Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Long/short all cap global Hedge Fund, Long/Short All Cap Global [Member] Hedge Fund, Long/Short All Cap Global Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] 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 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] Segment Reporting [Abstract] Segment Reporting [Abstract] Other assets Disposal Group, Including Discontinued Operation, Other Assets Accrued interest payment held outside of trust Repayments Of Accrued Interest Held Outside of Trust Repayments Of Accrued Interest Held Outside of Trust Schedule of fair value measurements for investment securities by level Fair Value, Assets Measured on Recurring Basis [Table Text Block] Net change in unearned insurance premiums Increase (Decrease) in Unearned Premiums Adjustments of redeemable noncontrolling interest Stockholders' Equity, Adjustments To Temporary Equity Stockholders' Equity, Adjustments To Temporary Equity Repayment of principal Repayments Of Debt, Principal Repayments Of Debt, Principal Other Disposal Group Other Disposal Group [Member] Other Disposal Group 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 Net Foreign Currency Gains (Losses) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Debt Securities, Trading, Net Foreign Currency Gain (Loss) Short-term investments, at fair value Business Combination, Recognized Asset Acquired, Investment in Debt and Equity Securities, Current Measurement Frequency [Axis] Measurement Frequency [Axis] Proceeds from the sale of Bamboo Group Net cash proceeds Proceeds from Divestiture of Businesses Attribution of acquisition date fair value estimates between goodwill and other intangible assets Goodwill, Attribution of Acquisition Adjustments Goodwill, Attribution of Acquisition Adjustments U.S. Government and agency obligations US Treasury and Government [Member] Total Investments Total Investments After TPC Providers - Investments [Member] Total Investments After TPC Providers - Investments Consolidated Entities [Axis] Consolidated Entities [Axis] Gross Unrealized Losses Equity Method Investment, Unrealized Loss Equity Method Investment, Unrealized Loss Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Developed technology Developed Technology Rights [Member] 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 Mortgage and asset-backed securities Total mortgage and asset-backed securities Asset-Backed Securities [Member] Subsidiary - Basesix Subsidiary - Basesix [Member] Subsidiary - Basesix 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 Restricted cash Business Combination, Recognized Asset Acquired, Restricted Cash Business Combination, Recognized Asset Acquired, Restricted Cash 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 Weighted average portfolio period, excluding short-term investment Debt Securities, Weighted Average Portfolio Period, Including Short-Term Investments Debt Securities, Weighted Average Portfolio Period, Including Short-Term Investments Pledged collateral Debt Instrument, Collateral Amount Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Commission and fee revenues Insurance Commissions And Fees, Adjusted Insurance Commissions And Fees, Adjusted Payments of principal received on the BAM Surplus Notes Payments on Long-term Debt Subsequent To Deconsolidation Payments on Long-term Debt Subsequent To Deconsolidation Number of reportable segments Number of Reportable Segments Distinguished Transaction, ScaleCo Program Distinguished Transaction, ScaleCo Program [Member] Distinguished Transaction, ScaleCo Program Customer relationships Customer Relationships [Member] Total Distinguished debt Distinguished Debt [Member] Distinguished Debt Concentration Risk [Table] Concentration Risk [Table] Mortgage and asset-backed securities and collateralized loan obligations Asset-Backed Securities And Collateralized Loan Obligations [Member] Asset-Backed Securities And Collateralized Loan Obligations Entity Address, City or Town Entity Address, City or Town 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. 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Commission and fee revenues Insurance Commissions and Fees Exchange-traded funds Common Stock, Exchange Traded Funds [Member] Common Stock, Exchange Traded Funds Investment Income [Table] Investment Income [Table] Total goodwill and other intangible assets Intangible Assets, Gross (Including Goodwill) Intangible Assets, Gross (Including Goodwill) Schedule of financial information for segments Segment Reporting [Table Text Block] Number of infrastructure funds Number of Prefunded Infrastructure Funds Number of Prefunded Infrastructure Funds Enterprise Solution Enterprise Solution [Member] Enterprise Electric, LLC Distinguished other debt, carrying value Distinguished other debt Distinguished Other Debt [Member] Distinguished Other Debt Gross Reinsurance Recoverables, Gross Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Citibank LOC Facility Citibank LOC Facility [Member] Citibank LOC Facility BAM Surplus Notes, at fair value Notes receivable, fair value disclosure Notes Receivable, Fair Value Disclosure 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 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 Additional interest rate Debt Instrument, Additional Interest Rate Debt Instrument, Additional Interest Rate 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 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 Reinsurance Recoverable Including Reinsurance Premium Paid Reinsurance Recoverable Including Reinsurance Premium Paid [Member] Net change in reinsurance payable Increase (Decrease) in Reinsurance Payables 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 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. Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Performance Period [Domain] Performance Period [Domain] This element represents minimum performance period for right to receive performance share from the entity. 