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Short-Term Debt and Credit Agreements (Narrative) (Detail) - USD ($)
1 Months Ended 12 Months Ended
May 31, 2020
Apr. 30, 2020
Mar. 31, 2020
Dec. 31, 2020
Oct. 31, 2020
Dec. 31, 2019
Nov. 30, 2017
Debt Instrument [Line Items]              
Debt maximum borrowing capacity [2]       $ 6,000,000,000 [1]   $ 6,000,000,000  
Short-term debt       895,000,000   849,000,000  
Letter of Credit              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity       2,000,000,000.0      
364-Day Revolving Credit Facility              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity     $ 900,000,000        
Credit facility, amount borrowed       $ 225,000,000      
Weighted average useful life     364 days        
Credit Facility, Maturing in June 2020 | Letter of Credit              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity $ 30,000,000 $ 21,000,000          
Credit facility, maturity date Jun. 30, 2022     Jun. 30, 2020      
Credit facility, amount borrowed       $ 30,000,000   21,000,000  
Term Loan Credit Agreement              
Debt Instrument [Line Items]              
Credit facility, amount borrowed   $ 625,000,000 $ 500,000,000        
Weighted average useful life   364 days 364 days        
Floating Rate Demand Notes              
Debt Instrument [Line Items]              
Short-term debt       268,000,000   75,000,000  
Floating Rate Demand Notes | Shelf Registration for Sale of Demand Notes              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity             $ 3,000,000,000.0
Debt instrument, maximum principal outstanding amount         $ 1,000,000,000.0    
Dominion Energy South Carolina Inc              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity       500,000,000      
Short-term indebtedness outstanding       $ 2,200,000,000      
Debt maturity month and year       2021-03      
Questar Gas              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity       $ 250,000,000      
GENCO              
Debt Instrument [Line Items]              
Short-term indebtedness outstanding       $ 200,000,000      
Debt maturity month and year       2021-03      
SBL Holdco | Credit Facilities, Maturing in December 2017 with 1 year Automatic Renewals through 2023              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity       $ 30,000,000      
Automatic renewal period       1 year      
Short-term debt       $ 0   0  
Dominion Solar Projects III, Inc | Credit Facilities, Maturing in May 2018 with 1 year Automatic Renewals through 2024              
Debt Instrument [Line Items]              
Debt maximum borrowing capacity       $ 25,000,000      
Automatic renewal period       1 year      
Short-term debt       $ 0   $ 0  
[1] In October 2020, the joint revolving credit facility was amended to remove Dominion Energy Gas as a co-borrower.
[2] 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.