Segmented Information (Segment And Geographical Information) (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
Segment Reporting Information [Line Items]
Product revenues									$ 2,999	$ 2,443	$ 3,350
Gains (losses) from risk management, net									482	(275)	592
Market optimization									863	647	368
Other revenues									99	103	112
Total Revenues	$ 1,210	$ 861	$ 1,083	$ 1,289	$ 822	$ 979	$ 364	$ 753	4,443	2,918	4,422
Production, mineral and other taxes									112	99	144
Transportation and processing									845	901	1,252
Operating									506	598	723
Purchased product									788	586	323
Depreciation, depletion and amortization									833	859	1,488
Asset Impairment Charges							484	912		1,396	6,473
Accretion of asset retirement obligation									37	51	45
Administrative									254	309	275
Total Operating Expenses									3,375	4,799	10,723
Operating Income (Loss), Total	262	(4)	321	489	(54)	128	(912)	(1,043)	1,068	(1,881)	(6,301)
Interest									363	397	614
Foreign exchange (gain) loss, net									(279)	(210)	1,082
(Gain) loss on divestitures, net	1	(406)		1	3	(395)	2		(404)	(390)	(14)
Other (gains) losses, net									(42)	(58)	27
Total Other (Income) Expenses									(362)	(261)	1,709
Net Earnings (Loss) Before Income Tax	147	522	327	434	(251)	379	(1,068)	(680)	1,430	(1,620)	(8,010)
Income tax expense (Recovery)	376	228	(4)	3	30	62	(467)	(301)	603	(676)	(2,845)
Net Earnings (Loss)	$ (229)	$ 294	$ 331	$ 431	$ (281)	$ 317	$ (601)	$ (379)	827	(944)	(5,165)
Corporate, Non-segment [Member]
Segment Reporting Information [Line Items]
Gains (losses) from risk management, net									442	(636)	(325)
Other revenues									69	71	69
Total Revenues									511	(565)	(256)
Transportation and processing										(22)	6
Operating									18	17	19
Depreciation, depletion and amortization									66	76	95
Accretion of asset retirement obligation									37	51	45
Administrative									254	309	275
Total Operating Expenses									375	431	440
Operating Income (Loss), Total									136	(996)	(696)
Canadian Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Product revenues									1,150	952	1,309
Gains (losses) from risk management, net									22	107	495
Other revenues									19	8	18
Total Revenues									1,191	1,067	1,822
Production, mineral and other taxes									20	23	28
Transportation and processing									578	576	654
Operating									122	152	152
Depreciation, depletion and amortization									236	260	305
Asset Impairment Charges										493
Total Operating Expenses									956	1,504	1,139
Operating Income (Loss), Total									235	(437)	683
USA Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Product revenues									1,849	1,491	2,041
Gains (losses) from risk management, net									18	255	425
Other revenues									11	24	25
Total Revenues									1,878	1,770	2,491
Production, mineral and other taxes									92	76	116
Transportation and processing									164	260	580
Operating									331	394	519
Depreciation, depletion and amortization									530	523	1,088
Asset Impairment Charges										903	6,473
Total Operating Expenses									1,117	2,156	8,776
Operating Income (Loss), Total									761	(386)	(6,285)
Market Optimization [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Gains (losses) from risk management, net										(1)	(3)
Market optimization									863	647	368
Total Revenues									863	646	365
Transportation and processing									103	87	12
Operating									35	35	33
Purchased product									788	586	323
Depreciation, depletion and amortization									1
Total Operating Expenses									927	708	368
Operating Income (Loss), Total									$ (64)	$ (62)	$ (3)