Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] Assets and goodwill acquired Business Combination, Recognized Identifiable Assets Acquired and Goodwill Business Combination, Recognized Identifiable Assets Acquired and Goodwill Title of 12(b) Security Title of 12(b) Security Diluted earnings (losses) per share numerators, Total Net Income (Loss) Available to Common Stockholders, Diluted Subsequent Event Subsequent Event [Member] Investment in MediaAlpha Equity Method Investments [Member] Debt Securities, Available-for-Sale [Table] Debt Securities, Available-for-Sale [Table] Adjustment to Compensation, Amount Adjustment to Compensation Amount Pledged Status [Axis] Pledged Status [Axis] Financial Instrument [Axis] Financial Instrument [Axis] WTM Partners Debt WTM Partners Debt [Member] WTM Partners Debt Segment Reporting [Abstract] Segment Reporting [Abstract] Other assets Disposal Group, Including Discontinued Operation, Other Assets Schedule of fair value measurements for investment securities by level Fair Value, Assets Measured on Recurring Basis [Table Text Block] Debt covenant, loan to value percent Debt Instrument, Covenant, Loan to Value Ratio, Maximum Debt Instrument, Covenant, Loan to Value Ratio, Maximum Net change in unearned insurance premiums Increase (Decrease) in Unearned Premiums Other Significant Transactions [Table] Other Significant Transactions [Table] Represents the disclosure of other significant transactions. Number of investments Number of Investments Number of Investments Incremental proceeds received Disposal Group, Including Discontinued Operation, Incremental Proceeds Received Disposal Group, Including Discontinued Operation, Incremental Proceeds Received Contract assets Contract with Customer, Asset, after Allowance for Credit Loss Amortization of restricted share awards APIC, Share-Based Payment Arrangement, Restricted Stock Unit, Increase for Cost Recognition Derivative Instrument [Axis] Derivative Instrument [Axis] Reinsurance payable Reinsurance Payable Other liabilities Business Combination, Recognized Liabilities Assumed, Other Liabilities Business Combination, Recognized Liabilities Assumed, Other Liabilities Short-term investments, at fair value Business Combination, Recognized Asset Acquired, Investment in Debt and Equity Securities, Current Deferred tax liabilities Deferred Tax Liability Reversal Deferred Tax Liability Reversal Measurement Frequency [Axis] Measurement Frequency [Axis] Proceeds from the sale of Bamboo Group Net cash proceeds Proceeds from Divestiture of Businesses U.S. Government and agency obligations US Treasury and Government [Member] Consolidated Entities [Axis] Consolidated Entities [Axis] Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Developed technology Developed Technology Rights [Member] Gross Unrealized Losses Other Long-Term Investments, Unrealized Loss Other Long-Term Investments, Unrealized Loss Schedule investments in 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. Mortgage and asset-backed securities Total mortgage and asset-backed securities Asset-Backed Securities [Member] Beginning nominal value Ending nominal value Notes Receivable, Nominal Fair Value Disclosure Notes Receivable, Nominal Fair Value Disclosure Intangible Asset, Finite-Lived, Acquired [Line Items] Intangible Asset, Finite-Lived, Acquired [Line Items] Consolidated Entities [Domain] Consolidated Entities [Domain] Distributions to other noncontrolling interests Payments to Noncontrolling Interests Premiums paid Derivative, Premium Paid Percentage of par value of policy reinsured Percentage of Par Value of Policy Reinsured Percentage of par value of policy reinsured. Number of private debt instruments Number of Private Debt Instruments Number of Private Debt Instruments Ark 2021 Subordinated Notes Tranche 2 Ark 2021 Subordinated Notes, Tranche 2 [Member] Ark 2021 Subordinated Notes, Tranche 2 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, par value per share (in dollars per share) Common Stock, Par or Stated Value Per Share Exchange [Domain] Exchange [Domain] Total other intangible assets Other intangible assets (excluding goodwill), beginning balance Other intangible assets (excluding goodwill), ending balance Intangible Asset, Excluding Goodwill, after Accumulated Amortization Beginning of period (in shares) End of period (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Number Net change in short-term investments Payments For (Proceeds From) Other Short-Term Investments Payments For (Proceeds From) Other Short-Term Investments General and administrative expenses General and Administrative Expense, Including Amortization of Other Intangible Assets General and Administrative Expense, Including Amortization of Other Intangible Assets Pledged collateral Debt Instrument, Collateral Amount Ark Ark Subsidiary - Ark Insurance Holdings Limited [Member] Subsidiary - Ark Insurance Holdings Limited Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Number of reportable segments Number of Reportable Segments Customer relationships Customer Relationships [Member] Concentration Risk [Table] Concentration Risk [Table] Entity Address, City or Town Entity Address, City or Town New grants (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period 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 Share-based compensation arrangement by share-based payment award Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Cash, restricted cash held for sale Restricted Cash, Held For Sale Restricted Cash, Held For Sale Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Other intangible assets, finite-lived, net carrying value Intangible Asset, Finite-Lived, after Accumulated Amortization Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Loss and LAE incurred relating to: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] 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 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 Level 2 Fair Value, Inputs, Level 2 [Member] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Solvency capital requirement Debt Instrument, Covenant, Solvency Capital, Minimum Debt Instrument, Covenant, Solvency Capital, Minimum United States UNITED STATES FHLMC Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] Net investment income Net Investment Income, Adjusted Net Investment Income, Adjusted Subsequent Event Type [Axis] Subsequent Event Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes Commission and fee revenues Commission and fee revenues Insurance Commissions and Fees Investment Income [Table] Investment Income [Table] Schedule of financial information for segments Segment Reporting [Table Text Block] 