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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 8,982 $ 1,384
Additions and revisions of prior estimates 7,546 7,084
Accretion expenses (26) 514
Settlement of liability (3,180)  
Ending balance $ 13,322 $ 8,982