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Summarized Financial Information Concerning Segments (Parenthetical) (Detail) (USD $)
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Segment Reporting Information [Line Items]    
Unrealized losses (gains) on derivatives $ 22,417,000 $ 127,836,000
Permian Properties
   
Segment Reporting Information [Line Items]    
Gain (loss) on sale of oil and gas property (399,100,000)  
Commodity Derivatives
   
Segment Reporting Information [Line Items]    
Unrealized losses (gains) on derivatives 24,812,000 129,190,000
Exploration and Production | Permian Properties
   
Segment Reporting Information [Line Items]    
Gain (loss) on sale of oil and gas property (399,100,000)  
Exploration and Production | Commodity Derivatives
   
Segment Reporting Information [Line Items]    
Unrealized losses (gains) on derivatives $ 24,812,000 $ 129,190,000