Derivative Instruments - Change in Unrealized (Losses) Gains in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains on cash flow hedges, net of taxes, beginning of period			$ 711
Change in fair value, net of tax	$ (4)	$ (8)	(3)	$ (3)
Net unrealized gains (losses) on cash flow hedges, net of taxes, end of period	583		583
Unrealized Gain (Loss) on Forward Exchange Contracts
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains on cash flow hedges, net of taxes, beginning of period	3	7	2	2
Change in fair value, net of tax	(5)	(3)	(4)	3
Reclassification into earnings, net of tax	1	(3)	1	(4)
Net unrealized gains (losses) on cash flow hedges, net of taxes, end of period	$ (1)	$ 1	$ (1)	$ 1