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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail)
$ in Thousands
3 Months Ended
Mar. 31, 2021
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2020 $ 14,996
Accretion expense 192
March 31, 2021 15,925
Additions $ 737