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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Defined Benefit Plan, Funded Status of Plan [Abstract]      
Amortization Period for Change in Benefit Obligation Related to 2016 Amendments to TVA Pension Plan 11 years    
Pension Benefits      
Change in benefit obligation      
Benefit obligation $ 13,083 $ 12,824 $ 12,265
Service cost 133 130 130
Interest cost 564 540 558
Plan participants' contributions 25 25  
Plan change 960 0  
Actuarial loss (gain) 1,188 556  
Net transfers from variable fund/401(k) plan 7 11  
Expenses paid (6) (6)  
Benefits paid (692) (697)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (133) 518  
Change in plan assets      
Fair value of net plan assets 7,145 6,797 $ 7,507
Actual return on plan assets 733 (325)  
Employer contributions 281 282  
Defined Benefit Plan, Funded Status of Plan [Abstract]      
Funded status $ (5,938) $ (6,027)  
Discount rate 3.65% 4.50% 4.45%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ 1,400 $ (79)  
Amount of defined benefit plan actuarial gain (loss) from plan demographic experience changes   349  
Amount of defined benefit plan actuarial gain (loss) from change in COLA 168 232  
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 571 657 $ 652
Service cost 16 16 18
Interest cost 29 29 32
Collections [1] 92 94  
Plan change 158 0  
Actuarial loss (gain) 68 3  
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid (133) (137)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (7) 21  
Change in plan assets      
Fair value of net plan assets 0 0 $ 0
Actual return on plan assets 0 0  
Employer contributions [2] 41 43  
Defined Benefit Plan, Funded Status of Plan [Abstract]      
Funded status $ (571) $ (657)  
Discount rate 3.70% 4.65% 4.50%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ 91 $ (13)  
Amount of defined benefit plan actuarial gain (loss) from plan demographic experience changes   20  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ 17 $ 30  
[1] Collections include retiree contributions as well as federal reinsurance payments and provider discounts and rebates.
[2] Other Post-Retirement Benefits Employer contributions are reduced by federal reinsurance payments and provider discounts and rebates.