Significant Accounting Policies (Schedule of Supplemental Cash Flow Information) (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Supplemental Cash Flow Information [Line Items]
Financing leases	$ 34	$ 42
Operating Leases	41	3
Virginia Electric and Power Company
Schedule Of Supplemental Cash Flow Information [Line Items]
Financing leases	24	$ 26
Operating Leases	$ 35