Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Operating leases:
Operating lease right-of-use assets	$ 46.8	$ 49.9
Accounts payable and accrued expenses	21.1	24.5
Operating lease liabilities, less current portion	37.5	42.1
Total operating lease liabilities	58.6	66.6
Finance leases:
Property, plant, equipment and mine development	13.8	20.4
Accumulated depreciation	(3.1)	(2.5)
Property, plant, equipment and mine development, net	10.7	17.9
Current portion of long-term debt	5.9	21.5
Long-term debt, less current portion	11.4	5.8
Total finance lease liabilities	$ 17.3	$ 27.3
Weighted average remaining lease term (years)
Operating leases	3 years 4 months 24 days
Finance leases	8 years 2 months 12 days
Weighted average discount rate
Operating leases	6.70%
Finance leases	10.10%