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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Asset retirement obligation          
Asset retirement obligation - Beginning $ 86,059 $ 58,460 $ 67,214 $ 57,454  
Accretion 1,990 1,435 3,614 2,752  
Additions and changes in estimates 0 0 18,270 0  
Settlements (448) (100) (1,497) (411)  
Asset retirement obligation - Ending 87,601 $ 59,795 87,601 $ 59,795  
Reclamation and Mine Closure (Textual) [Abstract]          
Accrued reclamation liabilities $ 3,700   $ 3,700   $ 3,600