shares, Restricted Stock Geographic Distribution [Domain] Geographic Distribution [Domain] United Kingdom UNITED KINGDOM Payments of principal and interest Payments of Notes Receivable, Principal and Interest Payments of Notes Receivable, Principal and Interest Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Member surplus contributions, insurance premium charges, percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Total goodwill and other intangible assets Intangible Assets, Gross (Including Goodwill) Intangible Assets, Gross (Including Goodwill) Total equity Beginning balances Ending balances Equity, Including Portion Attributable to Noncontrolling Interest 2024 – 2026 Performance Period One [Member] Range Of Performance Cycle From 2017-2019 [Member] P&C Insurance and Reinsurance (Ark/WM Outrigger) Ark Insurance Holdings Limited/Outrigger [Member] Ark Insurance Holdings Limited/Outrigger Document Type Document Type Fair Value Equity Method Investments, Fair Value Disclosure Level 3 Fair Value, Inputs, Level 3 [Member] Earnings Per Share Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Paid-in surplus Additional Paid in Capital Forfeited Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Forfeited The total reduction to unamortized compensation cost for performance shares forfeited during the period. Reinsurer, Name [Domain] Reinsurer, Name [Domain] Due after ten years Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 10 Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 10 Premiums and commissions payable Business Combination, Recognized Liability Assumed, Premiums and Commissions Payable Business Combination, Recognized Liability Assumed, Premiums and Commissions Payable Debt to capital requirement Debt Instrument, Covenant, Debt To Capital, Maximum Debt Instrument, Covenant, Debt To Capital, Maximum Assumed Assumed Premiums Earned Investment Income [Table] Investment Income [Table] Number of transactions Debt Instrument, Number of Transactions Debt Instrument, Number of Transactions Statistical Measurement [Axis] Statistical Measurement [Axis] Interest reserve account Short-term Investments, Interest Reserve Short-term Investments, Interest Reserve Bamboo CRV Bamboo CRV [Member] Bamboo CRV Total average common shares outstanding during the period (in shares) Total average common shares outstanding during the period (in shares) Weighted Average Number of Shares Issued, Basic Accrued Expense Accrued Expense [Roll Forward] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Disposal Group Name [Axis] Disposal Group Name [Axis] Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Due after ten years Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 Basic earnings (losses) per share numerators, Total Net earnings (losses) available to White Mountains’s common shareholders, net of restricted common share amounts Net Income (Loss) Available to Common Stockholders, Basic Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Due after five years through ten years Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 5 through 10 Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 5 through 10 Payments to acquire limited partnership interests Payments to Acquire Limited Partnership Interests BroadStreet BroadStreet [Member] BroadStreet Level 1 Fair Value, Inputs, Level 1 [Member] AAA Standard & Poor's, AAA Rating [Member] Diluted earnings (loss) per share (in dollars per share) Earnings Per Share, Diluted Other Standard & Poor's, Other [Member] Standard & Poor's, Other Other Other Funds [Member] Other Funds Basic units outstanding percentage Business Combination, Voting Equity Interest Acquired, Percentage of Basic Units Outstanding Business Combination, Voting Equity Interest Acquired, Percentage of Basic Units Outstanding Total revenues Revenues, Adjusted Revenues, Adjusted Consolidation Items [Domain] Consolidation Items [Domain] Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Equity Method Eligible Investments Equity Method Investments and Joint Ventures Disclosure [Text Block] Reinsurer, Name [Axis] Reinsurer, Name [Axis] Ownership percentage, after equity incentives Subsidiary, Ownership Percentage, Parent, After Equity Incentives Subsidiary, Ownership Percentage, Parent, After Equity Incentives Repurchases and retirements of common shares Stock Repurchased and Retired During Period, Value Reinsurance recoverables Reinsurance recoverables Collateral Reinsurance Recoverables, Including Reinsurance Premium Paid LUXEMBOURG LUXEMBOURG Noncontrolling Interest [Abstract] Noncontrolling Interest [Abstract] Gain (Loss) on Securities [Line Items] Gain (Loss) on Securities [Line Items] Sales of common equity securities and investment in MediaAlpha Proceeds from Sale of Equity Securities, FV-NI Distribution received Funds Held Under Reinsurance Agreements, Assets, Distribution Received Funds Held Under Reinsurance Agreements, Assets, Distribution Received Net income tax payments Income Taxes Paid, Net Financials Financial services Financial Services Sector [Member] Amount borrowed Beginning balance Ending balance Long-Term Debt, Gross Dividends declared to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Trade names Trade Names [Member] Investment owned (in shares) Investment Owned, Balance, Shares Expiration Date Trading Arrangement Expiration Date Ownership