No allowance for credit losses has been previously recognized Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded Non-credit Impairments AOCI, Gain (Loss), Debt Securities, Available-for-Sale, with Allowance for Credit Loss, Parent [Member] Adjustments to reconcile net income (loss) to net cash provided (used) by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Medical Technology Liability Medical Technology and Life Sciences Product Liability [Member] Medical Technology and Life Sciences Product Liability Statistical Measurement [Domain] Statistical Measurement [Domain] Gross Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Losses (gains) recorded Investment Program, Proportional Amortization Method, Elected, Equity Method Investment, Income (Loss) from Tax Credit Investment Cover [Abstract] Cover [Abstract] Fixed maturities Debt Securities [Member] Increase (decrease) in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Trading Symbol Trading Symbol Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Net realized gains (losses), equity investments Equity Securities, FV-NI, Realized Gain (Loss) All Trading Arrangements All Trading Arrangements [Member] Net Investment Income Net Investment Income [Abstract] Pledged Status [Domain] Pledged Status [Domain] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Non-NEOs Non-NEOs [Member] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Tax credit carryforwards Tax Credit Carryforward, Amount Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] An allowance for credit losses was recorded in a previous period Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) Cash dividends declared per common share (in usd per share) Common Stock, Dividends, Per Share, Declared Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Derivative [Line Items] Derivative [Line Items] Other Commitments [Line Items] Other Commitments [Line Items] Pay vs Performance Disclosure [Line Items] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Short-term investments Short-Term Investments [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Net unfavorable development in workers' compensation business Net Unfavorable Development in Workers' Compensation Business Net Unfavorable Development in Workers' Compensation Business Residential mortgage-backed securities Residential mortgage-backed securities Residential Mortgage-Backed Securities [Member] Equity investments Payments to Acquire Equity Securities, FV-NI Derivative [Table] Derivative [Table] Term Loan Term Loan [Member] Term Loan Income Tax Contingency [Table] Income Tax Contingency [Table] Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Hedging Relationship [Domain] Hedging Relationship [Domain] Interest rate swaps Interest Rate Cash Flow Hedge Liability at Fair Value Fair Value Measurement Fair Value Disclosures [Text Block] Award Timing Disclosures [Line Items] Investment, Name [Domain] Investment, Name [Domain] Other Performance Measure, Amount Other Performance Measure, Amount Net impairment losses recognized in earnings Impairment Loss, Debt Securities, Portion Recognized in Earnings, Excluding Non-credit Portion Impairment Loss, Debt Securities, Portion Recognized in Earnings, Excluding Non-credit Portion Goodwill [Line Items] Goodwill [Line Items] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Cash Flow Hedging Cash Flow Hedging [Member] Gross realized (losses), available-for-sale fixed maturities Debt Securities, Available-for-Sale, Realized Loss Entity Tax Identification Number Entity Tax Identification Number Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Number of limited partners not permitted for redemption Number of Limited Partners not Permitted for Redemption Number of Limited Partners Not Permitted for Redemption Pledged Status [Axis] Pledged Status [Axis] Schedule of Outstanding Debt Schedule of Long-Term Debt Instruments [Table Text Block] Common shares acquired (in shares) Treasury Stock, Shares, Acquired Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Operating segments Operating Segments [Member] Net cash provided (used) by operating activities Net Cash Provided by (Used in) Operating Activities Distributed earnings from unconsolidated subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Deferred policy acquisition costs Deferred Policy Acquisition Cost Workers' Compensation Insurance Workers' Compensation Insurance Reporting Unit [Member] Workers' Compensation Insurance Reporting Unit Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Treasury Stock Treasury Stock, Common [Member] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] Other Limited Partnerships and Limited Liability Company, Greater Than 25% Ownership Other Limited Partnerships and Limited Liability Company, Greater Than 25 Percent Ownership [Member] Other Limited Partnerships and Limited Liability Company, Greater Than 25 Percent Ownership [Member] U.S. Treasury obligations U.S. Treasury obligations U.S. Treasury obligations US Treasury Securities [Member] Schedule of Components of AOCI, Net of Tax Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Inter-segment Eliminations Intersegment Eliminations [Member] Tax Credit Partnership Investment Tax Credit Carryforward Tax Credit Partnership Investment Tax Credit Carryforward [Member] Tax Credit Partnership Investment Tax Credit Carryforward Securities sold during the period Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Equity Components [Axis] Equity Components [Axis] Equity Components [Axis] Gain on fair value of contingent consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Asset Award Timing Method Award Timing Method [Text Block] Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Fixed maturities, available-for-sale, at fair value (amortized cost, $3,849,715 and $3,758,720, respectively; allowance for expected credit losses, $3,403 as of September 30, 2024 and $555 as of December 31, 2023) Fixed maturities, available-for-sale Estimated Fair Value Total Fair Value Debt Securities, Available-for-Sale Measurement Frequency [Axis] Measurement Frequency [Axis] Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Aggregate Notional Amount Derivative Asset, Notional Amount Insider Trading Policies and Procedures [Line Items] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value State and municipal bonds State and municipal bonds US States and Political Subdivisions Debt Securities [Member] Accounting Policies Use of Estimates, Policy [Policy Text Block] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Number of corporate bonds Impairment Loss, Debt Securities, Number Of Corporate Bonds Impairment Loss, Debt Securities, Number Of Corporate Bonds Adjustment to Compensation, Amount Adjustment to Compensation Amount Investments [Domain] Investments [Domain] Additional paid-in capital Additional Paid in Capital, Common Stock Compensation Amount Outstanding Recovery Compensation Amount 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] Entity Small