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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Asset Retirement Obligations [Line Items]    
Beginning of period $ 146,478  
Liabilities incurred 3,846  
Liabilities settled (155)  
Disposition of wells (3,098)  
Accretion expense 5,324  
Change in estimate (6,231)  
End of period 146,164  
Less current portion (2,366) (2,470)
Long-term asset retirement obligations $ 143,798