Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Derivative liabilities, not subject to a master netting or similar arrangement	$ 2	$ 1
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Derivative liabilities, not subject to a master netting or similar arrangement	$ 2	$ 22

Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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Period Type:	instant