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Derivatives And Risk Management (Narrative) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Derivative [Line Items]    
Derivative, Fair Value, Amount Offset Against Collateral, Net $ 37,777 $ 51,747
Commodity Contracts [Member]    
Derivative [Line Items]    
Cash deposited as collateral 72,264 78,025
Letters of credit outstanding 56,100 6,500
Derivative, Fair Value, Amount Offset Against Collateral, Net 34,917 51,217
Liability position at aggregate fair value 1,406 2,193
Additional Collateral, Aggregate Fair Value 1,415 2,193
Interest Rate Swap [Member]    
Derivative [Line Items]    
Cash deposited as collateral 2,860 530
Derivative, Fair Value, Amount Offset Against Collateral, Net 2,860 530
Liability position at aggregate fair value 15,210 7,831
Additional Collateral, Aggregate Fair Value $ 10,064 $ 6,579