Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment losses for debt securities	$ 5
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment losses for debt securities	$ 2

Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284422-111562

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration