Capital Structure Capital Structure, Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended			12 Months Ended
	Jun. 30, 2014 USD ($) shares	Apr. 30, 2014	Mar. 31, 2014 USD ($) shares	Dec. 31, 2015 USD ($) director $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($)	Jun. 12, 2013 $ / shares
Class of Stock [Line Items]
Repurchased principal amounts of debt				$ 11.0	$ 10.0
Gains (Losses) on Extinguishment of Debt				0.0	(1.0)	$ (491.0)
Long-term debt				5,124.0	5,140.0
Unsecured revolving credit facility				1,000.0
Commercial Paper maximum borrowing capacity				1,000.0
Credit facility extended term		1 year
Additional borrowing capacity under credit facility increase provision				$ 500.0
Maximum debt to capital ratio allowed under credit facility covenant (as a percent)				37.50%
Interest paid on debt				$ 289.0	$ 332.0	$ 361.0
Other liabilities and accrued expenses				$ 107.0
Common stock
Common stock, shares issued | shares				900,000,000	900,000,000
Common stock, shares outstanding | shares				381,000,000	418,000,000
Treasury stock, shares | shares				519,000,000	482,000,000
Reacquired shares | shares				43,000,000
Average cost of reacquired shares (in dollars per share) | $ / shares				$ 65.55
Reissued net shares under equity incentive plans | shares				6,000,000
Preferred stock
Preferred stock, par value (in dollars per share) | $ / shares				$ 1	$ 1
Preferred stock, shares issued | shares				72,200	72,200
Aggregate liquidation preference				$ 1,805.0
Period of issuance of common stock prior to dividend declaration date during which dividend declaration is prohibited				90 days
Series E
Preferred stock
Preferred stock, shares issued | shares			29,900	29,900
Dividend rate				6.625%
Aggregate liquidation preference			$ 747.5	$ 747.5
Series F
Preferred stock
Preferred stock, shares issued | shares	10,000			10,000
Dividend rate				6.25%
Aggregate liquidation preference	$ 250.0			$ 250.0
Noncumulative Perpetual Preferred Stock
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			$ 25,000
Period after occurrence of certain tax and rating agency events in which redemption of debt in whole permissible				90 days
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
6.75% Senior Debentures, due 2018
Class of Stock [Line Items]
Note stated interest rate (as a percent)				6.75%	6.75%
Long-term debt				$ 176.0	$ 176.0
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
6.75% Senior Debentures, due 2018 | 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%
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
6.125% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Note stated interest rate (as a percent)				6.125%	6.125%
Long-term debt				$ 241.0	$ 252.0
Interest rate spread over LIBOR (as a percent)				1.935%
6.50% Junior Subordinated Debentures, due 2067
Class of Stock [Line Items]
Note stated interest rate (as a percent)				6.50%	6.50%
Long-term debt				$ 500.0	$ 500.0
Interest rate spread over LIBOR (as a percent)				2.12%
Subordinated Debentures
Class of Stock [Line Items]
Minimum principal outstanding amount for redemption of debt in whole or in part				$ 25.0
Interest on debentures payment terms				three-month LIBOR
Maximum period for deferment of interest payment				5 years
Preferred stock
Period after occurrence of certain tax and rating agency events in which redemption of debt in whole permissible				90 days
5.75% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)				5.75%	5.75%
Interest on debentures payment terms				three-month LIBOR
Long-term debt				$ 800.0	$ 800.0
Interest rate spread over LIBOR (as a percent)				2.938%
5.10% Subordinated Debentures, due 2053
Class of Stock [Line Items]
Note stated interest rate (as a percent)				5.10%	5.10%
Interest on debentures payment terms				three-month LIBOR
Long-term debt				$ 500.0	$ 500.0
Interest rate spread over LIBOR (as a percent)				3.165%
Federal Home Loan Bank (“FHLB”) advances, due 2018
Class of Stock [Line Items]
Long-term debt				$ 0.0	8.0
Commercial Paper
Class of Stock [Line Items]
Amount of commercial paper outstanding				0.0	$ 0.0
Limited Partnership [Member]
Class of Stock [Line Items]
Other liabilities and accrued expenses				89.0
Debt Related to Other Investments [Member]
Class of Stock [Line Items]
Other liabilities and accrued expenses				$ 18.0