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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2017 $ 8,982
Additions and revisions of prior estimates 2,086
Accretion expense 191
Settlement of liability (1,783)
June 30, 2018 $ 9,476