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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Asset retirement obligation      
Asset retirement obligation - Beginning $ 97,380 $ 82,072  
Accretion 2,338 1,960  
Additions and changes in estimates 0 251  
Settlements (478) (309)  
Asset retirement obligation - Ending 99,240 $ 83,974  
Reclamation and Mine Closure (Textual) [Abstract]      
Accrued reclamation liabilities $ 2,000   $ 1,900