Supplemental Information On Oil And Gas Operations (Standardized Measure Of Discounted Future Net Cash Flows Related To Proved Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 48,303	$ 32,519	$ 40,445
Future costs:
Development	(5,169)	(4,985)	(6,065)
Production	(21,512)	(17,983)	(21,439)
Future income tax expense	(988)	(121)	(580)
Future net cash flow	20,634	9,430	12,361
10% discount to reflect timing of cash flows	(9,297)	(3,990)	(4,478)
Standardized measure of discounted future net cash flows	11,337	5,440	7,883	$ 21,583
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	34,701	22,847	27,398
Future costs:
Development	(3,316)	(2,784)	(3,306)
Production	(15,526)	(11,934)	(14,938)
Future net cash flow	15,859	8,129	9,154
10% discount to reflect timing of cash flows	(7,541)	(3,524)	(3,230)
Standardized measure of discounted future net cash flows	8,318	4,605	5,924
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	13,602	9,672	13,047
Future costs:
Development	(1,853)	(2,201)	(2,759)
Production	(5,986)	(6,049)	(6,501)
Future income tax expense	(988)	(121)	(580)
Future net cash flow	4,775	1,301	3,207
10% discount to reflect timing of cash flows	(1,756)	(466)	(1,248)
Standardized measure of discounted future net cash flows	$ 3,019	$ 835	$ 1,959