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Income Taxes (Narrative) (Detail) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]            
Federal statutory income tax rate       35.00% 35.00% 35.00%
Percentage of bonus depreciation allowance for qualifying expenditures incurred   50.00%   50.00% 50.00% 50.00%
Percentage of investment tax credit for qualifying expenditures to be incurred in 2019   30.00%   30.00%    
Percentage of investment tax credit for qualifying expenditures to be incurred in 2020   26.00%   26.00%    
Percentage of investment tax credit for qualifying expenditures to be incurred in 2021   22.00%   22.00%    
Percentage of investment tax credit for qualifying expenditures to be incurred in 2022   10.00%   10.00%    
Percentage of investment tax credit for qualifying expenditures to be incurred thereafter   10.00%   10.00%    
Deferred income tax expense   $ (851)   $ (851)    
Provision for current income taxes       (27) $ (70) $ 51
Benefit for deferred income taxes         96  
Decrease in unrecognized tax benefits due to settlement       23 8 5
Unrecognized tax benefits that would impact effective tax rate   31   31 45 69
Decrease in accumulated deferred income tax liability   3,100   3,100    
Increase in regulatory liabilities       4,200    
Increase in deferred tax asset       1,100    
Increase (Decrease) in tax expense as a result of changes in unrecognized tax benefits       $ (9) $ (18) $ (6)
Virginia Electric and Power Company            
Income Taxes [Line Items]            
Federal statutory income tax rate       35.00% 35.00% 35.00%
Deferred income tax expense   (93)   $ (93)    
Provision for current income taxes       505 $ 258 $ 408
Decrease in unrecognized tax benefits due to settlement $ 8     8    
Unrecognized tax benefits that would impact effective tax rate 2 3   3 9 $ 8
Interest income $ 11          
Decrease in accumulated deferred income tax liability   1,900   1,900    
Increase in regulatory liabilities       2,600    
Increase in deferred tax asset       700    
Increase (Decrease) in tax expense as a result of changes in unrecognized tax benefits       $ (6) $ 1  
Dominion Energy Gas Holdings, LLC            
Income Taxes [Line Items]            
Federal statutory income tax rate       35.00% 35.00% 35.00%
Deferred income tax expense   (197)   $ (197)    
Provision for current income taxes       24 $ (23) $ 120
Decrease in unrecognized tax benefits due to settlement       7 4  
Unrecognized tax benefits that would impact effective tax rate         5 19
Decrease in accumulated deferred income tax liability   800   800    
Increase in regulatory liabilities       1,000    
Increase in deferred tax asset       200    
Increase (Decrease) in tax expense as a result of changes in unrecognized tax benefits       (5) (11)  
Maximum            
Income Taxes [Line Items]            
Decrease in Unrecognized tax benefits due to settlement negotiations and expiration of statutes of limitations   13   13    
Amount that earnings could potentially increase if changes were to occur       12    
Maximum | Virginia Electric and Power Company            
Income Taxes [Line Items]            
Increase (Decrease) in tax expense as a result of changes in unrecognized tax benefits           1
Decrease in Unrecognized tax benefits due to settlement negotiations and expiration of statutes of limitations   2   2    
Amount that earnings could potentially increase if changes were to occur       2    
Maximum | Dominion Energy Gas Holdings, LLC            
Income Taxes [Line Items]            
Unrecognized tax benefits that would impact effective tax rate   1   1    
Increase (Decrease) in tax expense as a result of changes in unrecognized tax benefits           1
Decrease in Unrecognized tax benefits due to settlement negotiations and expiration of statutes of limitations   $ 1   1    
Amount that earnings could potentially increase if changes were to occur       $ 1    
Scenario, Forecast            
Income Taxes [Line Items]            
Federal statutory income tax rate     21.00%      
Federal            
Income Taxes [Line Items]            
Federal tax credits         129  
Realizable domestic production activities deduction         17  
Federal | Dominion Midstream Partners, LP | Questar Pipeline, LLC | Disposal Group, Disposed of by Sale, Not Discontinued Operations            
Income Taxes [Line Items]            
Provision for current income taxes         $ 212  
Federal | IRS            
Income Taxes [Line Items]            
Benefit related to carryback recognized           $ 20
Tax year under ongoing examination       2017