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Commitments - Reconciliation of Liability for Plugging and Abandonment Costs (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]    
Asset retirement obligation at beginning of period $ 23,578 $ 17,505
Liabilities incurred 49 45
Liabilities settled (2,656) (676)
Accretion expense 1,120 768
Revisions in estimated liabilities (757) 5,936
Asset retirement obligation at end of period $ 21,334 $ 23,578