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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 9 Months Ended
Mar. 31, 2019
USD ($)
People
Sep. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
People
Mar. 31, 2018
USD ($)
Jun. 30, 2017
USD ($)
Accounting Policies [Abstract]            
Prior Period Reclassification Adjustment $ 65     $ 13 $ 128  
Deferred Costs (39)     (39)    
Regulatory Assets 7,063 $ 7,026   7,063    
Fuel cost tax equivalents (6)          
Other Deferred Costs, Net 19     19    
Other Depreciation and Amortization (26)          
Other Operating Activities, Cash Flow Statement (17)          
Accounts Payable and Accrued Liabilities, Current (12)     (12)    
Regulatory assets costs (5)          
Appropriation-investment power program   1,000        
Financing Receivable, Net $ 136 123   $ 136    
Population of TVA's service area | People 10     10    
Allowance for uncollectible accounts $ 1 1   $ 1    
Revenue capitalized during pre-commercial operations 0   $ 10 0 11  
Fuel cost capitalized during pre-commercial operations   $ 14 $ 11      
Depreciation expense $ 424     $ 732 $ 649 $ 331