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Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Offsetting Assets, Liabilities [Line Items]    
Initial margin $ 78 $ 91
Variation margin posted/(returned) 369 290
Letters of credit (81) (129)
Net broker receivable/(payable) 407 318
Exchange traded    
Offsetting Assets, Liabilities [Line Items]    
Letters of credit $ (40) $ (63)