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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions     $ 150  
Pension Benefits        
Retirement Plan Disclosure        
Service cost $ 14 $ 15 28 $ 27
Interest cost 93 104 184 208
Expected return on plan assets 123 (122) 246 (244)
Amortization of prior service cost (25) (24) (49) (48)
Recognized net actuarial loss (115) 109 (227) 218
Total net periodic benefit cost as actuarially determined 74 82 144 161
Amount capitalized due to actions of regulator 2 (6) 9 8
Total net periodic benefit cost 76 76 153 153
Other Post-retirement Benefits        
Retirement Plan Disclosure        
Defined benefit plan contributions     13  
Service cost 4 4 9 8
Interest cost 4 4 8 8
Expected return on plan assets 0   0 0
Amortization of prior service cost (4) (6) (9) (12)
Recognized net actuarial loss (2) 3 (5) 5
Total net periodic benefit cost as actuarially determined 6 5 13 9
Amount capitalized due to actions of regulator 0   0 0
Total net periodic benefit cost $ 6 $ 5 $ 13 $ 9