Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Weighted average common shares outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Trading Arrangement: Trading Arrangement [Axis] Common stock, shares outstanding: Common Stock, Shares Outstanding Roll Forward [Roll Forward] Common Stock, Shares Outstanding Roll Forward Maximum Maximum [Member] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Statistical Measurement [Domain] Statistical Measurement [Domain] Ceded Ceded Premiums Earned Title of 12(b) Security Title of 12(b) Security Carrying Amount Reported Value Measurement [Member] Debt Securities, Available-for-Sale [Table] Debt Securities, Available-for-Sale [Table] Net income (loss) available to common stockholders, diluted Net income (loss) available to common stockholders, diluted Net Income (Loss) Available to Common Stockholders, Diluted Document Fiscal Year Focus Document Fiscal Year Focus Adjustment to Compensation, Amount Adjustment to Compensation Amount Financial Instruments [Axis] Financial Instrument [Axis] Financial Instrument [Axis] Other investment assets measured at fair value Other Assets, Fair Value Disclosure Increase in accrued investment income Increase (Decrease) in Interest and Dividends Receivable Segment Reporting [Abstract] Segment Reporting [Abstract] Common stock Common Stock, Value, Issued Depreciation and amortization Depreciation, Amortization and Accretion, Net Revenue from External Customers by Products and Services [Table] Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Table] Retirement Plans Retirement Benefits [Text Block] Initial Allowance for Purchased Credit Deteriorated Assets with Credit Deterioration Debt Securities, Available-for-Sale, Allowance for Credit Loss, Purchased with Credit Deterioration, Increase Restatement Determination Date Restatement Determination Date Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Accrued salaries and benefits Employee-related Liabilities Long-term debt Carrying Amount Long-Term Debt and Lease Obligation, Including Current Maturities OCI before reclassifications Other Comprehensive Income (Loss) before Reclassifications, Tax Measurement Frequency [Axis] Measurement Frequency [Axis] U.S. government and government agencies U.S. government and government agencies US Treasury and Government [Member] Amendment Flag Amendment Flag Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Repayments of finance lease obligations Financing cash flows from finance leases Finance Lease, Principal Payments Loss and loss expense incurred: Policyholder Benefits and Claims Incurred, Net [Abstract] Net investment income earned Net investment income earned Net Investment Income Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS") CLO and other ABS Asset-Backed Securities [Member] Standard Personal Lines Standard Personal Lines [Member] This represents our Personal Lines segment of business. Investments [Domain] Investments [Domain] Other comprehensive income (loss), net of tax: Net Other Comprehensive Income (Loss), Net of Tax [Abstract] Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Table] Liability for Unpaid Claim and Claim Adjustment Expense, Change [Table] Illiquidity Spread Measurement Input, Credit Spread [Member] Related Party Transactions Related Party Transactions Disclosure [Text Block] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Transfers into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Reductions for Securities Identified as Intent (or Requirement) to Sell during the Period Debt Securities, Available-for-Sale, Allowance for Credit Loss, Sell before Recovery Schedule of Cash and Cash Equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Entity Address, State or Province Entity Address, State or Province Net realized and unrealized investment gains (losses) Return On Equity, Net Realized And Unrealized Investment Gains (Losses) Net realized and unrealized investment gains (losses), after-tax, divided by average stockholders equity. AFS Unrealized Losses Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Purchases of equity securities Payments to Acquire Equity Securities, FV-NI Non-catastrophe property loss and loss expense Non-Catastrophe Property Loss And Loss Expense Losses and loss expenses incurred that are attributable to property coverages that we have written throughout our lines of business, but exclude any such amounts that are related to catastrophe losses. PEO Total Compensation Amount PEO Total Compensation Amount Common stock, par or stated value (in dollars per share) Common Stock, Par or Stated Value Per Share Reclassification out of AOCI Reclassification out of Accumulated Other Comprehensive Income [Member] Weighted average common shares outstanding - basic (in shares) Weighted Average Number of Shares Outstanding, Basic Retirement Benefits [Abstract] Retirement Benefits [Abstract] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Premiums receivable Premiums Receivable, Gross Direct Direct Premiums Earned Number of reportable segments Number of Reportable Segments Increase in deferred policy acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Fixed income securities, held-to-maturity, net of allowance for credit losses Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss Corporate actions related to fixed income securities, available-for-sale ("AFS") Noncash or Part Noncash Acquisition, Investments Acquired Schedule of Debt Securities, Available-for-Sale, Allowance for Credit Loss Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table Text Block] Investments: Investments [Abstract] Due after five years through ten years fair value, AFS Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Unrealized gains (losses) on investment securities: Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Entity Address, City or Town Entity Address, City or Town Schedule of Cash Flow, Supplemental Disclosures Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Schedule of Unrealized Gain (Loss) on Equity Securities Unrealized Gain (Loss) on Securities [Table Text Block] Tabular disclosure that provides unrealized gains or losses recognized in income on equity securities separated by: (i) securities that continue to be held; and (ii) securities that were sold. Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Document Information [Line Items] Document Information [Line Items] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Dividends to preferred stockholders Payments of Ordinary Dividends, Preferred Stock and Preference Stock Class of Stock [Axis] Class of Stock [Axis] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Distributions from alternative investments and other investments Proceeds from Equity Method Investment, Distribution, Return of Capital Incurred loss and loss expense for claims occurring in the: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Federal Home Loan Bank Branch [Axis] Federal Home Loan Bank Branch [Axis] Net income (loss) available to common stockholders, basic Net income (loss) available to common stockholders, basic Net Income (Loss) Available to Common Stockholders, Basic Comprehensive income (loss) Comprehensive Income (Loss), Tax The tax effect of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners. Standard Commercial Lines Standard Commercial Lines [Member] This represents our Standard Commercial Lines of business. Dividend reinvestment plan Stock Issued During Period, Value, Dividend Reinvestment Plan Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] 3.03% borrowings from FHLBI 3.03% FHLBI Due 2026 [Member] 3.03% FHLBI Due 2026 Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Current period change for expected credit losses Reinsurance Recoverable, Credit Loss Expense (Reversal) Debt Securities, Available-for-Sale, Allowance for Credit Loss [Roll Forward] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Roll Forward] Schedule of Activity Under the Authorization Schedule Of Stock Repurchases [Table Text Block] Schedule Of Stock Repurchases Insurance Loss Reserves [Abstract] Insurance Loss Reserves [Abstract] Schedule of Alternative Investment Portfolio by Strategy and Remaining Commitment Amount and Maximum Exposure to Loss Associated with Each Strategy Schedule Of Alternative Investment Portfolio By Strategy, Remaining Commitment Amount, and Maximum Exposure to Loss Associated With Each Strategy [Table Text Block] Represents disclosure of information regarding the alternative investment portfolio, including carrying value, remaining commitments, and maximum exposure to loss broken out by strategy. OCI before reclassifications OCI, before Reclassifications, before Tax, Attributable to Parent Investment expenses Investment Income, Investment Expense Investment Income [Table] Investment Income [Table] Treasury stock – at cost (shares:  46,700,278 – 2026; 45,930,091 – 2025) Treasury Stock, Value Current period change for expected credit losses Premium Receivable, Credit Loss Expense (Reversal) Adoption of Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Schedule of Income (Loss) Before and After Federal Income Tax Segment Reporting [Table Text Block] (Gain) loss on disposal of fixed assets Gain (Loss) on Disposition of Property Plant Equipment Related and Nonrelated Parties [Axis] Related and Nonrelated Parties [Axis] Total reinsurance recoverable, gross Reinsurance Recoverables, Gross Other comprehensive income (loss) Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent A++ AM Best, A++ Rating [Member] Insurance Operations Insurance Operations [Member] This represents our the total of our Standard Commercial Lines, Standard Personal Lines, and E&S Lines business. Dividend reinvestment plan (in shares) Stock Issued During Period, Shares, Dividend Reinvestment Plan Proceeds from borrowings (net of debt issuance costs of $4.1 million in 2025) Proceeds from Debt, Net of Issuance Costs Due after one year through five years fair value, AFS Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Document Period End Date Document Period End Date Changes in assets and liabilities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Other insurance expenses Other insurance expenses Other Underwriting Expense Net cash provided by (used in) investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Preferred stock, shares outstanding, beginning balance (in shares) Preferred stock, shares outstanding, ending balance (in shares) Preferred Stock, Shares Outstanding Reinsurance recoverable Total reinsurance recoverable, gross Reinsurance Recoverables on Unpaid Losses, Gross Bonds Surety Product Line [Member] Due after five years through ten years fair value, HTM Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Equity Equity [Text Block] Security Exchange Name Security Exchange Name Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Statements of Cash Flows Cash Flow, Supplemental Disclosures [Text Block] Commercial mortgage loans – at carrying value (fair value: $267,408 – 2026 and $274,895 – 2025) Financing Receivable, before Allowance for Credit Loss Other comprehensive income (loss) Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Accumulated other comprehensive income (loss) (Note 11) Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Total non-rated reinsurers Total Non Rated Reinsurer [Member] Total Non Rated Reinsurers rated by a rating agency. Due after one year through five years carrying value, HTM Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year One through Five Sales of alternative investments and other investments Proceeds From Sale Of Alternative Investments And Other Investments Proceeds From Sale Of Alternative Investments And Other Investments AFS Fair Value, 12 months or longer Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer Purchases of commercial mortgage loans Payments to Acquire Mortgage Notes Receivable Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Businessowners' policies Business Owners Policies [Member] Represents insurance coverage provided for business owners' policies Schedule of Fair Value of Financial Assets Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Table Text Block] Earnings per Common Share Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Earnings per common share: Earnings Per Share [Abstract] Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Net investment income earned Return On Equity, Net Investment Income Earned Net investment income earned, after-tax, divided by average stockholders' equity. Document Fiscal Period Focus Document Fiscal Period Focus Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items] Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items] Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Net credit loss benefit (expense) on fixed income investments Credit Loss Expense (Benefit) On Investment Portfolio The credit loss expense (benefit) during the period on investment portfolio. Schedule of Revenue By Segment Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Table Text Block] Due in one year or less carrying value, HTM Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Year One Total liabilities Liabilities Trading Symbol Trading Symbol Purchases of property and equipment Payments to Acquire Property, Plant, and Equipment Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Workers compensation Workers Compensation [Member] Represents insurance coverage provided to policy holders for workers compensation. Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Less: allowance for credit losses Debt Securities, Held-to-Maturity, Allowance for Credit Loss Company Selected Measure Name Company Selected Measure Name Total net gains (losses) for the period included in OCI Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Purchases of fixed income securities, available-for-sale Payments to Acquire Debt Securities, Available-for-Sale Reclassifications Amounts reclassified from AOCI Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Schedule of Restrictions on Cash and Cash Equivalents Restrictions on Cash and Cash Equivalents [Table Text Block] Reclassifications Reclassification from AOCI, Current Period, before Tax, Attributable to Parent Net realized (gains) losses on disposals and losses on intent-to-sell available-for-sale securities OCI, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized amounts realized upon: (i) the disposal of; or (ii) the intent-to-sell write-down, of available-for-sale securities. Due in one year or less fair value, HTM Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Due after one year through five years