Derivative Instruments - Change in Unrealized (Losses) Gains in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains (losses) on cash flow hedges, net of taxes, beginning of period			$ 650
Change in fair value, net of tax	$ 3	$ 1	2	$ 3
Net unrealized gains on cash flow hedges, net of taxes, end of period	839		839
Unrealized Gain (Loss) on Forward Exchange Contracts
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains (losses) on cash flow hedges, net of taxes, beginning of period	1	1	2	(1)
Change in fair value, net of tax	5	2	8	6
Reclassification into earnings, net of tax	(2)	(1)	(6)	(3)
Net unrealized gains on cash flow hedges, net of taxes, end of period	$ 4	$ 2	$ 4	$ 2