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Derivative Instruments - Schedule of Location and Fair Value of Outstanding Commodity Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Derivatives, Fair Value [Line Items]    
Derivative instruments — current assets $ 7,353 $ 19,016
Derivative instruments — non-current assets 10,554 4,981
Derivative instruments — current liabilities 0 (1,048)
Derivative instruments — non-current liabilities (291) (380)
Total derivative instruments 17,616  
Crude oil [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative instruments — current assets 7,353 19,016
Derivative instruments — non-current assets 10,554 4,981
Derivative instruments — current liabilities 0 (1,048)
Derivative instruments — non-current liabilities (291) (380)
Total derivative instruments $ 17,616 $ 22,569