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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, 2021
Sep. 30, 2020
Offsetting Liabilities [Line Items]    
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements $ 1,722  
Derivative Liability   $ 2,253
Total derivatives not subject to master netting or similar arrangement 3 3
Total 1,725  
Currency Swap    
Offsetting Liabilities [Line Items]    
Derivative Liability 83 209 [1]
Interest Rate Contract    
Offsetting Liabilities [Line Items]    
Derivative Liability 1,639 2,041 [1]
Total derivatives subject to master netting or similar arrangement    
Offsetting Liabilities [Line Items]    
Derivative Liability   2,250
Letter of Credit    
Offsetting Liabilities [Line Items]    
Amount of letters of credit outstanding $ 1,167 $ 1,500
[1] Letters of credit of approximately $1.2 billion and $1.5 billion were posted as collateral at September 30, 2021 and 2020, 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.