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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 3 Months Ended 6 Months Ended
Mar. 31, 2021
USD ($)
Years
Mar. 31, 2020
USD ($)
Mar. 31, 2021
USD ($)
Years
Mar. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
Derivative          
Amount recognized for unrealized gains (losses)     $ 0    
Change in Unrealized gains (losses) on Interest Rate Derivatives $ (386,000,000) $ 556,000,000 (529,000,000) $ (385,000,000)  
Interest Rate Swap          
Derivative          
Gain (loss) recognized in income on derivatives (28,000,000) (23,000,000) [1] (57,000,000) [1] (44,000,000) [1]  
Fair value (419,000,000)   (419,000,000)   $ (588,000,000)
Commodity Contract Derivatives          
Derivative          
Gain (loss) recognized in income on derivatives 0 $ 0 0 [1] $ 1,000,000 [1]  
Fair value $ 27,000,000   $ 27,000,000   $ 46,000,000
Natural Gas [Member]          
Derivative          
Number of contracts 27   27   42
Derivative, Nonmonetary Notional Amount 274,000,000   274,000,000   302,000,000
Fair value $ 27,000,000   $ 27,000,000   $ 46,000,000
Number Commodity Contract Term Years | Years 3   3    
[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 and six months ended March 31, 2021 and 2020.(2) During the fourth quarter of 2020, TVA discontinued derivative accounting for forward coal contracts.