Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivatives - Effects of foreign currency forward which were not designated as hedging instruments (Details 6)

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Fair Value Measurements and Derivatives - Effects of foreign currency forward which were not designated as hedging instruments (Details 6) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Not Designated as Hedging Instrument | Foreign currency forward contracts      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) recognized in other income (expense), net $ 0 $ (6,133) $ 684