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. Gross Reinsurance Recoverables, Gross Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Total mortgage and asset-backed securities and collateralized loan obligations Mortgage and Asset-Backed Securities and Collateralized Loan Obligations [Member] Mortgage and Asset-Backed Securities and Collateralized Loan Obligations BAM Surplus Notes, at fair value Notes receivable, fair value disclosure Notes Receivable, Fair Value Disclosure Proceeds from debt, net of debt issuance costs Proceeds from Debt, Net of Issuance Costs Ceded Credit Risk, Collateralization [Domain] Ceded Credit Risk, Collateralization [Domain] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Document Period End Date Document Period End Date Net Foreign Currency Gains (Losses) Other Long-Term Investments, Net Foreign Currency Gain (Loss) Other Long-Term Investments, Net Foreign Currency Gain (Loss) Distinguished Management Distinguished Management [Member] Distinguished Management Face value Debt Instrument, Face Amount Basic and diluted earnings per share (in dollars): Earnings Per Share, Basic [Abstract] Debt Business Combination, Recognized Liability Assumed, Long-Term Debt, Noncurrent Effective yield (as a percent) Debt Instrument, Interest Rate, Effective Percentage Transaction costs Business Combination, Transaction Cost, Excluding Separately Recognized Transaction Loss and LAE Liability For Unpaid Claims And Claims Adjustment Expense, Incurred Claims, Adjusted Liability For Unpaid Claims And Claims Adjustment Expense, Incurred Claims, Adjusted Fair Value, Inputs, Level 1, Level 2, and Level 3 Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Contributions from other noncontrolling interests Proceeds from Noncontrolling Interests Distinguished - Management Equity Incentives Subsidiary - Distinguished Partners, Management Equity Incentives [Member] Subsidiary - Distinguished Partners, Management Equity Incentives Reinsurance Recoverable Including Reinsurance Premium Paid Reinsurance Recoverable Including Reinsurance Premium Paid [Member] Net change in reinsurance payable Increase (Decrease) in Reinsurance Payables Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Other Disposal Group Other Disposal Group [Member] Other Disposal Group Total undistributed net earnings (losses), net of restricted common share amounts Undistributed Continuing Operation Earnings (Loss), Basic Earnings Per Share Earnings Per Share [Text Block] Earnings (loss) per share attributable to White Mountains’s common shareholders: Earnings Per Share [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Purchase of surplus notes Payments to Acquire Surplus Notes Payments to Acquire Surplus Notes Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Net tangible assets acquired (liabilities assumed) Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Net Tangible Assets Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Net Tangible Assets Total liabilities Total liabilities Liabilities Trading Symbol Trading Symbol Discount Rate Measurement Input, Discount Rate [Member] Due in one year or less Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Year One Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Year One Total Goodwill and Other Intangible Assets Intangible Asset, Including Goodwill, after Accumulated Amortization [Abstract] Schedule of assets acquired and liabilities assumed Business Combination, Recognized Asset Acquired and Liability Assumed [Table Text Block] Company Selected Measure Name Company Selected Measure Name Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Fixed rate Long-Term Debt, Percentage Bearing Fixed Interest, Percentage Rate New and existing contracts deployed Other Long Term Investments, Increase (Decrease) in Participation Contracts Other Long Term Investments, Increase (Decrease) in Participation Contracts Concentration Risk [Line Items] Concentration Risk [Line Items] Payments to acquire interest in subsidiaries and affiliates, additional contribution Payments to Acquire Interest in Subsidiaries and Affiliates, Additional Contribution Business Combination, Right to Sell Units, Payment Receivable Post-closing purchase price adjustment Business Combination, Consideration Transferred, Post-Closing Purchase Price Adjustment Business Combination, Consideration Transferred, Post-Closing Purchase Price Adjustment Kudu Subsidiary - Kudu Investment Management, LLC [Member] Subsidiary - Kudu Investment Management, LLC Cost or Amortized Cost Equity Securities, FV-NI, Cost Credit Facility [Domain] Credit Facility [Domain] Loss and loss adjustment expense reserves Gross beginning balance Gross ending balance Liability for Claims and Claims Adjustment Expense Cash flows from operations: Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] 5 – 10 years Other Long-term Investments, Remaining Period, Five to Ten Years Other Long-term Investments, Remaining Period, Five to Ten Years Dividends paid per White Mountains’s common share (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Other Other Asset-Backed Securities [Member] Other Asset-Backed Securities Disposal Group Name [Domain] Disposal Group Name [Domain] Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Schedule of pre-tax net investment income Investment Income [Table Text Block] Deferred income tax expense (benefit) Deferred Income Taxes and Tax Credits Maturities, calls and paydowns of fixed maturity investments Proceeds from Maturity, Repayment, and Call, Debt Security, Trading, Held-for-Investment Dividends payable Dividends Payable Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table] Financial Guarantee Insurance Contracts, Claim Liability [Line Items] Financial Guarantee Insurance Contracts, Claim Liability [Line Items] ILS funds Insurance-Linked Securities Funds [Member] Insurance-Linked Securities Funds Schedule of fair value investments by level Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Other Other Intangible Assets [Member] Net other operating activities Increase (Decrease) in Other Operating Assets and Liabilities, Net Debt instrument, period for interest rate increase Debt Instrument, Increase In Interest Rate, Period Debt Instrument, Increase In Interest Rate, Period Debt Business Combination, Recognized Liability Assumed, Long-Term Debt Business Combination, Recognized Liability Assumed, Long-Term Debt Award Timing Predetermined Award Timing Predetermined [Flag] Nonredeemable noncontrolling interest Business Combination, Nonredeemable Noncontrolling Interest Acquired Business Combination, Nonredeemable Noncontrolling Interest Acquired Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Level 