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Debt and Other Obligations Debt Securities Activity (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Mar. 31, 2021
Mar. 31, 2020
Jun. 30, 2019
Debt Instrument          
Total long-term debt, net   $ 19,004 $ 19,023    
Redemptions and repurchases of power bonds     $ 1,501 [1] $ 1,218  
Variable Interest Entity, Financial or Other Support, Type     20    
Total redemptions/maturities of debt [1]     $ (1,521)    
Total          
Debt Instrument          
Percent of par value     100.00%    
2009 Series B [Member]          
Debt Instrument          
Redemptions and repurchases of power bonds $ 1,500 $ 1      
Debt of variable interest entities [Member]          
Debt Instrument          
Other Debt instrument, Interest Rate, Effective Percentage        
2009 Series B [Member]          
Debt Instrument          
Debt Instrument, Interest Rate, Effective Percentage 3.88% 3.77%      
[1] (1) All redemptions were at 100 percent of par.