Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value using NAV	$ 211	$ 88
Alternative Investment, Fair Value by Fair Value Hierarchy Level and NAV [Extensible List]	us-gaap:Assets	us-gaap:Assets
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	$ 171	$ 27
Alternative Investment, Fair Value by Fair Value Hierarchy Level and NAV [Extensible List]	us-gaap:Assets	us-gaap:Assets