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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Pension Benefits      
Defined Benefit Plan Disclosure      
Service cost $ 55 $ 44 $ 53
Interest cost 415 499 473
Expected return on plan assets 488 477 478
Amortization of prior service credit (97) (99) (99)
Recognized net actuarial loss (436) (336) (409)
Net periodic benefit cost as acutarially determined 321 303 358
Amount expensed (capitalized) due to actions of regulator 15 (1) 54
Total net period benefit cost 306 304 304
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Service cost 16 11 14
Interest cost 16 18 19
Expected return on plan assets 0 0 0
Amortization of prior service credit (24) (24) (22)
Recognized net actuarial loss (10) 4 8
Net periodic benefit cost as acutarially determined 18 9 19
Amount expensed (capitalized) due to actions of regulator 0 0 0
Total net period benefit cost $ 18 $ 9 $ 19