Proceeds from the divestiture of the Canadian business Proceeds from Divestiture of Businesses Guarantor obligations Guarantor Obligations [Line Items] Intangible assets subject to amortization, accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Other Companies [Member] Other Companies Not Rated By AM Best [Member] Reinsurance group of other companies not specifically noted not rated by AM Best.] Investment disclosure details Debt and Equity Securities, FV-NI [Line Items] Amortized cost and fair value of investments in fixed maturities [Table Text Block] Debt Securities, Available-for-Sale [Table Text Block] Net realized investment gains (losses) Net realized investment (gains) losses Reclassification adjustment impacting net realized investment gains (losses) on the consolidated statement of income Realized Investment Gains (Losses) Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Estimated decrease in claims and claim adjustment expenses for claims arising in prior years Net favorable development included in estimated claims and claim adjustment expenses incurred for claims arising in prior years Prior Year Claims and Claims Adjustment Expense Award Timing Predetermined Award Timing Predetermined [Flag] Total other intangible assets, gross carrying amount Intangible Assets, Gross (Excluding Goodwill) Entity file number Entity File Number Equity in income from other investments Income (Loss) from Equity Method Investments Tabular List, Table Tabular List [Table Text Block] Stock options vested at end of period, weighted average contractual life remaining (in years) Stock options vested at end of period, weighted average contractual life remaining Weighted average remaining contractual term for fully vested options outstanding. Leases [Abstract] Leases [Abstract] Unearned premium reserves Unearned Premiums Carrying value and fair value of the Company's financial assets and financial liabilities disclosed, but not carried, at fair value, and the level within the fair value hierarchy at which such assets and liabilities are categorized [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Major classes of assets and liabilities classified as held for sale [Table Text Block] Disposal Groups, Including Discontinued Operations [Table Text Block] Maximum amount of contingent obligation Guarantor Obligations, Maximum Exposure, Undiscounted Payables for reinsurance premiums Ceded Premiums Payable Goodwill by segment Goodwill [Line Items] Net shares acquired related to employee share-based compensation plans Cost of shares acquired to cover tax withholding costs and exercise costs Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation Gross unrealized losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Financial assets and financial liabilities measurement basis [Axis] Measurement Basis [Axis] Basic (in dollars per share) Net income per common share, basic (in dollars per share) Earnings Per Share, Basic Adjustment to Compensation: Adjustment to Compensation [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Reinsurance recoverables, allowance for uncollectible reinsurance [Roll Forward] Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Components of the Company's revenues and income (loss) by reportable business segment [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Reductions due to sales/defaults of credit-impaired securities Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Reinsurer Group Not Rated by A.M. Best [Member] Not Rated By AM Best [Member] Reinsurer group with no credit rating provided by AM Best. Assets held for sale Total assets held for sale Disposal Group, Including Discontinued Operation, Assets Adjustment to Compensation, Amount Adjustment to Compensation Amount Concentration risk percentage Concentration Risk, Percentage Pension Plans [Member] Pension Plan [Member] Employee Stock Option Share-Based Payment Arrangement, Option [Member] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Changes in Net Unrealized Gains (Losses) on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] AOCI, Gain (Loss), Debt Securities, Available-for-Sale, without Allowance for Credit Loss, Parent [Member] Total shareholders’ equity Balance, beginning of period Balance, end of period Equity, Attributable to Parent Other comprehensive income (loss) (OCI) before reclassifications, net of tax OCI, before Reclassifications, Net of Tax, Attributable to Parent Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-Sale [Table] Insider Trading Policies and Procedures [Line Items] Total compensation cost recognized in earnings for all share-based incentive compensation awards Share-Based Payment Arrangement, Expense Changes in the allowance for expected credit losses on fixed maturities classified as available for sale [Table Text Block] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table Text Block] Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost greater than 3 months, 6 months or less Available for Sale Securities for Which Fair Value is Less than Eighty Percent of Amortized Cost, Greater than Three Months Six Months or Less Amount of accumulated unrealized loss on investments in debt securities classified as available-for-sale in a continuous loss position of greater than 20% of amortized cost for greater than three months through six months. Common stock, shares authorized (in shares) Common Stock, Shares Authorized A.M. Best, Rating Provided [Member] Credit Rating, AM Best Provided Rating [Member] Credit rating provided by the external credit rating agency, AM Best. Assets by segment Segment Reporting, Asset Reconciling Item [Line Items] Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Total liabilities and shareholders’ equity Liabilities and Equity Premiums receivable Disposal Group, Including Discontinued Operation, Premium Receivable, after Allowance for Credit Loss Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Deferred acquisition costs Deferred Policy Acquisition Cost Current period change for expected credit losses Contractholder Receivables, Credit Loss Expense (Reversal) Amount of expense (reversal of expense) for expected credit loss on contractholder receivables. Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Premiums receivable (net of allowance for expected credit losses of $60 and $58) Premiums receivable (net of allowance for expected credit losses), balance, beginning of period Premiums receivable (net of allowance for expected credit losses), balance, end of period Premiums Receivable, Net Cover [Abstract] Amounts reclassified from accumulated other comprehensive income (loss), net of tax Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Disclosure items [Member] Portion at Fair Value Measurement [Member] Stock options exercisable at end of period, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Weighted Average Exercise Price Intangible assets subject to amortization, net Finite-Lived Intangible Assets, Net Continuous unrealized loss position, total, fair value Debt Securities, Available-for-Sale, Unrealized Loss Position Concentration Risk Type [Domain] Concentration Risk Type [Domain] Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Information for fully vested stock option awards [Table Text Block] Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block] Other investments Payments to Acquire Other Investments Total non-service cost (benefit) Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component Treasury stock, at cost [Member] Treasury Stock, Common [Member] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Pay vs Performance Disclosure [Line Items] Debt Debt, Long-Term and Short-Term, Combined Amount Captive Insurance Companies [Member] Captive Insurance Companies [Member] Reinsurance group of captive insurance companies not rated by AM