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Derivatives and Risk Management Activities - Embedded Derivatives (Details) - Preferred Distribution Rate Reset Option - Series A Preferred Units - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Embedded Derivatives      
Fair value of derivative liability $ 26   $ 22
Maximum      
Embedded Derivatives      
Loss recognized due to changes in fair value $ 4 $ 4