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Curtailment of Operations - Hawesville (Details) - Curtailment Of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Severance costs	$ 6.8	$ 16.6
Gain (loss) on material sales	$ 0.5	1.7
Temporary Facility Closing, Excess Capacity
Restructuring Cost and Reserve [Line Items]
Excess capacity charges		$ 9.0
Restructuring Incurred Cost, Statement Of Income Or Comprehensive Income, Extensible Enumeration, Not Disclosed Flag		demand capacity charges

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Gain (Loss) On Sale Of Nonproductive Assets

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

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