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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 | 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 | 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      
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      
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 | 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 | Pension Benefits | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 3,121 2,693  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 697 592  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Cash equivalents and other      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 18 12  
Collective Bargaining Agreement | Pension Benefits | Level 1 | 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 | Level 1 | Equity securities: | U.S.      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 1,902 1,705  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Equity securities: | International      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 1,151 928  
Collective Bargaining Agreement | Pension Benefits | Level 1 | 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 | Level 1 | Equity securities: | Dominion Energy Gas Holdings, LLC | International      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 257 203  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Corporate debt instruments      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 41 35  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Corporate debt instruments | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 9 8  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Government securities      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 9 13  
Collective Bargaining Agreement | Pension Benefits | Level 1 | Government securities | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 2 3  
Collective Bargaining Agreement | Pension Benefits | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 1,757 1,540  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 393 338  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Cash equivalents and other      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value   2  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Insurance contracts      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 352 334  
Collective Bargaining Agreement | Pension Benefits | Level 2 | 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 | Level 2 | Corporate debt instruments      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 729 682  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Corporate debt instruments | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 163 150  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Government securities      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 676 522  
Collective Bargaining Agreement | Pension Benefits | Level 2 | Government securities | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 151 115  
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 | 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 | 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      
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      
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 | 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 | Other Postretirement Benefits | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 836 718  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Dominion Energy Gas Holdings, LLC      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 163 145  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Cash equivalents and other      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 1 1  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Equity securities: | U.S.      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 636 571  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Equity securities: | International      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 196 143  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | 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 | Level 1 | Equity securities: | Dominion Energy Gas Holdings, LLC | International      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 33 24  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Corporate debt instruments      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 2 2  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 1 | Government securities      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 1 1  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 108 90  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 2 | Cash equivalents and other      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 2 1  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 2 | Insurance contracts      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 21 19  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 2 | Corporate debt instruments      
Defined Benefit Plan Disclosure [Line Items]      
Total recorded at fair value 44 40  
Collective Bargaining Agreement | Other Postretirement Benefits | Level 2 | 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.