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Fair Value Measurements (Narrative) (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Mar. 31, 2019
Oct. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Debt maximum borrowing capacity [1] $ 6,000,000,000 $ 6,000,000,000 $ 6,000,000,000      
Impairment of assets and other charges   1,535,000,000 403,000,000 $ 15,000,000    
Revolving Credit Facility            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Debt maximum borrowing capacity         $ 700,000,000  
Revolving Credit Facility | Atlantic Coast Pipeline | Financial Guarantee [Member]            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Debt maximum borrowing capacity           $ 3,400,000,000
Guarantee liability           $ 30,000,000
Dominion Energy Gas Holdings, LLC            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Debt maximum borrowing capacity [2] 1,500,000,000 1,500,000,000 1,500,000,000      
Impairment of assets and other charges 219,000,000 $ 13,000,000 163,000,000 $ 15,000,000    
Asset impairment charges, after tax 165,000,000          
Property, plant and equipment estimated fair value $ 190,000,000   $ 190,000,000      
[1] This credit facility matures in March 2023 and can be used by the borrowers under the credit facility to support bank borrowings and the issuance of commercial paper, as well as to support up to a combined $2.0 billion of letters of credit.
[2] A maximum of $1.5 billion of the facility is available to Dominion Energy Gas, assuming adequate capacity is available after giving effect to uses by co-borrowers Dominion Energy, Virginia Power, Questar Gas and DESC. The sub-limit for Dominion Energy Gas is set within the facility limit but can be changed at the option of the borrowers under the credit facility multiple times per year. At December 31, 2019, the sub-limit for Dominion Energy Gas was $750 million. If Dominion Energy Gas has liquidity needs in excess of its sub-limit, the sub-limit may be changed or such needs may be satisfied through short-term intercompany borrowings from Dominion Energy. This credit facility matures in March 2023 and can be used to support bank borrowings and the issuance of commercial paper, as well as to support up to $1.5 billion (or the sub-limit, whichever is less) of letters of credit.