interest Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Equity Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Foreign government and agency obligations Debt Security, Government, Non-US [Member] Schedule of significant equity method eligible investments Equity Method Investments [Table Text Block] Investments Investments [Member] Ceded Ceded premiums written Ceded Premiums Written Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Total unconsolidated entities Total Unconsolidated Entities [Member] Total Unconsolidated Entities [Member] Entity Current Reporting Status Entity Current Reporting Status Derivative [Line Items] Derivative [Line Items] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Redeemable noncontrolling interest Business Combination, Redeemable Noncontrolling Interest Acquired Business Combination, Redeemable Noncontrolling Interest Acquired Consolidated Entities [Domain] Consolidated Entities [Domain] Increase in investment owned (in dollars per share) Investment Owned, Change in Value, Per Share Investment Owned, Change in Value, Per Share Distributed earnings - dividends declared and paid (in dollars per share) Earnings Per Share, Diluted, Distributed Financial Instrument [Axis] Financial Instrument [Axis] Adjustments to reconcile net income to net cash provided from (used for) operations: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Noncash Item [Abstract] Expenses: Benefits, Losses and Expenses [Abstract] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Executive Category: Executive Category [Axis] Hedge funds Hedge Funds [Member] Recent transaction Valuation Technique, Recent or Expected Transaction Prices [Member] Valuation Technique, Recent or Expected Transaction Prices Gross Unrealized Gains Equity Method Investment, Unrealized Gain Equity Method Investment, Unrealized Gain Floating rate Debt Instrument, Floating Interest Rate, Percentage Debt Instrument, Floating Interest Rate, Percentage HG Global Senior Notes HG Global Senior Notes [Member] HG Global Senior Notes Schedule of insured obligations Schedule of Municipal Guarantee Insured Obligations [Table Text Block] [Table Text Block] for Schedule of Municipal Guarantee Insured Obligations [Table] Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple [Member] Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple Limited liability company or limited partnership, members or limited partners, ownership interest Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest Draw down of debt and revolving lines of credit Borrowings Proceeds from Issuance of Long-Term Debt Interest expense Interest expense Interest Expense, Operating and Nonoperating Current Fiscal Year End Date Current Fiscal Year End Date Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Kudu’s Participation Contracts Kudu's Participation Contracts Kudu Participation Contracts [Member] Kudu Participation Contracts Net change in reinsurance payable Increase (Decrease) in Reinsurance Payables 2029 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year Three Prior year losses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Accrued Expense Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Before Assumed Forfeitures Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Before Assumed Forfeitures Enterprise Solution Enterprise Solution [Member] Enterprise Electric, LLC Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Noncontrolling Interests Noncontrolling Interest Disclosure [Text Block] Dividend and income distributions from equity method eligible investments Proceeds from Equity Method Investment, Distribution Schedule of mortgage and asset-backed securities Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Net Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral Proceeds from debt, net of debt issuance costs Proceeds from Debt, Net of Issuance Costs Ark - Management Equity Incentive Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive [Member] Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive Diluted earnings per share denominator (in thousands): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Noncontrolling interest acquired Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Energy Energy Sector [Member] Investment portfolio duration Debt Securities, Weighted Average Portfolio Period Debt Securities, Weighted Average Portfolio Period Net cash provided from (used for) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Measure: Measure [Axis] Materials Materials Sector [Member] Materials Sector Total undistributed net earnings (losses), net of restricted common share amounts Undistributed Continuing Operation Earnings (Loss), Basic Private common shares and units Private Equity Securities [Member] Private Equity Securities Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Cost of sales Cost of Product and Service Sold Disposal Group Classification [Domain] Disposal Group Classification [Domain] Interest accrued multiplier Debt Instrument, Interest Reserve, Interest Accrued Multiplier Debt Instrument, Interest Reserve, Interest Accrued Multiplier Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Common shares and paid-in surplus Common Stock Including Additional Paid in Capital [Member] Third-party investment expenses Investment Income, Investment Expense Entity Tax Identification Number Entity Tax Identification Number Discounted cash flow Valuation Technique, Discounted Cash