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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail)
$ in Thousands
9 Months Ended
Sep. 30, 2020
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2019 $ 6,142
Accretion expense 239
September 30, 2020 14,805
Additions $ 8,424