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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Summary of Derivative Instruments That Receive Hedge Accounting Treatment    
Reclassification to earnings from cash flow hedges in the next 12 months $ (3)  
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax 19 $ 34 [1]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax 20 71
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 44 15
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 $ 10 $ 4
[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 $3 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.