Significant Accounting Policies (Checks the Outstanding Accounts Payable but not yet Presented for Payment and Recorded) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts Payable [Line Items]
Accounts payable for checks outstanding	$ 50	$ 29
Virginia Electric and Power Company
Accounts Payable [Line Items]
Accounts payable for checks outstanding	$ 30	$ 9