Related-Party Transactions (Detail) - Virginia Electric and Power Company - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Related Party Transaction [Line Items]
Commodity purchases from affiliates		$ 126	$ 104	$ 307	$ 315
Services provided by affiliates	[1]	122	114	247	235
Affiliated Entity
Related Party Transaction [Line Items]
Services provided to affiliates		$ 4	$ 4	$ 9	$ 9

[1] Includes capitalized expenditures of $44 million and $34 million for the three months ended June 30, 2021 and 2020, respectively, and $82 million and $68 million for the six months ended June 30, 2021 and 2020, respectively.