Statement of Cash Flows [Abstract] Interest Expense Debt To Affiliates Interest Expense Debt To Affiliates Interest expense debt to affiliates. Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Segment Reconciling Items [Member] Segment Reconciling Items [Member] Specialty Insurance Segment [Member] Specialty Insurance Segment [Member] Specialty Insurance Segment [Member] Specialty Insurance [Member] Specialty Insurance [Member] Specialty insurance. Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Acquisition And Operating Expenses Acquisition And Operating Expenses Acquisition expenses including the amortization of deferred acquisition costs (DAC) and general and administrative expenses. Number of reportable segments (segments) Number of Reportable Segments Investable assets Investable Assets Investable assets. Earnings Per Share [Abstract] Number of antidilutive options and awards Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Income Tax Disclosure [Abstract] Reconciliation Between Actual Federal Income Tax Benefit and Amount Computed at Indicated Statutory Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Class A Preferred Stock [Member] Preferred Class A [Member] Class B Preferred Stock [Member] Preferred Class B [Member] Statement [Line Items] Statement [Line Items] Fixed maturities, available-for-sale, amortized cost Debt Securities, Available-for-sale, Amortized Cost Deferred policy acquisitions costs from affiliates Deferred Policy Acquisitions Costs from Affiliates Net deferred policy acquisition costs Losses and loss expenses payable, affiliates Losses And Loss Expenses Payable From Affiliate Losses and loss expenses payable from affiliate. Unearned premiums, affiliates Unearned Premiums From Affiliates Unearned Premiums From Affiliates Notes payable, affiliates Due to Related Parties Net Postretirement and Pension Benefits Net Postretirement and Pension Benefits Net Postretirement and Pension Benefits Ceded to Affiliates Net Other Liabilities from Affiliates Net Other Liabilities from Affiliates Net Other Liabilities from Affiliates after Pooling Preferred stock, no par value (usd per share) Preferred Stock, No Par Value Preferred stock, shares authorized (shares) Preferred Stock, Shares Authorized Preferred stock, shares issued (shares) Preferred Stock, Shares Issued Common stock, no par value (usd per share) Common Stock, No Par Value Common stock, shares authorized (shares) Common Stock, Shares Authorized Common stock, shares issued (shares) Common Stock, Shares, Issued Common stock, stated value per share (usd per share) Common Stock, Par or Stated Value Per Share Treasury stock (shares) Treasury Stock, Shares Net Accumulated Other Comprehensive Income Net Accumulated Other Comprehensive Income Net Accumulated Other Comprehensive Income ceded to affiliates Income Taxes Income Tax Disclosure [Text Block] Deferred Policy Acquisition Costs Disclosures [Abstract] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Deferred Policy Acquisition Cost, Period Start Deferred Policy Acquisition Cost Deferred Policy Acquisition Cost, Capitalization Deferred Policy Acquisition Cost, Capitalization Deferred Policy Acquisition Costs, Disposition Deferred Policy Acquisition Costs, Disposition Deferred Policy Acquisition Cost, Period End Accounting Policies [Abstract] Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Retirement Benefits [Abstract] Company's Share of Net Periodic Cost Components Schedule of Net Benefit Costs [Table Text Block] Equity [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Unrealized Gains and Losses on Available-for-Sale Securities [Member] AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] Benefit Plan Items [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Beginning balance Accumulated Other Comprehensive Income (Loss), Net of Tax Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018 Cumulative Effect of New Accounting Principle in Period of Adoption Other comprehensive income before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from AOCI Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Other comprehensive income Other Comprehensive Income (Loss), Net of Tax Ending balance Insurance [Abstract] Unearned Premiums Ceded Unearned Premiums Ceded Unearned Premiums Ceded Premiums earned: Premiums Written and Earned [Abstract] Assumed from external insurers and reinsurers Assumed Premiums Earned Assumed under Pooling Arrangement Assumed Premiums Under Pooling Arrangement Assumed premiums under pooling arrangement. Ceded to external insurers and reinsurers Ceded Premiums Earned Ceded under Pooling Arrangement Premiums Ceded Under Pooling Arrangement Premiums ceded under pooling arrangement. Net assumed premiums earned Premiums Earned, Net, Other