Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period		$ 28,083		$ 28,083
Other	$ 1		$ 0	1	$ 0
Balance as of end of period	30,133			30,133
Foreign currency translation [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(478)	(511)		(511)
Foreign currency translation adjustments	(22)	36
Income taxes	5	(3)
Balance as of end of period	(495)	(478)		(495)
Cash flow hedges [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	118	297		297
Unrealized (losses) gains	(144)	(117)
Reclassification adjustments to income	133	(166)
Income taxes	5	104
Balance as of end of period	112	118		112
Available-for-sale securities [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	98	(260)		(260)
Unrealized (losses) gains	268	379
Reclassification adjustments to income	23	30
Income taxes	(107)	(51)
Balance as of end of period	282	98		282
Other [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(6)	(6)		(6)
Other	1
Balance as of end of period	(5)	(6)		(5)
AOCI [Member[
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(268)	(480)		(480)
Foreign currency translation adjustments	(22)	36
Unrealized (losses) gains	124	262
Reclassification adjustments to income	156	(136)
Income taxes	(97)	50
Balance as of end of period	$ (106)	$ (268)		$ (106)