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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 3 Months Ended 9 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Derivative          
Amount recognized for unrealized gains (losses)     $ 0    
Change in Unrealized gains (losses) on Interest Rate Derivatives $ (56,000,000) $ 14,000,000 (227,000,000) $ 445,000,000  
Interest Rate Swap          
Derivative          
Gain (loss) recognized in income on derivatives [1] (21,000,000) (25,000,000) (68,000,000) (77,000,000)  
Fair value (334,000,000)   (334,000,000)   $ (410,000,000)
Commodity Contract Derivatives          
Derivative          
Fair value (4,000,000)   (4,000,000)   (60,000,000)
Commodity derivatives under the financial trading program          
Derivative          
Gain (loss) recognized in income on derivatives [1] 0 $ (7,000,000) (8,000,000) $ (28,000,000)  
Fair value [2] $ 0   $ 0   $ (5,000,000)
Coal Contract Derivatives          
Derivative          
Number of contracts 13   13   20
Notional amount 18,000,000   18,000,000   17,000,000
Fair value $ 5,000,000   $ 5,000,000   $ (67,000,000)
Natural gas contract derivatives          
Derivative          
Number of contracts 47   47   53
Notional amount 296,000,000   296,000,000   271,000,000
Fair value $ (9,000,000)   $ (9,000,000)   $ 7,000,000
Natural gas contract derivatives          
Derivative          
Fair value $ 0   $ 0   $ (5,000,000)
[1] All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in incomebut instead are deferred as regulatory assets and liabilities. As such, there was no related gain (loss) recognized in income for these unrealized gains (losses) for the three and nine months ended June 30, 2018 and 2017.
[2] Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million at September 30, 2017, were recorded in TVA's margin cash accounts in Other current assets. These derivatives were transacted with futures commission merchants, and cash deposits were posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full. At June 30, 2018, TVA had no derivatives under the FTP in net liability positions.