Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Mar. 31, 2017
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period		$ 29,875
Balance as of end of period	$ 31,722
Foreign currency translation [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(586)	(610)
OCI, before reclassifications, before tax	37	21
Income taxes	(2)	3
Balance as of end of period	(551)	(586)
Cash flow hedges [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	209	282
OCI, before reclassifications, before tax	17	17
Reclassification adjustments to income	(330)	(131)
Income taxes	112	41
Balance as of end of period	8	209
Available-for-sale securities [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	20	(138)
OCI, before reclassifications, before tax	73	116
Reclassification adjustments to income	47	49
Income taxes	(40)	(7)
Balance as of end of period	100	20
Accumulated Other Adjustment Attributable to Parent [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(5)	(5)
OCI, before reclassifications, before tax	0	0
Other	(1)
Balance as of end of period	(6)	(5)
AOCI [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(362)	(471)
OCI, before reclassifications, before tax	90	133
Reclassification adjustments to income	(283)	(82)
Other	(1)
Income taxes	70	37
Balance as of end of period	$ (449)	$ (362)