fair value, HTM Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Non-cash items: Noncash Investing and Financing Items [Abstract] Purchases of alternative investments and other investments Payments to Acquire Other Investments Defined benefit pension and post-retirement life plans Defined Benefit Pension and Post-Retirement Plans Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Equity securities, cost Equity Securities, FV-NI, Cost Comprehensive income (loss) Comprehensive Income (Loss), Before Tax Amount before tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners. Dividends to policyholders (as a percent) Policyholder Dividend Ratio Ratio of policyholder dividend expenses to net premiums earned. Asset Class [Domain] Asset Class [Domain] Reserve for loss and loss expense (Note 8) Gross reserve for loss and loss expense, at beginning of period Gross reserve for loss and loss expense, at end of period Liability for Claims and Claims Adjustment Expense Related Party Transactions [Abstract] Operating Activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Compensation Amount Outstanding Recovery Compensation Amount Maximum exposure to credit concentration risk of a single issuer of the Company's stockholder's equity, other than to certain U.S. government agencies Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies Securities pledged as collateral with FHLB Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Individual: Individual [Axis] Weighted average common shares outstanding - diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Entity Emerging Growth Company Entity Emerging Growth Company Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Transfers out of Level 3 Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Underlying Security Market Price Change Underlying Security Market Price Change, Percent Schedule of Pre-Tax Net Investment Income Earned Investment Income [Table Text Block] Increase (Decrease) on Securities with Prior Allowance, excluding intent (or Requirement) to Sell Securities Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) 6.70% Senior Notes 6.70% Senior Notes Due 2035 [Member] 6.70% Senior Notes Due 2035 Maximum Exposure to Loss Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount ROE Return On Equity The after-tax income divided by average stockholders' equity. Schedule of Summary of the Changes in Fair Value of Securities Using Level 3 Inputs Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Percentage common stock owned by related party Percentage Common Stock Owned by Related Party Percentage of common stock owned by a related party Accrued investment income Accrued Investment Income Receivable Operating cash flows from operating leases Operating Lease, Payments Conversion of commercial mortgage loan ("CML") to alternative investment Conversion Of Commercial Mortgage Loan To Alternative Investment Conversion Of Commercial Mortgage Loan To Alternative Investment Award Timing Predetermined Award Timing Predetermined [Flag] Treasury stock: Treasury Stock, Common [Member] Deferred Deferred Federal Income Tax Expense (Benefit) Schedule of Components of Comprehensive Income-Gross and Net of Tax Comprehensive Income (Loss) [Table Text Block] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-Sale [Line Items] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Loss and loss expense incurred Net Loss and loss expense incurred Policyholder Benefits and Claims Incurred, Net Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) Fair Value, Inputs, Level 1 [Member] Net reserve for loss and loss expense, at beginning of period Net reserve for loss and loss expense, at end of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Effect of dilutive securities - stock compensation plans (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Schedule of Reinsurance Recoverable, Credit Quality Indicator Reinsurance Recoverable, Credit Quality Indicator [Table Text Block] Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table] Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table] All Executive Categories All Executive Categories [Member] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] E&S Lines E&S Lines [Member] This represents our E&S Lines of business Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Residential mortgage-backed securities ("RMBS") RMBS Residential Mortgage-Backed Securities [Member] General liability General Liability [Member] Preferred stock, par or stated value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Reinsurance Recoverable [Axis] Reinsurance Recoverable [Axis] Reinsurance Recoverable [Axis] Fair Value Long-term debt, fair value Long-Term Debt, Fair Value Ceded Policyholder Benefits and Claims Incurred, Ceded Schedule of Components of Net Periodic Benefit Cost Schedule of Net Benefit Costs [Table Text Block] Cash paid for amounts included in the measurement of lease liabilities: Cash Paid for Amounts Included in the Measurement of Lease Liabilities [Abstract] Cash Paid for Amounts Included in the Measurement of Lease Liabilities Ratio Insurance Ratios [Abstract] Federal Home Loan Bank Branch [Domain] Federal Home Loan Bank Branch [Domain] Conversion of AFS fixed income securities to equity securities Conversion of AFS fixed income securities to equity securities Conversion of AFS fixed income securities to equity securities Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Number of insurance subsidiaries Number Of Insurance Subsidiaries Number Of Insurance Subsidiaries Paid loss and loss expense for claims occurring in the: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Schedule of Earnings Per Common Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Reinsurance recoverable, net of allowance for credit losses Total reinsurance recoverable, net Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments Dividends to policyholders Policyholder Dividends, Expense Segment Information Segment Reporting [Text Block] Investments Investment in Debt and Equity Securities and Other Trading Assets [Text Block] Redemption and maturities of fixed income securities, available-for-sale Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] Dividends to common stockholders Payments of Ordinary Dividends, Common Stock Expiration Date Trading Arrangement Expiration Date Net increase (decrease) in cash and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Liabilities: Liabilities [Abstract] Location, Statement of Income, Balance [Axis] Location, Statement of Income, Balance [Axis] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Alternative investments Total alternative investments Partnership Interest [Member] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Issuances Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances Other liabilities Other Liabilities Affiliated Entity Affiliated Entity [Member] FHLBNY Collateral Federal Home Loan Bank of New York [Member] Dividends to common stockholders Dividends, Common Stock Total net periodic pension cost (benefit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Other than federal and state pools Other Federal And State Pools [Member] Represents federal and state pools not otherwise mentioned. Common stock, authorized (in shares) Common Stock, Shares