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Derivative Instruments - Effect of Cash Flow Hedges on the Statement of Operations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain Recognized in Other Comprehensive Income $ (84) $ 1,882 $ (1,080) $ 5,070
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain Recognized in Other Comprehensive Income (445) 2,179 (990) 5,045
Designated as Hedging Instrument | Oil and gas sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Reclassified into Oil and Gas Sales $ (307) $ 108 $ 233 $ (214)