Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents
Cash and money market funds	$ 780.3	$ 28.2
Commercial paper		236.1
Total	780.3	264.3
Trade Receivables, net
Allowance for doubtful receivables	(0.8)	(1.4)
Trade receivables, net	112.8	167.1
Inventories
Finished products	37.2	42.6
Work-in-process	46.0	63.5
Raw materials	62.3	65.0
Operating supplies	6.5	6.5
Total	152.0	177.6
Property, Plant and Equipment, net
Land and improvements	21.5	21.4
Buildings and leasehold improvements	117.2	104.5
Machinery and equipment	847.4	813.5
Construction in progress	37.8	33.2
Property, plant and equipment, gross	1,023.9	972.6
Accumulated depreciation and amortization	(398.4)	(352.2)
Assets held for sale	1.7	1.6
Property, plant and equipment, net	627.2	622.0
Other Accrued Liabilities
Uncleared cash disbursements	2.1	4.2
Accrued income taxes and other taxes payable	5.2	6.2
Accrued annual contribution to VEBAs	1.7	2.9
Accrued interest	11.6	2.3
Short-term environmental accrual	3.7	5.5
Other	17.1	22.9
Total	41.4	44.0
Long-Term Liabilities
Workers' compensation accruals	30.6	27.7
Long-term environmental accrual	15.1	11.5
Other long-term liabilities	32.9	27.8
Total	78.6	67.0
Billed
Trade Receivables, net
Billed trade receivables	$ 113.6	$ 168.5