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 Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Total liabilities Total liabilities Liabilities Trading Symbol Trading Symbol Kudu, Bamboo and Distinguished Credit Facility Change in Long-term Debt [Roll Forward] Change in Long-term Debt Discount Rate Measurement Input, Discount Rate [Member] Bank loan fund Bank Loan Fund [Member] Bank Loan Fund Total Goodwill and Other Intangible Assets Intangible Asset, Including Goodwill, after Accumulated Amortization [Abstract] 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 Post closing adjustment, consideration transferred Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred 1 – 3 years Other Long-term Investments, Remaining Period, One to Three Years Other Long-term Investments, Remaining Period, One to Three Years 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 Concentration Risk [Line Items] Concentration Risk [Line Items] Cost or Amortized Cost Other Long-Term Investments, Cost Other Long-Term Investments, Cost Other Other country [Member] Other country Cost or Amortized Cost Equity Securities, FV-NI, Cost Percentage of risk premium ceded Percentage of Risk Premium Ceded Percentage of Risk Premium Ceded 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. 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 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] Fixed maturity and short-term investments Fixed Maturities And Other Short-Term Investments [Member] Fixed Maturities And Other Short-Term Investments 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] 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 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. Maturities, calls and paydowns of fixed maturity investments Proceeds from Maturity, Repayment, and Call, Debt Security, Trading, Held-for-Investment Dividends payable Dividends Payable 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 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] 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 London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) 1 [Member] London Interbank Offered Rate (LIBOR) 1 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 Award Timing Predetermined Award Timing Predetermined [Flag] 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 (in percent) 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 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 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 Goodwill Goodwill, Impaired [Abstract] Fair Value Long-Term Debt, Fair Value Ceded Ceded Policyholder Benefits and Claims Incurred, Ceded Ark - Management Equity Incentive Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive [Member] Subsidiary - Ark Insurance Holdings Limited, Management Equity Incentive Purchase of surplus notes Payments to Acquire Surplus Notes Payments to Acquire Surplus Notes Scenario [Domain] Scenario [Domain] BroadStreet BroadStreet [Member] BroadStreet Other Significant Transactions [Table] Other Significant Transactions [Table] Represents the disclosure of other significant transactions. 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] Private Equity and Hedge Fund Concentration Risk Private Equity and Hedge Fund Concentration Risk [Member] Private Equity and Hedge Fund Concentration Risk Net assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less), and Goodwill 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] Schedule of undistributed earnings Schedule of Undistributed Earnings [Table Text Block] Schedule of Undistributed Earnings Cash dividends paid to common shareholders Payments of Ordinary Dividends, Common Stock Earned insurance premiums Premiums Earned, Net, Adjusted Premiums Earned, Net, Adjusted 2026 – 2028 Performance Period Three [Member] Performance Cycle 2019 to 2021 [Member] Expiration Date Trading Arrangement Expiration Date 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 Other Operations Debt Other Operations - Debt [Member] Other Operations - Debt Subordinated Debt Subordinated Debt [Member] Disposal Group, Including Discontinued Operations [Line Items] Disposal Group, Including Discontinued Operations [Line Items] Consumer Consumer Sector [Member] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Asset Management (Kudu) Kudu Investment Management, LLC - Segment [Member] Kudu Investment Management, LLC - Segment Total Revenues From External Customers Revenues From External Customers Cash flows from investing activities: Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Segments [Domain] Segments [Domain] Debt Debt Disclosure [Text Block] Scenario [Axis] Scenario [Axis] Net Foreign Currency Gains (Losses) Equity Method Investment, Net Foreign Currency Gain (Loss) Equity Method Investment, Net Foreign Currency Gain (Loss) Disposal Group Classification [Domain] Disposal Group Classification [Domain] Unamortized issuance cost Debt Issuance Costs, Net Distinguished Subsidiary - Distinguished Partners [Member] Subsidiary - Distinguished Partners 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 Gross Unrealized Gains Other Long-Term Investments, Unrealized Gain Other Long-Term Investments, Unrealized Gain PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Net deferred tax benefit Deferred Other Tax Expense (Benefit) Secured Debt Secured Debt [Member] Nonredeemable noncontrolling interest Business Combination, Nonredeemable Noncontrolling Interest Acquired Business Combination, Nonredeemable Noncontrolling Interest Acquired Net gain Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Disposal Group, Not Discontinued Operation, Net Gain (Loss) on Disposal Cash, restricted cash held for sale Restricted Cash, Held For Sale Restricted Cash, Held For Sale 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 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 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] New grants (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Grants In Period Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Grants