Flow [Member] Gross Unrealized Gains Equity Securities, FV-NI, Unrealized Gain Due after five years through ten years Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Other intangible assets, finite-lived, accumulated amortization General and Administrative Expense, Amortization of Intangible Assets General and Administrative Expense, Amortization of Intangible Assets Common shares, issued shares (in shares) Common Stock, Shares, Issued Loss and LAE: Losses and Loss Adjustment Expense [Abstract] Losses and Loss Adjustment Expense Schedule of noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] Schedule of HG's future premium revenues Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Table Text Block] Goodwill and Other Intangible Assets Goodwill and Intangible Assets Disclosure [Text Block] Total agency Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise [Member] Mortgage and asset-backed securities and collateralized loan obligations Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Debt instrument, measurement input Debt Instrument, Measurement Input Long/short all cap global Hedge Fund, Long/Short All Cap Global [Member] Hedge Fund, Long/Short All Cap Global Net realized and unrealized investment (losses) gains Total Investments Before TPC Providers - Investments [Member] Total Investments Before TPC Providers - Investments Other Operations Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] Security Exchange Name Security Exchange Name Ownership percentage, fully diluted Subsidiary, Ownership Percentage Fully Diluted, Parent Subsidiary, Ownership Percentage Fully Diluted, Parent Goodwill and intangible asset impairment Goodwill and Intangible Asset Impairment Common equity securities Equity Securities, FV-NI [Abstract] Mortgage-backed securities: Collateralized Mortgage-Backed Securities [Member] Total commitment under revolving credit facility Line of Credit Facility, Maximum Borrowing Capacity Shares paid or expired Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Shares 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. Beginning of period End of period Total Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. Ceded Ceded premiums earned Ceded Premiums Earned Financial Guarantee Insurance Contracts, Claim Liability [Line Items] Financial Guarantee Insurance Contracts, Claim Liability [Line Items] Indefinite-Lived Intangible Assets, Major Class Name [Domain] Indefinite-Lived Intangible Assets, Major Class Name [Domain] Reinsurance Disclosures [Abstract] Maturities, calls and paydowns of fixed maturity investments Proceeds from Maturity, Repayment, and Call, Debt Security, Trading, Held-for-Investment Net realized and unrealized gains Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Premiums, commissions and fees receivable Premiums and Commissions Receivable Premiums and Commissions Receivable Maximum interest rate Derivative, Maximum Interest Rate Derivative, Maximum Interest Rate Statement [Line Items] Statement [Line Items] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Fair Value Inputs Assets Quantitative Information [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) 1 [Member] London Interbank Offered Rate (LIBOR) 1 Common equity securities Common equity securities Common Stock [Member] Vested (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period Percentage of target payout - minimum actual Share-based Compensation Arrangement by Share-Based Payment Award, Percentage of Target Achieved Payout Represents the percentage of target achieved, low end of range. Net unrealized investment gains (losses) Total net unrealized investment gains (losses) recognized during the period on Level 3 investments held at the end of period Unrealized Gain (Loss) on Investments Dividends paid per White Mountains’s common share (in usd per share) Common Stock, Dividends, Per Share, Cash Paid Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Release of cash (pre-funding) of investment purchases Release Of Cash (Pre-Fundings) For Managed Funds Release Of Cash (Pre-Fundings) For Managed Funds Statistical Measurement [Domain] Statistical Measurement [Domain] Long/short equity financials and business services Hedge Fund, Long/Short Equity Banks and Financial [Member] Hedge Fund, Long/Short Equity Banks and Financial [Member] Prior year losses Favorable prior loss reserve development Prior Year Claims and Claims Adjustment Expense Total other intangible assets Intangible Assets, Gross (Excluding Goodwill) Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] 2026 – 2028 Performance Period Three [Member] Performance Cycle 2019 to 2021 [Member] Discount rate Other Long-Term Investments, Measurement Input Other Long-Term Investments, Measurement Input Measurement Input, Pre-Tax Discount Rate Measurement Input, Pre-Tax Discount Rate [Member] Measurement Input, Pre-Tax Discount Rate Unearned insurance premiums Unearned Premiums Basic and diluted earnings per share (in dollars): Earnings Per Share, Basic [Abstract] Variable Interest Entity, Primary Beneficiary - HG Global Variable Interest Entity, Primary Beneficiary - HG Global [Member] Variable Interest Entity, Primary Beneficiary - HG Global BB Standard & Poor's, BB Rating [Member] Intangible Asset, Acquired, Finite-Lived [Line Items] Intangible Asset, Acquired, Finite-Lived [Line Items] Municipal Bond Guarantee Insurance Financial Guarantee