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Commitments and Contingencies (Details) |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 01, 2021 USD ($) | Apr. 01, 2021 | Aug. 18, 2017 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2021 USD ($) Labor_Union MW | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) Labor_Union MW | Sep. 30, 2020 USD ($) | Dec. 31, 2013 USD ($) | Jul. 31, 2021 MW
Labor Commitments [Abstract] |  |  |  |  |  |  |  |  |  |
Percentage of total work force in union |  |  |  |  | 64.00% |  | 64.00% |  |  |
Percentage of Grundartangi work force represented by the labor unions |  |  |  |  | 86.00% |  | 86.00% |  |  |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | Labor_Union |  |  |  |  | 5 |  | 5 |  |  |
Percentage of U.S. based work force represented by a union |  |  |  |  | 55.00% |  | 55.00% |  |  |
PBGC [Member] |  |  |  |  |  |  |  |  |  |
PBGC Settlement [Abstract] |  |  |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  |  |  |  | $ 17,400,000 |
Payment for pension benefits |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 0 |  |
PBGC [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |
PBGC Settlement [Abstract] |  |  |  |  |  |  |  |  |  |
Pension contributions, amended term, annual contribution | $ 2,400,000 |  |  |  |  |  |  |  |  |
Pension contributions, amended term, total contribution | $ 9,600,000 |  |  |  |  |  |  |  |  |
Pension contributions, term | 4 years |  |  |  |  |  |  |  |  |
Hawesville [Member] |  |  |  |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  |  | 1 year |  |  |
Sebree [Member] |  |  |  |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  |  | 1 year |  |  |
Santee Cooper [Member] |  |  |  |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |  |  |  |
Power agreement, power supply, percentage |  | 100.00% |  |  |  |  |  |  |  |
Power supply agreement, increase production, at full capacity, percentage |  | 75.00% |  |  |  |  |  |  |  |
Grundartangi - HS, Landsvirkjun and OR [Member] |  |  |  |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  |  | 545 |  | 545 |  |  |
Grundartangi - Landsvirkjun [Member] |  |  |  |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  |  | 25 |  | 25 |  |  |
Power currently available under the power purchase agreement, extension (in megawatts) | MW |  |  |  |  |  |  |  |  |  | 161
Power currently available under the power purchase agreement, requested (in megawatts) | MW |  |  |  |  |  |  |  |  |  | 182
Netherlands [Member] |  |  |  |  |  |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |  |  |  |  |  |
Percentage of Vlissingen work force represented by the labor union |  |  |  |  | 100.00% |  | 100.00% |  |  |
Vernon [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |  |  |  |
Litigation settlement amount | $ 3,900,000 |  |  |  |  |  |  |  |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |  |  |  |
Litigation settlement amount |  |  | $ 23,000,000 |  |  |  |  |  |  |
Ravenswood litigation settlement installment period |  |  | 10 years |  |  |  | 9 years |  |  |
Litigation payment to trust |  |  |  | $ 5,000,000 |  |  |  |  |  |
Gain (loss) related to litigation settlement |  |  |  | 5,500,000 |  |  |  |  |  |
Loss contingency accrual |  |  |  | $ 12,500,000 |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  |  | $ 2,000,000 |  |  |
Other current liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  |  | 2,000,000 |  |  |
Other liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  |  | $ 6,000,000 |  |  |

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