Equity (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
	Jul. 31, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock, par value			$ 0	$ 0
Common stock, shares authorized			50,000,000	50,000,000
Common stock, shares issued			40,300,000	40,300,000
Common stock, shares outstanding			40,300,000	40,300,000
Preferred stock, par value			$ 0	$ 0
Preferred stock, shares authorized			20,000,000	20,000,000
Preferred stock, shares issued			1,000	1,000
Preferred stock, shares outstanding			1,000	1,000
Dominion Energy [Member] | Common Stock
Class Of Stock [Line Items]
Common stock issued to satisfy the settlement		$ 322
Dominion Energy [Member] | Subsequent Event [Member] | Common Stock | FILOT
Class Of Stock [Line Items]
Common stock issued to satisfy the settlement	$ 104
Dominion Energy [Member] | Subsequent Event [Member] | Common Stock | SCDOR
Class Of Stock [Line Items]
Common stock issued to satisfy the settlement	$ 45