Statistical Measurement [Domain] Statistical Measurement [Domain] Schedule of Other Information Regarding Available-For-Sale Securities Debt Securities, Available-for-Sale [Table Text Block] Medical Technology Liability Medical Technology and Life Sciences Product Liability [Member] Medical Technology and Life Sciences Product Liability (Favorable) unfavorable development of reserves established in prior years, net Prior Year Claims and Claims Adjustment Expense Award Timing Predetermined Award Timing Predetermined [Flag] Deferred policy acquisition costs and amortization Deferred Policy Acquisition Costs, Amortization Expense, Excluding Accrued Interest Entity File Number Entity File Number Additional Paid-in Capital Additional Paid-in Capital [Member] Cash Flow Hedge- Interest Rate Swaps Interest Rate Swap [Member] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Included in other comprehensive income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Tabular List, Table Tabular List [Table Text Block] Unearned premiums Unearned Premiums Accounting Changes Adopted and Accounting Changes Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Pledged Status [Axis] Pledged Status [Axis] Schedule of Financial Instruments Not Measured at Fair Value Fair Value, by Balance Sheet Grouping [Table Text Block] Prior year claims and claims adjustment expense, purchase accounting amortization Prior Year Claims and Claims Adjustment Expense, Purchase Accounting Amortization Prior Year Claims and Claims Adjustment Expense, Purchase Accounting Amortization Fixed maturities Debt Securities [Member] Gross Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Measurement Basis [Axis] Measurement Basis [Axis] Basic (in usd per share) Earnings Per Share, Basic Adjustment to Compensation: Adjustment to Compensation [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Liabilities: Liabilities, Fair Value Disclosure [Abstract] Term Loan Term Loan [Member] Term Loan Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Securities sold during the period Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Current expense (benefit) Current Income Tax Expense (Benefit) Measurement Input Type [Domain] Measurement Input Type [Domain] Schedule of Available-For-Sale Fixed Maturities Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] Adjustment to Compensation, Amount Adjustment to Compensation Amount Change in unrealized gains (losses) included in earnings Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) Other assets Increase (Decrease) in Other Operating Assets Share-based compensation Share-Based Payment Arrangement, Noncash Expense Aggregate Notional Amount Derivative Asset, Notional Amount Proceeds from sale of intangible assets Proceeds from Sale of Intangible Assets Employee Stock Option Share-Based Payment Arrangement, Option [Member] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Unrealized Investment Gains (Losses) AOCI, Gain (Loss), Debt Securities, Available-for-Sale, without Allowance for Credit Loss, Parent [Member] Total Shareholders' Equity Beginning balance Ending balance Equity, Attributable to Parent OCI, before reclassifications, net of tax OCI, before Reclassifications, Net of Tax, Attributable to Parent Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-Sale [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Insider Trading Policies and Procedures [Line Items] 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] Common shares, shares authorized (in shares) Common Stock, Shares Authorized Number of limited partners not permitted for redemption Number of Limited Partners not Permitted for Redemption Number of Limited Partners Not Permitted for Redemption Financial assets: Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Total Liabilities and Shareholders' Equity Liabilities and Equity Recurring Fair Value, Recurring [Member] Unrecognized Change in Defined Benefit Plan Liabilities Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Deferred policy acquisition costs Deferred Policy Acquisition Cost Liabilities and Shareholders' Equity Liabilities and Equity [Abstract] Distributed earnings from unconsolidated subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Premiums receivable, net (allowance for expected credit losses, $7,957 and $8,141, respectively) Premiums Receivable, Net Cover [Abstract] Cover [Abstract] Amounts reclassified from AOCI, net of tax Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] 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] Maximum Maximum Maximum [Member] Schedule of Credit Derivatives [Table] Credit Derivative [Table] Schedule of Equity Method Investments [Table] Equity Method Investment [Table] Fair Value Debt Securities, Available-for-Sale, Unrealized Loss Position Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Debt Instrument [Line Items] Debt Instrument [Line Items] 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] Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss) Other investments Payments to Acquire Other Investments Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table] Treasury Stock Treasury Stock, Common [Member] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Pay vs Performance Disclosure [Line Items] Operating