Retirement and Postretirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021
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	14,029	39,666
Total unrecognized in net periodic pension cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	14,029	39,666
Unrecognized prior service credit
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(51,200)	(64,666)
Unrecognized prior service credit | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(121)	(353)
Unrecognized prior service credit | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(51,079)	(64,313)
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(105,657)	8,465
Unrecognized actuarial (gain) loss | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(32,159)	(3,060)
Unrecognized actuarial (gain) loss | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(87,527)	(28,141)
Total unrecognized in net periodic pension cost | Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(32,280)	(3,413)
Total unrecognized in net periodic pension cost | Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(138,606)	(92,454)
Total unrecognized in net periodic pension cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	$ (156,857)	$ (56,201)