Segmented Information (Schedule Of Marketing Intersegment Eliminations) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product Information [Line Items]
Revenues	$ 1,210	$ 861	$ 1,083	$ 1,289	$ 822	$ 979	$ 364	$ 753	$ 4,443	$ 2,918	$ 4,422
Transportation and processing									845	901	1,252
Operating									506	598	723
Purchased product									788	586	323
Depreciation, depletion and amortization									833	859	1,488
Operating Income (Loss), Total	$ 262	$ (4)	$ 321	$ 489	$ (54)	$ 128	$ (912)	$ (1,043)	1,068	(1,881)	(6,301)
Market Optimization [Member] | Operating Segments [Member]
Product Information [Line Items]
Revenues									863	646	365
Transportation and processing									103	87	12
Operating									35	35	33
Purchased product									788	586	323
Depreciation, depletion and amortization									1
Operating Income (Loss), Total									(64)	(62)	(3)
Market Optimization [Member] | Reportable Subsegments [Member] | Marketing Sales [Member]
Product Information [Line Items]
Revenues									3,939	3,304	4,309
Transportation and processing									291	279	348
Operating									35	35	33
Purchased product									3,676	3,052	3,931
Depreciation, depletion and amortization									1
Operating Income (Loss), Total									(64)	(62)	(3)
Market Optimization [Member] | Intersubsegment Eliminations [Member]
Product Information [Line Items]
Revenues									(3,076)	(2,658)	(3,944)
Transportation and processing									(188)	(192)	(336)
Purchased product									$ (2,888)	$ (2,466)	$ (3,608)