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Proprietary Capital (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Appropriation Investment      
Amount of appropriation investment that was repaid $ 1,000    
Balance at beginning of year 14,465 $ 12,932 $ 11,625
Net income (loss) 1,108 1,512 1,352
Return on power program appropriation investment (4) (4) (6)
Balance at end of year 15,505 14,465 $ 12,932
Net proprietary capital at September 30 $ 15,505 $ 14,465  
Computed average interest rate payable 1.47% 1.64% 2.44%
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 13,689 12,177 10,823
Net income (loss) 1,115 1,520 1,360
Return on power program appropriation investment (4) (4) (6)
Balance at end of year 14,800 13,689 12,177
New Accounting Standard - CECL   (4)  
Nonpower Programs Appropriation Investment, Net      
Appropriation Investment      
Balance at beginning of year 540 548 556
Net income (loss) (7) (8) (8)
Return on power program appropriation investment 0 0 0
Balance at end of year 533 540 548
New Accounting Standard - CECL   0  
Affiliated Entity      
Appropriation Investment      
Return on power program appropriation investment $ (4) $ (4) $ (6)