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Investments (Investments Under Equity Method of Accounting) (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2015
--- | --- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |  |
Liabilities, other deferred credits and other liabilities | $ 16,631 | $ 16,424 |
Iroquois |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Nedpower Mount Storm Llc [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Liabilities, other deferred credits and other liabilities | $ 17 |  |
Dominion Energy Midstream Partners, LP | Iroquois |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 25.93% |  | 25.93%
Dominion Energy Gas Holdings, LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Liabilities, other deferred credits and other liabilities | $ 2,866 | $ 2,863 |
Dominion Energy Gas Holdings, LLC | Iroquois |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 24.07% |  |

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