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Significant Accounting Policies (Narrative) (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) Facility MW | Jan. 31, 2019 USD ($) | Mar. 31, 2020 USD ($) $ / shares | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) $ / shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impairment of assets and other charges |  |  |  | $ 768 | $ 835 |
Change in Estimated Useful Life | Merchant generation assets | Scenario Forecast |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Increase in EPS | $ / shares |  |  |  |  |  | $ 0.03
Virginia Electric and Power Company |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impairment of assets and other charges |  |  |  | 764 | $ 546 |
MW Capacity | MW |  | 1,292 |  |  |  |
Number of facilities in cold reserve units retired | Facility |  | 6 |  |  |  |
Virginia Electric and Power Company | Change In Depreciation Rates From New Depreciation Study |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Increase (decrease) in depreciation expense |  |  |  | (8) |  |
Increase (decrease) in depreciation expense, after tax |  |  |  | $ (6) |  |
Increase in EPS | $ / shares |  |  |  | $ 0.01 |  |
Virginia Electric and Power Company | Change in Estimated Useful Life | Merchant generation assets | Scenario Forecast |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Increase (decrease) in depreciation expense |  |  |  |  |  | $ (31)
Increase (decrease) in depreciation expense, after tax |  |  |  |  |  | $ (23)
Virginia Electric and Power Company | Impairment of Assets and Other Charges |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impairment of assets and other charges | $ 754 | $ 369 | $ 160 |  |  |
Asset impairment charges after tax | $ 561 | $ 275 | $ 119 |  |  |
Dominion Energy Midstream Partners, LP | Terra Nova Renewable Partners | Merchant Solar Projects | Call Option |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Percentage ownership in total units | 67.00% |  |  | 67.00% |  |
Dominion Energy Midstream Partners, LP | Dominion Energy Gas Holdings, LLC |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Percentage of equity interest sold to noncontrolling interest owners | 25.00% |  |  | 25.00% |  |
Four Brothers and Three Cedars |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Business Acquisition, Percentage of Voting Interests Acquired | 50.00% |  |  | 50.00% |  |
Four Brothers and Three Cedars | Dominion Energy Midstream Partners, LP | Terra Nova Renewable Partners | Merchant Solar Projects |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Percentage of equity interest sold to noncontrolling interest owners | 33.00% |  |  | 33.00% |  |

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