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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Asset Retirement Obligation [Abstract]    
Beginning of period $ 230,077  
Liabilities incurred 5,475  
Acquisitions 11,927  
Disposition of wells (12,121)  
Liabilities settled (3,301)  
Change in estimate 2,455  
Accretion expense 11,206  
End of period 245,718  
Less current portion (5,037) (5,037)
Long-term asset retirement obligations $ 240,681