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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Asset Retirement Obligations [Line Items]    
Beginning of period $ 146,478  
Liabilities incurred 1,690  
Liabilities settled (96)  
Disposition of wells (2,816)  
Accretion expense 2,717  
Change in estimate 0  
End of period 147,973  
Less current portion (2,366) (2,470)
Long-term asset retirement obligations $ 145,607