Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2015	Mar. 31, 2015
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, Foreign currency translation	$ (437)	$ (264)
Beginning balance, Cash flow hedges	468	290
Beginning balance, Available-for-sale securities	121	(19)
Beginning balance, Other	(15)	(15)
Beginning balance, AOCI	137	(8)
Other Comprehensive Income (Loss), before Tax [Abstract]
Foreign currency translation adjustments	24	(184)
Unrealized gains (losses), Cash flow hedges	44	168
Unrealized gains (losses), Available-for-sale securities	(180)	188
Unrealized gains (losses), AOCI	(136)	356
Reclassification adjustments to income, Cash flow hedges	(226)	114
Reclassification adjustments to income, Available-for-sale securities	9	35
Reclassification adjustments to income, AOCI	(217)	149
Income taxes, Foreign currency translation	(6)	11
Income taxes, Cash flow hedges	67	(104)
Income taxes, Available-for-sale securities	63	(83)
Income taxes, AOCI	124	(176)
Ending balance, Foreign currency translation	(419)	(437)
Ending balance, Cash flow hedges	353	468
Ending balance, Available-for-sale securities	13	121
Ending balance, Other	(15)	(15)
Ending balance, AOCI	$ (68)	$ 137