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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Asset retirement obligation      
Asset retirement obligation - Beginning $ 137,120 $ 134,543  
Accretion 2,863 2,804  
Settlements (629) (719)  
Asset retirement obligation - Ending 139,354 $ 136,628  
Property, Plant and Equipment [Line Items]      
Accrued reclamation liabilities, former mines $ 2,100   $ 1,900