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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, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income $ 1,882 $ (1,275) $ 5,070 $ (1,420)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income 2,179 (595) 5,045 206
Designated as Hedging Instrument | Oil and gas sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain Reclassified into oil and gas sales $ 108 $ 1,155 $ (214) $ 2,187