Earnings Per Common Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 619	$ 418	$ 987	$ 823
Preferred stock dividends	(33)	(29)	(82)	(60)
Dividends and undistributed earnings allocated to participating securities	(1)	0	(1)	(1)
Net income available to common shareholders	$ 585	$ 389	$ 904	$ 762
Basic average common shares	394,160	410,674	396,790	411,445
Effect of dilutive securities: common stock options and common stock awards (in shares)	4,687	6,038	4,323	6,198
Diluted average common shares	398,847	416,712	401,113	417,643
Anti-dilutive securities (in shares)	3,277	619	3,426	704
Earnings per Common Share:
Basic (in USD per share)	$ 1.48	$ 0.95	$ 2.28	$ 1.85
Diluted (in USD per share)	$ 1.47	$ 0.93	$ 2.25	$ 1.83