Leases Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	$ 82.4	$ 0.0
Accounts payable and accrued expenses	29.6	0.0
Operating lease liabilities, less current portion	52.8	0.0
Total operating lease liabilities	82.4
Accumulated depreciation	(1,409.8)	(957.4)
Property, plant, equipment and mine development, net	4,679.1	5,207.0	$ 5,111.9
Current portion of long-term debt	18.3	36.5
Long-term debt, less current portion	1,292.5	$ 1,330.5
Total finance lease liabilities	15.2
Operating lease [Member]
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	82.4
Accounts payable and accrued expenses	29.6
Operating lease liabilities, less current portion	52.8
Total operating lease liabilities	$ 82.4
Operating leases	3 years 9 months 18 days
Operating leases	7.30%
Finance lease [Member]
Lessee, Lease, Description [Line Items]
Property, plant, equipment and mine development	$ 89.6
Accumulated depreciation	(45.9)
Property, plant, equipment and mine development, net	43.7
Current portion of long-term debt	14.3
Long-term debt, less current portion	0.9
Total finance lease liabilities	$ 15.2
Finance leases	7 months 6 days
Finance leases	6.00%