Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Operating leases:
Operating lease right-of-use assets	$ 44.9	$ 82.4
Accounts payable and accrued expenses	54.4	29.6
Operating lease liabilities, less current portion	7.1	52.8
Total operating lease liabilities	61.5	82.4
Finance leases:
Property, plant, equipment and mine development	24.6	89.6
Accumulated depreciation	(20.7)	(45.9)
Property, plant, equipment and mine development, net	3.9	43.7
Current portion of long-term debt	8.0	14.3
Long-term debt, less current portion	0.0	0.9
Total finance lease liabilities	$ 8.0	$ 15.2
Weighted average remaining lease term (years)
Operating leases	3 years 6 months
Finance leases	16 years 3 months 18 days
Weighted average discount rate
Operating leases	7.40%
Finance leases	6.20%