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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, 2020
Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 6,142 $ 13,322
Additions and revisions of prior estimates 8,458 (5,114)
Accretion expenses 396 687
Settlement of liability 0 (2,753)
Ending balance $ 14,996 $ 6,142