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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
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning balance $ 150 $ 139
Total realized and unrealized gains (losses):    
Included in earnings (18) (15)
Included in other comprehensive income 1  
Included in regulatory assets/liabilities (21) (9)
Settlements 7 12
Transfers out of Level 3 1 3
Ending balance 120 130
Virginia Electric and Power Company    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning balance 147 143
Total realized and unrealized gains (losses):    
Included in earnings (17) (15)
Included in regulatory assets/liabilities (19) (8)
Settlements 6 12
Ending balance 117 132
Dominion Energy Gas Holdings, LLC    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning balance (2) (2)
Total realized and unrealized gains (losses):    
Included in other comprehensive income 1 (1)
Transfers out of Level 3 1 3
Ending balance $ 0 $ 0