Capital Structure - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) director $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 08, 2016 USD ($)
Class of Stock [Line Items]
Repurchase of debt		$ 17,000,000	$ 11,000,000
Long-term debt	$ 6,350,000,000	6,347,000,000
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	332,000,000	287,000,000	$ 289,000,000
Other liabilities and accrued expenses	$ 169,000,000	$ 132,000,000
Common stock
Common stock, shares issued (in shares) | shares	900,000,000	900,000,000
Common stock, shares outstanding (in shares) | shares	355,000,000	366,000,000
Treasury stock, shares (in shares) | shares	545,000,000	534,000,000
Reacquired shares (in shares) | shares	16,000,000
Average cost of reacquired shares (in dollars per share) | $ / shares	$ 89.95
Reissued net shares under equity incentive plans (in shares) | shares	5,000,000
Preferred stock
Preferred stock, par value (in dollars per share) | $ / shares	$ 1	$ 1
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 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
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%
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
3.28% Senior Notes, due 2026
Class of Stock [Line Items]
Debt instrument, face amount				$ 550,000,000
Note stated interest rate (as a percent)	3.28%			3.28%
4.20% Senior Notes, due 2046
Class of Stock [Line Items]
Debt instrument, face amount				$ 700,000,000
Note stated interest rate (as a percent)	4.20%			4.20%
5.10% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)	5.10%
5.75% 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
6.50% 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
6.125% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Note stated interest rate (as a percent)	6.125%
Long-term debt	$ 224,000,000
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%
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
Limited Partnership
Class of Stock [Line Items]
Other liabilities and accrued expenses	$ 132,000,000	$ 45,000,000
London Interbank Offered Rate (LIBOR) | 5.10% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)	3.165%
London Interbank Offered Rate (LIBOR) | 5.75% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)	2.938%
London Interbank Offered Rate (LIBOR) | 6.50% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)	2.12%
London Interbank Offered Rate (LIBOR) | 6.125% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Interest rate spread over LIBOR (as a percent)	1.935%