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Employee Benefit Plans (Narrative) (Detail)
3 Months Ended
Mar. 31, 2018
USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Contributions to defined benefit pension plans and OPEB plans $ 0
Expected contribution to voluntary employees' beneficiary association for the remainder of 2018 12,000,000
Dominion Energy Gas Holdings, LLC  
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Contributions to defined benefit pension plans and OPEB plans 0
Expected contribution to voluntary employees' beneficiary association for the remainder of 2018 $ 12,000,000