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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Pension Benefits      
Defined Benefit Plan Disclosure      
Service cost $ 130 $ 130 $ 154
Interest cost 540 558 468
Expected return on plan assets (437) (435) (428)
Amortization of prior service credit (21) (21) (22)
Recognized net actuarial loss 299 285 377
Net periodic benefit cost as acutarially determined 511 517 549
Amount capitalized due to actions of regulator (228) 0 0
Defined Benefit Plan, Net Periodic Benefit Cost 283 517 549
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Service cost 16 18 24
Interest cost 29 32 31
Expected return on plan assets 0 0 0
Amortization of prior service credit (6) (6) (6)
Recognized net actuarial loss 9 11 25
Net periodic benefit cost as acutarially determined 48 55 74
Amount capitalized due to actions of regulator 0 0 0
Defined Benefit Plan, Net Periodic Benefit Cost $ 48 $ 55 $ 74