Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Trade Receivables, Net
Allowance for doubtful receivables		$ (0.8)	$ (0.6)
Trade receivables, net		319.7	325.2
Inventories
Finished products		103.7	89.3
Work-in-process		193.3	210.8
Raw materials		192.8	161.5
Operating supplies		14.1	15.6
Inventories		503.9	477.2
Property, Plant and Equipment, Net
Land and improvements		37.2	38.0
Buildings and leasehold improvements		256.3	238.4
Machinery and equipment	[1]	1,337.4	1,265.3
Construction in progress		297.5	173.7
Property, plant and equipment, gross		1,928.4	1,715.4
Accumulated depreciation and amortization		(767.5)	(663.7)
Land held for sale		0.3	0.4
Property, plant and equipment, net		1,161.2	1,052.1
Other Assets
Assets to be conveyed associated with Warrick acquisition	[1]	18.3	56.8
Restricted cash - Note 16		19.5	18.3	$ 13.9
Long-term replacement parts		18.3	16.7
Other		22.5	25.9
Other assets		78.6	117.7
Other Accrued Liabilities
Uncleared cash disbursements		24.5	15.7
Accrued income taxes and other taxes payable		11.2	9.5
Accrued annual contribution to Salaried VEBA - Note 5		0.7	1.1
Accrued interest		9.9	9.9
Short-term environmental accrual - Note 10		0.7	2.8
Current operating lease liabilities - Note 3		6.3	8.0
Current finance lease liabilities - Note 3		2.4	2.1
Current deferred compensation plan liabilities - Note 5		6.7	0.4
Other - Note 8		17.0	14.8
Other accrued liabilities		79.4	64.3
Long-Term Liabilities
Workers' compensation accrual		26.8	29.9
Long-term environmental accrual - Note 10		17.7	14.2
Other long-term liabilities		39.5	37.6
Long-term liabilities		84.0	81.7
Billed
Trade Receivables, Net
Billed trade receivables		$ 320.5	$ 325.8

[1] During the year ended December 31, 2024, $38.5 million of certain assets associated with our acquisition of Warrick were conveyed to us and placed in service. At December 31, 2024, such assets are presented within Machinery and equipment.