Leases - Supplemental Balance Sheeet Information (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Operating leases:
Operating lease right-of-use assets	$ 93.1	$ 0.0
Operating lease liabilities, less current portion	(58.0)	0.0
Total operating lease liabilities	(87.4)
Finance leases:
Accumulated depreciation	(1,267.3)	(957.4)
Property, plant, equipment and mine development, net	4,974.8	5,207.0
Current portion of long-term debt	28.5	36.5
Long-term debt	1,327.1	1,330.5
Total finance lease liabilities	24.5	$ 40.0
Operating leases
Operating leases:
Operating lease right-of-use assets	93.1
Accounts payable and accrued expenses	(29.4)
Operating lease liabilities, less current portion	(58.0)
Total operating lease liabilities	$ (87.4)
Weighted average remaining lease term (years)
Operating leases	4 years 1 month 6 days
Weighted average discount rate
Operating leases	7.40%
Finance leases
Finance leases:
Property, plant, equipment and mine development	$ 96.8
Accumulated depreciation	(47.8)
Property, plant, equipment and mine development, net	49.0
Current portion of long-term debt	24.5
Long-term debt	0.0
Total finance lease liabilities	$ 24.5
Weighted average remaining lease term (years)
Finance leases	8 months 12 days
Weighted average discount rate
Finance leases	5.90%