Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		$ 308	$ 40	$ 20	$ 610
Less: Net gains recognized during the period on securities sold during the period		(15)	(17)	(6)	(61)
Unrealized gains (losses) recognized during the period on securities still held at September 30, 2020 and 2019	[1]	293	23	14	549
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		138	30	(16)	286
Less: Net gains recognized during the period on securities sold during the period		(6)	(7)	(3)	(15)
Unrealized gains (losses) recognized during the period on securities still held at September 30, 2020 and 2019	[1]	$ 132	$ 23	$ (19)	$ 271

[1] Included in other income and the nuclear decommissioning trust regulatory liability.