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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 870	$ 666	$ 2,521	$ 1,940
Less: Preferred stock dividends	37	29	105	87
Net income applicable to common shareholders	$ 833	$ 637	$ 2,416	$ 1,853
Denominator:
Weighted average common shares outstanding (in shares)	346.0	361.3	349.7	363.5
Effect of dilutive potential common shares:
Stock options (in shares)	3.8	4.4	3.8	4.3
Restricted stock units (non-participating) and performance stock awards (in shares)	1.9	1.4	1.9	1.3
Weighted average common and dilutive potential common shares outstanding (in shares)	351.7	367.1	355.4	369.1
Earnings per common share - Basic (in dollars per share)	$ 2.41	$ 1.76	$ 6.91	$ 5.10
Earnings per common share - Diluted (in dollars per share)	$ 2.37	$ 1.74	$ 6.80	$ 5.02