Insurance Losses and loss expenses incurred: Reinsurance Liabilities [Abstract] Assumed from external insurers and reinsurers Losses And Loss Expenses Assumed From External Insurers And Reinsurers Losses and loss expenses assumed from external insurers and reinsurers. Assumed under Pooling Arrangement Losses And Loss Expenses Assumed Under Pooling Arrangement Losses and loss expenses assumed under pooling arrangement. Ceded to external insurers and reinsurers Losses And Loss Expenses Ceded To External Insurers And Reinsurers Losses and loss expenses ceded to external insurers and reinsurers. Ceded under Pooling Arrangement Losses And Loss Expenses Ceded Under Pooling Arrangement Losses and loss expenses ceded under pooling arrangement. Net assumed losses and loss expenses incurred Net Assumed Losses And Loss Expenses Incurred Net assumed losses and loss expenses incurred. Pension and Postretirement Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Investments, Debt and Equity Securities [Abstract] Schedule of Gain (Loss) on Securities [Table] Schedule of Gain (Loss) on Securities [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Equity Securities [Member] Equity Securities [Member] Other Invested Assets [Member] Other Invested Assets [Member] Other invested assets. Gain (Loss) on Securities [Line Items] Gain (Loss) on Securities [Line Items] Debt Securities, Realized Gain (Loss): Debt Securities, Realized Gain (Loss) [Abstract] Debt Securities, Available-for-sale, Realized Gain Debt Securities, Available-for-sale, Realized Gain Debt Securities, Available-for-sale, Realized Loss Debt Securities, Available-for-sale, Realized Loss Debt Securities, Available-for-sale, Net Realized Gain (Loss) Debt Securities, Available-for-sale, Realized Gain (Loss) Equity Securities, FV-NI, Gain (Loss) Equity Securities, FV-NI, Gain (Loss) [Abstract] Equity Securities, FV-NI, Realized Gain (Loss) Equity Securities, FV-NI, Realized Gain (Loss) Equity Securities, FV-NI, Unrealized Gain (Loss) Equity Securities, FV-NI, Unrealized Gain (Loss) Equity Securities, FV-NI, Net Gain (Loss) Equity Securities, FV-NI, Gain (Loss) Other net realized loss Impairment of Long-Lived Assets to be Disposed of Gain (Loss) on Investments Gain (Loss) on Investments Change in unrealized holding gains (losses), net of tax: Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract] Change in net unrealized holding (losses) gains, net of tax Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Total net unrealized holding gain on available-for-sale investments Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net (loss) income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Adjustments to reconcile net income to net cash (used in) provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization, net Depreciation, Amortization and Accretion, Net Share-based compensation Share-based Payment Arrangement, Noncash Expense Net investment loss (gain) Gain (Loss) on Sale of Investments Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Deferred policy acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Accrued investment income and other assets Increase Decrease In Accrued Investment Income And Other Assets Increase decrease in accrued investment income and other assets. Postretirement and pension benefits Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits Other liabilities and due to/from affiliates, net Increase (Decrease) in Accrued Investment Income Receivable Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums Increase Decrease In Reinsurance Recoverable And Prepaid Reinsurance Premiums Increase decrease in reinsurance recoverable and prepaid reinsurance premiums. Losses and loss expenses payable Increase (Decrease) in Property and Casualty Insurance Liabilities Unearned premiums Increase (Decrease) in Unearned Premiums Excess Tax Benefit from Share-based Compensation, Operating Activities Excess Tax Benefit from Share-based Compensation, Operating Activities Federal income taxes Increase (Decrease) in Income Taxes Payable Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of fixed maturities available-for-sale Payments to Acquire Debt Securities, Available-for-sale Purchases of equity securities Payments to Acquire Available-for-sale Securities, Equity Purchases of other invested assets Payments to Acquire Other Investments Maturities, calls and pay downs of fixed maturities available-for-sale Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-sale Sales of fixed maturities available-for-sale Proceeds from Sale of Debt Securities, Available-for-sale Sales of equity securities Proceeds from Sale of Available-for-sale Securities, Equity Sales of other invested assets Proceeds from Sale of Other Investments Proceeds from Sale