Acquisitions and Dispositions (Results of Operations and Pro Forma Information) (Narrative) (Detail) - Dominion Energy Questar Corporation - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Increase in operating revenue		$ 379
Increase in net income		73
Other operations and maintenance
Business Acquisition [Line Items]
Transaction and transition costs incurred	$ 26	58
Interest And Related Charges
Business Acquisition [Line Items]
Transaction and transition costs incurred		$ 16

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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