Retirement Plan and Other Post-Retirement Benefits - Schedule of Benefit Obligations, Plan Assets and Funded Status (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 77,200	$ 95,800
Benefit Obligation at End of Period		69,500	77,200	$ 95,800
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Assets		200,301	196,597
Accumulated Benefit Obligation		$ 58,500	$ 64,900	$ 76,900
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		5.91%	5.49%	2.15%
Rate of compensation increase (as a percent)		8.00%	8.00%	8.00%
Components of Net Periodic Benefit Cost
Net Periodic Benefit Cost (Income)		$ 8,300	$ 8,900	$ 8,300
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]		Other Income (Deductions)	Other Income (Deductions)	Other Income (Deductions)
Amortization period		10 years
Retirement Plan
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 813,828	$ 1,098,456	$ 1,139,105
Service Cost		5,187	8,758	9,865
Interest Cost		42,516	22,827	21,686
Plan Participants’ Contributions		0	0	0
Retiree Drug Subsidy Receipts		0	0	0
Actuarial Gain		(27,313)	(251,173)	(8,141)
Benefits Paid		(65,468)	(65,040)	(64,059)
Benefit Obligation at End of Period		768,750	813,828	1,098,456
Change in Plan Assets
Fair Value of Assets at Beginning of Period		845,205	1,095,729	1,016,796
Actual Return on Plan Assets		4,975	(205,884)	122,992
Employer Contributions		0	20,400	20,000
Plan Participants’ Contributions		0	0	0
Benefits Paid		(65,468)	(65,040)	(64,059)
Fair Value of Assets at End of Period		784,712	845,205	1,095,729
Net Amount Recognized at End of Period (Funded Status)		15,962	31,377	(2,727)
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		0	0	(2,727)
Non-Current Assets		15,962	31,377	0
Net Amount Recognized at End of Period		15,962	31,377	(2,727)
Accumulated Benefit Obligation		$ 751,912	$ 793,555	$ 1,060,659
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		5.99%	5.57%	2.75%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.70%
Components of Net Periodic Benefit Cost
Service Cost		$ 5,187	$ 8,758	$ 9,865
Interest Cost		42,516	22,827	21,686
Expected Return on Plan Assets		(66,593)	(52,294)	(58,148)
Amortization of Prior Service Cost (Credit)		436	537	631
Recognition of Actuarial (Gain) Loss	[1]	(7,680)	26,405	36,814
Net Amortization and Deferral for Regulatory Purposes		21,512	16,854	14,063
Net Periodic Benefit Cost (Income)		$ (4,622)	$ 23,087	$ 24,911
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		5.57%	2.75%	2.66%
Effective Rate for Interest on Benefit Obligations (as a percent)		5.45%	2.14%	1.96%
Effective Discount Rate for Service Cost (as a percent)		5.49%	2.95%	3.01%
Effective Rate for Interest on Service Cost (as a percent)		5.53%	2.70%	2.60%
Expected Return on Plan Assets (as a percent)		6.90%	5.20%	6.00%
Rate of Compensation Increase (as a percent)		4.60%	4.70%	4.70%
Other Post-Retirement Benefits
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 299,283	$ 431,213	$ 476,722
Service Cost		587	1,328	1,602
Interest Cost		15,648	9,066	9,303
Plan Participants’ Contributions		3,297	3,271	3,216
Retiree Drug Subsidy Receipts		2,969	312	1,244
Actuarial Gain		(20,789)	(120,276)	(34,729)
Benefits Paid		(26,717)	(25,631)	(26,145)
Benefit Obligation at End of Period		274,278	299,283	431,213
Change in Plan Assets
Fair Value of Assets at Beginning of Period		461,438	575,565	547,885
Actual Return on Plan Assets		17,449	(94,849)	47,541
Employer Contributions		235	3,082	3,068
Plan Participants’ Contributions		3,297	3,271	3,216
Benefits Paid		(26,717)	(25,631)	(26,145)
Fair Value of Assets at End of Period		455,702	461,438	575,565
Net Amount Recognized at End of Period (Funded Status)		181,424	162,155	144,352
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		(2,915)	(3,065)	(4,799)
Non-Current Assets		184,339	165,220	149,151
Net Amount Recognized at End of Period		$ 181,424	$ 162,155	$ 144,352
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		5.99%	5.56%	2.76%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.70%
Components of Net Periodic Benefit Cost
Service Cost		$ 587	$ 1,328	$ 1,602
Interest Cost		15,648	9,066	9,303
Expected Return on Plan Assets		(25,612)	(29,359)	(28,964)
Amortization of Prior Service Cost (Credit)		(429)	(429)	(429)
Recognition of Actuarial (Gain) Loss	[1]	(8,755)	(7,610)	849
Net Amortization and Deferral for Regulatory Purposes		15,157	21,340	28,010
Net Periodic Benefit Cost (Income)		$ (3,404)	$ (5,664)	$ 10,371
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		5.56%	2.76%	2.71%
Effective Rate for Interest on Benefit Obligations (as a percent)		5.45%	2.17%	2.01%
Effective Discount Rate for Service Cost (as a percent)		5.35%	3.00%	3.20%
Effective Rate for Interest on Service Cost (as a percent)		5.47%	2.93%	2.98%
Expected Return on Plan Assets (as a percent)		5.70%	5.20%	5.40%
Rate of Compensation Increase (as a percent)		4.60%	4.70%	4.70%

[1] Distribution Corporation’s New York jurisdiction calculates the amortization of the actuarial loss on a vintage year basis over 10 years, as mandated by the NYPSC. All the other subsidiaries of the Company utilize the corridor approach.