Best. Retirement Benefits [Abstract] Retirement Benefits [Abstract] Unearned premium reserves Increase (Decrease) in Unearned Premiums Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Contractholder receivables, allowance for expected credit losses [Roll Forward] Contractholder Receivables, Allowance For Credit Loss [Roll Forward] A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. Liabilities Liabilities [Abstract] Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Income before income taxes Total reclassifications Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Entity shell company Entity Shell Company Amount of increase in gross claims and claim adjustment expense reserves Amount of increase (decrease) in gross claims and claim adjustment expense reserves Amount of increase (decrease) in the reserve for gross claims and claim adjustment expenses. Claims and claim adjustment expense reserves Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Reportable Segments [Member] Operating Segments [Member] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Company Selected Measure Amount Company Selected Measure Amount Cash (including restricted cash of $134 and $132) Cash Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Personal Insurance [Member] Personal Insurance [Member] Reportable segment of the Company writing a broad range of property and casualty insurance covering individuals' personal risks, primarily in the United States, as well as in Canada. Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Reclassification out of accumulated other comprehensive income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Common Share Repurchases Treasury Stock [Text Block] Disposal Group Name [Domain] Disposal Group Name [Domain] Name Measure Name Proceeds from maturities of fixed maturities Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale Other comprehensive income (loss) before income taxes Other comprehensive income (loss), before income taxes Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent Document fiscal period focus Document Fiscal Period Focus Net changes in unrealized foreign currency translation Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax, Portion Attributable to Parent Award Timing Method Award Timing Method [Text Block] Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost, total Available for Sale Securities for Which Fair Value is Less than Eighty Percent of Amortized Cost, Total Amount of accumulated unrealized loss on investments in debt securities classified as available-for-sale in a continuous loss position of greater than 20% of amortized cost. Award Type Award Type [Axis] Net Unrealized Foreign Currency Translation [Member] Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Pre-tax components of other comprehensive income (loss) and the related income tax expense (benefit) for each component [Table Text Block] Comprehensive Income (Loss) [Table Text Block] Deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs After-tax interest expense After-tax interest expense Interest expense, after tax. Total assets Total assets Assets Trading symbol Trading Symbol Geographical [Domain] Geographical [Domain] Changes in Net Unrealized Gains (Losses) on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] AOCI, Gain (Loss), Debt Securities, Available-for-Sale, with Allowance for Credit Loss, Parent [Member] Net shares issued under employee share-based compensation plans (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Entity address, city Entity Address, City or Town Contingencies, Commitments and Guarantees [Abstract] Contingencies, Commitments and Guarantees [Abstract] Cash flows from operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Unearned premium reserves Disposal Group, Including Discontinued Operation, Unearned Premiums Amount of unearned premium reserves attributable to disposal group held for sale. Concentration Risk Type [Axis] Concentration Risk Type [Axis] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Income Statement Location [Domain] Statement of Income Location, Balance [Domain] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Allowance for Expected Credit Losses Credit Loss, Financial Instrument [Text Block] Entity emerging growth company Entity Emerging Growth Company Net (purchases) sales of short-term securities Payments for (Proceeds from) Available-for-Sale Securities, Short-Term Other investments Other Investments Cash dividends declared per common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Common stock shares outstanding Entity Common Stock, Shares Outstanding Right-of-use assets obtained in exchange for new lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Investment disclosure details Debt Securities, Available-for-Sale [Line Items] 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] Ceded unearned premiums Prepaid Reinsurance Premiums Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Intangible assets not subject to amortization Indefinite-Lived Intangible Assets (Excluding Goodwill) Stock options vested at end of period, weighted average exercise price (in dollars per share) Stock options vested at end of period, weighted average exercise price (per share) As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested. Common stock, shares outstanding (in shares) Balance, beginning of period (in shares) Balance, end of period (in shares) Common Stock, Shares, Outstanding PEO PEO [Member] Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract] Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract] Commercial multi-peril [Member] Commercial multi-peril [Member] Product line providing a combination of the commercial property and general liability product lines. Reinsurance recoverables, after deducting mandatory pools and associations and before allowances for expected credit losses Reinsurance Recoverables, Gross Domestic [Member] UNITED STATES 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] Unrealized foreign exchange (gain) loss Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss) Retained earnings [Member] Retained Earnings [Member] Total payments Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Entity address, postal zip code Entity Address, Postal Zip Code Restatement Determination Date Restatement Determination Date Share-based Payment Arrangement [Abstract] Share-Based Payment Arrangement [Abstract] Cash and restricted cash at beginning of year Cash and restricted cash at end of period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Income Statement [Abstract] Income Statement [Abstract] Real estate investments Payment for Acquisition, Real Estate, Held-for-Investment Income tax expense Income tax expense (benefit) Income tax (expense) benefit Income Tax Expense (Benefit) Continuous unrealized loss position, 12 months or longer, fair value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer Purchases of investments: Payments to Acquire Investments [Abstract] Other assets Other Assets Homeowners and Other [Member] Homeowners and other [Member] Provides protection against losses to dwellings and contents from a variety of perils (excluding flooding) as well as coverage for personal liability. Total invested assets measured on a recurring basis Investments, Fair Value Disclosure Other intangible assets Total other intangible assets, net Intangible Assets, Net (Excluding Goodwill) Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Product and Service [Domain] Product and Service [Domain] Contractholder receivables (net of allowance for expected credit losses of $14 and $16) Contractholder receivables (net of the allowance for expected credit losses), balance, beginning of period Contractholder receivables (net of the allowance for expected credit losses), balance, end of period Contractholder receivables Under certain workers' compensation insurance contracts with deductible features, the Entity has an obligation to pay the full amount claimed. The Entity is subsequently reimbursed by the policyholder for the policy deductible. The amount to be reimbursed is included on a gross basis in the consolidated balance sheet in contractholder receivables and contractholder payables, respectively. Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured [Axis] Fair Value Hierarchy and NAV [Axis] Reinsurer Concentration Risk [Member] Reinsurer Concentration Risk [Member] Net investment income Net Investment Income Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Reinsurer Group Rated by A.M. Best [Member] Rated By AM Best [Member] Reinsurer group with a credit rating provided by AM Best. Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Indemnifications related to the sale of businesses [Member] Indemnification Agreement [Member] Schedule of guarantor obligations [Table] Guarantor Obligation [Table] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Issuance of common stock — employee share options Proceeds from Stock Options Exercised City area code City Area Code Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Issuance of common stock — net share settlement of employee options Stock Issued Net Share Settlement Employee Options Stock issued in noncash financing activity where options were exercised via the net share settlement method. Amount represents the strike price of options exercised. Investments Investment [Text Block] All Trading Arrangements All Trading Arrangements [Member] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Equity Securities [Table] Debt Securities, Trading, and Equity Securities, FV-NI [Table] Catastrophe losses Policyholder Benefits and Claims Incurred Net, Catastrophe Losses Catastrophe losses included in claims and claim adjustment expenses Other [Member] Other assets [Member] Other [Member] Items used in reconciling reportable segments' amounts to consolidated amount. Includes corporate-level activity. Amount indemnified by a third party Amount indemnified by a third party The amount of a guarantee that is indemnified by a third party. Cash flows from investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Net cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Disposal group, disposed of by sale, not discontinued operations [Member] Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] Fixed maturities Proceeds from sales of fixed maturities classified as available for sale Proceeds from Sale of Debt Securities, Available-for-Sale Reinsurance Recoverables [Member] Reinsurance Recoverables, After Deduction Of Mandatory Pools And Associations, Before Allowance [Member] Reinsurance recoverables, after deduction of mandatory pools and associations and before valuation allowance. Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss Defined Benefit Plan, Amortization of Gain (Loss) Net lease cost Lease, Cost General and administrative expenses [Member] Selling, General and Administrative Expenses [Member] Other liabilities Other Liabilities Goodwill Goodwill Equity securities, at fair value (cost $428 and $457) Equity securities, at fair value Equity Securities, FV-NI Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Claims and claim adjustment expense payments for claims arising in current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Additional information regarding real estate operating leases [Table Text Block] Lease, Cost [Table Text Block] PEO Total Compensation Amount PEO Total Compensation Amount Availability of financial information provided by private equity and real estate partnerships following the date of the reporting, maximum Availability of financial information provided by private equity and real estate partnerships following the date of the reporting, maximum Maximum number of months before which financial information provided by private equity and real estate partnerships is available following the reporting date. Held for Sale Reclassification [Member] Held for Sale Reclassification Elimination [Member] Identifies the contra/eliminating amount being reclassified as held for sale and is no longer part of the original balance sheet classification. Deferred federal income tax expense Deferred Federal Income Tax Expense (Benefit) Equity Components [Axis] Equity Components [Axis] International [Member] Non-US [Member] Less: sublease income Sublease Income Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Accretion expense Accretion Expense Number of reportable business segments Number of Reportable Segments Changes in the allowance for expected credit losses on contractholder receivables [Table Text Block] Contractholder Receivables, Allowance For Credit Loss [Table Text Block] Tabular disclosure of allowance for credit loss on contractholder receivables. Fixed maturities, available for sale, amortized cost Fixed maturities, amortized cost Debt Securities, Available-for-Sale, Amortized Cost Claims and claim adjustment expenses Reclassification adjustment impacting claims and claim adjustment expenses (benefit) on the consolidated statement of income Policyholder Benefits and Claims Incurred, Net Other Performance Measure, Amount Other Performance Measure, Amount U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member] US Treasury and Government [Member] Entity address, state Entity Address, State or Province Dividends Dividends, Common Stock Debt Securities, Available-for-sale, Allowance for Credit Loss [Table] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Individual: Individual [Axis] Restricted cash Restricted Cash Earnings Per Share Reconciliation [Abstract] Earnings Per Share Reconciliation [Abstract] Dividends paid to shareholders Payments of Ordinary Dividends, Common Stock Investments [Abstract] Investments [Abstract] Net cash used in investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activity Document fiscal year focus Document Fiscal Year Focus Basis of Presentation [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Contractholder receivables, allowance for expected credit losses Contractholder receivables, allowance for expected credit losses, beginning balance Contractholder receivables, allowance for expected credit losses, ending balance Contractholder Receivables, Allowance For Credit Loss Amount of allowance for credit loss on contractholder receivables. Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income Changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income The amount before tax, after reclassification adjustments, of changes in net unrealized gains on investment securities that have or have had credit losses recognized in the consolidated statement of income, attributable to parent. Amount therefore also includes non-credit changes on securities that had current or previous credit loss components, attributable to parent. Consolidation Reclassification [Axis] Consolidation Reclassification [Axis] Information by components and contra/elimination reclassifications used in consolidating amounts for reportable entity totals. Entity interactive data current Entity Interactive Data Current Operating leases Operating Lease, Cost Carrying Value [Member] Reported Value Measurement [Member] Weighted-average period of recognition for unrecognized compensation cost (in years) Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Canadian insurance business excluding surety business [Member] Canadian Insurance Business Excluding Surety Business [Member] Canadian insurance business (excluding surety business) being sold. Unfunded commitments to private equity limited partnerships, real estate partnerships and other investments Unfunded commitments to private equity limited partnerships and real estate partnerships The amount