Stockholders' equity (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended						9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Stock repurchase program
Stock repurchases, in shares	9.7	17.4	21.0	45.4	12.9	0	48.1	58.3
Stock repurchases, in dollars	$ 797	$ 1,203	$ 1,429	$ 2,421	$ 732	$ 0	$ 3,429	$ 3,153