Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivatives - Effects of foreign currency collar which were not designated as hedging instruments (Details 8)

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Fair Value Measurements and Derivatives - Effects of foreign currency collar which were not designated as hedging instruments (Details 8) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Not Designated as Hedging Instrument | Foreign currency collar      
Derivative Instruments Gain Loss [Line Items]      
Gain (loss) recognized in other income (expense), net $ 0 $ 10,312 $ (26,249)