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Commitments and Contingencies (Lease Commitments) (Narrative) (Detail) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Loss Contingencies [Line Items]        
Rental expense   $ 113 $ 104 $ 99
Corporate office | Agreement with Lessor to Construct and Lease Corporate Office Property        
Loss Contingencies [Line Items]        
Requested cash draws from lessor to fund project costs   139    
Required percentage payment of funded amount under certain events of default 89.90%      
Required percentage payment for specific full recourse events 100.00%      
Lease term 5 years      
Extension term of lease 5 years      
Required percentage payment to lessor for difference between project costs and sales proceeds 87.00%      
Virginia Electric and Power Company        
Loss Contingencies [Line Items]        
Rental expense   57 52 51
Dominion Gas Holdings, LLC        
Loss Contingencies [Line Items]        
Rental expense   $ 34 $ 37 $ 37
Lessor | Corporate office | Agreement with Lessor to Construct and Lease Corporate Office Property        
Loss Contingencies [Line Items]        
Amount of financing commitments to fund estimated project costs $ 365