| Benefits, Losses and Expenses | $10.6B | $10.3B | $9.4B | $11.3B | $10.2B | $10.1B | $9.3B | $9.8B |
| Payments to Acquire Debt Securities, Available-for-Sale | $4.3B | $4.3B | $4.8B | $4.3B | $4.8B | $5.4B | $4.3B | $5.4B |
| Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale | $2.5B | $1.8B | $2.5B | $2.8B | $3.1B | $2.9B | $2.9B | $3B |
| Proceeds from principal business dispositions | — | — | — | — | — | — | — | $2.4B |
| Cash Provided by (Used in) Operating Activity, Including Discontinued Operation | $1.7B | $3.9B | $2.1B | $1.4B | $2.3B | $4.2B | $2.7B | $2.2B |
| Cash Provided by (Used in) Financing Activity, Including Discontinued Operation | -$472M | -$454M | -$422M | -$535M | -$702M | — | -$1.8B | -$2.1B |
| Increase (Decrease) in Deferred Policy Acquisition Costs | $1.8B | $1.9B | $1.7B | $1.8B | $1.9B | $1.9B | $1.8B | $1.8B |
| Treasury stock, value, acquired | $250M | $250M | — | $250M | $500M | $625M | — | $1.8B |
| Payments for Repurchase of Common Stock | $249M | $248M | $256M | $250M | $500M | $619M | $1.6B | $1.8B |
| Deferred Policy Acquisition Costs, Amortization Expense | $1.7B | $1.8B | $1.8B | $1.8B | $1.8B | $1.8B | $1.8B | $1.8B |
| Core income (loss) | $585M | $1.2B | — | $443M | $1.5B | $1.9B | — | $1.7B |
| Loss and loss adjustment expense reserves | $1.4B | $755M | — | $1.8B | $725M | $845M | $0 | $1.2B |
| Net unrealized gains/(losses) on available-for-sale securities | -$324M | $2.4B | — | $436M | $341M | $1.3B | — | -$1.1B |
| Comprehensive Income (Loss), Net of Tax, Attributable to Parent | $255M | $3.2B | — | $794M | $2B | $2.9B | — | $1.1B |
| Net Investment Income | $885M | $904M | $955M | $930M | $942M | $1B | $1.1B | $1B |
| Payments for (Proceeds from) Available-for-Sale Securities, Short-Term | -$330M | $1.1B | -$712M | -$239M | — | $2.1B | — | $945M |
| Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent | -$350M | $2.5B | — | $498M | $569M | $1.3B | — | -$827M |
| All other operating activities | -$275M | $962M | $452M | -$899M | — | — | $289M | -$634M |
| Other comprehensive income/(loss) | -$279M | $2B | — | $399M | $483M | $991M | — | -$578M |
| Increase (Decrease) in Premiums Receivable | $664M | — | — | $459M | $438M | — | — | $434M |
| Securities transactions in the course of settlement | $247M | $24M | — | $308M | $64M | — | — | $430M |
| Unearned premiums | $788M | $659M | — | $419M | $495M | $331M | — | $351M |
| Cash Provided by (Used in) Investing Activity, Including Discontinued Operation | -$1.1B | -$3.4B | -$1.7B | -$808M | -$1.7B | -$4.6B | -$575M | -$335M |
| Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax, Portion Attributable to Parent | -$25M | $120M | — | $61M | $228M | -$72M | — | $327M |
| Sale of investments | $308M | $225M | $159M | $253M | $348M | $178M | $53M | $251M |
| Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent | -$71M | $513M | — | $99M | $86M | $284M | — | -$249M |
| Payments of Ordinary Dividends, Common Stock | $244M | $238M | $240M | $240M | $250M | $247M | $242M | $237M |
| Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation | $62M | $43M | — | $25M | -$65M | $70M | — | -$227M |
| Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation | $3M | $3M | — | $108M | $57M | $3M | — | $185M |
| Gross Unrealized Gains | — | $43M | — | $132M | $27M | $39M | — | $170M |
| Payment, Tax Withholding, Share-Based Payment Arrangement | $1M | $1M | $2M | $102M | $22M | $1M | $2M | $149M |
| Membership and other income | $105M | $120M | $112M | $112M | $123M | $148M | $125M | $141M |
| Payment for (Proceeds from) Other Investing Activity | $111M | $113M | $103M | $116M | $127M | $155M | $135M | $119M |
| Increase (Decrease) in Reinsurance Recoverable | $34M | -$74M | — | $97M | — | $250M | — | $108M |