of Property, Plant, and Equipment Proceeds from Sale of Property, Plant, and Equipment Net cash provided by investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of common stock Proceeds from Issuance of Common Stock Payments to acquire treasury stock Payments To Acquire Treasury Stock Payments to acquire treasury stock. Payment of dividends Payments of Ordinary Dividends, Common Stock Proceeds from Short-term Debt Proceeds from Short-term Debt Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash and cash equivalents at beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash and cash equivalents at end of period Cash and Cash Equivalents, at Carrying Value Supplemental disclosures: Supplemental Cash Flow Information [Abstract] Interest Paid, Excluding Capitalized Interest, Operating Activities Interest Paid, Excluding Capitalized Interest, Operating Activities Cost or Amortized Cost and Fair Value of Available-for-Sale Securities Cost Or Amortized Cost And Fair Value Of Available For Sale Securities [Table Text Block] Cost Or Amortized Cost And Fair Value Of Available For Sale Securities [Table Text Block] Gross Unrealized Losses and Fair Value on its Investments Available For Sale Securities Continuous Unrealized Loss Position Fair Value And Aggregate Losses [Table Text Block] Available For Sale Securities Continuous Unrealized Loss Position Fair Value And Aggregate Losses [Table Text Block] Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity Investments Classified by Contractual Maturity Date [Table Text Block] Components of Net Investment Income Investment Income [Table Text Block] Realized and Unrealized Holding Gains (Losses) Realized And Unrealized Holding Gains Losses Table [Table Text Block] Realized and unrealized holding gains losses. Commitments and Contingencies Disclosure [Abstract] Contingencies and Litigation Commitments and Contingencies Disclosure [Text Block] Earnings (Loss) per Common Share Earnings Per Share [Text Block] Current Expected Credit Losses [Abstract] Current Expected Credit Losses [Abstract] Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Reinsurance Recoverable, Allowance for Credit Loss Reinsurance Recoverable, Allowance for Credit Loss Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) Reinsurance Recoverable, Credit Loss Expense (Reversal) Reinsurance Recoverable, Credit Loss Expense (Reversal) Debt Disclosure [Abstract] Schedule of Federal Home Loan Bank Advances, by Branch of FHLB Bank [Table] Schedule of Federal Home Loan Bank Advances, by Branch of FHLB Bank [Table] Federal Home Loan Bank Branch [Axis] Federal Home Loan Bank Branch [Axis] Federal Home Loan Bank Branch [Domain] Federal Home Loan Bank Branch [Domain] Federal Home Loan Bank of Cincinnati [Member] Federal Home Loan Bank of Cincinnati [Member] Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items] Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items] Line of Credit Facility, Current Borrowing Capacity Line of Credit Facility, Current Borrowing Capacity Line of Credit Facility, Expiration Period Line of Credit Facility, Expiration Period Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances Federal Home Loan Bank, Advances, Maturity Period, Fixed Rate Federal Home Loan Bank, Advances, Maturity Period, Fixed Rate Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate Debt Prepayment Cost Payment for Debt Extinguishment or Debt Prepayment Cost Interest rate (percent) Debt Instrument, Interest Rate, Stated Percentage Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Notes Payable and Open Line of Credit Debt Disclosure [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments [Member] Operating Segments [Member] Eliminate Intersegment Revenues [Member] Intersegment Eliminations [Member] Corporate And Reconciling Items [Member] Corporate And Reconciling Items [Member] Corporate And Reconciling Items Personal Insurance Segments [Member] Personal Insurance Segments [Member] Personal insurance segments. Business Insurance Segments [Member] Business Insurance Segments [Member] Business insurance segments. Total Insurance Segments [Member] Insurance Segments [Member] Insurance segments. Net Investment Income [Member] Net Investment Income [Member] Net investment income. Net Realized Capital Gains [Member] Net Realized Capital Gains [Member] Net realized capital gains. Total Investment Operations [Member] Investment Operations Segment [Member] Investment operations segment. All Other Segments [Member] Other Segments [Member] Personal Insurance [Member] Personal Insurance [Member] Personal insurance. Business Insurance [Member] Business Insurance [Member] Business insurance. Segment Reconciling Items [Member] Subsegments [Axis] Subsegments [Axis] Subsegments [Domain] Subsegments [Domain] Revenues from external sources: Revenues From External Sources [Abstract] Revenues from external sources. Revenue Revenues Other Income Other Income Net Investment Income Net Investment Income Realized Investment Gains (Losses) Realized Investment Gains (Losses) Segment income before federal income tax: Segment Loss Income Before Federal Income Tax [Abstract] Segment (loss) income before federal income tax. Gaap Expense Adjustments Gaap Expense Adjustments GAAP expense adjustments. Interest expense on corporate debt Interest Expense, Debt Corporate Expenses Corporate Expenses Corporate expenses. Income before federal income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Premium Receivable, Credit Loss Expense (Reversal) Premium Receivable, Credit Loss Expense (Reversal) Fair Value Disclosures [Abstract] Fair Value Measurements Disclosure [Table] Fair Value Measurements Disclosure [Table] Fair Value Measurements Disclosure [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Affiliated Entity [Member] Affiliated Entity [Member] Corporate Securities [Member] Corporate Debt Securities [Member] International funds [Member] International funds [Member] International funds [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value Measurements Disclosure [Line Items] Fair Value Measurements Disclosure [Line Items] Fair Value Measurements Disclosure [Line Items] Number of available for sale securities Number of Available for Sale Securities Number of Available for Sale Securities Other invested assets Other Investments Federal home loan bank stock Federal Home Loan Bank Stock Other invested assets, at cost Other Investments and Securities, at Cost Number of Credit Agreements Number of Credit Agreement Number of Credit Agreement Notes receivable from affiliate Notes Receivable, Related Parties Notes receivable interest rate (percent) Notes Receivable Interest Rate, stated percentage Notes receivable interest rate, stated percentage Effective Income Tax Rate Reconciliation, Amount [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Amount at statutory rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Tax-exempt interest and dividends received deduction Income Tax Reconciliation Tax Exempt Income And Dividend Deduction Income tax reconciliation tax exempt income and dividend deduction. Other, net Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Federal income tax expense and effective rate Federal Income Tax Expense (Benefit), Continuing Operations Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount Income Tax Expense (Benefit) Income Tax Expense (Benefit) Effective Income Tax Rate Reconciliation, Percent [Abstract] Effective Income Tax Rate Reconciliation, Percent [Abstract] Amount at statutory rate (percent) Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Tax-exempt interest and dividends received deduction (percent) Income Tax Reconciliation Tax Exempt Income And Dividend Deductions Tax Rate Income tax reconciliation tax exempt income and dividend deductions tax rate. Other, net (percent) Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Effective Income Tax Rate Reconciliation, Deduction, Percent Effective Income Tax Rate Reconciliation, Deduction, Percent Liability for Claims and Claims Adjustment Expense [Abstract] Liability for Claims and Claims Adjustment Expense, Property Casualty Liability Liability for Claims and Claims Adjustment Expense, Property Casualty Liability Reinsurance Recoverables, Including Reinsurance Premium Paid Reinsurance Recoverables, Including Reinsurance Premium Paid Liability for Unpaid Claims and Claims Adjustment Expense, Net Liability for Unpaid Claims and Claims Adjustment Expense, Net Current Year Claims and Claims Adjustment Expense Current Year Claims and Claims Adjustment Expense Prior Year Claims and Claims Adjustment Expense Prior Year Claims and Claims Adjustment Expense Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Losses and Loss Expenses Payable [Abstract] Losses and Loss Expenses Payable [Abstract] Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification Out of Accumulated Other Comprehensive Income [Member] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Realized gain/(loss) on sale of securities Accumulated other comprehensive income components/Negative prior-service costs Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Accumulated other comprehensive income components/Net actuarial loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Total before tax Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Tax (expense) benefit Net of tax Net Income (Loss) Attributable to Parent Total reclassifications for the period Reinsurance Reinsurance [Text Block] Reinsurance Recoverable, Credit Quality Indicator [Table Text Block] Reinsurance Recoverable, Credit Quality Indicator [Table Text Block] Reinsurance Recoverable, Allowance for Credit Loss [Table Text Block] Reinsurance Recoverable, Allowance for Credit Loss [Table Text Block] Other Comprehensive Income and Accumulated Other Comprehensive Income Comprehensive Income (Loss) Note [Text Block] Numerator: Numerator [Abstract] Numerator. Denominator: Denominator [Abstract] Denominator. Weighted average shares for basic net earnings per common share (shares) Weighted Average Number of Shares Outstanding, Basic Effect of dilutive share-based awards (shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Adjusted weighted average shares for diluted net earnings per common share (shares) Weighted Average Number of Shares Outstanding, Diluted Basic net (loss) earnings per common share (usd per share) Earnings Per Share, Basic Diluted net (loss) earnings per common share (usd per share) Earnings Per Share, Diluted Reinsurance Recoverable, Credit Quality Indicator [Table] Reinsurance Recoverable, Credit Quality Indicator [Table] Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] AM Best, A++ Rating [Member] AM Best, A++ Rating [Member] AM Best, A+ Rating [Member] AM Best, A+ Rating [Member] AM Best, A Rating [Member] AM Best, A Rating [Member] Reinsurance Recoverable, Credit Quality Indicator [Line Items] Reinsurance Recoverable, Credit Quality Indicator [Line Items] Reinsurance Recoverables, Gross Reinsurance Recoverables, Gross Company's Investments within Fair Value Hierarchy Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities Note Receivables From Affiliates Fair Value Disclosure [Table Text Block] Note Receivables From Affiliates Fair Value Disclosure [Table Text Block] Included in Notes Payable are Senior Notes and Subordinated Debentures Notes Payable Fair Value Disclosure Table [Table Text Block] Notes payable fair value disclosure. Amortized cost Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract] Due in 1 year or less, Amortized cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost Due after 1 year through 5 years, Amortized cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost Due after 5 years through 10 years, Amortized cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost Due after 10 years, Amortized cost Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost U.S. government agencies mortgage-backed securities, Amortized cost Mortgage Backed Securities Amortized Cost Mortgage backed securities amortized cost. Total, Amortized cost Fair value Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] Due in 1 year or less, Fair value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Due after 1 year through 5 years, Fair value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Due after 5 years through 10 years, Fair value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Due after 10 years, Fair value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value U.S. government agencies mortgage-backed securities, Fair value Mortgage Backed Securities Fair Value Mortgage backed securities fair value. Total, Fair value Debt Securities, Available-for-sale Credit Loss, Financial Instrument [Text Block] Credit Loss, Financial Instrument [Text Block] Cover [Abstract] Entities [Table] Entities [Table] Common Class A [Member] Common Class A [Member] Entity Listings, Exchange [Axis] Entity Listings, Exchange [Axis] Exchange [Domain] Exchange [Domain] NASDAQ/NGS (GLOBAL SELECT MARKET) [Member] NASDAQ/NGS (GLOBAL SELECT MARKET) [Member] Entity Information [Line Items] Entity Information [Line Items] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, State or Province Entity Address, State or Province Entity Address, City or Town Entity Address, City or Town Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Shell Company Entity Shell Company Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Common Stock, Shares Outstanding (shares) Entity Common Stock, Shares Outstanding Statement of Comprehensive Income [Abstract] Other comprehensive income (loss), net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Net unrealized holding gain (loss) on fixed available-for-sale investments: Net Unrealized Holding Gains On Investments [Abstract] Net unrealized holding gains on investments. Unrealized holding gain Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax Reclassification adjustments for gains realized in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Income tax expense Other Comprehensive Income (Loss), Securities, Available-for-sale, Tax Net unrecognized benefit plan obligations: Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax [Abstract] Reclassification adjustments for amortization to statements of income: Reclassification Adjustments For Amortization To Net Income [Abstract] Reclassification adjustments for amortization to net income. Negative prior service cost Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, before Tax Net actuarial loss Other Comprehensive Income Reclassification Of Defined Benefit Plan Net Actuarial Loss Before Tax Other comprehensive income reclassification of defined benefit plan net actuarial loss before tax. Income tax expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Total net unrecognized benefit plan obligations Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Comprehensive (loss) income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Deferred Policy Acquisition Costs [Table Text Block] Deferred Policy Acquisition Costs [Table Text Block] Investment Income [Table] Investment Income [Table] Cash and Cash Equivalents, and Other [Member] Cash and Cash Equivalents [Member] Fixed Maturities [Member] Debt Securities [Member] Net Investment Income [Line Items] Net Investment Income [Line Items] Investment income Investment Income, Interest Investment expenses Investment Income, Investment Expense Net investment income Statement of Stockholders' Equity [Abstract] Common Stock [Member] Common Stock [Member] Treasury Stock [Member] Treasury Stock [Member] Additional Paid-in Capital [Member] Additional Paid-in Capital [Member] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Retained Earnings [Member] Retained Earnings [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance at beginning of year (shares) Shares, Outstanding Issuance of shares (shares) Stock Issued During Period, Shares, New Issues Balance at end of year (shares) Balance at beginning of year Stockholders' Equity Attributable to Parent Balance at end of year Issuance of shares Stock Issued During Period, Value, New Issues Shares acquired on stock award exercises and vested restricted shares Stock Issued During Period, Value, Stock Options Exercised Stock awards granted Adjustments To Additional Paid In Capital Stock Options Granted Adjustments to additional paid in capital, stock options granted. Change in net unrealized holding (losses) gains, net of tax Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments Net income (loss) Cash dividends paid Dividends, Common Stock, Cash Financial Information Regarding Company's Reportable Segments Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block] Investments Investment, Policy [Policy Text Block] Premiums Premiums Receivable, Basis of Accounting, Policy [Policy Text Block] New Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Deferred Policy Acquisition Costs [Text Block] Losses and Loss Expenses Payable - Additional Information [Abstract] Losses and Loss Expenses Payable - Additional Information [Abstract] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Catastrophic Event [Axis] Catastrophic Event [Axis] Catastrophic Event [Domain] Catastrophic Event [Domain] Catastrophe [Member] Catastrophe [Member] Noncatastrophic Event [Axis] Noncatastrophic Event [Axis] Noncatastrophic Event [Domain] Noncatastrophic Event [Domain] Unallocated loss adjustment expense [Member] Unallocated loss adjustment expense [Member] Unallocated loss adjustment expense [Member] Business Insurance Segments [Member] Personal And Business Insurance Segments [Member] Personal and Business Insurance Segments [Member] E&S Property [Member] E&S Property [Member] E&S Property [Member] Homeowners [Member] Homeowners [Member] Homeowners [Member] Middle Market Commercial [Member] Middle Market Commercial [Member] Commercial Insurance [Domain] Personal Auto [Member] Personal Auto [Member] Personal Auto [Member] Commercial Auto [Member] Commercial Auto [Member] Commercial Auto [Member] Workers' compensation [Member] Workers' compensation [Member] Workers' compensation [Member] E&S Casualty [Member] Excess & Surplus Casualty [Member] Excess & Surplus Casualty [Member] Programs [Member] Programs [Member] Programs [Member] Small commercial [Member] Small commercial [Member] Small commercial [Member] Other Personal [Member] Other Personal [Member] Other Personal [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Financial Instruments, Owned and Pledged as Collateral, at Fair Value Financial Instruments, Owned and Pledged as Collateral, at Fair Value Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Fixed maturities fair value of deposit with insurance regulators Available For Sale Debt Securities With Insurance Regulators Available for sale debt securities with insurance regulators. Proceeds on sale of available-for-sale securities Proceeds from Sale of Available-for-sale Securities New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets Due from Affiliate, Current Due from