1 Fair Value, Inputs, Level 1 [Member] Net loss and LAE reserves Net ending balance Liability for Unpaid Claims and Claims Adjustment Expense, Net Effective tax rate Effective Income Tax Rate Reconciliation, Percent Less: net realized and unrealized gains (losses) on investment securities sold during the period Gain (Loss) on Sale of Investments Gross earned premiums Gross earned premiums Gross Premiums Earned Gross Premiums Earned Schedule of performance shares outstanding and accrued expense for performance shares awarded 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. Recognition of equity-based compensation expense of subsidiaries APIC, Share-Based Payment Arrangement, Other, Increase for Cost Recognition Sales of fixed maturity investments Proceeds from Sale, Debt Security, Trading, and Equity Security, FV-NI, Held-for-Investment Private debt instruments Private Debt Instrument [Member] Private Debt Instrument [Member] Repurchases and retirements of common shares Stock Repurchased and Retired During Period, Value Goodwill Goodwill, Impaired [Abstract] Shares paid or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, 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. Debt Covenant, Period 1 Debt Covenant, Period 1 [Member] Debt Covenant, Period 1 Fair Value Long-Term Debt, Fair Value Ceded Ceded Policyholder Benefits and Claims Incurred, Ceded Scenario [Domain] Scenario [Domain] Other Operations Debt Other Operations - Debt [Member] Other Operations - Debt Concentration Risk Type [Domain] Concentration Risk Type [Domain] Schedule of carrying values for other long-term investments Other Investments Not Readily Marketable [Table Text Block] Loss and LAE paid relating to: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Net assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less), and Goodwill Schedule of undistributed earnings Schedule of Undistributed Earnings [Table Text Block] Schedule of Undistributed Earnings Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Noncontrolling Interest [Line Items] Noncontrolling Interest [Line Items] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Investment Securities Investment in Debt and Equity Securities and Other Trading Assets [Text Block] Cash dividends paid to common shareholders Payments of Ordinary Dividends, Common Stock Expiration Date Trading Arrangement Expiration Date Business combination, pro forma information, pro forma revenue Significant Transactions [Abstract] Significant Transactions [Abstract] Schedule of HG's future premium revenues Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Table Text Block] Restricted cash balance Restricted Cash and Cash Equivalent Total equity Beginning balances Ending balances Equity, Including Portion Attributable to Noncontrolling Interest Subordinated Debt Subordinated Debt [Member] Disposal Group, Including Discontinued Operations [Line Items] Disposal Group, Including Discontinued Operations [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 Consumer Consumer Sector [Member] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Outrigger Re Outrigger Re [Member] Outrigger Re Net contributions (distributions) and dilution from other noncontrolling interests Temporary Equity, Net Contribution and Dilution From Noncontrolling Interest During Period Temporary Equity, Net Contribution and Dilution From Noncontrolling Interest During Period Cash flows from investing activities: Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Limited Partnerships Limited Partnerships [Member] Limited Partnerships Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Segments [Domain] Segments [Domain] 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 Debt Debt Disclosure [Text Block] Scenario [Axis] Scenario [Axis] 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. Disposal Group Classification [Domain] Disposal Group Classification [Domain] Unamortized issuance cost Debt Issuance Costs, Net Debt covenant, maximum loan to value percent Debt Instrument, Loan To Value, Percentage Debt Instrument, Loan To Value, Percentage Bishop Street Bishop Street [Member] Bishop Street PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Net deferred tax benefit Deferred Other Tax Expense (Benefit) Secured Debt Secured Debt [Member] Surplus notes Funds Held Under Reinsurance Agreements, Assets, Surplus Notes Funds Held Under Reinsurance Agreements, Assets, Surplus Notes Supplement trust target Funds Held Under Reinsurance Agreements, Target Balance Funds Held Under Reinsurance Agreements, Target Balance Stated margin Debt Instrument, Interest Rate, Stated Percentage Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] 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 Entity Listings, Exchange [Axis] Entity Listings, Exchange [Axis] Disposal Group, Disposed of by Sale, Not Discontinued Operations Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] Proceeds held in escrow Disposal Group, Including Discontinued Operation, Incremental Proceeds Received, Held In Escrow Disposal Group, Including Discontinued Operation, Incremental Proceeds Received, Held In Escrow Assets Assets [Abstract] Vested Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Vested Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Vested Debt service coverage ratio Debt Instrument, Covenant, Debt Service Coverage Ratio, Minimum Debt Instrument, Covenant, Debt Service Coverage Ratio, Minimum Goodwill and intangible asset impairment Intangible Asset, Including Goodwill, Impairment Loss Insider Trading Policies and Procedures [Line Items] Derivative, cap interest rate Derivative, Cap Interest Rate Measurement period adjustments Goodwill, Measurement Period Adjustment Financial Guarantee Insurance Contracts, Claim Liability [Table] Financial Guarantee Insurance Contracts, Claim Liability [Table] Performance Share Performance Shares [Member] Loss and loss adjustment expenses Net incurred loss and LAE Loss and LAE Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Kudu, Bamboo and Distinguished Credit Facility Change in Long-term Debt [Roll Forward] Change in Long-term Debt Entity Central Index Key Entity Central Index Key Schedule of Guarantor Obligations [Table] Guarantor Obligation [Table] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Total liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities Asset Management (Kudu) Kudu Investment Management, LLC - Segment [Member] Kudu Investment Management, LLC - Segment Ownership [Axis] Ownership [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Investment, Name [Axis] Investment, Name [Axis] Fixed maturity investments Fixed Maturities [Member] 