Equity (Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends Payable [Line Items]
Preferred Stock, Shares Authorized	200,000,000				200,000,000								200,000,000	200,000,000
Preferred Stock, Shares Issued	26,032,200				25,500,000								26,032,200	25,500,000
Preferred Stock, Shares Outstanding	26,032,200				25,500,000								26,032,200	25,500,000
Preferred Stock [Member]
Preferred Stock
Declaration Date	Nov. 15, 2018	Aug. 15, 2018	May 15, 2018	Mar. 05, 2018	Nov. 15, 2017	Aug. 15, 2017	May 15, 2017	Mar. 06, 2017	Nov. 15, 2016	Aug. 15, 2016	May 16, 2016	Mar. 07, 2016
Record Date	Nov. 30, 2018	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016
Payment Date	Dec. 17, 2018	Sep. 17, 2018	Jun. 15, 2018	Mar. 15, 2018	Dec. 15, 2017	Sep. 15, 2017	Jun. 15, 2017	Mar. 15, 2017	Dec. 15, 2016	Sep. 15, 2016	Jun. 15, 2016	Mar. 15, 2016
Series A Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Dividend Payment Rate, Variable													Three-month LIBOR + 1.00%, with floor of 4.00%
Preferred Stock, Shares Authorized	27,600,000				27,600,000								27,600,000	27,600,000
Preferred Stock, Shares Issued	24,000,000				24,000,000								24,000,000	24,000,000
Preferred Stock, Shares Outstanding	24,000,000				24,000,000								24,000,000	24,000,000
Preferred Stock
Preferred Stock, Dividends Per Share, Declared	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.250	$ 0.253	$ 0.256	$ 0.256	$ 0.253	$ 1.015	$ 1.015	$ 1.018
Preferred stock, dividends	$ 6	$ 6	$ 7	$ 6	$ 6	$ 6	$ 7	$ 6	$ 6	$ 6	$ 7	$ 6	$ 25	$ 25	$ 25
Series C Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Dividend Payment Rate, Variable													5.25% Fixed-to-Floating Rate
Preferred Stock, Shares Authorized	1,500,000				1,500,000								1,500,000	1,500,000
Preferred Stock, Shares Issued	1,500,000				1,500,000								1,500,000	1,500,000
Preferred Stock, Shares Outstanding	1,500,000				1,500,000								1,500,000	1,500,000
Preferred Stock
Preferred Stock, Dividends Per Share, Declared	$ 26.250	$ 0.000	$ 26.250	$ 0.000	$ 26.250	$ 0.000	$ 26.250	$ 0.000	$ 26.250	$ 0.000	$ 26.250	$ 0.000	$ 52.500	$ 52.500	$ 52.500
Preferred stock, dividends	$ 40	$ 0	$ 39	$ 0	$ 39	$ 0	$ 39	$ 0	$ 39	$ 0	$ 39	$ 0	$ 79	$ 78	$ 78
Series D Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Dividend Payment Rate, Variable													5.875% Fixed-to-Floating Rate
Preferred Stock, Shares Authorized	500,000				0								500,000	0
Preferred Stock, Shares Issued	500,000				0								500,000	0
Preferred Stock, Shares Outstanding	500,000				0								500,000	0
Preferred Stock
Preferred Stock, Dividends Per Share, Declared	$ 0.000	$ 28.233	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 28.233	$ 0.000	$ 0.000
Preferred stock, dividends	$ 0	$ 14	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 14	$ 0	$ 0
Series E Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Shares Authorized	32,200				0								32,200	0
Preferred Stock, Shares Issued	32,200				0								32,200	0
Preferred Stock, Shares Outstanding	32,200				0								32,200	0
Preferred Stock
Preferred Stock, Dividends Per Share, Declared	$ 351.563	$ 394.531	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 746.094	$ 0.000	$ 0.000
Preferred stock, dividends	$ 11	$ 12	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 23	$ 0	$ 0
Series A Junior Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Shares Authorized	10,000,000				10,000,000								10,000,000	10,000,000
Preferred Stock, Shares Issued	0				0								0	0
Preferred Stock, Shares Outstanding	0				0								0	0
Not Designated Preferred Stock [Member]
Dividends Payable [Line Items]
Preferred Stock, Shares Authorized	160,367,800				160,900,000								160,367,800	160,900,000
Preferred Stock, Shares Issued	0				0								0	0
Preferred Stock, Shares Outstanding	0				0								0	0