Exploratory Costs (Narrative) (Details)	Mar. 31, 2018	Dec. 31, 2017
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Number of wells or projects with exploratory well costs that have been suspended for a period greater than one year	7	7
Eagle Ford Shale area
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Number of wells or projects with exploratory well costs that have been suspended for a period greater than one year	7

The number of projects for which exploratory well costs have been capitalized for a period of greater than one year after the completion of drilling.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 1B
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=68064819&loc=SL6806780-109447

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