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 Effects of reinsurance Effects of Reinsurance [Table Text Block] Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate 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 Percentage of borrowing base Debt Instrument, Borrowing Base, Percentage Debt Instrument, Borrowing Base, Percentage Variable Rate [Domain] Variable Rate [Domain] Restricted Stock Restricted Stock [Member] AOCI, after tax AOCI Attributable to Parent [Member] Investments, Excluding MediaAlpha Investments, Excluding MediaAlpha [Member] Investments, Excluding MediaAlpha All Individuals All Individuals [Member] Cash distribution received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received 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] Equity contributions Business Combination, Consideration Transferred, Equity Interest Statement [Line Items] Statement [Line Items] Basesix Basesix [Member] Basesix Total assets Total assets Assets Private equity funds Private Equity Funds [Member] 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. Regulation 114 Regulation 114 [Member] Regulation 114 Industry Sector [Axis] Industry Sector [Axis] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Operating Segments Operating Segments [Member] Liabilities acquired Business Combination, Recognized Liability Assumed, Liability Employee Share-Based Incentive Compensation Plans Share-Based Payment Arrangement [Text Block] Fidus Re Ltd. Fidus Re Ltd. [Member] Fidus Re Ltd. [Member] PEO PEO [Member] Net Foreign Currency Gains (Losses) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Other underwriting expenses General and Administrative Expense, Other Underwriting Expenses General and Administrative Expense, Other Underwriting Expenses Reinsurer, Name [Domain] Reinsurer, Name [Domain] Reinsurer Concentration Risk Reinsurer Concentration Risk [Member] Goodwill Net Carrying Value Goodwill, beginning Balance Goodwill, ending balance Goodwill Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Private preferred securities Mandatorily Redeemable Preferred Securities [Member] Mandatorily Redeemable Preferred Securities Retained earnings Retained Earnings [Member] Revolving Credit Facility Revolving Credit Facility [Member] Net change in premiums and commissions payable Increase (Decrease) in Premiums and Commissions Payable Increase (Decrease) in Premiums and Commissions Payable Increase (Decrease) in Shareholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Common shares, outstanding shares (in shares) Common Stock, Shares, Outstanding Award Timing Disclosures [Line Items] Schedule of the fair value and carrying value of financial instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Debt securities issued by corporations Corporate Debt Securities [Member] Diluted earnings (loss) per share (in dollars per share) Earnings Per Share, Diluted Other equity method eligible investments more than or equal to 50.0% Equity Method Eligible Investments, More Than or Equal to Fifty Percent [Member] Equity Method Eligible Investments, More Than Fifty Percent 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 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 Amortization of other intangible assets Amortization of Other Intangible Assets Amortization of Other Intangible Assets Amortization Intangible Asset, Finite-Lived, Amortization Expense 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] Ownership [Domain] Ownership [Domain] Gross Unrealized Gains Debt Securities, Trading, Unrealized Gain Reinsurer, Name [Axis] Reinsurer, Name [Axis] Net written premiums Premiums Written, Net Share-based Payment Arrangement [Abstract] Share-Based Payment Arrangement [Abstract] 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) Limited liability company or limited partnership, members or limited partners, ownership interest Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest Net gain Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Schedule of significant equity method eligible investments Equity Method Investments [Table Text Block] Utilities Utilities Sector [Member] Utilities Sector Noncontrolling interest acquired Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total other intangible assets Intangible Asset, Excluding Goodwill, before Accumulated Amortization Total agency Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise [Member] PassportCard/DavidShield DavidShield & PassportCard [Member] DavidShield & PassportCard [Member] Subsegments [Domain] Subsegments [Domain] Dividends declared, net of restricted common share amounts Dividends, Common Stock, Cash, Net of Restricted Stock Dividends, Common Stock, Cash, Net of Restricted Stock Sales and distributions Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Short-term investments Disposal Group, Including Discontinued Operation, Investment Assumed Assumed Premiums Written Net change in accrued incentive compensation — Other Operations Increase (Decrease) in Accrued Bonuses Increase (Decrease) in Accrued Bonuses A- to A AM Best, A- Rating [Member] Debt Covenant Period [Domain] Debt Covenant Period [Domain] Debt Covenant Period [Domain] Subsidiary - Basesix Subsidiary - Basesix [Member] Subsidiary - Basesix Income Tax Jurisdiction, Foreign Income Tax Jurisdiction, Foreign [Member] Delayed Draw Term Loan Delayed Draw Term Loan (DDTL) [Member] 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 Citibank LOC Facility Citibank LOC Facility [Member] Citibank LOC Facility Subsidiary - Distinguished Managing General Agent Subsidiary - Distinguished Managing [Member] Subsidiary - Distinguished Managing General Agent Foreign government and agency obligations Debt Security, Government, Non-US [Member] BAM Surplus Notes BAM Surplus Notes [Member] BAM Surplus Note [Member] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Common shares, issued shares (in shares) Common Stock, Shares, Issued Total Equity Total Equity Including Noncontrolling Interest [Member] Total Equity Including Noncontrolling Interest Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Aerospace/Defense/Government Aerospace, Defense Government Sector [Member] Aerospace Defense Government [Member] 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 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 Product and Service [Domain] Product and Service [Domain] Net distributions Proceeds From Return Of Capital On Subsidiaries Proceeds From Return Of Capital On Subsidiaries Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Gain (loss) on fair value of derivative instruments Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net Adjustments of redeemable noncontrolling interest