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Fair Value Measurements (Narrative) (Detail) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Unrealized gains or losses included in earnings in Level 3 fair value category | $ 0 | $ 0 | $ 0 | $ 0
Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Unrealized gains or losses included in earnings in Level 3 fair value category |  |  | 0 |
Dominion Energy Gas Holdings, LLC | Commodity |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Net changes in assets and liabilities measured at fair value on recurring basis | $ 0 | $ 0 | $ 0 |

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