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Financial Instruments - Effect of Derivative Instruments on Condensed Consolidated Statement of Operations (Detail) (Derivatives not designated as cash-flow hedging instruments [Member], USD $)
In Thousands, unless otherwise specified
 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income $ 2,471 $ 5,439
Unrealized 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 (437)   
Unrealized gain 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 2,834 5,098
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 331   
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 $ (257) $ 341