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Significant Accounting Policies (Narrative) (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) | May 31, 2019 USD ($) | Mar. 31, 2019 USD ($) Facility MW | Jan. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) $ / shares | Jun. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) $ / shares | Sep. 30, 2019 USD ($) | Dec. 31, 2020 USD ($) $ / shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment of assets and other charges |  |  |  |  |  | $ 1,151 |  | $ 85 | $ 1,963 | $ 1,219 |
Increase to nuclear decommissioning asset retirement obligation |  |  |  |  |  |  | $ 89 |  |  |  |
Change In Depreciation Rates From New Depreciation Study |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in EPS | $ / shares |  |  |  |  |  | $ 0.01 |  |  | $ 0.02 |  |
Change In Depreciation Rates From New Depreciation Study | Merchant generation assets | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in EPS | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 0.03
Impairment of Assets and Other Charges |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment of assets and other charges |  | $ 26 |  |  |  |  |  |  |  |  |
Asset impairment charges after tax |  | 19 |  |  |  |  |  |  |  |  |
Virginia Electric and Power Company |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment of assets and other charges |  |  |  |  |  | $ 200 |  | $ 38 | $ 1,008 | $ 781 |
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) |  |  | (24) |  |
Increase (decrease) in depreciation expense, after tax |  |  |  |  |  | $ (6) |  |  | $ (18) |  |
Virginia Electric and Power Company | Change In Depreciation Rates From New Depreciation Study | 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 | Electric Generation Facilities |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Charges recorded with dismantling |  |  |  |  |  |  | 30 |  |  |  |
Charges recorded with dismantling after tax |  |  |  |  |  |  | $ 22 |  |  |  |
Virginia Electric and Power Company | Impairment of Assets and Other Charges |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment of assets and other charges | $ 754 | 17 | $ 62 | $ 369 | $ 160 |  |  |  |  |  |
Asset impairment charges after tax | $ 561 | $ 12 | $ 46 | $ 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% |  |
Cove Point | Brookfield |  |  |  |  |  |  |  |  |  |  |
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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