Retirement and Postretirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
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	12,314	14,029
Total unrecognized in net periodic pension cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	12,314	14,029
Unrecognized prior service credit
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(37,937)	(51,200)
Unrecognized prior service credit | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	0	(121)
Unrecognized prior service credit | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(37,937)	(51,079)
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(177,976)	(105,657)
Unrecognized actuarial (gain) loss | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(72,129)	(32,159)
Unrecognized actuarial (gain) loss | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(118,161)	(87,527)
Total unrecognized in net periodic pension cost | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(72,129)	(32,280)
Total unrecognized in net periodic pension cost | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(156,098)	(138,606)
Total unrecognized in net periodic pension cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	$ (215,913)	$ (156,857)