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Financial Instruments - Effect of Derivative Instruments on Consolidated Statements of Operations (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income $ (7,533) $ 4,149
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Unrealized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (328) 1,852
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Realized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income 85 (205)
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Unrealized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (5,432) $ 2,502
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Realized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (1,634)  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Liquid Contracts [Member] | Unrealized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (197)  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Liquid Contracts [Member] | Realized (Loss) Gain on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income $ (27)