Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions		Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Trade Receivables, Net
Allowance for doubtful receivables		$ (0.8)	$ (0.6)
Trade receivables, net		370.1	325.2
Inventories
Finished products	[1]	72.1	89.3
Work-in-process	[1]	210.2	210.8
Raw materials	[1]	148.0	161.5
Operating supplies	[1]	16.6	15.6
Inventories	[1]	446.9	477.2
Property, Plant and Equipment, Net
Land and improvements		37.2	38.0
Buildings and leasehold improvements		241.6	238.4
Machinery and equipment		1,279.7	1,265.3
Construction in progress		240.1	173.7
Property, plant and equipment, gross		1,798.6	1,715.4
Accumulated depreciation and amortization		(715.5)	(663.7)
Land held for sale		1.5	0.4
Property, plant and equipment, net		1,084.6	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		17.3	16.7
Other		26.6	25.9
Other assets		119.0	117.7
Other Accrued Liabilities
Uncleared cash disbursements		20.0	15.7
Accrued income taxes and other taxes payable		10.8	9.5
Accrued annual contribution to Salaried VEBA			1.1
Accrued interest		9.9	9.9
Short-term environmental accrual - Note 7		0.5	2.8
Current operating lease liabilities		7.2	8.0
Current finance lease liabilities		2.5	2.1
Other - Note 5		14.8	15.2
Other accrued liabilities		65.7	64.3
Long-Term Liabilities
Workers' compensation accrual		29.7	29.9
Long-term environmental accrual - Note 7		14.8	14.2
Other long-term liabilities		43.8	37.6
Long-term liabilities		88.3	81.7
Billed
Trade Receivables, Net
Billed trade receivables		$ 370.9	$ 325.8

[1] At June 30, 2024 and December 31, 2023, the current cost of our inventory exceeded its stated last-in, first-out (“LIFO”) value by $83.5 million and $56.0 million, respectively.