Derivatives and Hedge Accounting Derivatives and Hedge Accounting - Location of Gain or Loss Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenues	$ 139,649	$ 123,734	$ 277,750	$ 242,806
General and administrative expenses	27,640	24,425	56,906	48,462
Selling & marketing expenses	15,151	13,095	29,103	26,435
Depreciation & amortization	10,582	9,535	21,086	18,907
Foreign exchange gain, net	(1,414)	(886)	(2,029)	(1,268)
Reclassification out of Accumulated Other Comprehensive Income | Derivatives Designated as Hedging Instruments | Derivatives in cash flow hedging relationships | Foreign currency exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenues	(1,191)	(1,566)	(3,336)	(2,296)
General and administrative expenses	(180)	(288)	(511)	(429)
Selling & marketing expenses	(17)	(30)	(50)	(48)
Depreciation & amortization	(79)	(99)	(241)	(140)
Reclassification from AOCI, before tax	(1,467)	(1,983)	(4,138)	(2,913)
Reclassification out of Accumulated Other Comprehensive Income | Derivatives not Designated as Hedging Instruments | Foreign currency exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange gain, net	$ 2,641	$ (151)	$ 5,569	$ (2,773)