Derivative Financial Instruments (Schedule Of Contracts Designated As Hedged Instruments) (Details) € in Thousands, £ in Thousands, NOK in Thousands, CAD in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 NOK	Dec. 31, 2016 GBP (£)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 EUR (€)	Sep. 30, 2017 CAD	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 CAD
Norwegian Kroner [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| NOK	NOK 7,449
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 \| CAD					CAD 13,345		CAD 11,001
Canadian Dollars [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Feb. 01, 2019	Feb. 01, 2018
British Pounds [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| £		£ 1,842	£ 2,343
British Pounds [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Feb. 01, 2019	Feb. 01, 2018
Euro [Member]
Foreign Exchange Contracts [Line Items]
Foreign exchange contracts, Notional Amount \| €				€ 22,200		€ 14,366
Euro [Member] \| Designated as Hedging Instrument [Member]
Foreign Exchange Contracts [Line Items]
Derivative, Maturity Date	Feb. 01, 2019	Feb. 01, 2018