Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions		Mar. 31, 2025	Dec. 31, 2024		Mar. 31, 2024
Trade Receivables, Net
Allowance for doubtful receivables		$ (0.8)	$ (0.8)	[1]
Trade receivables, net		347.4	319.7	[1],[2]
Inventories
Finished products		123.5	130.0	[1]
Work-in-process		229.2	229.1	[1]
Raw materials		205.7	228.7	[1]
Operating supplies		14.0	14.1	[1]
Inventories		572.4	601.9	[1],[2]	$ 536.4
Property, Plant and Equipment, Net
Land and improvements		37.7	37.2	[1]
Buildings and leasehold improvements		272.1	256.3	[1]
Machinery and equipment	[3]	1,343.2	1,337.4	[1]
Construction in progress		332.4	297.5	[1]
Property, plant and equipment, gross		1,985.4	1,928.4	[1]
Accumulated depreciation and amortization		(794.1)	(767.5)	[1]
Land held for sale		0.3	0.3	[1]
Property, plant and equipment, net		1,191.6	1,161.2	[1],[2]
Other Assets
Assets to be conveyed associated with Warrick acquisition	[1],[3]		18.3
Restricted cash - Note 12		20.0	19.5	[1]	$ 18.3
Long-term replacement parts		19.8	18.3	[1]
Other		22.4	22.5	[1]
Other assets		62.2	78.6	[1],[2]
Other Accrued Liabilities
Uncleared cash disbursements		11.6	24.5	[1]
Accrued income taxes and other taxes payable		14.3	11.1	[1]
Accrued annual contribution to Salaried VEBA	[1]		0.7
Accrued interest		10.3	9.9	[1]
Short-term environmental accrual - Note 7		0.8	0.7	[1]
Current operating lease liabilities		5.7	6.3	[1]
Current finance lease liabilities		2.2	2.4	[1]
Current deferred compensation plan liabilities - Note 3		6.4	6.7	[1]
Other - Note 5		15.6	17.0	[1]
Other accrued liabilities		66.9	79.3	[1],[2]
Long-Term Liabilities
Workers' compensation accrual		27.3	26.8	[1]
Long-term environmental accrual - Note 7		17.7	17.7	[1]
Other long-term liabilities		39.2	39.5	[1]
Long-term liabilities		84.2	84.0	[1],[2]
Billed
Trade Receivables, Net
Billed trade receivables		$ 348.2	$ 320.5	[1]

[1] Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.

[2] Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.

[3] During the quarter ended March 31, 2025, $18.3 million of certain assets associated with our acquisition of Warrick were conveyed to us and placed in service. At March 31, 2025, such assets are presented within Machinery and equipment.