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Investments (Investments Under Equity Method of Accounting) (Detail) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Equity Method Investments [Line Items] |  |  |
Investment Balance | $ 1,544 | $ 1,561 | $ 1,320
Iroquois |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Investment Balance | $ 311 | 316 |
Description | Gas transmission system |  |
Blue Racer Midstream Llc [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Investment Balance | $ 691 | 677 |
Description | Midstream gas and related services |  |
Atlantic Coast Pipeline |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 48.00% |  |
Investment Balance | $ 382 | 256 |
Description | Gas transmission system |  |
Fowler I Holdings LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Investment Balance | $ 81 | 116 |
Description | Wind-powered merchant generation facility |  |
Nedpower Mount Storm Llc [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 50.00% |  |
Investment Balance |  | 112 |
Description | Wind-powered merchant generation facility |  |
Other Investment [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Investment Balance | $ 79 | 84 |
Dominion Energy Gas Holdings, LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Investment Balance | $ 95 | 98 | $ 102
Dominion Energy Gas Holdings, LLC | Iroquois |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership% | 24.07% |  |
Investment Balance | $ 95 | $ 98 |
Description | Gas transmission system |  |

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