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, 2024	Sep. 30, 2023	Sep. 30, 2022
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 69,500	$ 77,200
Benefit Obligation at End of Period		66,000	69,500	$ 77,200
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Assets		180,230	200,301
Accumulated Benefit Obligation		$ 57,500	$ 58,500	$ 64,900
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		4.71%	5.91%	5.49%
Rate of compensation increase (as a percent)		8.00%	8.00%	8.00%
Components of Net Periodic Benefit Cost
Net Periodic Benefit Cost (Income)		$ 9,500	$ 8,300	$ 8,900
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		$ 768,750	$ 813,828	$ 1,098,456
Service Cost		4,197	5,187	8,758
Interest Cost		43,558	42,516	22,827
Plan Participants’ Contributions		0	0	0
Retiree Drug Subsidy Receipts		0	0	0
Actuarial (Gain) Loss		72,016	(27,313)	(251,173)
Benefits Paid		(66,215)	(65,468)	(65,040)
Benefit Obligation at End of Period		822,306	768,750	813,828
Change in Plan Assets
Fair Value of Assets at Beginning of Period		784,712	845,205	1,095,729
Actual Return on Plan Assets		105,488	4,975	(205,884)
Employer Contributions		0	0	20,400
Plan Participants’ Contributions		0	0	0
Benefits Paid		(66,215)	(65,468)	(65,040)
Fair Value of Assets at End of Period		823,985	784,712	845,205
Net Amount Recognized at End of Period (Funded Status)		1,679	15,962	31,377
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		0	0	0
Non-Current Assets		1,679	15,962	31,377
Net Amount Recognized at End of Period		1,679	15,962	31,377
Accumulated Benefit Obligation		$ 804,916	$ 751,912	$ 793,555
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		4.97%	5.99%	5.57%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.60%
Components of Net Periodic Benefit Cost
Service Cost		$ 4,197	$ 5,187	$ 8,758
Interest Cost		43,558	42,516	22,827
Expected Return on Plan Assets		(68,343)	(66,593)	(52,294)
Amortization of Prior Service Cost (Credit)		362	436	537
Recognition of Actuarial (Gain) Loss	[1]	(1,339)	(7,680)	26,405
Net Amortization and Deferral for Regulatory Purposes		16,231	21,512	16,854
Net Periodic Benefit Cost (Income)		$ (5,334)	$ (4,622)	$ 23,087
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		5.99%	5.57%	2.75%
Effective Rate for Interest on Benefit Obligations (as a percent)		5.93%	5.45%	2.14%
Effective Discount Rate for Service Cost (as a percent)		6.01%	5.49%	2.95%
Effective Rate for Interest on Service Cost (as a percent)		5.98%	5.53%	2.70%
Expected Return on Plan Assets (as a percent)		7.40%	6.90%	5.20%
Rate of Compensation Increase (as a percent)		4.60%	4.60%	4.70%
Other Post-Retirement Benefits
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 274,278	$ 299,283	$ 431,213
Service Cost		434	587	1,328
Interest Cost		15,561	15,648	9,066
Plan Participants’ Contributions		3,401	3,297	3,271
Retiree Drug Subsidy Receipts		1,208	2,969	312
Actuarial (Gain) Loss		54,443	(20,789)	(120,276)
Benefits Paid		(28,300)	(26,717)	(25,631)
Benefit Obligation at End of Period		321,025	274,278	299,283
Change in Plan Assets
Fair Value of Assets at Beginning of Period		455,702	461,438	575,565
Actual Return on Plan Assets		64,783	17,449	(94,849)
Employer Contributions		479	235	3,082
Plan Participants’ Contributions		3,401	3,297	3,271
Benefits Paid		(28,300)	(26,717)	(25,631)
Fair Value of Assets at End of Period		496,065	455,702	461,438
Net Amount Recognized at End of Period (Funded Status)		175,040	181,424	162,155
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		(3,511)	(2,915)	(3,065)
Non-Current Assets		178,551	184,339	165,220
Net Amount Recognized at End of Period		$ 175,040	$ 181,424	$ 162,155
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		4.98%	5.99%	5.56%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.60%
Components of Net Periodic Benefit Cost
Service Cost		$ 434	$ 587	$ 1,328
Interest Cost		15,561	15,648	9,066
Expected Return on Plan Assets		(26,642)	(25,612)	(29,359)
Amortization of Prior Service Cost (Credit)		(429)	(429)	(429)
Recognition of Actuarial (Gain) Loss	[1]	(2,266)	(8,755)	(7,610)
Net Amortization and Deferral for Regulatory Purposes		8,759	15,157	21,340
Net Periodic Benefit Cost (Income)		$ (4,583)	$ (3,404)	$ (5,664)
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		5.99%	5.56%	2.76%
Effective Rate for Interest on Benefit Obligations (as a percent)		5.93%	5.45%	2.17%
Effective Discount Rate for Service Cost (as a percent)		5.99%	5.35%	3.00%
Effective Rate for Interest on Service Cost (as a percent)		6.01%	5.47%	2.93%
Expected Return on Plan Assets (as a percent)		6.00%	5.70%	5.20%
Rate of Compensation Increase (as a percent)		4.60%	4.60%	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.