Authorized Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Due after ten years fair value, AFS Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 Exercise Price Award Exercise Price Investing Activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Fair Value, Recurring Fair Value, Recurring [Member] Share repurchase program, authorized, amount Share Repurchase Program, Authorized, Amount Segment Reporting [Line Items] Segment Reporting [Line Items] Document Quarterly Report Document Quarterly Report Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Segment [Domain] Segments [Domain] Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items] Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items] Obligations of states and political subdivisions US States and Political Subdivisions Debt Securities [Member] Non Rated Reinsurer [Domain] Non Rated Reinsurer [Domain] [Domain] for Reinsurers that are not rated by AM Best Rating Agency Sales of fixed income securities, available-for-sale Proceeds from Sale of Debt Securities, Available-for-Sale Other Other Commercial [Member] Represents insurance coverage provided to policy holders on other commercial line policies. Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Preferred stock, liquidation preference (in dollars per share) Preferred Stock, Liquidation Preference Per Share Unamortized debt issuance costs Debt Issuance Costs, Net Casualty lines Casualty Insurance Product Line [Member] Product line consisting of casualty-related insurance. Unrealized gains (losses) on securities with credit loss recognized in earnings Unrealized Gains (Losses) On Securities With Credit Loss Recognized In Earnings, Net Amount, after tax and before reclassification adjustment, of unrealized holding gain (loss) on investments in debt securities having credit losses recognized in the consolidated statement of income, and includes non-credit changes on debt securities currently having credit losses. Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Equity Component [Domain] Equity Component [Domain] Deferred policy acquisition costs Deferred Policy Acquisition Cost PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Measurement Input Type [Domain] Measurement Input Type [Domain] Total net gains (losses) for the period included in net income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Proceeds from commercial mortgage loans Proceeds from Sale and Collection of Mortgage Notes Receivable Expenses: Benefits, Losses and Expenses [Abstract] Adoption Date Trading Arrangement Adoption Date Remaining Commitment Remaining Commitment This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required. Cash paid (received) during the period for: Cash Paid (Received) During the Period [Abstract] Cash Paid (Received) During the Period Counterparty Name [Domain] Counterparty Name [Domain] Interest Rate Interest rate (as a percent) Debt Instrument, Interest Rate, Stated Percentage Total stockholders’ equity Beginning of period End of period Equity, Attributable to Parent Less: allowance for credit losses (Note 6) Balance at beginning of period Allowance for credit losses, end of period Premium Receivable, Allowance for Credit Loss Reinsurance Recoverable, Credit Quality Indicator [Line Items] Reinsurance Recoverable, Credit Quality Indicator [Line Items] Assumed Policyholder Benefits and Claims Incurred, Assumed Equity securities – at fair value (cost: $377,480 – 2026; $370,104 – 2025) Equity securities Equity Securities, FV-NI, Current Amounts reclassified into net income (loss): OCI, Reclassification Adjustments [Abstract] OCI, Reclassification Adjustments ASSETS Assets [Abstract] Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Other investments Other Investments Direct Direct Premiums Written Accounting Policies [Abstract] Fixed income securities, available-for-sale, amortized cost Cost/ Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Private credit Private Credit Funds [Member] This strategy includes the Mezzanine Financing and Distressed Debt strategies, providing privately negotiated fixed income securities, generally with an equity component, to LBO firms and private and publicly traded large, mid and small-cap companies to finance LBOs, recapitalizations, and acquisitions, and making direct and indirect investments in debt and equity securities of companies that are experiencing financial and/or operational distress. Accrued Investment Income [Table] Accrued Investment Income [Table] Accrued Investment Income Insider Trading Policies and Procedures [Line Items] Finance lease obligations Finance Lease, Liability Fixed income securities, held-to-maturity, fair value Total HTM fixed income securities, Fair Value Debt Securities, Held-to-Maturity, Fair Value Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Commercial property Commercial Property [Member] Commercial property line of business within Standard Commercial Lines segment. Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Total incurred loss and loss expense Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Financial Asset, Aging [Domain] Financial Asset, Aging [Domain] Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy [Table] Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy Sales of short-term investments Proceeds from Sale of Short-Term Investments Entity Central Index Key Entity Central Index Key Weighted Average Weighted Average [Member] Premiums written: Premiums Written, Net [Abstract] Property and equipment, accumulated depreciation and amortization Property, Plant, and Equipment, Accumulated Depreciation, Depletion, and Amortization Additional paid-in capital Additional Paid in Capital, Common Stock All Adjustments to Compensation All Adjustments to Compensation [Member] Other Reconciling Item, Corporate Reconciling Item, Corporate Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] Net underwriting expenses incurred Net Underwriting Expense Amount representing the sum of amortization of deferred policy acquisition costs and other insurance expenses, partially offset by other income Equity [Abstract] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Net cash provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Litigation Legal Matters and Contingencies [Text Block] Investments Investment Segment [Member] This represents our Investments segment. Prior years Incurred loss and loss expense for claims occurring in the prior year (Favorable)/unfavorable prior year casualty reserve development Prior Year Claims and Claims Adjustment Expense Income Statement [Abstract] Income Statement [Abstract] Stockholders’ Equity: Equity, Attributable to Parent [Abstract] Document Transition Report Document Transition Report Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Net realized and unrealized investment gains (losses) Net realized and unrealized (gains) losses Net realized and unrealized investment gains (losses) Gain (Loss) on Investments Gross losses on sales Debt and Equity Securities, Realized Losses Amount of realized losses from sale of investment securities. Dividends to preferred stockholders Dividends, Preferred Stock, Cash Expected return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-Term Rate of Return on Plan Assets Fixed income securities Fixed Maturities [Member] Deferred federal income