Risk Management and Hedging Activities - MEC - Designated as Hedging Contracts (Details) - Commodity derivative [Member] - Designated as Hedging Instrument [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]
Accumulated other comprehensive income (loss), net, cummulative net gain (loss) from cash flow hedges, effect before taxes	$ (21)	$ (26)	$ (16)	$ (46)
Changes in fair value recognized in OCI	5	15	28	35
Accumulated other comprehensive income (loss), net, cummulative net gain (loss) from cash flow hedges, effect before taxes	$ (19)	$ (35)	$ (19)	$ (35)

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration