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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, 2023 | Sep. 30, 2022
--- | --- | ---
Offsetting Liabilities [Line Items] |  |
Gross Amounts Offset in the Balance Sheet | $ 26 |
Amount of letters of credit outstanding | 6 |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 991 | $ 1,160
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 37 | 275
Commodity contract derivatives | 33 | 152
Currency Swap |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 141 | 240
Commodity Contract under FHP |  |
Offsetting Liabilities [Line Items] |  |
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements | 212 |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 186 | 8
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 0 | 123
Interest Rate Swap |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 662 | 905
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 4 | 0
Commodity Contract Derivatives |  |
Offsetting Liabilities [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 2 | 7
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 33 | 152
Letter of Credit |  |
Offsetting Liabilities [Line Items] |  |
Amount of letters of credit outstanding | $ 535 | $ 704

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