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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Change in benefit obligation      
Actuarial loss (gain)   $ (3)  
Change in plan assets      
Actual return on plan assets $ 88    
Amount related to experience loss 59    
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Amount related to change in number of participants 7    
Amount related to change in payment elections   46  
Pension Benefits      
Change in benefit obligation      
Benefit obligation 13,312 11,725  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 11,725 12,601  
Service cost 44 53 $ 60
Interest cost 499 473 $ 464
Plan participants' contributions 7 7  
Change in Plan Assets due to Collections [1] 0 0  
Collections [1] 0 0  
Actuarial loss (gain) 1,756 (658)  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 7 0  
Net transfers from variable fund/401(k) plan 1 (26)  
Expenses paid (6) (6)  
Benefits paid 721 719  
Increase (Decrease) in Obligation, Other Postretirement Benefits   110  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 7,980 8,003  
Change in plan assets      
Fair value of net plan assets 8,003 8,003  
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   7,989  
Actual return on plan assets 389 454  
Employer contributions 307 304  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (5,332) $ (3,722)  
Discount rate 3.20% 4.35% 3.85%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (1,600) $ (700)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (14) (138)  
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 499 428  
Postconfirmation, Other Postretirement Obligations 428 494  
Service cost 11 14 $ 18
Interest cost 18 19 $ 21
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections [1] 22 25  
Collections [1] 22 25  
Actuarial loss (gain) 78 (46)  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 0 (17)  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 58 61  
Increase (Decrease) in Obligation, Other Postretirement Benefits   17  
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 36 36  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (499) $ (428)  
Discount rate 3.30% 4.40%  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ (24) $ (8)  
Amount of change related to actual 7 23  
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (71) (28)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption   $ (6)  
Defined Benefit Plan, Assumptions Used in Calculation, Description 0.033 0.044 0.0395
Amount of defined benefit plan actuarial gain (loss) from change in health care trend rate assumptions $ 24    
[1] Collections include retiree contributions as well as provider discounts and rebates.