Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Fixed maturities Debt Securities [Member] Short-term investments Short-term Investments [Member] Non Government-Backed Non Government-Backed [Member] Securities that are not backed by the U.S. government or U.S. government agencies. Corporate Bond Securities Corporate Bond Securities [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum Maximum [Member] Investment, Name [Axis] Investment, Name [Axis] Investment, Name [Domain] Investment, Name [Domain] Tax Credit Partnerships Almost 100% Ownership Tax Credit Partnerships Almost 100% Ownership [Member] Tax Credit Partnerships with Almost a 100% Ownership Percentage [Member] Tax Credit Partnerships Less Than 20% Ownership Tax Credit Partnerships Less Than 20% Ownership [Member] Tax Credit Partnerships Less Than 20% Ownership [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] 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] Reclassifications from AOCI to net income: Schedule of Available-for-sale Securities [Line Items] Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit Threshold Limit Of Investments Based On Shareholders Equity Threshold Limit Of Investments Based On Shareholders Equity Threshold limit of investments based on shareholders equity. Securities on deposit with state insurance departments Assets Held by Insurance Regulators Required FAL Deposit Deposit Assets Number of debt securities in unrealized loss position Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions 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. Number of issuers in unrealized loss position Available For Sale Securities In Unrealized Loss Positions Number Of Issuers Available For Sale Securities In Unrealized Loss Positions. Single greatest unrealized loss position Single Greatest Unrealized Loss Position Single greatest unrealized loss position. Second greatest unrealized loss position Second Greatest Unrealized Loss Position Second greatest unrealized loss position. Business owned life insurance cost Business Owned Life Insurance Cost Business Owned Life Insurance Cost. Number of tax credit partnerships almost 100% ownership percentage Number of Tax Credit Partnerships Almost 100% Ownership Percentage Number of Tax Credit Partnerships Almost 100% Ownership Percentage Investment ownership percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Investment In Unconsolidated Subsidiaries, Ownership Percentage Number of LPs / LLCs with investment ownership percent over 25% 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 Investment in unconsolidated subsidiaries Investment In Unconsolidated Subsidiaries Investment In Unconsolidated Subsidiaries OTTI losses Other than Temporary Impairment Losses, Investments Net impairment losses recognized in earnings Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net Portion of noncredit OTTI losses recognized in OCI Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Portion Attributable to Parent Organization, Consolidation and Presentation of Financial Statements [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2016-15 Accounting Standards Update 2016-15 [Member] Accounting Standards Update 2016-01 Accounting Standards Update 2016-01 [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Number of reportable segments Number of Reportable Segments Distributions from unconsolidated subsidiaries, investing activities Proceeds from Equity Method Investment, Distribution, Return of Capital Distributed earnings from unconsolidated subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Return On Invested Capital From Unconsolidated Subsidiaries Cumulative-effect adjustment adoption of ASU Cumulative Effect of New Accounting Principle in Period of Adoption Changes in fair value of investments previously reported as cost method investments Equity Securities, FV-NI, Gain (Loss) Equity Securities, FV-NI, Gain (Loss) Reclassification from AOCI to retained earnings for stranded tax effects Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Statement of Comprehensive Income [Abstract] Revenues Revenues [Abstract] Net premiums earned Premiums Earned, Net, Property and Casualty Net investment income Net Investment Income Equity in earnings (loss) of unconsolidated subsidiaries Equity In Earnings (Loss) Of Unconsolidated Subsidiaries Equity In Earnings (Loss) Of Unconsolidated Subsidiaries Net realized investment gains (losses): Realized Investment Gains (Losses) [Abstract] OTTI losses Portion of OTTI losses recognized in other comprehensive income before taxes Net impairment losses recognized in earnings Other net realized investment gains (losses) Other net realized investment gains losses Other net realized investment gains (losses). Total net realized investment gains (losses) Gain (Loss) on Investments Other income Other Income Total revenues Revenues Expenses Operating Expenses [Abstract] Net losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Underwriting, policy acquisition and operating expenses Underwriting, Policy Acquisition And Operating Expenses [Abstract] Underwriting, Policy Acquisition And Operating Expenses [Abstract] Operating expense Operating Expenses DPAC amortization Deferred Policy Acquisition Costs, Amortization Expense Segregated portfolio cells dividend expense (income) Segregated Portfolio Cells Dividend Expense (Income) Segregated Portfolio Cells Dividend Expense (Income) Interest expense Interest