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Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Sep. 30, 2019
--- | --- | ---
Offsetting Assets [Line Items] |  |
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement | $ 2,505 | $ 2,025
Gross amounts of recognized liabilities | 2,445 | 1,972
Total derivatives not subject to master netting or similar arrangement | 4 | 12
Forward Contract Derivative Asset, at Fair Value | 10 | 22
Derivative Liability, Not Subject to Master Netting Arrangement | 60 | 53
Letter of Credit |  |
Offsetting Assets [Line Items] |  |
Amount of letters of credit outstanding for credit facilities | 1,400 | 1,300
Currency Swap |  |
Offsetting Assets [Line Items] |  |
Gross amounts of recognized liabilities | 295 | 208
Interest Rate Contract |  |
Offsetting Assets [Line Items] |  |
Gross amounts of recognized liabilities | 2,150 | 1,764
Fair Value, Inputs, Level 2 |  |
Offsetting Assets [Line Items] |  |
Forward Contract Derivative Asset, at Fair Value | $ 10 | $ 22

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