Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 1,153	$ 931	$ 2,952	$ 3,074
Less: Preferred stock dividends	27	42	89	103
Net income applicable to common shareholders	$ 1,126	$ 889	$ 2,863	$ 2,971
Denominator:
Weighted average common shares outstanding (in shares)	311.2	327.7	314.1	330.8
Effect of dilutive potential common shares:
Stock options (in shares)	1.6	3.4	2.3	3.2
Restricted stock units (non-participating) and performance stock awards (in shares)	1.3	1.9	1.5	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	314.1	333.0	317.9	335.7
Earnings per common share - Basic (in dollars per share)	$ 3.62	$ 2.71	$ 9.11	$ 8.98
Earnings per common share - Diluted (in dollars per share)	$ 3.58	$ 2.67	$ 9.01	$ 8.85
Anti-dilutive options excluded from diluted earnings per common share (in shares)	4.2	1.8	3.0	3.7