Benefit Plans Components of Benefit Plans (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) plans	Jun. 30, 2020 USD ($)
Retirement Plan Disclosure
Number of unfunded post-retirement health care plans | plans			2
Pension Plan
Retirement Plan Disclosure
Service cost	$ 14	$ 14	$ 42	$ 41
Interest cost	92	104	276	312
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(123)	122	(369)	366
Amortization of prior service cost	(24)	(25)	(73)	(73)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	113	(109)	340	(327)
Net periodic benefit cost as acutarially determined	72	80	216	241
Amount capitalized due to actions of regulator	(5)	3	(14)	(11)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	77	77	230	230
Other Postretirement Benefit Plan
Retirement Plan Disclosure
Service cost	5	4	14	12
Interest cost	4	4	12	12
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0		0	0
Amortization of prior service cost	(5)	(6)	(14)	(18)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	3	(2)	8	(7)
Net periodic benefit cost as acutarially determined	7	4	20	13
Amount capitalized due to actions of regulator	0		0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 7	$ 4	$ 20	$ 13