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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Asset Retirement Obligation [Abstract]    
Beginning of period $ 146,478  
Liabilities incurred 5,267  
Liabilities settled (398)  
Disposition of wells (3,104)  
Accretion expense 8,011  
Change in estimate (6,231)  
End of period 150,023  
Less current portion (2,366) (2,470)
Long-term asset retirement obligations $ 147,657