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)

v3.19.3
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)