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
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 368	$ 405
Preferred stock dividends	(49)	(31)
Dividends and undistributed earnings allocated to participating securities	0	(1)
Net income available to common shareholders	$ 319	$ 373
Basic (in shares)	399,421	412,225
Effect of dilutive securities: common stock options and common stock awards (in shares)	4,194	6,525
Diluted average common shares (in shares)	403,615	418,750
Anti-dilutive securities (in shares)	3,920	791
Earnings per Common Share:
Basic (in dollars per share)	$ 0.80	$ 0.90
Diluted (in dollars per share)	$ 0.79	$ 0.89