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Financial Instruments - Effect of Derivative Instruments on Condensed Consolidated Statements of Operations (Detail) (Derivatives not designated as cash-flow hedging instruments [Member], USD $)
In Thousands, unless otherwise specified
 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income $ (2,599) $ 2,828
Unrealized gain (loss) on derivative instruments, net [Member] | Natural gas commodity contracts [Member]
    
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income (495)   
Unrealized gain (loss) on derivative instruments, net [Member] | Crude oil commodity contracts [Member]
    
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income (977) 2,449
Realized gain (loss) on derivative instruments, net [Member] | Natural gas commodity contracts [Member]
    
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income 571   
Realized gain (loss) on derivative instruments, net [Member] | Crude oil commodity contracts [Member]
    
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income $ (1,698) $ 379