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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Asset retirement obligation          
Asset retirement obligation - Beginning $ 137,545 $ 122,907 $ 133,508 $ 118,799  
Accretion 3,024 2,830 8,878 8,141  
Settlements (833) (1,171) (2,650) (2,374)  
Asset retirement obligation - Ending 139,736 $ 124,566 139,736 $ 124,566  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 1,900   $ 1,900   $ 2,000