Capital Structure, Narrative (Details)	1 Months Ended		3 Months Ended		12 Months Ended
	Jun. 30, 2014 USD ($) shares	Mar. 31, 2014 USD ($) shares	Jun. 30, 2014 shares	Mar. 31, 2014 shares	Dec. 31, 2016 USD ($) director $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)	Dec. 08, 2016 USD ($)
Class of Stock [Line Items]
Repurchase of debt					$ 17,000,000	$ 11,000,000	$ 10,000,000
Loss on extinguishment of debt					0	0	1,000,000
Long-term debt					6,347,000,000	5,124,000,000
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					287,000,000	289,000,000	$ 332,000,000
Other liabilities and accrued expenses					$ 132,000,000	$ 107,000,000
Common stock
Common stock, shares issued (in shares) | shares					900,000,000	900,000,000
Common stock, shares outstanding (in shares) | shares					366,000,000	381,000,000
Treasury stock, shares (in shares) | shares					534,000,000	519,000,000
Reacquired shares (in shares) | shares					20,000,000
Average cost of reacquired shares (in dollars per share) | $ / shares					$ 66.45
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
Preferred stock, shares issued (in shares) | shares					72,200	72,200
Aggregate liquidation preference					$ 1,805,000,000
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 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 E
Preferred stock
Preferred stock, shares issued (in shares) | shares		29,900		29,900	29,900
Dividend rate				6.625%	6.625%
Aggregate liquidation preference		$ 747,500,000			$ 747,500,000
Series F
Preferred stock
Preferred stock, shares issued (in shares) | shares	10,000		10,000		10,000
Dividend rate			6.25%		6.25%
Aggregate liquidation preference	$ 250,000,000				$ 250,000,000
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%
Long-term debt					$ 550,000,000	$ 0
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%
Long-term debt					$ 700,000,000	0
5.10% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)					5.10%
Interest rate spread over LIBOR (as a percent)					3.165%
Long-term debt					$ 500,000,000	500,000,000
5.75% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)					5.75%
Interest rate spread over LIBOR (as a percent)					2.938%
Long-term debt					$ 800,000,000	800,000,000
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%
Interest rate spread over LIBOR (as a percent)					2.12%
Long-term debt					$ 500,000,000	500,000,000
6.125% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Note stated interest rate (as a percent)					6.125%
Interest rate spread over LIBOR (as a percent)					1.935%
Long-term debt					$ 224,000,000	241,000,000
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%
Long-term debt					$ 317,000,000	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
Limited Partnership
Class of Stock [Line Items]
Other liabilities and accrued expenses					45,000,000	89,000,000
Debt Related to Other Investments
Class of Stock [Line Items]
Other liabilities and accrued expenses					$ 87,000,000	$ 18,000,000