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Supplemental Financial Information - Other Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Other current liabilities:      
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Other current liabilities Other current liabilities Other current liabilities
Current portion of operating lease obligations $ 2,289 $ 1,883  
Current portion of mine reclamation liability 4,856 9,370  
Income and other taxes payable 2,409 1,305  
Other current liabilities 312 438  
Other current liabilities $ 9,866 $ 12,996  
Other long-term liabilities:      
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Other long-term liabilities Other long-term liabilities  
Mine reclamation liabilities $ 8,028 $ 12,077  
Operating lease obligations, long-term 4,582 1,109  
Other long-term liabilities 208 287  
Other long-term liabilities $ 12,818 $ 13,473