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Summary of Significant Accounting Policies General and Basis of Presentation (Details)
People in Millions, $ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2016
USD ($)
variable_interest_entities
People
Mar. 31, 2015
USD ($)
Mar. 31, 2016
USD ($)
variable_interest_entities
People
Mar. 31, 2015
USD ($)
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 144   144   117  
Allowance for loans receivable 8   8   $ 8  
Depreciation expense 369 $ 383 733 $ 763    
Change in depreciation 21   43      
Total estimated amount of BLEU payments 165   165      
Payments attributable to BLEU fuel program 131   131      
BLEU fuel obligation $ 25   $ 25