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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Asset retirement obligation          
Asset retirement obligation - Beginning $ 120,848 $ 88,397 $ 118,799 $ 86,754  
Accretion 2,766 2,116 5,311 4,180  
Settlements (707) (511) (1,203) (932)  
Asset retirement obligation - Ending 122,907 $ 90,002 122,907 $ 90,002  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 2,000   $ 2,000   $ 2,000