Supplemental Quarterly Financial Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	[1]	Dec. 31, 2015	[1]
Quarterly Financial Data [Line Items]
Asset dispositions									$ 217	$ 1,483		$ (7)
Asset dispositions per diluted share									$ 0.42
Asset impairments									$ 17	$ 1,310		$ 17,647
As Reported Under Sucessful Efforts
Quarterly Financial Data [Line Items]
Asset dispositions	$ 18	$ 169	$ 27	$ 3	$ 575	$ 830	$ 75	$ 3
Asset dispositions per diluted share					$ 1.10	$ 1.59	$ 0.14	$ 0.01
Asset impairments	$ 8	$ 2		$ 7	$ 81			$ 1,200
Asset impairment per diluted share					$ 0.15			$ 2.59

[1] Prior year amounts have been recast due to change in accounting principle. See Note 2 in “Item 8. Financial Statements and Supplementary Data” of this report.