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Related Party Transactions - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Apr. 29, 2019
Related Party Transaction [Line Items]
Net sales to Glencore		$ 1,337.0	$ 1,025.5	$ 1,191.6
Glencore | Dispute Related to Alumina Purchases
Related Party Transaction [Line Items]
Gain (loss) related to litigation settlement	$ (4.0)
Glencore
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners		42.90%
Economic ownership percentage by related party		46.40%
Glencore
Related Party Transaction [Line Items]
Net sales to Glencore		$ 1,337.0	$ 1,025.5	$ 1,191.6
Hawesville
Related Party Transaction [Line Items]
Due to related parties, noncurrent					$ 40.0
Customer Concentration Risk | Sales Revenue | Glencore
Related Party Transaction [Line Items]
Major customer, percentage revenue, net		60.00%	64.00%	65.00%
Alumina | Glencore
Related Party Transaction [Line Items]
Net sales to Glencore		$ 18.3	$ 17.9	$ 28.7

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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