Leases (Cash Paid for Amounts Included in Measurement of Lease Liabilities) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee Lease Disclosure [Line Items]
Operating cash flows for finance leases		$ 4
Operating cash flows for operating leases	$ 96	111
Financing cash flows for finance leases	33	20
Virginia Electric and Power Company
Lessee Lease Disclosure [Line Items]
Operating cash flows for operating leases	$ 52	$ 56