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Summary of Significant Accounting Policies General and Basis of Presentation (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2020
USD ($)
People
Dec. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Reclassification of Amounts on Cash Flow and Statement of Operations [Line Items]        
Cash and Cash Equivalents, at Carrying Value $ 509   $ 500  
Restricted Cash and Cash Equivalents 20   21  
Regulatory Assets 9,816   10,375  
Appropriation-investment power program     1,000  
Financing Receivable, after Allowance for Credit Loss $ 96   93  
Population of TVA's service area | People 10      
Allowance for uncollectible accounts $ 1      
Depreciation expense 345 $ 539    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents $ 529 327 $ 521 $ 322
Cash and Cash Equivalents, Period Increase (Decrease)   $ 5