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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Asset retirement obligation          
Asset retirement obligation - Beginning $ 87,601 $ 59,795 $ 67,214 $ 57,454  
Accretion 2,038 1,452 5,652 4,205  
Additions and changes in estimates 0 0 18,270 0  
Settlements (2,363) (58) (3,860) (470)  
Asset retirement obligation - Ending 87,276 $ 61,189 87,276 $ 61,189  
Reclamation and Mine Closure (Textual) [Abstract]          
Accrued reclamation liabilities $ 4,000   $ 4,000   $ 3,600