Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Trade Receivables, Net
Allowance for doubtful receivables		$ (0.6)	$ (0.6)
Trade receivables, net		341.4	325.2
Inventories
Finished products	[1]	81.0	89.3
Work-in-process	[1]	209.5	210.8
Raw materials	[1]	165.0	161.5
Operating supplies	[1]	15.8	15.6
Inventories	[1]	471.3	477.2
Property, Plant and Equipment, Net
Land and improvements		37.2	38.0
Buildings and leasehold improvements		240.0	238.4
Machinery and equipment		1,272.8	1,265.3
Construction in progress		193.8	173.7
Property, plant and equipment, gross		1,743.8	1,715.4
Accumulated depreciation and amortization		(690.3)	(663.7)
Land held for sale		1.5	0.4
Property, plant and equipment, net		1,055.0	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	$ 18.6
Long-term replacement parts		17.2	16.7
Other		25.8	25.9
Other assets		118.1	117.7
Other Accrued Liabilities
Uncleared cash disbursements		16.8	15.7
Accrued income taxes and other taxes payable		13.1	9.5
Accrued annual contribution to Salaried VEBA			1.1
Accrued interest		10.3	9.9
Short-term environmental accrual - Note 7		0.5	2.8
Current operating lease liabilities		7.7	8.0
Current finance lease liabilities		2.3	2.1
Other - Note 5		14.7	15.2
Other accrued liabilities		65.4	64.3
Long-Term Liabilities
Workers' compensation accrual		29.2	29.9
Long-term environmental accrual - Note 7		15.6	14.2
Other long-term liabilities		38.8	37.6
Long-term liabilities		83.6	81.7
Billed
Trade Receivables, Net
Billed trade receivables		$ 342.0	$ 325.8

[1] At March 31, 2024 and December 31, 2023, the current cost of our inventory exceeded its stated last-in, first-out (“LIFO”) value by $42.6 million and $56.0 million, respectively.