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Acquisitions and Dispositions (Narrative) (Detail) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended | 30 Months Ended |
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Apr. 01, 2015 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) MW | Sep. 30, 2015 USD ($) mi MW shares | Jun. 30, 2015 USD ($) Project MW | Jan. 31, 2015 USD ($) mi | Sep. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) | Nov. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Project Cost |  |  |  |  |  |  |  | $ 405 | $ 40 | $ 418 |  |
Goodwill | [1] |  | $ 3,294 |  |  |  |  | 6,405 | 6,399 | 3,294 | $ 6,405 |
DECG |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  |  |  |  | 100.00% |  |  |  |  |  |
Purchase price |  |  |  |  |  | $ 497 |  |  |  |  |  |
Net property plant and equipment acquired |  |  |  |  |  | $ 277 |  |  |  |  |  |
Length of FERC regulated interstate natural gas pipeline (in miles) (nearly 1,500 miles) | mi |  |  |  |  |  | 1,500 |  |  |  |  |  |
Goodwill |  |  |  |  |  | $ 250 |  |  |  |  |  |
Amount expected to be deductible for tax purposes |  |  |  |  |  | 225 |  |  |  |  |  |
Amount of regulatory liabilities |  |  |  |  |  | $ 38 |  |  |  |  |  |
Four Brothers Solar, LLC |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  |  |  | 50.00% |  |  |  |  |  |  |
Number of Projects | Project |  |  |  |  | 4 |  |  |  |  |  |  |
Purchase price |  |  |  |  | $ 64 |  |  |  |  |  |  |
Acquisition price in cash |  |  |  |  | 2 |  |  |  |  |  |  |
Acquisition price payable |  |  |  |  | $ 62 |  |  |  |  |  |  |
MW Capacity | MW |  |  |  |  | 320 |  |  |  |  |  |  |
Project Cost |  |  |  |  |  |  | $ 670 |  |  |  |  |
Purchase price |  |  |  |  | $ 64 |  |  |  |  |  |  |
Net property plant and equipment acquired |  |  |  |  | 89 |  |  |  |  |  |  |
Amount of noncontrolling interest |  |  |  |  | $ 25 |  |  |  |  |  |  |
Three Cedars |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  |  | 50.00% |  |  |  |  |  |  |  |
Number of Projects |  |  |  | 3 |  |  |  |  |  |  |  |
Purchase price |  |  |  | $ 43 |  |  |  |  |  |  |  |
Acquisition price in cash |  |  |  | 6 |  |  |  |  |  |  |  |
Acquisition price payable |  |  |  | $ 37 |  |  |  |  |  |  |  |
MW Capacity | MW |  |  |  | 210 |  |  |  |  |  |  |  |
Project Cost |  |  |  |  |  |  | $ 450 |  |  |  |  |
Purchase price |  |  |  | $ 43 |  |  |  |  |  |  |  |
Net property plant and equipment acquired |  |  |  | 65 |  |  |  |  |  |  |  |
Amount of noncontrolling interest |  |  |  | $ 22 |  |  |  |  |  |  |  |
Three Cedars | Current Liabilities |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Payable included in other current liabilities |  |  |  |  |  |  |  | $ 0 | 2 |  | $ 0 |
Four Brothers and Three Cedars | Investment Credits | IRS |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of federal investment tax credits on projects claimed |  |  |  |  |  |  |  | 99.00% |  |  | 99.00% |
Iroquois |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  | 50.00% |  |  | 50.00% |
Partnership Interest | Iroquois | National Grid |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest percentage |  |  |  | 20.40% |  |  |  |  |  |  |  |
Partnership Interest | Iroquois | NJNR Pipeline Company |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest percentage |  |  |  | 5.53% |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Iroquois |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 25.93% |  |  |  | 25.93% |  |  | 25.93% |
Dominion Energy Midstream Partners, LP | Partnership Interest | Iroquois |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Length of FERC regulated interstate natural gas pipeline (in miles) (nearly 1,500 miles) | mi |  |  |  | 416 |  |  |  |  |  |  |  |
Common units included in initial public offering | shares |  |  |  | 8,600,000 |  |  |  |  |  |  |  |
Value of common units at closing |  |  |  | $ 216 |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Partnership Interest | Iroquois | National Grid |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common units included in initial public offering | shares |  |  |  | 6,800,000 |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Partnership Interest | Iroquois | NJNR Pipeline Company |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common units included in initial public offering | shares |  |  |  | 1,800,000 |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Disposal Group, Disposed of by Sale, Not Discontinued Operations | DECG | Affiliated Entity |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest percentage |  | 100.00% |  |  |  |  |  |  |  |  |  |
Sales price |  | $ 501 |  |  |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Disposal Group, Disposed of by Sale, Not Discontinued Operations | DECG | Affiliated Entity | Unsecured Promissory Note Receivable | Notes Receivable |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Term of senior unsecured promissory note payable |  | 2 years |  |  |  |  |  |  |  |  |  |
Senior unsecured promissory note payable by Dominion Midstream |  | $ 301 |  |  |  |  |  |  |  |  |  |
Interest rate (percentage) |  | 0.60% |  |  |  |  |  |  |  |  |  |
Dominion Energy Midstream Partners, LP | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Partnership Interest | DECG | Affiliated Entity |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common units included in initial public offering | shares |  | 5,112,139 |  |  |  |  |  |  |  |  |  |
Value of common units at closing |  | $ 200 |  |  |  |  |  |  |  |  |  |
Number of trading days |  | 10 days |  |  |  |  |  |  |  |  |  |
Price per unit | $ / shares |  | $ 39.12 |  |  |  |  |  |  |  |  |  |
Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Goodwill | [1] |  | $ 542 |  |  |  |  | $ 542 | 542 | 542 | $ 542 |
Dominion Energy Gas Holdings, LLC | Iroquois |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  | 24.07% |  |  | 24.07% |
Virginia Electric and Power Company |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Project Cost |  |  |  |  |  |  |  | $ 41 | 7 | $ 43 |  |
Virginia Electric and Power Company | Morgans Corner Solar Energy, LLC |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  | 100.00% |  |  |  |  |  |  | 100.00% |  |
Purchase price |  |  | $ 47 |  |  |  |  |  |  |  |  |
MW Capacity | MW |  |  | 20 |  |  |  |  |  |  |  |  |
Virginia Electric and Power Company | Morgans Corner Solar Energy, LLC | Supply Agreement with US Navy |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Duration of supply agreement |  |  | 10 years |  |  |  |  |  |  |  |  |
Virginia Electric and Power Company | Morgans Corner Solar Energy, LLC | Solar Development Project |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total cost of project placed into service |  |  | $ 49 |  |  |  |  |  |  | $ 49 |  |
NRG Energy, Inc |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contributions from SunEdison and NRG to Four Brothers and Three Cedars |  |  |  |  |  |  |  | $ 9 |  |  |  |
NRG Energy, Inc | Four Brothers and Three Cedars |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity interests acquired |  |  |  |  |  |  |  |  |  |  |  | 50.00%
Sun Edison Inc |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contributions from SunEdison and NRG to Four Brothers and Three Cedars |  |  |  |  |  |  |  |  | $ 189 | $ 103 |  |
Sun Edison Inc | Four Brothers and Three Cedars |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contributions from SunEdison and NRG to Four Brothers and Three Cedars |  |  |  |  |  |  |  |  |  |  | $ 301 |
[1] | Goodwill amounts do not contain any accumulated impairment losses.

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