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Leases - Supplemental Balance Sheet Information (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Operating leases:    
Operating lease right-of-use assets $ 78.0 $ 82.4
Accounts payable and accrued expenses 25.2 29.6
Operating lease liabilities, less current portion 44.5 52.8
Total operating lease liabilities 69.7 82.4
Finance leases:    
Property, plant, equipment and mine development 88.0 89.6
Accumulated depreciation (47.6) (45.9)
Property, plant, equipment and mine development, net 40.4 43.7
Current portion of long-term debt 8.6 14.3
Long-term debt, less current portion 0.5 0.9
Total finance lease liabilities $ 9.1 $ 15.2
Weighted average remaining lease term (years)    
Operating leases 3 years 8 months 12 days  
Finance leases 14 years 9 months 18 days  
Weighted average discount rate    
Operating leases 7.30%  
Finance leases 6.00%