Insurance Contracts [Text Block] Debt Covenant, Period 4 Debt Covenant, Period 4 [Member] Debt Covenant, Period 4 Pledged Status [Axis] Pledged Status [Axis] Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Basic earnings (loss) per share (in dollars per share) Earnings Per Share, Basic Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Subsidiary or Equity Method Investee, Sale of Stock, Type [Table] Fixed maturity and short-term investments Fixed Maturities And Other Short-Term Investments [Member] Fixed Maturities And Other Short-Term Investments Discount Rate Measurement Input, Discount Rate [Member] Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Schedule of financial information for segments Schedule of Segment Reporting Information, by Segment [Table Text Block] Measurement Input Type [Domain] Measurement Input Type [Domain] Performance Period [Axis] Performance Period [Axis] Disclosure of minimum performance period for right to receive performance share from the entity. Premiums and commissions payable Premiums and Commissions Payable Premiums and Commissions Payable Lloyds Bank Lloyds Bank [Member] Lloyds Bank Supplemental Trust Supplemental Trust [Member] Supplemental Trust [Member] Commission and fee revenues Commission and fee revenues Insurance Commissions and Fees Gross and net reinsurance recoverables by reinsurers ratings Ceded Credit Risk [Table Text Block] Gross unearned insurance premiums (in millions) Total gross unearned insurance premiums Financial Guarantee Insurance Contracts, Unearned Premium Revenue Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Gross realized investment gains Gain on Sale of Investments Distinguished Transaction Distinguished Transaction [Member] Distinguished Transaction Premiums, commissions and fees receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Total liabilities, redeemable noncontrolling interests and equity Liabilities and Equity AA Standard & Poor's, AA Rating [Member] Deferred acquisition costs Deferred Policy Acquisition Cost Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Net income (loss) Temporary Equity, Net Income Guarantees [Abstract] Guarantees [Abstract] 2027 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year One Concentration Risk Type [Domain] Concentration Risk Type [Domain] Purchases of other long-term investments Payments to Acquire Other Investments Sales of fixed maturity investments Proceeds from Sale, Debt Security, Trading, and Equity Security, FV-NI, Held-for-Investment Pay vs Performance Disclosure [Line Items] Goodwill, intangible assets and other liabilities acquired, measurement period adjustment Goodwill, Intangible Assets and Other Liabilities Acquired, Measurement Period Adjustment Goodwill, Intangible Assets and Other Liabilities Acquired, Measurement Period Adjustment Net change in unearned insurance premiums Increase (Decrease) in Unearned Premiums Effects of Reinsurance [Table] Effects of Reinsurance [Table] Liabilities Liabilities [Abstract] Acquisition of noncontrolling interests Noncontrolling Interest, Increase from Business Combination Percent over a target trust balance Funds Held under Reinsurance Agreements, Balance Limit, Percentage Funds Held under Reinsurance Agreements, Balance Limit, Percentage Gross Unrealized Gains Other Long-Term Investments, Unrealized Gain Other Long-Term Investments, Unrealized Gain Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Cost or Amortized Cost Other Long-Term Investments, Cost Other Long-Term Investments, Cost Impairments Goodwill, Impairment Loss Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Company Selected Measure Amount Company Selected Measure Amount Cash Cash Comprehensive income (loss) attributable to White Mountains’s common shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Measurement Input Type [Axis] Measurement Input Type [Axis] Disposal Group Name [Domain] Disposal Group Name [Domain] Name Measure Name Secured Debt Secured Debt [Member] Industry Sector [Domain] Industry Sector [Domain] Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Table] Consolidation, Less-than-Wholly-Owned Subsidiary, Parent Ownership Interest, Effect of Change [Table] Assumed Assumed Premiums Written Ark Ark Insurance Holdings Limited [Member] Ark Insurance Holdings Limited [Member] Income Tax Contingency [Table] Income Tax Contingency [Table] Debt Total debt Carrying Value Long-Term Debt Legal Entity [Axis] Legal Entity [Axis] Total assets Total assets Assets Geographical [Domain] Geographical [Domain] Cash flows from operations: Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Consumer Consumer Sector [Member] Other intangible assets, weighted average useful life (in years) Acquired Finite-Lived Intangible Assets, Weighted Average Useful Life Private equity funds and hedge funds Total private equity funds and hedge funds Private Equity Funds and Hedge Funds [Member] Investments in hedge funds and private equity funds. Concentration Risk Type [Axis] Concentration Risk Type [Axis] Private equity funds Private Equity Funds [Member] Reinsurance Recoverable Including Reinsurance Premium Paid Reinsurance Recoverable Including Reinsurance Premium Paid [Member] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Distinguished other debt, carrying value Distinguished other debt Distinguished Other Debt [Member] Distinguished Other Debt Interest receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable Dividends declared per White Mountains’s common share (in usd per share) Common Stock, Dividends, Per Share, Declared Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Kudu Credit Facility Kudu Credit Facility [Member] Kudu Credit Facility Total leverage ratio Debt Instrument, Covenant, Total Leverage Ratio Debt Instrument, Covenant, Total Leverage Ratio Subsidiary - HG Global HG Global Subsidiary - HG Global [Member] Subsidiary - HG Global Ceded unearned premiums Prepaid Reinsurance Premiums Level 2 Fair Value, Inputs, Level 2 [Member] Investment in MediaAlpha, at fair value Carrying Value Equity Method Investments Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Business Combination [Domain] Business Combination [Domain] Net gain on sale of the Bamboo Group Net gain on sale of the Bamboo Group Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal Investment in MediaAlpha Equity Method Investments [Member] Asset Pledged as Collateral Asset Pledged as Collateral [Member] Lloyd’s trust deposits Foreign Regulatory Deposits [Member] Foreign Regulatory Deposits Document Information [Table] Document Information [Table] PEO PEO [Member] Unrestricted cash held for sale Unrestricted Cash, Held For Sale Unrestricted Cash, Held For Sale Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Other long-term investments Other Long-Term Investments [Abstract] Other Long-Term Investments Schedule of contractual maturities of fixed maturities Investments Classified by Contractual Maturity Date [Table Text Block] Fixed maturity investments, at fair value Carrying Value Total fixed maturity investments Debt Securities, Trading Subsidiary - 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Direct Direct Premiums Written Arrangement Duration Trading Arrangement Duration Change in fair value of BAM Surplus Notes Gain (Loss) on Fair Value Adjustment of Surplus Notes Gain (Loss) on Fair Value Adjustment of Surplus Notes Schedule of equity securities and other long-term investments Debt Securities, Trading, and Equity Securities, FV-NI [Table Text Block] Segments [Axis] Segments [Axis] Local Phone Number Local Phone Number Other equity method eligible investments Equity Method Eligible Investments, Less Than Fifty Percent [Member] Equity Method Eligible Investments, Less Than Fifty Percent Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Net contributions (distributions) and dilution from other noncontrolling interests Noncontrolling Interest, Net Contribution and Dilution During Period Noncontrolling Interest, Net Contribution and Dilution During Period Net other investing activities Payment for (Proceeds from) Other Investing Activity 2025 – 2027 Performance Period Two [Member] Performance Cycle 2018 to 2020 [Member] Employee Share-Based Incentive Compensation Plans Share-Based Payment Arrangement [Text Block] Subsidiary, ownership percentage economic interest falls Subsidiary, Ownership Percentage, Economic Interest Subsidiary, Ownership Percentage, Economic Interest Other intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill Cash (restricted $0.4) Business Combination, Recognized Asset Acquired, Cash and Cash Equivalent Assets Assets [Abstract] Credit Facility [Axis] Credit Facility [Axis] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Debt Instrument [Axis] Debt Instrument [Axis] Annual payments Debt Instrument, Annual Principal Payment Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Credit Facility [Domain] Credit Facility [Domain] White Mountains’s common shareholders’ equity Equity, Attributable to Parent [Abstract] WM Outrigger Re WM Outrigger Re - Segment [Member] WM Outrigger Re - Segment Kudu - Management Equity Incentive Subsidiary - Kudu Investment Management, Management Equity Incentive [Member] Subsidiary - Kudu Investment Management, Management Equity Incentive Stated margin Debt Instrument, Interest Rate, Stated Percentage Investment, Name [Axis] Investment, Name [Axis] Written notice period Other Long-Term Investments, Redemption, Written Notice Period Other Long-Term Investments, Redemption, Written Notice Period Other long-term investments Carrying Value Total Other Long-Term Investments Amortization (accretion) and depreciation Depreciation, Amortization and Accretion, Net Entity Registrant Name Entity Registrant Name Communications Communications Sector [Member] Communications Sector Ceded Credit Risk, Collateralization [Axis] Ceded Credit Risk, Collateralization [Axis] Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Asset Management (Kudu) Kudu Investment Management, LLC [Member] Kudu Investment Management, LLC [Member] Mortgage and asset-backed securities and collateralized loan obligations Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date Debt Covenant, Period 3 Debt Covenant, Period 3 [Member] Debt Covenant, Period 3 Total liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities Performance cycle period Share-based Compensation Arrangement by Share-based Payment Award, Performance Cycle Period Represents the performance cycle on conclusion of which shares become payable. Discount rate Equity Method Investment, Measurement Input Equity Method Investment, Measurement Input Ark Ark Insurance Holdings Limited - Segment [Member] Ark Insurance Holdings Limited - Segment Undistributed earnings (losses) (in dollars per share) Earnings Per