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Reclamation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Asset retirement obligation          
Asset retirement obligation - Beginning $ 135,741 $ 120,848 $ 133,508 $ 118,799  
Accretion 2,959 2,766 5,854 5,311  
Settlements (1,155) (707) (1,817) (1,203)  
Asset retirement obligation - Ending 137,545 $ 122,907 137,545 $ 122,907  
Property, Plant and Equipment [Line Items]          
Accrued reclamation liabilities, former mines $ 2,100   $ 2,100   $ 2,000