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Acquisitions and Dispositions (Acquisition of Dominion Energy Questar) (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 01, 2016 | Sep. 16, 2016 | Jul. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Aug. 31, 2016 | Apr. 30, 2016 | Sep. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock |  |  |  |  |  |  |  |  |  | $ 1,302 | $ 2,152 | $ 786
Issuance of short-term notes |  |  |  |  |  |  |  |  |  |  | 1,200 | 600
Issuance of common stock |  |  |  |  |  |  |  |  |  | 1,302 | 2,152 | 786
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Repayment of short-term notes |  |  |  |  |  |  |  |  |  | $ 250 | $ 1,800 | $ 400
Dominion Energy Questar Pipeline LLC | Dominion Energy Midstream Partners, LP | Subsidiary of Common Parent | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Cash consideration | $ 823 |  |  |  |  |  |  |  |  |  |  |
Debt financed distribution | 300 |  |  |  |  |  |  |  |  |  |  |
Amount of consideration | 1,300 |  |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Pipeline LLC | Dominion Energy Midstream Partners, LP | Subsidiary of Common Parent | Partnership Interest | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Combined value of common and convertible preferred units | $ 467 |  |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Pipeline LLC | Dominion Energy Midstream Partners, LP | Limited Partner | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Senior Unsecured Promissory Note Payable | Notes Payable, Other Payables |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Repayments of promissory note |  |  |  | $ 301 |  |  |  |  |  |  |  |
Dominion Energy Questar Pipeline LLC | Dominion Energy Midstream Partners, LP | Limited Partner | Common Units | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Number of common units to be purchased |  |  |  | 6,656,839 |  |  |  |  |  |  |  |
Questar Fueling Company | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Cash consideration |  |  |  | $ 28 |  |  |  |  |  |  |  |
Underwritten Public Offering |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock |  |  |  |  |  |  | $ 756 |  |  |  |  |
Term Loans | Dominion Energy Questar Pipeline LLC | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Private Placement Term Loan Agreement Maturing in September 2017 |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Repayment of short-term notes |  |  |  | $ 1,200 |  |  |  |  |  |  |  |
Capital Unit, Class A |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock |  |  | $ 1,000 |  |  |  |  |  |  |  |  |
Dominion Energy Questar Corporation |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Price per share |  | $ 25.00 |  |  |  |  |  |  |  |  |  |
Total consideration |  | $ 4,400 |  |  |  |  |  |  |  |  |  |
Number of shares of Dominion Questar outstanding at closing |  | 175,500,000 |  |  |  |  |  |  |  |  |  |
Issuance of senior notes |  |  |  |  |  | $ 1,300 |  |  |  |  |  |
Increase (decrease) to goodwill related modifications to preliminary valuation amounts |  |  |  |  |  |  |  | $ 6 | $ (6) |  |  |
Contributions to fund Dominion Questar's qualified and non-qualified defined benefit pension plans |  | $ 75 |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Corporation | Commitment for Charitable Contributions |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions, next twelve months |  | 1 |  |  |  |  |  |  |  |  |  |
Charitable contributions, second year |  | 1 |  |  |  |  |  |  |  |  |  |
Charitable contributions, third year |  | 1 |  |  |  |  |  |  |  |  |  |
Charitable contributions, fourth year |  | 1 |  |  |  |  |  |  |  |  |  |
Charitable contributions, fifth year |  | $ 1 |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Corporation | Commitment for Charitable Contributions | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Other Commitment, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Period for charitable contributions |  | 5 years |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Corporation | White River Hub LLC |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  | 50.00% |  |  |  |  |  |  |  |  |  |
Dominion Energy Questar Corporation | Underwritten Public Offering |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock |  |  |  |  |  |  | $ 500 |  |  |  |  |
Dominion Energy Questar Corporation | Term Loans |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of short-term notes |  |  |  |  | $ 1,200 |  |  |  |  |  |  |
Dominion Energy Questar Corporation | Capital Unit, Class A |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock |  |  |  |  |  | $ 1,400 |  |  |  |  |  |

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