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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
Jun. 30, 2019
Dec. 31, 2018
Offsetting Assets and Liabilities [Line Items]    
As Presented with Effects of Netting $ 24,843 $ 65,114
As Presented with Effects of Netting (20,914) (17,920)
Current assets: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 20,044 78,091
Effects of Netting (9,365) (12,977)
As Presented with Effects of Netting 10,679 65,114
Long-term assets: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 4,420  
Effects of Netting (2,081)  
As Presented with Effects of Netting 2,339  
Current liabilities: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (26,616) (23,457)
Effects of Netting 9,365 12,977
As Presented with Effects of Netting (17,251) (10,480)
Long-term liabilities: Fair value of commodity derivatives    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (5,744) (7,440)
Effects of Netting 2,081 0
As Presented with Effects of Netting $ (3,663) $ (7,440)