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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Pension Benefits      
Change in benefit obligation      
Benefit obligation $ 10,536 $ 13,348  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 10,099 10,536  
Service cost 32 53 $ 57
Plan participants' contributions 4 5  
Change in Plan Assets due to Collections 0 0  
Collections [1] 0 0  
Actuarial loss (gain) 284 (2,509)  
Net transfers from variable fund/401(k) plan 4 11  
Expenses paid (7) (6)  
Benefits paid 754 744  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 8,129 8,094  
Change in plan assets      
Fair value of net plan assets 8,094 9,110  
Actual return on plan assets 482 (590)  
Employer contributions 306 308  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (1,970) $ (2,442)  
Discount rate 5.95% 5.60% 2.90%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (334) $ (3,500)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (61) (78)  
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 12 527  
Amount of defined benefit plan actuarial gain (loss) from assumption change in elections   411  
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 347 388  
Postconfirmation, Other Postretirement Obligations 388 498  
Service cost 10 17 $ 18
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections 13 15  
Collections [1] 13 15  
Actuarial loss (gain) 44 114  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 38 43  
Actuarial Gain (Loss) from Changes in Discount Rate 17 168  
Actuarial Gain (Loss) Reflecting Plan Experience for Contribution Costs 27    
Actuarial Gain (Loss) from Delaying post-Medicare Trend Rate Assumption   31  
Actuarial Gain (Loss) from pre-Medicare Trend Rate Assumptions   67  
Actuarial Gain (Loss) from Changes in Mortality Assumptions   18  
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 25 28  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (347) $ (388)  
Discount rate 6.05% 5.65% 305.00%
[1] Collections include retiree contributions as well as provider discounts and rebates.