Business Entity Small Business Company Selected Measure Amount Company Selected Measure Amount Trading securities, cost Debt Securities, Trading, Amortized Cost Premiums receivable, net (allowance for expected credit losses, $8,177 as of September 30, 2024 and $7,809 as of December 31, 2023) Premiums Receivable, Net BOLI BOLI Cash Surrender Value [Member] Tabular List, Table Tabular List [Table Text Block] Level 3 Fair Value, Inputs, Level 3 [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Weighted Average Weighted Average [Member] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Other Liabilities Other Liabilities [Member] Debt Securities, Available-for-Sale [Line Items] Debt Securities, Available-for-Sale [Line Items] Antidilutive shares excluded from computation of earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Asset Pledged as Collateral Asset Pledged as Collateral [Member] Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Line Items] Payment period for redemption of LP valued at NAV Payment Period for Redemption of LP Valued at NAV The number of days required for payment upon redemption of a particular LP that is valued at NAV. Fair Value Debt Securities, Available-for-Sale, Unrealized Loss Position Credit Facility [Domain] Credit Facility [Domain] Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents Business owned life insurance Life Insurance, Corporate or Bank Owned, Amount No impairment has been previously recognized Impairment Loss, Debt Securities, No Impairment Previously Recognized Impairment Loss, Debt Securities, No Impairment Previously Recognized Reserve for losses and loss adjustment expenses Balance, beginning of year Balance, end of period Liability for Claims and Claims Adjustment Expense Equity investments, at fair value (cost, $145,891 and $164,262, respectively) Equity investments Equity Securities, FV-NI, Current 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] Industrial Equity Industrial Securities [Member] Equity securities in financial sector. Net favorable development in workers' compensation business Net Favorable Development in Workers' Compensation Business Net Favorable Development in Workers' Compensation Business Policy liabilities and accruals Policy liabilities and accruals [Abstract] Policy liabilities and accruals. Goodwill Goodwill Goodwill Goodwill Tax Credit Partnerships Less Than 20% Ownership Tax Credit Partnerships Less Than 20% Ownership [Member] Tax Credit Partnerships Less Than 20% Ownership [Member] Consolidation Items [Axis] Consolidation Items [Axis] Underwriting, policy acquisition and operating expenses Underwriting Policy Acquisition And Operating Expense Underwriting, policy acquisition and operating expense. Treasury shares (in shares) Treasury Stock, Common, Shares Accrued liability for interest related to unrecognized tax benefits Unrecognized Tax Benefits, Interest on Income Taxes Accrued Bond funds Fixed Income Funds [Member] Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Product and Service [Domain] Product and Service [Domain] Employee Stock Option Share-Based Payment Arrangement, Option [Member] Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-Sale [Table] Number of instruments (in derivative instruments) Number of Interest Rate Derivatives Held Goodwill [Roll Forward] Goodwill [Roll Forward] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] (Favorable) unfavorable development of reserves established in prior years, net Prior Year Claims and Claims Adjustment Expense Discounted Cash Flows Valuation, Income Approach [Member] Other Proceeds from (Payments for) Other Financing Activities Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets and liabilities held at period-end Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) Investment in unconsolidated subsidiaries Investments In Unconsolidated Subsidiaries [Member] Investments In Unconsolidated Subsidiaries [Member] Borrowings (repayments) under Revolving Credit Agreement Repayments of Lines of Credit Net impairment losses recognized in earnings Net impairment losses recognized in earnings Net impairment losses recognized in earnings Impairment Loss, Debt Securities, Portion Recognized in Earnings Impairment Loss, Debt Securities, Portion Recognized in Earnings Security Exchange Name Security Exchange Name Basis spread on variable rate for debt Debt Instrument, Basis Spread on Variable Rate Schedule of Available-For-Sale Securities by Contractual Maturity Investments Classified by Contractual Maturity Date [Table Text Block] Award Type [Axis] Award Type [Axis] Schedule of a Roll Forward of Cumulative Credit Losses Recorded in Earnings Related to Impaired Debt Securities Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table Text Block] SPC dividend expense (income) SPC dividend (expense) income Segregated Portfolio Cells Dividend Expense (Income) Segregated Portfolio Cells Dividend Expense (Income) Corporate Debt Corporate debt Corporate Debt Securities [Member] Equity investments Proceeds from Sale and Maturity, Debt Security, Trading, and Equity Security, FV-NI, Held-for-Investment Purchases of capital assets Payments to Acquire Other Productive Assets Included in other comprehensive income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) Real estate, net Property, Plant and Equipment, Net Total Liabilities Liabilities Liabilities Unrealized Loss Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] Number of reporting units Number of Reporting Units Reinsurance related assets and liabilities Increase (Decrease) In Reinsurance Related Assets And Liabilities The change in reinsurance related assets and liabilities on the balance sheet. Common shares, par value (in usd per share) Common Stock, Par or Stated Value Per Share Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Commitments total Other Commitment Schedule of Other Information Regarding Available-For-Sale Securities Debt Securities, Available-for-Sale [Table Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table] Revolving Credit Facility Revolving Credit Facility [Member] Expiration Date Trading Arrangement Expiration Date Prepaid reinsurance premiums Prepaid Reinsurance Premiums Fair Value, Nonrecurring Fair Value, Nonrecurring [Member] Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Deferred tax (benefit) expense included in OCI Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent All other investments, primarily investment fund LPs/LLCs Other Limited Partnerships and Limited Liability Company [Member] Other Limited Partnerships and Limited Liability Company [Member] Operating expense Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Operating Expenses Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Operating Expenses Total Shareholder Return Amount Total Shareholder Return Amount Common shares, shares issued (in shares) Common Stock, Shares, Issued Entity [Domain] Entity [Domain] Short-term investments Other Short-Term Investments Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Additional