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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Pension Benefits        
Defined Benefit Plan Disclosure        
Service cost   $ 53 $ 60 $ 133
Interest cost   473 464 564
Expected return on plan assets   478 (457) (446)
Amortization of prior service credit   (99) (99) (23)
Recognized net actuarial loss   (409) 472 310
Curtailments   0 0 (78)
Net periodic benefit cost as acutarially determined   358 440 460
Amount expensed (capitalized) due to actions of regulator   54 365 (179)
Total net period benefit cost   304 805 281
Other Post-retirement Benefits        
Defined Benefit Plan Disclosure        
Service cost   14 18 16
Interest cost   19 21 29
Expected return on plan assets   0 0 0
Amortization of prior service credit   (22) (22) (6)
Recognized net actuarial loss   (8) 14 7
Curtailments   0 0 0
Net periodic benefit cost as acutarially determined   19 31 46
Amount expensed (capitalized) due to actions of regulator   0 0 0
Total net period benefit cost   $ 19 $ 31 $ 46
Scenario, Forecast | Pension Benefits        
Defined Benefit Plan Disclosure        
Amount expensed (capitalized) due to actions of regulator $ (10)