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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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Balance at January 1 $ 120 $ 130 $ 150 $ 139
Total realized and unrealized gains/income (losses):        
Included in earnings (2) (10) (20) (25)
Included in other comprehensive income (loss)     1  
Included in regulatory assets/liabilities 11 32 (10) 23
Settlements (10)   (3) 12
Transfers out of Level 3     1 3
Balance at December 31 119 152 119 152
Virginia Electric and Power Company        
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Balance at January 1 117 132 147 143
Total realized and unrealized gains/income (losses):        
Included in earnings (2) (10) (19) (25)
Included in regulatory assets/liabilities 8 31 (11) 23
Settlements (8) (1) (2) 11
Balance at December 31 115 152 115 152
Dominion Energy Gas Holdings, LLC        
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Balance at January 1     (2) (2)
Total realized and unrealized gains/income (losses):        
Included in other comprehensive income (loss)     1 (1)
Transfers out of Level 3     1 3
Balance at December 31 $ 0 $ 0 $ 0 $ 0