Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Investment Holdings [Line Items]
Net losses recognized during the period	$ (65)
Less: Net gains recognized during the period on securities sold during the period	(19)
Unrealized gain (loss) on securities recorded in other income, before tax	(84)	[1]
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net losses recognized during the period	(32)
Less: Net gains recognized during the period on securities sold during the period	(15)
Unrealized gain (loss) on securities recorded in other income, before tax	$ (47)	[1]

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