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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 931	$ 979	$ 3,074	$ 2,702
Less: Preferred stock dividends	42	37	103	105
Net income applicable to common shareholders	$ 889	$ 942	$ 2,971	$ 2,597
Denominator:
Weighted average common shares outstanding (in shares)	327.7	346.0	330.8	349.7
Effect of dilutive potential common shares:
Stock options (in shares)	3.4	3.8	3.2	3.8
Restricted stock units (non-participating) and performance stock awards (in shares)	1.9	1.9	1.7	1.9
Weighted average common and dilutive potential common shares outstanding (in shares)	333.0	351.7	335.7	355.4
Earnings per common share - Basic (in dollars per share)	$ 2.71	$ 2.72	$ 8.98	$ 7.43
Earnings per common share - Diluted (in dollars per share)	$ 2.67	$ 2.68	$ 8.85	$ 7.31
Anti-dilutive options excluded from diluted earnings per common share (in shares)	1.8	2.3	3.7	1.9