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)