Liability for Future Policy Benefits [Abstract] Liability for Future Policy Benefit, by Product Segment [Table] Liability for Future Policy Benefit, by Product Segment [Table] Category [Axis] Industry Sector [Axis] Category [Domain] Industry Sector [Domain] Life policy reserves: Life Policy Reserves [Member] Life Policy Reserves [Member] Investment contract reserves: Investment contract reserves [Member] Investment contract reserves [Member] Insurance Policy [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Ordinary/traditional life Individual Ordinary Life Insurance [Member] Individual Ordinary Life Insurance [Member] Other Other Life Policy Reserves [Member] Other Life Policy Reserves [Member] Deferred annuities Deferred Annuities [Member] Deferred Annuities [Member] Universal life Universal Life [Member] Structured Settlement Annuity [Member] Structured Settlement Annuity [Member] Other Other Investment Contract Reserves [Member] Other Investment Contract Reserves [Member] Liability for Future Policy Benefit, by Product Segment [Line Items] Liability for Future Policy Benefit, by Product Segment [Line Items] Life policy and investment contract reserves Liability for Future Policy Benefits, Life Fair Value Disclosures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring Basis [Table] Fair Value, Assets and Liabilities Measured on Recurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation By Major Type [Axis] Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation By Major Type [Axis] Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, by Major Type [Axis] Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation By Major Types [Domain] Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation By Major Types [Domain] Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, by Major Types [Domain] Taxable Fixed Maturities Taxable Fixed Maturities [Member] Taxable Fixed Maturities [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Quoted prices in active markets for identical assets (Level 1) Fair Value, Inputs, Level 1 [Member] Significant other observable inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant unobservable inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Investment Securities Class [Axis] Investment Securities Class [Axis] Investment Securities Class [Domain] Investment Securities Class [Domain] Debt Securities Debt Securities [Member] Equity securities Equity Securities [Member] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Corporate Corporate Bond Securities [Member] States, municipalities and political subdivisions US States and Political Subdivisions Debt Securities [Member] Commercial mortgage-backed Commercial Mortgage Backed Securities [Member] Government-sponsored enterprises US Government-sponsored Enterprises Debt Securities [Member] United States government US Government Agencies Debt Securities [Member] Foreign government Foreign Government Debt Securities [Member] Common equities Common Stock [Member] Nonredeemable preferred equities Preferred Stock [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] Fixed maturities, at fair value Available-for-sale Securities, Debt Securities Equity securities, at fair value Available-for-sale Securities, Equity Securities Separate accounts Separate Account Assets Top Hat savings plan mutual funds and common equity (included in Other assets) Other Investment Not Readily Marketable, Fair Value Fair value of asset Investments, Fair Value Disclosure Reinsurance Disclosures [Abstract] Reinsurance Reinsurance [Text Block] Investments [Abstract] Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Senior Debentures 6.900% Due 2028 Senior Debentures Six Point Nine Percent Due Twenty Twenty Eight [Member] Senior Debentures 6.9 Percent Due 2028 [Member] Senior Debentures 6.920% Due 2028 Senior Debentures Six Point Nine Two Percent Due Twenty Twenty Eight [Member] Senior Debentures 6.92 Percent Due 2028 [Member] Senior Notes 6.125% Due 2034 Senior Notes Six Point One Two Five Percent Due Twenty Thirty Four [Member] Senior Notes 6.125 Percent Due 2034 [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Interest rate Debt Instrument, Interest Rate, Stated Percentage Debt issuance year Debt Instrument Issuance Year Debt instrument issuance year. Book value Long-term Debt Principal amount of debt Debt Instrument, Face Amount Commitments and Contingencies Disclosure [Abstract] Commitments And Contingent Liabilities Commitments and Contingencies Disclosure [Text Block] Deferred Costs [Abstract] Deferred Acquisition Costs Deferred Policy Acquisition Costs [Text Block] Reinsurance for Insurance Companies, by Product Segment [Table] Reinsurance Premiums for Insurance Companies, by Product Segment [Table] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Life Insurance Segment Life Insurance Product Line [Member] Reinsurance for Insurance Companies, by Product Segment [Line Items] Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items] Direct earned premiums Direct Premiums Earned Ceded earned premiums Ceded Premiums Earned Earned premiums Premiums Earned, Net, Life Direct contract holders' benefits incurred Policyholder Benefits and Claims Incurred, Direct Policyholder Benefits and Claims Incurred, Assumed Policyholder Benefits and Claims Incurred, Assumed Ceded contract holders' benefits incurred Policyholder Benefits and Claims Incurred, Ceded Insurance losses and contract holders' benefits