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Derivatives And Risk Management Derivatives and Risk Management (Collateral) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Derivative [Line Items]    
Derivative, Fair Value, Amount Offset Against Collateral, Net $ 62,408 $ 44,758
Commodity Contract [Member]    
Derivative [Line Items]    
Liability position at aggregate fair value 1,336 1,124
Collateral Already Posted, Aggregate Fair Value 39,458 17,134
Letters of credit outstanding 23,000 24,400
Derivative, Fair Value, Amount Offset Against Collateral, Net 27,438 9,858
Additional Collateral, Aggregate Fair Value 1,336 1,046
Interest Rate Swap [Member]    
Derivative [Line Items]    
Liability position at aggregate fair value 73,514 73,978
Collateral Already Posted, Aggregate Fair Value 34,970 34,900
Letters of credit outstanding 5,000 3,600
Derivative, Fair Value, Amount Offset Against Collateral, Net 34,970 34,900
Additional Collateral, Aggregate Fair Value $ 18,770 $ 21,100