Stockholders' Equity, Adjustments To Temporary Equity Stockholders' Equity, Adjustments To Temporary Equity Termination Date Trading Arrangement Termination Date Noncontrolling Interest [Table] Noncontrolling Interest [Table] Payments of principal and interest Payments of Notes Receivable, Principal and Interest Payments of Notes Receivable, Principal and Interest Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] Held for Sale Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Hawkeye Electric, LLC Hawkeye Electric, LLC [Member] Hawkeye Electric, LLC Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Member surplus contributions, insurance premium charges, percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Current year losses Current Year Claims and Claims Adjustment Expense Total goodwill and other intangible assets Intangible Assets, Gross (Including Goodwill) Intangible Assets, Gross (Including Goodwill) 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 Fair Value as of Grant Date Award Grant Date Fair Value 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. 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 B++ or lower and not rated AM Best, B++ Rating [Member] Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Accrued incentive compensation Accrued Bonuses Debt to capital requirement Debt Instrument, Covenant, Debt To Capital, Maximum Debt Instrument, Covenant, Debt To Capital, Maximum Arrangement Duration Trading Arrangement Duration Number of transactions Debt Instrument, Number of Transactions Debt Instrument, Number of Transactions Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Interest reserve account Short-term Investments, Interest Reserve Short-term Investments, Interest Reserve Basis of Presentation and Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Bamboo CRV Bamboo CRV [Member] Bamboo CRV Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Other intangible assets, finite-lived, acquisition date fair value Intangible Asset, Finite-Lived, before Accumulated Amortization Total revenues Revenues Accrued Expense Accrued Expense [Roll Forward] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] 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 Vesting period Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period Goodwill [Roll Forward] Goodwill [Roll Forward] Intangible Asset, Finite-Lived, Class [Domain] Intangible Asset, Finite-Lived, Class [Domain] Trade names Trade Names [Member] Schedule of mortgage and asset-backed securities Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Award Type [Domain] Award Type [Domain] 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 Commission and fee revenues Insurance Commissions And Fees, Adjusted Insurance Commissions And Fees, Adjusted Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Cost or Amortized Cost Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date [Abstract] Level 3 Fair Value, Inputs, Level 3 [Member] BroadStreet BroadStreet [Member] BroadStreet Asset Pledged as Collateral Asset Pledged as Collateral [Member] Nonredeemable noncontrolling percentage Subsidiary, Ownership Percentage, Noncontrolling Owner Geographical [Axis] Geographical [Axis] Restricted cash Restricted Cash Contract liabilities Contract with Customer, Liability Other Standard & Poor's, Other [Member] Standard & Poor's, Other NAV Fair Value Measured at Net Asset Value Per Share [Member] 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 Other assets Other Assets Legal Entity [Axis] Legal Entity [Axis] Total revenues Revenues, Adjusted Revenues, Adjusted Cash balance at beginning of period (includes restricted cash balances of $1.2 and $14.1, unrestricted cash held for sale of $0.7 and $16.0 and restricted cash held for sale of $0.0 and $59.5) Cash balance at end of period (includes restricted cash balances of $28.8 and $4.6, unrestricted cash held for sale of $0.3 and $23.8 and restricted cash held for sale of $— and $94.9) Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Ownership percentage, after equity incentives Subsidiary, Ownership Percentage, Parent, After Equity Incentives Subsidiary, Ownership Percentage, Parent, After Equity Incentives Effects of Reinsurance [Table] Effects of Reinsurance [Table] Undistributed earnings (losses) (in dollars per share) Earnings Per Share, Basic, Undistributed Non-NEOs Non-NEOs [Member] Ownership interest (as a percent) Ownership Interest Equity Method Investment, Ownership Percentage Gain (loss) on investments, exchange rate effect Gain (Loss), Foreign Currency Transaction, after Tax Distribution received Funds Held Under Reinsurance Agreements, Assets, Distribution Received Funds Held Under Reinsurance Agreements, Assets, Distribution Received Investment, Name [Domain] Investment, Name [Domain] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Repurchases and retirements of common shares Payments for Repurchase of Common Stock Mortgage-backed securities: Collateralized Mortgage-Backed Securities [Member] Statistical Measurement [Axis] Statistical Measurement [Axis] Award Type [Axis] Award Type [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Retained earnings Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] Common equity securities, at fair value Carrying Value Equity Securities, FV-NI Change in fair value of contingent consideration Change in fair value of contingent consideration — Ark Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease) Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Common equity securities Common equity securities Common Stock [Member] Issued (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Shares Issued in Period Noncontrolling Interest [Abstract] Noncontrolling Interest [Abstract] Total unconsolidated entities Total Unconsolidated Entities [Member] Total Unconsolidated Entities [Member] Derivative Contract [Domain] Derivative Contract [Domain] Cost or Amortized Cost Equity Method Investment, Aggregate Cost Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Redeemable noncontrolling interest Business Combination, Redeemable Noncontrolling Interest Acquired Business Combination, Redeemable Noncontrolling Interest Acquired Total liabilities, redeemable noncontrolling interests and equity Liabilities and Equity Increase in investment owned (in dollars per share) Investment Owned, Change in Value, Per Share Investment Owned, Change in Value, Per Share Goodwill and other intangible assets Total goodwill and other intangible assets Total goodwill and other intangible assets, beginning balance Total goodwill and other intangible assets, ending balance Intangible Asset, Including Goodwill, after Accumulated Amortization