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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Curtailment Of Operations
Restructuring Cost and Reserve [Line Items]
Severance costs	$ 4.4		$ 11.4
Gain (loss) on material sales	0.5		1.2
Curtailment Of Operations | Temporary Facility Closing, Excess Capacity
Restructuring Cost and Reserve [Line Items]
Excess capacity charges	$ 3.6		$ 9.0
WARN Notice
Restructuring Cost and Reserve [Line Items]
Severance costs		$ 8.2		$ 8.2

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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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