Regulatory Matters - Narrative (Details) - USD ($) $ in Thousands						3 Months Ended	12 Months Ended			77 Months Ended
	Oct. 31, 2023	Aug. 01, 2023	Jul. 31, 2023	Oct. 28, 2022	Oct. 01, 2021	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2023
Regulatory Matters [Line Items]
Reduction of other post-retirement regulatory liability							$ 0	$ 18,533	$ 0
Supply Corporation
Regulatory Matters [Line Items]
Public utilities proposed annual cost of service			$ 385,400
Public utilities proposed rate base			$ 1,320,000
Proposed return on equity (as a percent)			15.12%
New York
Regulatory Matters [Line Items]
Approved return on equity (as a percent)										8.70%
Authorized annual recovery from customers for pension and OPEB expense										$ 15,000
New York | Subsequent Event [Member]
Regulatory Matters [Line Items]
Proposed base rate increase	$ 88,800
Proposed rate recovery of pension and OPEB costs	0
Savings reflected in new base delivery rates	$ 15,000
Pennsylvania
Regulatory Matters [Line Items]
Proposed base rate increase				$ 28,100
Public utilities authorized rate increase, amount		$ 23,000
Approved base rate reduction for OPEB expenses					$ 7,700
Proposed refund to customers from over-collection of OPEB expenses					$ 50,000
Reduction of other post-retirement regulatory liability						$ 18,500
Approved refund to customers from over-collection of OPEB expenses						$ 54,000