In Period Measurement period adjustments Goodwill and Intangible Assets, Measurement Period Adjustments Goodwill and Intangible Assets, Measurement Period Adjustments Assets Assets [Abstract] 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 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 ILS funds Insurance-Linked Securities Funds [Member] Insurance-Linked Securities Funds Ark 2021 Subordinated Notes Tranche 1 Ark 2021 Subordinated Notes, Tranche 1 [Member] Ark 2021 Subordinated Notes, Tranche 1 Entity Central Index Key Entity Central Index Key P&C Insurance Distribution (Bamboo) Bamboo Insurance Services, LLC - Segment [Member] Bamboo Insurance Services, LLC - Segment 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] Distinguished, net Distinguished Partners - Segment, Net of Reconciling Items [Member] Distinguished Partners - Segment, Net of Reconciling Items Liabilities held for sale - Other Disposal Group, Including Discontinued Operation, Liabilities 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] Attribution of acquisition date fair value estimates between goodwill and other intangible assets Goodwill and Finite-Lived Intangible Assets, Attribution of Acquisition Adjustments Goodwill and Finite-Lived Intangible Assets, Attribution of Acquisition Adjustments Schedule of Effects of reinsurance Effects of Reinsurance [Table Text Block] Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate Underwriting Capacity Underwriting Capacity [Member] Underwriting Capacity Variable Rate [Domain] Variable Rate [Domain] Restricted Stock Restricted Stock [Member] AOCI, after tax AOCI Attributable to Parent [Member] All Individuals All Individuals [Member] Fidus Re Ltd. Fidus Re Ltd. [Member] Fidus Re Ltd. [Member] 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] Total assets Total assets Assets 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 Private equity funds Private Equity Funds [Member] 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] PEO PEO [Member] Fair Value Trading Security, Fair Value Trading Security, Fair Value Measurement period adjustments Intangible Assets, Measurement Period Adjustment Intangible Assets, Measurement Period Adjustment 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] 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. Retained earnings Retained Earnings [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 Revolving Credit Facility Revolving Credit Facility [Member] 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 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] Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period Net Unrealized Gains (Losses) On Investments Net Unrealized Gains (Losses) On Investments Diluted earnings (loss) per share (in dollars per share) Earnings Per Share, Diluted 2025 – 2027 Performance Period Two [Member] Performance Cycle 2018 to 2020 [Member] 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 Investment in MediaAlpha Equity Method Investments [Abstract] Equity Method Investments 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] Net change in accrued incentive compensation — Other Operations Increase (Decrease) in Accrued Bonuses Increase (Decrease) in Accrued Bonuses Ownership [Domain] Ownership [Domain] Gross Unrealized Gains Debt Securities, Trading, Unrealized Gain Reinsurer, Name [Axis] Reinsurer, Name [Axis] Net written premiums Premiums Written, Net 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 Share-based Payment Arrangement [Abstract] Share-Based Payment Arrangement [Abstract] Limited liability company or limited partnership, members or limited partners, ownership interest Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest Schedule of significant equity method eligible investments Equity Method Investments [Table Text Block] Outstanding par value of policies assumed (in millions) Financial Guarantee Insurance Contracts, Principal Balance Financial Guarantee Insurance Contracts, Principal Balance 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] Subsegments [Domain] Subsegments [Domain] 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 Written notice period Other Long-Term Investments, Redemption, Written Notice Period Other Long-Term Investments, Redemption, Written Notice Period Short-term investments Other Short-Term Investments [Member] Other Short-Term Investments Asset Management (Kudu) Kudu Investment Management, LLC [Member] Kudu Investment Management, LLC [Member] HG Global 2026 Senior Notes HG Global 2026 Senior Notes [Member] HG Global 2026 Senior Notes A- to A AM Best, A- Rating [Member] Supplemental Trust Supplemental Trust [Member] Supplemental Trust [Member] 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. Income Tax Jurisdiction, Foreign Income Tax Jurisdiction, Foreign [Member] Delayed Draw Term Loan Delayed Draw Term Loan (DDTL) [Member] Subsidiary - Distinguished Managing General Agent Subsidiary - Distinguished Managing [Member] Subsidiary - Distinguished Managing General Agent Dividends declared, net of restricted common share amounts Dividends, Common Stock, Cash, Net of Restricted Stock Dividends, Common Stock, Cash, Net of Restricted Stock Foreign government and agency obligations Debt Security, Government, Non-US [Member] 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 Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Common shares, issued shares (in shares) Common Stock, Shares, Issued Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 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 Cash distribution received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received Funds Held Under Reinsurance Agreements, Assets, Cash Distribution Received Product and Service [Domain] Product and Service [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. 