Hedging - Derivative Instruments on the Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Total derivative instrument expense	$ 433	$ 881	$ 627
Cost of sales
Derivatives, Fair Value [Line Items]
Total derivative instrument expense	152	281	187
Research and development
Derivatives, Fair Value [Line Items]
Total derivative instrument expense	211	436	305
Selling, general and administrative
Derivatives, Fair Value [Line Items]
Total derivative instrument expense	$ 70	$ 164	$ 135