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Financial Instruments - Effect of Derivative Instruments on Consolidated Statements of Operations (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income $ (6,881) $ 6,895
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Unrealized gain (loss) on derivative instruments, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (1,085) 87
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Realized gain (loss) on derivative instruments, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (277) 409
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Unrealized (loss) gain on derivative instruments, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (4,883) 854
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Realized (loss) on derivative instruments, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income $ (636) $ 5,545