Supplemental Information On Oil And Gas Operations (Results Of Operations) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / Boe	Dec. 31, 2017 USD ($) $ / Boe	Dec. 31, 2016 USD ($) $ / Boe
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil, gas and NGL sales	$ 5,677	$ 5,150	$ 4,182
Production expenses	(2,225)	(1,823)	(1,805)
Exploration expenses	(177)	(380)	(215)
Depreciation, depletion and amortization	(1,559)	(1,419)	(1,446)
Asset dispositions	262	212	947
Asset impairments	(109)		(435)
Accretion of asset retirement obligations	(59)	(62)	(75)
Income tax (expense) benefit	(409)	(104)	(13)
Results of operations	$ 1,401	$ 1,574	$ 1,140
Depreciation, depletion and amortization per Boe | $ / Boe	7.98	7.15	6.47
US [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil, gas and NGL sales	$ 4,863	$ 3,746	$ 3,198
Production expenses	(1,620)	(1,232)	(1,313)
Exploration expenses	(129)	(346)	(176)
Depreciation, depletion and amortization	(1,234)	(1,050)	(1,066)
Asset dispositions	262	211	946
Asset impairments	(109)		(435)
Accretion of asset retirement obligations	(35)	(38)	(49)
Income tax (expense) benefit	(460)
Results of operations	$ 1,538	$ 1,291	$ 1,105
Depreciation, depletion and amortization per Boe | $ / Boe	8.08	6.97	6.11
Canada [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Oil, gas and NGL sales	$ 814	$ 1,404	$ 984
Production expenses	(605)	(591)	(492)
Exploration expenses	(48)	(34)	(39)
Depreciation, depletion and amortization	(325)	(369)	(380)
Asset dispositions		1	1
Accretion of asset retirement obligations	(24)	(24)	(26)
Income tax (expense) benefit	51	(104)	(13)
Results of operations	$ (137)	$ 283	$ 35
Depreciation, depletion and amortization per Boe | $ / Boe	7.63	7.73	7.75