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Commitments and Contingencies - Legal Proceedings (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 USD ($) Units | Sep. 30, 2024 USD ($)
Legal Proceedings |  |
Contribution related to TVARS case | $ 5 | $ 5
Amount remaining to be spent under environmental agreements | $ 7 |
Megawatts of Cumberland Combined Cycle Plant [Abstract] | Units | 1,450 |
General |  |
Legal Proceedings |  |
Legal loss contingency accrual | $ 11 |
Environmental Agreements |  |
Legal Proceedings |  |
Amount to be invested in certain environmental projects | 290 |
Amount invested in certain environmental projects | 285 |
Other long-term liabilities | General |  |
Legal Proceedings |  |
Legal loss contingency accrual | 9 |
Accounts payable and accrued liabilities | General |  |
Legal Proceedings |  |
Legal loss contingency accrual | $ 2 |

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