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