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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Asset retirement obligation    
Asset retirement obligation - Beginning $ 134,543 $ 133,508
Accretion 2,804 2,895
Settlements (719) (662)
Asset retirement obligation - Ending 136,628 $ 135,741
Property, Plant and Equipment [Line Items]    
Accrued reclamation liabilities, former mines $ 1,900