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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Asset retirement obligation      
Asset retirement obligation - Beginning $ 133,508 $ 118,799  
Accretion 2,895 2,545  
Settlements (662) (496)  
Asset retirement obligation - Ending 135,741 $ 120,848  
Property, Plant and Equipment [Line Items]      
Accrued reclamation liabilities, former mines $ 2,100   $ 2,000