Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details)

v3.19.2
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)