Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating leases:
Operating lease right-of-use assets	$ 35.5	$ 49.9	$ 82.4
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	$ 16.4	$ 24.5
Operating lease liabilities, less current portion	27.2	42.1
Total operating lease liabilities	43.6	66.6
Finance leases:
Property, plant, equipment and mine development	32.2	20.4
Accumulated depreciation	(7.4)	(2.5)
Property, plant, equipment and mine development, net	$ 24.8	$ 17.9
Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration]	Long-term debt	Long-term debt
Total finance lease liabilities	$ 29.3	$ 27.3
Weighted average remaining lease term (years)
Operating leases	3 years
Finance leases	6 years 8 months 12 days
Weighted average discount rate
Operating leases	6.90%
Finance leases	8.50%
Operating Leased Assets [Line Items]
Operating lease liabilities	$ 16.4	24.5
Total lease liabilities	29.3	27.3
Long-term Debt
Finance leases:
Current portion of long-term debt	15.3	21.5
Long-term debt, less current portion	14.0	5.8
Operating Leased Assets [Line Items]
Long-term debt, less current portion	14.0	5.8
Current portion of long-term debt	$ 15.3	$ 21.5