Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		$ 279	$ (898)
Less: Net (gains) losses recognized during the period on securities sold during the period		(178)	14
Unrealized gains (losses) recognized during the period on securities still held at period end	[1]	101	(884)
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		143	(423)
Less: Net (gains) losses recognized during the period on securities sold during the period		(88)	6
Unrealized gains (losses) recognized during the period on securities still held at period end		$ 55	$ (417)

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