Earnings per share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income (Numerator):
Net income for basic and diluted EPS	$ 788	$ 1,107	$ 1,266	$ 1,184	$ 934	$ 454	[1]	$ 1,170	$ 1,125	$ 4,345	$ 3,683	$ 4,627
Shares (Denominator):
Weighted-average shares for basic EPS										775	905	960
Effect of dilutive securities										12	7	5
Weighted-average shares for diluted EPS										787	912	965
Basic EPS	$ 1.03	$ 1.44	$ 1.63	$ 1.50	$ 1.09	$ 0.50	[1]	$ 1.26	$ 1.21	$ 5.61	$ 4.07	$ 4.82
Diluted EPS	$ 1.01	$ 1.41	$ 1.61	$ 1.48	$ 1.08	$ 0.50	[1]	$ 1.25	$ 1.20	$ 5.52	$ 4.04	$ 4.79

[1] We recorded a $780 million legal settlement charge ($705 million, net of tax) in connection with an agreement in principle to settle allegations related to our sales and marketing practices.