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Risk Management and Derivatives (Changes In Foreign Currency Exchange Rates And Derivative Foreign Currency Forward Contracts) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Foreign Currency Exchange Gain Loss [Line Items]    
Unrealized foreign currency exchange rate gains (losses) $ 5,030 $ 8,313
Foreign Currency Exchange Rates [Member]    
Foreign Currency Exchange Gain Loss [Line Items]    
Unrealized foreign currency exchange rate gains (losses) (5,030) 8,313
Realized foreign currency exchange rate gains (losses) (2,374) (272)
Gain (Loss) on Derivative Instruments [Member]    
Foreign Currency Exchange Gain Loss [Line Items]    
Unrealized derivative gains (losses) 35 (704)
Realized derivative gains (losses) $ 4,226 $ (6,366)