Policyholder Benefits and Claims Incurred, Net Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Corporate Fixed Maturities Corporate Debt Securities [Member] Taxable Fixed Maturities [Member] States, Municipalities And Political Subdivisions Fixed Maturities Preferred Equities 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 [Roll Forward] 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 or losses (realized/unrealized): Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Total Gains Losses Realized And Unrealized [Abstract] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Assets, Total Gains (Losses) Realized and Unrealized [Abstract] Included in net income 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 into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Ending balance Income Tax Disclosure [Abstract] Differences Between The 35 Percent Statutory Income Tax Rate And Effective Income Tax Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Property Casualty Insurance Assumed And Ceded Business [Table] Property Casualty Insurance Premiums Written On Assumed And Ceded Business [Table] Property Casualty Insurance Premiums Written On Assumed And Ceded Business Consolidated Property and Casualty Insurance Entity Consolidated Property and Casualty Insurance Entity [Member] Property Casualty Insurance Assumed And Ceded Business [Line Items] Property Casualty Insurance Premiums Written On Assumed And Ceded Business [Line Items] [Line Items] for Property Casualty Insurance Premiums Written On Assumed And Ceded Business Direct written premiums Direct Premiums Written Assumed written premiums Assumed Premiums Written Ceded written premiums Ceded Premiums Written Net written premiums Increase (Decrease) in Property and Casualty Insurance Liabilities Assumed earned premiums Assumed Premiums Earned Earned premiums Premiums Earned, Net, Property and Casualty Direct incurred loss and loss expenses Assumed incurred loss and loss expenses Ceded incurred loss and loss expenses Incurred loss and loss expenses Premiums Written, Net [Abstract] Property Casualty Loss And Loss Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Segment Reporting [Abstract] Segment Information Segment Reporting Disclosure [Text Block] Schedule of Gain (Loss) on Investments [Table] Schedule of Gain (Loss) on Investments [Table] Fixed maturities Gain (Loss) on Investments [Line Items] Gain (Loss) on Investments [Line Items] Investment income: Net Investment Income [Abstract] Interest Investment Income, Interest Dividends Investment Income, Dividend Other Investment Income Other Long-Term Investments Income derived from those other than interest and dividends. Total Gross Investment Income, Operating Less investment expenses Investment Income, Investment Expense Total Net Investment Income Gain (Loss) on Sale of Investments Gain (Loss) on Sale of Investments Unrealized Gain (Loss) on Investments Unrealized Gain (Loss) on Investments Investment gains and losses, net: Realized Investment Gains (Losses) [Abstract] Gross realized gains Available-for-sale Securities, Gross Realized Gains Gross realized losses Available-for-sale Securities, Gross Realized Losses Other Than Temporary Impairment Losses Other Than Temporary Impairment Losses Other Than Temporary Impairment Losses Other Other Realized Investment Gains Losses Net This item represents realized investment gains (losses) included in earnings for the period excluding those relating to other than temporary impairment losses on investments in debt securities. Includes realized investment gains and losses generated from the sale of debt securities, equity securities, investments in joint ventures and limited partnerships and from other than temporary impairments on investments other than debt securities. Also includes realized investment gains and losses are generated from prepayment premiums, recoveries of principal on previously impaired securities, provisions for losses on commercial mortgage and other loans, and fair value changes on embedded derivatives and derivatives that do not qualify for hedge accounting treatment, except those derivatives used in the Company's capacity as a broker or dealer. Investment gains and losses, net Realized Investment Gains (Losses) Schedule of Deferred Policy Acquisition Costs [Table] Schedule of Deferred Policy Acquisition Costs [Table] Schedule of Deferred Policy Acquisition Costs [Table] Life Insurance Product Line [Member] Schedule of Deferred Policy Acquisition Costs [Line Items] Schedule of Deferred Policy Acquisition Costs [Line Items] [Line Items] for Schedule of Deferred Policy Acquisition Costs [Table] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Deferred policy acquisition costs asset, beginning of period Deferred Policy Acquisition Costs Capitalized deferred policy acquisition costs Deferred Policy Acquisition Costs, Additions Deferred Policy Acquisition Costs, Amortization Expense Deferred Policy Acquisition Costs, Amortization Expense Deferred Policy Acquisition Cost, Amortization Expense, Other Deferred Policy Acquisition Cost, Amortization Expense, Other Deferred policy acquisition costs asset, end of period Accounting Policies [Abstract] Accounting Policies Significant Accounting Policies [Text Block] Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Previously Reported Scenario, Previously Reported [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Paid-In Capital Additional