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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% |

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- Definition 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) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959 + Details
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- Definition 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 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611 + Details
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