Affiliate, Current Premium Receivable, Allowance for Credit Loss Premium Receivable, Allowance for Credit Loss Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Changes in Accumulated Other Comprehensive Income by Component, Net of Tax Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Compilation of Basic and Diluted Earnings Per Common Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Options to Purchase Shares of Common Stock Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Income Statement [Abstract] Earned premiums, from affiliates Premiums Earned Net Property And Casualty To Affiliates Premiums earned net property and casualty to affiliates. Net investment income, from affiliate Net Investment Income From Affiliates Net investment income, affiliates. Losses and loss expenses, from affiliates Incurred Claim Property Casualty And Liability To Affiliates Incurred claim property casualty and liability to affiliates. Net Acquisition and Operating Expenses, from affiliates Net Acquisition and Operating Expenses Net Acquisition and Operating Expenses Debt and Equity Securities, Gain (Loss) Debt and Equity Securities, Gain (Loss) Earned premiums (affiliated net assumed $71.9 and $115.3, respectively, and $251.1 and $349.0, respectively) Premiums Earned, Net, Property and Casualty Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively) Net realized gains on investments: Realized Investment Gains (Losses) [Abstract] Total other-than-temporary impairment losses Other than Temporary Impairment Losses, Investments Total net realized gains on investments Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest Other income from affiliates Total revenues Losses and loss expenses (affiliated net assumed $34.7 and $128.9, respectively, and $105.4 and $298.8, respectively) Acquisition and operating expenses (affiliated net ceded $26.8 and affiliated net assumed $78.9, respectively, and $52.3 and $231.0, respectively) Interest expense (affiliates $0.2 and $0.2, respectively, and $0.7 and $0.6, respectively) Other Expenses Other Expenses Costs and Expenses Costs and Expenses Income before federal income taxes Current Income Tax Expense (Benefit) Current Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Federal income tax expense Net income (Loss) earnings per common share: Basic (usd per share) Diluted (usd per share) Dividends paid per common share (usd per share) Common Stock, Dividends, Per Share, Cash Paid Contributions by employer Defined Benefit Plan, Plan Assets, Contributions by Employer Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Short-term Debt, Type [Axis] Short-term Debt, Type [Axis] Short-term Debt, Type [Domain] Short-term Debt, Type [Domain] Federal Home Loan Bank Advances [Member] Federal Home Loan Bank Advances [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Reported Value Measurement [Member] Reported Value Measurement [Member] Estimate of Fair Value Measurement [Member] Estimate of Fair Value Measurement [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Notes Payable to Banks [Member] Notes Payable to Banks [Member] Affiliate Subordinated Debentures Due 2033 [Member] Subordinated Debt [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Total notes payable Notes Payable, Fair Value Disclosure FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest Debt Instrument, Face Amount Summary of External Reinsurance Transactions with State Auto Mutual Under Pooling Arrangement Schedule Of Premiums Earned And Losses Incurred Table [Text Block] Schedule of premiums earned and losses incurred. Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value [Table] Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value [Table] U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] US Treasury and Government [Member] Obligations of States and Political Subdivisions [Member] US States and Political Subdivisions Debt Securities [Member] U.S. Government Agencies Mortgage-Backed Securities [Member] Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items] Debt Securities, Available-for-sale, 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, Accumulated Loss Less than 12 months, Number of positions Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year 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, Accumulated Loss 12 months or more, Number of positions Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Total, Fair value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Total, Unrealized losses Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Total, Number of positions Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Receivable Type [Axis] Receivable Type [Axis] Receivable [Domain] Receivable [Domain] Note Receivable from Affiliate, 2009 [Member] Note Receivable