Paid-in Capital [Member] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income AOCI Attributable to Parent [Member] Treasury Stock Treasury Stock [Member] Statement [Line Items] Statement [Line Items] Beginning Balance Stockholders' Equity Attributable to Parent Share-based awards Stock Issued During Period, Value, Stock Options Exercised Share-based compensation Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Other Stockholders' Equity, Other Unrealized gains reclassified to retained earnings for ASU 2016-01, after tax Unrealized gains reclassified to retained earnings for ASU 2016-01, after tax Unrealized gains on equity securities reclassified from accumulated other comprehensive income to retained earnings, after tax upon adoption of ASU 2016-01. Net Income Net Income (Loss) Attributable to Parent Dividends declared Dividends, Common Stock, Cash Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Shares acquired - share repurchase authorization Treasury Stock, Value, Acquired, Cost Method Shares acquired - share-based compensation plans Shares Acquired During Period, Value, Employee Share-Based Compensation Plans Shares Acquired During Period, Value, Employee Share-Based Compensation Plans Ending Balance Beginning Balance (in shares) Common Stock, Shares, Outstanding Share-based awards (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Shares acquired - share repurchase authorization (in shares) Treasury Stock, Shares, Acquired Shares acquired - share-based compensation plans (in shares) Shares Acquired During Period, Employee Share-Based Compensation Plans Shares Acquired During Period, Employee Share-Based Compensation Plans Ending Balance (in shares) Reserve For Losses Loss Adjustment Expenses Table [Text Block] Schedule of Liability for Future Policy Benefits, by Product Segment [Table Text Block] Subsequent Events [Abstract] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Events Subsequent Event [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] MSP Underwriting Limited MSP Underwriting Limited [Member] MSP Underwriting Limited [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Net asset value of acquisition Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Exchange ratio (usd to gbp) Foreign Currency Exchange Rate, Translation Amortized cost Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract] Due in one year or less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis Due after one year through five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis Due after five years through ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis Due after ten years Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis Total Available-for-sale Debt Securities, Amortized Cost Basis Fair value Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract] Due in one year or less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Due after one year through five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Due after five years through ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value Due after ten years Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Total % of fair value Available-for-sale Securities, Debt Maturities, Average, Percent [Abstract] Available-for-sale Securities, Debt Maturities, Average, Percent [Abstract] Due in one year or less Available For Sale Securities Debt Maturities Within One Year Average Available For Sale Securities Debt Maturities Within One Year Average Due after one year through five years Available For Sale Securities Debt Maturities After One Through Five Years Average Available For Sale Securities Debt Maturities After One Through Five Years Average Due after five years through ten years Available For Sale Securities Debt Maturities After Five Through Ten Years Average Available For Sale Securities Debt Maturities After Five Through Ten Years Average Due after ten years Available For Sale Securities Debt Maturities After Ten Years Average Available For Sale Securities Debt Maturities After Ten Years Average Total Available For Sale Securities Debt Maturities Average Available For Sale Securities Debt Maturities Average Adopted Accounting Updates and Pending Accounting Updates New Accounting Pronouncements, Policy [Policy Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Derivative, by Nature [Axis] Derivative Instrument [Axis] Derivative, Name [Domain] Derivative Contract [Domain] Debt Debt [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value Hierarchy for Assets Measured at Fair Value on a Recurring Basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Asset Fair Value Measurements Using Significant Unobservable Inputs (Level 3) Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Fair Values of Deferred Annuities, Structured Settlements and Other Items Fair Value, Liabilities Measured on Recurring and Nonrecurring Basis [Table Text Block] Fair Value of Life Policy Loans Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Table Text Block] Investments, Unrealized Loss Position [Table] Investments Unrealized Loss Position [Table] Investments, Unrealized Loss Position [Table] Investments, Unrealized Loss Position [Line Items] Investments Unrealized Loss Position [Line Items] Investments, Unrealized Loss Position [Line Items] Less than 12 months Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Less than 12 months Unrealized Losses Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months Aggregate Loss 3 Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months. 