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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, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]        
Net gain (loss) on derivative instruments $ (54,310) $ 20,847 $ 52,297 $ (55,624)