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Derivatives (Net Asset or Liability Positions in Balance Sheet) (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral      
Net derivative asset position, Collateral paid (received) $ 23,997 $ 14,944 $ 14,942
Net derivative asset position, Fair value of derivatives 10,199 22,285 36,653
Net derivative asset position, net 34,196 37,229 51,595
Net derivative liability position, Collateral paid 0 0 0
Net derivative liability position, Fair value of derivatives 0 0 0
Net derivative liability position, net $ 0 $ 0 $ 0