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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, 2019
USD ($)
People
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
People
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
Accounting Policies [Abstract]                
Cash and Cash Equivalents, at Carrying Value $ 300 $ 299   $ 300        
Prior Period Reclassification Adjustment       13        
Deferred Costs (6)     (6)        
Regulatory Assets 6,938 7,026   6,938        
Fuel cost tax equivalents (8)              
Other Deferred Costs, Net 29     29        
Other Depreciation and Amortization (15)              
Other Operating Activities, Cash Flow Statement (20)              
Accounts Payable and Accrued Liabilities, Current (12)     (12)        
Regulatory assets costs (8)              
Appropriation-investment power program   1,000            
Financing Receivable, after Allowance for Credit Loss $ 111 123   $ 111        
Population of TVA's service area | People 10     10        
Allowance for uncollectible accounts $ 1 1   $ 1        
Revenue capitalized during pre-commercial operations 0   $ 1 0 $ 11      
Fuel cost capitalized during pre-commercial operations   20 $ 5          
Depreciation expense 536     1,268 $ 950 $ 301    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents $ 323 $ 322   $ 323     $ 312 $ 300