Derivatives and Hedge Accounting Derivatives and Hedge Accounting - Location of Gain or Loss Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenues	$ 157,240	$ 138,101
General and administrative expenses	32,531	29,266
Selling & marketing expenses	18,047	13,952
Depreciation & amortization	13,667	10,504
Foreign exchange gain, net	1,260	615
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	531	2,145
General and administrative expenses	80	331
Selling & marketing expenses	7	33
Depreciation & amortization	53	162
Reclassification from AOCI, before tax	671	2,671
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	$ 1,396	$ (2,928)