Significant Accounting Policies (Reconciliation of Total Cash, Restricted Cash and Equivalents) (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020		Sep. 30, 2020	Dec. 31, 2019
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	$ 180	$ 172	[1]
Current Assets Held for Sale
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	15	7		$ 49	$ 31
Restricted cash and equivalents	$ 22	$ 3		$ 16	$ 12

[1] Dominion Energy’s Consolidated Balance Sheet at December 31, 2020 has been derived from the audited Consolidated Balance Sheet at that date.