Share, Diluted, Undistributed Largest Investment Largest Investment [Member] Largest Investment Investment in MediaAlpha Equity Method Investments [Abstract] Equity Method Investments Restatement Determination Date: Restatement Determination Date [Axis] Net other financing activities Proceeds from (Payment for) Other Financing Activity Ark 2021 Subordinated Notes Tranche 3 Ark 2021 Subordinated Notes, Tranche 3 [Member] Ark 2021 Subordinated Notes, Tranche 3 Increase in book value per share (in dollars per share) Investment Owned, Change in Value, Book Value Per Share Investment Owned, Change in Value, Book Value Per Share Name Trading Arrangement, Individual Name Exchange-traded funds Common Stock, Exchange Traded Funds [Member] Common Stock, Exchange Traded Funds Subsidiary - Distinguished Interest Rate Cap Subsidiary - Distinguished Interest Rate Cap [Member] Subsidiary - Distinguished Interest Rate Cap Fair Value Trading Security, Fair Value Trading Security, Fair Value Face value Debt Instrument, Face Amount Distinguished Subsidiary - Distinguished Partners [Member] Subsidiary - Distinguished Partners Business combination, basic ownership interest acquired, percentage Business Combination, Voting Equity Interest Acquired, Basic, Percentage Business Combination, Voting Equity Interest Acquired, Basic, Percentage Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Net change in insurance premiums, commissions and fees receivable Increase (Decrease) in Premiums Commissions and Fees Receivable Increase (Decrease) in Premiums Commissions and Fees Receivable Preferred Stock Preferred Stock [Member] Name Awards Close in Time to MNPI Disclosures, Individual Name Broker commission expenses Broker Commission Expense Broker Commission Expense Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Beginning of period (in shares) End of period (in shares) Total target performance shares outstanding (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Nonvested, Number Measurement period adjustments Intangible Assets, Measurement Period Adjustment Intangible Assets, Measurement Period Adjustment Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items] Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items] BBB Standard & Poor's, BBB Rating [Member] Foreign Tax Jurisdiction Foreign Tax Jurisdiction [Member] Segments [Domain] Segments [Domain] Income Statement Location [Axis] Statement of Income Location, Balance [Axis] Fixed maturity investments Fixed Maturities [Member] Investment Securities Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Expense recognized Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Expense In Period Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Expense In Period Renegotiation period Reinsurance Agreement, Renegotiation Period Reinsurance Agreement, Renegotiation Period Deferred tax liabilities related to investment basis differences Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill, Deferred Tax Liability Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill, Deferred Tax Liability Accrued incentive compensation Accrued Bonuses Ark 2021 Subordinated Notes Ark 2021 Subordinated Notes [Member] Ark 2021 Subordinated Notes BroadStreet SPV BroadStreet SPV [Member] BroadStreet SPV Purchases of common equity securities and investment in MediaAlpha Payments to Acquire Equity Securities and Equity Method Investments Payments to Acquire Equity Securities and Equity Method Investments Due after one year through five years Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Senior Notes Senior Notes [Member] Short-term investments Other Short-Term Investments [Member] Other Short-Term Investments Carrying Value Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Liabilities acquired Business Combination, Recognized Liability Assumed, Liability Subordinated Debt Subordinated Debt [Member] All Executive Categories All Executive Categories [Member] Current year losses Current Year Claims and Claims Adjustment Expense Acquisition expenses Deferred Policy Acquisition Costs, Amortization Expense Interest income from BAM Surplus Notes Other Operating Income White Mountains’s common shares at $1 par value per share - authorized 50,000,000 shares; issued and outstanding 2,476,670 and 2,479,677 shares Common Stock, Value, Issued Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Earned insurance premiums Net earned premiums Earned insurance premiums Premiums Earned, Net Net Foreign Currency Gains (Losses) Equity Method Investment, Net Foreign Currency Gain (Loss) Equity Method Investment, Net Foreign Currency Gain (Loss) Other Significant Transactions [Line Items] Other Significant Transactions [Line Items] Other Significant Transactions [Line Items] NAV Fair Value Measured at Net Asset Value Per Share [Member] Business Combination [Axis] Business Combination [Axis] Comprehensive income (loss) Total comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Net Foreign Currency Gains (Losses) Equity Securities, FN-NI, Net Foreign Currency Gain (Loss) Equity Securities, FN-NI, Net Foreign Currency Gain (Loss) Total Distinguished debt Distinguished Debt [Member] Distinguished Debt Gain (loss) on investments, exchange rate effect Gain (Loss), Foreign Currency Transaction, after Tax Nonredeemable noncontrolling interest Noncontrolling Interest [Member] 2030 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year Four