Supplementary Oil And Gas Information (Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Costs Incurred (Line Items)
Unproved oil and gas properties	$ 52	$ 4	$ 17
Proved	2	206	19
Total acquisitions	54	210	36
Exploration costs	5	14	6
Development costs	1,779	1,115	2,221
Total costs incurred	1,838	1,339	2,263
Canada [Member]
Costs Incurred (Line Items)
Unproved oil and gas properties	31		2
Proved		1	7
Total acquisitions	31	1	9
Exploration costs	1	1	3
Development costs	425	255	377
Total costs incurred	457	257	389
United States [Member]
Costs Incurred (Line Items)
Unproved oil and gas properties	21	4	15
Proved	2	205	12
Total acquisitions	23	209	27
Exploration costs	4	13	3
Development costs	1,354	860	1,844
Total costs incurred	$ 1,381	$ 1,082	$ 1,874