Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Operating leases:
Operating lease right-of-use assets	$ 50.7	$ 82.4
Accounts payable and accrued expenses	24.9	29.6
Operating lease liabilities, less current portion	42.0	52.8
Total operating lease liabilities	66.9	82.4
Finance leases:
Property, plant, equipment and mine development	44.2	89.6
Accumulated depreciation	(27.6)	(45.9)
Property, plant, equipment and mine development, net	16.6	43.7
Current portion of long-term debt	6.9	14.3
Long-term debt, less current portion	1.0	0.9
Total finance lease liabilities	$ 7.9	$ 15.2
Weighted average remaining lease term (years)
Operating leases	3 years 7 months 6 days
Finance leases	17 years 3 months 18 days
Weighted average discount rate
Operating leases	7.40%
Finance leases	6.20%