Supplemental Information on Oil and Gas Operations (Unaudited) (Results Of Operations) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / Boe	Dec. 31, 2019 USD ($) $ / Boe	Dec. 31, 2018 USD ($) $ / Boe
Oil And Gas Exploration And Production Industries Disclosures [Abstract]
Oil, gas and NGL sales	$ 2,695	$ 3,809	$ 4,085
Production expenses	(1,123)	(1,197)	(1,153)
Exploration expenses	(167)	(58)	(128)
Depreciation, depletion and amortization	(1,207)	(1,398)	(1,134)
Asset dispositions		37	276
Asset impairments	(2,664)		(109)
Accretion of asset retirement obligations	(20)	(21)	(26)
Income tax expense		(270)	(416)
Results of operations	$ (2,486)	$ 902	$ 1,395
Depreciation, depletion and amortization per Boe | $ / Boe	9.90	11.72	10.51