Regulatory Assets and Liabilities (Narrative) (Detail) $ in Billions	3 Months Ended
	Mar. 31, 2021 USD ($)
Public Utilities General Disclosures [Line Items]
Regulatory assets not expect to earn return	$ 4.1
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	$ 2.8
Period for which expenditures are expected to be recovered	2 years