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Financial Instruments (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]    
Long Term Debt Percentage Bearing Fixed Interest Percentage 77.00%  
Gain/Loss on FX Effectiveness   $ 0
Designated as Hedging Instrument [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months $ 2,000  
Designated as Hedging Instrument [Member] | Cross Currency Swap [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Amount of Hedged Item $ 100,000  
Derivative, Fixed Interest Rate 0.01%  
Gain/Loss on FX Effectiveness $ 0  
Designated as Hedging Instrument [Member] | 2.52% $50 Million Interest Rate Swap [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Line Of Credit Facility Forecasted Borrowing Amount $ 50,000  
Derivative, Fixed Interest Rate 2.52%  
Gain/Loss on IR Effectiveness $ 0 0
Designated as Hedging Instrument [Member] | 2.25% $100 Million Interest Rate Swap [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Line Of Credit Facility Forecasted Borrowing Amount $ 100,000  
Derivative, Fixed Interest Rate 2.25%  
Not Designated as Hedging Instrument [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative, Notional Amount $ 394,800 353,000
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments $ (9,400) $ 3,300