Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value using NAV	$ 340	$ 274
Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value using NAV	$ 167	$ 159