Derivative Financial Instruments - Balance Sheet Location and Fair Value of Assets Associated with Derivative Financial Instruments (Details) - Not Designated as Hedging Instrument - Foreign exchange contract - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Foreign exchange contracts: other current liabilities	$ 458	$ 314
Other Current Assets
Derivatives, Fair Value [Line Items]
Foreign exchange contracts: other current assets	$ 36	$ 840