Risk Management and Hedging Activities - Designated as Hedging Contracts (Details) - Designated as Hedging Instrument [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]
Cash flow hedge ineffectiveness	insignificant	Insignificant	Insignificant
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ (36)
Commodity Contract [Member]
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]
Beginning balance	32	$ 12	$ 32
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	52	(6)	(9)
Net gains reclassified to operating revenue	9	0	0
Net (losses) gains reclassified to cost of sales	(47)	26	(11)
Ending balance	$ 46	$ 32	$ 12