Stockholders' equity (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended													12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011
Stock repurchase program
Stock repurchases (in shares)	0	0	0	9.1	14.2	9.7	17.4	21.0	86.0		45.4	12.9	0	9.1	62.3	144.3
Stock Repurchase Program [Member]
Stock repurchase program
Stock repurchases	$ 0	$ 0	$ 0	$ 771	$ 1,233	$ 797	$ 1,203	$ 1,429	$ 5,154	[1]	$ 2,421	$ 732	$ 0	$ 771	$ 4,662	$ 8,307

[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.