Annual report pursuant to Section 13 and 15(d)

Financial Instruments and Risk Management (Balance Sheet Classification and Fair Value of Derivative Instruments) (Details)

v3.8.0.1
Financial Instruments and Risk Management (Balance Sheet Classification and Fair Value of Derivative Instruments) (Details) - Forward Contracts [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Not Designated as Hedging Instrument [Member] | Prepaid Expenses And Other Current Assets [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative assets $ 683 $ 1,361
Not Designated as Hedging Instrument [Member] | Accrued Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative liabilities 1,229 180
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Prepaid Expenses And Other Current Assets [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative assets 1,648 9,805
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative assets 335 1,969
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Accrued Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative liabilities 9,336 106
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Other Long Term Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Fair value of derivative liabilities $ 3,820 $ 0