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Proprietary Capital (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Appropriation Investment      
Amount of appropriation investment that was repaid $ 1,000    
Balance at beginning of year 15,505 $ 14,465 $ 12,932
Net income (loss) 500 1,108 1,512
Return on power program appropriation investment (6) (4) (4)
Balance at end of year 16,056 15,505 $ 14,465
Net proprietary capital at September 30 $ 16,056 $ 15,505  
Computed average interest rate payable 1.99% 1.47% 1.64%
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 14,800 13,689 12,177
Net income (loss) 508 1,115 1,520
Return on power program appropriation investment (6) (4) (4)
Balance at end of year 15,302 14,800 13,689
New Accounting Standard - CECL     (4)
Nonpower Programs Appropriation Investment, Net      
Appropriation Investment      
Balance at beginning of year 533 540 548
Net income (loss) (8) (7) (8)
Return on power program appropriation investment 0 0 0
Balance at end of year 525 533 540
New Accounting Standard - CECL     0
Affiliated Entity      
Appropriation Investment      
Return on power program appropriation investment $ (6) $ (4) $ (4)