Net realized investment gains (losses) Realized Investment Gains (Losses) Employee Stock Option Share-Based Payment Arrangement, Option [Member] Purchases and contributions Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Nonredeemable noncontrolling interests Nonredeemable Noncontrolling Equity Equity, Attributable to Noncontrolling Interest Segments [Axis] Segments [Axis] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] 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 HG Global 2022 Senior Notes HG Global 2022 Senior Notes [Member] HG Global 2022 Senior Notes HG Global 2022 Senior Notes HG Global Senior Notes [Member] HG Global Senior Notes Floating rate Debt Instrument, Floating Interest Rate, Percentage Debt Instrument, Floating Interest Rate, Percentage Schedule of insured obligations Schedule of Municipal Guarantee Insured Obligations [Table Text Block] [Table Text Block] for Schedule of Municipal Guarantee Insured Obligations [Table] Fixed maturity investments, at fair value Carrying Value Total fixed maturity investments Debt Securities, Trading Tabular List, Table Tabular List [Table Text Block] 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 AA Standard & Poor's, AA Rating [Member] Paid-in surplus Additional Paid in Capital 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 Net change in reinsurance recoverables Increase (Decrease) in Reinsurance Recoverable Kudu’s Participation Contracts Kudu's Participation Contracts Kudu Participation Contracts [Member] Kudu Participation Contracts Gross Unrealized Losses Equity Securities, FV-NI, Unrealized Loss Name Awards Close in Time to MNPI Disclosures, Individual Name 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] Ceded Ceded premiums written Ceded Premiums Written Schedule of equity securities and other long-term investments Debt Securities, Trading, and Equity Securities, FV-NI [Table Text Block] 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 July 1, 2026 - December 31, 2026 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, First Quarter Enterprise Solution Enterprise Solution [Member] Enterprise Electric, LLC 2027 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year One Debt instrument, measurement input Debt Instrument, Measurement Input Adjustment to Compensation: Adjustment to Compensation [Axis] Net Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral Cost of Sales Cost of Goods and Service [Policy Text Block] Ark - Management Equity Incentive Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive [Member] Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive Ceded Credit Risk, Collateralization [Axis] Ceded Credit Risk, Collateralization [Axis] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activity Accumulated other comprehensive income (loss), after-tax: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Investment portfolio duration Debt Securities, Weighted Average Portfolio Period Debt Securities, Weighted Average Portfolio Period Gross realized investment gains Gain on Sale of Investments Credit Facility [Axis] Credit Facility [Axis] Payments to acquire limited partnership interests Payments to Acquire Limited Partnership Interests Materials Materials Sector [Member] Materials Sector Name Measure Name Private common shares and units Private Equity Securities [Member] Private Equity Securities Earned insurance premiums Premiums Earned, Net, Adjusted Premiums Earned, Net, Adjusted Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] GNMA Government National Mortgage Association Certificates and Obligations (GNMA) [Member] Debt Instrument Debt Instrument [Line Items] Sales of common equity securities and investment in MediaAlpha Proceeds from Sale of Equity Securities, FV-NI Net change in loss and loss adjustment expense reserves Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Intangible Asset, Indefinite-Lived, Class [Axis] Intangible Asset, Indefinite-Lived, Class [Axis] Impairments Goodwill, Impairment Loss Interest receivable Interest Receivable Name Trading Arrangement, Individual Name 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 Average unvested restricted common shares (in shares) Weighted Average Number of Shares, Restricted Stock 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 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 Cash Disposal Group, Including Discontinued Operation, Cash Company Selected Measure Amount Company Selected Measure Amount Secured Overnight Financing Rate (SOFR) Secured Overnight Financing Rate (SOFR) [Member] Loss and LAE: Losses and Loss Adjustment Expense [Abstract] Losses and Loss Adjustment Expense Vehicle receivables Collateralized Auto Loans [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 Non-PEO NEO Non-PEO NEO [Member] 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 Ownership percentage, fully diluted Subsidiary, Ownership Percentage Fully Diluted, Parent Subsidiary, Ownership Percentage Fully Diluted, Parent Cost of sales Cost of Product and Service Sold Award Timing Method Award Timing Method [Text Block] Product and service revenues Product and Service, Other [Member] Forecast Forecast [Member] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] 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. Net income (loss) attributable to White Mountains’s common shareholders Net income (loss) attributable to White Mountains’s common shareholders Net income (loss) Net Income (Loss) Attributable to Parent Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Investments Investments [Member] Restatement Determination Date: Restatement Determination Date [Axis] BBB Standard & Poor's, BBB Rating [Member] BB Standard & Poor's, BB Rating [Member] Premiums, commissions and fees receivable Premiums and Commissions Receivable Premiums and Commissions Receivable Other liabilities Disposal Group, Including Discontinued Operation, Other Liabilities Reinsurance Disclosures [Abstract] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) 1 [Member] London Interbank Offered Rate (LIBOR) 1 Revenues: Revenues [Abstract] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Gross realized investment losses Loss on Sale of Investments 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. Diluted earnings per share denominator (in thousands): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Schedule of intangible assets and goodwill Intangible Asset and Goodwill [Table Text Block] Net 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] Ceded Ceded premiums earned Ceded Premiums Earned 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] Document Fiscal Year Focus Document Fiscal Year Focus Target Performance