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Summary of Significant Accounting Policies - Schedule of Gains and Losses from Derivative Financial Instruments (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative [Line Items]        
Gain (loss) from derivative financial instruments $ (1,638) $ 5,295 $ 964 $ 13,155
Gain (Loss) from Derivative Financial Instruments | Natural Gas Price Derivatives        
Derivative [Line Items]        
Gain (loss) from derivative financial instruments $ (1,638) $ 5,295 $ 964 $ 13,155