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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 676	$ 579	$ 1,651	$ 1,274
Less: Preferred stock dividends	39	29	68	58
Net income applicable to common shareholders	$ 637	$ 550	$ 1,583	$ 1,216
Denominator:
Weighted average common shares outstanding (in shares)	349.2	363.6	351.6	364.6
Effect of dilutive potential common shares:
Stock options (in shares)	3.8	4.3	3.9	4.2
Restricted stock units (non-participating) and performance stock awards (in shares)	1.6	1.1	1.7	1.3
Weighted average common and dilutive potential common shares outstanding (in shares)	354.6	369.0	357.2	370.1
Earnings per common share - Basic (in dollars per share)	$ 1.82	$ 1.51	$ 4.50	$ 3.34
Earnings per common share - Diluted (in dollars per share)	$ 1.80	$ 1.49	$ 4.43	$ 3.29