lease liabilities Operating Lease, Liability Unearned premiums Increase (Decrease) in Unearned Premiums Comparability Adjustment Measurement Input, Comparability Adjustment [Member] Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Liabilities Liabilities [Abstract] Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Number of corporate bonds Impairment Loss, Debt Securities, Number Of Corporate Bonds Impairment Loss, Debt Securities, Number Of Corporate Bonds Income (loss) before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Entity Shell Company Entity Shell Company Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Proceeds from sale of property Proceeds from Sale of Property, Plant, and Equipment 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] Reserve for losses and loss adjustment expenses Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Operating segments Operating Segments [Member] Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Intangible assets, net Other Intangible Assets, Net Company Selected Measure Amount Company Selected Measure Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Interest expense Interest expense Interest Expense, Operating Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Measurement Input Type [Axis] Measurement Input Type [Axis] Name Measure Name Net investment gains (losses) Net Investment Gains (Losses) [Member] Net Investment Gains (Losses) Document Fiscal Period Focus Document Fiscal Period Focus No impairment has been previously recognized Impairment Loss, Debt Securities, No Impairment Previously Recognized Impairment Loss, Debt Securities, No Impairment Previously Recognized Award Timing Method Award Timing Method [Text Block] 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 Award Type [Axis] Award Type [Axis] Policy liabilities and accruals Policy liabilities and accruals [Abstract] Policy liabilities and accruals. Tax Credit Partnerships Less Than 20% Ownership Tax Credit Partnerships Less Than 20% Ownership [Member] Tax Credit Partnerships Less Than 20% Ownership [Member] Debt less unamortized debt issuance costs Debt less unamortized debt issuance costs Long-Term Debt Legal Entity [Axis] Legal Entity [Axis] Policy acquisition costs, net of amortization (net deferral) Increase (Decrease) in Deferred Policy Acquisition Costs Total Assets VIEs carrying value Assets Investment, Name [Domain] Investment, Name [Domain] Trading Symbol Trading Symbol Operating expenses Underwriting Policy Acquisition And Operating Expense Underwriting, policy acquisition and operating expense. Non-credit Impairments AOCI, Gain (Loss), Debt Securities, Available-for-Sale, with Allowance for Credit Loss, Parent [Member] Entity Address, City or Town Entity Address, City or Town Operating Activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Change in unrealized holding gains (losses), trading fixed maturities Debt Securities, Trading, Unrealized Gain (Loss) Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Investment in unconsolidated subsidiaries Investments In Unconsolidated Subsidiaries [Member] Investments In Unconsolidated Subsidiaries [Member] Restricted Share Units Restricted Stock Units (RSUs) [Member] Income Statement Location [Domain] Statement of Income Location, Balance [Domain] Non-public equity funds Private Equity Funds [Member] 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 Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] SPC dividend expense (income) SPC dividend (expense) income Segregated Portfolio Cells Dividend Expense (Income) Segregated Portfolio Cells Dividend Expense (Income) Assets: Assets: Assets, Fair Value Disclosure [Abstract] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Entity Emerging Growth Company Entity Emerging Growth Company Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Operating lease liabilities arising from obtaining ROU assets Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Debt Securities, Available-for-Sale [Line Items] Debt Securities, Available-for-Sale [Line Items] Borrowings (repayments) under Revolving Credit Agreement Repayments of Lines of Credit 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] Prepaid reinsurance premiums Prepaid Reinsurance Premiums Level 2 Fair Value, Inputs, Level 2 [Member] Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Business Combination [Domain] Business Combination [Domain] Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Asset Pledged as Collateral Asset Pledged as Collateral [Member] 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. Insurance [Abstract] Insurance [Abstract] Other assets Other assets Other Assets, Fair Value Disclosure PEO PEO [Member] Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Tax credits recognized Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization Expense All other investments, primarily investment fund LPs/LLCs Other Limited Partnerships and Limited Liability Company [Member] Other Limited Partnerships and Limited Liability Company [Member] Schedule of Available-For-Sale Securities by Contractual Maturity Investments Classified by Contractual Maturity Date [Table Text Block] Fixed maturities, trading, at fair value (cost, $12,304 and $52,493, respectively) Fixed maturities, trading Debt