Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales	$ 1,712	$ 1,804	$ 1,831
Realized gains	195	140	166
Realized losses	96	91	71
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales	858	887	849
Realized gains	58	60	75
Realized losses	$ 22	$ 27	$ 30