Stockholders' Equity (Dividends Declared) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Dividends declared	$ 34	$ 34	$ 115	$ 117
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0	$ 0	$ 14	$ 14
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 35.00	$ 35.00
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 9	$ 9	$ 27	$ 27
Dividends declared, per share amount (USD per share)	$ 15.00	$ 15.00	$ 45.00	$ 45.00
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 11	$ 11	$ 33	$ 33
Dividends declared, per share amount (USD per share)	$ 14.88	$ 14.88	$ 44.64	$ 44.64
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 14	$ 14	$ 28	$ 28
Dividends declared, per share amount (USD per share)	$ 2,312.50	$ 2,312.50	$ 4,625.00	$ 4,625.00
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0	$ 0	$ 13	$ 15
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 2,500.00	$ 2,930.56