Securities, Trading Significant Non-Cash Transactions Noncash Investing and Financing Items [Abstract] Due in one year or less Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value Fair value, net asset (liability) Fair Value, Net Asset (Liability) Change in: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] 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] Nature of Operation, Product Information, Concentration of Risk [Table] Nature of Operation, Product Information, Concentration of Risk [Table] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] 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 Retained Earnings Retained Earnings [Member] Total paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Effective interest rate on debt Debt Instrument, Interest Rate, Effective Percentage Pledged Status [Domain] Pledged Status [Domain] 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 Entity Address, Postal Zip Code Entity Address, Postal Zip Code Restatement Determination Date Restatement Determination Date Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Estimated fair value Derivative Asset Total income tax expense (benefit) Income tax expense (benefit) Income Tax Expense (Benefit) 12 months or longer, Fair Value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer Basis spread on variable rate for debt Debt Instrument, Basis Spread on Variable Rate Right to receive cash per share of common stock converted (in USD per share) Business Combination, Common Stock Conversion, Per Share Business Combination, Common Stock Conversion, Per Share Purchases of: Payments to Acquire Investments [Abstract] Interest Expense Interest Expense [Member] Other assets Other Assets Fair Value, Nonrecurring Fair Value, Nonrecurring [Member] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Short-term and other investments Investments, Fair Value Disclosure Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Product and Service [Domain] Product and Service [Domain] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 Net losses: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Effect of dilutive shares on earnings (loss) per share (in usd per share) Income (Loss) from Continuing Operations, Per Diluted Share Net investment income Net investment income Net investment income Net Investment Income Equity securities measured at fair value Equity Securities, FV-NI, Measurement Input Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Total liabilities categorized within the fair value hierarchy Liabilities, Fair Value Disclosure Derivative, notional amount Derivative, Notional Amount Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Net cash impact of deconsolidation of closed years of account from Lloyd's Syndicates operations Closed Block Operations, Net Results Purchases of capital assets Payments to Acquire Other Productive Assets 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] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] City Area Code City Area Code Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Receivable from reinsurers on paid losses and loss adjustment expenses Reinsurance Recoverable for Paid Claims and Claims Adjustments All Trading Arrangements All Trading Arrangements [Member] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [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] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Reinsurance premiums payable Reinsurance Payable Segment results Segment Profit (Loss) Segment Profit (Loss) Utilities/Energy Utilities Energy Equity Securities [Member] Equity securities in the utilities and energy sectors. Investing Activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Net cash provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Proceeds from sales (exclusive of maturities and paydowns) Proceeds from Sale of Debt Securities, Available-for-Sale Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Equity investments Equity Securities, FV-NI Other liabilities Other Liabilities Goodwill Goodwill Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year PEO Total Compensation Amount PEO Total Compensation Amount Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Total net investment gains (losses) Gain (Loss) on Investments BOLI BOLI Cash Surrender Value [Member] Income taxes receivable Income Taxes Receivable Equity Components [Axis] Equity Components [Axis] Equity Components [Axis] Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Number of reportable segments Number of Reportable Segments Available for sale securities, amortized cost Amortized Cost Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost Net losses and loss adjustment expenses Net losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Other Performance Measure, Amount Other Performance Measure, Amount Entity Address, State or Province Entity Address, State or Province Interest and dividend income, operating Interest and Dividend Income, Operating Debt Securities, Available-for-sale, Allowance for Credit Loss [Table] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Individual: Individual [Axis] Financial liabilities: Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Additional paid-in capital Additional Paid in Capital, Common Stock