Schedule of pre-tax net investment income Investment Income [Table Text Block] Ownership interest (as a percent) Ownership Interest Equity Method Investment, Ownership Percentage Short-term investments Short-Term Investments [Member] Basic and diluted earnings per share numerators (in millions): Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Common equity securities and investment in MediaAlpha Common Stock And Equity Method Investments [Member] Common Stock And Equity Method Investments Noncontrolling Interest [Table] Noncontrolling Interest [Table] Concentration Risk [Line Items] Concentration Risk [Line Items] Ceded Ceded Policyholder Benefits and Claims Incurred, Ceded PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations [Member] Represents the fixed maturity investments and convertible fixed maturity investments excluding mortgage-backed securities, asset-backed securities and preferred stock. HG Global HG Global - Segment [Member] HG Global - Segment Lock up period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Fund Lock Up Period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Funds Lock Up Period P&C Insurance and Reinsurance (Ark/WM Outrigger) P&C Insurance and Reinsurance (Ark/WM Outrigger) Ark Insurance Holdings Limited/WM Outrigger Re - Segment [Member] Ark Insurance Holdings Limited/WM Outrigger Re - Segment Bamboo Group Bamboo Group [Member] Bamboo Group Interest income from BAM Surplus Notes Interest Income, Operating, Net of Interest Payments Received Interest Income, Operating, Net of Interest Payments Received Scenario [Axis] Scenario [Axis] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Reinsured amount Reinsurance Agreements, Asset Limit Amount Reinsurance Agreements, Asset Limit Amount Number of interest rate cap agreements Derivative, Number of Instruments Held Derivative Contract [Domain] Derivative Contract [Domain] All Individuals All Individuals [Member] Financial Guarantee Insurance Contracts, Claim Liability [Table] Financial Guarantee Insurance Contracts, Claim Liability [Table] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Name Forgone Recovery, Individual Name Net change in cash during the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect, Continuing Operation Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Assumed forfeitures Share-Based Compensation Arrangement by Share-Based 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Terminated Rule 10b5-1 Arrangement Terminated [Flag] Forecast Forecast [Member] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Title of 12(b) Security Title of 12(b) Security Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] Due after one year through five years Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year One through Five Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year One through Five Earnings (loss) per share attributable to White Mountains’s common shareholders: Earnings Per Share [Abstract] Derivative, cap interest rate Derivative, Cap Interest Rate Loss and LAE paid relating to: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Lloyds LOC Facility Lloyds LOC Facility [Member] Lloyds LOC Facility Basic ownership Subsidiary, Ownership Percentage, Parent Business combination, right to purchase units Business Combination, Right To Purchase Units, Unit Price Multiplier Business Combination, Right To Purchase Units, Unit Price Multiplier Gross Unrealized Losses Equity Securities, FV-NI, Unrealized Loss Other asset-backed securities: Asset-Backed Securities, Securitized Loans and Receivables [Member] Changes In Nominal Value Of Surplus Notes [Roll Forward] Changes In Nominal Value Of Surplus Notes [Roll Forward] Changes In Nominal Value Of Surplus Notes Agency relationships Agency Relationships [Member] Agency Relationships MediaAlpha Investment in MediaAlpha MediaAlpha [Member] MediaAlpha Schedule of unrecognized compensation costs associated with outstanding restricted share awards Nonvested Restricted Stock Shares Activity [Table Text Block] Award Timing Disclosures [Line Items] Schedule of net realized and unrealized investment gains and losses Gain (Loss) on Securities [Table Text Block] Measurement Frequency [Domain] Measurement Frequency [Domain] Secured credit facilities Number of Secured Credit Facilities Number of Secured Credit Facilities Net loss and LAE reserves Net ending balance Liability for Unpaid Claims and Claims Adjustment Expense, Net Due in one year or less Debt Securities, Trading, Fair Value, Maturity, Allocated And Single Maturity Date, Year One Debt Securities, Trading, Fair Value, Maturity, Allocated And Single Maturity Date, Year One Unfunded Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Third-Party Reinsurance Reinsurance [Text Block] Net income (loss) attributable to White Mountains’s common shareholders Net income (loss) attributable to White 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Unconsolidated Private Equity Investments [Member] Other Unconsolidated Private Equity Investments [Member] Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Ark 2021 Subordinated Notes Tranche 1 Ark 2021 Subordinated Notes, Tranche 1 [Member] Ark 2021 Subordinated Notes, Tranche 1 Notice period Investment Redemption, Notice Period Investment Redemption, Notice Period 3 – 5 years Other Long-term Investments, Remaining Period, Three to Five Years Other Long-term