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Asset Retirement Obligations - Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Asset Retirement Obligation [Abstract]    
Beginning of period $ 276,855  
Liabilities incurred 2,050  
Acquisitions 13,438  
Liabilities settled (2,080)  
Accretion expense 8,210  
Change in estimate 4,073  
End of period 302,546  
Less current portion (6,327) $ (6,327)
Long-term asset retirement obligations $ 296,219