12 months or more Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 12 months or more Unrealized Losses Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss 3 Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer. Total Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Total Unrealized Losses Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3 Amount of unrealized loss on securities classified as available-for-sale. Income Taxes Income Tax Disclosure [Text Block] Earnings Per Share [Abstract] Calculations For Basic And Diluted Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Retirement Benefits [Abstract] Employee Retirement Benefits Pension and Other Postretirement Benefits Disclosure [Text Block] Segment Information Schedule of Segment Reporting Information, by Segment [Table Text Block] Property Casualty Insurance Assumed And Ceded Business Property Casualty Insurance Assumed And Ceded Business [Text Block] Property Casualty Insurance Assumed And Ceded Business [Text Block] Life Insurance Assumed and Ceded Business Life Insurance Assumed And Ceded Business Table [Text Block] Life Insurance Assumed And Ceded Business Table [Text Block] Fair Value Measurements Fair Value Disclosures [Text Block] Statement of Financial Position [Abstract] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Assets Assets [Abstract] Investments Fixed maturities, at fair value (amortized cost: 2018—$10,653; 2017—$10,314) Equity securities, at fair value (cost: 2018—$3,294; 2017—$3,094) Other invested assets Other Investments Total investments Investments Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Investment income receivable Accrued Investment Income Receivable Finance receivable Loans and Leases Receivable, Net Amount Premiums receivable Premiums Receivable, Net Reinsurance recoverable Reinsurance Recoverables, Including Reinsurance Premium Paid Prepaid reinsurance premiums Prepaid Reinsurance Premiums Deferred policy acquisition costs Land, building and equipment, net, for company use (accumulated depreciation: 2018—$259; 2017—$253) Property, Plant and Equipment, Net Other assets Other Assets Total assets Assets Liabilities Liabilities [Abstract] Insurance reserves Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] Loss and loss expense reserves Liability for Claims and Claims Adjustment Expense, Property Casualty Liability Unearned premiums Unearned Premiums Other liabilities Other Liabilities Deferred income tax Deferred Tax Liabilities, Gross Note payable Notes Payable Long-term debt and capital lease obligations Long-term Debt and Capital Lease Obligations Separate accounts Separate Accounts, Liability Total liabilities Liabilities Commitments and contingent liabilities (Note 12) Commitments and Contingencies Shareholders' Equity Stockholders' Equity Attributable to Parent [Abstract] Common stock, par value—$2 per share; (authorized: 2018 and 2017—500 million shares; issued: 2018 and 2017—198.3 million shares) Common Stock, Value, Issued Paid-in capital Additional Paid in Capital, Common Stock Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Treasury stock at cost (2018—35.6 million shares and 2017—34.4 million shares) Treasury Stock, Value Total shareholders' equity Total liabilities and shareholders' equity Liabilities and Equity Fixed maturities, amortized cost Available-for-sale Equity Securities, Amortized Cost Basis Available-for-sale Equity Securities, Amortized Cost Basis Land, building and equipment, accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Common stock, par value Common Stock, Par or Stated Value Per Share Common stock, authorized Common Stock, Shares Authorized Common stock, issued Common Stock, Shares, Issued Common stock, outstanding Treasury stock, shares Treasury Stock, Shares Tax at statutory rate, percentage Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Effective Income Tax Rate Reconciliation, Tax Accounting Method Changes, Percent Effective Income Tax Rate Reconciliation, Tax Accounting Method Changes, Percent Effective Income Tax Rate Reconciliation, Tax Accounting Method Changes, Percent Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Products and Services [Axis] Insurance premiums Earned Premiums [Member] Earned Premiums [Member] Fee revenues Fee Revenues [Member] Fee Revenues [Member] Investment income, net of expenses Investment Income, Net Of Expenses [Member] Investment Income, Net Of Expenses [Member] Investment gains and losses, net Investment Gains And Losses, Net [Member] Investment Gains And Losses, Net [Member] Other Other Revenues [Member] Other Revenues [Member] Subsegments [Axis] Subsegments [Axis] Subsegments [Domain] Subsegments [Domain] Commercial Casualty Commercial Casualty Segment [Member] Commercial Casualty Segment [Member] Commercial Property Commercial Property Segment [Member] Commercial Property Segment [Member] Commercial Auto Commercial Auto Segment [Member] Commercial Auto Segment [Member] Workers' Compensation Workers Compensation [Member] Workers' Compensation [Member] Other Commercial Lines Other Commercial Lines [Member] Other Commercial Lines [Member] Personal Auto Personal Auto [Member] Personal Auto [Member] Home Owner Segment Home Owner Segment [Member] Home Owner Segment [Member] Other Personal Lines Other Personal Lines [Member] Other Personal Lines [Member] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Corporate, Non-Segment [Member] Corporate, Non-Segment [Member] Operating Segments Operating Segments [Member] Commercial Lines Property and Casualty, Commercial Insurance Product Line [Member] Personal Lines Property and Casualty, Personal Insurance Product Line [Member] Excess and Surplus Lines Insurance Excess and Surplus Lines Insurance [Member] Excess and Surplus Lines Insurance [Member] Investments Segment Investments Product Line [Member] Reinsurance assumed and other non segment [Member] Reinsurance assumed and other non segment [Member] Reinsurance assumed and other non segment [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Total revenues Revenue from Contract with Customer, Excluding Assessed Tax Income (loss) before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Identifiable assets Statement of Comprehensive Income [Abstract] Change in unrealized gains on investments, net of tax (benefit) of ($17), $66, ($81) and $189, respectively Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax Amortization of pension actuarial loss and prior service cost, net of tax of $0, $1, $0 and $1, respectively Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax Change in life deferred acquisition costs, life policy reserves and other, net of tax of $0, ($1), $2 and $0, respectively Other Comprehensive Income, Change in Life Deferred Acquistion Costs, Life Policy Reserves and Other, Net of Tax Other Comprehensive Income, Change in Life Deferred Acquistion Costs, Life Policy Reserves and Other, Net of Tax Other comprehensive income (loss) Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Number of Industries Operating In Number of Industries Operating In Number of Industries Operating In Number of Reportable Segments Number of Reportable Segments 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] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Available-for-sale Equity Securities, Unrealized Gain, After Tax Net investment gain (loss) Cumulative effect adjustment Cumulative Effect of New Accounting Principle in Period of Adoption Investment Secondary Categorization [Axis] Investment Secondary Categorization [Axis] Investments by Secondary Categorization [Domain] Investments by Secondary Categorization [Domain] Investment, Name [Axis] Investment, Name [Axis] Investment, Name [Domain] Investment, Name [Domain] Largest Single Common Stock Holding Largest Single Common Stock Holding Largest Single Common Stock Holding Largest equity holdings to total equity portfolio largest equity holdings to total equity portfolio largest equity holdings to total equity portfolio Largest equity holdings to total portfolio Largest equity holdings to total portfolio Largest equity holdings to total portfolio Number Of Securities Other Than Temporary Impairment Losses1 Number Of Securities Other Than Temporary Impairment Losses1 Number Of Securities Other Than Temporary Impairment Losses Provision for Other Credit Losses Provision for Other Credit Losses Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months Unrealized losses 12 months or more (equity investment less than $500,000) Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost Number of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost Fixed maturity investments, Percentage of amortized costs Fixed maturity investments, Percentage of amortized costs Fixed maturity investments, Percentage of amortized costs. Accumulated Other Comprehensive Income [Abstract] Accumulated Other Comprehensive Income [Abstract] Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Fixed Maturities And Equity Securities Fixed Maturities And Equity Securities [Member] Fixed Maturities And Equity Securities Member Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Cost or Amortized Cost Available-for-sale Securities, Amortized Cost Basis Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Table] Schedule Of Earnings Per Share Basic And Diluted By Common Class [Table] Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Table] Award Type [Axis] Award Type [Axis] Share-based Compensation Arrangements by Share-based Payment Award, Award Type and Plan Name [Domain] Equity Award [Domain] Stock Options Employee Stock Option [Member] Nonvested shares Restricted Stock Units (RSUs) [Member] Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items] Schedule Of Earnings Per Share Basic And Diluted By Common Class [Line Items] Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items] Numerator: Net Income (Loss) Attributable to Parent [Abstract] Net income—basic and diluted Denominator: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic weighted-average common shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Effect of stock-based awards (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Diluted weighted-average shares Weighted Average Number of Shares Outstanding, Diluted Earnings per share: Earnings Per Share, Basic and Diluted [Abstract] Basic (in USD per share) Earnings Per Share, Basic Diluted (in USD per share) Earnings Per Share, Diluted Number of anti-dilutive stock-based awards (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward] Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward] Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward] Gross loss and loss expense reserves, beginning of period Liability for Claims and Claims Adjustment Expense Less reinsurance recoverable Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Net