Derivative instruments (Effective Potion of Unrealized Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Effective portion of the unrealized gain (loss) recognized in Other Comprehensive Income for our derivative instruments designated as cash flow hedges
Unrealized gain (loss) on derivative instruments recognized in Other Comprehensive Income, effective portion, net	$ 44	$ 8	$ 212	$ 25
Foreign currency contracts [Member]
Effective portion of the unrealized gain (loss) recognized in Other Comprehensive Income for our derivative instruments designated as cash flow hedges
Unrealized gain (loss) on derivative instruments recognized in Other Comprehensive Income, effective portion, net	(99)	(13)	293	0
Cross currency swap contracts [Member]
Effective portion of the unrealized gain (loss) recognized in Other Comprehensive Income for our derivative instruments designated as cash flow hedges
Unrealized gain (loss) on derivative instruments recognized in Other Comprehensive Income, effective portion, net	$ 143	$ 21	$ (81)	$ 25