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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sales |  | $ 419 | $ 756
Realized gains | [1] | 36 | 94
Realized losses | [1] | 19 | 20
Virginia Electric and Power Company |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sales |  | 218 | 330
Realized gains | [1] | 18 | 45
Realized losses | [1] | $ 5 | $ 10
[1] | Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.

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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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- Definition The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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