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Benefit Plans Postretirement Benefit Costs Deferred as Regulatory Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2015
Sep. 30, 2014
Defined Benefit Plan Disclosure    
Regulatory assets $ 10,418 $ 8,994
Pension Benefits    
Defined Benefit Plan Disclosure    
Unrecognized prior service cost (credit) (158) (180)
Unrecognized net loss 5,355 4,337
Amount capitalized due to actions of regulator (228) 0
Regulatory assets 5,425 4,157
Other Post-retirement Benefits    
Defined Benefit Plan Disclosure    
Unrecognized prior service cost (credit) (33) (39)
Unrecognized net loss 173 179
Amount capitalized due to actions of regulator 0 0
Regulatory assets $ 140 $ 140