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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jun. 22, 2022	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Asset impairment		$ 159.4	$ 0.0	$ 159.4	$ 0.0
Property, plant and equipment - net		738.4		$ 738.4		$ 892.5
Hawesville
Restructuring Cost and Reserve [Line Items]
Asset impairment	$ 159.4
Property, plant and equipment - net	$ 15.0
Severance costs		$ 8.2