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Employee Benefit Plans (Narrative) (Detail) - USD ($) |  |  |  | 3 Months Ended | 12 Months Ended
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Sep. 30, 2017 | Jun. 30, 2017 | Jan. 31, 2017 | Dec. 31, 2017 | Mar. 31, 2017 | Sep. 30, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | $ 1,600,000,000 | $ 534,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 767,000,000 | 691,000,000 |
Net actuarial loss |  |  |  |  |  |  |  |  |  | $ (25,000,000)
Accumulated benefit obligation |  |  |  | $ 8,200,000,000 |  |  |  | 8,200,000,000 | 7,300,000,000 |
Contributions to qualified defined benefit pension plans |  |  |  |  |  |  |  | 0 |  |
Contributions to qualified defined benefit pension plans in 2018 |  |  |  | $ 0 |  |  |  | 0 |  |
Estimated reduction in net cumulative required contributions |  |  |  |  |  |  |  | $ 200,000,000 |  |
Cumulative required contribution period (years) |  |  |  |  |  |  |  | 10 years |  |
Expected contribution to voluntary employees beneficiary association |  |  |  |  |  |  |  | $ 12,000,000 | 12,000,000 |
Amounts recognized in employer matching contributions |  |  |  |  |  |  |  | $ 45,000,000 | 44,000,000 | 43,000,000
Special Termination Benefits | Organizational Design Initiative |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Organizational design initiative |  |  |  |  |  |  |  |  | 65,000,000 |
Organizational design initiative, after tax |  |  |  |  |  |  |  |  | 40,000,000 |
Common and Preferred Stocks |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Defined benefit plan, actual plan asset allocation percentages |  |  |  | 28.00% |  |  |  | 28.00% |  |
Government Securities |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Defined benefit plan, actual plan asset allocation percentages |  |  |  | 18.00% |  |  |  | 18.00% |  |
Fixed Income |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Defined benefit plan, actual plan asset allocation percentages |  |  |  | 35.00% |  |  |  | 35.00% |  |
Real Estate Funds |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Defined benefit plan, actual plan asset allocation percentages |  |  |  | 3.00% |  |  |  | 3.00% |  |
Other Alternative Investments |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Defined benefit plan, actual plan asset allocation percentages |  |  |  | 16.00% |  |  |  | 16.00% |  |
Scenario, Forecast |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Expected contribution to voluntary employees beneficiary association |  |  |  |  |  |  | $ 12,000,000 |  |  |
Pension or other postretirement plan assets to be returned |  |  |  |  |  |  | 0 |  |  |
Dominion Energy Questar Corporation | Capital Contribution to Dominion Energy Questar to Fund Pension Contributions |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Contribution to qualified pension plan |  |  |  |  |  |  |  | $ 75,000,000 |  |
Medical Coverage for Local retirees |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Decrease in accumulated postretirement benefit obligation |  |  |  |  | $ (73,000,000) | $ 37,000,000 |  |  |  |
Discount rate percentage |  |  |  |  | 4.30% | 3.71% |  |  |  |
Charge recorded from plan amendment and remeasurement |  |  |  |  | $ 7,000,000 | $ (9,000,000) |  |  |  |
Charge recorded from plan amendment and remeasurement, after tax |  |  |  |  | 4,000,000 |  |  |  |  |
Pension Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | 1,327,000,000 | 426,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 639,000,000 | 573,000,000 | $ 531,000,000
Net actuarial loss |  |  |  | $ 193,000,000 |  |  |  | 193,000,000 |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  | 830,000,000 | $ 784,000,000 |
Decrease in accumulated postretirement benefit obligation |  |  |  | (75,000,000) |  |  |  |  |  |
Discount rate percentage | 4.05% | 4.05% | 4.51% |  |  |  |  |  | 4.46% | 4.40%
Contributions to qualified defined benefit pension plans |  |  |  |  |  |  |  | 118,000,000 | $ 15,000,000 |
Net periodic benefit cost |  |  |  |  |  |  |  | 7,000,000 | (25,000,000) | $ 44,000,000
Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | 236,000,000 | 108,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 128,000,000 | 118,000,000 | $ 117,000,000
Net actuarial loss |  |  |  | 11,000,000 |  |  |  | 11,000,000 |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  | $ 119,000,000 | 166,000,000 |
Decrease in accumulated postretirement benefit obligation |  |  |  | $ 2,000,000 |  |  |  |  |  |
Discount rate percentage |  |  |  |  |  |  |  |  |  | 4.40%
Discount rate percentage |  |  |  | 3.76% |  |  |  | 3.76% |  |
Contributions to qualified defined benefit pension plans |  |  |  |  |  |  |  | $ 13,000,000 | 12,000,000 |
Net periodic benefit cost |  |  |  |  |  |  |  | (80,000,000) | $ (49,000,000) | $ (31,000,000)
Collective Bargaining Agreement |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Period for experience study of employee demographics |  |  |  |  |  |  |  |  | 5 years |
Increase in net periodic benefit costs for 2017 |  |  |  |  |  |  |  |  | $ 42,000,000 |
