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Commitments - Reconciliation of Liability for Plugging and Abandonment Costs (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]    
Asset retirement obligation at beginning of period $ 11,737 $ 12,501
Liabilities incurred 68 28
Liabilities settled (288) (1,112)
Accretion expense 498 517
Revisions in estimated liabilities 5,490 (197)
Asset retirement obligation at end of period $ 17,505 $ 11,737