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Related Party Transactions - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Apr. 29, 2019
Related Party Transaction [Line Items]
Related parties		$ 350.6	$ 235.8	$ 925.3	$ 792.4
Glencore [Member]
Related Party Transaction [Line Items]
Related parties		350.6	235.8	925.3	792.4
Glencore [Member] | Dispute related to alumina purchases [Member]
Related Party Transaction [Line Items]
Gain (loss) related to litigation settlement	$ (4.0)
Hawesville [Member]
Related Party Transaction [Line Items]
Term loan						$ 40.0
Alumina [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Related parties		$ 5.5	$ 16.4	$ 16.9	$ 17.6
Consolidated sales [Member] | Customer concentration risk [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Major customer, percentage of revenue, net (percent)		60.30%	60.00%	59.60%	65.20%
Glencore [Member]
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners		42.90%		42.90%
Economic ownership percentage by related party		46.70%		46.70%

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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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