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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Amount related to change in number of participants $ 7    
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 13,312 11,725  
Service cost 55 44 $ 53
Interest cost 415 499 $ 473
Plan participants' contributions 6 7  
Change in Plan Assets due to Collections [1] 0 0  
Collections [1] 0 0  
Actuarial loss (gain) 614 1,756  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 2 7  
Net transfers from variable fund/401(k) plan 2 1  
Expenses paid (5) (6)  
Benefits paid 726 721  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 7,959 7,980  
Change in plan assets      
Fair value of net plan assets 7,980 7,980  
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   8,003  
Actual return on plan assets 397 389  
Employer contributions 305 307  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (5,716) $ (5,332)  
Discount rate 2.75% 3.20% 4.35%
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 (714) $ (1,600)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (137) (14)  
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 74 147  
Amount of defined benefit plan actuarial gain (loss) from assumption change in elections 32    
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 544 499  
Postconfirmation, Other Postretirement Obligations 499 428  
Service cost 16 11 $ 14
Interest cost 16 18 $ 19
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections [1] 20 22  
Collections [1] 20 22  
Actuarial loss (gain) 39 78  
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 46 58  
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 26 36  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (544) $ (499)  
Discount rate 3.05% 3.30%  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ (30) $ (24)  
Amount of change related to actual 4 7  
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (20) $ (71)  
Defined Benefit Plan, Assumptions Used in Calculation, Description 3.05 3.30 percent 4.40
Amount of defined benefit plan actuarial gain (loss) from change in health care trend rate assumptions $ 15 $ 24  
[1] Collections include retiree contributions as well as provider discounts and rebates.