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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Asset retirement obligation          
Asset retirement obligation - Beginning $ 139,354 $ 136,628 $ 137,120 $ 134,543  
Accretion 2,922 2,863 5,785 5,667  
Settlements (1,071) (587) (1,700) (1,306)  
Asset retirement obligation - Ending 141,205 $ 138,904 141,205 $ 138,904  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 2,000   $ 2,000   $ 1,900