Capital Structure (Details) - USD ($)		1 Months Ended
	Mar. 29, 2018	May 13, 2018	Apr. 13, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt outstanding:
Preferred stock, par value (in dollars per share)				$ 1	$ 1
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% | Subsequent Event
Debt outstanding:
Debt instrument, face amount			$ 224,000,000
Note stated interest rate (as a percent)			6.125%
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%
Scenario, Forecast | Junior Subordinated Debentures Due 2067 at 6.125%
Debt outstanding:
Redemption price, percentage of principal amount redeemed		100.00%
Debt instrument, repurchased face amount			$ 224,000,000
Debt instrument, accrued interest			$ 2,000,000