Shares Outstanding Restricted Shares Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] 2026 – 2028 Performance Period Three [Member] Performance Cycle 2019 to 2021 [Member] White Mountains’s common shares at $1 par value per share - authorized 50,000,000 shares; issued and outstanding 2,386,721 and 2,479,677 shares Common Stock, Value, Issued 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 Amortization (accretion) and depreciation Depreciation, Amortization and Accretion, Net Interest expense Interest expense Interest Expense, Operating and Nonoperating Schedule of debt outstanding Schedule of Long-Term Debt Instruments [Table Text Block] Variable Interest Entity, Primary Beneficiary - HG Global Variable Interest Entity, Primary Beneficiary - HG Global [Member] Variable Interest Entity, Primary Beneficiary - HG Global Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Debt Covenant, Period 4 Debt Covenant, Period 4 [Member] Debt Covenant, Period 4 Amendment Flag Amendment Flag Debt and Equity Securities, FV-NI [Line Items] Debt and Equity Securities, FV-NI [Line Items] Fixed maturity and short-term investments Fixed Maturities And Other Short-Term Investments [Member] Fixed Maturities And Other Short-Term Investments Disposal Group Classification [Axis] Disposal Group Classification [Axis] Net investment income Net investment income Net Investment Income 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 Investments [Domain] Investments [Domain] Supplemental Trust Supplemental Trust [Member] Supplemental Trust [Member] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Redeemable noncontrolling interest Redeemable Noncontrolling Interest, Equity, Redemption Value Entity Address, State or Province Entity Address, State or Province Distinguished Transaction Distinguished Transaction [Member] Distinguished Transaction PEO Total Compensation Amount PEO Total Compensation Amount Premiums, commissions and fees receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Basic earnings (losses) per share denominator (in shares) Weighted Average Number of Shares Outstanding, Basic Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Allocation of losses (earnings) to participating restricted common shares Undistributed Continuing Operation Earnings (Loss), Allocation to Participating Securities, Basic Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Direct Direct Direct Premiums Earned Net change in deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Product and service revenues Revenue from Contract with Customer, Including Assessed Tax Fair Value Equity method investments, fair value Equity Method Investments, Fair Value Disclosure 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 Document Information [Line Items] Document Information [Line Items] Terminal Revenue Growth Rate Measurement Input, Long-Term Revenue Growth Rate [Member] Assets held for sale - Other Disposal Group, Including Discontinued Operation, Assets Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Vested (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period Basis of Presentation Basis of Accounting, Policy [Policy Text Block] 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 Percent over a target trust balance Funds Held under Reinsurance Agreements, Balance Limit, Percentage Funds Held under Reinsurance Agreements, Balance Limit, Percentage Other intangible assets, finite-lived, accumulated amortization Intangible Asset, Finite-Lived, Accumulated Amortization Acquisition of businesses Intangible Asset, Indefinite-Lived, Acquired Total commitment under revolving credit facility Line of Credit Facility, Maximum Borrowing Capacity Gross Unrealized Gains Other Long-Term Investments, Unrealized Gain Other Long-Term Investments, Unrealized Gain Cost or Amortized Cost Other Long-Term Investments, Cost Other Long-Term Investments, Cost Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Subsidiary or Equity Method Investee, Sale of Stock, Type [Table] Third-party investment expenses Investment Income, Investment Expense Nonredeemable noncontrolling interest Noncontrolling Interest [Member] 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 WTM Partners Subsidiary - WTM Partners [Member] Subsidiary - WTM Partners Noncontrolling Interests Noncontrolling Interest Disclosure [Text Block] Other intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill Gross unearned insurance premiums (in millions) Total gross unearned insurance premiums Financial Guarantee Insurance Contracts, Unearned Premium Revenue Debt Total debt Carrying Value Long-Term Debt Ark Ark Insurance Holdings Limited [Member] Ark Insurance Holdings Limited [Member] Hedge funds Hedge Funds [Member] Other operating items: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Net cash provided from (used for) investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Security Exchange Name Security Exchange Name 2029 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year Three Loss and Loss Adjustment Expense Reserves Insurance Disclosure [Text Block] Gross written premiums Premiums written, gross Premiums Written, Gross Investments in unconsolidated affiliates Subsidiary or Equity Method Investee [Line Items] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] 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. Reportable Segments Segment Reporting, Policy [Policy Text Block] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Name Outstanding Recovery, Individual Name Distinguished other debt, carrying value Distinguished other debt Distinguished Other Debt [Member] Distinguished Other Debt Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Interest receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable 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 Geographical [Domain] Geographical [Domain] Other intangible assets, indefinite-lived Intangible Asset, Excluding Goodwill, Indefinite-Lived Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Basic earnings per share denominators (in thousands): Weighted Average Number of Shares Outstanding, Basic [Abstract] Lloyd’s trust deposits Foreign Regulatory Deposits [Member] Foreign Regulatory Deposits Unrestricted cash held for sale Unrestricted Cash, Held For Sale Unrestricted Cash, Held For Sale Other long-term investments Other Long-Term Investments [Abstract] Other Long-Term Investments Other long-term investments Carrying Value Total Other Long-Term Investments Purchases of other long-term investments Payments to Acquire Other Investments Basic ownership Subsidiary, Ownership Percentage, Parent Subsidiary - WM Outrigger Re Subsidiary - WM Outrigger Re [Member] Subsidiary - WM Outrigger Re Amount borrowed Beginning balance Ending balance Long-Term Debt, Gross New grants Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Grants In Period Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Accrued Liability, Grants In Period Ceded Credit Risk, Secured Ceded Credit Risk, Secured [Member] Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Diluted earnings (losses) per share denominator (in shares) Weighted Average Number of Shares Outstanding, Diluted Percentage of risk premium ceded Percentage of Risk Premium Ceded Percentage of Risk Premium Ceded Entity Emerging Growth Company Entity Emerging Growth Company Gross and net reinsurance recoverables by reinsurers ratings Ceded Credit Risk [Table Text Block] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Other Intangible Assets Intangible Asset, Excluding Goodwill, before Accumulated Amortization [Abstract] Total Investments Total Investments After TPC Providers - 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Share-based Compensation Arrangement by Share-based Payment Award, Performance Cycle Period Represents the performance cycle on conclusion of which shares become payable. 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Disposal Contingent consideration Business Combination, Contingent Consideration, Liability Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Bank loan fund Bank Loan Fund [Member] Bank Loan Fund Cover [Abstract] Cover [Abstract] Insurance [Abstract] Common equity securities Equity Securities, FV-NI [Abstract] United Kingdom UNITED KINGDOM Fair Value Inputs Assets Quantitative Information [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] % of Total Concentration Risk, Percentage Equity Awards Adjustments Equity Awards Adjustments [Member] Lloyd’s trust deposits Trust Deposits [Member] Trust Deposits Assumed Assumed Premiums Earned Other unconsolidated entities Other Unconsolidated Private Equity Investments [Member] Other Unconsolidated Private Equity Investments [Member] Balance Sheet Grouping, Financial Statement Captions Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Industry Sector [Domain] Industry Sector [Domain] Geographic Distribution [Domain] Geographic Distribution [Domain] Services, Industrial and Consumer (WTM Partners) Services, Industrial and Consumer (WTM Partners) WTM Partners WTM Partners LLC - Segment [Member] WTM Partners LLC - Segment Income Tax Jurisdiction [Domain] Income Tax Jurisdiction [Domain] 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 Total Parent [Member] Short-term investments Short-Term Investments [Member] Reinsurer, Type [Domain] Reinsurer, Type [Domain] Reinsurer, Type [Domain] Equity Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Forfeited (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period Schedule of noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] General and administrative expenses General and Administrative Expense 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 Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Contracts outstanding Financial Guarantee Insurance Contracts, Number of Contracts Municipal bond contracts issued and outstanding. Income Tax Jurisdiction [Axis] Income Tax Jurisdiction [Axis] Discontinued Operations and Disposal Groups [Abstract] Discontinued Operations and Disposal Groups [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Income Taxes Income Tax Disclosure [Text Block] Acquisition of noncontrolling interests Noncontrolling Interest, Increase from Business Combination Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Business combination, consideration transferred Business Combination, Consideration Transferred Entity Filer Category Entity Filer Category Total Revenues From External Customers Revenues From External Customers Distributed earnings - dividends declared and paid (in dollars per share) Earnings Per Share, Basic, Distributed 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. LUXEMBOURG LUXEMBOURG Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Ownership interest Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Dividend and income distributions from equity method eligible investments Proceeds from Equity Method Investment, Distribution Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Bamboo SPV Bamboo SPV [Member] Bamboo SPV Earned premiums: Premiums Earned, Net [Abstract] Schedule of net realized and unrealized investment gains and losses Gain (Loss) on Securities [Table Text Block] Other Other country [Member] Other country Municipal Bond Guarantee Reinsurance Financial Guarantee Insurance Contracts [Text Block] Net income (loss) Net income (loss) Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Total investment income Gross Investment Income, Operating Surplus Notes Surplus Notes [Member] Surplus Notes Adjustments of redeemable noncontrolling interest Temporary Equity, Other Changes Interest Rate Cap Interest Rate Cap [Member] Pro forma revenue Business Combination, Pro Forma Information, Pro Forma Revenue Senior Notes Senior Notes [Member] Entity Address, Address Line One Entity Address, Address Line One Other intangible assets, impairments Intangible Asset, Finite-Lived, Impairment Loss Total expenses Benefits, Losses and Expenses Carrying Value Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Income tax (expense) benefit Income Tax Expense (Benefit) Collateralized loan obligations Collateralized Loan Obligations [Member] 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 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] Accrued interest Funds Held Under Reinsurance Agreements, Assets, Accrued Interest Funds Held Under Reinsurance Agreements, Assets, Accrued Interest 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] Insurance premiums receivable Premiums Receivable, Net Bermuda Stock Exchange Bermuda Stock Exchange [Member] Bermuda Stock Exchange Discounted cash flow Valuation Technique, Discounted Cash Flow [Member] Statement [Table] Statement [Table] 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 Credit Rating, Standard & Poor's [Axis] Credit Rating, Standard & Poor's [Axis] Infrastructure Fund Infrastructure Fund [Member] Infrastructure Fund Geographic Distribution [Axis] Geographic Distribution [Axis] Other Long-Term Investments Total Other Long-Term Investments [Member] Total Other Long-Term Investments Ownership percentage of basic units outstanding Subsidiary, Ownership Percentage Of Basic Units Outstanding, Parent Subsidiary, Ownership Percentage Of Basic Units Outstanding, Parent Other long-term investments Other Long-Term Investments [Member] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]