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Commitments and Contingencies - Legal Contingencies (Details) |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 27 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 01, 2024 | Oct. 01, 2021 USD ($) | Aug. 18, 2017 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2023 USD ($) MW | Dec. 31, 2013 USD ($) | Dec. 31, 2023 USD ($) MW | Sep. 30, 2022 MW | 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 |  |  |  |
Loss contingency, accrual, current |  |  |  |  |  |  |  |  | $ 2,000,000 |  | $ 2,000,000 |  |
Other liabilities |  |  |  |  |  |  |  |  | $ 3,300,000 |  | 3,300,000 |  |
Pension Benefit Guarantee Corporation |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  |  |  |  |  | $ 17,400,000 |  |  |
Pension contributions |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 0 |  |  | $ 6,900,000 |  |
Pension contributions, amended term, annual contribution |  | $ 2,400,000 |  |  |  |  |  |  |  |  |  |  |
Pension contributions, amended term, total contribution |  | $ 9,600,000 |  |  |  |  |  |  |  |  |  |  |
Pension contributions, term |  | 4 years |  |  |  |  |  |  |  |  |  |  |
Grundartangi - HS, Landsvirkjun |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Underlying, derivative power, available | MW |  |  |  |  |  |  |  |  | 545 |  | 545 |  |
Grundartangi - Landsvirkjun |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Underlying, derivative power, available | MW |  |  |  |  |  |  |  |  |  |  |  | 25 |
Underlying, derivative power, available, extension | MW |  |  |  |  |  |  |  |  |  |  |  | 42 | 161
Underlying, derivative power, available, requested | MW |  |  |  |  |  |  |  |  |  |  |  |  | 182
Underlying, derivative power, rate | MW |  |  |  |  |  |  |  |  |  |  |  | 119 |
Subsequent Event | Mt. Holly |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term power agreement, supply percentage | 100.00% |  |  |  |  |  |  |  |  |  |  |  |

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