Income taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current provision:
Federal	$ 251	$ 54	$ 438
State	58	26	47
Foreign	194	191	158
Total current provision	503	271	643
Deferred provision (benefit):
Federal	(22)	(86)	83
State	(4)	19	(43)
Foreign	(50)	(20)	(19)
Total deferred provision (benefit)	(76)	(87)	21
Total provision	$ 427	$ 184	$ 664