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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits    
Defined Benefit Plan Disclosure    
Service cost $ 32 $ 34
Interest cost 140 133
Expected return on plan assets (111) (109)
Amortization of prior service cost (6) (5)
Recognized net actuarial loss 73 69
Total net periodic benefit cost as actuarially determined 128 122
Amount capitalized due to actions of regulator (58) (51)
Total net periodic benefit cost 70 71
Other Post-retirement Benefits    
Defined Benefit Plan Disclosure    
Service cost 4 5
Interest cost 7 8
Expected return on plan assets 0 0
Amortization of prior service cost (1) (2)
Recognized net actuarial loss 2 2
Total net periodic benefit cost as actuarially determined 12 13
Amount capitalized due to actions of regulator 0 0
Total net periodic benefit cost $ 12 $ 13