Earnings Per Common Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic and diluted:
Net income (loss)		$ 1,438	$ 1,073	$ 257	$ 367	$ 1,693	$ 1,755	$ 1,761	$ 1,550	$ 3,135	$ 6,759	$ 6,921
Preferred dividends		(14)	(16)	(17)	(32)	(20)	(21)	(19)	(21)	(79)	(81)	(80)
Net income available to common shareholders										3,056	6,678	6,841
Earnings allocated to participating share awards		$ (9)	$ (7)	$ (2)	$ (2)	$ (12)	$ (11)	$ (13)	$ (11)	(20)	(47)	(54)
Net income attributable to common shareholders										$ 3,036	$ 6,631	$ 6,787
Denominator:
Basic: Weighted-average common stock (in shares)										805.0	828.0	856.0
Add: Weighted-average stock options (in shares)										1.0	2.0	3.0
Diluted (in shares)										806.0	830.0	859.0
Basic EPS:
Basic (in dollars per share)	[1]									$ 3.77	$ 8.00	$ 7.93
Diluted EPS:
Diluted (in dollars per share)	[1]									$ 3.77	$ 7.99	$ 7.91
Stock Option [Member]
Diluted EPS:
Antidilutive securities excluded from computation of earnings per share (in shares)										0.5	0.2	0.7

[1] Represents net income less (i) earnings allocated to participating share awards of $20 million, $47 million and $54 million for the years ended December 31, 2020, 2019 and 2018, respectively, and (ii) dividends on preferred shares of $79 million, $81 million and $80 million for the years ended December 31, 2020, 2019 and 2018, respectively.