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