Stockholders' Equity (Preferred Stock Issued and Outstanding) (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Shares issued (in shares)	1,761,000	1,761,000
Shares outstanding (in shares)	1,761,000	1,761,000
Carrying Value	$ 2,793	$ 2,793
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	600,000	600,000
Shares outstanding (in shares)	600,000	600,000
Liquidation preference per share (USD per share)	$ 1,000
Carrying Value	$ 585	$ 585
Dividend Rate in Effect	6.00%
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	750,000	750,000
Shares outstanding (in shares)	750,000	750,000
Liquidation preference per share (USD per share)	$ 1,000
Carrying Value	$ 728	$ 728
Dividend Rate in Effect	5.95%
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	400,000	400,000
Shares outstanding (in shares)	400,000	400,000
Liquidation preference per share (USD per share)	$ 1,000
Carrying Value	$ 397	$ 397
Dividend Rate in Effect	7.00%
Series A Preferred Stock [Member] | LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	4.82%
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	6,000	6,000
Shares outstanding (in shares)	6,000	6,000
Liquidation preference per share (USD per share)	$ 100,000
Carrying Value	$ 591	$ 591
Dividend Rate in Effect	4.625%
Series E Preferred Stock [Member] | LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	3.315%
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Shares issued (in shares)	5,000	5,000
Shares outstanding (in shares)	5,000	5,000
Liquidation preference per share (USD per share)	$ 100,000
Carrying Value	$ 492	$ 492
Dividend Rate in Effect	5.00%
Series F Preferred Stock [Member] | LIBOR [Member]
Class of Stock [Line Items]
Floating Annual Rate	2.575%