CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Net income (loss)	$ 1,360	$ 1,135	$ 500
Total other comprehensive income (loss)	(2)	(23)	57
Return on power program appropriation investment	(8)	(7)	(6)
Appropriation-investment power program	258	258
Retained Earnings Power Program	17,797	16,437
Appropriation investment nonpower programs, net	510	518
Accumulated other comprehensive income (loss)	(54)	(52)	(29)	$ (86)
Proprietary Capital	18,511	17,161	16,056	15,505
Power Program Appropriation Investment
Net income (loss)	0	0	0
Total other comprehensive income (loss)	0	0	0
Return on power program appropriation investment	0	0	0
Appropriation-investment power program	258	258	258	258
Power Program Retained Earnings
Net income (loss)	1,368	1,142	508
Total other comprehensive income (loss)	0	0	0
Return on power program appropriation investment	(8)	(7)	6
Retained Earnings Power Program	17,797	16,437	15,302	14,800
Nonpower Programs Appropriation Investment, Net
Net income (loss)	(8)	(7)	(8)
Total other comprehensive income (loss)	0	0	0
Return on power program appropriation investment	0	0	0
Appropriation investment nonpower programs, net	510	518	525	$ 533
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Net income (loss)	0	0	0
Total other comprehensive income (loss)	(2)	(23)	57
Return on power program appropriation investment	$ 0	$ 0	$ 0