Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income	$ 975	$ 695
Less: Preferred stock dividends	29	29
Net income applicable to common shareholders	$ 946	$ 666
Denominator:
Weighted average common shares outstanding (in shares)	354.1	365.7
Effect of dilutive potential common shares:
Stock options (in shares)	4.1	4.2
Restricted stock units (non-participating) and performance stock awards (in shares)	1.7	1.4
Weighted average common and dilutive potential common shares outstanding (in shares)	359.9	371.3
Earnings per common share - Basic (in dollars per share)	$ 2.67	$ 1.82
Earnings per common share - Diluted (in dollars per share)	$ 2.63	$ 1.79