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Employee Benefit Plans (Fair values of pension and post retirement plan assets by asset category) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total investments |  | $ 1,378 | $ 1,078 |
Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total investments |  | 1,828 | 1,557 |
Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 8,062 | 7,016 | $ 6,166
Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,803 | 1,542 | 1,467
Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,729 | 1,512 | 1,382
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 333 | 299 | $ 283
Collective Bargaining Agreement | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 4,878 | 4,233 |
Total recorded at NAV | [1] | 3,169 | 2,765 |
Total investments | [2] | 8,047 | 6,998 |
Collective Bargaining Agreement | Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,090 | 930 |
Total recorded at NAV | [1] | 709 | 608 |
Total investments | [3] | 1,799 | 1,538 |
Collective Bargaining Agreement | Pension Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 18 | 14 |
Collective Bargaining Agreement | Pension Benefits | Cash equivalents and other | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 4 | 3 |
Collective Bargaining Agreement | Pension Benefits | Equity securities: | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,902 | 1,705 |
Collective Bargaining Agreement | Pension Benefits | Equity securities: | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,151 | 928 |
Collective Bargaining Agreement | Pension Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 425 | 375 |
Collective Bargaining Agreement | Pension Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 257 | 203 |
Collective Bargaining Agreement | Pension Benefits | Insurance contracts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 352 | 334 |
Collective Bargaining Agreement | Pension Benefits | Insurance contracts | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 79 | 73 |
Collective Bargaining Agreement | Pension Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 770 | 717 |
Collective Bargaining Agreement | Pension Benefits | Corporate debt instruments | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 172 | 158 |
Collective Bargaining Agreement | Pension Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 685 | 535 |
Collective Bargaining Agreement | Pension Benefits | Government securities | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 153 | 118 |
Collective Bargaining Agreement | Pension Benefits | Common/collective trust funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 2,272 | 1,960 |
Collective Bargaining Agreement | Pension Benefits | Common/collective trust funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 509 | 430 |
Collective Bargaining Agreement | Pension Benefits | Real Estate Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 111 | 121 |
Collective Bargaining Agreement | Pension Benefits | Real Estate Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 25 | 27 |
Collective Bargaining Agreement | Pension Benefits | Private Equity Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 606 | 506 |
Collective Bargaining Agreement | Pension Benefits | Private Equity Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 135 | 111 |
Collective Bargaining Agreement | Pension Benefits | Fixed Income Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 161 | 153 |
Collective Bargaining Agreement | Pension Benefits | Fixed Income Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 36 | 34 |
Collective Bargaining Agreement | Pension Benefits | Hedge Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 19 | 25 |
Collective Bargaining Agreement | Pension Benefits | Hedge Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 4 | 6 |
Collective Bargaining Agreement | Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 944 | 808 |
Total recorded at NAV | [1] | 783 | 702 |
Total investments | [4] | 1,727 | 1,510 |
Collective Bargaining Agreement | Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 163 | 145 |
Total recorded at NAV | [1] | 170 | 154 |
Total investments |  | 333 | 299 |
Collective Bargaining Agreement | Other Postretirement Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 3 | 2 |
Collective Bargaining Agreement | Other Postretirement Benefits | Equity securities: | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 636 | 571 |
Collective Bargaining Agreement | Other Postretirement Benefits | Equity securities: | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 196 | 143 |
Collective Bargaining Agreement | Other Postretirement Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 130 | 121 |
Collective Bargaining Agreement | Other Postretirement Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 33 | 24 |
Collective Bargaining Agreement | Other Postretirement Benefits | Insurance contracts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 21 | 19 |
Collective Bargaining Agreement | Other Postretirement Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 46 | 42 |
Collective Bargaining Agreement | Other Postretirement Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 42 | 31 |
Collective Bargaining Agreement | Other Postretirement Benefits | Common/collective trust funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 689 | 621 |
Collective Bargaining Agreement | Other Postretirement Benefits | Common/collective trust funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 154 | 140 |
Collective Bargaining Agreement | Other Postretirement Benefits | Real Estate Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 9 | 9 |
Collective Bargaining Agreement | Other Postretirement Benefits | Real Estate Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 1 | 1 |
Collective Bargaining Agreement | Other Postretirement Benefits | Private Equity Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 73 | 59 |
Collective Bargaining Agreement | Other Postretirement Benefits | Private Equity Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 15 | 12 |
Collective Bargaining Agreement | Other Postretirement Benefits | Fixed Income Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 11 | 12 |
Collective Bargaining Agreement | Other Postretirement Benefits | Fixed Income Funds | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] |  | 1 |
Collective Bargaining Agreement | Other Postretirement Benefits | Hedge Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at NAV | [1] | 1 | 1 |
Collective Bargaining Agreement | Level 1 | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 3,121 | 2,693 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 697 | 592 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 18 | 12 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Cash equivalents and other | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 4 | 3 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Equity securities: | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,902 | 1,705 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Equity securities: | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,151 | 928 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 425 | 375 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 257 | 203 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 41 | 35 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Corporate debt instruments | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 9 | 8 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 9 | 13 |
Collective Bargaining Agreement | Level 1 | Pension Benefits | Government securities | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 2 | 3 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 836 | 718 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 163 | 145 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1 | 1 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Equity securities: | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 636 | 571 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Equity securities: | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 196 | 143 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | U.S. |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 130 | 121 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Equity securities: | Dominion Energy Gas Holdings, LLC | International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 33 | 24 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 2 | 2 |
Collective Bargaining Agreement | Level 1 | Other Postretirement Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1 | 1 |
Collective Bargaining Agreement | Level 2 | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 1,757 | 1,540 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 393 | 338 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  |  | 2 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Insurance contracts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 352 | 334 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Insurance contracts | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 79 | 73 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 729 | 682 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Corporate debt instruments | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 163 | 150 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 676 | 522 |
Collective Bargaining Agreement | Level 2 | Pension Benefits | Government securities | Dominion Energy Gas Holdings, LLC |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 151 | 115 |
Collective Bargaining Agreement | Level 2 | Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 108 | 90 |
Collective Bargaining Agreement | Level 2 | Other Postretirement Benefits | Cash equivalents and other |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 2 | 1 |
Collective Bargaining Agreement | Level 2 | Other Postretirement Benefits | Insurance contracts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 21 | 19 |
Collective Bargaining Agreement | Level 2 | Other Postretirement Benefits | Corporate debt instruments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | 44 | 40 |
Collective Bargaining Agreement | Level 2 | Other Postretirement Benefits | Government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total recorded at fair value |  | $ 41 | $ 30 |
[1] | These investments that are measured at fair value using the NAV per share (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.
[2] | Excludes net assets related to pending sales of securities of $11 million, net accrued income of $19 million, and includes net assets related to pending purchases of securities of $15 million at December 31, 2017. Excludes net assets related to pending sales of securities of $46 million, net accrued income of $19 million, and includes net assets related to pending purchases of securities of $47 million at December 31, 2016.
[3] | Excludes net assets related to pending sales of securities of $3 million, net accrued income of $4 million, and includes net assets related to pending purchases of securities of $3 million at December 31, 2017. Excludes net assets related to pending sales of securities of $10 million, net accrued income of $4 million, and includes net assets related to pending purchases of securities of $10 million at December 31, 2016.
[4] | Excludes net assets related to pending sales of securities of $1 million, net accrued income of $2 million, and includes net assets related to pending purchases of securities of $1 million at December 31, 2017. Excludes net assets related to pending sales of securities of $5 million, net accrued income of $2 million, and includes net assets related to pending purchases of securities of $5 million at December 31, 2016.

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