loss and loss expense reserves, beginning of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Current accident year Current Year Claims and Claims Adjustment Expense Prior accident years Prior Year Claims and Claims Adjustment Expense Total incurred Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current accident year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior accident 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 loss and loss expense reserves, end of period Plus reinsurance recoverable Gross loss and loss expense reserves, end of period Analysis Of Cost Or Amortized Cost, Gross Unrealized Gains And Losses And Fair Value for Investments Available-for-sale Securities [Table Text Block] Fair Values And Unrealized Losses by Investment Category And By The Duration Of The Securities' Continuous Unrealized Loss Position Schedule of Unrealized Loss on Investments [Table Text Block] Investments Classified by Contractual Maturity Date [Table Text Block] Investments Classified by Contractual Maturity Date [Table Text Block] Investment Income, Realized Investment Gains And Losses And Change In Unrealized Investment Gains And Losses Schedule Of Investment Income Realized Gain Loss Comprehensive Income Loss Table [Text Block] Tabular disclosure of investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums; proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales; components of comprehensive income. Reclassification Out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Accumulated Net Unrealized Investment Gain (Loss) Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Net Unrealized Gain (Loss) on Deferred Costs, Reserves And Other Accumulated Net Unrealized Gain (Loss) on Deferred Costs, Reserves And Other [Member] Accumulated Net Unrealized Gain (Loss) on Deferred Costs, Reserves And Other [Member] Accumulated Other Comprehensive Income (Loss) Reclassification Out of Accumulated Other Comprehensive Income [Line Items] Reclassification Out of Accumulated Other Comprehensive Income [Line Items] [Line Items] for Reclassification Out of Accumulated Other Comprehensive Income [Table] Change In Accumulated Other Comprehensive Income [Roll Forward] Change In Accumulated Other Comprehensive Income [Roll Forward] Change In Accumulated Other Comprehensive Income [Roll Forward] Accumulated Other Comprehensive Income Loss Before Tax [Abstract] Other Comprehensive Income (Loss), before Tax [Abstract] Accumulated other comprehensive income, before tax, beginning of period AOCI Including Portion Attributable to Noncontrolling Interest, before Tax Cumulative effect of change in accounting for equity securities, before tax Cumulative Effect of New Accounting Principle in Period of Adoption, before Tax Cumulative Effect of New Accounting Principle in Period of Adoption, before Tax Other comprehensive income before reclassification, before tax Other Comprehensive Income (Loss), before Reclassifications, before Tax Reclassification adjustment, before tax Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax Effect on other comprehensive income, before tax Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent Change in unrealized gains, net, on investments available for sale, before tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, before Tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, before Tax Change in pension obligations, before tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax Change in life deferred acquisition costs, life policy reserves and other, before tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, before Tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, before Tax Effect on other comprehensive income, before tax Increase (Decrease) in Other Comprehensive Income (Loss), before Tax Increase (Decrease) in Other Comprehensive Income (Loss), before Tax Accumulated other comprehensive income, before tax, end of period Accumulated Other Comprehensive Income (Loss), Tax [Abstract] Accumulated Other Comprehensive Income Loss Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Tax [Abstract] Accumulated other comprehensive income, tax, beginning of period Accumulated Other Comprehensive Income Loss Tax Accumulated Other Comprehensive Income (Loss), Tax Cumulative effect of change in accounting for equity securities, tax Cumulative Effect of New Accounting Principle in Period of Adoption, Tax Cumulative Effect of New Accounting Principle in Period of Adoption, Tax Other comprehensive income before reclassification, tax Other Comprehensive Income (Loss) before Reclassifications, Tax Reclassification adjustment, tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax Effect on other comprehensive income, tax Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Change in unrealized gains, net, on investments available for sale, tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, Tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, Tax Change in pension obligations, tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Change in life deferred acquisition costs, life policy reserves and other, tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, Tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, Tax Effect on other comprehensive income, tax Increase (Decrease) in Other Comprehensive Income (Loss), Tax Increase (Decrease) in Other Comprehensive Income (Loss), Tax Accumulated other comprehensive income, tax, end