Stockholders' equity (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2009	Jun. 30, 2009	Mar. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stock repurchase program
Stock repurchases, in shares	86.0	45.4	12.9	0	20.5	6.6	10.3	29.1	21.7	0	0	37.5	144.3	66.5	59.2
Stock repurchases, in dollars													$ 8,307	$ 3,800
Stock repurchase program
Stock repurchase program
Stock repurchases, in dollars	5,154	2,421	732	0	1,136	364	616	1,684	1,211	0	0	1,997	$ 8,307	$ 3,800	3,208