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Risk Management and Hedging Activities - Designated as Hedging Contracts (Details) - Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]        
Cash flow hedge ineffectiveness insignificant insignificant insignificant insignificant
Cash flow hedge loss to be reclassified within twelve months     $ 13  
Commodity derivative [Member]        
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]        
Beginning balance $ 23 $ 72 16 $ 46
Changes in fair value recognized in OCI 7 (28) 23 20
Net losses reclassified to cost of sales (9) (18) (18) (40)
Ending balance $ 21 $ 26 $ 21 $ 26