credit losses recognized during the period, related to securities for which: Additional Credit Losses Recognized During the Period, Related to Securities [Abstract] Additional Credit Losses Recognized During the Period, Related to Securities Gross realized gains, available-for-sale fixed maturities Debt Securities, Available-for-Sale, Realized Gain Unrealized Loss Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Other assets Other assets Other Assets, Fair Value Disclosure Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Fair Value Asset Recurring Basis Still Held Unrealized Gain Loss Statement Of Income Extensible List Not Disclosed Flag Fair Value Asset Recurring Basis Still Held Unrealized Gain Loss Statement Of Income Extensible List Not Disclosed Flag Fair Value Asset Recurring Basis Still Held Unrealized Gain Loss Statement Of Income Extensible List Not Disclosed Flag Reserve for losses and loss adjustment expenses Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Liquidating investments remaining period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period Debt Instrument [Line Items] Debt Instrument [Line Items] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] Other assets Other Assets Common shares, shares authorized (in shares) Common Stock, Shares Authorized Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Insurance [Abstract] Insurance [Abstract] Diluted (in shares) Weighted average number of common shares outstanding, diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Other asset-backed securities Other asset-backed securities Asset-Backed Securities, Securitized Loans and Receivables [Member] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] AOCI Attributable to Parent, Net of Tax [Roll Forward] AOCI Attributable to Parent, Net of Tax [Roll Forward] Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Total Shareholders' Equity Beginning balance Ending balance Equity, Attributable to Parent Total gains (losses) realized and unrealized: Gains Losses Realized and Unrealized Liabilities [Abstract] Gains Losses Realized and Unrealized Liabilities Underwriting, policy acquisition and operating expenses: Underwriting, Policy Acquisition And Operating Expenses [Abstract] Underwriting, Policy Acquisition And Operating Expenses [Abstract] Balance Sheet Location [Axis] Statement of Financial Position Location, Balance [Axis] Deferred expense (benefit) Deferred income tax expense (benefit) Deferred Income Tax Expense (Benefit) Schedule of Goodwill Schedule of Goodwill [Table Text Block] Schedule of Investment in Unconsolidated Subsidiaries Schedule Of Investments In Unconsolidated Subsidiaries [Table Text Block] Schedule Of Investments In Unconsolidated Subsidiaries [Table Text Block] Net sales or (purchases) of fixed maturities, trading Proceeds from (Payments for) Trading Securities, Short-Term Change in unrealized holding gains (losses), trading fixed maturities Debt Securities, Trading, Unrealized Gain (Loss) Other investments Proceeds from Sale and Maturity of Other Investments Other comprehensive income (loss), after tax, net of reclassification adjustments Net OCI, current period Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Recurring Fair Value, Recurring [Member] Segment results Segment Profit (Loss) Segment Profit (Loss) 12 months or longer, Fair Value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer Utilities/Energy Utilities Energy Equity Securities [Member] Equity securities in the utilities and energy sectors. Other liabilities Increase (Decrease) in Other Operating Liabilities Debt less unamortized debt issuance costs Debt less unamortized debt issuance costs Long-Term Debt Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Document Fiscal Period Focus Document Fiscal Period Focus Transaction-related costs, net Business Combination, Acquisition Related Costs Common shares issued for compensation Stock Issued During Period, Value, Other All Executive Categories All Executive Categories [Member] Dilutive effect of securities (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Balance Sheet Location [Domain] Statement of Financial Position Location, Balance [Domain] Equity investments Equity Investments Equities Equity Securities [Member] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Assets Assets [Abstract] Deferred income tax credits Deferred Income Tax Credits Deferred Income Tax Credits Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Document Type Document Type Derivative Contract [Domain] Derivative Contract [Domain] Effective interest rate on debt Debt Instrument, Interest Rate, Effective Percentage Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Investment fees and expenses Investment Income, Investment Expense Net investment gains (losses) Gain (Loss) on Investments [Member] Net balance, beginning of year Net balance, end of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Available for sale securities, amortized cost Amortized Cost Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost Fair value of funded deferred compensation assets Deferred Compensation Plan Assets Depreciation and amortization, net of accretion Depreciation and amortization, net of accretion Depreciation, Amortization and Accretion, Net Maximum Maximum Maximum [Member] Weighted Average Cost of Capital Measurement Input Weighted Average Cost Of Capital [Member] Measurement Input Weighted Average Cost Of Capital Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Accumulated impairment loss Goodwill, Impaired, Accumulated Impairment Loss Schedule of Reserve for Losses and Loss Adjustment Expenses Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Investment Type [Axis] Schedule of Investment Income, Reported Amounts, by Category [Axis] Investment Type [Axis] Liabilities Liabilities [Abstract] Revenues Revenues [Abstract] Current Fiscal Year End Date Current Fiscal Year End Date Due in one year or less Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value Statistical Measurement [Axis] Statistical Measurement [Axis] PEO Name PEO Name Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Income (loss) before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Premium receivable, allowance for credit loss Premium Receivable, Allowance for Credit Loss Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Award Type [Domain] Award Type [Domain] Required FAL deposit Deposit Assets Stochastic Model/Discounted Cash Flows Valuation Technique, Monte Carlo Simulation And Discounted Cash Flows [Member] Valuation Technique, Monte Carlo Simulation And Discounted Cash Flows Name Outstanding Recovery, Individual Name Goodwill impairment Impairment losses Goodwill, Impairment Loss Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Deferred tax asset, net Deferred Income Tax Assets, Net Non-PEO NEO Non-PEO NEO [Member] Paid related to: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Short-term and other investments Investments, Fair