of unfunded commitments to private equity limited partnerships and real estate partnerships in which the Company invests. Real estate investments Real Estate Investments, Net Stock options exercisable at end of period, aggregate intrinsic value Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Intrinsic Value Segment Information Segment Reporting Disclosure [Text Block] Goodwill by segment [Table] Goodwill [Table] Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Reinsurance recoverables (net of allowance for estimated uncollectible reinsurance of $130 and $129) Reinsurance recoverables (net of allowance for estimated uncollectible reinsurance), balance, beginning of period Reinsurance recoverables (net of allowance for estimated uncollectible reinsurance), balance, end of period Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Other intangible assets by major asset class Finite-Lived Intangible Assets [Line Items] Summary of the components of net periodic benefit cost (benefit) for the Company's pension and postretirement benefit plans recognized in the consolidated statement of income [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Proceeds from sales of investments: Proceeds from Sale, Maturity and Collection of Investments [Abstract] Corporate and all other bonds [Member] Other Debt Obligations [Member] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured on a recurring basis [Table] Fair Value, Recurring and Nonrecurring [Table] Obligations of U.S. states, municipalities and political subdivisions, pre-refunded [Member] Obligations of states, municipalities and political subdivisions, pre-refunded [Member] Bonds or similar securities issued by state, city, or local US governments or the agencies operated by state, city, or local governments for which said states or municipalities have established irrevocable trusts, almost exclusively comprised of U.S. Treasury securities, which were created to satisfy their responsibility for payments of principal and interest. Commercial paper Commercial Paper General and administrative expenses Reclassification adjustment impacting general and administrative expenses (benefit) on the consolidated statement of income Selling, General and Administrative Expense Deferred taxes Deferred Income Tax Assets, Net Changes in accumulated other comprehensive income (loss) (AOCI) [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Depreciation and amortization Amortization and depreciation Depreciation, Depletion and Amortization Central index key Entity Central Index Key Other comprehensive income (loss): Pre-tax components of other comprehensive income (loss) and the related income tax expense (benefit) Other Comprehensive Income (Loss), before Tax [Abstract] PEO Name PEO Name Changes in the allowance for expected credit losses on premiums receivable [Table Text Block] Premium Receivable, Allowance for Credit Loss [Table Text Block] Total revenues Revenues 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] Financial Instruments Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Supplemental disclosure of cash flow information Supplemental Cash Flow Elements [Abstract] Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] Residential Mortgage-Backed Securities [Member] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Total increases Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Benefit Plans [Table] Defined Benefit Plan [Table] Arrangement Duration Trading Arrangement Duration Schedule of segment reporting information, by segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Cost and fair value of investments in equity securities [Table Text Block] Debt Securities, Trading, and Equity Securities, FV-NI [Table Text Block] Held for sale investments, at fair value Disposal Group, Including Discontinued Operation, Investment Premiums receivable, allowance for expected credit losses Premiums receivable, allowance for expected credit losses, beginning balance Premiums receivable, allowance for expected credit losses, ending balance Premium Receivable, Allowance for Credit Loss Segments [Axis] Segments [Axis] Exercise Price Award Exercise Price Filer category Entity Filer Category Local phone number Local Phone Number Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Other investing activities Payment for (Proceeds from) Other Investing Activity Other operating activities Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Other Item Share-Based Incentive Compensation Share-Based Payment Arrangement [Text Block] Sales price on sale of business Disposal Group, Including Discontinued Operation, Consideration Assets Assets [Abstract] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Total segment revenues Total segment revenues Total revenues excluding realized gains (losses). Reinsurance Recoverable, Credit Quality Indicator [Table] Reinsurance Recoverable, Credit Quality Indicator [Table] Pension Plans, Retirement Benefits and Savings Plans Retirement Benefits [Text Block] Fair Value Measurements Fair Value Disclosures [Text Block] Non-redeemable preferred stock [Member] Nonredeemable Preferred Stock [Member] Intangible assets subject to amortization, gross carrying amount Finite-Lived Intangible Assets, Gross Shareholders’ equity Equity, Attributable to Parent [Abstract] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Total claims and expenses Benefits, Losses and Expenses Entity address, address line one Entity Address, Address Line One Treasury stock acquired — share repurchase authorizations Cost of shares repurchased under share repurchase authorization Treasury Stock, Value, Acquired, Cost Method Accumulated other comprehensive loss, net of tax [Member] Accumulated Other Comprehensive Income (Loss) [Member] AOCI Attributable to Parent [Member] Number of shares acquired to cover tax withholding costs and exercise costs (in shares) Share-Based Payment Arrangement, Shares Withheld for Tax Withholding Obligation Reinsurance recoverables on unpaid losses Reinsurance recoverables on unpaid losses Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Automobile [Member] Automobile [Member] Provides coverage for losses relating to automobiles including but not limited to bodily injury and property damage. Other investments Proceeds from Sale and Maturity of Other Investments Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Contingencies, Commitments and Guarantees Commitments Contingencies and Guarantees [Text Block] Disposal group disclosure Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Fair Value as of Grant Date Award Grant Date Fair Value Equity securities, cost Equity Securities, FV-NI, Cost Registrant name Entity Registrant Name Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Net periodic benefit cost (benefit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Stock options vested at end of period, number (in shares) Stock options vested at end of period, number (in shares) As of the balance sheet date, the number of shares into which fully vested stock options outstanding can be converted under the option plan. Disposal Group, Held-for-Sale, Not Discontinued Operations [Member] Amounts classified as held for sale [Member] Disposal Group, Held-for-Sale, Not Discontinued Operations [Member] Liabilities held for sale Total liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities Weighted average effects of dilutive securities - stock options and performance shares (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Realized losses resulting from the early redemption of fixed maturities by the issuer prior to the bonds' maturity date Debt Securities, Available-for-Sale, Early Redemption, Realized Loss Amount of realized loss from early redemption of investment in debt security classified as available-for-sale Total unrecognized compensation cost related to all nonvested share-based incentive compensation awards Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Current period change for estimated uncollectible reinsurance Reinsurance Recoverable, Credit Loss Expense (Reversal) Document quarterly report Document Quarterly Report Continuous unrealized loss position, less than 12 months, fair value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months Debt Long-Term Debt, Fair Value Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Current period change for expected credit losses Premium Receivable, Credit Loss Expense (Reversal) Credit losses on fixed maturities recognized in the consolidated statement of income, as a percentage of the fixed maturity portfolio on a pre-tax basis (less than) Debt Securities, Available-For-Sale, Credit Losses Recognized In Earnings For The Period As Percentage Of Fixed Maturity Portfolio On Pretax Basis Credit losses recognized in earnings during the period related to debt securities held, as a percentage of the fixed maturity portfolio on a pretax basis. Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Equity securities Payments to Acquire Equity Securities, FV-NI Investment income accrued Accrued Investment Income Receivable Changes in net unrealized gains (losses) on investment securities having no credit losses recognized in the consolidated statement of income Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, before Tax, Portion Attributable to Parent Changes in the allowance for estimated uncollectible reinsurance [Table Text Block] Reinsurance Recoverable, Allowance for Credit Loss [Table Text Block] Interest cost on benefit obligation Defined Benefit Plan, Interest Cost Total investments Investments Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Other intangible assets subject to amortization [Table Text Block] Schedule of Finite-Lived Intangible Assets [Table Text Block] Restatement Determination Date: Restatement Determination Date [Axis] Employee share-based compensation Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Non-PEO NEO Non-PEO NEO [Member] General liability [Member] General Liability [Member] Fixed maturities, available for sale, allowance for expected credit losses Allowance for expected credit losses Allowance for expected credit losses on fixed maturities classified as available-for-sale, balance, beginning of period Allowance for expected credit losses on fixed maturities classified as available-for-sale, balance, end of period Debt Securities, Available-for-Sale, Allowance for Credit Loss Accumulated Other Comprehensive Income [Table] Accumulated Other Comprehensive Income (Loss) [Table] Other Comprehensive Income And Accumulated Other Comprehensive Income Disclosure [Abstract] Other Comprehensive Income And Accumulated Other Comprehensive Income Disclosure [Abstract] Other Comprehensive Income and Accumulated Other Comprehensive Income disclosure [Abstract] Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss) [Line Items] Gross unrealized gains Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax Name Trading Arrangement, Individual Name Assets Disposal Group, Including Discontinued Operation, Assets [Abstract] All Award Types Award Type [Domain] Equity Awards Adjustments Equity Awards Adjustments [Member] Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less Available for Sale Securities for Which Fair Value is Less than Eighty Percent of Amortized Cost, Greater than Six Months Twelve Months or Less Amount of accumulated unrealized loss on investments in debt securities classified as available-for-sale in a continuous loss position of greater than 20% of amortized cost for greater than six months through twelve months. Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Marketing-related [Member] Marketing-Related Intangible Assets [Member] Other revenues Other Income Defined benefit plan disclosure Defined Benefit Plan Disclosure [Line Items] Compensation Amount Outstanding Recovery Compensation Amount Workers' compensation [Member] Workers' compensation [Member] Product line providing coverage for employers for specified benefits payable under state or federal law for workplace injuries to employees. Write-offs of uncollectible contractholder receivables Contractholder Receivables, Loss On Uncollectible Accounts In Period Amount of the write-off of uncollectible contractholder receivables. Reconciliation of beginning and ending property casualty reserve balances for claims and claim adjustment expenses [Table Text Block] Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Fair Value [Member] Estimate of Fair Value Measurement [Member] Commercial property [Member] Commercial property [Member] Product line providing coverage for loss or damage to buildings, inventory and equipment from a variety of events, including, among others, hurricanes and other windstorms, earthquakes, hail, wildfires, severe winter weather, floods, volcanic eruptions, tsunamis, theft, vandalism, fires, explosions, terrorism and financial loss due to business interruption resulting from covered property damage. Other [Member] Bond & Specialty Other [Member] Grouping of product lines not specifically outlined for Bond & Specialty including Property, Workers' Compensation, Commercial Automobile and Commercial Multi-Peril Pension and postretirement benefit plans [Domain] Retirement Plan Type [Domain] Percent of fixed maturities for which a pricing service estimates fair value Percent of fixed maturities for which a pricing service estimates fair value Percentage of fixed maturities for which a pricing service was used in determining fair value. MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Name Awards Close in Time to MNPI Disclosures, Individual Name Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Revenues Revenues [Abstract] Credit Loss [Abstract] Credit Loss [Abstract] Other Investments, at fair value Alternative Investment Compensation amortization under share-based plans and other changes APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Financial Instruments [Domain] Financial Instruments [Domain] Reportable Segments [Domain] Segments [Domain] Income Statement Location [Axis] Statement of Income Location, Balance [Axis] Other intangible assets not subject to amortization [Table Text Block] Schedule of Indefinite-Lived Intangible Assets [Table Text Block] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] A.M. Best, A- Rating Or Better [Member] AM Best, A Minus Rating Or Better [Member] A- credit rating or better as defined by the external credit rating agency, AM Best. Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured [Domain] Fair Value Hierarchy and NAV [Domain] Liabilities Disposal Group, Including Discontinued Operation, Liabilities [Abstract] Other intangible assets by major asset class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Remaining assets held for sale Disposal Group, Including Discontinued Operation, Other Assets Company Selected Measure Name Company Selected Measure Name Premiums receivable Increase (Decrease) in Premiums Receivable Fair Value Measurement [Domain] Fair Value Measurement [Domain] Fixed maturities Payments to Acquire Debt Securities, Available-for-Sale Net reserves disposed of related to the divestiture of the Canadian business Liability for Unpaid Claims and Claims Adjustment Expense, Net, Business Divestiture Amount, after deduction of reinsurance recovery, of unpaid claim liability divested in sale of business. Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Obligations of U.S. states, municipalities and political subdivisions [Member] US States and Political Subdivisions Debt Securities [Member] Cash payments to settle a lease liability reported in cash flows Operating Lease, Payments Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Core income (loss) Core income (loss) Net income (loss) excluding the after-tax impact of net realized gains (losses). Net income available to common shareholders -- diluted Net Income (Loss) Available to Common Stockholders, Diluted All Executive Categories All Executive Categories [Member] Estimated claims and claim adjustment expenses for claims arising in the current year Current Year Claims and Claims Adjustment Expense Amortization of deferred acquisition costs Deferred Policy Acquisition Costs, Amortization Expense Participating share-based awards — allocated income Undistributed Earnings (Loss) Allocated to Participating Securities, Basic Reconciliation of beginning and ending property casualty reserve balances for claims and claim adjustment expenses [Table] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Premiums Premiums Earned, Net Diluted (in shares) Weighted average shares outstanding, diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Obligations of U.S. states, municipalities and political subdivisions, revenue [Member] Obligations of states, municipalities and political subdivisions, revenue [Member] Bonds or similar securities issued by state or local governments that finance income-producing projects and are secured by a specified revenue source. Assets by segment [Table] Reconciliation of Assets from Segment to Consolidated [Table] Contract-based [Member] Contract-Based Intangible Assets [Member] Entity small business Entity Small Business Before Reclassifications of Held For Sale [Member] Before Reclassifications of Held For Sale [Member] Identifies the amount before any reclassifications needed for held for sale. Net favorable prior year reserve development Net favorable prior year reserve development impacting the Company's results of operations Net favorable prior year reserve development impacting the Company's results of operations Amount of benefit (expense) for claims incurred in prior reporting periods and related claims settlement costs impacting the Company's results of operations. Obligations of U.S. states, municipalities and political subdivisions, state general obligation [Member] Obligations of states, municipalities and political subdivisions, state general obligation [Member] Bonds or similar securities issued by state governments or the agencies operated by the state government which are secured by the issuers general taxing powers. Document transition report Document Transition Report Premiums receivable, allowance for expected credit losses [Roll Forward] Premium Receivable, Allowance for Credit Loss [Roll Forward] Underlying Securities Award Underlying Securities Amount Gross realized gains Debt Securities, Available-for-Sale, Realized Gain Equity Components [Domain] Equity Component [Domain] Document period end date Document Period End Date PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Gross realized losses on sales of fixed maturities Debt Securities, Available-for-sale, Sales, Realized Loss Amount of realized loss from sales of investment in debt security classified as available-for-sale. Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Fixed maturities, available for sale, at fair value Disposal Group, Including Discontinued Operation, Debt Securities, Available-for-sale Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) attributable to disposal group held for sale or disposed of. Short-term leases Short-Term Lease, Cost Securities transactions in the course of settlement Securities transactions in course of settlement The impact of timing on market settlement of cash and securities. Stock options exercisable at end of period, weighted average contractual life remaining (in years) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Average cost per share repurchased under share repurchase authorization (in dollars per share) Shares Acquired, Average Cost Per Share Goodwill Disposal Group, Including Discontinued Operation, Goodwill Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses Liability for claims and claim adjustment expense Liability for Claims and Claims Adjustment Expense [Line Items] Document type Document Type Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Asset reconciliation [Table Text Block] Reconciliation of Assets from Segment to Consolidated [Table Text Block] Earnings per Share Earnings Per Share [Text Block] Insurance Claim Reserves Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Name Outstanding Recovery, Individual Name Investment in non-public common and preferred equity securities where the fair value estimate is determined either internally or by an external fund manager [Member] Investments In Various Nonpublic Common And Preferred Equity Securities [Member] Investments in various non-public common and preferred equity securities, disclosed within other investments. Product and Service [Axis] Product and Service [Axis] Ceded Credit Risk, Reinsurer Group [Domain] Reinsurer, Name [Domain] All Individuals All Individuals [Member] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Business Insurance [Member] Business Insurance [Member] Reportable segment of the Company offering a broad array of property and casualty insurance and insurance related services to its customers primarily in the United States, as well as in Canada, the United Kingdom, the Republic of Ireland, Brazil and throughout other parts of the world as a corporate member of Lloyd's. Name Forgone Recovery, Individual Name Allowance for expected credit losses [Roll Forward] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Roll Forward] Allowance for expected credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss [Line Items] 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] Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Write-offs of uncollectible reinsurance recoverables Reinsurance, Loss on Uncollectible Accounts in Period, Amount Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Disposal Group Name [Axis] Disposal Group Name [Axis] Treasury stock acquired — net employee share-based compensation Payment, Tax Withholding, Share-Based Payment Arrangement Other [Member] Business Insurance Other [Member] Product line grouping within Business Insurance including the Special Liability Group (which manages the Company's asbestos and environmental liabilities) and the assumed reinsurance and certain other runoff operations. Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Remaining liabilities held for sale Disposal Group, Including Discontinued Operation, Other Liabilities Short-term securities Other Short-Term Investments Net income available to common shareholders -- basic Net Income (Loss) Available to Common Stockholders, Basic Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Other intangible assets by major asset class [Table] Intangible Asset, Finite-Lived [Table] Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost, greater than 12 months Available for Sale Securities for Which Fair Value is Less than Eighty Percent of Amortized Cost, Greater than Twelve Months Amount of accumulated unrealized loss on investments in debt securities classified as available-for-sale in a continuous loss position of greater than 20% of amortized cost for greater than twelve months. Other comprehensive income (loss), net of taxes Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Geographical [Axis] Geographical [Axis] Guarantor obligations, nature [Domain] Guarantor Obligations, Nature [Domain] Customer-related [Member] Customer-Related Intangible Assets [Member] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Accident and health [Member] Accident and Health Insurance, Excluding Workers' Compensation [Member] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Diluted (in dollars per share) Net income per common share, diluted (in dollars