Derivative Financial Instruments (Schedule Of Contracts Designated As Hedged Instruments) (Details) € in Thousands, £ in Thousands, kr in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 GBP (£)	Dec. 31, 2017 NOK (kr)	Jun. 30, 2018 EUR (€)	Jun. 30, 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 \| $				$ 10,535			$ 9,538
Canadian Dollars [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Aug. 01, 2019	Feb. 01, 2019
British Pounds [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| £	£ 266				£ 1,737
British Pounds [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Sep. 01, 2018	Feb. 01, 2019
Euro [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| €			€ 11,693			€ 15,928
Euro [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Aug. 01, 2019	Feb. 01, 2019