Investments (Investments Under Equity Method of Accounting) (Parenthetical) (Detail) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Liabilities, other deferred credits and other liabilities		$ 17,075	$ 16,631
Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership%	[1]	50.00%
NedPower Mount Storm LLC
Schedule of Equity Method Investments [Line Items]
Liabilities, other deferred credits and other liabilities		$ 1	17
Dominion Energy Midstream Partners, LP | Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership%		25.93%
Dominion Energy Gas Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Liabilities, other deferred credits and other liabilities		$ 2,944	$ 2,866
Dominion Energy Gas Holdings, LLC | Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership%		24.07%

[1] Comprised of Dominion Energy Midstream's interest of 25.93% and Dominion Energy Gas' interest of 24.07%. See Note 15 for more information.