Proprietary Capital (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Appropriation Investment
Amount of appropriation investment that was repaid	$ 1,000
Net income (loss)	1,360	$ 1,135	$ 500
Return on power program appropriation investment	(8)	(7)	(6)
Net proprietary capital at September 30	$ 18,511	$ 17,161	$ 16,056	$ 15,505
Computed average interest rate payable	3.39%	3.02%	1.99%
Power Program Appropriation Investment
Appropriation Investment
Net income (loss)	$ 0	$ 0	$ 0
Return on power program appropriation investment	0	0	0
Power Program Retained Earnings
Appropriation Investment
Net income (loss)	1,368	1,142	508
Return on power program appropriation investment	(8)	(7)	6
Nonpower Programs Appropriation Investment, Net
Appropriation Investment
Net income (loss)	(8)	(7)	(8)
Return on power program appropriation investment	0	0	0
Affiliated Entity
Appropriation Investment
Return on power program appropriation investment	$ (8)	$ (7)	$ (6)