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Related Party Transactions - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 29, 2019
Related Party Transaction [Line Items]
Related parties	$ 306.4	$ 285.6	$ 574.7	$ 556.6
Glencore [Member]
Related Party Transaction [Line Items]
Related parties	306.4	285.6	574.7	556.6
Hawesville [Member]
Related Party Transaction [Line Items]
Term loan - less current portion					$ 40.0
Alumina [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Related parties	$ 11.4	$ 0.9	$ 11.4	$ 1.2
Consolidated sales [Member] | Customer concentration risk [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Major customer, percentage of revenue, net (percent)	58.00%	71.00%	59.10%	67.60%
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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