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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Asset retirement obligation          
Asset retirement obligation - Beginning $ 138,904 $ 137,545 $ 134,543 $ 133,508  
Accretion 2,924 3,024 8,591 8,878  
Settlements (930) (833) (2,236) (2,650)  
Asset retirement obligation - Ending 140,898 $ 139,736 140,898 $ 139,736  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 1,800   $ 1,800   $ 1,900