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Significant Accounting Policies (Depreciation Rates and Estimated Useful Life) (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Nonregulated Generation-Nuclear |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful lives |  | 44 years |  |
Nonregulated Generation-Other | Minimum |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful lives |  | 15 years |  |
Nonregulated Generation-Other | Maximum |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful lives |  | 30 years |  |
General and Other | Minimum |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful lives |  | 5 years |  |
General and Other | Maximum |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful lives |  | 59 years |  |
Generation |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | [1] | 2.51% | 2.84% | 2.71%
Generation | Virginia Electric and Power Company |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) |  | 2.52% | 2.94% | 2.71%
Transmission |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | [1] | 2.48% | 2.50% | 2.50%
Transmission | Virginia Electric and Power Company |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) |  | 2.52% | 2.54% | 2.52%
Distribution |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | [1] | 2.76% | 2.80% | 2.97%
Distribution | Virginia Electric and Power Company |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) |  | 3.19% | 3.14% | 3.31%
Storage |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | [1] | 1.59% | 1.49% | 2.20%
General and Other |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | [1] | 4.35% | 3.99% | 4.11%
General and Other | Virginia Electric and Power Company |  |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) |  | 5.09% | 4.40% | 4.52%
[1] | Excludes rates for depreciation reported as discontinued operations.

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