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Derivative Instruments - Effect of Cash Flow Hedges on the Statement of Operations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income $ (996) $ 3,188
Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income (545) 2,867
Designated as Hedging Instrument | Oil and gas sales    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Reclassified into Oil and Gas Sales $ 540 $ (321)