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Benefit Plans Postretirement Benefit Costs Deferred as Regulatory Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2016
Sep. 30, 2015
Defined Benefit Plan Disclosure    
Regulatory assets $ 10,164 $ 10,418
Pension Benefits    
Defined Benefit Plan Disclosure    
Unrecognized prior service cost (credit) (1,017) (158)
Unrecognized net loss 5,946 5,355
Amount capitalized due to actions of regulator (407) (228)
Regulatory assets 5,336 5,425
Other Post-retirement Benefits    
Defined Benefit Plan Disclosure    
Unrecognized prior service cost (credit) (185) (33)
Unrecognized net loss 234 173
Amount capitalized due to actions of regulator 0 0
Regulatory assets $ 49 $ 140