Leases - Supplemental Balance Sheeet Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Operating leases:
Operating lease right-of-use assets	$ 97.0	$ 0.0
Operating lease liabilities, less current portion	58.2	0.0
Total operating lease liabilities	88.9
Finance leases:
Accumulated depreciation	(1,112.6)	(957.4)
Property, plant, equipment and mine development, net	5,069.5	5,207.0
Current portion of long-term debt	34.8	36.5
Long-term debt	1,326.9	1,330.5
Total finance lease liabilities	32.7	$ 40.0
Operating leases
Operating leases:
Operating lease right-of-use assets	97.0
Accounts payable and accrued expenses	30.7
Operating lease liabilities, less current portion	58.2
Total operating lease liabilities	$ 88.9
Weighted average remaining lease term
Operating leases	4 years 2 months 12 days
Weighted average discount rate
Operating leases	7.40%
Finance leases
Finance leases:
Property, plant, equipment and mine development	$ 97.8
Accumulated depreciation	(33.4)
Property, plant, equipment and mine development, net	64.4
Current portion of long-term debt	30.8
Long-term debt	1.9
Total finance lease liabilities	$ 32.7
Weighted average remaining lease term
Finance leases	5 years 4 months 24 days
Weighted average discount rate
Finance leases	7.60%