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Employee Benefit Plans (Fair values of pension and post retirement plan assets by asset category) (Parenthetical) (Detail) - Collective Bargaining Agreement - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Pension Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pending sales of securities | $ 11 | $ 46
Net accrued income | 19 | 19
Pending purchases of securities | 15 | 47
Other Postretirement Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pending sales of securities | 1 | 5
Net accrued income | 2 | 2
Pending purchases of securities | 1 | 5
Dominion Energy Gas Holdings, LLC | Pension Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pending sales of securities | 3 | 10
Net accrued income | 4 | 4
Pending purchases of securities | $ 3 | $ 10

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- Details
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- Details
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