Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended			6 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017
Dividends Payable [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	[1]		$ 1.050	$ 1.050
Preferred Stock, Dividends Per Share, Declared		$ 17.1875	$ 17.1875	$ 17.1875
Dividends, Common Stock, Cash	[1]		$ 438	$ 439
Dividends, Preferred Stock, Cash		$ 28	$ 28	$ 28
Common stock repurchased during period, shares					0.3
Purchases of capital stock					$ 34	$ 23
Common Stock [Member]
Dividends Payable [Line Items]
Dividends Payable, Date Declared	[1]		May 17, 2018	Feb. 21, 2018
Dividends Payable, Date of Record	[1]		Jun. 15, 2018	Mar. 16, 2018
Dividends Payable, Date to be Paid	[1]		Jun. 29, 2018	Mar. 30, 2018
Preferred Stock [Member]
Dividends Payable [Line Items]
Dividends Payable, Date Declared		Jun. 22, 2018	Mar. 19, 2018	Dec. 15, 2017
Dividends Payable, Date of Record		Jul. 15, 2018	Apr. 15, 2018	Jan. 15, 2018
Dividends Payable, Date to be Paid		Aug. 01, 2018	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.