Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 32	$ 29	$ 32
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	(1)	0
Expected return on plan assets	505	494	492
Net periodic benefit cost as acutarially determined	137	124	232
Amount expensed due to actions of regulator	0	0	(77)
Total net period benefit cost	137	124	155
Actuarial loss (gain)	173	99	135
Defined Benefit Plan, Interest Cost	526	579	568
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(89)	(89)	(88)
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Service cost	11	11	10
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits	1	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0	0
Expected return on plan assets	0	0	0
Net periodic benefit cost as acutarially determined	11	14	9
Amount expensed due to actions of regulator	0	0	0
Total net period benefit cost	10	14	9
Actuarial loss (gain)	(1)	1	2
Defined Benefit Plan, Interest Cost	17	21	18
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	$ (17)	$ (17)	$ (17)