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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions     $ 150  
Pension Benefits        
Retirement Plan Disclosure        
Service cost $ 8 $ 7 15 $ 16
Interest cost 145 142 289 284
Expected return on plan assets 123 (123) 247 (246)
Amortization of prior service cost (23) (22) (45) (44)
Recognized net actuarial loss (25) 33 (50) 67
Total net periodic benefit cost as actuarially determined 32 37 62 77
Amount capitalized due to actions of regulator 0 20 0 (39)
Total net periodic benefit cost 32 57 62 116
Other Post-retirement Benefits        
Retirement Plan Disclosure        
Defined benefit plan contributions     11  
Service cost 2 3 5 6
Interest cost 6 6 11 11
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (5) (5) (9) (9)
Recognized net actuarial loss (1) 0 0 0
Total net periodic benefit cost as actuarially determined 4 4 7 8
Amount capitalized due to actions of regulator 0 0 0 0
Total net periodic benefit cost $ 4 $ 4 $ 7 $ 8