Supplementary Oil And Gas Information (Capitalized Costs) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Proved oil and gas properties	$ 40,228	$ 39,610	$ 40,647
Unproved oil and gas properties	4,480	5,198	5,616
Total capital cost	44,708	44,808	46,263
Accumulated DD&A	37,350	38,254	38,050
Net capitalized costs	7,358	6,554	8,213
Canada [Member]
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Proved oil and gas properties	14,555	13,159	14,866
Unproved oil and gas properties	311	285	334
Total capital cost	14,866	13,444	15,200
Accumulated DD&A	14,047	12,896	14,170
Net capitalized costs	819	548	1,030
United States [Member]
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Proved oil and gas properties	25,610	26,393	25,723
Unproved oil and gas properties	4,169	4,913	5,282
Total capital cost	29,779	31,306	31,005
Accumulated DD&A	23,240	25,300	23,822
Net capitalized costs	6,539	6,006	7,183
Other Countries [Member]
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Proved oil and gas properties	63	58	58
Total capital cost	63	58	58
Accumulated DD&A	$ 63	$ 58	$ 58