Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of securities		$ 425	$ 363	$ 844	$ 1,119
Realized gains	[1]	36	23	72	117
Realized losses	[1]	23	16	42	36
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of securities		196	168	414	498
Realized gains	[1]	15	10	33	55
Realized losses	[1]	$ 7	$ 8	$ 12	$ 18

[1] Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.