Derivatives - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on foreign currency exchange contracts	$ (3,603)	$ (1,251)	$ 294
Not designated as hedging instrument | Foreign Currency Exchange Contract
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on foreign currency exchange contracts	$ (3,600)	$ (1,300)	$ 300