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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions		12 Months Ended
	Jun. 22, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Restructuring Cost and Reserve [Line Items]
Asset impairment		$ 159.4	$ 0.0	$ 0.0
Property, plant and equipment - net		744.4	892.5
OPEB curtailment gain		(8.9)	$ 0.0	$ 0.0
Hawesville
Restructuring Cost and Reserve [Line Items]
Asset impairment	$ 159.4
Property, plant and equipment - net	$ 15.0
Severance costs		18.1
OPEB curtailment gain		$ 8.9