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Derivative Financial Instruments (Schedule Of Fair Values Of Derivative Financial Instruments) (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Asset Derivatives, Fair Value $ 18,969 $ 12,045
Liability Derivatives, Fair Value 22,952 12,976
Net unrealized gains on exchange traded futures and options contracts 11,700 8,500
Net unrealized gains (losses) on cash flow hedges 1,100 300
Derivative Financial Instruments [Member]    
Derivatives, Fair Value [Line Items]    
Asset Derivatives, Fair Value 18,969 12,045
Liability Derivatives, Fair Value   12,884
Accrued and Other Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives, Fair Value 22,868  
Other Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives, Fair Value $ 84 $ 92