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Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]        
Net gain (loss) on derivative instruments $ (120,285) $ 50,532 $ (191,401) $ 106,607