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Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Offsetting Assets, Liabilities [Line Items]    
Initial margin $ 71 $ 95
Variation margin returned (85) (91)
Letters of credit (153) (184)
Net broker payable (73) (80)
Exchange traded    
Offsetting Assets, Liabilities [Line Items]    
Letters of credit $ (59) $ (84)