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Proprietary Capital (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Appropriation Investment      
Amount of appropriation investment that was repaid $ 1,000    
Remaining appropriation investment 258    
Balance at beginning of year 12,932 $ 11,625 $ 10,283
Net income (loss) 1,512 1,352 1,417
Return on power program appropriation investment (4) (6) (6)
Balance at end of year 14,465 12,932 $ 11,625
Net proprietary capital at September 30 $ 14,465 $ 12,932  
Computed average interest rate payable 1.64% 2.44% 2.37%
New Accounting Standard - CECL $ (4)    
Nonpower Programs Appropriation Investment      
Appropriation Investment      
Balance at beginning of year 4,351    
Balance at end of year 4,351 $ 4,351  
Power Program Appropriation Investment      
Appropriation Investment      
Balance at beginning of year 258 258 $ 258
Net income (loss) 0 0 0
Return on power program appropriation investment 0 0 0
Balance at end of year 258 258 258
New Accounting Standard - CECL 0    
Power Program Retained Earnings      
Appropriation Investment      
Balance at beginning of year 12,177 10,823 9,404
Net income (loss) 1,520 1,360 1,425
Return on power program appropriation investment (4) (6) (6)
Balance at end of year 13,689 12,177 10,823
Net proprietary capital at September 30 13,947 12,435  
New Accounting Standard - CECL (4) 0  
Nonpower Programs Appropriation Investment, Net      
Appropriation Investment      
Balance at beginning of year 548 556 564
Net income (loss) (8) (8) (8)
Return on power program appropriation investment 0 0 0
Balance at end of year 540 548 556
New Accounting Standard - CECL 0 0  
Nonpower Programs Retained Earnings      
Appropriation Investment      
Balance at beginning of year (3,803) (3,795)  
Return on power program appropriation investment 0 0  
Balance at end of year (3,811) (3,803) (3,795)
Net proprietary capital at September 30 540 548  
Affiliated Entity      
Appropriation Investment      
Return on power program appropriation investment $ (4) $ (6) $ (6)