Expense Total expenses Benefits, Losses and Expenses Income before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Provision for income taxes Income Tax Expense (Benefit), Continuing Operations [Abstract] Current expense (benefit) Current Income Tax Expense (Benefit) Deferred expense (benefit) Deferred Income Tax Expense (Benefit) Total income tax expense (benefit) Income Tax Expense (Benefit) Net income Net Income (Loss) Attributable to Parent Other comprehensive income (loss), after tax, net of reclassification adjustments Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Earnings per share Earnings Per Share [Abstract] Basic (USD per share) Earnings Per Share, Basic Diluted (USD per share) Earnings Per Share, Diluted Weighted average number of common shares outstanding: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Cash dividends declared per common share (USD per share) Common Stock, Dividends, Per Share, Declared Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Adjustments for Error Corrections [Axis] Adjustments for Error Corrections [Axis] Adjustments for Error Correction [Domain] Adjustments for Error Correction [Domain] Previously Unrecognized Segregated Portfolio Cells (SPC) Dividend Expense [Member] Previously Unrecognized Segregated Portfolio Cells (SPC) Dividend Expense [Member] Previously Unrecognized Segregated Portfolio Cells (SPC) Dividend Expense [Member] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments Operating Segments [Member] Inter-segment Eliminations Intersegment Eliminations [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Specialty P&C Specialty Property and Casualty [Member] Specialty Property and Casualty [Member] Workers' Compensation Workers Compensation [Member] Workers Compensation [Member] Lloyd's Syndicates Lloyd's Syndicates [Member] Lloyd's Syndicates [Member] Corporate Corporate Segment [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Net premiums earned Net investment income Equity in earnings (loss) of unconsolidated subsidiaries Income (Loss) from Equity Method Investments Net realized gains (losses) Other income (expense) Net losses and loss adjustment expenses Underwriting, policy acquisition and operating expenses Underwriting Policy Acquisition And Operating Expense Underwriting, policy acquisition and operating expense. Segregated portfolio cells dividend (expense) income Interest expense Income tax benefit (expense) Depreciation and amortization, net of accretion Depreciation, Depletion and Amortization Pre-tax expense Tax Impact Equity [Abstract] Share-based compensation expense Allocated Share-based Compensation Expense Related tax benefits Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Fair Value Disclosures [Abstract] Fair Value Measurement Fair Value Disclosures [Text Block] Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Equity Components [Axis] Common Stock Common Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Treasury Stock Treasury Stock [Member] Accounting Standards Update 2018-02 Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2016-09 Accounting Standards Update 2016-09 [Member] Statement [Line Items] Statement [Line Items] Increase (Decrease) in Stockholders' Equity: Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance Stockholders' Equity Attributable to Parent Common shares issued for compensation and effect of shares reissued to stock purchase plan Stock Issued During Period, Value, Other Share-based compensation Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Net effect of restricted and performance shares issued Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Dividends to shareholders Dividends, Common Stock Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Net income Ending Balance Segment Information Segment Reporting Disclosure [Text Block] Insurance [Abstract] Liabilities for Guarantees on Long-Duration Contracts [Table] Liabilities for Guarantees on Long-Duration Contracts [Table] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Retroactive Insurance Contract Retroactive Insurance Contract [Member] Retroactive Insurance Contract [Member] Retroactive Insurance Contract, Prospective Coverage Retroactive Insurance Contract, Prospective Coverage [Member] Retroactive Insurance Contract, Prospective Coverage [Member] Retroactive Insurance Contract, Retroactive Coverage Retroactive Insurance Contract, Retroactive Coverage [Member] Retroactive Insurance Contract, Retrospective Coverage [Member] Liabilities for Guarantees on Long-Duration Contracts [Line Items] Liabilities for Guarantees on Long-Duration Contracts [Line Items] Reserve for losses and loss adjustment expenses Liability for Claims and Claims Adjustment Expense Reserve for Losses and Loss Adjustment Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Earnings Per Share Earnings Per Share [Text Block] Information regarding sales and purchases of available-for-sale securities Schedule of Sales And Purchases Of Available For Sale Securities [Abstract] Schedule of Sales And Purchases Of Available For Sale Securities [Abstract] Proceeds from sales (exclusive of maturities and paydowns) Proceeds from Sale of Available-for-sale Securities Purchases Payments to Acquire Available-for-sale Securities, Debt Other than Temporary Impairment, Credit Losses Recognized in Earnings: Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Balance beginning of period Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held