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Related Party Transactions - Narrative (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Apr. 29, 2019
Related Party Transaction [Line Items] |  |  |  |
Major customer, percentage revenue, net | 10.00% |  |  |
Net sales to Glencore | $ 1,025,500,000 | $ 1,191,600,000 | $ 1,204,500,000 |
Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Ownership percentage by noncontrolling owners | 42.90% |  |  |
Economic ownership percentage by related party | 46.80% |  |  |
Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Net sales to Glencore | $ 1,025,500,000 | 1,191,600,000 | 1,204,500,000 |
Hawesville [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Due to related parties, noncurrent |  |  |  | $ 40,000,000.0
Customer Concentration Risk [Member] | Sales Revenue [Member] | Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Major customer, percentage revenue, net | 64.00% |  |  |
Alumina [Member] | Glencore [Member] |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Net sales to Glencore | $ 17,900,000 | $ 28,700,000 | $ 0 |

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