Derivatives and Hedge Accounting Derivatives and Hedge Accounting - Location of Gain or Loss Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue	$ 495,586	$ 447,956	$ 402,917
General & administrative expenses	102,567	88,648	77,293
Selling & marketing expenses	53,383	50,582	49,474
Depreciation & amortization	38,549	34,580	31,465
Foreign exchange gain, net	(2,839)	(5,597)	$ (2,744)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives Designated as Hedging Instruments | Derivatives in cash flow hedging relationships | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue	(5,465)
General & administrative expenses	(960)
Selling & marketing expenses	(103)
Depreciation & amortization	(371)
Foreign exchange gain, net	0	(2,669)
Reclassification from AOCI, before tax	(6,899)	(2,669)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives Designated as Hedging Instruments | Derivatives in fair value hedging relationships | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange gain, net	$ (5,056)	$ (4,790)