Supplementary Oil And Gas Information (Changes In Standardized Measure Of Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Balance, beginning of year	$ 1,675	$ 2,048	$ 11,550
Sales of oil and gas produced during the period	(1,762)	(1,356)	(2,264)
Discoveries and extensions, net of related costs	1,723	478	613
Purchases of proved reserves in place	13	47
Sales and transfers of proved reserves in place	(425)	(291)	(2,278)
Net change in prices and production costs	3,076	345	(6,314)
Revisions to quantity estimates	(225)	(85)	(3,538)
Accretion of discount	168	205	1,398
Development costs incurred during the period	1,820	1,159	2,334
Changes in Estimated Future Development Costs	(1,712)	(760)	(1,822)
Other	15	(115)	(61)
Net change in income taxes	(53)		2,430
Balance, end of year	4,313	1,675	2,048
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Balance, beginning of year	439	635	4,476
Sales of oil and gas produced during the period	(471)	(316)	(988)
Discoveries and extensions, net of related costs	582	211	109
Sales and transfers of proved reserves in place	(12)	(71)	(674)
Net change in prices and production costs	893	20	(3,075)
Revisions to quantity estimates	(22)	(124)	(1,355)
Accretion of discount	44	64	565
Development costs incurred during the period	454	286	460
Changes in Estimated Future Development Costs	(279)	(304)	(13)
Other	7	38	(45)
Net change in income taxes	(53)		1,175
Balance, end of year	1,582	439	635
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Balance, beginning of year	1,236	1,413	7,074
Sales of oil and gas produced during the period	(1,291)	(1,040)	(1,276)
Discoveries and extensions, net of related costs	1,141	267	504
Purchases of proved reserves in place	13	47
Sales and transfers of proved reserves in place	(413)	(220)	(1,604)
Net change in prices and production costs	2,183	325	(3,239)
Revisions to quantity estimates	(203)	39	(2,183)
Accretion of discount	124	141	833
Development costs incurred during the period	1,366	873	1,874
Changes in Estimated Future Development Costs	(1,433)	(456)	(1,809)
Other	8	(153)	(16)
Net change in income taxes			1,255
Balance, end of year	$ 2,731	$ 1,236	$ 1,413