No OTTI has been previously recognized Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment Securities sold during the period (realized) Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold Balance ending of period Commitments and Contingencies Disclosure [Abstract] Other Commitments [Table] Other Commitments [Table] Other Commitments [Axis] Other Commitments [Axis] Other Commitments [Domain] Other Commitments [Domain] Commitment to Provide Capital Commitment to Provide Capital [Member] Commitment to Provide Capital [Member] Other Commitments [Line Items] Other Commitments [Line Items] Commitments Total Other Commitment Current lending capacity under revolving credit agreement Syndicate Credit Agreement, Current Lending Capacity Amount of current lending capacity under the Syndicate credit agreement without considering any amounts currently outstanding under the facility. Interest rate on revolving credit agreement Syndicate Credit Agreement, Interest Rate Syndicate Credit Agreement, Interest Rate on Lendings Unused commitments to extend credit Unused Commitments to Extend Credit Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative [Table] Derivative [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest Rate Cap Interest Rate Cap [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other assets Other Assets [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Mortgage Loans Mortgages [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] LIBOR London Interbank Offered Rate (LIBOR) [Member] Derivative [Line Items] Derivative [Line Items] Premium paid on derivative for right to receive cash payments Debt Instrument, Unamortized Premium Derivative asset, notional amount Derivative Asset, Notional Amount Floor interest rate on derivative Derivative, Floor Interest Rate Basis spread on variable rate for debt Debt Instrument, Basis Spread on Variable Rate Cap interest rate on derivative Derivative, Cap Interest Rate Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Mortgage fund Mortgage Fund [Member] Mortgage Fund [Member] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Level 3 Fair Value, Inputs, Level 3 [Member] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] U.S. Treasury obligations US Treasury Securities [Member] U.S. Government-sponsored enterprise obligations US Government-sponsored Enterprises Debt Securities [Member] State and municipal bonds US States and Political Subdivisions Debt Securities [Member] 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. Corporate debt, limited observable inputs Corporate Debt, Limited Observable Inputs [Member] Corporate Debt, Limited Observable Inputs [Member] Residential mortgage-backed securities Residential Mortgage Backed Securities [Member] Agency commercial mortgage-backed securities Government National Mortgage Association Certificates and Obligations (GNMA) [Member] Other commercial mortgage-backed securities Commercial Mortgage Backed Securities [Member] Other asset-backed securities Asset-backed Securities, Securitized Loans and Receivables [Member] Corporate Debt Corporate Debt Securities [Member] Financial Equity Financial Securities [Member] Equity securities in financial sector. Utilities/Energy Utilities Energy Equity Securities [Member] Equity securities in the utilities and energy sectors. Consumer oriented Consumer Oriented [Member] Equity securities in consumer oriented sector. Industrial Equity Industrial Securities [Member] Equity securities in financial sector. Bond funds Fixed Income Funds [Member] All other All Other [Member] Equity securities in industry sectors that are not listed separately. Other investments Other Investments [Member] Equities Equity Securities [Member] Investment in unconsolidated subsidiaries Investments In Unconsolidated Subsidiaries [Member] Investments In Unconsolidated Subsidiaries [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] Assets: Assets, Fair Value Disclosure [Abstract] Fixed maturities, available for sale Available-for-sale Securities Trading Securities Trading Securities Equity investments Equity Securities, FV-NI Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Short-term and other investments Investments, Fair Value Disclosure Other assets Other Assets, Fair Value Disclosure Total assets categorized within the fair value hierarchy Assets, Fair Value Disclosure Fair Value Alternative Investment, Fair Value Disclosure Total assets at fair value Assets, Fair Value Disclosure, Including NAV Value Not Within Fair Value Hierarchy Assets, Fair Value Disclosure, Including NAV Value Not Within Fair Value Hierarchy Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Share Units Restricted Stock Units (RSUs) [Member] Performance Share Units Performance Shares [Member] Performance Shares Granted in 2015 Performance Shares Granted in 2015 [Member] Performance Shares Granted in 2015 [Member] Minimum Minimum [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Authorized common stock (in shares) Common Stock, Shares Authorized Authorized preferred stock (in shares) Preferred Stock, Shares Authorized Quarterly dividend declared, per share (USD per share) Dividend declared Dividends, Common Stock, Cash 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. Common shares acquired (in shares) Treasury Stock, Shares, Acquired Awards in period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Fair value of each unit awarded (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Award vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Award payout rate Share Based Compensation Arrangement By Share Based Payment Award Payout Rate Share based compensation arrangement by share based payment award payout rate. Shares issued in period for share-based compensation (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period Bonus compensation shares issued (in shares) Bonus Compensation Shares Issued Bonus compensation shares issued. Non-credit loss, net of tax Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Purchase Match Units Employee Purchase Plan [Member] Employee Purchase Plan Weighted average number of common shares outstanding, basic (in shares) Dilutive securities (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Weighted average number of common shares outstanding, diluted (in shares) Effect of dilutive shares on earnings per share (usd per share) Income (Loss) from Continuing Operations, Per Diluted Share Antidilutive shares excluded from computation of earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Fair Value Inputs, Assets, Quantitative Information [Table] Fair Value Inputs, Assets, Quantitative Information [Table] Corporate debt, limited observable inputs Residential mortgage-backed and other asset-backed securities Residential Mortgage-Based And Other Asset-Backed Securities [Member] Residential Mortgage-Based And Other Asset-Backed Securities [Member] Other investments Valuation Technique [Axis] Valuation Technique [Axis] Valuation Technique [Domain] Valuation Technique [Domain] Market Comparable Securities Market Approach Valuation Technique [Member] Discounted Cash Flows Income Approach Valuation Technique [Member] Weighted Average Weighted Average [Member] Fair Value Inputs, Assets, Quantitative Information [Line Items] Fair Value Inputs, Assets, Quantitative Information [Line Items] Fair Value at Comparability Adjustment Fair Value Inputs, Comparability Adjustments Amortized cost and estimated fair value of available-for-sale fixed maturities and equity securities Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Schedule of available for sale securities by contractual maturity Investments Classified by Contractual Maturity Date [Table Text Block] Investments held in an unrealized loss position Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block] Information regarding sales and purchases of available-for-sale securities Available-for-sale Securities [Table Text Block] Net Investment Income Investment Income [Table Text Block] Investments in unconsolidated subsidiaries Schedule Of Investments In Unconsolidated Subsidiaries [Table Text Block] Schedule Of Investments In Unconsolidated Subsidiaries [Table Text Block] Equity in Earnings (Loss) of Unconsolidated Subsidiaries [Table Text Block] Equity Method Investments [Table Text Block] Net realized investment gains (losses) Gain (Loss) on Investments [Table Text Block] Other than temporary impairment, credit losses recognized in earnings Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table Text Block] Healthcare professional liability Healthcare Professional Liability [Member] Healthcare Professional Liability [Member] Legal professional liability Legal Professional Liability [Member] Legal Professional Liability [Member] Medical technology liability Medical Technology and Life Sciences Product Liability [Member] Medical Technology and Life Sciences Product Liability [Member] Other Other Premiums [Member] Other Premiums [Member] Traditional business Workers' Compensation Traditional Business [Member] Activity associated with our traditional workers' compensation business. Alternative market business Workers' Compensation Alternative Market Business [Member] Activity associated with our Alternative Market Business of our Workers' Compensation segment. Property and casualty Property, Liability and Casualty Insurance Product Line [Member] Gross premiums earned Gross Premiums Earned Amount of premiums earned before premiums ceded to other entities, including directed and assumed premiums earned. Ceded premiums earned Ceded Premiums Earned Premiums assumed under Quota Share Agreement Premiums Assumed Under Quota Share Agreement Premiums Assumed Under Quota Share Agreement Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Reclassifications Reclassification, Policy [Policy Text Block] Retroactive Insurance Contracts Retroactive Insurance Contracts [Policy Text Block] [Text Block] Retroactive Insurance Contracts [Policy Text Block] [Text Block] Accounting Changes Adopted and Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Fair Value Policy Fair Value Measurement, Policy [Policy Text Block] Shareholders' Equity Stockholders' Equity Note Disclosure [Text Block] Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Private debt funds Private Debt Funds [Member] Private Debt Funds [Member] Long equity fund Private Equity Funds, Foreign [Member] Long/short equity funds Long/Short Equity Funds [Member] Long/Short Equity Funds [Member] Non-public equity funds Private Equity Funds [Member] Multi-strategy fund of funds Multi-Strategy Fund of Funds [Member] Investment LPs/LLCs that are structured to build and manage low volatility, multi-manager portfolios that have little or no correlation to broader fixed income and equity security markets. Credit fund 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. Long/short