Earnings per share (EPS) (Details) (USD $) In Millions, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net income	$ 601	$ 859	$ 1,938	$ 2,286
Basic EPS:
Income allocated to common stock for basic EPS calculation	591	846	1,908	2,254
Weighted average number of shares outstanding, basic (in shares)	1,144	1,184	1,156	1,208
Basic EPS (in dollars per share)	$ 0.52	$ 0.71	$ 1.65	$ 1.87
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	13	12	21	13
Diluted EPS:
Income allocated to common stock for dilutive EPS calculation	591	846	1,909	2,255
Weighted average number of shares outstanding, diluted (in shares)	1,157	1,196	1,177	1,221
Diluted EPS (in dollars per share)	$ 0.51	$ 0.71	$ 1.62	$ 1.85
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	50	96	40	96
Less income allocated to RSUs | Basic EPS
Net income	(10)	(13)	(30)	(32)
Less income allocated to RSUs | Diluted EPS
Net income	$ (10)	$ (13)	$ (29)	$ (31)