Capital Structure (Details) - USD ($)				9 Months Ended
	Oct. 15, 2019	Aug. 08, 2019	May 16, 2019	Sep. 30, 2019	Sep. 30, 2018	Oct. 15, 2024	Jun. 10, 2019	Dec. 31, 2018
Debt outstanding:
Preferred stock, par value (in dollars per share)				$ 1				$ 1
Redemption and repayment of long-term debt				$ 317,000,000	$ 401,000,000
Senior Notes, Due 2049
Debt outstanding:
Debt instrument, face amount							$ 500,000,000
Note stated interest rate (as a percent)							3.85%
Senior Notes Due 2019 at 7.45 Percent
Debt outstanding:
Note stated interest rate (as a percent)			7.45%
Redemption and repayment of long-term debt			$ 317,000,000
Series H Preferred Stock
Debt outstanding:
Shares Issued (in shares)		46,000
Dividend Rate (in dollars per share)		5.10%
Preferred stock, par value (in dollars per share)		$ 1.00
Liquidation preference per share (in dollars per shares)		$ 25,000
Proceeds from issuance of preferred stock Gross		$ 1,150,000,000
Redemption price (in dollars per share)		$ 25,000
Period after occurrence in which redemption of debt in whole permissible		90 days
Redemption price, rating agency event (in dollars per share)		$ 25,500
Subsequent Event
Debt outstanding:
Payments for repurchase of preferred stock	$ 1,130,000,000
Redemption price in excess of carrying value	$ 37,000,000
Subsequent Event | Series D Preferred Stock
Debt outstanding:
Preferred stock, par value (in dollars per share)	$ 1.00
Liquidation preference per share (in dollars per shares)	$ 25,000
Stock redeemed (in shares)	5,400
Subsequent Event | Series E Preferred Stock
Debt outstanding:
Preferred stock, par value (in dollars per share)	$ 1.00
Liquidation preference per share (in dollars per shares)	$ 25,000
Stock redeemed (in shares)	29,900
Subsequent Event | Series F Preferred Stock
Debt outstanding:
Preferred stock, par value (in dollars per share)	$ 1.00
Liquidation preference per share (in dollars per shares)	$ 25,000
Stock redeemed (in shares)	10,000
Forecast | Series H Preferred Stock
Debt outstanding:
Redemption price (in dollars per share)						$ 25,000