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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Asset retirement obligation          
Asset retirement obligation - Beginning $ 83,974 $ 86,059 $ 82,072 $ 67,214  
Accretion 2,009 1,990 3,968 3,614  
Additions and changes in estimates (130) 0 121 18,270  
Settlements (308) (448) (616) (1,497)  
Asset retirement obligation - Ending 85,545 $ 87,601 85,545 $ 87,601  
Reclamation and Mine Closure (Textual) [Abstract]          
Accrued reclamation liabilities $ 1,300   $ 1,300   $ 3,200