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Nature of Operations (Narrative) (Detail) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 shares | Oct. 31, 2014 shares | Dec. 31, 2017 Segments
Subsidiary, Sale of Stock [Line Items] |  |  |
Number of operating segments |  |  | 3
Dominion Energy Midstream Partners, LP | Iroquois | Partnership Interest |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Common units included in initial public offering | shares | 8,600,000 |  |
Noncontrolling partnership percentage interest |  |  | 25.93%
Dominion Energy Midstream Partners, LP | Common and Subordinated Units |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Ownership interest percentage of limited partner interests |  |  | 50.60%
Dominion Energy Midstream Partners, LP | Convertible Preferred Units |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Ownership interest percentage of limited partner interests |  |  | 37.50%
Dominion Energy Midstream Partners, LP | IPO [Member] |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Common units included in initial public offering | shares |  | 20,125,000 |
Virginia Electric and Power Company |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Number of operating segments |  |  | 2
Dominion Energy Gas Holdings, LLC |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |
Number of operating segments |  |  | 1

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