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Derivative Financial Instruments (Schedule Of Contracts Designated As Hedged Instruments) (Details)
€ in Thousands, £ in Thousands, kr in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
GBP (£)
Dec. 31, 2017
NOK (kr)
Mar. 31, 2018
EUR (€)
Mar. 31, 2018
CAD ($)
Dec. 31, 2017
GBP (£)
Dec. 31, 2017
EUR (€)
Dec. 31, 2017
CAD ($)
Norwegian Kroner [Member]              
Foreign Exchange Contracts [Line Items]              
Foreign exchange contracts, Notional Amount | kr   kr 2,629          
Norwegian Kroner [Member] | Designated as Hedging Instrument [Member]              
Foreign Exchange Contracts [Line Items]              
Derivative, Maturity Date   Feb. 01, 2018          
Canadian Dollars [Member]              
Foreign Exchange Contracts [Line Items]              
Foreign exchange contracts, Notional Amount | $       $ 11,981     $ 9,538
Canadian Dollars [Member] | Designated as Hedging Instrument [Member]              
Foreign Exchange Contracts [Line Items]              
Derivative, Maturity Date Feb. 01, 2019 Feb. 01, 2019          
British Pounds [Member]              
Foreign Exchange Contracts [Line Items]              
Foreign exchange contracts, Notional Amount | £ £ 1,290       £ 1,737    
British Pounds [Member] | Designated as Hedging Instrument [Member]              
Foreign Exchange Contracts [Line Items]              
Derivative, Maturity Date Feb. 01, 2019 Feb. 01, 2019          
Euro [Member]              
Foreign Exchange Contracts [Line Items]              
Foreign exchange contracts, Notional Amount | €     € 14,899     € 15,928  
Euro [Member] | Designated as Hedging Instrument [Member]              
Foreign Exchange Contracts [Line Items]              
Derivative, Maturity Date Feb. 01, 2019 Feb. 01, 2019