Income taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current provision [Abstract]
Federal	$ 438	$ 551	$ 620
State	47	54	52
Foreign	158	148	153
Total current provision	643	753	825
Deferred (benefit) provision [Abstract]
Federal	83	(273)	(180)
State	(43)	(12)	43
Foreign	(19)	(1)	2
Total deferred provision (benefit)	21	(286)	(135)
Total provision	$ 664	$ 467	$ 690