Capital Structure - Narrative (Details)						12 Months Ended
	Jan. 15, 2020 USD ($) $ / shares shares	Nov. 08, 2019 USD ($) $ / shares shares	Oct. 15, 2019 USD ($) $ / shares shares	Aug. 08, 2019 USD ($) $ / shares shares	May 16, 2019 USD ($)	Dec. 31, 2019 USD ($) director $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Jun. 10, 2019 USD ($)
Class of Stock [Line Items]
Repayments of long-term debt						$ 317,000,000	$ 400,000,000	$ 0
Long-term debt						6,631,000,000	6,451,000,000
Unsecured revolving credit facility						1,000,000,000.00
Commercial Paper maximum borrowing capacity						1,000,000,000.00
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						312,000,000	330,000,000	332,000,000
Other liabilities and accrued expenses						$ 389,000,000	$ 260,000,000
Common stock
Common stock, shares issued (in shares) | shares						900,000,000	900,000,000
Common stock, shares outstanding (in shares) | shares						319,000,000	332,000,000
Treasury stock, shares (in shares) | shares						581,000,000	568,000,000
Reacquired shares (in shares) | shares						16,000,000
Average cost of reacquired shares (in dollars per share) | $ / shares						$ 110.37
Reissued net shares under equity incentive plans (in shares) | shares						3,000,000
Preferred stock
Preferred stock, par value (in dollars per share) | $ / shares						$ 1	$ 1
Aggregate liquidation preference						$ 2,313,000,000	$ 1,995,000,000
Redemption of preferred stock			$ 1,130,000,000			1,132,000,000	385,000,000	$ 0
Preferred Stock Redemption Premium						$ 37,000,000
Period of issuance of common stock prior to dividend declaration date during which dividend declaration is prohibited						90 days
Series D
Preferred stock
Stock Redeemed (in shares) | shares			5,400
Preferred stock, par value (in dollars per share) | $ / shares			$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares			$ 25,000
Series E
Preferred stock
Stock Redeemed (in shares) | shares			29,900
Preferred stock, par value (in dollars per share) | $ / shares			$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares			$ 25,000
Series F
Preferred stock
Stock Redeemed (in shares) | shares			10,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares			$ 25,000
Series I
Preferred stock
Preferred stock, par value (in dollars per share) | $ / shares		$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares		$ 25,000
Aggregate liquidation preference		$ 300,000,000				$ 300,000,000.0	0
Stock issued during period (in shares) | shares		12,000
Dividend rate		4.75%				4.75%
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 value (in dollars 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 H
Preferred stock
Preferred stock, par value (in dollars per share) | $ / shares				$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares				$ 25,000
Aggregate liquidation preference				$ 1,150,000,000		$ 1,150,000,000.0	0
Stock issued during period (in shares) | shares				46,000
Dividend rate				5.10%		5.10%
Series E
Preferred stock
Aggregate liquidation preference						$ 0	747,500,000
Dividend rate						6.625%
Series F
Preferred stock
Aggregate liquidation preference						$ 0	$ 250,000,000.0
Dividend rate						6.25%
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
Floating Rate Senior Notes, due 2021
Class of Stock [Line Items]
Debt instrument, face amount									$ 500,000,000
Senior Notes, Due 2049 [Member]
Class of Stock [Line Items]
Note stated interest rate (as a percent)									3.85%
3.28% Senior Notes, due 2026
Class of Stock [Line Items]
Note stated interest rate (as a percent)						3.28%
4.20% Senior Notes, due 2046
Class of Stock [Line Items]
Note stated interest rate (as a percent)						4.20%
5.100% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)						5.10%
5.750% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)						5.75%
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%	6.50%
Long-term debt							$ 500,000,000
6.125% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Note stated interest rate (as a percent)						6.125%
6.75% Senior Debentures, due 2018
Class of Stock [Line Items]
Note stated interest rate (as a percent)
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
7.45% Senior Notes, due 2019
Class of Stock [Line Items]
Note stated interest rate (as a percent)					7.45%	7.45%
Repayments of long-term debt					$ 317,000,000
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%
Commercial Paper
Class of Stock [Line Items]
Amount of commercial paper outstanding						$ 0	$ 0
London Interbank Offered Rate (LIBOR) | 5.100% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)						3.165%
London Interbank Offered Rate (LIBOR) | 5.750% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)						2.938%
London Interbank Offered Rate (LIBOR) | 6.500% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)						2.12%
Subsequent Event
Preferred stock
Redemption of preferred stock	$ 288,000,000
Preferred Stock Redemption Premium	$ 10,000,000
Subsequent Event | Series A Preferred Stock
Preferred stock
Stock Redeemed (in shares) | shares	11,500
Preferred stock, par value (in dollars per share) | $ / shares	$ 1.00
Preferred stock, shares liquidation preference (in dollars per share) | $ / shares	$ 25,000