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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, 2019
Dec. 31, 2018
Summary of Derivative Instruments That Receive Hedge Accounting Treatment    
Net unrealized gain (loss) on cash flow hedges $ 76 $ (52)
Reclassification to earnings from cash flow hedges [1] (59) $ 18
Reclassification to earnings from cash flow hedges in the next 12 months $ (15)  
[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 $15 million of gains from AOCI to interest expense within the next 12 months to offset amounts anticipated to be recorded in interest expense related to exchange gain on the debt.