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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, 2021
USD ($)
Years
Jun. 30, 2021
USD ($)
 [1]
Dec. 31, 2020
USD ($)
Sep. 30, 2021
USD ($)
Derivative        
Amount recognized for unrealized gains (losses) $ 0      
Change in Unrealized gains (losses) on Interest Rate Derivatives (11,000,000)   $ 143,000,000  
Derivative, Notional Amount 1,500,000,000      
Interest Rate Swap        
Derivative        
Gain (loss) recognized in income on derivatives (29,000,000) $ (29,000,000)    
Fair value (452,000,000)     $ (455,000,000)
Commodity Contract Derivatives        
Derivative        
Fair value $ 105,000,000     $ 247,000,000
Natural Gas [Member]        
Derivative        
Number of contracts 42     40
Derivative, Nonmonetary Notional Amount 361,000,000     263,000,000
Fair value $ 105,000,000     $ 247,000,000
Number Commodity Contract Term Years | Years 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 months ended December 31, 2021 and 2020.