Benefit Plans Postretirement Benefit Costs Deferred as Regulatory Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Defined Benefit Plan Disclosure
Regulatory assets	$ (8,047)	$ (9,408)
Non-current regulatory liabilities	141	83
Pension Benefits
Defined Benefit Plan Disclosure
Unrecognized prior service cost (credit)	(158)	(247)
Unrecognized net loss	1,689	2,226
Regulatory assets	(1,531)	(1,979)
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Unrecognized prior service cost (credit)	(25)	(42)
Unrecognized net loss	(106)	(39)
Regulatory assets	(131)	$ (81)
Non-current regulatory liabilities	$ 131