Supplementary Oil And Gas Information (Standardized Measure Of Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves-Before Discounting) (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	$ 19,309	$ 13,878	$ 15,746
Production costs	7,177	6,415	7,759
Development costs	5,100	4,754	4,872
Income taxes	76
Future net cash flows	6,956	2,709	3,115
Less 10% annual discount for estimated timing of cash flows	2,643	1,034	1,067
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	4,313	1,675	2,048	$ 11,550
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	7,850	5,341	6,284
Production costs	3,516	2,876	3,800
Development costs	2,058	1,949	1,742
Income taxes	76
Future net cash flows	2,200	516	742
Less 10% annual discount for estimated timing of cash flows	618	77	107
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	1,582	439	635	4,476
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	11,459	8,537	9,462
Production costs	3,661	3,539	3,959
Development costs	3,042	2,805	3,130
Future net cash flows	4,756	2,193	2,373
Less 10% annual discount for estimated timing of cash flows	2,025	957	960
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	$ 2,731	$ 1,236	$ 1,413	$ 7,074