commodities fund Long/Short Commodity Fund [Member] Long/Short Commodity Fund Strategy focused fund Strategy Focused Fund [Member] Strategy Focused Fund [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] Unfunded Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Schedule of Share-based compensation expense and related tax benefits Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Reclassification adjustments related to available-for-sale securities Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] SPC dividends payable Segregated Portfolio Cell Dividends Payable The cumulative undistributed earnings of segregated portfolio cells that are contractually payable to external preferred shareholders of the cells Unpaid dividends Dividends Payable All other Other Liabilities Not Detailed Other Liabilities Not Detailed Total other liabilities Other Liabilities Corporate debt Available-for-sale securities cost basis and estimated fair value Available-for-sale Securities, Debt Maturities [Abstract] Fixed maturities, available for sale Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract] Amortized Cost Available-for-sale Securities, Amortized Cost Basis Due in one year or less Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Fair Value Due after one year through five years Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value Due after five years through ten years Available-for-sale Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value Due after ten years Available-for-sale Securities, Debt Maturities, Rolling after Year Ten, Fair Value Estimated Fair Value Net impairment losses recognized in earnings Gross realized gains, available-for-sale fixed maturities Available-for-sale Securities, Gross Realized Gains Gross realized (losses), available-for-sale fixed maturities Available-for-sale Securities, Gross Realized Losses Net realized gains (losses), trading fixed maturities Trading Securities, Realized Gain (Loss) Net realized gains (losses), equity investments Equity Investments, Realized Gain (Loss) Equity Investments, Realized Gain (Loss) 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. Change in unrealized holding gains (losses), trading fixed maturities Trading Securities, Change in Unrealized Holding Gain (Loss) Change in unrealized holding gains (losses), equity investments Equity Securities, Change in Unrealized Holding Gain (Loss) Equity Securities, Change in Unrealized Holding Gain (Loss) Change in unrealized holding gains (losses), convertible securities, carried at fair value Unrealized Gain (Loss) on Hybrid Instrument, Net Other Gain (Loss) on Sale of Other Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Asset-backed Securities Asset-backed Securities [Member] Other investments Equity Securities and Other [Member] Equity Securities and Other [Member] Income Statement Location [Axis] Income Statement Location [Axis] Interest expense Income Statement Location [Domain] Net investment income Investment Income [Member] Net realized investment gains (losses) Gain (Loss) on Investments [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation: Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Total gains (losses) realized and unrealized: Gains Losses Realized and Unrealized Assets [Abstract] Total gains (losses) realized and unrealized. Included in earnings, as a part of: Gains Losses By Earnings Category Assets [Abstract] Included in Earnings, as a part of. Total gains (losses) realized and unrealized, included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Included in other comprehensive income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Transfers in Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Transfers out Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Ending Balance Change in unrealized gains (losses) included in earnings for the above period for Level 3 assets held at period-end Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Variable Interest Entities [Abstract] Variable interest entities. Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments Schedule of Long-term Debt Instruments [Table Text Block] Schedule of interest rate caps Schedule of Interest Rate Derivatives [Table Text Block] Debt Long-term Debt [Text Block] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Variable Interest Entities by Classification of Entity [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] VIE interests carrying value Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets Summary of reserve for losses and loss adjustment expenses Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Senior Notes Senior Notes [Member] Line of Credit Line of Credit [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Senior notes due 2023 Senior notes due 2023 [Member] Senior notes due 2023 [Member] Revolving Credit Agreement Revolving Credit Facility [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Gross debt Long-term Debt, Gross Less debt issuance costs Unamortized Debt Issuance Expense Debt less debt issuance costs Long-term Debt Stated interest rate on debt Debt Instrument, Interest Rate, Stated Percentage Weighted average interest rate on debt Line of Credit Facility, Interest Rate During Period Line of credit maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Retroactive Insurance Insurance Disclosure [Text Block] Statement of Financial Position [Abstract] Assets Assets [Abstract] Investments