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Reclamation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset retirement obligation    
Asset retirement obligation - Beginning $ 134,398 $ 133,508
Accretion 11,574 11,968
Additions and changes in estimates (6,132) (7,538)
Settlements (2,720) (3,540)
Asset retirement obligation - Ending 137,120 134,398
Property, Plant and Equipment [Line Items]    
Additions and changes in estimates (6,132) (7,538)
Accrued reclamation liabilities, former mines $ 2,200 $ 1,900