Derivative Instruments - Location and Amounts of Pre-Tax Gains (Losses) on Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Accumulated Other Comprehensive Loss (effective portion)	$ (5)	$ (10)	$ (4)	$ (3)
Foreign exchange forward contracts | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Accumulated Other Comprehensive Loss (effective portion)	(4)	(6)	(3)	(1)
Foreign exchange forward contracts | Revenue, Selling and general expenses | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Income (effective portion)	$ (1)	$ 3	$ (1)	$ 4