Value Disclosure Fixed maturities, trading, at fair value (cost, $51,947 and $48,449, respectively) Fixed maturities, trading Debt Securities, Trading OCI, before reclassifications, net of tax OCI, before Reclassifications, Net of Tax, Attributable to Parent Additional Paid-in Capital Additional Paid-in Capital [Member] Hedging Relationship [Axis] Hedging Relationship [Axis] Award Timing Predetermined Award Timing Predetermined [Flag] Goodwill Goodwill Disclosure [Text Block] Dividends to shareholders Payments of Ordinary Dividends, Common Stock Other Limited Partnerships and Limited Liability Company Less than 25% Ownership Other Limited Partnerships and Limited Liability Company Less than 25 Percent Ownership [Member] Other Limited Partnerships and Limited Liability Company Less than 25 Percent Ownership [Member] Investment, Name [Axis] Investment, Name [Axis] Accounting Changes Adopted and Accounting Changes Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Debt Long-Term Debt [Text Block] Diluted (in usd per share) Earnings Per Share, Diluted Name Measure Name Net realized gains (losses), other investments Other Investments, Realized Gain (Loss) This item represents the gain (loss) realized during the period from the sale of Other Investments. Entity Interactive Data Current Entity Interactive Data Current Net investment gains (losses) Gain (Loss) On Investments, Excluding Business Combination, Contingent Consideration Arrangements, Change In Amount Of Contingent Consideration, Liability Gain (Loss) On Investments, Excluding Business Combination, Contingent Consideration Arrangements, Change In Amount Of Contingent Consideration, Liability Reserve for Losses and Loss Adjustment Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Current expense (benefit) Current Income Tax Expense (Benefit) Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Proceeds from sales (exclusive of maturities and paydowns) Proceeds from Sale of Debt Securities, Available-for-Sale Credit funds Structured Credit Fund [Member] Investment LPs/LLCs that seek to obtain superior risk-adjusted absolute returns by acquiring and actively managing a diversified portfolio of debt securities, including bonds, loans and other asset-backed instruments. Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 Number of operating segments Number of Operating Segments Number of LPs / LLCs with investment ownership percent over 25% (in businesses) Number of LPs / LLCs with Equity Method Investment Ownership Percentage Over 25 percent Number of LPs / LLCs with Equity Method Investment Ownership Percentage Over 25 percent Net losses and loss adjustment expenses Net losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Schedule of Fair Value of Assets Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Workers' compensation Workers' Compensation Alternative Market Business [Member] Activity associated with our Alternative Market Business of our Workers' Compensation segment. Tax Year 2019 Tax Year 2019 [Member] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Asset-backed Securities Asset-backed securities Asset-Backed Securities [Member] Share-based compensation Share-Based Payment Arrangement, Noncash Expense Included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Other Gain (Loss) on Sale of Other Investments Fair Value Measured at Net Asset Value Per Share Fair Value Measured at Net Asset Value Per Share [Member] Schedule of Credit Derivatives [Table] Credit Derivative [Table] Unrecognized Change in Defined Benefit Plan Liabilities Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Total income tax expense (benefit) Income Tax Expense (Benefit) Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Stated interest rate on debt Debt Instrument, Interest Rate, Stated Percentage Derivative, notional amount Derivative, Notional Amount Due after ten years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value Estimated fair value Derivative Asset Other commercial mortgage-backed securities Other Commercial Mortgage-Backed Securities [Member] Other Commercial Mortgage-Backed Securities Tax credits recognized Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization Expense Equity investments Equity Securities, FV-NI Traditional business Traditional Business [Member] Traditional Business [Member] Schedule of Derivative Instruments, Gain (Loss) Derivative Instruments, Gain (Loss) [Table Text Block] Other investments, measurement input Alternative Investment, Measurement Input Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Net cash provided (used) by investing activities Net Cash Provided by (Used in) Investing Activities Settlement received Syndicate Credit Agreement, Settlement Received Syndicate Credit Agreement, Settlement Received Corporate debt, multiple observable inputs Corporate Bonds Multiple Observable Inputs [Member] Debt securities that are issued by either a domestic or foreign corporate business entity with a promise of repayment and fair value has been determined using multiple observable inputs. Financial Equity Financial Securities [Member] Equity securities in financial sector. Total Liabilities and Shareholders' Equity Liabilities and Equity Investment ownership percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] All Adjustments to Compensation All Adjustments to Compensation [Member] Ceded premiums earned Ceded Premiums Earned Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Schedule of Equity Method Investments Equity Method Investments [Table Text Block] Due after five years through ten years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after 5 through 10 Years, Fair Value Intangible assets, net Other Intangible Assets, Net Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Tax Credit Carryforward [Axis] Tax Credit Carryforward [Axis] Contribution Certificates Long-term Debt and Contribution Certificates, Fair Value Disclosure Long-term Debt and Contribution Certificates, Fair Value Disclosure Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Legal Entity [Axis] Legal Entity [Axis] Investment in unconsolidated subsidiaries Investment in unconsolidated subsidiaries Investment In Unconsolidated Subsidiaries Investment In Unconsolidated Subsidiaries Operating lease liabilities arising from obtaining ROU assets Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Unearned premiums Increase (Decrease) in Unearned Premiums Underlying Security Market Price Change Underlying Security Market Price Change, Percent Tax Credit Carryforward, Name [Domain] Tax Credit Carryforward, Name [Domain] Measurement Input Type [Axis] Measurement Input Type [Axis] Individual: Individual [Axis] Assets measured at fair value Debt Securities, Available-for-Sale, Measurement Input Total gains (losses) realized and unrealized: Gains Losses Realized and Unrealized Assets [Abstract] Total gains (losses) realized and unrealized. Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Cash Flow Hedging Gains (Losses) Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Product and Service [Axis] Product and Service [Axis] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Entity Address, State or Province Entity Address, State or Province Statement [Line Items] Statement [Line Items] Measurement Input Type [Domain] Measurement Input Type [Domain] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] Portion of impairment losses recognized in other comprehensive income (loss) before taxes Corporate debt Impairment Losses, Investments, Portion In Other Comprehensive Loss, Before Tax, Portion Attributable To Parent Impairment Losses, Investments, Portion In Other Comprehensive Loss, Before Tax, Portion Attributable To Parent Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Number of investment in any entity or affiliates greater than 10% of stockholders' equity Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Schedule of Investments in LLCs and Limited Partnerships Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Expenses Operating Expenses [Abstract] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Financial Instruments [Domain] Financial Instruments [Domain] Operating expense Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liabilities, Operating Expenses Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liabilities, Operating Expenses Liabilities and Shareholders' Equity Liabilities and Equity [Abstract] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Included in earnings, as a part of: Gains Losses By Earnings Category Assets [Abstract] Included in Earnings, as a part of. Other investments Fair Value Alternative Investment Accordion feature borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity, Accordion Feature Line of Credit Facility, Maximum Borrowing Capacity, Accordion Feature Schedule of Fair Value Measurements - Level 3 Assets & Liabilities Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Other liabilities Other liabilities Other Liabilities, Fair Value Disclosure Derivative, fixed interest rate Derivative, Fixed Interest Rate Minimum Minimum Minimum [Member] Restatement Determination Date Restatement Determination Date Number of limited partners to allow redemption by special consent Number of Limited Partners to Allow Redemption by Special Consent Number of Limited Partners to Allow Redemption by Special Consent Fair Value Estimate of Fair Value Measurement [Member] Impairment losses Total impairment losses: Impairment Losses, Investments Impairment Losses, Investments Less unamortized debt issuance costs Unamortized Debt Issuance Expense Net investment income Net investment income Net investment income Net Investment Income Adoption Date Trading Arrangement Adoption Date Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Corporate debt, limited observable inputs Corporate debt, limited observable inputs Corporate Debt, Limited Observable Inputs [Member] Corporate Debt, Limited Observable Inputs [Member] Schedule of Available-For-Sale Fixed Maturities Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] DPAC amortization Deferred Policy Acquisition Costs, Amortization Expense Lloyd's Syndicates Lloyd's Syndicates [Member] Lloyd's Syndicates [Member] Operating lease ROU assets Operating Lease, Right-of-Use Asset Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] BOLI Investment Income (Loss), Bank Owned Life Insurance Investment Income (Loss), Bank Owned Life Insurance Segments [Axis] Segments [Axis] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Tax Period [Axis] Tax Period [Axis] Derivative Instrument [Axis] Derivative Instrument [Axis] Number of limited partners, redemption allowed with prior notice Number of Limited Partners, Redemption Allowed with Prior Notice Number of Limited Partners, Redemption Allowed with Prior Notice Total principal Long-Term Debt, Gross Return of invested capital from unconsolidated subsidiaries Proceeds from Equity Method Investment, Distribution, Return of Capital Exercise Price Award Exercise Price Earnings (Loss) Per Share Earnings Per Share [Text Block] Arrangement Duration Trading Arrangement Duration Threshold limit of investments based on shareholders' equity Threshold Limit Of Investments Based On Shareholders Equity Threshold Limit Of Investments Based On Shareholders Equity Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] Net favorable prior year reserve development Net Favorable Prior Year Reserve Development Net Favorable Prior Year Reserve Development Shareholders' Equity Equity, Attributable to Parent [Abstract] Less than 12 months, Fair Value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months Total net investment gains (losses) Net investment (gains) losses Gain (Loss) on Investments Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Total Investments Investments Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Other income (expense) Other income (expense) Other Income Segments [Domain] Segments [Domain] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] All Individuals All Individuals [Member] Surplus notes Surplus Notes Unrealized Investment Gains (Losses) AOCI, Gain (Loss), Debt Securities, Available-for-Sale, without Allowance for Credit Loss, Parent [Member] Reporting Unit [Axis] Reporting Unit [Axis] PEO PEO [Member] Premiums receivable Increase (Decrease) in Premiums Receivable Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Other net investment gains (losses) Other Net Realized Investment Gain (Loss) Other net realized investment gains (losses). Revolving Credit Agreement and Term Loan Debt Instrument, Fair Value Disclosure Net sales or maturities (purchases) of short-term investments Payments for (Proceeds from) Short-Term Investments Name Trading Arrangement, Individual Name Goodwill, gross Goodwill, Gross All other All Other [Member] Equity securities in industry sectors that are not listed separately. Qualified affordable housing project tax credit partnerships Qualified Affordable Housing Tax Credit Partnerships [Member] Qualified Affordable Housing Tax Credit Partnerships [Member] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Investment redemption notice period Fair Value, Investments, Entities that Calculate Net Asset Value, Investment Redemption, Notice Period, One Fund Fair Value, Investments, Entities that Calculate Net Asset Value, Investment Redemption, Notice Period, One Fund Gross Unrealized Gains Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax Private debt funds Private Debt Funds [Member] Private Debt Funds [Member] Treasury shares, at cost (12,606,968 shares as of each respective period end) Treasury Stock, Common, Value Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Other Commitments [Domain] Other Commitments [Domain] Net losses: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Assets: Assets: Assets, Fair Value Disclosure [Abstract] Number of limited partners in investment Number of Limited Partners In Investment Number of Limited Partners In Investment 