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Acquisition of Marshall Mine - Additional Information (Details)
$ in Millions
Sep. 30, 2020
USD ($)
Business Acquisition [Line Items]  
Reclamation reimbursements $ 10.2
Marshall Mine  
Business Acquisition [Line Items]  
Membership interest 100.00%
Capped cost $ 19.7
Direct cost $ 3.6
Estimated reclamation period 10 years
Term of contract 15 years
Marshall Mine | Cabot  
Business Acquisition [Line Items]  
Reclamation reimbursements $ 10.2
Term of contract 13 years