Investments [Abstract] Fixed maturities, at fair value; cost or amortized cost, $2,178,066 and $2,257,188, respectively Debt Securities, Trading And Available-For-Sale Debt Securities, Trading And Available-For-Sale Equity investments, at fair value; cost, $474,306 and $425,942, respectively Short-term investments Other Short-term Investments Business owned life insurance Life Insurance, Corporate or Bank Owned, Amount Investment in unconsolidated subsidiaries Other investments, $33,020 and $52,301 at fair value, respectively, otherwise at cost or amortized cost Other Investments Total Investments Investments Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Premiums receivable Premiums Receivable, Net Receivable from reinsurers on paid losses and loss adjustment expenses Reinsurance Recoverable for Paid Claims and Claims Adjustments Receivable from reinsurers on unpaid losses and loss adjustment expenses Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Prepaid reinsurance premiums Prepaid Reinsurance Premiums Deferred policy acquisition costs Deferred Policy Acquisition Costs Deferred tax asset, net Deferred Tax Assets, Net Real estate, net Property, Plant and Equipment, Net Intangible assets, net Other Intangible Assets, Net Goodwill Goodwill Other assets Other Assets Total Assets Assets Liabilities and Shareholders' Equity Liabilities and Equity [Abstract] Liabilities Liabilities [Abstract] Policy liabilities and accruals Policy liabilities and accruals [Abstract] Policy liabilities and accruals. Unearned premiums Unearned Premiums Reinsurance premiums payable Reinsurance Payable Total Policy Liabilities Policy Liabilities Total policy liabilities. Other liabilities Debt less debt issuance costs Total Liabilities Liabilities Shareholders' Equity Stockholders' Equity Attributable to Parent [Abstract] Common shares, par value $0.01 per share, 100,000,000 shares authorized, 62,986,474 and 62,824,523 shares issued, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Accumulated other comprehensive income (loss), net of deferred tax expense (benefit) of ($4,027) and $5,218, respectively Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Treasury shares, at cost, 9,367,545 shares and 9,367,502 shares, respectively Treasury Stock, Value Total Shareholders' Equity Total Liabilities and Shareholders' Equity Liabilities and Equity Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Accumulated Other-than-Temporary Impairment Attributable to Parent [Member] Accumulated Other-than-Temporary Impairment Attributable to Parent [Member] Realized investment gains (losses) Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax Tax effect Reclassification from AOCI, Current Period, Tax Net reclassification adjustments Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Deferred tax expense (benefit) included in OCI Other Comprehensive Income (Loss), Tax Available-for-sale securities [Abstract] Available-for-sale Securities [Abstract] Fixed maturities, available for sale Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract] Amortized Cost Gross Unrealized Gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Estimated Fair Value Statement of Cash Flows [Abstract] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization, net of accretion Depreciation, Amortization and Accretion, Net (Increase) decrease in cash surrender value of BOLI Life Insurance, Corporate or Bank Owned, Change in Value Net realized investment (gains) losses Share-based compensation Share-based Compensation Deferred income taxes Policy acquisition costs, net of amortization (net deferral) Increase (Decrease) in Deferred Policy Acquisition Costs Equity in (earnings) loss of unconsolidated subsidiaries Other Other Noncash Income (Expense) Other changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Premiums receivable Increase (Decrease) in Premiums Receivable 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. Other assets Increase (Decrease) in Other Operating Assets Reserve for losses and loss adjustment expenses Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Unearned premiums Increase (Decrease) in Unearned Premiums Other liabilities Increase (Decrease) in Other Operating Liabilities Net cash provided (used) by operating activities Net Cash Provided by (Used in) Operating Activities Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of: Payments to Acquire Investments [Abstract] Fixed maturities, available for sale Fixed maturities, trading Payments to Acquire Trading Securities Held-for-investment Equity investments Payments To Acquire Equity Securities, FV-NI Payments To Acquire Equity Securities, FV-NI Other investments Payments to Acquire Other Investments Funding of qualified affordable housing project tax credit partnerships Payments For Funding Of Tax Credit Limited Partnerships Payments for funding of tax credit limited partnerships. 