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Credit Risk (Narrative) (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) Counterparty Customer | Dec. 31, 2017 USD ($) | Dec. 31, 2016
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Gross credit exposure | $ 145 |  |
Additional collateral to be posted if the credit related contingent features were triggered | 1 | $ 62 |
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position | 1 | $ 65 |
Virginia Electric and Power Company |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Gross credit exposure | 10 |  |
Dominion Energy Gas Holdings, LLC |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Gross credit exposure | $ 81 |  |
Credit Concentration Risk |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Number of counterparties | Counterparty | 0 |  |
Amount of exposure for single counterparty | $ 47 |  |
Credit Concentration Risk | Virginia Electric and Power Company |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Amount of exposure for single counterparty | $ 9 |  |
Credit Concentration Risk | Dominion Energy Gas Holdings, LLC |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Number of counterparties | Counterparty | 0 |  |
Amount of exposure for single counterparty | $ 15 |  |
Credit Concentration Risk | Investment Grade | Investment Grade Counterparty |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Concentration risk, percentage (percentage) | 69.00% |  |
Credit Concentration Risk | Investment Grade | Investment Grade Counterparty | Dominion Energy Gas Holdings, LLC |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Concentration risk, percentage (percentage) | 94.00% |  |
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 | 307 |  |
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) | 71.00% |  |
Number of customers | Customer | 30 |  |
Customer Concentration Risk | Sales Revenue, Net | Dominion Energy Gas Holdings, LLC | Largest Customer |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Concentration risk, percentage (percentage) | 12.00% | 5.00% | 14.00%
Customer Concentration Risk | Sales Revenue, Net | Dominion Energy Gas Holdings, LLC | Export Customers |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Concentration risk, percentage (percentage) | 32.00% | 9.00% | 18.00%
Customer Concentration Risk | Sales Revenue, Net | East Ohio |  |  |
Concentration Risk And Guarantor Obligations [Line Items] |  |  |
Concentration risk, percentage (percentage) | 99.00% |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959 + Details
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