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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Asset retirement obligation      
Asset retirement obligation - Beginning $ 82,072 $ 67,214  
Accretion 1,960 1,412  
Additions and changes in estimates 251 18,292  
Settlements (309) (859)  
Asset retirement obligation - Ending 83,974 $ 86,059  
Reclamation and Mine Closure (Textual) [Abstract]      
Accrued reclamation liabilities $ 3,400   $ 3,200