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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details) (USD $)
 3 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 9 Months Ended
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Sep. 30, 2012
Jun. 30, 2013
Interest Rate Swap
Jun. 30, 2012
Interest Rate Swap
Jun. 30, 2013
Interest Rate Swap
Jun. 30, 2012
Interest Rate Swap
Jun. 30, 2013
Coal Contract Derivatives
Sep. 30, 2012
Coal Contract Derivatives
Jun. 30, 2013
Commodity Contract Derivatives
Jun. 30, 2012
Commodity Contract Derivatives
Jun. 30, 2013
Commodity Contract Derivatives
Jun. 30, 2012
Commodity Contract Derivatives
Jun. 30, 2013
Commodity Derivatives Under Financial Trading Program
Jun. 30, 2012
Commodity Derivatives Under Financial Trading Program
Jun. 30, 2013
Commodity Derivatives Under Financial Trading Program
Jun. 30, 2012
Commodity Derivatives Under Financial Trading Program
Sep. 30, 2012
Commodity Derivatives Under Financial Trading Program
Jun. 30, 2013
Natural Gas Contract Derivatives
Sep. 30, 2012
Natural Gas Contract Derivatives
Jun. 30, 2013
Maximum
Coal Contract Derivatives
Jun. 30, 2013
Maximum
Natural Gas Contract Derivatives
Derivative                                              
Amount of gain (loss) recognized in income on derivatives   $ 0     $ 0 [1] $ 0 [1] $ 0 [1] $ 0 [1]     $ (2,000,000) [1] $ (6,000,000) [1] $ (2,000,000) [1] $ (22,000,000) [1] $ (21,000,000) [1] $ (104,000,000) [1] $ (99,000,000) [1] $ (248,000,000) [1]          
Unrealized gains (losses) on investments         252,000,000   465,000,000                                
Fair value                 (109,000,000) (267,000,000) (111,000,000)   (111,000,000)   (181,000,000)   (181,000,000)   (229,000,000) (2,000,000) 0    
Derivative, Term of Contract                                           5 years 2 years
Number of contracts                 23 23                   28 25    
Notional amount                 44,000,000 46,000,000                   49,000,000 51,000,000    
Unrealized gain (loss) on derivatives $ 0 $ 0 $ 0 $ 0                                      
[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 and nine months ended June 30, 2013, and 2012.