Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Dividends Payable [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	[1]	$ 1.050
Preferred Stock, Dividends Per Share, Declared		$ 17.1875	$ 17.1875
Dividends, Common Stock, Cash	[1]	$ 439
Dividends, Preferred Stock, Cash		$ 28	$ 28
Common stock repurchased during period, shares		0.3
Purchases of capital stock		$ 33		$ 22
Common Stock [Member]
Dividends Payable [Line Items]
Dividends Payable, Date Declared	[1]	Feb. 21, 2018
Dividends Payable, Date of Record	[1]	Mar. 16, 2018
Dividends Payable, Date to be Paid	[1]	Mar. 30, 2018
Preferred Stock [Member]
Dividends Payable [Line Items]
Dividends Payable, Date Declared		Mar. 19, 2018	Dec. 15, 2017
Dividends Payable, Date of Record		Apr. 15, 2018	Jan. 15, 2018
Dividends Payable, Date to be Paid		May 01, 2018	Feb. 01, 2018

[1] Inclusive of dividends accrued for holders of unvested restricted stock units, which will be paid when and if the restricted stock units vest.