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 Common equity securities and investment in MediaAlpha Common Stock And Equity Method Investments [Member] Common Stock And Equity Method Investments 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 Termination Date Trading Arrangement Termination Date Other revenues Other Income, Net of Premiums Earned and Insurance Commissions and Fees Other Income, Net of Premiums Earned and Insurance Commissions and Fees Noncontrolling Interest [Table] Noncontrolling Interest [Table] 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] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Current year losses Current Year Claims and Claims Adjustment Expense Fair Value as of Grant Date Award Grant Date Fair Value Loss and LAE: Losses and Loss Adjustment Expense [Abstract] Losses and Loss Adjustment Expense Contributions to Kudu’s Participation Contracts Payments for Participation Contracts Payments for Participation Contracts 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. 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] P&C Insurance and Reinsurance (Ark/WM Outrigger) Ark Insurance Holdings Limited/Outrigger [Member] Ark Insurance Holdings Limited/Outrigger Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Accrued incentive compensation Accrued Bonuses Arrangement Duration Trading Arrangement Duration WTM Partners Subsidiary - WTM Partners [Member] Subsidiary - WTM Partners 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 Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Basis of Presentation and Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] 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 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] Vesting period Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period 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 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] Ark Ark Insurance Holdings Limited [Member] Ark Insurance Holdings Limited [Member] Other Common Stock, Other [Member] Common Stock, Other 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] Asset Pledged as Collateral Asset Pledged as Collateral [Member] Regulation 114 Regulation 114 [Member] Regulation 114 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 Nonredeemable noncontrolling percentage Subsidiary, Ownership Percentage, Noncontrolling Owner Geographical [Axis] Geographical [Axis] Restricted cash Restricted Cash Contract liabilities Contract with Customer, Liability NAV Fair Value Measured at Net Asset Value Per Share [Member] Other assets Other Assets Significant Transactions Significant Transactions [Text Block] Description of various transactions that are significant to White Mountains financial position, results of operations and/or cash flows. 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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 Additional Investments, Insurance Regulators and Reinsurance Counterparties Additional Investments, Insurance Regulators and Reinsurance Counterparties [Member] Additional Investments, Insurance Regulators and Reinsurance Counterparties Acquisition Date Fair Value Indefinite-Lived Intangible Assets (Excluding Goodwill), Acquisition Date, Fair Value Indefinite-Lived Intangible Assets (Excluding Goodwill), Acquisition Date, Fair Value 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] October 1, 2026 - December 31, 2026 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, First Quarter 2027 Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Year One Variable Interest Entity, Primary Beneficiary - HG Global Variable Interest Entity, Primary Beneficiary - HG Global [Member] Variable Interest Entity, Primary Beneficiary - HG Global 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 Debt instrument, measurement input Debt Instrument, Measurement Input Adjustment to 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The interest is eliminated in consolidation with the primary beneficiary. Ceded Credit Risk, Collateralization [Axis] Ceded Credit Risk, Collateralization [Axis] Basesix Basesix [Member] Basesix 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] 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 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 Gross realized investment gains Gain on Sale of Investments Subsidiary - Distinguished Interest Rate Cap Subsidiary - Distinguished Interest Rate Cap [Member] Subsidiary - Distinguished Interest Rate Cap Deferred tax expense (benefit) Gain (Loss) on Economic Transition Adjustment, Tax Expense (Benefit) Gain (Loss) on Economic Transition Adjustment, Tax Expense (Benefit) Credit Facility [Axis] Credit Facility [Axis] Payments to acquire limited partnership interests Payments to Acquire Limited Partnership Interests Cash on hand Business Combination, Consideration Transferred, Cash on Hand Business Combination, Consideration Transferred, Cash on Hand 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 Collateral trusts, investments and cash Funds Held Under Reinsurance Agreements, Assets, Cash And Investments Funds Held Under Reinsurance Agreements, Assets, Cash And Investments Name Measure Name 3 – 5 years Other Long-term Investments, Remaining Period, Three to Five Years Other Long-term Investments, Remaining Period, Three to Five Years 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 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 Amortization of other intangible assets Amortization of Other Intangible Assets Amortization of Other Intangible Assets Distinguished Transaction Distinguished Transaction [Member] Distinguished Transaction Intangible Asset, Indefinite-Lived, Class [Axis] Intangible Asset, Indefinite-Lived, Class [Axis] Impairments Goodwill, Impairment Loss 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 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 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 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 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 Interest payments received Interest Income, Operating, Interest Payments Received Interest Income, Operating, Interest Payments Received Secured Overnight Financing Rate (SOFR) Secured Overnight Financing Rate (SOFR) [Member] Total unconsolidated entities Total Unconsolidated Entities [Member] Total Unconsolidated Entities [Member] Vehicle receivables Collateralized Auto Loans [Member] Non-PEO NEO Non-PEO NEO [Member] Euro Interbank Offered Rate (EURIBOR) Euro Interbank Offered Rate (EURIBOR) [Member] Euro Interbank Offered Rate (EURIBOR) 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] BroadStreet SPV BroadStreet SPV [Member] BroadStreet SPV Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] WTM Partners Debt WTM Partners Debt [Member] WTM Partners Debt 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] >10 years Other Long-term Investments, Remaining Period, Greater than 10 Years Other Long-term Investments, Remaining Period, Greater than 10 Years 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] Beginning nominal value Ending nominal value Notes Receivable, Nominal Fair Value Disclosure Notes Receivable, Nominal Fair Value Disclosure BB Standard & Poor's, BB Rating [Member] Other liabilities Disposal Group, Including Discontinued Operation, Other Liabilities Reinsurance Disclosures [Abstract] Revenues: Revenues [Abstract] Percentage of borrowing base Debt Instrument, Borrowing Base, Percentage Debt Instrument, Borrowing Base, Percentage Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Gross realized investment losses Loss on Sale of Investments 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] Statistical Measurement [Domain] Statistical Measurement [Domain] Gross earned premiums Gross Premiums Earned Gross Premiums Earned Net change in premiums and commissions payable Increase (Decrease) in Premiums and Commissions Payable Increase (Decrease) in Premiums and Commissions Payable Ceded Ceded premiums earned Ceded Premiums Earned Document Fiscal Year Focus Document Fiscal Year Focus 2031 and thereafter Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Due after Year Four Financial Guarantee Insurance Contract, Premium Revenue to be Recognized, Due after Year Four 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] Recent transaction Valuation Technique, Recent or Expected Transaction Prices [Member] Valuation Technique, Recent or Expected Transaction Prices White Mountains’s common shares at $1 par value per share - authorized 50,000,000 shares; issued and outstanding 2,386,823 and 2,479,677 shares Common Stock, Value, Issued 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] Line of Credit Line of Credit [Member] Net Foreign Currency Gains (Losses) Equity Securities, FN-NI, Net Foreign Currency Gain (Loss) Equity Securities, FN-NI, Net Foreign Currency Gain (Loss) Restatement Determination Date Restatement Determination Date Accrued Expense Accrued Expense [Roll Forward] HG Global 2022 Senior Notes HG Global 2022 Senior Notes [Member] HG Global 2022 Senior Notes Amendment Flag Amendment Flag Debt and Equity Securities, FV-NI [Line Items] Debt and Equity Securities, FV-NI [Line Items] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Net investment income Net investment income Net Investment Income Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes [Abstract] Effects of Reinsurance Footnotes Debt instrument, period for interest rate increase Debt Instrument, Increase In Interest Rate, Period Debt Instrument, Increase In Interest Rate, Period Debt Covenant Period [Axis] Debt Covenant Period [Axis] Debt Covenant Period 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. Investments [Domain] Investments [Domain] Surplus Notes Surplus Notes [Member] Surplus Notes Subsidiary - WM Outrigger Re Subsidiary - WM Outrigger Re [Member] Subsidiary - WM Outrigger Re 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 PEO Total Compensation Amount PEO Total Compensation Amount Assets and goodwill acquired Business Combination, Recognized Identifiable Assets Acquired and Goodwill Business Combination, Recognized Identifiable Assets Acquired and Goodwill 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] Interest reserve account Short-term Investments, Interest Reserve Short-term Investments, Interest Reserve Direct Direct Direct Premiums Earned Measurement Input, Pre-Tax Discount Rate Measurement Input, Pre-Tax Discount Rate [Member] Measurement Input, Pre-Tax Discount Rate Net change in deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Percentage of par value of policy reinsured Percentage of Par Value of Policy Reinsured Percentage of par value of policy reinsured. Purchases of common equity securities Payments to Acquire Equity Securities and Equity Method Investments Payments to Acquire Equity Securities and Equity Method Investments Product and service revenues Revenue from Contract with Customer, Including Assessed Tax Broker commission expenses Broker Commission Expense Broker Commission Expense Insurance acquisition expenses Deferred Policy Acquisition Costs, Amortization Expense, Adjusted Deferred Policy Acquisition Costs, Amortization Expense, Adjusted Fair Value Equity method investments, fair value Equity Method Investments, Fair Value Disclosure 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 Kudu Credit Facility Kudu Credit Facility [Member] Kudu Credit Facility 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 Lloyd’s trust deposits Trust Deposits [Member] Trust Deposits Gross Unrealized Gains Equity Method Investment, Unrealized Gain Equity Method Investment, Unrealized Gain HG Global HG Global - Segment [Member] HG Global - Segment Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Subsidiary or Equity Method Investee, Sale of Stock, Type [Table] Secured credit facilities Number of Secured Credit Facilities Number of Secured Credit Facilities Third-party investment expenses Investment Income, Investment Expense 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 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 Redemption value Business Combination, Right to Sell, Redemption Value Business Combination, Right to Sell, Redemption Value Debt Covenant, Period 3 Debt Covenant, Period 3 [Member] Debt Covenant, Period 