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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Asset retirement obligation          
Asset retirement obligation - Beginning $ 122,907 $ 90,002 $ 118,799 $ 86,754  
Accretion 2,830 2,167 8,141 6,347  
Additions and changes in estimates 0 3,116 0 3,116  
Settlements (1,171) (656) (2,374) (1,588)  
Asset retirement obligation - Ending 124,566 $ 94,629 124,566 $ 94,629  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 2,200   $ 2,200   $ 2,000