Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Operating leases:
Operating lease right-of-use assets	$ 42.4	$ 49.9
Accounts payable and accrued expenses	18.7	24.5
Operating lease liabilities, less current portion	34.8	42.1
Total operating lease liabilities	53.5	66.6
Finance leases:
Property, plant, equipment and mine development	34.0	20.4
Accumulated depreciation	(7.8)	(2.5)
Property, plant, equipment and mine development, net	26.2	17.9
Current portion of long-term debt	15.3	21.5
Long-term debt, less current portion	17.0	5.8
Total finance lease liabilities	$ 32.3	$ 27.3
Weighted average remaining lease term (years)
Operating leases	3 years 3 months 18 days
Finance leases	6 years 8 months 12 days
Weighted average discount rate
Operating leases	6.80%
Finance leases	9.00%