3 Noncontrolling Interests Noncontrolling Interest Disclosure [Text Block] Other intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill Ark Ark Subsidiary - Ark Insurance Holdings Limited [Member] Subsidiary - Ark Insurance Holdings Limited 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 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 Utilities Utilities Sector [Member] Utilities Sector 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] Lloyds Bank Lloyd's [Member] Lloyd's Solvency capital requirement Debt Instrument, Covenant, Solvency Capital, Minimum Debt Instrument, Covenant, Solvency Capital, Minimum Reportable Segments Segment Reporting, Policy [Policy Text Block] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Name Outstanding Recovery, Individual Name Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] 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] PassportCard/DavidShield PassportCard/DavidShield [Member] PassportCard/DavidShield 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 Amount borrowed Beginning balance Ending balance Long-Term Debt, Gross Purchase price (in dollars per share) Equity Method Investment, Share Price Equity Method Investment, Share Price Ceded Credit Risk, Secured Ceded Credit Risk, Secured [Member] Compensation Amount Outstanding Recovery Compensation Amount 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 Individual: Individual [Axis] Diluted earnings (losses) per share denominator (in shares) Weighted Average Number of Shares Outstanding, Diluted Entity Emerging Growth Company Entity Emerging Growth Company Schedule of Gross and net reinsurance recoverables by reinsurers ratings Ceded Credit Risk [Table Text Block] Communications Communications Sector [Member] Communications Sector Underlying Security Market Price Change Underlying Security Market Price Change, Percent Discount rate Equity Method Investment, Measurement Input Equity Method Investment, Measurement Input Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Other Intangible Assets Intangible Asset, Excluding Goodwill, before Accumulated Amortization [Abstract] Deferred tax liabilities Deferred Tax Liability Reversal Deferred Tax Liability Reversal Number of investments Number of Investments Number of Investments Partnership interest, carrying value Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount MediaAlpha Investment in MediaAlpha MediaAlpha [Member] MediaAlpha WM Outrigger Re WM Outrigger Re - 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Favorable prior loss reserve development Prior Year Claims and Claims Adjustment Expense Lloyds LOC Facility Lloyds LOC Facility [Member] Lloyds LOC Facility Post-closing purchase price adjustment Business Combination, Consideration Transferred, Post-Closing Purchase Price Adjustment Business Combination, Consideration Transferred, Post-Closing Purchase Price Adjustment Energy Energy Sector [Member] Income Statement [Abstract] Income Statement [Abstract] White Mountains’s common shareholders’ equity Equity, Attributable to Parent [Abstract] Document Transition Report Document Transition Report Quantitative information for Level 3 Fair Value Measurements Assets Fair Value, Option, Quantitative Disclosures [Line Items] Net realized and unrealized investment gains (losses) Net realized and unrealized investment (gains) losses Net realized and unrealized investment gains (losses) Gain (Loss) on Investments Ark Ark Insurance Holdings Limited - Segment [Member] Ark Insurance Holdings Limited - Segment Total revenues Revenues, Adjusted Revenues, Adjusted WTM Credit Facility WTM Credit Facility [Member] WTM Credit Facility Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Geographical presence Limited Liability Company or Limited Partnership, Geographical Presence Limited Liability Company or Limited Partnership, Geographical Presence Intangible Asset, Finite-Lived, Acquired [Table] Intangible Asset, Finite-Lived, Acquired [Table] Reinsurance recoverables Reinsurance recoverables Collateral Reinsurance Recoverables, Including Reinsurance Premium Paid City Area Code City Area Code Document Information [Table] Document Information [Table] Name Forgone Recovery, Individual Name Ark 2021 Subordinated Notes Tranche 3 Ark 2021 Subordinated Notes, Tranche 3 [Member] Ark 2021 Subordinated Notes, Tranche 3 Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Schedule of insured obligations Schedule of Municipal Guarantee Insured Obligations [Table Text Block] [Table Text Block] for Schedule of Municipal Guarantee Insured Obligations [Table] Other Other Funds [Member] Other Funds Entity [Domain] Entity [Domain] Equity Infrastructure Funds Equity Infrastructure Funds [Member] Equity Infrastructure Funds Other Operations Other Noncontrolling Interest [Member] Other Noncontrolling Interest Debt Covenant, Period 4 Debt Covenant, Period 4 [Member] Debt Covenant, Period 4 Other Significant Transactions [Line Items] Other Significant Transactions [Line Items] Other Significant Transactions [Line Items] Premiums, commissions and fees receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Business Combination, Recognized Assets Acquired, Premiums and Commissions Receivable Other Long-Term Investments Total Other Long-Term Investments [Member] Total Other Long-Term Investments Acquisition of businesses Goodwill, Acquired During Period Reinsured amount Reinsurance Agreements, Asset Limit 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(Loss) Entity Current Reporting Status Entity Current Reporting Status Net cash provided from (used for) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Other Standard & Poor's, Other [Member] Standard & Poor's, Other Net change in short-term investments Payments For (Proceeds From) Other Short-Term Investments Payments For (Proceeds From) Other Short-Term Investments Interest receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable Funds Held Under Reinsurance Agreements, Assets, Interest Receivable Performance Period [Axis] Performance Period [Axis] Disclosure of minimum performance period for right to receive performance share from the entity. 