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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Net unrealized gain (loss) on cash flow hedges $ 44,000,000 $ 25,000,000 $ 83,000,000 $ 17,000,000
Reclassification to earnings from cash flow hedges [1] $ (28,000,000) $ (12,000,000) (31,000,000) $ 26,000,000
Ineffective portion excluded from testing     0  
Reclassification to earnings from cash flow hedges in the next 12 months     $ (17,000,000)  
[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 $17 million of gains from AOCI to interest expense within the next twelve months to offset amounts anticipated to be recorded in interest expense related to net exchange gain on the debt.