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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Pension Benefits        
Defined Benefit Plan Disclosure        
Service cost   $ 133 $ 130 $ 130
Interest cost   564 540 558
Expected return on plan assets   (446) (437) (435)
Amortization of prior service credit   (23) (21) (21)
Recognized net actuarial loss   310 299 285
Curtailments   (78) 0 0
Net periodic benefit cost as acutarially determined   460 511 517
Amount capitalized due to actions of regulator   (179) (228) 0
Total net period benefit cost   281 283 517
Other Post-retirement Benefits        
Defined Benefit Plan Disclosure        
Service cost   16 16 18
Interest cost   29 29 32
Expected return on plan assets   0 0 0
Amortization of prior service credit   (6) (6) (6)
Recognized net actuarial loss   7 9 11
Curtailments   0 0 0
Net periodic benefit cost as acutarially determined   46 48 55
Amount capitalized due to actions of regulator   0 0 0
Total net period benefit cost   $ 46 $ 48 $ 55
Scenario, Forecast | Pension Benefits        
Defined Benefit Plan Disclosure        
Amount capitalized due to actions of regulator $ (136)