per share) Earnings Per Share, Diluted Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Title of 12(b) security Title of 12(b) Security Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Claims and claim adjustment expense reserves [Table Text Block] Claims and claim adjustment expense reserves [Table Text Block] Tabular disclosure of claims and claim adjustment reserves by insurance product line. Accumulated other comprehensive income (loss) [Roll Forward] AOCI Attributable to Parent, Net of Tax [Roll Forward] Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Credit Rating, A.M. Best [Domain] Credit Rating, AM Best [Domain] Write-offs of uncollectible premiums receivable Premium Receivable, Allowance for Credit Loss, Writeoff Treasury stock, at cost (in shares) Treasury Stock, Common, Shares Net income per share Earnings Per Share [Abstract] Consolidation Items [Domain] Consolidation Items [Domain] Ceded Credit Risk, Reinsurer [Axis] Reinsurer, Name [Axis] Gross losses Equity Securities, FV-NI, Unrealized Loss Availability of financial information provided by hedge funds following the date of the reporting Availability of financial information provided by hedge funds following the date of the reporting Number of months within which financial information provided by hedge funds is available following the reporting date. Bond & Specialty Insurance [Member] Bond & Specialty Insurance [Member] Reportable segment of the Company providing surety, fidelity, management liability, professional liability, and other property and casualty coverages and related risk management services to its customers in the United States and certain specialty insurance products in Canada, the United Kingdom, the Republic of Ireland and Brazil, utilizing various degrees of financially-based underwriting approaches. Additions for expected credit losses on securities where no credit losses were previously recognized Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded Claims and claim adjustment expenses [Member] Claims and claim adjustment expenses [Member] Primary financial statement caption encompassing claims and claim adjustment expenses. Contractholder payables Contractholder payables Under certain workers' compensation insurance contracts with deductible features, the Entity has an obligation to pay the full amount claimed. The Entity is subsequently reimbursed by the policyholder for the policy deductible. The amount to be reimbursed is included on a gross basis in the consolidated balance sheet in contractholder receivables and contractholder payables, respectively. Equity securities Proceeds from Sale of Equity Securities, FV-NI Income taxes paid Income Taxes Paid, Net Treasury stock acquired — share repurchase authorizations (in shares) Number of shares repurchased under share repurchase authorization (in shares) Treasury Stock, Shares, Acquired Award Timing Disclosures [Line Items] Service cost Defined Benefit Plan, Service Cost Net reserves at beginning of year Net reserves at end of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Credit Rating, A.M. Best [Axis] Credit Rating, AM Best [Axis] Net realized investment gains (losses), net of tax Net realized investment gains (losses), net of tax The net realized gains or losses on investments during the period, after tax. Property-casualty [Member] Property, Liability and Casualty Insurance Product Line [Member] Net income Net income Net income Amounts reclassified from accumulated other comprehensive income (loss), net of taxes Net Income (Loss) Attributable to Parent Unrealized investment losses [Table Text Block] Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Expiration Date Trading Arrangement Expiration Date Reinsurance recoverables Disposal Group, Including Discontinued Operation, Reinsurance Recoverables On Paid and Unpaid Losses Amount, after valuation allowance, recoverable under reinsurance contracts, attributable to disposal group held for sale or disposed of. Net recognized losses on equity securities still held Equity Securities, FV-NI, Unrealized Gain (Loss) Debt securities issued by foreign governments [Member] Debt Security, Government, Non-US [Member] Segment reporting information Segment Reporting Information [Line Items] Treasury stock acquired — share repurchase authorizations Payments for Repurchase of Common Stock Adoption Date Trading Arrangement Adoption Date Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Entity current reporting status Entity Current Reporting Status Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Retained earnings Retained Earnings (Accumulated Deficit) Financial Instrument [Axis] Financial Instrument [Axis] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Claims and expenses Benefits, Losses and Expenses [Abstract] Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Executive Category: Executive Category [Axis] Continuous unrealized loss position, 12 months or longer, gross unrealized losses Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Fee income Insurance Fee Income Fee income includes servicing carrier fees and revenues from large deductible policies and service contracts. Interest expense Interest Expense, Operating and Nonoperating Current fiscal year end date Current Fiscal Year End Date Continuous unrealized loss position, less than 12 months, gross unrealized losses Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Other intangible assets subject to amortization [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] 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] Benefit Plans [Axis] Retirement Plan Type [Axis] Claims and claim adjustment expense payments for claims arising in prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Statement [Table] Statement [Table] Reductions for impairments of securities which the Company intends to sell or more likely than not will be required to sell Debt Securities, Available-for-Sale, Allowance for Credit Loss, Sell before Recovery Effect of exchange rate changes on cash and restricted cash Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Including Discontinued Operation Adjustments to reconcile net income to net cash provided by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Cash flows from financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Reinsurance recoverables, credit quality indicator Reinsurance Recoverable, Credit Quality Indicator [Line Items] Common stock, shares issued (in shares) Shares, Issued Continuous unrealized loss position, total, gross unrealized losses Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured on a recurring basis [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Reinsurance Recoverables Rated by A.M. Best [Member] Reinsurance Recoverables, After Deduction Of Mandatory Pools And Associations, Before Allowance, Rated By AM Best [Member] Reinsurance recoverables, after deduction of mandatory pools and associations and before valuation allowance with a credit rating provided by AM Best. Basis of Presentation and Accounting Policies Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Weighted average number of common shares outstanding Weighted Average Number of Shares Outstanding, Diluted [Abstract] All Adjustments to Compensation All Adjustments to Compensation [Member] Carrying value and fair value of the financial assets and financial liabilities disclosed, but not carried, at fair value and the level within the fair value hierarchy at which such assets and liabilities are categorized [Table] Fair Value, by Balance Sheet Grouping [Table] Amendment flag Amendment Flag Tax benefit recognized in the consolidated statement of income related to compensation costs Share-Based Payment Arrangement, Expense, Tax Benefit Postretirement Benefit Plans [Member] Other Postretirement Benefits Plan [Member] Claims and claim