Earnings Per Common Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 1,980	$ 2,022	$ 2,050
Preferred stock dividends	(130)	(61)	(26)
Dividends and undistributed earnings allocated to participating securities	(2)	(3)	(8)
Net income available to common shareholders	$ 1,848	$ 1,958	$ 2,016
Basic (in shares)	407,856	424,223	446,245
Effect of dilutive securities: common stock options and common stock awards (in shares)	5,782	7,784	8,910
Diluted average common shares (in shares)	413,638	432,007	455,155
Anti-dilutive securities (in shares)	661	1,498	1,855
Earnings per Common Share:
Basic (in dollars per share)	$ 4.53	$ 4.62	$ 4.52
Diluted (in dollars per share)	$ 4.47	$ 4.53	$ 4.43