Employee Benefit Plans (Net Periodic Benefit Cost (Credit)) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 39	$ 35	$ 118	$ 104
Interest cost	85	86	253	259
Expected return on plan assets	(165)	(160)	(498)	(480)
Amortization of prior service cost (credit)			1	1
Amortization of net actuarial loss	48	40	145	121
Settlements		1	0	2
Net periodic benefit cost (credit)	7	2	19	7
Pension Benefits | Dominion Energy Gas Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	3	13	11
Interest cost	7	7	21	22
Expected return on plan assets	(36)	(34)	(111)	(105)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net actuarial loss	4	4	14	12
Net periodic benefit cost (credit)	(21)	(20)	(63)	(60)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	7	20	20
Interest cost	14	15	42	45
Expected return on plan assets	(36)	(32)	(107)	(95)
Amortization of prior service cost (credit)	(13)	(13)	(39)	(38)
Amortization of net actuarial loss	3	3	8	9
Settlements			0	0
Net periodic benefit cost (credit)	(25)	(20)	(76)	(59)
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	3	3
Interest cost	3	3	8	9
Expected return on plan assets	(7)	(7)	(21)	(19)
Amortization of prior service cost (credit)	(1)	(1)	(3)	(2)
Amortization of net actuarial loss	0	1	2	2
Net periodic benefit cost (credit)	$ (4)	$ (3)	$ (11)	$ (7)