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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Sep. 30, 2023
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Offsetting Liabilities [Line Items] |  |
Gross Amounts Offset in the Balance Sheet | $ 4 | $ 26
Amount of letters of credit outstanding | 6 |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 1,122 | 991
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 7 | 37
Commodity contract derivatives | 7 | 33
Currency Swap |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 116 | 141
Commodity Contract under FHP |  |
Offsetting Liabilities [Line Items] |  |
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements | 165 | 212
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 161 | 186
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 0 | 0
Interest Rate Swap |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 840 | 662
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 0 | 4
Commodity Contract Derivatives |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 5 | 2
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 7 | 33
Letter of Credit |  |
Offsetting Liabilities [Line Items] |  |
Amount of letters of credit outstanding | $ 566 | 535
Interest rate swap collateral [Member] |  |
Offsetting Liabilities [Line Items] |  |
Amount of letters of credit outstanding |  | $ 509

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