Supplemental Information On Oil And Gas Operations (Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property acquisition costs:
Proved properties	$ 237	$ 195	$ 5,210
Unproved properties	1,358	717	1,177
Exploration costs	394	587	322
Development costs	1,143	3,671	5,463
Costs incurred	3,132	5,170	12,172
United States [Member]
Property acquisition costs:
Proved properties	237	193	5,210
Unproved properties	1,356	634	1,176
Exploration costs	345	478	270
Development costs	1,034	3,269	4,400
Costs incurred	2,972	4,574	11,056
Canada [Member]
Property acquisition costs:
Proved properties		2
Unproved properties	2	83	1
Exploration costs	49	109	52
Development costs	109	402	1,063
Costs incurred	$ 160	$ 596	$ 1,116