Risk Management and Hedging Activities - Designated as Hedging Contracts (Details) - Designated as Hedging Instrument [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
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			$ 24
Commodity derivative [Member]
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]
Beginning balance	$ 26	$ 38	46	$ 32
Changes in fair value recognized in OCI	15	20	35	37
Net gains reclassified to operating revenue	1	1	1	4
Net losses reclassified to cost of sales	(7)	(14)	(47)	(28)
Ending balance	$ 35	$ 45	$ 35	$ 45