Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivatives - Effects of foreign currency options designated as cash flow hedges (Details 4)

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Fair Value Measurements and Derivatives - Effects of foreign currency options designated as cash flow hedges (Details 4) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency options      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount reclassified from accumulated comprehensive income (loss) into depreciation and amortization expense $ 1,320 $ 1,320 $ 1,320