Derivative Instruments - Change in Unrealized (Losses) Gains in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	1	$ 5
Net unrealized gains on cash flow hedges, net of taxes, end of period	7
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	2	2
Change in fair value, net of tax	2	6
Reclassification into earnings, net of tax	(1)	(1)
Net unrealized gains on cash flow hedges, net of taxes, end of period	$ 3	$ 7