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Retirement and Postretirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Unrecognized prior service credit | Supplemental Executive Retirement Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | $ 0 | $ 0
Unrecognized actuarial (gain) loss | Supplemental Executive Retirement Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | 17,661 | 16,136
Total unrecognized in net periodic pension cost | Supplemental Executive Retirement Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | 17,661 | 16,136
Unrecognized prior service credit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (9,984) | (24,897)
Unrecognized prior service credit | Employee Pension Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | 0 | 0
Unrecognized prior service credit | Postretirement Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (9,984) | (24,897)
Unrecognized actuarial (gain) loss |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (278,380) | (222,353)
Unrecognized actuarial (gain) loss | Employee Pension Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (179,351) | (126,989)
Unrecognized actuarial (gain) loss | Postretirement Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (116,690) | (111,500)
Total unrecognized in net periodic pension cost | Employee Pension Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (179,351) | (126,989)
Total unrecognized in net periodic pension cost | Postretirement Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | (126,674) | (136,397)
Total unrecognized in net periodic pension cost | Pension and postretirement benefit costs |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Net regulatory assets | $ (288,364) | $ (247,250)

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