Income Taxes (Reconciliation of Income Taxes at the U.S. Statutory Federal Income Tax Rate) (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 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	3.30%	2.90%
Investment tax credits	(0.90%)	(5.70%)
Production tax credits	(0.70%)	(0.70%)
Reversal of excess deferred income taxes	(1.50%)
Federal legislative change	2.00%
State legislative change	(0.80%)
AFUDC - equity	(0.90%)	(1.30%)
Other, net	(1.00%)	(2.60%)
Effective tax rate	20.50%	27.60%
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	4.60%	3.70%
Investment tax credits	(1.40%)	(0.80%)
Production tax credits	(0.70%)	(0.50%)
Reversal of excess deferred income taxes	(1.90%)
Federal legislative change	(0.20%)
AFUDC - equity	(0.50%)	(0.60%)
Other, net	(0.30%)	0.20%
Effective tax rate	20.60%	37.00%
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	3.90%	2.70%
Reversal of excess deferred income taxes	(1.30%)
Federal legislative change	2.00%
State legislative change	0.10%
AFUDC - equity	(0.40%)	(0.80%)
Other, net	0.70%	(0.10%)
Effective tax rate	26.00%	36.80%