Earnings per share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator:
Net income (loss)	$ 423	$ (624)	$ 1,189	$ (100)
Denominator:
Weighted average common shares outstanding	490.6	523.1	494.9	528.2
Effect of dilutive potential common shares:
Stock options (in shares)	2.3		2.2
Restricted stock units and performance stock awards (non-participating) (in shares)	0.9		0.8
Weighted average common and dilutive potential common shares outstanding	493.8	523.1	497.9	528.2
Earnings per share - Basic	$ 0.86	$ (1.19)	$ 2.40	$ (0.19)
Earnings per share - Diluted	$ 0.86	$ (1.19)	$ 2.39	$ (0.19)
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)	22.5	28.3	25.3	28.4
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)	$ 27.75	$ 27.36	$ 26.09	$ 27.36
Antidilutive stock options, exercise price exceeds market price, exercise price range, high end of range (in dollars per share)	$ 62.84	$ 62.84	$ 62.84	$ 62.84
Weighted average dilutive potential common shares outstanding resulting from stock options (in shares)		2.1		2.1
Weighted average dilutive potential common shares outstanding resulting from restricted stock options (in shares)		0.5		0.5
Weighted average common and dilutive potential common shares outstanding in the absence of the net loss (in shares)		525.7		530.8