Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		$ 352	$ 610	$ 631	$ (288)
Less: Net (gains) losses recognized during the period on securities sold during the period		(134)	(5)	(312)	9
Unrealized gains (losses) recognized during the period on securities still held at period end	[1]	218	605	319	(279)
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		170	269	313	(154)
Less: Net (gains) losses recognized during the period on securities sold during the period		(85)	(3)	(173)	3
Unrealized gains (losses) recognized during the period on securities still held at period end	[1]	$ 85	$ 266	$ 140	$ (151)

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