Collective Bargaining Agreement | Pension Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  |  | 290,000,000 |
Collective Bargaining Agreement | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  |  | 38,000,000 |
Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | 335,000,000 | 130,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 165,000,000 | 157,000,000 |
Net actuarial loss |  |  |  |  |  |  |  |  |  | (3,000,000)
Accumulated benefit obligation |  |  |  | $ 724,000,000 |  |  |  | 724,000,000 | 640,000,000 |
Contributions to qualified defined benefit pension plans |  |  |  |  |  |  |  | 0 |  |
Contributions to qualified defined benefit pension plans in 2018 |  |  |  | 0 |  |  |  | 0 |  |
Contribution to qualified pension plan |  |  |  |  |  |  |  | 15,000,000 | 150,000,000 | 692,000,000
Amounts recognized in employer matching contributions |  |  |  |  |  |  |  | 7,000,000 | 7,000,000 | 7,000,000
Dominion Energy Gas Holdings, LLC | Special Termination Benefits | Organizational Design Initiative |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Organizational design initiative |  |  |  |  |  |  |  |  | 8,000,000 |
Organizational design initiative, after tax |  |  |  |  |  |  |  |  | 5,000,000 |
Dominion Energy Gas Holdings, LLC | Other operations and maintenance | Dominion Retiree Health And Welfare Plan | Multiemployer Plans, Postretirement Benefit |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Net periodic benefit cost |  |  |  |  |  |  |  | (5,000,000) | (4,000,000) | (5,000,000)
Dominion Energy Gas Holdings, LLC | Scenario, Forecast |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Pension or other postretirement plan assets to be returned |  |  |  |  |  |  | $ 0 |  |  |
Dominion Energy Gas Holdings, LLC | Medical Coverage for Local retirees |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Decrease in accumulated postretirement benefit obligation |  |  |  |  | (61,000,000) |  |  |  |  |
Charge recorded from plan amendment and remeasurement |  |  |  |  | 6,000,000 |  |  |  |  |
Charge recorded from plan amendment and remeasurement, after tax |  |  |  |  | $ 4,000,000 |  |  |  |  |
Dominion Energy Gas Holdings, LLC | Pension Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | 294,000,000 | 107,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 141,000,000 | 134,000,000 | $ 126,000,000
Net actuarial loss |  |  |  | $ 19,000,000 |  |  |  | 19,000,000 |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  | $ 78,000,000 | $ 64,000,000 |
Discount rate percentage |  |  |  |  |  |  |  | 4.50% | 4.99% | 4.40%
Discount rate percentage |  |  |  | 3.81% |  |  |  | 3.81% | 4.50% |
Net periodic benefit cost |  |  |  |  |  |  |  | $ (80,000,000) | $ (78,000,000) | $ (63,000,000)
Dominion Energy Gas Holdings, LLC | Pension Benefits | Other operations and maintenance |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Net periodic benefit cost |  |  |  |  |  |  |  | (37,000,000) | (45,000,000) | (38,000,000)
Dominion Energy Gas Holdings, LLC | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actual return (loss) on plan assets |  |  |  |  |  |  |  | 41,000,000 | 23,000,000 |
Expected return on pension and other postretirement plan assets |  |  |  |  |  |  |  | 24,000,000 | 23,000,000 | $ 24,000,000
Net actuarial loss |  |  |  | $ 3,000,000 |  |  |  | 3,000,000 |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  | $ 34,000,000 | $ 28,000,000 |
Discount rate percentage |  |  |  |  |  |  |  | 4.47% | 4.93% | 4.40%
Discount rate percentage |  |  |  | 3.76% |  |  |  | 3.76% | 4.47% |
Contributions to qualified defined benefit pension plans |  |  |  |  |  |  |  | $ 12,000,000 | $ 12,000,000 |
Net periodic benefit cost |  |  |  |  |  |  |  | (9,000,000) | (2,000,000) | $ (2,000,000)
Dominion Energy Gas Holdings, LLC | Collective Bargaining Agreement | Pension Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  |  | 24,000,000 |
Dominion Energy Gas Holdings, LLC | Collective Bargaining Agreement | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Actuarial (gains) losses during the year |  |  |  |  |  |  |  |  | 9,000,000 |
Virginia Electric and Power Company |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Amounts recognized in employer matching contributions |  |  |  |  |  |  |  | 19,000,000 | 19,000,000 | 18,000,000
Virginia Electric and Power Company | Special Termination Benefits | Organizational Design Initiative |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Organizational design initiative |  |  |  |  |  |  |  |  | 33,000,000 |
Organizational design initiative, after tax |  |  |  |  |  |  |  |  | 20,000,000 |
Virginia Electric and Power Company | Other operations and maintenance | Dominion Retiree Health And Welfare Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Net periodic benefit cost |  |  |  |  |  |  |  | (42,000,000) | (29,000,000) | (16,000,000)
Virginia Electric and Power Company | Pension Benefits | Other operations and maintenance |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |
Net periodic benefit cost |  |  |  |  |  |  |  | $ 110,000,000 | $ 79,000,000 | $ 97,000,000

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