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, 2025	Sep. 30, 2024	Sep. 30, 2023
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 66,000	$ 69,500
Benefit Obligation at End of Period		58,400	66,000	$ 69,500
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Assets		169,228	180,230
Accumulated Benefit Obligation		$ 50,900	$ 57,500	$ 58,500
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		4.86%	4.71%	5.91%
Rate of compensation increase (as a percent)		8.00%	8.00%	8.00%
Components of Net Periodic Benefit Cost
Net Periodic Benefit Income		$ 4,600	$ 9,500	$ 8,300
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Amortization period		10 years
Retirement Plan
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 822,306	768,750	813,828
Service Cost		4,092	4,197	5,187
Interest Cost		36,892	43,558	42,516
Plan Participants’ Contributions		0	0	0
Retiree Drug Subsidy Receipts		0	0	0
Actuarial (Gain) Loss		(17,232)	72,016	(27,313)
Benefits Paid		(66,395)	(66,215)	(65,468)
Benefit Obligation at End of Period		779,663	822,306	768,750
Change in Plan Assets
Fair Value of Assets at Beginning of Period		823,985	784,712	845,205
Actual Return on Plan Assets		20,527	105,488	4,975
Employer Contributions		0	0	0
Plan Participants’ Contributions		0	0	0
Benefits Paid		(66,395)	(66,215)	(65,468)
Fair Value of Assets at End of Period		778,117	823,985	784,712
Net Amount Recognized at End of Period (Funded Status)		(1,546)	1,679	15,962
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		(1,546)	0	0
Non-Current Assets		0	1,679	15,962
Net Amount Recognized at End of Period		(1,546)	1,679	15,962
Accumulated Benefit Obligation		$ 765,747	$ 804,916	$ 751,912
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		5.28%	4.97%	5.99%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.60%
Components of Net Periodic Benefit Cost
Service Cost		$ 4,092	$ 4,197	$ 5,187
Interest Cost		36,892	43,558	42,516
Expected Return on Plan Assets		(58,587)	(68,343)	(66,593)
Amortization of Prior Service Cost (Credit)		302	362	436
Recognition of Actuarial (Gain) Loss	[1]	6,481	(1,339)	(7,680)
Net Amortization and Deferral for Regulatory Purposes		(1,375)	16,231	21,512
Net Periodic Benefit Income		$ (12,195)	$ (5,334)	$ (4,622)
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		4.97%	5.99%	5.57%
Effective Rate for Interest on Benefit Obligations (as a percent)		4.68%	5.93%	5.45%
Effective Discount Rate for Service Cost (as a percent)		5.14%	6.01%	5.49%
Effective Rate for Interest on Service Cost (as a percent)		4.96%	5.98%	5.53%
Expected Return on Plan Assets (as a percent)		6.60%	7.40%	6.90%
Rate of Compensation Increase (as a percent)		4.60%	4.60%	4.60%
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)
Other Post-Retirement Benefits
Change in Benefit Obligation
Benefit Obligation at Beginning of Period		$ 321,025	$ 274,278	$ 299,283
Service Cost		519	434	587
Interest Cost		14,501	15,561	15,648
Plan Participants’ Contributions		3,597	3,401	3,297
Retiree Drug Subsidy Receipts		1,514	1,208	2,969
Actuarial (Gain) Loss		17,072	54,443	(20,789)
Benefits Paid		(27,644)	(28,300)	(26,717)
Benefit Obligation at End of Period		330,584	321,025	274,278
Change in Plan Assets
Fair Value of Assets at Beginning of Period		496,065	455,702	461,438
Actual Return on Plan Assets		23,494	64,783	17,449
Employer Contributions		594	479	235
Plan Participants’ Contributions		3,597	3,401	3,297
Benefits Paid		(27,644)	(28,300)	(26,717)
Fair Value of Assets at End of Period		496,106	496,065	455,702
Net Amount Recognized at End of Period (Funded Status)		165,522	175,040	181,424
Amounts Recognized in the Balance Sheets Consist of:
Non-Current Liabilities		(3,706)	(3,511)	(2,915)
Non-Current Assets		169,228	178,551	184,339
Net Amount Recognized at End of Period		$ 165,522	$ 175,040	$ 181,424
Weighted Average Assumptions Used to Determine Benefit Obligation
Discount rate (as a percent)		5.30%	4.98%	5.99%
Rate of compensation increase (as a percent)		4.60%	4.60%	4.60%
Components of Net Periodic Benefit Cost
Service Cost		$ 519	$ 434	$ 587
Interest Cost		14,501	15,561	15,648
Expected Return on Plan Assets		(26,144)	(26,642)	(25,612)
Amortization of Prior Service Cost (Credit)		(429)	(429)	(429)
Recognition of Actuarial (Gain) Loss	[1]	37	(2,266)	(8,755)
Net Amortization and Deferral for Regulatory Purposes		(3,857)	8,759	15,157
Net Periodic Benefit Income		$ (15,373)	$ (4,583)	$ (3,404)
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost
Effective Discount Rate for Benefit Obligations (as a percent)		4.98%	5.99%	5.56%
Effective Rate for Interest on Benefit Obligations (as a percent)		4.69%	5.93%	5.45%
Effective Discount Rate for Service Cost (as a percent)		5.21%	5.99%	5.35%
Effective Rate for Interest on Service Cost (as a percent)		5.14%	6.01%	5.47%
Expected Return on Plan Assets (as a percent)		5.40%	6.00%	5.70%
Rate of Compensation Increase (as a percent)		4.60%	4.60%	4.60%
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)

[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.