Income Taxes (Reconciliation of Income Taxes at the U.S. Statutory Federal Income Tax Rate) (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Effective Income Tax Computation [Line Items]
U.S. statutory rate	21.00%	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	1.50%	(898.50%)
Investment tax credits	(30.50%)	(325.20%)
Production tax credits	(2.40%)	(66.30%)
Reversal of excess deferred income taxes	(14.50%)	(375.60%)
Write-off of regulatory assets	0.00%	16565.90%
Change in tax status	(6.10%)	0.00%
AFUDC - equity	(1.10%)	(55.80%)
Changes in state deferred taxes associated with assets held for sale	(11.60%)	0.00%
Absence of tax on noncontrolling interest	14.10%	0.00%
Other, net	(1.80%)	(987.30%)
Effective tax rate	(31.40%)	13878.20%
Virginia Electric and Power Company
Effective Income Tax Computation [Line Items]
U.S. statutory rate	21.00%	21.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.70%	4.50%
Investment tax credits	(5.60%)	(5.30%)
Production tax credits	(0.90%)	(0.80%)
Reversal of excess deferred income taxes	(1.90%)	(4.10%)
Write-off of regulatory assets	0.00%	0.00%
Change in tax status	0.00%	0.00%
AFUDC - equity	(0.30%)	0.00%
Changes in state deferred taxes associated with assets held for sale	0.00%	0.00%
Absence of tax on noncontrolling interest	0.00%	0.00%
Other, net	0.00%	(1.20%)
Effective tax rate	17.00%	14.10%