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Summary of Significant Accounting Policies - Summary of Changes in Reserve for Future Abandonment Costs (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Asset Retirement Obligations Noncurrent [Abstract]    
Future abandonment costs — beginning of period $ 10,407 $ 15,804
Accretion expense 280 431
New wells placed on production 4 4
Liabilities settled and assets disposed of (69) (36)
Future abandonment costs — end of period $ 10,622 $ 16,203