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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Pension Benefits        
Defined Benefit Plan Disclosure        
Service cost $ 34 $ 32 $ 100 $ 97
Interest cost 141 135 423 405
Expected return on plan assets (112) (110) (335) (328)
Amortization of prior service cost (6) (5) (17) (16)
Recognized net actuarial loss 78 75 233 225
Total net periodic benefit cost as actuarially determined 135 127 404 383
Amount capitalized due to actions of regulator (64) (57) (193) (171)
Total net periodic benefit cost 71 70 211 212
Other Post-retirement Benefits        
Defined Benefit Plan Disclosure        
Service cost 4 4 12 12
Interest cost 7 7 22 22
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (2) (2) (5) (5)
Recognized net actuarial loss 2 3 5 7
Total net periodic benefit cost as actuarially determined 11 12 34 36
Amount capitalized due to actions of regulator 0 0 0 0
Total net periodic benefit cost $ 11 $ 12 $ 34 $ 36