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Acquisitions and Dispositions (Results of Operations and Pro Forma Information) (Narrative) (Detail) - Dominion Energy Questar Corporation - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
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 $ 9 $ 26 58
Interest And Related Charges      
Business Acquisition [Line Items]      
Transaction and transition costs incurred     $ 16