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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 851	$ 717	$ 2,143	$ 1,723
Less: Preferred stock dividends	30	39	61	68
Net income applicable to common shareholders	$ 821	$ 678	$ 2,082	$ 1,655
Denominator:
Weighted average common shares outstanding (in shares)	332.0	349.2	332.3	351.6
Effect of dilutive potential common shares:
Stock options (in shares)	3.3	3.8	3.2	3.9
Restricted stock units (non-participating) and performance stock awards (in shares)	1.6	1.6	1.7	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	336.9	354.6	337.2	357.2
Earnings per common share - Basic (in dollars per share)	$ 2.47	$ 1.94	$ 6.27	$ 4.71
Earnings per common share - Diluted (in dollars per share)	$ 2.44	$ 1.91	$ 6.17	$ 4.63
Anti-dilutive options excluded from diluted earnings per common share (in shares)	4.1	2.3	4.5	1.7