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Benefit Plans Obligations and Funded Status (Details) - USD ($)
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Change in benefit obligation      
Actuarial loss (gain) $ (3,000,000)    
Change in plan assets      
Employer contributions   $ 500,000,000  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Discount rate   3.70%  
Amount related to change in payment elections 46,000,000    
Pension Benefits      
Change in benefit obligation      
Benefit obligation 11,725,000,000 $ 12,601,000,000  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 12,601,000,000 13,083,000,000  
Service cost 53,000,000 60,000,000 $ 133,000,000
Interest cost 473,000,000 464,000,000 564,000,000
Plan participants’ contributions 7,000,000 9,000,000  
Change in Plan Assets due to Collections [1] 0 0  
Collections [1] 0 0  
Actuarial loss (gain) (658,000,000) (286,000,000)  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 0 0  
Net transfers from variable fund/401(k) plan (26,000,000) (12,000,000)  
Expenses paid (6,000,000) (5,000,000)  
Benefits paid 719,000,000 712,000,000  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 8,003,000,000 7,989,000,000  
Change in plan assets      
Fair value of net plan assets 7,989,000,000 7,989,000,000  
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   7,145,000,000  
Actual return on plan assets 454,000,000 759,000,000  
Employer contributions 304,000,000 805,000,000  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (3,722,000,000) $ (4,612,000,000)  
Discount rate 4.35% 3.85%  
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (676,000,000) $ (3.85) (3.65)
Increase (Decrease) in Obligation, Pension Benefits   292,000,000  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (138,000,000) $ (117,000,000)  
Defined Benefit Plan, Assumptions Used in Calculation, Description   119  
Amount related to change in payment elections 110,000,000    
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 428,000,000 $ 494,000,000  
Postconfirmation, Other Postretirement Obligations 494,000,000 571,000,000  
Service cost 14,000,000 18,000,000 16,000,000
Interest cost 19,000,000 21,000,000 $ 29,000,000
Plan participants’ contributions 0 0  
Change in Plan Assets due to Collections [1] 25,000,000 47,000,000  
Collections [1] 25,000,000 47,000,000  
Actuarial loss (gain) (28,000,000) (80,000,000)  
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit 46,000,000    
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment (17,000,000) 0  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 61,000,000 83,000,000  
Increase (Decrease) in Obligation, Other Postretirement Benefits 17,000,000    
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions [2] 36,000,000 36,000,000  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (428,000,000) $ (494,000,000)  
Discount rate 4.40% 3.95%  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ (8,000,000) $ (66,000,000)  
Amount of defined benefit plan actuarial gain (loss) from discount rate change (4.40) (18,000,000)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption $ (6,000,000) $ (6,000,000)  
Defined Benefit Plan, Assumptions Used in Calculation, Description 23 3.95  
[1] Collections include retiree contributions as well as provider discounts and rebates.
[2] Other Post-Retirement Benefits Employer contributions are reduced by provider discounts and rebates.