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Reclamation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset retirement obligation    
Asset retirement obligation - Beginning $ 82,072 $ 67,214
Accretion 8,136 7,738
Additions and changes in estimates 8,688 11,939
Settlements (1,516) (4,819)
Asset retirement obligation - Ending 97,380 82,072
Reclamation and Mine Closure (Textual) [Abstract]    
Accrued reclamation liabilities $ 1,900 $ 3,200