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Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Dividends declared per White Mountains’s common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Entity File Number Entity File Number Private debt instruments Private Debt Instrument [Member] Private Debt Instrument [Member] Schedule of contractual maturities of fixed maturities Investments Classified by Contractual Maturity Date [Table Text Block] Summary of loss and LAE reserve activity Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Dividends declared to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Credit card receivables Collateralized Credit Card Securities [Member] Ownership percentage of basic units outstanding Subsidiary, Ownership Percentage Of Basic Units Outstanding, Parent Subsidiary, Ownership Percentage Of Basic Units 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change in short-term investments — Kudu Increase (Decrease) in Short Term Investments Increase (Decrease) in Short Term Investments Entity Small Business Entity Small Business Comprehensive income (loss) attributable to White Mountains’s common shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Unearned insurance premiums Unearned Premiums Other intangible assets, weighted average useful life (in years) Intangible Asset, Finite-Lived, Acquired, Weighted-Average Amortization Period Kudu - Management Equity Incentive Subsidiary - Kudu Investment Management, Management Equity Incentive [Member] Subsidiary - Kudu Investment Management, Management Equity Incentive Net Foreign Currency Gains (Losses) Other Long-Term Investments, Net Foreign Currency Gain (Loss) Other Long-Term Investments, Net Foreign Currency Gain (Loss) 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] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Document Type Document Type Purchases of consolidated subsidiaries, net of cash acquired of $7.4 and $1.9 Payments to Acquire Businesses, Net of Cash Acquired Private common shares and units Private Equity Securities [Member] Private Equity Securities Limited Partnerships Limited Partnerships [Member] Limited Partnerships Loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Business combination, basic ownership interest acquired, percentage Business Combination, Voting Equity Interest Acquired, Basic, Percentage Business Combination, Voting Equity Interest Acquired, Basic, Percentage Subsidiary, ownership percentage economic interest falls Subsidiary, Ownership Percentage, Economic Interest Subsidiary, Ownership Percentage, Economic Interest Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Debt 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Average Compensation Actually Paid Amount Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] 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. Supplemental cash flows information: Supplemental Cash Flow Information [Abstract] PEO Name PEO Name Net paid loss and LAE Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Net realized and unrealized investment (losses) gains Total Investments Before TPC Providers - Investments [Member] Total Investments Before TPC Providers - Investments Remaining weighted average contract period (in years) Financial Guarantee Insurance Contracts, Remaining Weighted Average Contract Period Financial Guarantee Insurance Contracts, Remaining Weighted Average Contract Period Debt Covenant, Period 1 Debt Covenant, Period 1 [Member] Debt Covenant, Period 1 Cash acquired from acquisition Cash Acquired from Acquisition Bermuda BERMUDA AAA Standard & Poor's, AAA Rating [Member] Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Revenue Recognition Revenue [Policy Text Block] Gross Policyholder Benefits and Claims Incurred, Direct Minimum interest reserve period required Short-term Investments, Interest Reserve, Minimum Period Required Short-term Investments, Interest Reserve, Minimum Period Required 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. Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Equity interest percent of ownership sold Disposal Group, Including Discontinued Operation, Ownership Percentage Disposed Of Disposal Group, Including Discontinued Operation, Ownership Percentage Disposed Of Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] 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] Incremental proceeds received Disposal Group, Including Discontinued Operation, Incremental Proceeds Received Disposal Group, Including Discontinued Operation, Incremental Proceeds Received Comprehensive income (loss) Total comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Debt Instrument [Axis] Debt Instrument [Axis] Measurement Frequency [Domain] Measurement Frequency [Domain] Number of transactions Debt Instrument, Number of Transactions Debt Instrument, Number of Transactions Amortization of restricted share awards Share-Based Payment Arrangement, Noncash Expense 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. 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Schedule of Nominal and Fair Value of Notes Receivable [Table Text Block] Schedule of Nominal and Fair Value of Notes Receivable Insider Trading Arrangements [Line Items] Ark 2021 Subordinated Notes Ark 2021 Subordinated Notes [Member] Ark 2021 Subordinated Notes PassportCard/DavidShield DavidShield & PassportCard [Member] DavidShield & PassportCard [Member] Bishop Street Bishop Street [Member] Bishop Street Accounts receivable Accounts Receivable, after Allowance for Credit Loss, Current Insurance acquisition expenses Deferred Policy Acquisition Costs, Amortization Expense Distributed earnings - dividends declared and paid (in dollars per share) Earnings Per Share, Diluted, Distributed Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Industrial Commercial and Industrial Sector [Member] Temporary equity, beginning balance Temporary equity, ending balance Temporary Equity, Including Noncontrolling