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Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | $ 307 |  | $ 279
Derivative Liabilities |  | 284 |  | 155
Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 166 |  | 194
Derivative Liabilities |  | 66 |  | 31
Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 32 |  |
Derivative Liabilities |  | 8 |  | 11
Fair Value, Measurements, Recurring |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 5,379 |  | 4,722
Total Liabilities |  | 284 |  | 155
Fair Value, Measurements, Recurring | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 2,466 |  | 2,200
Total Liabilities |  | 66 |  | 31
Fair Value, Measurements, Recurring | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 32 |  |
Total Liabilities |  | 8 |  | 11
Fair Value, Measurements, Recurring | Equity securities: | U.S. |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 3,493 |  | 2,913
Fair Value, Measurements, Recurring | Equity securities: | U.S. | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 1,566 |  | 1,302
Fair Value, Measurements, Recurring | Fixed Income [Member] | Corporate debt instruments |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 444 |  | 487
Fair Value, Measurements, Recurring | Fixed Income [Member] | Corporate debt instruments | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 224 |  | 277
Fair Value, Measurements, Recurring | Fixed Income [Member] | Government Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 1,101 |  | 1,038
Fair Value, Measurements, Recurring | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 494 | [2] | 427
Fair Value, Measurements, Recurring | Cash equivalents and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 34 |  | 5
Fair Value, Measurements, Recurring | Cash equivalents and other | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 16 |  |
Fair Value, Measurements, Recurring | Commodity |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 258 |  | 262
Derivative Liabilities |  | 197 |  | 96
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 166 |  | 188
Derivative Liabilities |  | 9 |  | 10
Fair Value, Measurements, Recurring | Commodity | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Liabilities |  | 6 |  | 5
Fair Value, Measurements, Recurring | Interest rate |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 17 |  | 17
Derivative Liabilities |  | 85 |  | 53
Fair Value, Measurements, Recurring | Interest rate | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  |  |  | 6
Derivative Liabilities |  | 57 |  | 21
Fair Value, Measurements, Recurring | Foreign currency |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 32 |  |
Derivative Liabilities |  | 2 |  | 6
Fair Value, Measurements, Recurring | Foreign currency | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 32 |  |
Derivative Liabilities |  | 2 |  | 6
Fair Value, Measurements, Recurring | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 3,834 |  | 3,342
Fair Value, Measurements, Recurring | Level 1 | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 1,750 |  | 1,438
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S. |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 3,493 |  | 2,913
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S. | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 1,566 |  | 1,302
Fair Value, Measurements, Recurring | Level 1 | Fixed Income [Member] | Government Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 307 |  | 424
Fair Value, Measurements, Recurring | Level 1 | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 168 | [2] | 136
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 34 |  | 5
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 16 |  |
Fair Value, Measurements, Recurring | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 1,388 |  | 1,233
Total Liabilities |  | 277 |  | 147
Fair Value, Measurements, Recurring | Level 2 | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 564 |  | 617
Total Liabilities |  | 61 |  | 29
Fair Value, Measurements, Recurring | Level 2 | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 32 |  |
Total Liabilities |  | 6 |  | 9
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Corporate debt instruments |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 444 |  | 487
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Corporate debt instruments | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [2] | 224 |  | 277
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Government Securities [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | [1] | 794 |  | 614
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 326 | [2] | 291
Fair Value, Measurements, Recurring | Level 2 | Commodity |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 101 |  | 115
Derivative Liabilities |  | 190 |  | 88
Fair Value, Measurements, Recurring | Level 2 | Commodity | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 14 |  | 43
Derivative Liabilities |  | 4 |  | 8
Fair Value, Measurements, Recurring | Level 2 | Commodity | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Liabilities |  | 4 |  | 3
Fair Value, Measurements, Recurring | Level 2 | Interest rate |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 17 |  | 17
Derivative Liabilities |  | 85 |  | 53
Fair Value, Measurements, Recurring | Level 2 | Interest rate | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  |  |  | 6
Derivative Liabilities |  | 57 |  | 21
Fair Value, Measurements, Recurring | Level 2 | Foreign currency |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 32 |  |
Derivative Liabilities |  | 2 |  | 6
Fair Value, Measurements, Recurring | Level 2 | Foreign currency | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 32 |  |
Derivative Liabilities |  | 2 |  | 6
Fair Value, Measurements, Recurring | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 157 |  | 147
Total Liabilities |  | 7 |  | 8
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total assets |  | 152 |  | 145
Total Liabilities |  | 5 |  | 2
Fair Value, Measurements, Recurring | Level 3 | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total Liabilities |  | 2 |  | 2
Fair Value, Measurements, Recurring | Level 3 | Commodity |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 157 |  | 147
Derivative Liabilities |  | 7 |  | 8
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Asset |  | 152 |  | 145
Derivative Liabilities |  | 5 |  | 2
Fair Value, Measurements, Recurring | Level 3 | Commodity | Dominion Energy Gas Holdings, LLC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative Liabilities |  | $ 2 |  | $ 2
[1] | Includes investments held in the nuclear decommissioning and rabbi trusts. Excludes $88 million and $89 million of assets at December 31, 2017 and 2016, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.
[2] | Includes investments held in the nuclear decommissioning trusts. Excludes $27 million and $26 million of assets at December 31, 2017 and 2016, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.

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