Credit Risk (Narrative) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Counterparty Customer	Dec. 31, 2016 USD ($)
Concentration Risk And Guarantor Obligations [Line Items]
Gross credit exposure	$ 95
Additional collateral to be posted if the credit related contingent features were triggered	62	$ 3
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position	65	$ 9
Virginia Electric and Power Company
Concentration Risk And Guarantor Obligations [Line Items]
Gross credit exposure	60
Dominion Energy Gas Holdings, LLC
Concentration Risk And Guarantor Obligations [Line Items]
Gross credit exposure	$ 15
Credit Concentration Risk
Concentration Risk And Guarantor Obligations [Line Items]
Number of counterparties \| Counterparty	0
Amount of exposure for single counterparty	$ 13
Credit Concentration Risk \| Virginia Electric and Power Company
Concentration Risk And Guarantor Obligations [Line Items]
Number of counterparties \| Counterparty	0
Amount of exposure for single counterparty	$ 13
Credit Concentration Risk \| Dominion Energy Gas Holdings, LLC
Concentration Risk And Guarantor Obligations [Line Items]
Number of counterparties	0
Amount of exposure for single counterparty	$ 4
Credit Concentration Risk \| Investment Grade \| Investment Grade Counterparty
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	26.00%
Credit Concentration Risk \| Investment Grade \| Investment Grade Counterparty \| Virginia Electric and Power Company
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	9.00%
Credit Concentration Risk \| Investment Grade \| Investment Grade Counterparty \| Dominion Energy Gas Holdings, LLC
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	22.00%
Customer Concentration Risk \| Sales Revenue, Net \| Dominion Energy Gas Holdings, LLC
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	96.00%
Number of customers \| Customer	289
Customer Concentration Risk \| Sales Revenue, Net \| Dominion Energy Gas Holdings, LLC \| Ten Largest Customers
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	38.00%
Number of customers \| Customer	10
Customer Concentration Risk \| Sales Revenue, Net \| Dominion Energy Gas Holdings, LLC \| Thirty Largest Customers
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	68.00%
Number of customers \| Customer	30
Customer Concentration Risk \| Sales Revenue, Net \| East Ohio
Concentration Risk And Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	98.00%

Concentration Risk and Guarantor Obligations [Line Items]

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Concentration risk, credit risk, financial instrument, maximum exposure for single counterparty.

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Concentration risk, number of customers provided service.

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Number of counterparties.

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The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

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-Name Accounting Standards Codification
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