tax Deferred Income Tax Assets, Net Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Schedule of Direct, Assumed, and Ceded Reinsurance Amounts Effects of Reinsurance [Table Text Block] Increase in unearned premiums, net of prepaid reinsurance Increase Decrease In Unearned Premiums Net Of Prepaid Reinsurance Change during the period in the unearned portion of premiums written, net of prepaid reinsurance. Premiums written are initially booked as unearned premiums and are recognized as revenue over the known or estimated life of the policy Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] City Area Code City Area Code Accumulated other comprehensive income (loss): Total AOCI AOCI Attributable to Parent [Member] Document Information [Table] Document Information [Table] All Individuals All Individuals [Member] Vanguard Capital Management Vanguard Capital Management [Member] Vanguard Capital Management Name Forgone Recovery, Individual Name 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] Net realized gains (losses) on disposals Debt and Equity Securities, Realized Gain (Loss) Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments, Gross Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments, Gross [Member] Reinsurance recoverable for unpaid claims and claims adjustments prior to the unpaid claims allowance. Statement [Line Items] Statement [Line Items] Total assets Assets Losses on securities for which we have the intent to sell Losses On Investment Securities For Which We Have The Intent To Sell Losses On Investment Securities For Which We Have The Intent To Sell Private equity Private Equity Funds [Member] Write-offs charged against the allowance for credit losses Premium Receivable, Allowance for Credit Loss, Writeoff Net actuarial loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Preferred stock, issued (in shares) Preferred Stock, Shares Issued Operating Segments Operating Segments [Member] Recoveries Reinsurance Recoverable, Allowance for Credit Loss, Recovery Total defined benefit pension and post-retirement plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent Unrealized Gains Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax OCI before reclassifications OCI before reclassifications OCI, before Reclassifications, Net of Tax, Attributable to Parent (Increase) decrease in other assets Increase (Decrease) in Other Operating Assets Issuance of preferred stock (in shares) Stock Issued During Period, Shares, New Issues PEO PEO [Member] Measurement Basis [Axis] Measurement Basis [Axis] Goodwill Goodwill Retirement Plan Type [Axis] Retirement Plan Type [Axis] Corporate actions related to fixed income securities, available-for-sale ("AFS") Fixed Income Securities [Member] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Depositary Shares Noncumulative Preferred Stock [Member] Total investments (Note 4 and 5) Investments Due in one year or less fair value, AFS Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Retained earnings: Retained Earnings [Member] Investments Subtotal AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Net actuarial (gain) loss Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Schedule of Fair Value Measurement Inputs and Valuation Techniques Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Common stock, shares outstanding, beginning of period (in shares) Common stock, shares outstanding, end of period (in shares) Common Stock, Shares, Outstanding Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Unrealized holding gains (losses) arising during period OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Award Timing Disclosures [Line Items] Schedule of Carrying Values and Estimated Fair Values of Financial Liabilities Fair Value, by Balance Sheet Grouping [Table Text Block] Debt issuance costs paid Payments of Debt Issuance Costs Corporate securities Corporate Debt Securities [Member] Net income (loss) available to common stockholders - Diluted (in dollars per share) Diluted (in dollars per share) Earnings Per Share, Diluted Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Entity Current Reporting Status Entity Current Reporting Status Net cash provided by (used in) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Acquisition of treasury stock - shares acquired related to employee share-based compensation plans Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation Unrealized gains (losses) recognized in income on equity securities remaining in our portfolio at end of period Unrealized Gain (Loss) on Investments Increase (decrease) in accrued salaries and benefits Increase (Decrease) in Employee Related Liabilities Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Vanguard Portfolio Management Vanguard Portfolio Management [Member] Vanguard Portfolio Management Net Premiums Written, Net Reclassifications Reclassification from AOCI, Current Period, Tax Financing Activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Property and equipment – at cost, net of accumulated depreciation and amortization of: $311,425 – 2026; $297,211 – 2025 Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Schedule of Securities Pledged as Collateral Schedule of Financial Instruments Owned and Pledged as Collateral [Table Text Block] Interest cost Defined Benefit Plan, Interest Cost Net Investment Income [Line Items] Net Investment Income [Line Items] Financial Instruments [Domain] Financial Instruments [Domain] Tax Other Comprehensive Income (Loss), Tax [Abstract] Prepaid reinsurance premiums Prepaid Reinsurance Premiums Other income Other Income Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Underlying Securities Award Underlying Securities Amount Purchases of short-term investments Payments to Acquire Short-Term Investments Issuance of preferred stock Stock Issued During Period, Value, New Issues Subsegments [Domain] Subsegments [Domain] Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Write-offs charged against the allowance for credit losses Reinsurance, Loss on Uncollectible Accounts in Period, Amount Net proceeds from stock purchase and compensation plans Proceeds, Issuance of Shares, Share-Based Payment Arrangement, Including Option Exercised Cash Cash Cash Assumed Assumed Premiums Written Subsegments [Axis] Subsegments [Axis] Preferred stock: Preferred Stock [Member] Segment Reporting [Table] Segment Reporting [Table] Personal automobile Personal Automobile [Member] Represents insurance coverage provided to policy holders on personal automobiles. Income tax expense (benefit): Income Tax Expense (Benefit), Continuing Operations [Abstract] Related Party Transaction [Table] Related Party Transaction [Table] A- AM Best, A- Rating [Member] Schedule of Available-for-Sale Securities Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] Federal and state pools Total Federal And State Pools [Member] Represents the total federal and state pools. A AM Best, A Rating [Member] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] AFS Fair Value, Less than 12 Months Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Current Fiscal Year End Date Current Fiscal Year End Date Homeowners Homeowners [Member] Represents insurance coverage provided to policy holders on dwellings. Fixed income securities, held-to-maturity – at carrying value (fair value: $19,540 – 2026; $23,939 – 2025) Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Foreign government Foreign government Debt Security, Government, Non-US [Member] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Dividends declared per common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Entity File Number Entity File Number Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Stock purchase and compensation plans Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Schedule of Fixed Maturity Securities by Contractual Maturities Investments Classified by Contractual Maturity Date [Table Text Block] Equity securities Equity Securities [Member] Schedule of Roll Forward of Reserve for Loss and Loss Expense Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Debt securities, available-for-sale, measurement input Debt Securities, Available-for-Sale, Measurement Input Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Dividends declared per preferred share (in dollars per share) Preferred Stock, Dividends Per Share, Declared Common stock, issued (in shares) Common Stock, Shares, Issued Measure: Measure [Axis] Fair value, beginning of year Fair value, ending year Total Level 3 securities Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Treasury stock, common (in shares) Treasury Stock, Common, Shares Net premiums earned Net Premiums Earned, Net Undistributed gains of equity method investments Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Supplemental Cash Flow [Table] Supplemental Cash Flow [Table] Supplemental Cash Flow [Table] Fixed income securities, available-for-sale, allowance for credit loss Allowance for Credit Losses Beginning Balance Ending Balance Debt Securities, Available-for-Sale, Allowance for Credit Loss Gross Other Comprehensive Income (Loss), Gross [Abstract] Other Comprehensive Income (Loss), Gross AFS Unrealized Losses, 12 months or longer Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Equity Components [Axis] Equity Components [Axis] Product and Service [Domain] Product and Service [Domain] Federal income tax Income Tax Paid, after Refund Received Credit loss (benefit) expense Debt Securities, Available-for-sale, Allowance for Credit Loss, Net Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized amounts realized upon a credit-related impairment loss. Entity Interactive Data Current Entity Interactive Data Current A+ AM Best, A+ Rating [Member] Non-cash purchase of property and equipment Noncash or Part Noncash Acquisition, Fixed Assets Acquired Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] AFS, Fair Value Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Entity Small Business Entity Small Business Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Commercial automobile Commercial Automobile [Member] Represents insurance coverage provided to policy holders on commercial automobiles. Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Unearned premiums Unearned Premiums Premium Receivable, Allowance for Credit Loss [Roll Forward] Premium Receivable, Allowance for Credit Loss [Roll Forward] Termination Date Trading Arrangement Termination Date Allowance for Credit Losses on Premiums Receivable Allowance for Uncollectible Premiums Receivable [Text Block] The entire disclosure for allowance for credit losses on Premiums Receivable. Acquisition of treasury stock - shares acquired related to employee share-based compensation plans (in shares) Share-Based Payment Arrangement, Shares Withheld for Tax Withholding Obligation Non Rated Reinsurer [Axis] Non Rated Reinsurer [Axis] Reinsurers that are not rated by a rating agency. Gross gains on sales Debt and Equity Securities, Realized Gains Amount of realized gains from sale of investment securities. Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] (Increase) decrease in net federal income taxes Increase (Decrease) in Income Taxes Unrealized gains (losses) recognized in income on equity securities sold in period Unrealized Gain Loss on Equity Securities Sold The unrealized gains (losses) on equity securities sold during the period. Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Current year Current year casualty loss costs Current Year Claims and Claims Adjustment Expense Document Type Document Type Commissions to distribution partners Insurance Commissions Due after ten years fair value, HTM Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 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] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value as of Grant Date Award Grant Date Fair Value Schedule of Quantitative Information about Unrealized Losses on AFS Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Preferred stock, shares outstanding: Increase (Decrease) in Temporary Equity [Roll Forward] Distributions in excess of current year income of equity method investments Proceeds From Equity Investments, In Excess Of Current Year Earnings Proceeds From Equity Investments, In Excess Of Current Year Earnings Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Remaining Authorization Share Repurchase Program, Remaining Authorized, Amount Corporate expenses Corporate expenses Operating Costs and Expenses Arrangement Duration Trading Arrangement Duration Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Financial Instruments Owned and Pledged as Collateral [Table] Financial Instruments Owned and Pledged as Collateral [Table] Local Phone Number Local Phone Number Supplemental Cash Flow [Line Items] Supplemental Cash Flow [Line Items] Supplemental Cash Flow [Line Items] Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Short-term investments Other Short-Term Investments AFS Unrealized Losses, Less than 12 months Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Other comprehensive income (loss) Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent Income (loss) before income tax Net income (loss) Income (loss) before federal income tax Income (loss) before income tax Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Less: allowance for credit losses (Note 7) Less: allowance for credit losses Balance at beginning of period Allowance for credit losses, end of period Reinsurance Recoverable, Allowance for Credit Loss Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Schedule of Quantitative Information of Financial Assets and Liabilities, Disclosed at Fair Value Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table Text Block] Commitments and contingencies Commitments and Contingencies Total revenues Total segment revenues Revenues 5.375% Senior Notes 5.375% Senior Notes Due 2049 [Member] This item represents the 5.375% Senior Notes of the company due 2049. Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Alternative investments Carrying Value Alternative Investment Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] Executive Category: Executive Category [Axis] Current Provision for Securities without Prior Allowance Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded Net realized (gains) losses on disposals and intent-to-sell AFS securities All Other AOCI, Gain (Loss), Debt Securities, Available-for-Sale, without Allowance for Credit Loss, Parent [Member] Counterparty Name [Axis] Counterparty Name [Axis] LIABILITIES AND STOCKHOLDERS’ EQUITY Liabilities and Equity [Abstract] Reinsurance Reinsurance [Text Block] Amounts reclassified into net income (loss): Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax [Abstract] All Award Types Award Type [Domain] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Fair Value Measurements Fair Value Disclosures [Text Block] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] HTM, Carrying Value Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date [Abstract] Total net underwriting expenses incurred Net Underwriting Expense, Excluding Other Income Amount representing primarily the sum of amortization of deferred policy acquisition costs and other insurance expenses. Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Total rated reinsurers AM Best Rated Reinsurer [Member] Total of all rated reinsurer's by credit quality with ratings by AM Best. Acquisition of treasury stock - share repurchase authorization (in shares) Total Number of Shares Purchased Treasury Stock, Shares, Acquired PEO Name PEO Name Total paid loss and loss expense Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Restricted cash Restricted cash Restricted Cash Fair Value Measured at Net Asset Value Per Share Fair Value Measured at Net Asset Value Per Share [Member] Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Line Items] Other segment expenses Segment Reporting, Other Segment Item, Amount Other assets Other Assets Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Current Financial Asset, Not Past Due [Member] Interest expense Interest expense Interest Expense, Debt Property lines Property Insurance [Member] 5.90% Senior Notes 5.90% Senior Notes Due 2035 [Member] 5.90% Senior Notes Due 2035 Acquisition of treasury stock - share repurchase authorization Total Cost Treasury Stock, Value, Acquired, Cost Method Cash and restricted cash, beginning of period Cash and restricted cash, end of period Total cash and restricted cash shown in the Consolidated Statements of Cash Flows Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Preferred stock dividends Preferred stock dividends Preferred Stock Dividends, Income Statement Impact Direct Policyholder Benefits and Claims Incurred, Direct Premiums Receivable Disclosure [Abstract] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Schedule of Reclassifications Out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Non-NEOs Non-NEOs [Member] Net catastrophe losses Net Catastrophe Loss Severe loss, as defined by the Insurance Services Office's Property Claims Service (PCS) unit, either natural or man-made, usually involving, but not limited to, many risks from one occurrence such as fire, hurricane, tornado, earthquake, windstorm, explosion, hail, severe winter weather, and terrorism. Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Other Other Personal [Member] Represents insurance coverage provided to policy holders on other personal line policies. Measurement Frequency [Domain] Measurement Frequency [Domain] Debt Instrument [Axis] Debt Instrument [Axis] Stock-based compensation expense Share-Based Payment Arrangement, Noncash Expense Salaries and employee benefits Labor and Related Expense Redemption and maturities of fixed income securities, held-to-maturity Proceeds from Maturities, Prepayments and Calls of Held-to-Maturity Securities Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Net income (loss) available to common stockholders - Basic (in dollars per share) Basic (in dollars per share) Earnings Per Share, Basic Measurement Input Type [Axis] Measurement Input Type [Axis] Acquisition of treasury stock Payments for Repurchase of Common Stock Statistical Measurement [Axis] Statistical Measurement [Axis] Insider Trading Arrangements [Line Items] Preferred stock Nonredeemable Preferred Stock [Member] Award Type Award Type [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Total unrealized gains (losses) on investment securities Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent Retained earnings Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] Amortization of deferred policy acquisition costs Deferred Policy Acquisition Costs, Amortization Expense Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] EPS: Earnings Per Share, Basic and Diluted EPS [Abstract] Earnings Per Share, Basic and Diluted EPS Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Defined benefit pension plan Pension Plan [Member] Common Stock Common stock: Common stock Common Stock [Member] Fixed income securities, available-for-sale – at fair value (allowance for credit losses: $39,102 – 2026 and $31,287 – 2025; amortized cost: $10,131,816 – 2026 and $9,576,878 – 2025) Fair Value Total AFS fixed income securities, Fair Value Fixed income securities Debt Securities, Available-for-Sale Reinsurance Recoverable [Domain] Reinsurance Recoverable [Domain] [Domain] for Reinsurance Recoverable [Axis] Fair Value Measurement[Domain] Fair Value Measurement [Domain] Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Change in unrealized gains (losses) for the period included in OCI for assets held at period end Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Equity Awards Adjustments Equity Awards Adjustments [Member] Past Due Financial Asset, Past Due [Member] Assumed Assumed Premiums Earned Total liabilities and stockholders’ equity Liabilities and Equity Employee Stock Option Share-Based Payment Arrangement, Option [Member] Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Reinsurance Recoverable, Credit Quality Indicator [Table] Reinsurance Recoverable, Credit Quality Indicator [Table] Schedule of Weighted-Average Expense Assumptions Defined Benefit Plan, Assumptions [Table Text Block] Segments [Axis] Segments [Axis] Increase (decrease) in other liabilities Increase (Decrease) in Other Operating Liabilities Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Asset Class [Axis] Asset Class [Axis] Related Party [Domain] Related and Nonrelated Parties [Domain] Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy [Line Items] Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy [Line Items] Other Investment Portfolio By Strategy And Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy Increase in premiums receivable Increase (Decrease) in Premiums Receivable Tabular List, Table Tabular List [Table Text Block] Credit loss related Credit Loss Related AOCI, Gain (Loss), Debt Securities, Available-for-Sale, with Allowance for Credit Loss, Parent [Member] Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of period Add: Reinsurance recoverable on unpaid loss and loss expense, at end of period Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Reserve for Loss and Loss Expense Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Purchases of fixed income securities, held-to-maturity Payments to Acquire Held-to-Maturity Securities 7.25% Senior Notes 7.25% Senior Notes Due 2034 [Member] 7.25% Senior Notes Due 2034 Commercial mortgage loans ("CMLs") Commercial Mortgage Loans [Member] Commercial mortgage loans are loans secured by commercial property, such as an office building, multi-family apartment complex, industrial warehouse, or shopping center. These investments may be acquired through direct originations under a loan syndication arrangement or purchased in the marketplace. Short-term investments Short-Term Investments [Member] Name Awards Close in Time to MNPI Disclosures, Individual Name Due after five years through ten years carrying value, HTM Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 5 through 10 Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Ceded Ceded Premiums Written Schedule of Reinsurance Recoverable, Allowance for Credit Loss Reinsurance Recoverable, Allowance for Credit Loss [Table Text Block] Other investments Other Investment [Member] Other Investments AFS Securities Debt Securities, Available-For-Sale [Member] Debt Securities, Available-For-Sale Commercial mortgage loans, fair value Commercial Mortgage Loan, Fair Value Fair value of investment in commercial loan measured at original cost. Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Preferred stock Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Commercial mortgage-backed securities ("CMBS") CMBS Commercial Mortgage-Backed Securities [Member] Class of Stock [Domain] Class of Stock [Domain] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Operating cash flows from financing leases Finance Lease, Interest Payment on Liability Interest Interest Paid, Excluding Capitalized Interest, Operating Activity Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Net underwriting expenses incurred (as a percent) Underwriting Expense Ratio Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Total assets measured at fair value Assets, Fair Value Disclosure Accrued Investment Income [Line Items] Accrued Investment Income [Line Items] Accrued Investment Income Name Measure Name Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Pay vs Performance Disclosure [Line Items] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Sales of equity securities Proceeds from Sale of Equity Securities, FV-NI Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Allowance for Credit Loss Rollforward [Table] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Current Current Federal, State and Local, Tax Expense (Benefit) Retirement Plan Type [Domain] Retirement Plan Type [Domain] Additional paid-in capital: Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category AFS Fair Value Debt Securities, Available-for-Sale, Unrealized Loss Position Securities owned and pledged as collateral, fair value Fair Value of Security Owned and Pledged as Collateral Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. Name Trading Arrangement, Individual Name Defined benefit pension and post-retirement plans: Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent [Abstract] Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Loss and loss expense incurred (as a percent) Loss Ratio Company Selected Measure Amount Company Selected Measure Amount Financial Asset, Aging [Axis] Financial Asset, Aging [Axis] Schedule of Reconciliation of Segment Results to Income (Loss) Before Federal Income Tax Segment Reporting, Reconciliation of Profit (Loss) by Segment to Consolidated [Table Text Block] Schedule of Allowance for Credit Losses on Premiums Receivable Premium Receivable, Allowance for Credit Loss [Table Text Block] Net unrealized gains (losses) on equity securities Total unrealized gains (losses) recognized in income on equity securities Equity Securities, FV-NI, Unrealized Gain (Loss) Current federal income tax Income Taxes Receivable Premiums earned: Premiums Earned, Net [Abstract] Schedule of Realized and Unrealized Gains (Losses) on Securities Gain (Loss) on Securities [Table Text Block] Change in unrealized gains (losses) for the period included in earnings for assets held at period end Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) Increase in reserve for loss and loss expense, net of reinsurance recoverable Increase (Decrease) In Reserves For Losses And Loss Expenses, Net Of Reinsurance Recoverables The net change during the reporting period in the reserve account established to account for expected losses net of reinsurance recoverables. Stock purchase and compensation plan (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Less: allowance for credit losses Allowance for Loan and Lease Losses, Real Estate Reductions for Securities Sold Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Non-PEO NEO Non-PEO NEO [Member] Senior Notes Senior Notes [Member] Entity Address, Address Line One Entity Address, Address Line One Underwriting income (loss) (as a percent) Combined Ratio Assets acquired under operating lease arrangements Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Total expenses Benefits, Losses and Expenses HTM, Fair Value Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Total income tax expense (benefit) Income tax (expense) benefit Net income Total income tax expense (benefit) Income Tax Expense (Benefit) Vanguard Group Vanguard Group [Member] Vanguard Group Financial Instruments Owned and Pledged as Collateral [Line Items] Financial Instruments Owned and Pledged as Collateral [Line Items] Securities pledged as collateral on deposit with various state and regulatory agencies Assets Held by Insurance Regulators Award Timing Method Award Timing Method [Text Block] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Premiums receivable, net of allowance for credit losses Premiums Receivable, Net Amortization of unrecognized net actuarial loss Defined Benefit Plan, Amortization of Gain (Loss) Adoption of Accounting Pronouncements Accounting Standards Update and Change in Accounting Principle [Text Block] AOCI Attributable to Parent, Net of Tax [Roll Forward] AOCI Attributable to Parent, Net of Tax [Roll Forward] Net income (loss) Net income (loss) Net Income (Loss) Attributable to Parent Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Investments Investments [Member] Valuation Technique, Discounted Cash Flow Valuation Technique, Discounted Cash Flow [Member] Statement [Table] Statement [Table] Restatement Determination Date: Restatement Determination Date [Axis] Recoveries Premium Receivable, Allowance for Credit Loss, Recovery Real assets Real Assets [Member] This strategy includes the Energy & Power Generation and Real Estate strategies, making energy and power generation investments in cash flow generating infrastructure assets and investing opportunistically in real estate in North America, Europe, and Asia via direct property ownership, joint ventures, mortgages, and investments in equity and debt instruments. Schedule of Components of Accumulated Other Comprehensive Income Net of Tax Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Effective interest rate for calculation of interest cost Effective Interest Rate for Calculation of Interest Cost The effective interest rate used to calculate interest cost related to the Pension Plan. FHLBI Collateral Federal Home Loan Bank of Indianapolis [Member] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Commercial mortgage loans, net of allowance for credit losses Mortgage Loans on Real Estate, Commercial and Consumer, Net Reinsurance Disclosures [Abstract] Reinsurance Disclosures [Abstract] Current federal income tax Accrued Income Taxes Revenues: Revenues [Abstract] Basis of Presentation Basis of Presentation and Significant Accounting Policies [Text Block] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Due after ten years carrying value, HTM Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 10