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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Sep. 30, 2020
Letter of Credit    
Offsetting Assets [Line Items]    
Amount of letters of credit outstanding $ 1,167 $ 1,500
Other Contract    
Offsetting Assets [Line Items]    
Net Amounts of Assets Presented in the Balance Sheet $ 250 $ 49