Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		$ 512	$ 919	$ (245)
Less: Net gains recognized during the period on securities sold during the period		(16)	(80)	(58)
Unrealized gains (losses) recognized during the period on securities still held at period end(1)	[1]	496	839	(303)
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains (losses) recognized during the period		224	423	(105)
Less: Net gains recognized during the period on securities sold during the period		(6)	(20)	(32)
Unrealized gains (losses) recognized during the period on securities still held at period end(1)	[1]	$ 218	$ 403	$ (137)

[1] Included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.