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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Pension Benefits      
Defined Benefit Plan Disclosure      
Service cost $ 32 $ 53 $ 57
DefinedBenefitPlanNetPeriodicBenefitCostCreditInterestCostStatementOfIncome 568 378 368
DefinedBenefitPlanNetPeriodicBenefitCostCreditExpectedReturnLossStatementOfIncome (492) (435) (493)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfPriorServiceCostCreditStatementOfIncome (88) (93) (97)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfGainLossStatementOfIncome 135 392 452
Net periodic benefit cost as acutarially determined 155 295 287
Amount expensed due to actions of regulator (77) (13) (19)
Total net period benefit cost 232 308 306
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Service cost 10 17 18
DefinedBenefitPlanNetPeriodicBenefitCostCreditInterestCostStatementOfIncome 18 15 16
Expected return on plan assets 0 0 0
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfPriorServiceCostCreditStatementOfIncome (17) (17) (18)
DefinedBenefitPlanNetPeriodicBenefitCostCreditAmortizationOfGainLossStatementOfIncome (2) (5) (11)
Net periodic benefit cost as acutarially determined 9 20 27
Amount expensed due to actions of regulator 0 0 0
Total net period benefit cost $ 9 $ 20 $ 27