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Fair Value Measurements (Narrative) (Detail) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Oct. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Facility Limit |  | $ 5,500,000,000 | $ 5,500,000,000 |  |
Revolving Credit Facility | Atlantic Coast Pipeline | Financial Guarantee [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Facility Limit |  | 3,400,000,000 |  |  | $ 3,400,000,000
Guarantee liability |  | 30,000,000 |  |  |
Virginia Electric and Power Company |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Facility Limit | [1] | 5,500,000,000 | 5,500,000,000 |  |
Unrealized gains and losses included in earnings in Level 3 fair value category |  | 0 | 0 | $ 0 |
Dominion Energy Gas Holdings, LLC |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Facility Limit | [2] | 1,500,000,000 | 1,500,000,000 |  |
Unrealized gains and losses included in earnings in Level 3 fair value category |  | $ 0 | $ 0 | $ 0 |
[1] | The full amount of the facilities is available to Virginia Power, less any amounts outstanding to co-borrowers Dominion Energy, Dominion Energy Gas and Questar Gas. Sub-limits for Virginia Power are set within the facility limit but can be changed at the option of the Companies multiple times per year. At December 31, 2017, the sub-limit for Virginia Power was an aggregate $1.5 billion. If Virginia Power has liquidity needs in excess of its sub-limit, the sub-limit may be changed or such needs may be satisfied through short-term intercompany borrowings from Dominion Energy. These facilities mature in April 2020, and can be used to support bank borrowings and the issuance of commercial paper, as well as to support up to $2.0 billion (or the sub-limit, whichever is less) of letters of credit.
[2] | A maximum of a combined $1.5 billion of the facilities is available to Dominion Energy Gas, assuming adequate capacity is available after giving effect to uses by co-borrowers Dominion Energy, Virginia Power and Questar Gas. Sub-limits for Dominion Energy Gas are set within the facility limit but can be changed at the option of the Companies multiple times per year. At December 31, 2017, the sub-limit for Dominion Energy Gas was an aggregate $500 million. If Dominion Energy Gas has liquidity needs in excess of its sub-limit, the sub-limit may be changed or such needs may be satisfied through short-term intercompany borrowings from Dominion Energy. These credit facilities mature in April 2020 and can be used to support bank borrowings and the issuance of commercial paper, as well as to support up to $1.5 billion (or the sub-limit, whichever is less) of letters of credit.

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