Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

Name:	us-gaap_AssetRetirementObligationRevisionOfEstimate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Asset Retirement Obligations (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Asset Retirement Obligations [Line Items]
Decrease in asset retirement obligation			$ 64	$ 26
Revision of estimated obligation		$ (184)	$ 23	$ (184)
Barnett Shale [Member]
Asset Retirement Obligations [Line Items]
Decrease in asset retirement obligation	$ 34