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Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Initial margin $ 55 $ 119
Variation margin posted/(returned) (43) 291
Net broker receivable/(payable) $ 12 $ 410