Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Operating leases:
Operating lease right-of-use assets	$ 85.6	$ 0.0
Operating lease liabilities, less current portion	55.1	0.0
Total operating lease liabilities	82.7
Finance leases:
Accumulated depreciation	(1,383.9)	(957.4)
Property, plant, equipment and mine development, net	4,899.2	5,207.0
Current portion of long-term debt	23.4	36.5
Long-term debt, less current portion	1,329.4	1,330.5
Total finance lease liabilities	19.6	$ 40.0
Operating leases
Operating leases:
Operating lease right-of-use assets	85.6
Accounts payable and accrued expenses	27.6
Operating lease liabilities, less current portion	55.1
Total operating lease liabilities	$ 82.7
Weighted average remaining lease term (years)
Operating leases	4 years
Weighted average discount rate
Operating leases	7.30%
Finance leases
Finance leases:
Property, plant, equipment and mine development	$ 95.4
Accumulated depreciation	(49.3)
Property, plant, equipment and mine development, net	46.1
Current portion of long-term debt	19.4
Long-term debt, less current portion	0.2
Total finance lease liabilities	$ 19.6
Weighted average remaining lease term (years)
Finance leases	7 months 6 days
Weighted average discount rate
Finance leases	5.90%