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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 3 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2017
USD ($)
Derivative      
Amount recognized for unrealized gains (losses) $ 0 $ 0  
Change in Unrealized gains (losses) on Interest Rate Derivatives (28,000,000) 441,000,000  
Interest Rate Swap      
Derivative      
Gain (loss) recognized in income on derivatives [1] (24,000,000) (26,000,000)  
Fair value (394,000,000)   $ (410,000,000)
Commodity Contract Derivatives      
Derivative      
Gain (loss) recognized in income on derivatives 3,000,000 [1] (2,000,000)  
Fair value (77,000,000)   (60,000,000)
Commodity derivatives under the financial trading program      
Derivative      
Gain (loss) recognized in income on derivatives [1] 8,000,000 $ (14,000,000)  
Fair value [2] $ 0   $ (5,000,000)
Coal Contract Derivatives      
Derivative      
Number of contracts 11   20
Notional amount 23,000,000   17,000,000
Fair value $ (73,000,000)   $ (67,000,000)
Natural gas contract derivatives      
Derivative      
Number of contracts 46   53
Notional amount 253,000,000   271,000,000
Fair value $ (4,000,000)   $ 7,000,000
Natural gas contract derivatives      
Derivative      
Fair value $ 0   $ (5,000,000)
[1] All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in incomebut instead are deferred as regulatory assets and liabilities. As such, there was no related gain (loss) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2017 and 2016.
[2] Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million at September 30, 2017, were recorded in TVA's margin cash accounts in Other current assets. These derivatives were transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full. At December 31, 2017, TVA had no derivatives under the FTP in net liability positions.