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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
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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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