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Capital Structure - Narrative (Details) |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jan. 15, 2020 USD ($) $ / shares shares | Dec. 31, 2020 USD ($) director $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) | Nov. 30, 2020 USD ($) | Nov. 19, 2020 USD ($)
Class of Stock [Line Items] |  |  |  |  |  |
Long-term debt |  | $ 7,825,000,000 | $ 6,631,000,000 |  |  |
Unsecured revolving credit facility |  |  |  |  | $ 750,000,000 |
Additional borrowing capacity under credit facility increase provision |  | $ 500,000,000 |  |  |  |
Maximum debt to capital ratio allowed under credit facility covenant (as a percent) |  | 37.50% |  |  |  |
Credit facility, amount outstanding |  | $ 0 | 0 |  |  |
Interest paid on debt |  | 311,000,000 | 312,000,000 | $ 330,000,000 |  |
Other liabilities and accrued expenses |  | $ 430,000,000 | $ 389,000,000 |  |  |
Common stock |  |  |  |  |  |
Common stock, shares, issued (in shares) | shares |  | 900,000,000 | 900,000,000 |  |  |
Common stock, shares, outstanding (in shares) | shares |  | 304,000,000 | 319,000,000 |  |  |
Treasury stock, shares (in shares) | shares |  | 596,000,000 | 581,000,000 |  |  |
Reacquired shares (in shares) | shares |  | 17,000,000 |  |  |  |
Average cost of reacquired shares (in dollars per share) | $ / shares |  | $ 97.54 |  |  |  |
Reissued net shares under equity incentive plans (in shares) | shares |  | 3,000,000 |  |  |  |
Preferred stock |  |  |  |  |  |
Preferred stock, par or stated value per share (in USD per share) | $ / shares |  | $ 1 | $ 1 |  |  |
Redemption of preferred stock | $ 288,000,000 |  |  |  |  |
Preferred stock redemption premium | $ 10,000,000 |  |  |  |  |
Period of issuance of common stock prior to dividend declaration date during which dividend declaration is prohibited |  | 90 days |  |  |  |
Noncumulative Perpetual Preferred Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Period after occurrence of certain tax and rating agency events in which redemption of debt in whole permissible |  | 90 days |  |  |  |
Preferred stock |  |  |  |  |  |
Preferred stock, par or stated value per share (in USD per share) | $ / shares |  | $ 1.00 |  |  |  |
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares |  | 25,000 |  |  |  |
Preferred stock, redemption price (in dollars per share) | $ / shares |  | $ 25,000 |  |  |  |
Series A Preferred Stock |  |  |  |  |  |
Preferred stock |  |  |  |  |  |
Preferred stock, par or stated value per share (in USD per share) | $ / shares | $ 1.00 |  |  |  |  |
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares | $ 25,000 |  |  |  |  |
Stock redeemed (in shares) | shares | 11,500 |  |  |  |  |
Minimum |  |  |  |  |  |
Preferred stock |  |  |  |  |  |
Collective risk-based capital ratios of subsidiaries as a percentage of net written premiums of insurance business |  | 80.00% |  |  |  |
Percentage of decline in consolidated shareholders' equity from benchmark quarter |  | 20.00% |  |  |  |
Minimum | Noncumulative Perpetual Preferred Stock |  |  |  |  |  |
Preferred stock |  |  |  |  |  |
Preferred stock, redemption price (in dollars per share) | $ / shares |  | $ 25,000 |  |  |  |
Maximum |  |  |  |  |  |
Preferred stock |  |  |  |  |  |
Risk-based capital ratios as a percentage of action level risk-based capital |  | 175.00% |  |  |  |
Percentage by which consolidated shareholders' equity should increase or decrease to avoid restriction on dividends |  | 20.00% |  |  |  |
Maximum | Noncumulative Perpetual Preferred Stock |  |  |  |  |  |
Preferred stock |  |  |  |  |  |
Additional members of the board of directors | director |  | 2 |  |  |  |
Preferred stock, redemption price (in dollars per share) | $ / shares |  | $ 25,500 |  |  |  |
Commercial Paper |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Commercial Paper maximum borrowing capacity |  | $ 750,000,000 |  |  |  |
Amount of commercial paper outstanding |  | $ 0 | $ 0 |  |  |
Senior Notes Due 2025 at 0.750 Percent |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  | $ 600,000,000
Note stated interest rate (as a percent) |  | 0.75% |  |  |  | 0.75%
Long-term debt |  |  |  |  |  | $ 600,000,000
Senior Notes Due 2030 at 1.45 Percent |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  | $ 600,000,000
Note stated interest rate (as a percent) |  | 1.45% |  |  |  | 1.45%
Long-term debt |  |  |  |  |  | $ 600,000,000
7.45% Senior Notes, due 2019 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Threshold percentage of repurchase or redemption of outstanding principal debentures for non applicability of promises and covenants contained in the new RCC |  | 10.00% |  |  |  |
Period within which threshold percentage of outstanding principal of debentures is to be purchased for non applicability of promises and covenants contained in the new RCC |  | 1 year |  |  |  |
Specified period within which percentage of outstanding principal of debentures is to be purchased for non applicability of promises and covenants contained in the new RCC |  | 10 years |  |  |  |
7.45% Senior Notes, due 2019 | Maximum |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Percentage of repurchase or redemption of outstanding principal debentures within specified period for non applicability of promises and covenants contained in the new RCC |  | 25.00% |  |  |  |
5.100% Subordinated Debentures, due 2053 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Note stated interest rate (as a percent) |  | 5.10% |  |  |  |
5.100% Subordinated Debentures, due 2053 | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Interest rate spread over LIBOR (as a percent) |  | 3.165% |  |  |  |
5.750% Subordinated Debentures, due 2053 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Note stated interest rate (as a percent) |  | 5.75% |  |  |  |
5.750% Subordinated Debentures, due 2053 | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Interest rate spread over LIBOR (as a percent) |  | 2.938% |  |  |  |
Subordinated Debentures |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Minimum principal outstanding amount for redemption of debt in whole or in part |  | $ 25,000,000 |  |  |  |
Period after occurrence of certain tax and rating agency events in which redemption of debt in whole permissible |  | 90 days |  |  |  |
Maximum period for deferment of interest payment |  | 5 years |  |  |  |
6.500% Junior Subordinated Debentures, due 2067 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Note stated interest rate (as a percent) |  | 6.50% |  |  |  |
Long-term debt |  | $ 500,000,000 |  |  |  |
6.500% Junior Subordinated Debentures, due 2067 | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Interest rate spread over LIBOR (as a percent) |  | 2.12% |  |  |  |
The Debentures |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Maximum period for deferment of interest payment |  | 10 years |  |  |  |
Reserved common stock authorized and unissued for debentures obligations (in shares) | shares |  | 75,000,000 |  |  |  |

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