Significant Accounting Policies (Schedule of Supplemental Cash Flow Information) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Supplemental Cash Flow Information [Line Items]
Financing leases	$ 46	$ 113
Operating leases	127	44
Virginia Electric and Power Company
Schedule Of Supplemental Cash Flow Information [Line Items]
Financing leases	$ 32	20
Operating leases		$ 35