Summary of Significant Accounting Policies - General (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) Units	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2015 USD ($)
Accounting Policies [Abstract]
Cash and Cash Equivalent	$ 1,576	$ 502
Reimbursements from DOE	334
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1,597	523	$ 521	$ 520
Accounts receivable from DOE	6
Appropriation-investment power program	$ 258	258			$ 1,000
Population of Service Area [Line Items]
Population of service area | Units	10,000,000
Cash and Cash Equivalent	$ 1,576	502
Restricted Cash and Cash Equivalent, Noncurrent	21	21
Accumulated depreciation	$ 38,716	$ 38,793