Income Taxes (Reconciliation of Income Taxes at the U.S. Statutory Federal Income Tax Rate) (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Effective Income Tax Computation [Line Items]
U.S. statutory rate	21.00%	35.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	3.80%	2.80%
Investment tax credits	(0.90%)	(5.50%)
Production tax credits	(0.70%)	(0.80%)
Reversal of excess deferred income taxes	(1.50%)
State legislative change	(1.60%)
AFUDC - equity	(1.00%)	(1.40%)
Other, net	(1.00%)	(2.60%)
Effective tax rate	18.10%	27.50%
Virginia Electric and Power Company
Effective Income Tax Computation [Line Items]
U.S. statutory rate	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.50%	3.70%
Investment tax credits	(1.40%)	(0.80%)
Production tax credits	(0.70%)	(0.50%)
Reversal of excess deferred income taxes	(2.00%)
AFUDC - equity	(0.50%)	(0.60%)
Other, net	0.30%	(0.10%)
Effective tax rate	21.20%	36.70%
Dominion Energy Gas Holdings, LLC
Effective Income Tax Computation [Line Items]
U.S. statutory rate	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	3.60%	1.50%
Reversal of excess deferred income taxes	(1.20%)
State legislative change	0.30%
AFUDC - equity	(0.40%)	(1.00%)
Other, net	0.20%
Effective tax rate	23.50%	35.50%