Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Trade Receivables, Net
Allowance for doubtful receivables	$ (0.6)	$ (0.5)
Trade receivables, net	325.2	297.2
Inventories
Finished products	89.3	98.0
Work-in-process	210.8	242.5
Raw materials	161.5	174.0
Operating supplies	15.6	10.9
Inventories	477.2	525.4
Property, Plant and Equipment, Net
Land and improvements	38.0	28.4
Buildings and leasehold improvements	238.4	185.5
Machinery and equipment	1,265.3	1,232.7
Construction in progress	173.7	141.3
Property, plant and equipment, gross	1,715.4	1,587.9
Accumulated depreciation and amortization	(663.7)	(574.9)
Land held for sale	0.4	0.2
Property, plant and equipment, net	1,052.1	1,013.2
Other Assets
Assets to be conveyed associated with Warrick acquisition	56.8	56.8
Restricted cash - Note 16	18.3	13.9	$ 13.8
Long-term replacement parts	16.7	15.5
Other	25.9	26.1
Other assets	117.7	112.3
Other Accrued Liabilities
Uncleared cash disbursements	15.7	13.6
Accrued income taxes and other taxes payable	9.5	8.9
Accrued annual contribution to Salaried VEBA - Note 5	1.1
Accrued interest	9.9	9.9
Short-term environmental accrual - Note 10	2.8	1.1
Current operating lease liabilities - Note 3	8.0	9.1
Current finance lease liabilities - Note 3	2.1	2.1
Other - Note 8	15.2	23.7
Other accrued liabilities	64.3	68.4
Long-Term Liabilities
Workers' compensation accrual	29.9	30.9
Long-term environmental accrual	14.2	16.6
Other long-term liabilities	37.6	27.2
Long-term liabilities	81.7	74.7
Billed
Trade Receivables, Net
Billed trade receivables	$ 325.8	$ 297.7