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Summary of Significant Accounting Policies General and Basis of Presentation (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2021
USD ($)
People
Dec. 31, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Accounting Policies [Abstract]        
Cash and Cash Equivalents, at Carrying Value $ 507   $ 499  
Regulatory Assets 7,956   8,152  
Appropriation-investment power program     1,000  
Financing Receivable, after Allowance for Credit Loss $ 99   94  
Population of TVA's service area | People 10      
Allowance for uncollectible accounts $ 1      
Depreciation expense 453 $ 345    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents 526 529 518 $ 521
Cash and Cash Equivalents, Period Increase (Decrease)   8    
Restricted Cash and Cash Equivalents, Noncurrent 19   $ 19  
Property, Plant and Equipment [Line Items]        
Depreciation expense 453 $ 345    
Increase due to new depreciation rates        
Accounting Policies [Abstract]        
Depreciation expense 98      
Property, Plant and Equipment [Line Items]        
Depreciation expense $ 98