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Financial Instruments - Effect of Derivative Instruments on Consolidated Balance Sheets (Detail) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Derivatives, Fair Value [Line Items]    
Derivative assets $ 5,649 $ 16,927
Derivative liabilities (76) (170)
Total derivative instruments 5,573 16,757
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Derivative Contracts [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets   12
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Other Assets [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets   13
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Derivative Liability Short-Term [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities (76) (170)
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Derivative Contracts [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 4,861 14,629
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Derivative Contracts [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets $ 788 $ 2,273