Employee Benefit Plans (Fair values of pension and post retirement plan assets by asset category) (Parenthetical) (Detail) - Collective Bargaining Agreement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Common stock	$ 508
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pending sales of securities	52	$ 12
Net accrued income	24	21
Pending purchases of securities	99	22
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Pending sales of securities	2	3
Net accrued income	2	5
Pending purchases of securities	6	5
Dominion Energy Gas Holdings, LLC | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pending sales of securities	2	1
Net accrued income	2	2
Pending purchases of securities	$ 5	$ 2