Stockholders' equity (Details) (USD $) In Millions, 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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock repurchase program
Stock repurchases (in shares)	14.2	9.7	17.4	21.0	86.0		45.4	12.9	0	20.5	6.6	10.3	29.1	62.3	144.3	66.5
Stock Repurchase Program [Member]
Stock repurchase program
Stock repurchases	1,233	797	1,203	1,429	5,154	[1]	2,421	732	0	1,136	364	616	1,684	4,662	8,307	3,800

[1] Includes the repurchase of 83.3 million shares of our common stock at an average price paid per share of $60.08, including related expenses, for an aggregate cost of $5.0 billion, under a modified Dutch auction tender offer.