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Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Offsetting Assets and Liabilities [Line Items]    
As Presented with Effects of Netting $ 36,241 $ 65,114
As Presented with Effects of Netting (11,514) (17,920)
Current assets: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 35,936 78,091
Effects of Netting (11,020) (12,977)
As Presented with Effects of Netting 24,916 65,114
Long-term assets: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 7,464  
Effects of Netting (3,728)  
As Presented with Effects of Netting 3,736  
Current liabilities: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (19,961) (23,457)
Effects of Netting 11,020 12,977
As Presented with Effects of Netting (8,941) (10,480)
Long-term liabilities: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (6,301) (7,440)
Effects of Netting 3,728 0
As Presented with Effects of Netting $ (2,573) $ (7,440)