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Derivatives and Risk Management Activities - Embedded Derivatives (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Embedded Derivatives          
Gain/(loss) recognized due to changes in fair value     $ 0 $ 42  
Preferred Distribution Rate Reset Option | Series A Preferred Units          
Embedded Derivatives          
Fair value of derivative liability $ 33   33   $ 32
Gain/(loss) recognized due to changes in fair value $ 2 $ 17   $ 42  
Maximum | Preferred Distribution Rate Reset Option | Series A Preferred Units          
Embedded Derivatives          
Gain/(loss) recognized due to changes in fair value     $ 1