Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		$ (15)	$ 308	$ 616	$ 20
Less: Net (gains) losses recognized during the period on securities sold during the period		(11)	(15)	(323)	(6)
Unrealized gains (losses) recognized during the period on securities still held at period end	[1]	(26)	293	293	14
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		6	138	319	(16)
Less: Net (gains) losses recognized during the period on securities sold during the period		(9)	(6)	(182)	(3)
Unrealized gains (losses) recognized during the period on securities still held at period end	[1]	$ (3)	$ 132	$ 137	$ (19)

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