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Income Taxes - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Income Tax Contingency [Line Items] |  |  |
Charitable contribution carryforwards | $ 11,800 |  |
Remeasured federal effects | 600 |  |
Unrecognized tax benefits, period increase | 60,300 | $ 57,800 |
Unrecognized tax benefits, deferred tax liabilities | 10,100 | 11,600 |
Interest and penalties | 100 | 100 | $ 3,400
Payment of interest and penalties accrued | 15,100 | 15,100 | $ 15,100
Regulatory liabilities | 1,269,648 | 1,227,882 |
Regulatory excess net deferred tax liability | 140,500 |  |
Deferred tax liabilities, net | 2,918,347 | 2,593,342 |
Regulatory excess deferred taxes |  |  |
Income Tax Contingency [Line Items] |  |  |
Regulatory liabilities | 190,274 | 257,001 |
Regulatory excess deferred taxes to be returned tranche one |  |  |
Income Tax Contingency [Line Items] |  |  |
Deferred tax liabilities, net | $ 91,800 |  |
Regulatory excess deferred taxes to be returned tranche one | Minimum |  |  |
Income Tax Contingency [Line Items] |  |  |
Return basis, term | 36 months |  |
Regulatory excess deferred taxes to be returned tranche one | Maximum |  |  |
Income Tax Contingency [Line Items] |  |  |
Return basis, term | 60 months |  |
Regulatory excess deferred taxes to be returned tranche two |  |  |
Income Tax Contingency [Line Items] |  |  |
Regulatory liabilities | $ 47,700 |  |
Regulatory excess deferred taxes to be returned tranche two | Minimum |  |  |
Income Tax Contingency [Line Items] |  |  |
Return basis, term | 15 years |  |
Regulatory excess deferred taxes to be returned tranche two | Maximum |  |  |
Income Tax Contingency [Line Items] |  |  |
Return basis, term | 69 years |  |
Regulatory excess deferred taxes to be returned tranche three |  |  |
Income Tax Contingency [Line Items] |  |  |
Deferred tax liabilities, net | $ 1,000 |  |
Other current liabilities | Regulatory excess deferred taxes |  |  |
Income Tax Contingency [Line Items] |  |  |
Regulatory liabilities | 72,800 | $ 79,700 |
Domestic Tax Jurisdiction |  |  |
Income Tax Contingency [Line Items] |  |  |
Federal net operating loss carryforwards | 435,300 |  |
State and Local Jurisdiction |  |  |
Income Tax Contingency [Line Items] |  |  |
Federal net operating loss carryforwards | 38,300 |  |
Federal effects | 10,100 |  |
Federal alternative minimum tax credit carryforwards | $ 2,400 |  |

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