Earnings per share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income (Numerator):
Net income for basic and diluted earnings per share	$ 1,107	$ 454	$ 3,557	$ 2,749
Shares (Denominator):
Weighted-average shares for basic earnings per share	771	907	779	922
Effect of dilutive securities	12	7	10	8
Weighted-average shares for diluted earnings per share	783	914	789	930
Basic earnings per share	$ 1.44	$ 0.50	$ 4.57	$ 2.98
Diluted earnings per share	$ 1.41	$ 0.50	$ 4.51	$ 2.96