Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Fair value of securities in an unrealized loss position	$ 1,100	$ 565
Net assets related to pending sales and purchases of securities		5
Virginia Electric and Power Company
Investment Holdings [Line Items]
Fair value of securities in an unrealized loss position	539	234
Net assets related to pending sales and purchases of securities	$ 1	$ 6