Derivative Instruments - Narrative (Details) - Foreign currency forward exchange contracts - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	$ 936,800	$ 2,460,000	$ 936,800	$ 2,460,000	$ 987,700
Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Maximum term for foreign currency forward exchange contracts			18 months
Cost of sales | Derivatives designated as hedging instruments:
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of gain (loss) reclassified from accumulated OCI to statements of operations	(787)	(9,241)	$ (10,004)	(6,648)
Derivative Instruments | Reclassification Out of Accumulated Other Comprehensive Income (Loss) | Cost of sales | Derivatives designated as hedging instruments:
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of gain (loss) reclassified from accumulated OCI to statements of operations	$ (800)	$ (9,200)	$ (10,000)	$ (6,600)