Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income	$ 520	$ 650	$ 1,037	$ 1,682
Less: Preferred stock dividends	29	29	87	87
Net income applicable to common shareholders	$ 491	$ 621	$ 950	$ 1,595
Denominator:
Weighted average common shares outstanding	371.5	397.0	374.4	406.5
Effect of dilutive potential common shares:
Stock options (in shares)	3.2	3.6	3.3	4.2
Restricted stock units(non-participating) and performance stock awards (in shares)	1.2	1.5	1.2	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	375.9	402.1	378.9	412.4
Earnings per common share - Basic (in dollars per share)	$ 1.32	$ 1.56	$ 2.54	$ 3.92
Earnings per common share - Diluted (in dollars per share)	$ 1.31	$ 1.54	$ 2.51	$ 3.87
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)	3.6	2.2	4.7	2.2
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)	$ 58.14	$ 52.22	$ 57.29	$ 57.98
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