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Summarized Financial Information Concerning Segments (Parenthetical) (Detail) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Segment Reporting Information        
Unrealized losses (gains) on derivatives     $ (63,520,000) $ (455,138,000)
Permian Properties
       
Segment Reporting Information        
Gain (loss) on sale of oil and gas property     (399,100,000)  
Commodity Derivatives
       
Segment Reporting Information        
Unrealized losses (gains) on derivatives (85,937,000) (580,730,000) (61,125,000) (451,540,000)
Exploration and Production | Permian Properties
       
Segment Reporting Information        
Gain (loss) on sale of oil and gas property     (399,100,000)  
Exploration and Production | Commodity Derivatives
       
Segment Reporting Information        
Unrealized losses (gains) on derivatives $ (85,937,000) $ (580,730,000) $ (61,125,000) $ (451,540,000)