Equity Equity (Preferred Stock Dividends) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Preferred Stock
Dividends Payable [Line Items]
Dividends Payable, Date Declared	Aug. 15, 2018	May 15, 2018	Mar. 05, 2018	Aug. 15, 2017	May 15, 2017	Mar. 06, 2017
Dividends Payable, Date of Record	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017
Dividends Payable, Date to be Paid	Sep. 17, 2018	Jun. 15, 2018	Mar. 15, 2018	Sep. 15, 2017	Jun. 15, 2017	Mar. 15, 2017
Series A Preferred Stock
Dividends Payable [Line Items]
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 0.256	$ 0.256	$ 0.250	$ 0.256	$ 0.256	$ 0.250
Dividends, Preferred Stock, Cash	$ 6	$ 7	$ 6	$ 6	$ 7	$ 6	$ 19	$ 19
Series C Preferred Stock
Dividends Payable [Line Items]
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 0.000	$ 26.250	$ 0.000	$ 0.000	$ 26.250	$ 0.000
Dividends, Preferred Stock, Cash	$ 0	$ 39	$ 0	$ 0	$ 39	$ 0	39	39
Series D Preferred Stock
Dividends Payable [Line Items]
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 28.233	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000
Dividends, Preferred Stock, Cash	$ 14	$ 0	$ 0	$ 0	$ 0	$ 0	14	0
Series E Preferred Stock
Dividends Payable [Line Items]
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 394.531	$ 0.000	$ 0.000	$ 0.000	$ 0.000	$ 0.000
Dividends, Preferred Stock, Cash	$ 12	$ 0	$ 0	$ 0	$ 0	$ 0	$ 12	$ 0