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v3.21.2

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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