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Asset Retirement Obligations - Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation [Abstract]    
Beginning of period $ 257,943 $ 264,137
Liabilities incurred 7,724 2,694
Acquisitions 0 21,900
Liabilities settled (7,965) (11,511)
Disposition of wells (8,078) (10,540)
Accretion expense 14,711 18,021
Change in estimate 12,520 (26,758)
End of period 276,855 257,943
Less current portion (6,327) (7,271)
Long-term asset retirement obligations $ 270,528 $ 250,672