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Marshall Mine Asset Retirement Obligation and Cabot Receivable - ARO and Cabot Receivable (Details) - USD ($)
3 Months Ended 6 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Jun. 30, 2021
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation and related Long-Term Receivable [Abstract]                
Reclamation costs $ 21,300,000              
Expected future cash flows $ 23,700,000              
Risk-free rate 7.00%   7.00%     7.00%    
Long-term receivable $ 9,700,000   $ 9,700,000     $ 9,700,000    
Discount rate 1.50%   1.50%     1.50%    
Effect of discounting $ 500,000   $ 500,000     $ 500,000    
Reclamation reimbursements $ 10,200,000              
Allowance for credit losses   $ 0     $ 0   $ 0  
Property, Plant and Equipment [Line Items]                
Asset retirement obligation   12,884,000     12,884,000   21,447,000 $ 2,721,000
Gain on change in estimate, asset retirement obligation   0 $ 0   (1,942,000) $ 0    
Marshall Mine                
Property, Plant and Equipment [Line Items]                
Asset retirement obligation   $ 9,400,000     $ 9,400,000   $ 18,100,000  
Gain on change in estimate, asset retirement obligation       $ 1,900,000