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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)
 3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Net unrealized gain (loss) on cash flow hedges $ (65,000,000) $ 14,000,000 $ 17,000,000 $ 31,000,000
Reclassification to earnings from cash flow hedges [1] $ 43,000,000 $ (29,000,000) 13,000,000 $ (3,000,000)
Ineffective portion excluded from testing     0  
Reclassification to earnings from cash flow hedges in the next 12 months     $ (42,000,000)  
[1] There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented. Based on forecasted foreign currency exchange rates, TVA expects to reclassify approximately $42 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 net exchange gain on the debt.