Earnings Per Common Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic and diluted:
Net income		$ 1,693	$ 1,755	$ 1,761	$ 1,550	$ 2,010	$ 1,654	$ 1,623	$ 1,634	$ 6,759	$ 6,921	$ 2,748
Dividends on preferred Stock		(20)	(21)	(19)	(21)	(19)	(20)	(20)	(21)	(81)	(80)	(81)
Net income available to common shareholders										6,678	6,841	2,667
Earnings allocated to participating share awards		$ (12)	$ (11)	$ (13)	$ (11)	$ (16)	$ (13)	$ (12)	$ (13)	(47)	(54)	(21)
Net income attributable to common shareholders										$ 6,631	$ 6,787	$ 2,646
Denominator:
Basic: Weighted-average common stock (in shares)										828.0	856.0	883.0
Add: Weighted-average stock options (in shares)										2.0	3.0	3.0
Diluted (in shares)										830.0	859.0	886.0
Basic EPS:
Basic (in dollars per share)	[1]									$ 8.00	$ 7.93	$ 3.00
Diluted EPS:
Diluted (in dollars per share)	[1]									$ 7.99	$ 7.91	$ 2.99
Stock Option [Member]
Diluted EPS:
Antidilutive securities excluded from computation of earnings per share (in shares)										0.2	0.7	0.6

[1] Represents net income less (i) earnings allocated to participating share awards of $47 million, $54 million and $21 million for the years ended December 31, 2019, 2018 and 2017, respectively, and (ii) dividends on preferred shares of $81 million, $80 million and $81 million for the years ended December 31, 2019, 2018 and 2017, respectively.