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Reclamation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset retirement obligation    
Asset retirement obligation - Beginning $ 67,214 $ 55,966
Accretion 7,738 4,826
Additions and changes in estimates 11,939 6,748
Settlements (4,819) (326)
Asset retirement obligation - Ending 82,072 67,214
Reclamation and Mine Closure (Textual) [Abstract]    
Accrued reclamation liabilities $ 3,200 $ 3,600