Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Trade Receivables, Net
Allowance for doubtful receivables	$ (0.9)	$ (0.8)
Trade receivables, net	305.8	112.8
Inventories
Finished products	83.0	37.2
Work-in-process	178.5	46.0
Raw materials	117.7	62.3
Operating supplies	6.1	6.5
Total	385.3	152.0
Property, Plant and Equipment, Net
Land and improvements	26.9	21.5
Buildings and leasehold improvements	176.0	117.2
Machinery and equipment	1,144.7	847.4
Construction in progress	68.4	37.8
Property, plant and equipment, gross	1,416.0	1,023.9
Accumulated depreciation and amortization	(411.1)	(398.4)
Assets held for sale	1.7	1.7
Property, plant and equipment, net	1,006.6	627.2
Other Accrued Liabilities
Uncleared cash disbursements	7.0	2.1
Accrued income taxes and other taxes payable	10.4	5.2
Accrued annual contribution to Salaried VEBA		1.7
Accrued interest	11.5	11.6
Short-term environmental accrual	4.0	3.7
Other	26.0	17.1
Total	58.9	41.4
Long-Term Liabilities
Workers' compensation accrual	30.2	30.6
Long-term environmental accrual	14.0	15.1
Other long-term liabilities	32.1	31.6
Total	76.3	77.3
Billed
Trade Receivables, Net
Billed trade receivables	$ 306.7	$ 113.6