Regulatory Assets and Liabilities (Narrative) (Detail) $ in Billions	12 Months Ended
	Dec. 31, 2020 USD ($)
Public Utilities General Disclosures [Line Items]
Regulatory assets not expect to earn return	$ 4.5
Period for which expenditures are expected to be recovered	2 years
Virginia Electric and Power Company
Public Utilities General Disclosures [Line Items]
Regulatory assets not expect to earn return	$ 3.2
Period for which expenditures are expected to be recovered	2 years