Operating Revenue (Narrative) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenues From Contract With Customer [Line Items]
Revenue recognized from contract liability balances	$ 124	$ 95
Other Current Liabilities and Other Deferred Credits and Other Liabilities
Revenues From Contract With Customer [Line Items]
Contract liability balances	124		$ 130
Virginia Electric and Power Company
Revenues From Contract With Customer [Line Items]
Revenue recognized from contract liability balances	36	$ 24
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	$ 29		$ 36