Equity (Accumulated Other Comprehensive Income (Loss)) (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred Gains and Losses on Derivatives-Hedging Activities
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax	$ 141	$ 135
Unrealized Gains and Losses on Investment Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax	(21)	(13)
Unrecognized Pension and Other Postretirement Benefit Costs
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax	478	492
Other Comprehensive Loss From Equity Method Investees
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax		1
Virginia Electric and Power Company | Deferred Gains and Losses on Derivatives-Hedging Activities
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax	21	11
Virginia Electric and Power Company | Unrealized Gains and Losses on Investment Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Amount of tax	$ (3)	$ (1)