from Affiliate, 2009 [Member] An amount representing an agreement for an unconditional promise by the maker to pay the Entity (holder) a definite sum of money at a future date(s) within one year of the balance sheet date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among myriad other features and characteristics. Note Receivable from Affiliate, 2019 [Member] Note Receivable from Affiliate, 2019 [Member] An amount representing an agreement for an unconditional promise by the maker to pay the Entity (holder) a definite sum of money at a future date(s) within one year of the balance sheet date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among myriad other features and characteristics. Level 2 [Member] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Notes receivable from affiliate Notes Receivable, Fair Value Disclosure Segment Information Segment Reporting Disclosure [Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension [Member] Pension Plan [Member] Postretirement [Member] Other Postretirement Benefits Plan [Member] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] 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 Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of: Defined Benefit Plan Amortization [Abstract] Defined Benefit Plan Amortization [Abstract] Defined Benefit Plan, Amortization of Gain (Loss) Defined Benefit Plan, Amortization of Gain (Loss) Net periodic cost (benefit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Assets Assets [Abstract] Fixed maturities, available-for-sale, at fair value (amortized cost $2,051.8 and $2,080.0, respectively) Equity securities Available-for-sale Securities, Equity Securities Total investments Investments Cash and cash equivalents Accrued investment income and other assets Accrued Investment Income Receivable Deferred policy acquisition costs (affiliated net assumed $21.8 and $20.2, respectively) Reinsurance recoverable on losses and loss expenses payable Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments Prepaid reinsurance premiums Prepaid Reinsurance Premiums Due from Affiliates Due from Affiliates Current federal income taxes Income Taxes Receivable, Current Net deferred federal income taxes Deferred Income Tax Assets, Net Property and equipment, held for sale Property, Plant and Equipment, Net Total assets Assets Liabilities and Stockholders’ Equity Liabilities and Equity [Abstract] Losses and loss expenses payable (affiliated net assumed $457.3 and $500.8, respectively) Unearned premiums (affiliated net assumed $126.3 and $116.7, respectively) Unearned Premiums Notes payable (affiliates $15.2 and $15.2, respectively) Notes Payable Pension and postretirement benefits Liability, Defined Benefit Plan Due to Affiliate Due to Affiliate Other liabilities (affiliated net assumed $10.7 and $19.5, respectively) Other Liabilities Total liabilities Liabilities Stockholders’ equity: Stockholders' Equity Attributable to Parent [Abstract] Preferred stock value Preferred Stock, Value, Issued Common stock, without par value. Authorized 100.0 shares; 50.6 and 50.4 shares issued, respectively, at stated value of $2.50 per share Common Stock, Value, Issued Treasury stock, 6.9 and 6.9 shares, respectively, at cost Treasury Stock, Value Additional paid-in capital Additional Paid in Capital, Common Stock Accumulated other comprehensive loss Retained earnings Retained Earnings (Accumulated Deficit) Total stockholders’ equity Total liabilities and stockholders’ equity Liabilities and Equity Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-sale [Table] Large-Cap Equity Securities [Member] Large Cap Securities [Member] Large-cap securities. Mutual and exchange traded funds [Domain] [Member] Mutual and exchange traded funds [Domain] [Member] Mutual and exchange traded funds [Domain] [Member] Cost or amortized cost Available-for-sale Securities, Amortized Cost Basis Gross unrealized holding gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross unrealized holding losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair value Available-for-sale Securities Schedule Of Activity In Liability For Unpaid Losses And Loss Adjustment Expenses Table [Table Text Block] Schedule Of Activity In Liability For Unpaid Losses And Loss Adjustment Expenses Table [Table Text Block] Schedule Of Activity In Liability For Unpaid Losses And Loss Adjustment Expenses Table [Text Block] Fair Value Option, Disclosures [Table] Fair Value Option, Disclosures [Table] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fixed Maturities [Member] Fixed Maturities [Member] Fair Value, Option, Quantitative Disclosures [Line Items] Fair Value, Option, Quantitative Disclosures [Line Items] Investments, Fair Value Disclosure Investments, Fair Value Disclosure