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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Asset retirement obligation          
Asset retirement obligation - Beginning $ 141,205 $ 138,904 $ 137,120 $ 134,543  
Accretion 2,984 2,924 8,769 8,591  
Settlements (1,406) (930) (3,106) (2,236)  
Asset retirement obligation - Ending 142,783 $ 140,898 142,783 $ 140,898  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 2,000   $ 2,000   $ 2,200