Derivative Financial Instruments (Schedule Of Contracts Designated As Hedged Instruments) (Details) - Designated as Hedging Instrument [Member] € in Thousands, £ in Thousands, kr in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 EUR (€)	Dec. 31, 2017 NOK (kr)	Sep. 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
Derivative, Maturity Date		Feb. 01, 2018
Canadian Dollars [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| $			$ 9,129			$ 9,538
Derivative, Maturity Date	Aug. 01, 2019	Feb. 01, 2019
British Pounds [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| £				£ 1,737
Derivative, Maturity Date		Feb. 01, 2019
Euro [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| €	€ 10,244				€ 15,928
Derivative, Maturity Date	Aug. 01, 2019	Feb. 01, 2019