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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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Offsetting Assets [Line Items] |  |
Amount of letters of credit outstanding | $ 6 |
Letter of Credit |  |
Offsetting Assets [Line Items] |  |
Amount of letters of credit outstanding | 498 | $ 566
Commodity Contract under FHP |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets, subject to master netting or similar arrangements | $ 28 | $ 4

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