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Derivative Instruments (Schedule of Location and Fair Value of Outstanding Commodity Derivative Instruments) (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Derivatives, Fair Value [Line Items]    
Derivative instruments -current assets $ 17,320  
Derivative instruments - non-current assets 10,047 4,362
Derivative instruments - current liabilities (1,273) (5,907)
Derivative instruments - non-current liabilities (360) (3,505)
Total derivative instruments 25,734  
Crude Oil Collar [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative instruments -current assets 17,320  
Derivative instruments - non-current assets 10,047 4,362
Derivative instruments - current liabilities (1,273) (5,907)
Derivative instruments - non-current liabilities (360) (3,505)
Total derivative instruments $ 25,734 $ (5,050)