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] Due after one year through five years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Local Phone Number Local Phone Number Net realized gains (losses), trading fixed maturities Debt Securities, Trading, Realized Gain (Loss) Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 Other investments, portion carried at fair value Other Investments, Fair Value Disclosure Other Investments, Fair Value Disclosure Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Schedule of Equity Method Investments [Table] Equity Method Investment [Table] Interest Expense Interest Expense [Member] Debt securities in unrealized loss position (in securities) Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Other assets Increase (Decrease) in Other Operating Assets Change in: Increase (Decrease) in Operating Capital [Abstract] Fair value Investments continuous unrealized loss position [Abstract] Investments Continuous Unrealized Loss Position. Allowance for expected credit losses Allowance for Expected Credit Losses Allowance for credit loss beginning balance Allowance for credit loss ending balance Debt Securities, Available-for-Sale, Allowance for Credit Loss PEO Total Compensation Amount PEO Total Compensation Amount Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Equity investments at fair value, cost Equity Securities, FV-NI, Cost Income Statement Location [Axis] Statement of Income Location, Balance [Axis] Fixed maturities, available-for-sale Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] U.S. Government-sponsored enterprise obligations U.S. Government-sponsored enterprise obligations US Government-sponsored Enterprises Debt Securities [Member] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Impairment has been previously recognized Impairment Loss, Debt Securities, Impairment Previously Recognized Impairment Loss, Debt Securities, Impairment Previously Recognized Common Stock Common Stock [Member] Measure: Measure [Axis] Unearned premiums Unearned Premiums Net effect of restricted and performance shares issued Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Workers' Compensation Insurance Workers' Compensation Insurance Segment Workers' Compensation Insurance Segment [Member] Workers' Compensation Insurance Segment Proceeds from sales or maturities of: Proceeds from Sale, Maturity and Collection of Investments [Abstract] Investments Investments [Abstract] Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Line of Credit Line of Credit [Member] Unfunded Contractual Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Entity Emerging Growth Company Entity Emerging Growth Company SPC U.S. federal income tax expense (benefit) SPC U.S. federal income tax benefit (expense) Segregated Portfolio Cells Income Tax Expense (Benefit) Segregated Portfolio Cells Income Tax Expense (Benefit) Total liabilities categorized within the fair value hierarchy Liabilities, Fair Value Disclosure Segment Reporting [Abstract] Segment Reporting [Abstract] Short-term investments, including Other Short-Term Investments and Other Invested Assets [Member] Short-term investments and Other invested assets [Member] Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Other investments Other Investments Other investments Other Investments [Member] Entity Central Index Key Entity Central Index Key Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Total Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Total authorizations which remain available for use Stock Repurchase Or Debt Repayment Remaining Authorized Amount The remaining amount authorized by an entity's Board of Directors for stock repurchase or debt repayment. Corporate Corporate Segment [Member] Shareholders' Equity Equity [Text Block] Unsettled security transactions, net change Unsettled security transactions net Unsettled security transactions, net. Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Equity Component [Domain] Equity Component [Domain] Financial Instrument [Axis] Financial Instrument [Axis] Segment Information Segment Reporting Disclosure [Text Block] Fair Value, Inputs, Level 1, 2 and 3 Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Second greatest unrealized loss position Second Greatest Unrealized Loss Position Second greatest unrealized loss position. Interest and dividend income, operating Interest and Dividend Income, Operating Agency commercial mortgage-backed securities Agency commercial mortgage-backed securities Government National Mortgage Association Certificates and Obligations (GNMA) [Member] Net investment gains (losses) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Increase (decrease) in fair value of contingent consideration issued in NORCAL acquisition Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Share-based compensation APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Entity Shell Company Entity Shell Company Measurement Basis [Axis] Measurement Basis [Axis] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Title Trading Arrangement, Individual Title Line of credit, borrowing capacity including accordion feature Line Of Credit Facility, Maximum Borrowing Capacity, Including Accordion Feature Line Of Credit Facility, Maximum Borrowing Capacity, Including Accordion Feature Receivable from reinsurers on paid losses and loss adjustment expenses Reinsurance Recoverable for Paid Claims and Claims Adjustments Consolidated Entities [Axis] Consolidated Entities [Axis] Statement [Table] Statement [Table] Income taxes payable Taxes Payable Measurement Frequency [Domain] Measurement Frequency [Domain] Other investments (at fair value, $2,084 and $62,604, respectively, otherwise at cost or amortized cost) Other Long-Term Investments Unrecognized tax benefits Unrecognized Tax Benefits Consolidation Items [Domain] Consolidation Items [Domain] City Area Code City Area Code Debt Securities, Available-for-sale, Allowance for Credit Loss [Table] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Fair value of contingent consideration in NORCAL acquisition Business Combination, Contingent Consideration, Liability Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Level 1 Fair Value, Inputs, Level 1 [Member] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Significant Non-Cash Transactions Noncash Investing and Financing Items [Abstract] Tax Period [Domain] Tax Period [Domain] Repurchase of common stock Payments for Repurchase of Common Stock Provision for income taxes: Income Tax Expense (Benefit), Continuing Operations [Abstract] Equity in earnings (loss) of unconsolidated subsidiaries Equity in (earnings) loss of unconsolidated subsidiaries Equity in earnings (loss) of unconsolidated subsidiaries Equity In Earnings (Loss) Of Unconsolidated Subsidiaries Equity In Earnings (Loss) Of Unconsolidated Subsidiaries NORCAL Group NORCAL Group [Member] NORCAL Group [Member] Schedule of Net Investment Income Investment Income [Table Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Financial