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Derivatives and Hedge Accounting Activities (Schedule of Offsetting Assets) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | $ 303 | $ 272
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 303 | 272
Total derivatives, not subject to a master netting or similar arrangement | 4 | 7
Gross Amounts of Recognized Assets, Total | 307 | 279
Derivative Asset | 307 | 279
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 99 | 67
Net Amounts | 204 | 205
Virginia Electric and Power Company |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 155 | 153
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 155 | 153
Total derivatives, not subject to a master netting or similar arrangement | 11 | 41
Gross Amounts of Recognized Assets, Total | 166 | 194
Derivative Asset | 166 | 194
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 4 | 2
Net Amounts | 151 | 151
Dominion Energy Gas Holdings, LLC |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 32 | 0
Gross Amounts Offset in the Consolidated Balance Sheet | 0 | 0
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 32 | 0
Derivative Asset | 32 |
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 2 | 0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received | 0 | 0
Net Amounts | 30 | 0
Commodity | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 174 | 211
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 174 | 211
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 9 | 14
Net Amounts | 165 | 197
Commodity | Exchange |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 80 | 44
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 80 | 44
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 80 | 44
Commodity | Virginia Electric and Power Company | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 155 | 147
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 155 | 147
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 4 | 2
Net Amounts | 151 | 145
Interest rate | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 17 | 17
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 17 | 17
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 8 | 9
Net Amounts | 9 | 8
Interest rate | Virginia Electric and Power Company | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets |  | 6
Net Amounts of Assets Presented in the Consolidated Balance Sheet |  | 6
Net Amounts |  | 6
Foreign currency | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 32 |
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 32 |
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 2 |
Net Amounts | 30 |
Foreign currency | Dominion Energy Gas Holdings, LLC | Over-the-counter |  |
Offsetting Assets [Line Items] |  |
Gross Amounts of Recognized Assets | 32 | 0
Gross Amounts Offset in the Consolidated Balance Sheet | 0 | 0
Net Amounts of Assets Presented in the Consolidated Balance Sheet | 32 | 0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments | 2 | 0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received | 0 | 0
Net Amounts | $ 30 | $ 0

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