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. Proceeds from sales or maturities of: Proceeds from Sale, Maturity and Collection of Investments [Abstract] Fixed maturities, available for sale Proceeds from Sale and Maturity of Available-for-sale Securities Equity investments Proceeds From Sale And Maturity Of Equity Securities, FV-NI Proceeds From Sale And Maturity Of Equity Securities, FV-NI Other investments Proceeds from Sale and Maturity of Other Investments Return of invested capital from unconsolidated subsidiaries Proceeds from Limited Partnership Investments Net sales or maturities (purchases) of short-term investments Payments for (Proceeds from) Short-term Investments Unsettled security transactions, net change Unsettled security transactions net Unsettled security transactions, net. Purchases of capital assets Payments to Acquire Productive Assets Repayments (advances) under Syndicate Credit Agreement Repayments (advances) under Syndicate Credit Agreement Repayments (advances) under Syndicate Credit Agreement Other Payments for (Proceeds from) Other Investing Activities Net cash provided (used) by investing activities Net Cash Provided by (Used in) Investing Activities Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Borrowings (repayments) under Revolving Credit Agreement Proceeds from (Repayments of) Lines of Credit Repayments of Mortgage Loans Repayments of Secured Debt Dividends to shareholders Payments of Ordinary Dividends, Common Stock Capital contribution received from (return of capital to) external segregated portfolio cell owners External Capital Contribution Received (Return Of Capital) For Segregated Portfolio Cells External Capital Contribution Received (Return Of Capital) For Segregated Portfolio Cells Other Proceeds from (Payments for) Other Financing Activities Net cash provided (used) by financing activities Net Cash Provided by (Used in) Financing Activities Increase (decrease) in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Significant Non-Cash Transactions Significant Non Cash Transactions Abstract Significant non cash transactions. Dividends declared and not yet paid Secured debt fund Investment redemption notice period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period 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. Liquidating investments remaining period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period Redemption percentage of LP at NAV for which initial payment is limited Redemption Percentage of LP at NAV above which initial payment is limited The redemption percentage of a particular LP valued at NAV above which initial payment is limited. Interest expense Interest Expense [Member] Gains (Losses) Recognized in Income on Derivatives Derivative, Gain (Loss) on Derivative, Net Investments held in an unrealized loss position Investments continuous unrealized loss position [Abstract] Investments Continuous Unrealized Loss Position. Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Less than 12 months, Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value More than 12 months, Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Unrealized Loss Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] Unrealized Loss Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Less than 12 months, Unrealized Loss Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss More than 12 months, Unrealized Loss Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Number of Instruments Number of Interest Rate Derivatives Held Notional Amount Estimated Fair Value Derivative Asset Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Qualified affordable housing project tax credit partnerships Qualified Affordable Housing Tax Credit Partnerships [Member] Qualified Affordable Housing Tax Credit Partnerships [Member] Other tax credit partnerships Other Tax Credit Partnerships [Member] Other Tax Credit Partnerships [Member] 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 Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Unconsolidated Subsidiaries Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures [Abstract] Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Gross Premiums by Product Revenue from External Customers by Products and Services [Table Text Block] Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Schedule of Investment Income, Reported Amounts, by Category [Axis] Short-term investments, including Other Short-term investments and Other invested assets [Member] Short-term investments and Other invested assets [Member] BOLI Cash Surrender Value [Member] Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Net Investment Income Net Investment Income [Abstract] Investment Income Interest and Dividend Income, Operating Investment fees and expenses Investment Income, Investment Expense Net investment income Syndicate 1729 Syndicate 1729 [Member] Syndicate 1729 [Member] Syndicate 6131 Syndicate 6131 [Member] Syndicate 6131 [Member] Number of operating segments Number of Operating Segments Worker's compensation SPC percentage ceded Alternative Market Solutions Percentage Ceded Alternative Market Solutions Percentage Ceded Proportion of Capital Provided to support Lloyd's Syndicate 1729 Proportion Of Capital Provided To Support Segment This item represents the capital provided in support of a segment by a corporate member as a percentage of total capital provided by all members to support the segment. Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Debt Securities, at cost Debt Securities, Trading And Available For Sale, Amortized Cost Debt Securities, Trading And Available For Sale, Amortized Cost Equity Investments, fair value, cost Equity Securities, FV-NI, Cost Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. Other investments, portion carried at fair value Other Investments, Fair Value Disclosure Other Investments, Fair Value Disclosure Common shares, par value (USD per share) Common Stock, Par or Stated Value Per Share Common shares, shares authorized (in shares) Common shares, shares issued (in shares) Common Stock, Shares, Issued Deferred tax expense (benefit) on accumulated other comprehensive income (loss) AOCI Tax, Attributable to Parent Treasury shares, number of shares (in shares) Treasury Stock, Shares Income taxes liabilities Taxes Payable Unrecognized tax benefits Unrecognized Tax Benefits Accrued liability for interest related to unrecognized tax benefits Unrecognized Tax Benefits, Interest on Income Taxes Accrued Other Liabilities Other Liabilities [Table Text Block] Assets and liabilities measured at fair value Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Summary of quantitative information about Level 3 fair value measurements Fair Value Inputs, Assets, Quantitative Information [Table Text Block] Summary of changes in the fair value of assets measured at fair value Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Investments in Limited liability companies and limited partnerships Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Financial instruments not measured at fair value Fair Value, by Balance Sheet Grouping [Table Text Block] Schedule of Credit Derivatives [Table] Schedule of Credit Derivatives [Table] Fair Value, Measurements, Nonrecurring Fair Value, Measurements, Nonrecurring [Member] Early Stage Business Development Early Stage Business Development [Member] Early Stage Business Development [Member] Credit Rating Source [Axis] Credit Rating Source [Axis] Credit Rating Source [Domain] Credit Rating Source [Domain] NRSRO Nationally Recognized Statistical Rating Organization (NRSRO) [Member] Nationally Recognized Statistical Rating Organization (NRSRO) [Member] Internal Credit Assessment [Axis] Internal Credit Assessment [Axis] Internal Credit Assessment [Domain] Internal Credit Assessment [Domain] NRSRO, Rating BBB Nationally Recognized Statistical Rating Organization (NRSRO), Rating BBB [Member] Nationally Recognized Statistical Rating Organization (NRSRO), Rating BBB [Member] NRSRO, Rating BBBplus Nationally Recognized Statistical Rating Organization (NRSRO), Rating BBBplus [Member] Nationally Recognized Statistical Rating Organization (NRSRO), Rating BBBplus [Member] NRSRO, Rating AAA Nationally Recognized Statistical Rating Organization (NRSRO), Rating AAA [Member] Nationally Recognized Statistical Rating Organization (NRSRO), Rating AAA [Member] Credit Derivatives [Line Items] Credit Derivatives [Line Items] Credit rating Concentration Risk, Percentage Transfer of equity securities from Level 2 to Level 1 Fair Value, Assets, Level 2 to Level 1 Transfers, Amount Fair value, net asset (liability) Fair Value, Net Asset (Liability) Fair value of funded deferred compensation assets Deferred Compensation Plan Assets Number of line of credit agreement Line of Credit, Number of Agreement Line of Credit, Number of Agreement Document and Entity Information [Abstract] Document And Entity Information. Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract] Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract] Minimum period for claims resolution (years) Minimum Period For Claims Resolution Minimum Period For Claims Resolution Number of business partitions Number of Business Partitions Number of Business Partitions Summary of reserve for losses and loss adjustment expenses Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Balance, beginning of year Less reinsurance recoverables on unpaid losses and loss adjustment expenses Net balance, beginning of year Liability for Unpaid Claims and Claims Adjustment Expense, Net Net losses: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Current year Current Year Claims and Claims Adjustment Expense Favorable development of reserves established in prior years, net Prior Year Claims and Claims Adjustment Expense Total Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Paid related to: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Net balance, end of period Plus reinsurance recoverables on unpaid losses and loss adjustment expenses Balance, end of period Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Not Measured At Fair Value Not Measured At Fair Value [Member] Not Measured At Fair Value [Member] Fair Value, by Balance Sheet Grouping, Disclosure Item Amounts [Axis] Measurement Basis [Axis] Fair Value, Disclosure Item Amounts [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Financial instruments not measured at fair value Financial Instruments Not Measured At Fair Value [Abstract] Financial instruments not measured at fair value. Financial assets: Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] BOLI Other investments Other assets Financial liabilities: Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Debt Debt, Long-term and Short-term, Combined Amount Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Losses recorded from affordable housing projects Income (Loss) from Affordable Housing Projects, Equity Method Investments Tax credits recognized related to affordable housing projects Affordable Housing Tax Credits and Other Tax Benefits, Amount Losses from historic tax credit investments Income (Loss) from Historical Projects, Equity Method Investments Income (Loss) from Historical Projects, Equity Method Investments Tax credits recognized related to historic tax credit investments Historic Tax Credits Investments and Other Tax Benefits, Amount Historic Tax Credits Investments and Other Tax Benefits, Amount