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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Sep. 30, 2023
--- | --- | ---
Offsetting Assets [Line Items] |  |
Derivative Asset, Subject to Master Netting Arrangement, after Offset | $ 19 | $ 37
Amount of letters of credit outstanding for credit facilities | 6 |
Forward Contract Derivative Asset, at Fair Value | 5 | 11
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 1,149 | 991
Offsetting Derivative Assets [Abstract] |  |
Derivative Asset, Fair Value, Gross Liability | 7 |
Letter of Credit |  |
Offsetting Assets [Line Items] |  |
Amount of letters of credit outstanding for credit facilities | 516 | 535
Currency Swap |  |
Offsetting Assets [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 125 | 141
Commodity Contract under FHP |  |
Offsetting Assets [Line Items] |  |
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 267 | 186
Offsetting Derivative Assets [Abstract] |  |
Derivative Asset, Subject to Master Netting Arrangement, before Offset | 7 |
Gross amounts of recognized liabilities | 274 |
Interest Rate Swap |  |
Offsetting Assets [Line Items] |  |
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 1 | 4
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 753 | 662
Commodity Contract Derivatives |  |
Offsetting Assets [Line Items] |  |
Derivative Asset, Subject to Master Netting Arrangement, after Offset | 18 | 33
Derivative Liability, Subject to Master Netting Arrangement, after Offset | 4 | 2
Fair Value, Inputs, Level 2 |  |
Offsetting Assets [Line Items] |  |
Forward Contract Derivative Asset, at Fair Value | $ 5 | $ 11

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