Investments Investments [Abstract] Entity [Domain] Entity [Domain] Net cash provided by (used in) investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation 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] Document Fiscal Year Focus Document Fiscal Year Focus Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Operating lease ROU assets Operating Lease, Right-of-Use Asset Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Fair value of funded deferred compensation assets Deferred Compensation Plan Assets Entity Interactive Data Current Entity Interactive Data Current 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] Carrying Value Reported Value Measurement [Member] Transaction-related costs, net Business Combination, Acquisition-Related Cost, Expense Equity [Abstract] Equity [Abstract] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] 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. 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] 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 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. Segment Information Segment Reporting Disclosure [Text Block] 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 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] U.S. Treasury obligations U.S. Treasury obligations U.S. Treasury obligations US Treasury Securities [Member] Reductions due to: Debt Securities, Available-for-Sale, Allowance for Credit Loss, Reductions [Abstract] Debt Securities, Available-for-Sale, Allowance for Credit Loss, Reductions Other Commitments [Line Items] Other Commitments [Line Items] Increase (Decrease) in Stockholders' Equity: Increase (Decrease) in Stockholders' Equity [Roll Forward] Unrealized Loss Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] Workers' compensation Workers' Compensation Alternative Market Business [Member] Activity associated with our Alternative Market Business of our Workers' Compensation segment. Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 Proceeds from sales or maturities of: Proceeds from Sale, Maturity and Collection of Investments [Abstract] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Weighted Average Weighted Average [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] Deferred tax asset, net Deferred Income Tax Assets, Net Assets measured at fair value Debt Securities, Available-for-Sale, Measurement Input Schedule of Components of AOCI, Net of Tax Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Agency commercial mortgage-backed securities Agency commercial mortgage-backed securities Government National Mortgage Association Certificates and Obligations (GNMA) [Member] Accident Year [Domain] Accident Year [Domain] Accident Year [Domain] Trading securities, cost Debt Securities, Trading, Amortized Cost Entity Central Index Key Entity Central Index Key PEO Name PEO Name Total revenues Revenues Foreign Currency Forwards Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 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] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Other commercial mortgage-backed securities Other Commercial Mortgage-Backed Securities [Member] Other Commercial Mortgage-Backed Securities Residential mortgage-backed securities Residential mortgage-backed securities Residential Mortgage-Backed Securities [Member] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Total Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Traditional business Traditional Business [Member] Traditional Business [Member] Revolving Credit Facility Revolving Credit Facility [Member] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Arrangement Duration Trading Arrangement Duration Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Securities on deposit with state insurance departments Assets Held by Insurance Regulators Premium receivable, allowance for credit loss Premium Receivable, Allowance for Credit Loss Segments [Axis] Segments [Axis] 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. Exercise Price Award Exercise Price Entity Filer Category Entity Filer Category Local Phone Number Local Phone Number Financial Equity Financial Securities [Member] Equity securities in financial sector. Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Investment ownership percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Other Payment for (Proceeds from) Other Investing Activity Assets Assets [Abstract] Credit Facility [Axis] Credit Facility [Axis] Contribution Certificates Long-term Debt and Contribution Certificates, Fair Value Disclosure Long-term Debt and Contribution Certificates, Fair Value Disclosure Underlying Security Market Price Change Underlying Security Market Price Change, Percent Balance Sheet Location [Axis] Statement of Financial Position Location, Balance [Axis] Other Commitments [Axis] Other Commitments [Axis] Debt Instrument [Axis] Debt Instrument [Axis] Investment in unconsolidated subsidiaries Investment in unconsolidated subsidiaries Investment In Unconsolidated Subsidiaries Investment In Unconsolidated Subsidiaries Fair Value Measurement Fair Value Disclosures [Text Block] Credit Facility [Domain] Credit Facility [Domain] Shareholders' Equity Equity, Attributable to Parent [Abstract] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Total