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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Asset Retirement Obligations [Line Items]    
Beginning of period $ 84,810  
Liabilities incurred 7,400  
Liabilities settled (3,073)  
Disposition of wells (1,286)  
Accretion expense 6,444  
Change in estimate 22,295  
End of period 116,590  
Less current portion (5,005) (5,005)
Long-term asset retirement obligations $ 111,585