Related Parties Related Parties (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Related Parties
Other income	$ 189	$ 191	$ 166
Long-term Investments	5,573	4,968
Return on power program appropriation investment	(8)	(7)	(6)
Related Party Transactions
Related Parties
Revenue from sales of electricity	132	118	120
Other income	309	273	282
Operating expenses	264	241	234
Additions to property, plant, and equipment	13	12	8
Cash and cash equivalents	31	31	31
Receivables from Customers	89	81	87
Long-term Investments	534	495	391
Receivables, Long-term Contracts or Programs	43	35	38
Accounts payable and accrued liabilities	41	45	42
Long-term power bonds, net	4	3	1
Return on power program appropriation investment	$ (8)	$ (7)	$ (6)