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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Pension Benefits      
Change in benefit obligation      
Benefit obligation $ 10,099 $ 10,536  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 11,002 10,099  
Service cost 29 32 $ 53
Plan participants' contributions 3 4  
Change in Plan Assets due to Collections 0 0  
Collections [1] 0 0  
Actuarial loss (gain) (1,059) (284)  
Net transfers from variable fund/401(k) plan 5 4  
Expenses paid (6) (7)  
Benefits paid 766 754  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 8,673 8,129  
Change in plan assets      
Fair value of net plan assets 8,129 8,094  
Actual return on plan assets 1,004 482  
Employer contributions 304 306  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (2,329) $ (1,970)  
Discount rate 4.95% 5.95% 5.60%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (981) $ (334)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (46) (12)  
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 37    
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 353 347  
Postconfirmation, Other Postretirement Obligations   388  
Service cost 11 10 $ 17
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections 12 13  
Collections [1] 12 13  
Actuarial loss (gain) 4 44  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 34 38  
Actuarial Gain (Loss) from Changes in Discount Rate 43    
Actuarial Gain (Loss) from pre-Medicare Trend Rate Assumptions   27  
Actuarial Gain (Loss) from Changes in Mortality Assumptions   17  
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 22 25  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (353) $ (347)  
Discount rate 5.00% 6.05% 565.00%
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience   $ 10  
Amount of defined benefit plan actuarial gain (loss) from assumption change in elections   $ 61  
[1] Collections include retiree contributions as well as provider discounts and rebates.