Derivatives and Hedge Accounting Activities (Schedule of Offsetting Assets) (Detail) - USD ($) $ in Millions	Sep. 30, 2018		Dec. 31, 2017
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 287		$ 303
Net Amounts of Assets Presented in the Consolidated Balance Sheet	287		303
Total derivatives, not subject to a master netting or similar arrangement	10		4
Gross Amounts of Recognized Assets, Total	297		307
Derivative Asset	297		307
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	51		99
Net Amounts	236		204
Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	154		155
Net Amounts of Assets Presented in the Consolidated Balance Sheet	154		155
Total derivatives, not subject to a master netting or similar arrangement	16		11
Gross Amounts of Recognized Assets, Total	170		166
Derivative Asset	170	[1]	166
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	5		4
Net Amounts	149		151
Dominion Energy Gas Holdings, LLC
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	40		32
Gross Amounts Offset in the Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in the Consolidated Balance Sheet	40		32
Derivative Asset	40		32
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	4		2
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received	0		0
Net Amounts	36		30
Commodity contracts | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	109		174
Net Amounts of Assets Presented in the Consolidated Balance Sheet	109		174
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	10		9
Net Amounts	99		165
Commodity contracts | Exchange
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	29		80
Net Amounts of Assets Presented in the Consolidated Balance Sheet	29		80
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	29		80
Commodity contracts | Virginia Electric and Power Company | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	89		155
Net Amounts of Assets Presented in the Consolidated Balance Sheet	89		155
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	5		4
Net Amounts	84		151
Interest rate | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	117		17
Net Amounts of Assets Presented in the Consolidated Balance Sheet	117		17
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	9		8
Net Amounts	108		9
Interest rate | Virginia Electric and Power Company | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	65
Net Amounts of Assets Presented in the Consolidated Balance Sheet	65
Net Amounts	65
Interest rate | Dominion Energy Gas Holdings, LLC | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	8		0
Gross Amounts Offset in the Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in the Consolidated Balance Sheet	8		0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	1		0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received	0		0
Net Amounts	7		0
Foreign currency | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	32		32
Net Amounts of Assets Presented in the Consolidated Balance Sheet	32		32
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	3		2
Net Amounts	29		30
Foreign currency | Dominion Energy Gas Holdings, LLC | Over-the-counter
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	32		32
Gross Amounts Offset in the Consolidated Balance Sheet	0		0
Net Amounts of Assets Presented in the Consolidated Balance Sheet	32		32
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments	3		2
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received	0		0
Net Amounts	$ 29		$ 30

[1] Noncurrent derivative assets are presented in other deferred charges and other assets in Virginia Power’s Consolidated Balance Sheets.