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Commitments and Contingencies - Legal 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 ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2021 USD ($) MW | Dec. 31, 2013 USD ($) | Jul. 31, 2021 MW
Ravenswood Retiree Medical Benefits Changes |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Litigation settlement amount |  |  | $ 23,000,000 |  |  |  |  |  |  |  |
Litigation settlement period |  |  |  |  |  |  |  |  | 10 years |  |
Ravenswood retiree legal settlement |  |  |  | $ 5,000,000 |  |  |  |  |  |  |
Gain (loss) related to litigation settlement |  |  |  | 5,500,000 |  |  |  |  |  |  |
Loss contingency accrual |  |  |  | $ 12,500,000 |  |  |  |  |  |  |
Litigation settlement, annual installment |  |  |  |  |  |  |  |  | $ 2,000,000 |  |
Litigation settlement installment period |  |  |  |  |  |  |  |  | 9 years |  |
Other liabilities, current |  |  |  |  |  |  |  |  | $ 2,000,000 |  |
Other liabilities |  |  |  |  |  |  |  |  | $ 6,200,000 |  |
Pension Benefit Guarantee Corporation |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  |  |  |  |  | $ 17,400,000 |
Pension contributions |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 0 |  |  |
Pension contributions, amended term, annual contribution | $ 2,400,000 |  |  |  |  |  |  |  |  |  |
Pension contributions, amended term, total contribution | $ 9,600,000 |  |  |  |  |  |  |  |  |  |
Pension contributions, term | 4 years |  |  |  |  |  |  |  |  |  |
Hawesville |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  |  |  |  | 1 year |  |
Sebree |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  |  |  |  | 1 year |  |
Mt. Holly |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term power agreement, supply percentage |  | 100.00% |  |  |  |  |  |  |  |  |
Power supply agreement, increase production at full capacity, percentage |  | 75.00% |  |  |  |  |  |  |  |  |
Grundartangi - HS, Landsvirkjun |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Underlying, derivative power, available | MW |  |  |  |  |  |  |  |  | 545 |  |
Grundartangi - Landsvirkjun |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Underlying, derivative power, available, extension | MW |  |  |  |  |  |  |  |  |  |  | 161
Underlying, derivative power, available, requested | MW |  |  |  |  |  |  |  |  |  |  | 182
Vernon |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Litigation settlement amount | $ 3,900,000 |  |  |  |  |  |  |  |  |  |

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