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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Pension Benefits      
Change in benefit obligation      
Benefit obligation $ 13,675 $ 13,312  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 13,348 13,675  
Service cost 57 55 $ 44
Interest cost 368 415 $ 499
Plan participants' contributions 5 6  
Change in Plan Assets due to Collections 0 0  
Collections [1] 0 0  
Actuarial loss (gain) (25) 614  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 0 2  
Net transfers from variable fund/401(k) plan 2 2  
Expenses paid (6) (5)  
Benefits paid 728 726  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 9,110 7,959  
Change in plan assets      
Fair value of net plan assets 7,959 7,980  
Actual return on plan assets 1,572 397  
Employer contributions 306 305  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (4,238) $ (5,716)  
Discount rate 2.90% 2.75% 3.20%
Amount of defined benefit plan actuarial gain (loss) from change in COLA   $ 69  
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (248) (714)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (28) (137)  
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 104 74  
Amount of defined benefit plan actuarial gain (loss) from assumption change in elections 91 32  
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 498 544  
Postconfirmation, Other Postretirement Obligations 544 499  
Service cost 18 16 $ 11
Interest cost 16 16 $ 18
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections 17 20  
Collections [1] 17 20  
Actuarial loss (gain) (53) 39  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 0 0  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 44 46  
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 27 26  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (498) $ (544)  
Discount rate 3.05% 3.05%  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ (47) $ (30)  
Amount of change related to actual $ 1 4  
Defined Benefit Plan, Assumptions Used in Calculation, Description     3.30 percent
Amount of defined benefit plan actuarial gain (loss) from change in health care trend rate assumptions   $ 15  
[1] Collections include retiree contributions as well as provider discounts and rebates.