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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, 2020 | Sep. 30, 2019
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Offsetting Liabilities [Line Items] |  |  |
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements |  | $ 2,250 |
Derivative Liability |  |  | $ 2,025
Total derivatives not subject to master netting or similar arrangement |  | 3 | 53
Total |  | 2,253 |
Currency Swap |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements |  | 209 |
Derivative Liability | [1] |  | 208
Interest Rate Contract |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements |  | 2,041 |
Derivative Liability | [1] |  | 1,764
Total derivatives subject to master netting or similar arrangement |  |  |
Offsetting Liabilities [Line Items] |  |  |
Derivative Liability |  |  | 1,972
Letter of Credit |  |  |
Offsetting Liabilities [Line Items] |  |  |
Amount of letters of credit outstanding |  | $ 1,457 | $ 1,300
[1] | Letters of credit of approximately $1.5 billion and $1.3 billion were posted as collateral at September 30, 2020 and 2019, respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.

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