Related-Party Transactions (Detail) - Virginia Electric and Power Company - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Commodity purchases from affiliates		$ 135	$ 170	$ 450	$ 561
Services provided by affiliates	[1]	108	107	343	387
Affiliated Entity
Related Party Transaction [Line Items]
Services provided to affiliates		$ 5	$ 5	$ 14	$ 19

[1] Includes capitalized expenditures of $39 million and $33 million for the three months ended September 30, 2020 and 2019, respectively, and $107 million and $100 million for the nine months ended September 30, 2020 and 2019, respectively.