Accounting For Certain Long-Lived Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Impaired Long-Lived Assets Held and Used [Line Items]
Proved oil and gas properties carrying amount	$ 370	$ 643
Proved oil and gas properties written down to fair value	146	527
Pretax impairment charges for proved oil and gas properties	224	116
Amortization and impairments of unproved oil and gas property costs including amortization of capitalized interest	$ 211	$ 291	$ 288