Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 29,875	$ 28,083	$ 25,778
Ending Balance	25,241	29,875	28,083
Foreign currency translation [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(610)	(511)	(264)
Foreign currency translation adjustments	77	(93)	(257)
Income taxes	4	(6)	10
Ending Balance	(529)	(610)	(511)
Cash flow hedges [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	282	297	290
Unrealized gains (losses)	192	(176)	150
Reclassification adjustments to income	(638)	139	(143)
Income taxes	158	22	0
Ending Balance	(6)	282	297
Available-for-sale securities [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(138)	(260)	(19)
Unrealized gains (losses)	(46)	63	(299)
Reclassification adjustments to income	41	61	76
Income taxes	(1)	(2)	(18)
Ending Balance	(144)	(138)	(260)
Other [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(5)	(6)	(15)
Unrealized gains (losses)	0	0	8
Other	5	1	1
Ending Balance	0	(5)	(6)
AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(471)	(480)	(8)
Foreign currency translation adjustments	77	(93)	(257)
Unrealized gains (losses)	146	(113)	(141)
Reclassification adjustments to income	(597)	200	(67)
Other	5	1	1
Income taxes	161	14	(8)
Ending Balance	$ (679)	$ (471)	$ (480)