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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits    
Defined Benefit Plan Disclosure    
Service cost $ 17 $ 32
Interest cost 116 140
Expected return on plan assets (114) (111)
Amortization of prior service cost (25) (6)
Recognized net actuarial loss 116 73
Total net periodic benefit cost as actuarially determined 110 128
Amount capitalized due to actions of regulator (34) (58)
Total net periodic benefit cost 76 70
Other Post-retirement Benefits    
Defined Benefit Plan Disclosure    
Service cost 5 4
Interest cost 5 7
Expected return on plan assets 0 0
Amortization of prior service cost (6) (1)
Recognized net actuarial loss 3 2
Total net periodic benefit cost as actuarially determined 7 12
Amount capitalized due to actions of regulator 0 0
Total net periodic benefit cost $ 7 $ 12