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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, 2020
USD ($)
People
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
People
Mar. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Reclassification of Amounts on Cash Flow and Statement of Operations [Line Items]            
Lease obligation related to Purchased Power [Table Text Block]     296      
Cash and Cash Equivalents, at Carrying Value $ 835   $ 835   $ 299  
Restricted Cash and Cash Equivalents 23   23   23  
Regulatory Assets 9,594   9,594   8,919  
Other Deferred Costs, Net (10)   (10)      
Appropriation-investment power program         1,000  
Financing Receivable, after Allowance for Credit Loss $ 128   $ 128   120  
Population of TVA's service area | People 10   10      
Allowance for uncollectible accounts $ 1   $ 1   1  
Depreciation expense 407 $ 424 946 $ 732    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents $ 858 $ 323 $ 858 $ 323 $ 322 $ 322