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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions     $ 150  
Pension Benefits        
Retirement Plan Disclosure        
Service cost $ 15 $ 10 27 $ 22
Interest cost 104 126 208 249
Expected return on plan assets 122 (120) 244 (239)
Amortization of prior service cost (24) (24) (48) (49)
Recognized net actuarial loss (109) 86 (218) 168
Total net periodic benefit cost as actuarially determined 82 78 161 151
Amount capitalized due to actions of regulator 6 (2) 8 1
Total net periodic benefit cost 76 76 153 152
Other Post-retirement Benefits        
Retirement Plan Disclosure        
Defined benefit plan contributions     16  
Service cost 4 3 8 6
Interest cost 4 5 8 9
Expected return on plan assets 0   0 0
Amortization of prior service cost (6) (6) (12) (12)
Recognized net actuarial loss (3) 1 (5) 2
Total net periodic benefit cost as actuarially determined 5 3 9 5
Amount capitalized due to actions of regulator 0   0 0
Total net periodic benefit cost $ 5 $ 3 $ 9 $ 5