Fair Value Measurements (Summary of Derivative Instruments) (Details) - Derivatives not designated as hedging instruments - Foreign currency derivative contracts - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Prepaid expenses and other current assets
Derivative Assets
Foreign currency derivative contracts	$ 0	$ 65
Accrued expenses and other liabilities
Derivative Liabilities
Foreign currency derivative contracts	$ 0	$ 2