expenses Benefits, Losses and Expenses Entity Address, Address Line One Entity Address, Address Line One Expected gains reclassified Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net 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] Stated interest rate on debt Debt Instrument, Interest Rate, Stated Percentage Investment, Name [Axis] Investment, Name [Axis] Schedule of Gross Premiums by Product Revenue from External Customers by Products and Services [Table Text Block] Investments [Domain] Investments [Domain] 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 Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Other investments Proceeds from Sale and Maturity of Other Investments 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 Other investments (at fair value, $2,137 and $2,077, respectively, otherwise at cost or amortized cost) Other Long-Term Investments 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 Net Investment Income Net Investment Income [Abstract] Other Liabilities Other Liabilities [Member] Depreciation and amortization, net of accretion Depreciation and amortization, net of accretion Depreciation, Amortization and Accretion, Net Fair Value as of Grant Date Award Grant Date Fair Value Equity investments at fair value, cost Equity Securities, FV-NI, Cost Entity Registrant Name Entity Registrant Name Liquidating investments remaining period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Deferred compensation liabilities Deferred Compensation Liability, Current and Noncurrent Other investments, measurement input Alternative Investment, Measurement Input Fair Value, Inputs, Level 1, 2 and 3 Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Schedule of Outstanding Debt Schedule of Long-Term Debt Instruments [Table Text Block] Included in earnings, as a part of: Gains Losses By Earnings Category Assets [Abstract] Included in Earnings, as a part of. Dilutive effect of securities (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Accordion feature borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity, Accordion Feature Line of Credit Facility, Maximum Borrowing Capacity, Accordion Feature Document Quarterly Report Document Quarterly Report Less than 12 months, Fair Value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 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 Total assets categorized within the fair value hierarchy Assets, Fair Value Disclosure Impairment losses Total impairment losses: Impairment Losses, Investments Impairment Losses, Investments Debt securities in unrealized loss position (in securities) Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions Schedule of Quantitative Information Regarding Level 3 Valuations Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Corporate debt, limited observable inputs Corporate debt, limited observable inputs Corporate Debt, Limited Observable Inputs [Member] Corporate Debt, Limited Observable Inputs [Member] Number of operating segments Number of Operating Segments U.S. Government-sponsored enterprise obligations U.S. Government-sponsored enterprise obligations US Government-sponsored Enterprises Debt Securities [Member] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Lloyd's Syndicates Lloyd's Syndicates [Member] Lloyd's Syndicates [Member] Equity investments Payments to Acquire Equity Securities, FV-NI Schedule of Investments in LLCs and Limited Partnerships Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Investment Type [Axis] Schedule of Investment Income, Reported Amounts, by Category [Axis] Investment Type [Axis] Segment reconciling items Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member] Total Investments Investments Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Operating expense Other Underwriting Expense BOLI Investment Income (Loss), Bank Owned Life Insurance Investment Income (Loss), Bank Owned Life Insurance Restatement Determination Date: Restatement Determination Date [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 Net effect of restricted and performance shares issued Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Non-PEO NEO Non-PEO NEO [Member] Allowance for 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Increase (decrease) in fair value of contingent consideration issued in NORCAL acquisition Contingent Consideration Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease) Weighted average number of common shares outstanding: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Commitments total Other Commitment All other All Other [Member] Equity securities in industry sectors that are not listed separately. 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Treasury shares (in shares) Treasury Stock, Common, Shares Earnings (loss) per share: Earnings Per Share [Abstract] Earnings Per Share [Abstract] Paid related to: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Consolidation Items [Domain] Consolidation Items [Domain] 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. Return of invested capital from unconsolidated subsidiaries Proceeds from Equity Method Investment, Distribution, Return of Capital Total Policy Liabilities and Accruals Policy Liabilities Total policy liabilities. Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Other asset-backed securities Other asset-backed securities Asset-Backed Securities, Securitized Loans and Receivables [Member] No allowance for credit losses has been previously recognized Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded Tax credit carryforwards Tax Credit Carryforward, Amount Portfolio Cell Reinsurance Portfolio Cell Reinsurance [Member] Portfolio Cell Reinsurance Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Award Timing Disclosures [Line Items] Total principal Long-Term Debt, Gross Schedule of Net Realized Investment Gains (Losses) Gain (Loss) on Securities [Table Text Block] Measurement Frequency [Domain] Measurement Frequency [Domain] Net balance, beginning of year Net balance, end of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Unfunded Contractual Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Shareholders' Equity Equity [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 Net income (loss) Net income (loss) Net Income (Loss) Attributable to Parent Inter-segment Eliminations Intersegment Eliminations [Member] Schedule of Investments Held in an Unrealized Loss Position Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Expiration Date Trading Arrangement Expiration Date Other Commitments [Domain] Other Commitments [Domain] Interest rate swaps Interest Rate Cash Flow Hedge Liability at Fair 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. Other liabilities Increase (Decrease) in Other Operating Liabilities Product Information [Line Items] Product Information [Line Items] Change in unrealized holding gains (losses), equity investments Equity Securities, FV-NI, Unrealized Gain (Loss) 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 Other investments Other Investments Other investments Other Investments [Member] Schedule of Equity Method Investments Equity Method Investments [Table Text Block] Real estate, net Property, Plant and Equipment, Net Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Long/short equity funds Long/Short Equity Funds [Member] Long/Short Equity Funds [Member] Income taxes payable Taxes Payable Adoption Date Trading Arrangement Adoption Date Reclassifications Reclassification, Comparability Adjustment [Policy Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Contribution Certificates Contribution Certificate due 2031 [Member] Contribution Certificate due 2031 Entity Current Reporting Status Entity Current Reporting Status Funding Commitments Funding Commitments [Member] Funding Commitments [Member] Derivative [Line Items] Derivative [Line Items] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Consolidated Entities [Domain] Consolidated Entities [Domain] Line of Credit Line of Credit [Member] Retained earnings Retained Earnings (Accumulated Deficit) Corporate Corporate Segment [Member] Financial Instrument [Axis] Financial Instrument [Axis] Alternative market business Alternative Market Business [Member] Alternative Market Business [Member] Statement of Financial Position [Abstract] Statement of Financial Position [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] Executive Category: Executive Category [Axis] Foreign Currency Forwards Foreign Exchange Forward [Member] 12 months or longer, Unrealized Loss Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss (Increase) decrease in cash surrender value of BOLI Life Insurance, Corporate or Bank Owned, Change in Value Market Comparable Securities Valuation, Market Approach [Member] Current Fiscal Year End Date Current Fiscal Year End Date Cash Flow Hedging Gains (Losses) Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Less than 12 months, Unrealized Loss Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 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] Credit Derivatives [Line Items] Credit Derivatives [Line Items] Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Net investment gains (losses): Realized Investment Gains (Losses) [Abstract] Statement [Table] Statement [Table] Number of employees Workers Compensation Insurance Products, Maximum Employees Per Company Workers Compensation Insurance Products, Maximum Employees Per Company Securities sold during the period Debt Securities, Available-for-Sale, Allowance for Credit Loss, Sell before Recovery 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. Adjustments to reconcile net income (loss) to net cash provided (used) by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Deferred expense (benefit) Deferred income tax expense (benefit) Deferred Income Tax Expense (Benefit) Financing Activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Non government-backed Non Government-Backed [Member] Securities that are not backed by the U.S. government or U.S. government agencies. Unrealized Loss Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Schedule of Fair Value of Assets Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Equity investments, at fair value (cost, $120,665 and $145,411, respectively) Equity investments Equity Securities, FV-NI, Current Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Antidilutive shares excluded from computation of earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Schedule of Fair Value Measurements - Level 3 Assets & Liabilities Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] 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] All Adjustments to Compensation All Adjustments to Compensation [Member] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Amendment Flag Amendment Flag Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales Reserve for losses and loss adjustment expenses Balance, beginning of year Balance, end of period Liability for Claims and Claims Adjustment Expense Net investment (gains) losses Net investment gains (losses) Gain (Loss) on Sale of Investments, Excluding Sale of Properties Gain (Loss) on Sale of Investments, Excluding Sale of Properties Treasury shares, at cost (12,606,968 shares as of each respective period end) Treasury Stock, Common, Value PPM RRG PPM RRG [Member] PPM RRG Other Commitments [Table] Other Commitments [Table] 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) Termination Date Trading Arrangement Termination Date Net cash provided by (used in) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Measure: Measure [Axis] Number of instruments (in derivative instruments) Number of Interest Rate Derivatives Held Foreign currency exchange rate (gains) losses Realized Gain (Loss), Foreign Currency Transaction, before Tax Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] MPL MPL [Member] MPL Basic (in shares) Weighted average number of common shares outstanding, basic (in shares) Weighted Average Number of Shares Outstanding, Basic Other asset-backed securities Other Asset-Backed Securities [Member] Other Asset-Backed Securities [Member] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Income tax benefit (expense) Income Tax Expense (Benefit), Excluding Business Acquisition Tax Effect Income Tax Expense (Benefit), Excluding Business Acquisition Tax Effect Segment Reporting [Abstract] Segment Reporting [Abstract] Minimum period for claims resolution Minimum Period For Claims Resolution Minimum Period For Claims Resolution Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Due after ten years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value Debt Disclosure [Abstract] Debt Disclosure [Abstract] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Investment fees and expenses Investment Income, Investment Expense Entity Tax Identification Number Entity Tax Identification Number Scenario [Domain] Scenario [Domain] Other Other Premiums [Member] Other Premiums [Member] Net realized gains (losses), trading fixed maturities Debt Securities, Trading, Realized Gain (Loss) Strategy focused funds Strategy Focused Fund [Member] Strategy Focused Fund [Member] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Consolidated Entities [Axis] Consolidated Entities [Axis] Common shares, par value (in usd per share) Common Stock, Par or Stated Value Per Share Common shares, shares issued (in shares) Common Stock, Shares, Issued Other commercial mortgage-backed securities Other commercial mortgage-backed securities Commercial Mortgage-Backed Securities [Member] Net realized gains (losses), equity investments Equity Securities, FV-NI, Realized Gain (Loss) Accounting Policies Use of Estimates, Policy [Policy Text Block] Number of 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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 Ceded premiums earned Ceded Premiums Earned Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Fixed maturities, available-for-sale Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale Expenses Operating Expenses [Abstract] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Segregated Portfolio Cell Reinsurance Segregated Portfolio Cell Reinsurance Segment Segregated Portfolio Cell Reinsurance [Member] Segregated Portfolio Cell Reinsurance [Member] Included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 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 Proceeds from sale of capital assets Proceeds from Sale of Other Productive Assets Statement [Line Items] Statement [Line Items] Asset-backed Securities Asset-backed securities Asset-Backed Securities [Member] Less unamortized debt issuance costs Unamortized Debt Issuance Expense Consolidation Items [Axis] Consolidation Items [Axis] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents Cash and Cash Equivalent Specialty P&C Specialty P&C Specialty P&C Segment Specialty Property and Casualty [Member] Specialty Property and Casualty [Member] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Revolving Credit Agreement Revolving Credit Agreement [Member] Revolving Credit Agreement Common Stock Common Stock [Member] Fixed maturities, available-for-sale, at fair value (amortized cost, $3,798,274 and $3,803,835, respectively; allowance for expected credit losses, $4,031 and $3,399, respectively) Fixed maturities, available-for-sale Estimated Fair Value Total Fair Value Debt Securities, Available-for-Sale Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Corporate Debt Corporate debt Corporate Debt Securities [Member] Gross premiums earned: Gross Premiums Earned Amount of premiums earned before premiums ceded to other entities, including directed and assumed premiums earned. Increase (decrease) in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation