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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Regulatory Assets and Liabilities    
Current regulatory assets $ 506 $ 481
Regulatory Assets 10,924 9,475
Yearly amortization of Deferred nuclear generating units $ 237  
Period collected in rates 15 years  
Deferred Nuclear Generating Units and Construction Costs    
Regulatory Assets and Liabilities    
Remaining balance to be included in plant asset balance at completion $ 619  
Environmental Cleanup Costs - Kingston Ash Spill    
Regulatory Assets and Liabilities    
Current regulatory asset amortization period 12 months  
Environmental Agreements    
Regulatory Assets and Liabilities    
Initial regulatory asset $ 360  
Investment in environmental agreement projects 290  
Amount to be provided to fund environmental projects 60  
Amount to be paid in civil penalties 10  
Energy Efficiency Projects | Environmental Agreements    
Regulatory Assets and Liabilities    
Regulatory asset amount expensed 188  
Paid to States | Environmental Agreements    
Regulatory Assets and Liabilities    
Regulatory asset amount expensed 60  
Civil Penalties | Environmental Agreements    
Regulatory Assets and Liabilities    
Regulatory asset amount expensed 10  
Derivatives Under FTP    
Regulatory Assets and Liabilities    
Net unrealized gains (losses) [1] 116 103
Other current regulatory assets    
Regulatory Assets and Liabilities    
Current regulatory assets 2 0
Deferred Nuclear Generating Units and Construction Costs    
Regulatory Assets and Liabilities    
Current regulatory assets 237 $ 237
Regulatory Assets $ 1,300  
[1] Fair values of certain derivatives under the FTP that were in net liability positions totaling $89 million and $69 million at September 30, 2015 and September 30, 2014, respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are 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.