assets: Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Expected gains reclassified Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net Schedule of Quantitative Information Regarding Level 3 Valuations Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Other Commitments [Axis] Other Commitments [Axis] Total paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Income Taxes Income Tax Disclosure [Text Block] Earnings (loss) per share: Earnings Per Share [Abstract] Earnings Per Share [Abstract] Schedule of Gross Premiums by Product Revenue from External Customers by Products and Services [Table Text Block] Equity [Abstract] Equity [Abstract] Policy acquisition costs, net of amortization (net deferral) Increase (Decrease) in Deferred Policy Acquisition Costs Retained earnings Retained Earnings (Accumulated Deficit) Net unfavorable development medical professional liability business Net Unfavorable Development Medical Professional Liability Business Net Unfavorable Development Medical Professional Liability Business Single greatest unrealized loss position Single Greatest Unrealized Loss Position Single greatest unrealized loss position. Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Net investment income (loss) Net Investment Income [Member] Net Investment Income Change in unrealized holding gains (losses), equity investments Equity Securities, FV-NI, Unrealized Gain (Loss) Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Debt securities in unrealized loss position as percentage of total debt securities held Number Of Debt Securities In Unrealized Loss Position As Percentage Of Debt Securities Number of debt securities in unrealized loss position as percentage of total debt securities. Total Policy Liabilities and Accruals Policy Liabilities Total policy liabilities. Equity Awards Adjustments Equity Awards Adjustments [Member] Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Roll Forward] Included in other comprehensive income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Operating lease liabilities Operating Lease, Liability Net cash provided (used) by financing activities Net Cash Provided by (Used in) Financing Activities Underlying Securities Award Underlying Securities Amount Performance Share Units Performance Shares [Member] Deferred compensation liabilities Deferred Compensation Liability, Current and Noncurrent Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Credit Facility [Axis] Credit Facility [Axis] Portfolio Cell Reinsurance Portfolio Cell Reinsurance [Member] Portfolio Cell Reinsurance Dividend declared Dividends, Common Stock, Cash Period to advance funds Syndicate Credit Agreement, Advance Funds Period Syndicate Credit Agreement, Advance Funds Period Amendment Flag Amendment Flag Carrying Value Reported Value Measurement [Member] Entity Registrant Name Entity Registrant Name Other commercial mortgage-backed securities Other commercial mortgage-backed securities Commercial Mortgage-Backed Securities [Member] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Receivable from reinsurers on unpaid losses and loss adjustment expenses Less reinsurance recoverables on unpaid losses and loss adjustment expenses Plus reinsurance recoverables on unpaid losses and loss adjustment expenses Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Fair Value as of Grant Date Award Grant Date Fair Value Fair value, net asset (liability) Fair Value, Net Asset (Liability) Level 2 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Interest Rate Swaps Interest Rate Swap [Member] Restatement Determination Date: Restatement Determination Date [Axis] (Increase) decrease in cash surrender value of BOLI Life Insurance, Corporate or Bank Owned, Change in Value Title of 12(b) Security Title of 12(b) Security Net investment gains (losses): Realized Investment Gains (Losses) [Abstract] Common shares (par value $0.01 per share, 100,000,000 shares authorized, 63,763,271 and 63,576,932 shares issued, respectively) Common Stock, Value, Issued Deferred tax expense (benefit) on accumulated other comprehensive income (loss) AOCI Tax, Attributable to Parent Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Issuers in unrealized loss position (in issuers) Available For Sale Securities In Unrealized Loss Positions Number Of Issuers Available For Sale Securities In Unrealized Loss Positions. Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets and liabilities held at period-end Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss) Other Payments for (Proceeds from) Other Investing Activities Balance beginning of period Balance ending of period Credit Losses Recognized In Earnings In Which A Portion Of Losses Are Recognized In Comprehensive Income (Loss), Credit Losses On Debt Securities Held Credit Losses Recognized In Earnings In Which A Portion Of Losses Are Recognized In Comprehensive Income (Loss), Credit Losses On Debt Securities Held Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Long/short equity funds Long/Short Equity Funds [Member] Long/Short Equity Funds [Member] Contribution Certificates Contribution Certificate due 2031 [Member] Contribution Certificate due 2031 Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] 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] Funding Commitments Funding Commitments [Member] Funding Commitments [Member] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Reporting Unit [Domain] Reporting Unit [Domain] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Basic (in usd per share) Earnings Per Share, Basic Operating expense Other Underwriting Expense Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Alternative market business Alternative Market Business [Member] Alternative Market Business [Member] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Schedule of Goodwill [Table] Goodwill [Table] Market Comparable Securities Valuation, Market Approach [Member] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Schedule of Investments Held in an Unrealized Loss Position Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table 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] Total Assets VIEs carrying value Assets Contingent consideration, maximum estimate Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Number of employees Workers Compensation Insurance Products, Maximum Employees Per Company Workers Compensation Insurance Products, Maximum Employees Per Company Investment in unconsolidated subsidiaries Payments For (Proceeds From) Investment In Unconsolidated Subsidiaries The cash outflow associated with an entity's investment in equity method investments. Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Other liabilities, measurement input Other Liabilities, Measurement Input Other Liabilities, Measurement Input Non government-backed Non Government-Backed [Member] Securities that are not backed by the U.S. government or U.S. government agencies. Termination Date Trading Arrangement Termination Date Number of limited partners to allow redemption by special consent of partner Number of Limited Partners to Allow Redemption by Special Consent of Partner Number of Limited Partners to Allow Redemption by Special Consent of Partner Non-public equity funds Private Equity Funds [Member] Credit Derivatives [Line Items] Credit Derivatives [Line Items] Residential mortgage-backed, other commercial mortgage-backed and other asset-backed securities Collateralized Mortgage-Backed Securities [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Total revenues Revenues Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales Other liabilities Other Liabilities Entity Address, City or Town Entity Address, City or Town PPM RRG PPM RRG [Member] PPM RRG Debt Instrument [Axis] Debt Instrument [Axis] Reinsurance premiums payable Reinsurance Payable Net income (loss) Net income (loss) Net Income (Loss) Attributable to Parent Trading Arrangement: Trading Arrangement [Axis] Schedule of Financial Instruments Not Measured at Fair Value Fair Value, by Balance Sheet Grouping [Table Text Block] Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Securities on deposit with state insurance departments Assets Held by Insurance Regulators Other Commitments [Table] Other Commitments [Table] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Business Acquisition [Axis] Business Acquisition [Axis] MPL MPL [Member] MPL Other asset-backed securities Other Asset-Backed Securities [Member] Other Asset-Backed Securities [Member] Equity securities measured at fair value Equity Securities, FV-NI, Measurement Input Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] 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 expenses Benefits, Losses and Expenses Comparability Adjustment Measurement Input, Comparability Adjustment [Member] Entity File Number Entity File Number Income tax benefit (expense) Income Tax Expense (Benefit), Excluding Business Acquisition Tax Effect Income Tax Expense (Benefit), Excluding Business Acquisition Tax Effect Minimum period for claims resolution Minimum Period For Claims Resolution Minimum Period For Claims Resolution Liabilities: Liabilities: Liabilities, Fair Value Disclosure [Abstract] Consolidated Entities [Domain] Consolidated Entities [Domain] Document Fiscal Year Focus Document Fiscal Year Focus Entity Address, Address Line One Entity Address, Address Line One Weighted average number of common shares outstanding: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Common shares reacquired Common shares, at cost Treasury Stock, Value, Acquired, Cost Method Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Change in unrealized holding gains (losses), convertible securities, carried at fair value Unrealized Gain (Loss) on Hybrid Instrument, Net Dividends to shareholders Dividends, Common Stock Total assets categorized within the fair value hierarchy Assets, Fair Value Disclosure Name Forgone Recovery, Individual Name Other Other Premiums [Member] Other Premiums [Member] Document Period End Date Document Period End Date Schedule of Interest Rate Swaps Schedule of Interest Rate Derivatives [Table Text Block] Strategy focused funds Strategy Focused Fund [Member] Strategy Focused Fund [Member] Other, net Other Noncash Income (Expense) Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Insider Trading Arrangements [Line Items] Segment reconciling items Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Number of proassurance's insurance subsidiaries Number of Insurance Subsidiaries Number of Insurance Subsidiaries PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Effect of dilutive shares on earnings (loss) per share (in usd per share) Income (Loss) from Continuing Operations, Per Diluted Share Purchases of: Payments to Acquire Investments [Abstract] Adjustment to Compensation: Adjustment to Compensation [Axis] Document Transition Report Document Transition Report Document Quarterly Report Document Quarterly Report Current year Current Year Claims and Claims Adjustment Expense Qualifying Cash Flow Hedges - Gains (Losses) Reclassified from AOCI, net of tax, to Earnings Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Financial liabilities: Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Interest expense Interest expense Interest Expense, Operating Capital contribution received from (return of capital to) external segregated portfolio cell participants External Capital Contribution Received (Return Of Capital) For Segregated Portfolio Cells External Capital Contribution Received (Return Of Capital) For Segregated Portfolio Cells Other investments Payments to Acquire Other Investments Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] Accumulated other comprehensive income (loss) (net of deferred tax expense (benefit) of ($33,848) and ($55,738), respectively) Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Current Reporting Status Entity Current Reporting Status Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Schedule of Net Realized Investment Gains (Losses) Gain (Loss) on Securities [Table Text Block] Retained Earnings Retained Earnings [Member] Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Basic (in shares) Weighted average number of common shares outstanding, basic (in shares) Weighted Average Number of Shares Outstanding, Basic Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Segregated Portfolio Cell Reinsurance Segregated Portfolio Cell Reinsurance Segment Segregated Portfolio Cell Reinsurance [Member] Segregated Portfolio Cell Reinsurance [Member] Net premiums earned Net premiums earned Segment net premiums earned Premiums Earned, Net 12 months or longer, Unrealized Loss Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Summary of reserve for losses and loss adjustment expenses Liability for Claims and Claims Adjustment Expense [Roll Forward] Liability for Claims and Claims Adjustment Expense Restricted Share Units Restricted Stock Units (RSUs) [Member] Increase (Decrease) in Stockholders' Equity: Increase (Decrease) in Stockholders' Equity [Roll Forward] Executive Category: Executive Category [Axis] Specialty P&C Specialty P&C Specialty P&C Segment Specialty Property and Casualty [Member] Specialty Property and Casualty [Member] Name Awards Close in Time to MNPI Disclosures, Individual Name Revolving Credit Agreement Revolving Credit Agreement [Member] Revolving Credit Agreement Less than 12 months, Unrealized Loss Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Entity Filer Category Entity Filer Category Amounts reclassified from AOCI, net of tax Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Gross premiums earned: Gross Premiums Earned Amount of premiums earned before premiums ceded to other entities, including directed and assumed premiums earned. Income Statement Location [Domain] Statement of Income Location, Balance [Domain] Fixed maturities, available-for-sale Purchases Payments to Acquire Debt Securities, Available-for-Sale Company Selected Measure Name Company Selected Measure Name