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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Reclassification to earnings from cash flow hedges in the next 12 months     $ (10)  
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax $ 7 $ 22 [1] 20 [1] $ 65
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax 1 5 17 79
Fuel Expense | Commodity Contract under FHP        
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (59) 78 (186) 205
Purchased Power Expense | Commodity Contract under FHP        
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ (13) $ 19 $ (41) $ 51
[1] There were no amounts excluded from effectiveness testing for any of the periods presented. Based on forecasted foreign currency exchange rates, TVA expects to reclassify approximately $10 million of gains from Accumulated other comprehensive income (loss) ("AOCI") to Interest expense within the next 12 months to offset amounts anticipated to be recorded in Interest expense related to the forecasted exchange loss on the debt.