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Summary of Significant Accounting Policies General and Basis of Presentation (Details)
People in Millions, $ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2016
USD ($)
variable_interest_entities
People
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
variable_interest_entities
People
Jun. 30, 2015
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
General and Basis of Presentation            
Population of TVA's service area | People 9   9      
Amount of appropriation investment that was repaid           $ 1,000
Number of variable interest entities | variable_interest_entities 3   3      
Reclassification from Other Long-Term Assets to Long-Term Debt, Net         $ 80  
Reclassification from Other Long-Term Assets to Long-term Power Bonds, Net         67  
Reclassification from Other Long-Term Assets to Long-Term Variable Interest Entities, Net         13  
Allowance for uncollectible accounts, accounts receivable $ 1   $ 1   1  
Loans Receivable, Carrying Value 160   160   117  
Allowance for loans receivable 8   8   $ 8  
Depreciation expense 353 $ 462 1,100 $ 1,200    
Change in depreciation 22   65      
CGLP lease liability incurred     76      
CGLP capital lease asset 76          
Total estimated amount of BLEU payments 165   165      
Payments attributable to BLEU fuel program 131   131      
BLEU fuel obligation $ 25   $ 25