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Derivative Instruments - Schedule of Location and Fair Value of Outstanding Commodity Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Derivatives, Fair Value [Line Items]    
Derivative instruments - current assets $ 19,016  
Derivative instruments - non-current assets 4,981 4,362
Derivative instruments - current liabilities (1,048) (5,907)
Derivative instruments - non-current liabilities (380) (3,505)
Total derivative instruments 22,569  
Crude oil collar [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative instruments - current assets 19,016   
Derivative instruments - non-current assets 4,981 4,362
Derivative instruments - current liabilities (1,048) (5,907)
Derivative instruments - non-current liabilities (380) (3,505)
Total derivative instruments $ 22,569 $ (5,050)