of period Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated other comprehensive income, net of tax, beginning of period Cumulative effect of change in accounting for equity securities, net Other comprehensive income before reclassification, net of tax Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Reclassification adjustment, net of tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Effect on other comprehensive income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Change in unrealized gains, net, on investments available for sale, net of tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, Net of Tax Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, Net of Tax Change in pension obligations, net of tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Change in life deferred acquisition costs, life policy reserves and other, net of tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, Net of Tax Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, Net of Tax Effect on other comprehensive income, net of tax Increase (Decrease) in Other Comprehensive Income (Loss), Net of Tax Increase (Decrease) in Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive income, net of tax, end of period Schedule of Defined Benefit Plans Disclosures Schedule of Defined Benefit Plans Disclosures [Table Text Block] Life Policy And Investment Contract Reserves Liability For Future Policy Benefits Life [Text Block] The disclosure of the sum of the known and estimated amounts payable as of the balance sheet date to life insurance policyholders that relates to insured events which can be viewed as either (a) the present value of future benefits to be paid to or on behalf of policyholders and expenses less the present value of future net premiums payable under the insurance contracts or (b) the accumulated amount of net premiums already collected less the accumulated amount of benefits and expenses already paid to or on behalf of policyholders. Statement of Cash Flows [Abstract] Cash Flows From 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 Depreciation, Amortization and Accretion, Net Gain (Loss) on Sale of Investments Share-based compensation Share-based Compensation Interest credited to contract holders' Interest Credited to Policy Owner Account Deferred income tax expense Deferred Income Tax Expense (Benefit) Changes in: Increase (Decrease) in Operating Capital [Abstract] Investment income receivable Increase (Decrease) in Accrued Investment Income Receivable Premiums and reinsurance receivable Increase (Decrease) in Insurance Assets Deferred policy acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Other assets Increase (Decrease) in Other Operating Assets Loss and loss expense reserves Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Life policy and investment contract reserves Increase (Decrease) in Future Policy Benefit Reserves Unearned premiums Increase (Decrease) in Unearned Premiums Other liabilities Increase (Decrease) in Operating Liabilities Current income tax receivable/payable Increase (Decrease) in Deferred Income Taxes Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash Flows From Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Sale of fixed maturities Proceeds from Sale of Available-for-sale Securities, Debt Call or maturity of fixed maturities Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities Sale of equity securities Proceeds from Sale of Available-for-sale Securities, Equity Purchase of fixed maturities Payments to Acquire Available-for-sale Securities, Debt Purchase of equity securities Payments to Acquire Available-for-sale Securities, Equity Investment in finance receivables Payments to Acquire Finance Receivables Collection of finance receivables Proceeds from Sale and Collection of Finance Receivables Investment in buildings and equipment Payments to Acquire Property, Plant, and Equipment Change in other invested assets, net Payments for (Proceeds from) Other Investing Activities Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash Flows From Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Payment of cash dividends to shareholders Payments of Ordinary Dividends, Common Stock Shares acquired - share repurchase authorization Payments for Repurchase of Common Stock Changes in note payable Proceeds from (Repayments of) Notes Payable Proceeds from stock options exercised Proceeds from Stock Options Exercised Contract holders' funds deposited Additions to Contract Holders Funds Contract holders' funds withdrawn Withdrawal from Contract Holders Funds Other Proceeds from (Payments for) Other Financing Activities Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Net change in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of year Cash and cash equivalents at end of period Supplemental Disclosures of Cash Flow Information: Supplemental Cash Flow Information [Abstract] Interest paid Interest Paid, Net Income taxes paid Income Taxes Paid, Net Noncash Activities Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Conversion of securities Conversion of Stock, Amount Converted Equipment acquired under capital lease obligations Capital Lease Obligations Incurred Cashless exercise of stock options Purchase Of Treasury Shares Non Cash Purchase Of Treasury Shares Non Cash Other assets and other liabilities Liabilities Assumed Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Qualified Pension Plans [Member] Pension Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Matching contributions to 401(k) and Top Hat plans Defined Contribution Plan, Employer Discretionary Contribution Amount Pension contributions Payment for Pension Benefits Document Documentand Entity Information [Abstract] Document - Document and Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key 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 Consolidated Property Casualty Loss And Loss Expense Reserves Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Subsequent Events Subsequent Events [Text Block] Income Statement [Abstract] Earned premiums Investment income, net of expenses Investment gains and losses, net Fee revenues Other revenues Revenues Revenues [Abstract] Total revenues Benefits and Expenses Benefits, Losses and Expenses [Abstract] Underwriting, acquisition and insurance expenses Underwriting Acquisition And Insurance Expenses Amount of commissions, insurance operating expenses, increase in deferred acquisition costs and taxes, licenses and fees. Interest expense Interest Expense Other operating expenses Other Cost and Expense, Operating Total benefits and expenses Benefits, Losses and Expenses Income Before Income Taxes Provision (Benefit) for Income Taxes Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Current Income Tax Expense (Benefit) Deferred Total provision for income taxes Income Tax Expense (Benefit) Net Income Per Common Share Net income—basic Net income—diluted Net Income Per Common Share Earnings Per Share [Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum Maximum [Member] Investment Type [Axis] Investment Type [Axis] Three Primary Contributors Investments [Domain] Life Policy Loans Policy Loans [Member] Structured Finance [Member] Structured Finance [Member] Percentage Of Level 3 Assets To Total Financial Instruments Measured At Fair Value Percentage Of Level 3 Assets To Total Financial Instruments Measured At Fair Value Percentage of Level 3 Assets to Total Financial Instruments Measured At Fair Value Loss and loss expense reserves Liability for Future Policy Benefits, Individual and Group Annuities and Supplementary Contracts Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax Deferred Policy Acquisition Costs And Asset Reconciliation, Including the Amortized Deferred Policy Acquisition Costs Deferred Policy Acquisition Costs [Table Text Block] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of actuarial loss and prior service cost Defined Benefit Plan, Amortization of Gain (Loss) Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Investments [Domain] Policy Loans [Member] Fair value of life policy loans Loans Receivable, Fair Value Disclosure Liabilities Related to Investment Contracts, Fair Value Disclosure Liabilities Related to Investment Contracts, Fair Value Disclosure Supplemental Information for Property, Casualty Insurance Underwriters [Table] Supplemental Information for Property, Casualty Insurance Underwriters [Table] Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Domain] Short-duration Insurance Contracts, Accident Year [Domain] Catastrophe Development Catastrophe Development [Member] Catastrophe Development Member. Commercial Casualty Segment [Member] Other Commercial Lines [Member] Homeowner [Member] Homeowner [Member] Homeowner [Member]. Personal Auto [Member] Commercial Property Segment [Member] Life and Health Loss Reserves Life and Health Loss Reserves [Member] Life and Health Loss Reserves [Member] Supplemental Information for Property, Casualty Insurance Underwriters [Line Items] Supplemental Information for Property, Casualty Insurance Underwriters [Line Items] Reserve for loss and loss expenses Tax at statutory rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (decrease) resulting from: Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount [Abstract] Tax-exempt income from municipal bonds Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount Dividend received exclusion Effective Income Tax Rate Reconciliation, Deduction, Dividends, Amount Impact of tax reform Effective Income Tax Rate Reconciliation, Tax Accounting Method Changes, Amount Effective Income Tax Rate Reconciliation, Tax Accounting Method Changes, Amount Other Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Tax at statutory rate Increase (decrease) resulting from: Effective Income Tax Rate Reconciliation, Percent [Abstract] Tax-exempt income from municipal bonds Effective Income Tax Rate Reconciliation, Tax Exempt Income, Percent Dividend received exclusion Effective Income Tax Rate Reconciliation, Deduction, Dividend, Percent Impact of tax reform Other Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Provision for income taxes Effective Income Tax Rate Reconciliation, Percent Senior Debentures 6.900 % Due 2028 Senior Debentures 6.9 Percent Due 2028 [Member] Senior Debentures 6.9 Percent Due 2028 [Member] Senior Debentures 6.920 % Due 2028 Senior Debentures 6.92 Percent Due 2028 [Member] Senior Debentures 6.92 Percent Due 2028 [Member] Senior Notes 6.125 % Due 2034 Senior Notes 6.125 Percent Due 2034 [Member] Senior Notes 6.125 Percent Due 2034 [Member] Debt instrument, year of maturity Debt Instrument, Maturity Date, Description Fair value of note payable Notes Payable, Fair Value Disclosure Fair value of senior debt Long-term Debt, Fair Value Total Debt Instrument, Fair Value Disclosure Comprehensive Income (Loss) Note Comprehensive Income (Loss) Note [Text Block]