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Derivatives and Risk Management Activities - Embedded Derivatives (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Embedded Derivatives          
Gain/(loss) recognized due to changes in fair value     $ (2) $ 25  
Preferred Distribution Rate Reset Option | Series A Preferred Units          
Embedded Derivatives          
Fair value of derivative liability $ 35   35   $ 32
Gain/(loss) recognized due to changes in fair value $ 2 $ 25 $ (2) $ 25