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Significant Accounting Policies (Narrative) (Detail) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) | May 31, 2019 USD ($) | Mar. 31, 2019 USD ($) Facility | Jan. 31, 2019 USD ($) MW | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2018 | Jan. 01, 2019 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Asset impairment charges |  |  |  |  | $ 85 | $ 12 | $ 1,232 | $ 147 |  |
Option to extend, description, operating lease |  |  |  |  |  |  | Certain of the Companies’ leases include one or more options to renew, with renewal terms that can extend the lease from one to 70 years. |  |  |
Option to extend, existence, operating lease |  |  |  |  |  |  | true |  |  |
Estimated useful life of finite-lived intangible asset |  |  |  |  |  |  | 30 years |  |  |
Operating lease assets | $ 455 |  |  |  | $ 455 |  | $ 455 |  |  | $ 504
Minimum |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Lease renewal term, operating lease | 1 year |  |  |  | 1 year |  | 1 year |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Lease renewal term, operating lease | 70 years |  |  |  | 70 years |  | 70 years |  |  |
Virginia Electric and Power Company |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Asset impairment charges |  |  |  |  | $ 38 |  | $ 781 |  |  |
MW Capacity | MW |  |  |  | 1,292 |  |  |  |  |  |
Number of facilities in cold reserve units retired | Facility |  |  | 6 |  |  |  |  |  |  |
Operating lease assets | $ 184 |  |  |  | 184 |  | 184 |  |  | 209
Virginia Electric and Power Company | Impairment of Assets and Other Charges |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Asset impairment charges | 17 | $ 62 | $ 369 | $ 160 |  |  |  |  |  |
Asset impairment charges after tax | 12 | $ 46 | $ 275 | $ 119 |  |  |  |  |  |
Dominion Energy | Impairment of Assets and Other Charges |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Asset impairment charges | 26 |  |  |  |  |  |  |  |  |
Asset impairment charges after tax | 19 |  |  |  |  |  |  |  |  |
Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Asset impairment charges |  |  |  |  |  | $ 1 | 13 | $ 127 |  |
Operating lease assets | $ 56 |  |  |  | $ 56 |  | $ 56 |  |  | $ 64
Dominion Energy Midstream Partners, LP | Common And Subordinated Units |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest percentage of limited partner interests |  |  |  |  |  |  |  |  | 60.90% |
Dominion Energy Midstream Partners, LP | Convertible Preferred Stock |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest percentage of limited partner interests |  |  |  |  |  |  |  |  | 37.50% |
Merchant Solar Projects | Terra Nova Renewable Partners |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Business Acquisition, Percentage of Voting Interests Acquired | 50.00% |  |  |  | 50.00% |  | 50.00% |  |  |
Percentage of equity interest sold to noncontrolling interest owners | 33.00% |  |  |  | 33.00% |  | 33.00% |  |  |
Merchant Solar Projects | Terra Nova Renewable Partners | Call Option |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage ownership in total units | 67.00% |  |  |  | 67.00% |  | 67.00% |  |  |

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