adjustment expense reserves Disposal Group, Including Discontinued Operation, Liability for Claims and Claims Adjustment Expense Claims and claim adjustment expense reserves Claims and claim adjustment expense reserves at beginning of year Claims and claim adjustment expense reserves at end of period Liability for Claims and Claims Adjustment Expense Treasury stock, at cost (582.5 and 575.9 shares) Treasury Stock, Common, Value Additions (reductions) for expected credit losses on securities where credit losses were previously recognized Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) Fidelity and surety [Member] Fidelity and surety [Member] Product line providing fidelity insurance coverage, which protects an insured for loss due to embezzlement or misappropriation of funds by an employee, and surety, which is a three-party agreement whereby the insurer agrees to pay a third party to make complete an obligation in response to the default, acts or omissions of an insured. Leases Lessee, Operating Leases [Text Block] Termination Date Trading Arrangement Termination Date Net cash used in financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Measure: Measure [Axis] Obligations of U.S. states, municipalities and political subdivisions, local general obligation [Member] Obligations of states, municipalities and political subdivisions, local general obligation [Member] Bonds or similar securities issued by city or local governments or the agencies operated by the local government which are secured by the issuers general taxing powers. Common shares outstanding Stock Transactions, Parenthetical Disclosure [Abstract] Weighted average discount rate (percent) Operating Lease, Weighted Average Discount Rate, Percent Stock options vested at end of period, aggregate intrinsic value Stock options vested at end of period, aggregate intrinsic value Amount of difference between fair value of the underlying shares reserved for issuance and exercise prices of fully vested options outstanding. Basic (in shares) Weighted average shares outstanding, basic (in shares) Weighted Average Number of Shares Outstanding, Basic Remaining capacity under share repurchase authorization Share Repurchase Program, Remaining Authorized, Amount Noncash Investing and Financing Activities Cash Flow, Supplemental Disclosures [Text Block] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Earnings per share reconciliation [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Stock options exercisable at end of period, number (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Number Pre-tax and related income tax (expense) benefit components of the amounts reclassified from AOCI to the consolidated statement of income [Table Text Block] Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Segment Reporting [Abstract] Segment Reporting [Abstract] Gross unrealized investment losses on securities continuously in an unrealized loss position of greater than 20% of amortized cost by length of time [Table Text Block] Period for Which Fair Value is Less Than Eighty Percent of Amortized Cost [Table Text Block] Tabular disclosure of gross unrealized investment loss for securities with fair values less than 80% of amortized loss, by length of time in an unrealized loss position of greater than 20% of amortized cost. Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Common stock [Member] Common Stock Including Additional Paid in Capital [Member] Entity tax identification number Entity Tax Identification Number Gross gains Equity Securities, FV-NI, Unrealized Gain Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost, 3 months or less Available for Sale Securities for Which Fair Value is Less than Eighty Percent of Amortized Cost, Three Months or Less Amount of accumulated unrealized loss on investments in debt securities classified as available-for-sale in a continuous loss position of greater than 20% of amortized cost for three months or less. Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Weighted average remaining lease term (in years) Operating Lease, Weighted Average Remaining Lease Term Consolidation Reclassification [Domain] Consolidation Reclassification [Domain] Components and contra/elimination reclassifications used in consolidating amounts for reportable entity totals. Goodwill and Other Intangible Assets Goodwill and Intangible Assets Disclosure [Text Block] Guarantor obligations by nature [Axis] Guarantor Obligations, Nature [Axis] Income tax expense (benefit) Income tax expense (benefit) Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Goodwill by segment [Table Text Block] Schedule of Goodwill [Table Text Block] Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Insider Trading Arrangements [Line Items] Security exchange name Security Exchange Name Business segment reconciliations of revenue and income, net of tax [Table Text Block] Reconciliation of Revenue from Segments to Consolidated [Table Text Block] Lease expense Operating Lease And Short-Term Lease, Cost Total operating and short-term lease costs. Reinsurance recoverables Increase (Decrease) in Reinsurance Recoverable Reinsurance recoverables, allowance for estimated uncollectible reinsurance Reinsurance recoverables, allowance for uncollectible reinsurance, beginning balance Reinsurance recoverables, allowance for uncollectible reinsurance, ending balance Reinsurance Recoverable, Allowance for Credit Loss Insurance Loss Reserves [Abstract] Insurance Loss Reserves [Abstract] Guarantees of certain insurance policy obligations [Member] Guarantees of certain insurance policy obligations [Member] An agreement (contract) that contingently requires the guarantor to make payments to the guaranteed party for that party's loss related to certain insurance policy obligations of a former subsidiary. Total liabilities Liabilities Common stock (1,750.0 shares authorized; 212.6 and 217.5 shares issued and outstanding) Common Stock, Value, Outstanding Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Credit losses on fixed maturities recognized in the consolidated statement of income, as a percentage of shareholders' equity on an after-tax basis (less than) Debt Securities, Available-For-Sale, Credit Losses Recognized In Earnings For The Period As Percentage Of Shareholders' Equity On After Tax Basis Credit losses recognized in earnings during the period related to debt securities held, as a percentage of shareholders' equity on an after-tax basis. Consolidated Statement of Changes in Shareholders' Equity Statement [Line Items] Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Consolidation Items [Axis] Consolidation Items [Axis] Voluntary Pools [Member] Voluntary Pools [Member] Reinsurance group relating to the Company's participation in voluntary pools not rated by AM Best. Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Total net impairment charges, including credit impairments, reported in net realized investment gains (losses) in the consolidated statement of income Total Net Impairment Charges, Including Credit Impairments, Reported in Net Realized Investment Gains (Losses) Total amount of net impairment changes, including credit impairments, reported in net realized investment gains (losses) in the consolidated statement of income. Common shares outstanding [Member] Common stock [Member] Common Stock [Member] Fixed maturities, available for sale, at fair value (amortized cost $93,742 and $91,717; allowance for expected credit losses of $3 and $3) Fixed maturities, at fair value Debt Securities, Available-for-Sale Entity incorporation, state Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Net increase (decrease) in cash and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Net changes in benefit plan assets and obligations Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent