Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Operating leases:
Operating lease right-of-use assets	$ 33.0	$ 35.5
Accounts payable and accrued expenses	16.1	16.4
Operating lease liabilities, less current portion	24.6	27.2
Total operating lease liabilities	40.7	43.6
Finance leases:
Property, plant, equipment and mine development	32.4	32.2
Accumulated depreciation	(9.0)	(7.4)
Property, plant, equipment and mine development, net	23.4	24.8
Current portion of long-term debt	15.0	15.3
Long-term debt, less current portion	12.6	14.0
Total finance lease liabilities	$ 27.6	$ 29.3
Weighted average remaining lease term (years)
Operating leases	2 years 10 months 24 days
Finance leases	6 years 8 months 12 days
Weighted average discount rate
Operating leases	7.00%
Finance leases	8.20%