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Debt and Other Obligations Debt Securities Activity (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Jun. 30, 2021
Jun. 30, 2020
Sep. 30, 2020
Sep. 30, 2019
Debt Instrument            
Total long-term debt, net     $ 19,000   $ 19,004  
Redemptions and repurchases of power bonds     $ 1,860 [1] $ 1,286    
Variable Interest Entity, Financial or Other Support, Type     20      
Total redemptions/maturities of debt [1]     $ (1,880)      
Proceeds from Issuance of Debt     0 997    
Repayments of Notes Payable     $ 0 $ 23    
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          
1998 Series D            
Debt Instrument            
Redemptions and repurchases of power bonds     $ 28      
1999 Series A Member            
Debt Instrument            
Redemptions and repurchases of power bonds     $ 331      
1998 Series D            
Debt Instrument            
Debt Instrument, Interest Rate, Effective Percentage     3.77%      
1999 Series A Member            
Debt Instrument            
Debt Instrument, Interest Rate, Effective Percentage     5.805%      
2009 Series B [Member]            
Debt Instrument            
Debt Instrument, Interest Rate, Effective Percentage 3.875% 3.77%        
[1] (1) All redemptions were at 100 percent of par.