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Risk Management Activities and Derivative Transactions Offsetting for Derivative Liabilities (Details) $ in Millions | Mar. 31, 2025 USD ($) | Sep. 30, 2024 USD ($) | Jun. 30, 2024 Bond_issues
--- | --- | --- | ---
Offsetting Liabilities [Line Items] |  |  |
Number of currency swaps outstanding | Bond_issues |  |  | 2
Accounts Receivable, before Allowance for Credit Loss, Current | $ 1,604 | $ 1,683 |
Amount of letters of credit outstanding | 6 |  |
Forward Contract Derivative Asset, at Fair Value | 16 | 11 |
Letter of Credit |  |  |
Offsetting Liabilities [Line Items] |  |  |
Amount of letters of credit outstanding | 571 | 566 |
Interest Swap Collateral |  |  |
Offsetting Liabilities [Line Items] |  |  |
Amount of letters of credit outstanding | 465 |  |
Fair Value, Inputs, Level 2 |  |  |
Offsetting Liabilities [Line Items] |  |  |
Forward Contract Derivative Asset, at Fair Value | $ 16 | $ 11 |

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