Earnings Per Common Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		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, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic and diluted:
Net Income from continuing operations										$ 2,736	$ 5,408	$ 5,163
Dividends Preferred Stock		$ (20)	$ (21)	$ (19)	$ (21)	$ (19)	$ (21)	$ (19)	$ (21)	(81)	(80)	(62)
Earnings allocated to participating share awards		$ (2)	$ (11)	$ (11)	$ (10)	$ (6)	$ (9)	$ (17)	$ (11)	$ (21)	$ (43)	$ (38)
Denominator:
Basic: Weighted-average common stock										883.0	933.0	999.0
Add: Weighted-average stock options										3.0	2.0	4.0
Diluted										886.0	935.0	1,003.0
Basic EPS:
Basic	[1]									$ 2.98	$ 5.67	$ 5.07
Diluted EPS:
Diluted										$ 2.97	$ 5.65	$ 5.05
Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per Share, amount										0.6	2.4	0.5

[1] Represents net income less (i) earnings allocated to participating share awards of $ 21 million, $ 43 million and $ 38 million for the years ended December 31, 2017 , 2016 and 2015 , respectively, and (ii) dividends on preferred shares of $ 81 million, $ 80 million and $ 62 million for the years ended December 31, 2017 , 2016 and 2015 , respectively.