Operating Revenue (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenues From Contract With Customer [Line Items]
Revenue recognized from contract liability balances	$ 97	$ 85
Other Current Liabilities and Other Deferred Credits and Other Liabilities
Revenues From Contract With Customer [Line Items]
Contract liability balances	130	102
Virginia Electric and Power Company
Revenues From Contract With Customer [Line Items]
Revenue recognized from contract liability balances	24	22
Virginia Electric and Power Company | Other Current Liabilities and Other Deferred Credits and Other Liabilities
Revenues From Contract With Customer [Line Items]
Contract liability balances	$ 36	$ 24