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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 3 Months Ended
Dec. 31, 2020
USD ($)
Years
Dec. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Derivative      
Amount recognized for unrealized gains (losses) $ 0    
Change in Unrealized gains (losses) on Interest Rate Derivatives (143,000,000) $ 171,000,000  
Interest Rate Swap      
Derivative      
Gain (loss) recognized in income on derivatives (29,000,000) (21,000,000) [1]  
Fair value (533,000,000)   $ (588,000,000)
Commodity Contract Derivatives      
Derivative      
Gain (loss) recognized in income on derivatives 0 $ 1,000,000  
Fair value $ 22,000,000   $ 46,000,000
Natural Gas [Member]      
Derivative      
Number of contracts 32   42
Derivative, Nonmonetary Notional Amount 300,000,000   302,000,000
Fair value $ 22,000,000   $ 46,000,000
Number Commodity Contract Term Years | Years 4    
[1] All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in income but instead are deferred as regulatory assets and liabilities. As such, there were no related gains (losses) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2020 and 2019.(2) During the fourth quarter of 2020, TVA discontinued derivative accounting for forward coal contracts.