Investments, Remaining Period, Three to Five Years Other intangible assets, impairments Impairment of Intangible Assets, Finite-Lived Reinsurer, Type [Domain] Reinsurer, Type [Domain] Reinsurer, Type [Domain] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Undistributed earnings (losses) (in dollars per share) Earnings Per Share, Basic, Undistributed Statement [Table] Statement [Table] Short-term investments, at fair value Business Combination, Recognized Asset Acquired, Investment in Debt and Equity Securities, Current Cash flows from financing activities: Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Net assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less), and Goodwill Variable Rate [Domain] Variable Rate [Domain] New York Stock Exchange New York Stock Exchange [Member] New York Stock Exchange Collateral trusts, investments and cash Funds Held Under Reinsurance Agreements, Assets, Cash And Investments Funds Held Under Reinsurance Agreements, Assets, Cash And Investments Schedule of fair value investments by level Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] All Adjustments to Compensation All Adjustments to Compensation [Member] Amendment Flag Amendment Flag Loss and loss adjustment expense reserves Gross beginning balance Gross ending balance Liability for Claims and Claims Adjustment Expense Termination Date Trading Arrangement Termination Date Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Contracts outstanding Financial Guarantee Insurance Contracts, Number of Contracts Municipal bond contracts issued and outstanding. Basic earnings (losses) per share denominator (in shares) Weighted Average Number of Shares Outstanding, Basic Schedule of fair value measurements for investment securities by level Fair Value, Assets Measured on Recurring Basis [Table Text Block] Schedule of earnings per share from continuing operations Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Segment Reporting [Abstract] Segment Reporting [Abstract] Total Revenues From External Customers Revenues From External Customers Debt Disclosure [Abstract] Debt Disclosure [Abstract] Scenario [Domain] Scenario [Domain] Schedule of credit ratings of debt securities issued Investment Fixed Maturity Securities Credit Quality Indicators [Table Text Block] Tabular disclosure of fixed maturity securities by credit quality indicator. Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Gross Unrealized Losses Debt Securities, Trading, Unrealized Loss Share-based compensation arrangement by share-based payment award Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Consolidated Entities [Axis] Consolidated Entities [Axis] Common shares, par value per share (in dollars per share) Common Stock, Par or Stated Value Per Share Bamboo SPV Bamboo SPV [Member] Bamboo SPV Other Other country [Member] Other country Geographic Distribution [Axis] Geographic Distribution [Axis] Surplus Notes Surplus Notes [Member] Surplus Notes Schedule of carrying values for other long-term investments Other Investments Not Readily Marketable [Table Text Block] Forfeitures and cancellations Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Forfeited and Cancelled In Period Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Forfeited and Cancelled In Period Insider Trading Arrangements [Line Items] Net change in reinsurance recoverables Increase (Decrease) in Reinsurance Recoverable Accrued interest Funds Held Under Reinsurance Agreements, Assets, Accrued Interest Funds Held Under Reinsurance Agreements, Assets, Accrued Interest Total liabilities Total liabilities Liabilities Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Bermuda Stock Exchange Bermuda Stock Exchange [Member] Bermuda Stock Exchange Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] P&C Insurance Distribution (Bamboo) Bamboo Insurance Services, LLC - Segment [Member] Bamboo Insurance Services, LLC - Segment Deferred tax expense (benefit) Gain (Loss) on Economic Transition Adjustment, Tax Expense (Benefit) Gain (Loss) on Economic Transition Adjustment, Tax Expense (Benefit) Forfeitures and cancellations (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Forfeited and Cancelled In Period Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Forfeited and Cancelled In Period Other long-term investments Other Long-Term Investments [Member] Debt Business Combination, Recognized Liability Assumed, Long-Term Debt, Noncurrent Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Mortgage and asset-backed securities Total mortgage and asset-backed securities Asset-Backed Securities [Member] Variable Rate [Axis] Variable Rate [Axis] Consolidation Items [Axis] Consolidation Items [Axis] Infrastructure Fund Infrastructure Fund [Member] Infrastructure Fund Total investment income Gross Investment Income, Operating Other Long-Term Investments Total Other Long-Term Investments [Member] Total Other Long-Term Investments Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Ownership percentage of basic units outstanding Subsidiary, Ownership Percentage Of Basic Units Outstanding, Parent Subsidiary, Ownership Percentage Of Basic Units Outstanding, Parent Non-NEOs Non-NEOs [Member] Debt securities issued by corporations Corporate Debt Securities [Member] Proceeds from derivative instrument Proceeds From Derivative Instrument, Operating Activities Proceeds From Derivative Instrument, Operating Activities