Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income	$ 1,250	$ 851	$ 1,799	$ 2,143
Less: Preferred stock dividends	26	30	62	61
Net income applicable to common shareholders	$ 1,224	$ 821	$ 1,737	$ 2,082
Denominator:
Weighted average common shares outstanding (in shares)	313.7	332.0	315.6	332.3
Effect of dilutive potential common shares:
Stock options (in shares)	2.1	3.3	2.7	3.2
Restricted stock units (non-participating) and performance stock awards (in shares)	1.2	1.6	1.5	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	317.0	336.9	319.8	337.2
Earnings per common share - Basic (in dollars per share)	$ 3.90	$ 2.47	$ 5.50	$ 6.27
Earnings per common share - Diluted (in dollars per share)	$ 3.86	$ 2.44	$ 5.43	$ 6.17
Anti-dilutive options excluded from diluted earnings per common share (in shares)	3.2	4.1	1.4	4.5