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Income Taxes (Narrative) (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Effective Income Tax Computation [Line Items]      
Federal statutory income tax rate 21.00% 35.00% 35.00%
Percentage of deductibility of interest expense 30.00%    
Percentage of federal net operating loss carryforwards 21.00%    
Income tax reconciliation increase in deferred income tax expense $ 23    
Dominion Energy Gas Holdings, LLC      
Effective Income Tax Computation [Line Items]      
Federal statutory income tax rate 21.00% 35.00%  
Increase (decrease) in regulatory liabilities $ 35    
Increase in income tax expense returned to customers 8    
Increase in regulatory liabilities returned to customers $ 11