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Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Offsetting Assets, Liabilities [Line Items]    
Initial margin $ 146 $ 48
Variation margin posted 357 164
Letter of credit (168) (166)
Net broker receivable 426 $ 212
Exchange Traded    
Offsetting Assets, Liabilities [Line Items]    
Letter of credit $ (77)