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Income Taxes - Narrative (Details) - USD ($) |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2022 | Jun. 30, 2022 | Sep. 30, 2022 | Sep. 30, 2032 | Sep. 30, 2024 | Sep. 30, 2022 | Oct. 01, 2024 | Sep. 30, 2023
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Taxes refundable to customers |  |  |  |  | $ 305,645,000 |  |  | $ 268,562,000
Recoverable future taxes |  |  |  |  | 80,084,000 |  |  | 69,045,000
Unrecognized tax benefits | $ 0 |  | $ 0 |  | 0 | $ 0 |  | 0
Sale of California Assets |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Adjustments to valuation allowance |  | $ (27,200,000) |  |  |  |  |  |
Pennsylvania Tax Rate Change |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Adjustments to valuation allowance |  |  | (5,500,000) |  |  |  |  |
Remaining State Deferred Tax Assets |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Adjustments to valuation allowance | $ (24,900,000) |  |  |  |  |  |  |
Deferred Income Taxes |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Taxes refundable to customers |  |  |  |  | 305,600,000 |  |  | 268,600,000
Recoverable future taxes |  |  |  |  | $ 80,100,000 |  |  | $ 69,000,000.0
Pennsylvania |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  | 8.99% |  |  |
Increase (decrease) in deferred income taxes |  |  |  |  |  | $ (28,400,000) |  |
Reduction in deferred taxes for rate regulated activities due to remeasurement of deferred income tax assets and liabilities |  |  | 37,200,000 |  |  |  |  |
Pennsylvania | Taxes Refundable to Customers |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Increase to taxes refundable to customers, impact of changes in corporate tax rate |  |  | 17,400,000 |  |  |  |  |
Pennsylvania | Recoverable Future Taxes |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Decrease to recoverable future taxes, impact of change in corporate tax rate |  |  | $ 19,800,000 |  |  |  |  |
Pennsylvania | Forecast |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  | 4.99% |  |  |  |
Annual reduction in corporate income tax Rate from fiscal 2025 to fiscal 2032 (as a percent) |  |  |  |  |  |  | 0.50% |

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