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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
Sep. 30, 2017
Pension Benefits        
Defined Benefit Plan Disclosure        
Service cost   $ 44 $ 53 $ 60
Interest cost   499 473 464
Expected return on plan assets   477 478 457
Amortization of prior service credit   (99) (99) (99)
Recognized net actuarial loss   (336) (409) (472)
Net periodic benefit cost as acutarially determined   303 358 440
Amount expensed (capitalized) due to actions of regulator   (1) 54 (365)
Total net period benefit cost   304 304 805
Other Post-retirement Benefits        
Defined Benefit Plan Disclosure        
Service cost   11 14 18
Interest cost   18 19 21
Expected return on plan assets   0 0 0
Amortization of prior service credit   (24) (22) (22)
Recognized net actuarial loss   (4) 8 14
Net periodic benefit cost as acutarially determined   9 19 31
Amount expensed (capitalized) due to actions of regulator   0 0 0
Total net period benefit cost   $ 9 $ 19 $ 31
Scenario, Forecast | Pension Benefits        
Defined Benefit Plan Disclosure        
Amount expensed (capitalized) due to actions of regulator $ (13)