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Financial Instruments - Effect of Derivative Instruments on Consolidated Statements of Operations (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income $ 4,149 $ (7)
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Interest Expense [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income   $ (7)
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 1,852  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Realized Gain (Loss) on Derivative Instruments, Net [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gain/loss recognized in income (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 $ 2,502