| Interest Paid, Excluding Capitalized Interest, Operating Activity | $135M | $60M | $135M | $61M | $136M | $60M | $136M | $94M |
| Pre-tax stock-based compensation cost | $60M | $63M | — | $83M | $57M | $59M | — | $87M |
| Cash received from option exercises | $22M | $33M | $76M | $57M | $70M | $36M | $51M | $86M |
| Payments to Acquire Other Investments | $95M | $98M | — | $96M | $80M | $86M | — | $69M |
| Other investments, net of payments   | $55M | $101M | $211M | $63M | $79M | $68M | $111M | $60M |
| Proceeds from disposal of certain strategic investments | $41M | $31M | $50M | $68M | $32M | $31M | $29M | $52M |
| Federal | -$85M | -$59M | -$50M | $31M | -$83M | $320M | — | $51M |
| Gross realized gains (losses) | -$65M | $55M | -$55M | -$61M | $6M | $27M | -$20M | $49M |
| Payments to Acquire Equity Securities, FV-NI | $21M | $33M | — | $25M | $35M | $34M | — | $25M |
| Gross realized losses on sales of fixed maturities | $2M | $16M | — | $12M | $14M | $3M | — | $19M |
| Operating Lease Payments | $20M | $18M | — | $18M | $18M | $19M | — | $18M |
| Current period change for expected credit losses | $13M | $16M | — | $16M | $19M | $13M | — | $18M |
| Lease Cost | $19M | $19M | — | $17M | $16M | $17M | — | $16M |
| Operating Lease And Short-Term Lease, Cost | $19M | $19M | — | $17M | $16M | $17M | — | $16M |
| Premium Receivable, Allowance for Credit Loss, Writeoff | $12M | $15M | — | $16M | $16M | $16M | — | $16M |
| Net realized investment gains (losses), net of tax | -$51M | $42M | — | -$48M | $5M | $21M | — | $15M |
| Share-Based Payment Arrangement, Expense, Tax Benefit | $10M | $11M | — | $13M | $9M | $11M | — | $13M |
| Income Taxes Paid, Net | $831M | $92M | $363M | $24M | $538M | $194M | $518M | $12M |
| Payment for Acquisition, Real Estate, Held-for-Investment | $11M | $10M | $14M | $7M | $6M | $10M | $25M | $12M |
| Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent | $2M | $1M | — | $0 | $0 | $0 | — | $8M |
| Right Of Use Asset Obtained In Exchange For Operating Lease Liability | $4M | $31M | — | $7M | $9M | $9M | — | $8M |
| Equity Securities, FV-NI, Unrealized Loss | $1M | — | $4M | $10M | $3M | $0 | $5M | $7M |
| Number of shares purchased (in shares) | 1.1M | 1.1M | — | 1M | 1.8M | 2.3M | — | 6M |
| Effect of currency exchange rate changes on cash, cash equivalents, and restricted cash | $0 | $13M | -$22M | $8M | $19M | — | $2M | -$5M |
| Total net impairment charges, including credit impairments, reported in net realized investment gains (losses) in the consolidated statement of income | $0 | $5M | — | $2M | $0 | $0 | — | $4M |
| Add - Incremental shares under stock-based compensation plans (in shares) | 2.9M | 3.2M | — | 3.5M | 3.4M | 3.4M | — | 3.2M |
| Current period change for expected credit losses | -$1M | $0 | — | -$1M | $0 | $0 | — | -$2M |
| Current period change for estimated uncollectible reinsurance | $0 | $3M | — | $9M | -$1M | $14M | — | $1M |
| Tax withholding obligation, shares withheld (in shares) | 12.2K | 14.3K | — | 400K | 300K | 10.4K | — | 600K |
| Average price paid (in dollars per share) | $211.22 | $222.58 | — | $256.95 | $269.90 | $271.76 | — | $300.30 |
| Number of reportable segments | — | — | — | 3.00 | — | — | — | 3.00 |
| Dividends declared (in dollars per share) | $1.1 | $1.1 | — | $1.1 | $1.1 | $1.1 | — | $1.1 |
| Changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income | $1M | $1M | — | $1M | $0 | $0 | — | $0 |
| Write-offs of uncollectible contractholder receivables | $0 | $0 | — | $0 | $0 | $0 | — | $0 |
| Realized losses resulting from the early redemption of fixed maturities by the issuer prior to the bonds' maturity date | $33M | $1M | — | $18M | $5M | — | — | $0 |
| Write-offs of uncollectible reinsurance recoverables | $0 | $0 | — | $0 | $0 | $0 | — | $0 |
