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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Pension Benefits      
Defined Benefit Plan Disclosure      
Service cost $ 57 $ 55 $ 44
Interest cost 368 415 499
Expected return on plan assets 493 488 477
Amortization of prior service credit (97) (97) (99)
Recognized net actuarial loss (452) (436) (336)
Net periodic benefit cost as acutarially determined 287 321 303
Amount expensed (capitalized) due to actions of regulator (19) 15 (1)
Total net period benefit cost 306 306 304
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Service cost 18 16 11
Interest cost 16 16 18
Expected return on plan assets 0 0 0
Amortization of prior service credit (18) (24) (24)
Recognized net actuarial loss (11) 10 4
Net periodic benefit cost as acutarially determined 27 18 9
Amount expensed (capitalized) due to actions of regulator 0 0 0
Total net period benefit cost $ 27 $ 18 $ 9