Related-Party Transactions (Detail) - Virginia Electric and Power Company - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Commodity purchases from affiliates		$ 219	$ 135	$ 526	$ 450
Services provided by affiliates	[1]	116	108	363	343
Affiliated Entity
Related Party Transaction [Line Items]
Services provided to affiliates		$ 6	$ 5	$ 15	$ 14

[1] Includes capitalized expenditures of $39 million for both the three months ended September 30, 2021 and 2020, and $121 million and $107 million for the nine months ended September 30, 2021 and 2020, respectively.