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Leases - Supplemental Balance Sheet Information (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Operating leases:      
Operating lease right-of-use assets $ 26.9 $ 35.5 $ 49.9
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Accounts Payable and Accrued Liabilities, Current Accounts Payable and Accrued Liabilities, Current  
Accounts payable and accrued expenses $ 16.8 $ 16.4  
Operating lease liabilities, less current portion 11.0 27.2  
Total operating lease liabilities 27.8 43.6  
Finance leases:      
Property, plant, equipment and mine development 36.1 32.2  
Accumulated depreciation (12.6) (7.4)  
Property, plant, equipment and mine development, net $ 23.5 $ 24.8  
Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration] Long-term debt Long-term debt  
Total finance lease liabilities $ 23.6 $ 29.3  
Weighted average remaining lease term (years)      
Operating leases 2 years    
Finance leases 6 years 8 months 12 days    
Weighted average discount rate      
Operating leases 5.70%    
Finance leases 7.20%    
Operating Leased Assets [Line Items]      
Operating lease liabilities $ 16.8 16.4  
Long-term Debt      
Finance leases:      
Current portion of long-term debt 13.2 15.3  
Long-term debt, less current portion 10.4 14.0  
Operating Leased Assets [Line Items]      
Long-term debt, less current portion 10.4 14.0  
Current portion of long-term debt $ 13.2 $ 15.3