Earnings per share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income	$ 709	$ 766
Denominator:
Weighted average common shares outstanding	475.4	498.7
Effect of dilutive potential common shares:
Stock options (in shares)	3.7	2.0
Restricted stock units and performance stock awards (non-participating) (in shares)	1.7	0.8
Weighted average common and dilutive potential common shares outstanding	480.8	501.5
Earnings per share - Basic (in dollars per share)	$ 1.49	$ 1.54
Earnings per share - Diluted (in dollars per share)	$ 1.47	$ 1.53
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)	17.0	28.5
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)	$ 37.40	$ 25.91
Antidilutive stock options, exercise price exceeds market price, exercise price range, high end of range (in dollars per share)	$ 62.84	$ 62.84