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Acquisitions and Dispositions (Schedule of Preliminary Allocation of Purchase Price to Assets Acquired and Liabilities Assumed) (Parenthetical) (Detail) - Dominion Energy Questar Corporation - USD ($) $ in Millions | Sep. 16, 2016 | Dec. 31, 2017
--- | --- | ---
Business Acquisition [Line Items] |  |
Short-term debt | $ 301 |
White River Hub LLC |  |
Business Acquisition [Line Items] |  |
Equity method investment | $ 40 |
Minimum |  |
Business Acquisition [Line Items] |  |
Estimated Useful Lives | 9 years |
Maximum |  |
Business Acquisition [Line Items] |  |
Estimated Useful Lives | 18 years |
Unsecured Senior Notes | Minimum |  |
Business Acquisition [Line Items] |  |
Interest rate (percentage) |  | 2.98%
Unsecured Senior Notes | Maximum |  |
Business Acquisition [Line Items] |  |
Interest rate (percentage) |  | 7.20%
Variable Interest Rate Short Term Loan Due in August 2017 [Member] |  |
Business Acquisition [Line Items] |  |
Short-term note outstanding | $ 250 |

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