Interest Member surplus contributions, insurance premium charges, percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Member Surplus Contributions, Insurance Premium Charges, Percentage Business combination, right to purchase units Business Combination, Right To Purchase Units, Unit Price Multiplier Business Combination, Right To Purchase Units, Unit Price Multiplier 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 Contingent consideration Business Combination, Contingent Consideration, Liability Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] 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 Bermuda Stock Exchange Bermuda Stock Exchange [Member] Bermuda Stock Exchange Net investment income Net Investment Income, Adjusted Net Investment Income, Adjusted Equity Awards Adjustments Equity Awards Adjustments [Member] Agency relationships Agency Relationships [Member] Agency Relationships Assumed Premiums Earned Assumed Premiums Earned Assumed forfeitures Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Accrued Liability, Forfeited in Period Share-Based Compensation Arrangement by Share-Based Payment Award, Performance Shares, Accrued Liability, Forfeited in Period Balance Sheet Grouping, Financial Statement Captions Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Industry Sector [Domain] Industry Sector [Domain] Hawkeye Electric, LLC Hawkeye Electric, LLC [Member] Hawkeye Electric, LLC Other liabilities Business Combination, Recognized Liabilities Assumed, Other Liabilities Business Combination, Recognized Liabilities Assumed, Other Liabilities Geographic Distribution [Domain] Geographic Distribution [Domain] Income Tax Jurisdiction [Domain] Income Tax Jurisdiction [Domain] 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 Total Parent [Member] Short-term investments Short-Term Investments [Member] Equity Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Recognition of equity-based compensation expense of subsidiaries Temporary Equity, Share-Based Payment Arrangement, Other, Increase for Cost Recognition Temporary Equity, Share-Based Payment Arrangement, Other, Increase for Cost Recognition Forfeited (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period 5 – 10 years Other Long-term Investments, Remaining Period, Five to Ten Years Other Long-term Investments, Remaining Period, Five to Ten Years Schedule of noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] General and administrative expenses General and Administrative Expense Renegotiation period Reinsurance Agreement, Renegotiation Period Reinsurance Agreement, Renegotiation Period Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Income Tax Jurisdiction [Axis] Income Tax Jurisdiction [Axis] Private preferred securities Mandatorily Redeemable Preferred Securities [Member] Mandatorily Redeemable Preferred Securities Discontinued Operations and Disposal Groups [Abstract] Discontinued Operations and Disposal Groups [Abstract] Surplus notes Funds Held Under Reinsurance Agreements, Assets, Surplus Notes Funds Held Under Reinsurance Agreements, Assets, Surplus Notes 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] Materials Materials Sector [Member] Materials Sector Acquisition of noncontrolling interests Noncontrolling Interest, Increase from Business Combination Discount rate Other Long-Term Investments, Measurement Input Other Long-Term Investments, Measurement Input Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Business combination, consideration transferred Business Combination, Consideration Transferred Beginning fair value discount Ending fair value discount Notes Receivable, Interest Rate Fair Value Discount Notes Receivable, Interest Rate Fair Value Discount Entity Filer Category Entity Filer Category Change in fair value of BAM Surplus Notes 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 Distributed earnings - dividends declared and paid (in dollars per share) Earnings Per Share, Basic, Distributed Net Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral Reinsurance Recoverables, Including Reinsurance Premium Paid, Net of Collateral 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] Earned premiums: Premiums Earned, Net [Abstract] Schedule of net realized and unrealized investment gains and losses Gain (Loss) on Securities [Table Text Block] 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 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] 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] 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 Unamortized Issue Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grant Date Fair Value [Roll Forward] Other equity method eligible investments less than 50.0% Equity Method Eligible Investments, Less Than Fifty Percent [Member] Equity Method Eligible Investments, Less Than Fifty Percent Terminal Cash Flow Exit Multiple Measurement Input, Terminal Cash Flow Exit Multiple [Member] Measurement Input, Terminal Cash Flow Exit Multiple Discounted cash flow Valuation Technique, Discounted Cash Flow [Member] Statement [Table] Statement [Table] 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 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 New York Stock Exchange New York Stock Exchange [Member] New York Stock Exchange Credit Rating, Standard & Poor's [Axis] Credit Rating, Standard & Poor's [Axis] 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 Purchase price adjustments Disposal Group, Including Discontinued Operation, Incremental Proceeds Received, Purchase Price Adjustments Disposal Group, Including Discontinued Operation, Incremental Proceeds Received, Purchase Price Adjustments Geographic Distribution [Axis] Geographic Distribution [Axis] Services, Industrial and Consumer (WTM Partners) Services, Industrial and Consumer (WTM Partners) WTM Partners WTM Partners LLC - Segment [Member] WTM Partners LLC - Segment Other long-term investments Other Long-Term Investments [Member] Bamboo Group Bamboo Group [Member] Bamboo Group Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]