Total Equity (Preferred Stock Outstanding) (Details) - USD ($) $ / shares in Units, $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Class of Stock
Preferred stock carrying value	$ 8,520	$ 8,520
Series A
Class of Stock
Preferred stock shares outstanding	44,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 1,100	1,100
Series C
Class of Stock
Preferred stock shares outstanding	519,882
Liquidation preference per share	$ 1,000
Preferred stock carrying value	$ 408	408
Series C | MUFG
Class of Stock
Preferred stock shares issued	1,160,791
Preferred stock aggregate purchase price	$ 911
Preferred stock redemption, shares	640,909
Preferred stock redemption amount	$ 503
Preferred stock converted to common shares	$ 705
Series E
Class of Stock
Preferred stock shares outstanding	34,500
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 862	862
Series F
Class of Stock
Preferred stock shares outstanding	34,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 850	850
Series G
Class of Stock
Preferred stock shares outstanding	20,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 500	500
Series H
Class of Stock
Preferred stock shares outstanding	52,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 1,300	1,300
Series I
Class of Stock
Preferred stock shares outstanding	40,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 1,000	1,000
Series J
Class of Stock
Preferred stock shares outstanding	60,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 1,500	1,500
Series K
Class of Stock
Preferred stock shares outstanding	40,000
Liquidation preference per share	$ 25,000
Preferred stock carrying value	$ 1,000	$ 1,000