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Significant Accounting Policies (Depreciation Rates and Estimated Useful Life) (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Generation |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.94% | 2.83% | 2.78%
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) | 2.94% | 2.83% | 2.78%
Transmission |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.55% | 2.47% | 2.42%
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) | 2.54% | 2.36% | 2.33%
Transmission | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.40% | 2.43% | 2.46%
Distribution |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 3.00% | 3.02% | 3.11%
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) | 3.32% | 3.32% | 3.33%
Storage |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.48% | 2.29% | 2.42%
Storage | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.45% | 2.19% | 2.44%
Gas Gathering and Processing |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.21% | 2.66% | 3.19%
Gas Gathering and Processing | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.42% | 2.58% | 3.20%
General and Other |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 4.89% | 4.12% | 3.67%
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) | 4.68% | 3.49% | 3.40%
General and Other | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 4.96% | 4.54% | 4.72%
Distribution | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Average composite depreciation rates on utility property, plant and equipment (percentage) | 2.42% | 2.55% | 2.45%
Merchant Generation Nuclear |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 44 years |  |
Minimum | Merchant Generation-Other |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 15 years |  |
Minimum | Nonutility Gas Gathering and Processing |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 3 years |  |
Minimum | Nonutility Gas Gathering and Processing | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 10 years |  |
Minimum | General and Other |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 5 years |  |
Maximum | Merchant Generation-Other |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 40 years |  |
Maximum | Nonutility Gas Gathering and Processing |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 50 years |  |
Maximum | Nonutility Gas Gathering and Processing | Dominion Energy Gas Holdings, LLC |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 50 years |  |
Maximum | General and Other |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated Useful Lives | 59 years |  |

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