Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions		Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Trade Receivables, Net
Allowance for doubtful receivables		$ (0.8)	$ (0.6)
Trade receivables, net		370.2	325.2
Inventories
Finished products	[1]	89.3	89.3
Work-in-process	[1]	191.7	210.8
Raw materials	[1]	175.2	161.5
Operating supplies	[1]	17.7	15.6
Inventories	[1]	473.9	477.2
Property, Plant and Equipment, Net
Land and improvements		37.2	38.0
Buildings and leasehold improvements		251.1	238.4
Machinery and equipment		1,293.2	1,265.3
Construction in progress		259.8	173.7
Property, plant and equipment, gross		1,841.3	1,715.4
Accumulated depreciation and amortization		(741.2)	(663.7)
Land held for sale		0.3	0.4
Property, plant and equipment, net		1,100.4	1,052.1
Other Assets
Assets to be conveyed associated with Warrick acquisition		56.8	56.8
Restricted cash - Note 12		18.3	18.3	$ 18.1
Long-term replacement parts		18.0	16.7
Other		23.7	25.9
Other assets		116.8	117.7
Other Accrued Liabilities
Uncleared cash disbursements		9.6	15.7
Accrued income taxes and other taxes payable		10.7	9.5
Accrued annual contribution to Salaried VEBA			1.1
Accrued interest		10.3	9.9
Short-term environmental accrual - Note 7		0.9	2.8
Current operating lease liabilities		6.7	8.0
Current finance lease liabilities		2.4	2.1
Other - Note 5		14.3	15.2
Other accrued liabilities		54.9	64.3
Long-Term Liabilities
Workers' compensation accrual		29.5	29.9
Long-term environmental accrual - Note 7		17.2	14.2
Other long-term liabilities		44.1	37.6
Long-term liabilities		90.8	81.7
Billed
Trade Receivables, Net
Billed trade receivables		$ 371.0	$ 325.8

[1] At September 30, 2024 and December 31, 2023, the current cost of our inventory exceeded its stated last-in, first-out (“LIFO”) value by $81.6 million and $56.0 million, respectively.