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Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]      
Net gain (loss) on derivative instruments $ 28,457 $ (71,657) $ (105,317)