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Fair Value Measurements (Net Change in the Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category) (Detail) - Commodity - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Beginning balance $ 150 $ 139 $ 95
Total realized and unrealized gains (losses):      
Included in other comprehensive income (loss) 1 (2)  
Included in regulatory assets/liabilities (44) 42 (39)
Settlements (27) 6 38
Purchases     87
Transfers out of Level 3 1 3 (7)
Ending balance 64 150 139
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date   2 (1)
Virginia Electric and Power Company      
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Beginning balance 147 143 93
Total realized and unrealized gains (losses):      
Included in regulatory assets/liabilities (45) 40 (37)
Settlements (25) 7 35
Purchases     87
Ending balance 60 147 143
Dominion Energy Gas Holdings, LLC      
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Beginning balance (2) (2) 6
Total realized and unrealized gains (losses):      
Included in other comprehensive income (loss) 1 (3)  
Transfers out of Level 3 1 3 (8)
Ending balance   (2) (2)
Operating Revenue      
Total realized and unrealized gains (losses):      
Included in earnings (2) 3  
Electric fuel and other energy-related purchases      
Total realized and unrealized gains (losses):      
Included in earnings (15) (42) (35)
Electric fuel and other energy-related purchases | Virginia Electric and Power Company      
Total realized and unrealized gains (losses):      
Included in earnings $ (17) (43) $ (35)
Purchased Gas      
Total realized and unrealized gains (losses):      
Included in earnings   $ 1