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Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]        
Net gain (loss) on derivative instruments $ (48,544) $ (54,310) $ (239,945) $ 52,297