Derivative instruments (Effective Portion of Unrealized Gain (Loss)) (Details) - Cash flow hedge [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ 65	$ 35	$ 99	$ (226)
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	(110)	(26)	(360)	(88)
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	165	67	446	(128)
Forward interest rate contract [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	$ 10	$ (6)	$ 13	$ (10)