Capital Structure (Details) - USD ($)					9 Months Ended
	Oct. 15, 2018	May 15, 2018	May 13, 2018	Mar. 29, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt outstanding:
Preferred stock, par value (in dollars per share)					$ 1		$ 1
Repayments of long-term debt					$ 401,000,000	$ 0
Noncumulative Preferred Stock
Debt outstanding:
Preferred stock, issued shares (in shares)				23,000
Preferred stock, dividend rate				5.625%
Preferred stock, par value (in dollars per share)				$ 1.00
Preferred stock, shares liquidation preference (in dollars per share)				$ 25,000
Proceeds from issuance of redeemable preferred stock				$ 575,000,000
Preferred stock, redemption price per share (in dollars per share)				$ 25,000
Period after occurrence of certain rating agency events in which redemption of debt in whole permissible				90 days
Senior Notes, Due 2021
Debt outstanding:
Debt instrument, face amount				$ 250,000,000
Senior Notes, Due 2023
Debt outstanding:
Debt instrument, face amount				$ 250,000,000
Junior Subordinated Debentures Due 2067 at 6.125%
Debt outstanding:
Debt instrument, face amount			$ 224,000,000
Note stated interest rate (as a percent)			6.125%
Redemption price, percentage of principal amount redeemed			100.00%
Senior Debentures Due 2018 at 6.75%
Debt outstanding:
Note stated interest rate (as a percent)		6.75%
Redemption price, percentage of principal amount redeemed		100.00%
Repayments of long-term debt		$ 176,000,000
London Interbank Offered Rate (LIBOR) | Senior Notes, Due 2021
Debt outstanding:
Basis spread on variable rate				0.43%
London Interbank Offered Rate (LIBOR) | Senior Notes, Due 2023
Debt outstanding:
Basis spread on variable rate				0.63%
Subsequent Event | Noncumulative Preferred Stock
Debt outstanding:
Preferred stock, par value (in dollars per share)	$ 1.00
Preferred stock, shares liquidation preference (in dollars per share)	$ 25,000
Stock redeemed (in shares	15,400
Payments for repurchase of preferred stock	$ 385,000,000