Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Trade Receivables, Net
Allowance for doubtful receivables	$ (0.8)	$ (0.8)
Trade receivables, net	361.6	112.8
Inventories
Finished products	81.9	37.2
Work-in-process	163.2	46.0
Raw materials	159.6	62.3
Operating supplies	8.9	6.5
Total	413.6	152.0
Property, Plant and Equipment, Net
Land and improvements	26.0	21.5
Buildings and leasehold improvements	174.7	117.2
Machinery and equipment	1,119.7	847.4
Construction in progress	82.3	37.8
Property, plant and equipment, gross	1,402.7	1,023.9
Accumulated depreciation and amortization	(456.1)	(398.4)
Assets held for sale	2.1	1.7
Property, plant and equipment, net	948.7	627.2
Other Accrued Liabilities
Uncleared cash disbursements	7.1	2.1
Accrued income taxes and other taxes payable	11.0	5.2
Accrued annual contribution to Salaried VEBA		1.7
Accrued interest	11.0	11.6
Short-term environmental accrual	3.9	3.7
Other	22.2	17.1
Total	55.2	41.4
Long-Term Liabilities
Workers' compensation accrual	30.9	30.6
Long-term environmental accrual	13.3	15.1
Other long-term liabilities	30.3	31.6
Total	74.5	77.3
Billed
Trade Receivables, Net
Billed trade receivables	$ 362.4	$ 113.6