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Financial Instruments - Effect of Derivative Instruments on Condensed Consolidated Balance Sheets (Detail) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Derivatives, Fair Value [Line Items]    
Derivative assets $ 506  
Derivative liabilities (860) $ (7)
Total derivative instruments (354) (7)
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Other Current Assets [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 178  
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Other Assets [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 328  
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Derivative Liability Short-Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities   $ (7)
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Derivative Liability Short-Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities (132)  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Derivative liability Long Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities (197)  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Derivative Liability Short-Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities (207)  
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Derivative liability Long Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities $ (324)