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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail)
$ in Thousands
6 Months Ended
Jun. 30, 2020
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
December 31, 2019 $ 6,142
Accretion expense 151
June 30, 2020 $ 6,293