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Commitments - Reconciliation of Liability for Plugging and Abandonment Costs (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]    
Asset retirement obligation at beginning of period $ 12,501 $ 10,537
Liabilities incurred 28 1,627
(Gain) loss on settlement of obligations   (1,797)
Liabilities settled (1,112) (1,074)
Accretion expense 517 370
Revisions in estimated liabilities (197) 2,838
Asset retirement obligation at end of period $ 11,737 $ 12,501