Employee Benefit Plans (Net Periodic Benefit Cost (Credit)) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 40	$ 34	$ 79	$ 69
Interest cost	84	87	168	173
Expected return on plan assets	(168)	(161)	(333)	(320)
Amortization of prior service cost (credit)	1	1	1	1
Amortization of net actuarial loss	49	41	97	81
Settlements			0	1
Net periodic benefit cost (credit)	6	2	12	5
Pension Benefits | Dominion Energy Gas Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	4	9	8
Interest cost	7	8	14	15
Expected return on plan assets	(38)	(36)	(75)	(71)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net actuarial loss	5	4	10	8
Net periodic benefit cost (credit)	(21)	(20)	(42)	(40)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	6	13	13
Interest cost	14	14	28	30
Expected return on plan assets	(35)	(31)	(71)	(63)
Amortization of prior service cost (credit)	(13)	(13)	(26)	(25)
Amortization of net actuarial loss	2	3	5	6
Settlements			0	0
Net periodic benefit cost (credit)	(26)	(21)	(51)	(39)
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	2	3	5	6
Expected return on plan assets	(6)	(6)	(14)	(12)
Amortization of prior service cost (credit)	(1)	(1)	(2)	(1)
Amortization of net actuarial loss	1	0	2	1
Net periodic benefit cost (credit)	$ (3)	$ (3)	$ (7)	$ (4)