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Risk Management Activities and Derivative Transactions Currency Swaps Outstanding (Details)
$ in Millions
9 Months Ended
Jun. 30, 2021
USD ($)
Bond_issues
Sep. 30, 2020
Bond_issues
Derivative    
Number of British pound sterling denominated bond transactions 3  
Number of currency swaps outstanding   2
Associated TVA bond issues currency exposure | $ $ 400  
Minimum    
Derivative    
Expiration date range of swaps 2032  
Maximum    
Derivative    
Expiration date range of swaps 2043