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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Asset retirement obligation        
Asset retirement obligation - Beginning $ 118,799 $ 86,754    
Accretion 2,545 2,064    
Settlements (496) (421)    
Asset retirement obligation - Ending $ 120,848 $ 88,397    
Property, Plant and Equipment [Line Items]        
Accrued reclamation liabilities, former mines     $ 2,100 $ 2,000