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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 |  |  |

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