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Commitments and Contingencies (Details) $ in Millions |  | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 18, 2017 USD ($) | Oct. 31, 2021 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2025 USD ($) laborUnion MW | Dec. 31, 2013 USD ($)
Labor Commitments [Abstract] |  |  |  |  |
Percentage of total work force in union |  |  |  | 58.00% |
Percentage of Grundartangi work force represented by the labor unions |  |  |  | 87.00% |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | laborUnion |  |  |  | 5 |
Percentage of domestic based work force represented by a union |  |  |  | 37.00% |
Percentage of foreign work force, represented by the union |  |  |  | 62.00% |
Contingent consideration, accrued interest and principal |  |  |  | $ 33.4 |
E ON Contingent Obligation | Long-Term Debt |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |
Stated interest rate, percentage |  |  |  | 10.94% |
Netherlands |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |
Percentage of total work force in union |  |  |  | 100.00% |
PBGC |  |  |  |  |
PBGC Settlement [Abstract] |  |  |  |  |
Required pension contributions above minimum |  |  |  |  | $ 17.4
Pension contributions, amended term, annual contribution |  | $ 2.4 |  |  |
Pension contributions, amended term, total contribution |  | $ 9.6 |  |  |
Pension contributions, term |  | 4 years |  |  |
Kenergy | Century Marketer, LLC |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Extension term |  |  |  | 1 year |
Required termination period |  |  |  | 1 year |
Hawesville | Kenergy |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Extension term |  |  |  | 1 year |
Required termination period |  |  |  | 1 year |
Sebree | Kenergy |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Extension term |  |  |  | 1 year |
Required termination period |  |  |  | 1 year |
Santee Cooper |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power agreement, power supply, percentage |  |  |  | 100.00% |
Grundartangi - HS, Landsvirkjun and OR |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  | 545 |
Grundartangi - Landsvirkjun |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  | 25 |
Ravenswood Retiree Medical Benefits Changes |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |
Litigation settlement amount | $ 23.0 |  |  |  |
Ravenswood litigation settlement installment period | 10 years |  |  |  |
Litigation payment to trust |  |  | $ 5.0 |  |
Gain (loss) related to litigation settlement |  |  | 5.5 |  |
Loss contingency accrual |  |  | $ 12.5 |  |
Litigation settlement, annual installment |  |  |  | $ 2.0 |
Litigation settlement installment period |  |  |  | 9 years |
Other current liabilities |  |  |  | $ 1.9 |

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