Supplementary Oil And Gas Information (Results Of Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil and gas revenues, net of royalties and transportation	$ 2,327	$ 2,001	$ 3,079
Operating costs, production and mineral taxes and accretion of asset retirement obligations	602	696	860
Depreciation, depletion and amortization	766	783	1,393
Results of Operations, Impairment of Oil and Gas Properties	0	1,396	6,473
Operating income (loss)	959	(874)	(5,647)
Income taxes	219	(273)	(2,106)
Results of operations	740	(601)	(3,541)
Canada [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil and gas revenues, net of royalties and transportation	613	491	1,168
Operating costs, production and mineral taxes and accretion of asset retirement obligations	164	197	199
Depreciation, depletion and amortization	236	260	305
Results of Operations, Impairment of Oil and Gas Properties		493
Operating income (loss)	213	(459)	664
Income taxes	58	(123)	179
Results of operations	155	(336)	485
United States [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil and gas revenues, net of royalties and transportation	1,714	1,510	1,911
Operating costs, production and mineral taxes and accretion of asset retirement obligations	438	499	661
Depreciation, depletion and amortization	530	523	1,088
Results of Operations, Impairment of Oil and Gas Properties		903	6,473
Operating income (loss)	746	(415)	(6,311)
Income taxes	161	(150)	(2,285)
Results of operations	$ 585	$ (265)	$ (4,026)