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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Pension Benefits      
Defined Benefit Plan Disclosure      
Service cost $ 29 $ 32 $ 53
DefinedBenefitPlanNetPeriodicBenefitCostCreditInterestCostStatementOfIncome 579 568 378
DefinedBenefitPlanNetPeriodicBenefitCostCreditExpectedReturnLossStatementOfIncome (494) (492) (435)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfPriorServiceCostCreditStatementOfIncome (89) (88) (93)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfGainLossStatementOfIncome 99 135 392
Net periodic benefit cost as acutarially determined 124 155 295
Amount expensed due to actions of regulator 0 (77) (13)
Total net period benefit cost 124 232 308
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Service cost 11 10 17
DefinedBenefitPlanNetPeriodicBenefitCostCreditInterestCostStatementOfIncome 21 18 15
Expected return on plan assets 0 0 0
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfPriorServiceCostCreditStatementOfIncome (17) (17) (17)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfGainLossStatementOfIncome (1) 2 (5)
Net periodic benefit cost as acutarially determined 14 9 20
Amount expensed due to actions of regulator 0 0 0
Total net period benefit cost $ 14 $ 9 $ 20