Retirement and Postretirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Sep. 30, 2023
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	16,136	12,314
Total unrecognized in net periodic pension cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	16,136	12,314
Unrecognized prior service credit
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(24,897)	(37,937)
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	(24,897)	(37,937)
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(222,353)	(177,976)
Unrecognized actuarial (gain) loss | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(126,989)	(72,129)
Unrecognized actuarial (gain) loss | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(111,500)	(118,161)
Total unrecognized in net periodic pension cost | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(126,989)	(72,129)
Total unrecognized in net periodic pension cost | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(136,397)	(156,098)
Total unrecognized in net periodic pension cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	$ (247,250)	$ (215,913)