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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring Cost and Reserve [Line Items]
OPEB curtailment gain, net	$ 0.0	$ (8.0)	$ 0.0	$ (8.0)
Hawesville
Restructuring Cost and Reserve [Line Items]
OPEB curtailment gain, net		8.0		8.0
Curtailment Of Operations
Restructuring Cost and Reserve [Line Items]
Severance costs	2.0		13.4
Gain (loss) on material sales	0.0		1.2
Curtailment Of Operations | Temporary Facility Closing, Excess Capacity
Restructuring Cost and Reserve [Line Items]
Excess capacity charges	$ 0.0		$ 9.0
WARN Notice
Restructuring Cost and Reserve [Line Items]
Severance costs		$ 4.5		$ 12.6

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