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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2017 $ 8,982
Additions and revisions of prior estimates 250
Accretion expense 134
Settlement of liability (1,249)
March 31, 2018 $ 8,117