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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Asset retirement obligation          
Asset retirement obligation - Beginning $ 85,545 $ 87,601 $ 82,072 $ 67,214  
Accretion 2,059 2,038 6,027 5,652  
Additions and changes in estimates (239) 0 (118) 18,270  
Settlements (183) (2,363) (799) (3,860)  
Asset retirement obligation - Ending 87,182 $ 87,276 87,182 $ 87,276  
Reclamation and Mine Closure (Textual) [Abstract]          
Accrued reclamation liabilities $ 1,500   $ 1,500   $ 3,200