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Summary of Significant Accounting Policies - Pre-commercial Plant Ops (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Revenue capitalized during pre-commercial operations | $ 4 | $ 3 | $ 3
Pre-commercial Operations - Capitalized Revenue |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Unregulated Operating Revenue | (4) | 3 | $ 3
Paradise CT Units 5-7 |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Fuel capitalized during pre-commercial ops |  | 3 |
Paradise CT Units 5-7 | Pre-commercial Operations - Capitalized Revenue |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Unregulated Operating Revenue |  | $ 3 |
Johnsonville Aeroderivative CT Units 21-30 |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Fuel capitalized during pre-commercial ops | 7 |  |
Johnsonville Aeroderivative CT Units 21-30 | Pre-commercial Operations - Capitalized Revenue |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Unregulated Operating Revenue | $ 4 |  |

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