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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2025
Mar. 31, 2024
Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions     $ 150  
Pension Benefits        
Retirement Plan Disclosure        
Service cost $ 8 $ 8 16 $ 15
Interest cost 132 145 263 289
Expected return on plan assets 127 (123) 253 (247)
Amortization of prior service cost (23) (23) (45) (45)
Recognized net actuarial loss (45) 25 (87) 50
Total net periodic benefit cost as actuarially determined 35 32 68 62
Other Post-retirement Benefits        
Retirement Plan Disclosure        
Defined benefit plan contributions     13  
Service cost 3 2 6 5
Interest cost 5 6 9 11
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (5) (5) (9) (9)
Recognized net actuarial loss 0 1 0 0
Total net periodic benefit cost as actuarially determined $ 3 $ 4 $ 6 $ 7