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Leases - Supplemental Balance Sheet Information (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Operating leases:    
Operating lease right-of-use assets $ 38.4 $ 49.9
Accounts payable and accrued expenses 17.0 24.5
Operating lease liabilities, less current portion 31.6 42.1
Total operating lease liabilities 48.6 66.6
Finance leases:    
Property, plant, equipment and mine development 31.3 20.4
Accumulated depreciation (5.8) (2.5)
Property, plant, equipment and mine development, net 25.5 17.9
Current portion of long-term debt 15.5 21.5
Long-term debt, less current portion 14.9 5.8
Total finance lease liabilities $ 30.4 $ 27.3
Weighted average remaining lease term (years)    
Operating leases 3 years 1 month 6 days  
Finance leases 6 years 8 months 12 days  
Weighted average discount rate    
Operating leases 6.80%  
Finance leases 8.80%