Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net income	$ 271	$ 355	$ 517	$ 1,032
Less: Preferred stock dividends	29	29	58	58
Net income applicable to common shareholders	$ 242	$ 326	$ 459	$ 974
Denominator:
Weighted average common shares outstanding	373.6	407.0	375.8	411.4
Effect of dilutive potential common shares:
Stock options (in shares)	3.4	4.2	3.4	4.5
Restricted stock units(non-participating) and performance stock awards (in shares)	1.1	1.4	1.3	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	378.1	412.6	380.5	417.6
Earnings per common share - Basic (in dollars per share)	$ 0.65	$ 0.80	$ 1.22	$ 2.37
Earnings per common share - Diluted (in dollars per share)	$ 0.64	$ 0.79	$ 1.21	$ 2.33
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)	4.8	2.2	4.8	2.2
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)	$ 57.29	$ 60.81	$ 57.29	$ 60.81
Antidilutive stock options, exercise price exceeds market price, exercise price range, high end of range (in dollars per share)	$ 71.29	$ 71.29	$ 71.29	$ 71.29