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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Sep. 30, 2020
--- | --- | ---
Offsetting Assets [Line Items] |  |
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement | $ 1,603 | $ 2,253
Gross amounts of recognized liabilities | 1,598 | 2,250
Total derivatives not subject to master netting or similar arrangement | 32 | 49
Forward Contract Derivative Asset, at Fair Value | 8 | 13
Derivative Liability, Not Subject to Master Netting Arrangement | 5 | 3
Letter of Credit |  |
Offsetting Assets [Line Items] |  |
Amount of letters of credit outstanding for credit facilities | 1,153 | 1,500
Currency Swap |  |
Offsetting Assets [Line Items] |  |
Gross amounts of recognized liabilities | 86 | 209
Interest Rate Contract |  |
Offsetting Assets [Line Items] |  |
Gross amounts of recognized liabilities | 1,512 | 2,041
Fair Value, Inputs, Level 2 |  |
Offsetting Assets [Line Items] |  |
Forward Contract Derivative Asset, at Fair Value | $ 8 | $ 13

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