Related-Party Transactions (Narrative) (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Derivative Asset	$ 297,000,000			$ 297,000,000			$ 307,000,000
Derivative Liabilities	145,000,000			145,000,000			284,000,000
Virginia Electric and Power Company
Related Party Transaction [Line Items]
Derivative Asset	170,000,000	[1]		170,000,000	[1]		166,000,000
Derivative Liabilities	8,000,000			8,000,000			66,000,000
Short term demand note	15,000,000			15,000,000			33,000,000	[2]
Outstanding borrowings, net of repayments, under money pool for non-regulated subsidiaries	$ 0			$ 0			0
Issuance of common stock (in shares)	0		0	0		0
Virginia Electric and Power Company | Maximum
Related Party Transaction [Line Items]
Interest charges on borrowings	$ 1,000,000		$ 1,000,000	$ 1,000,000		$ 1,000,000
Virginia Electric and Power Company | Pension Benefits | Amounts Associated with the Dominion Pension Plan
Related Party Transaction [Line Items]
Amounts due to Dominion, noncurrent	600,000,000			600,000,000			505,000,000
Virginia Electric and Power Company | Medical Coverage for Local retirees | Amounts Associated with the Dominion Retiree Health and Welfare Plan
Related Party Transaction [Line Items]
Amounts due from Dominion, noncurrent	241,000,000			241,000,000			199,000,000
Virginia Electric and Power Company | Affiliated Entity
Related Party Transaction [Line Items]
Derivative Asset	16,000,000			16,000,000			11,000,000
Derivative Liabilities	3,000,000			3,000,000			5,000,000
Virginia Electric and Power Company | Principal Owner
Related Party Transaction [Line Items]
Short term demand note	15,000,000			15,000,000			33,000,000
Dominion Energy Gas Holdings, LLC
Related Party Transaction [Line Items]
Derivative Asset	40,000,000			40,000,000			32,000,000
Derivative Liabilities	13,000,000			13,000,000			8,000,000
Short term demand note	24,000,000			24,000,000			18,000,000	[3]
Dominion Energy Gas Holdings, LLC | Revolving Credit Facility | IRCA
Related Party Transaction [Line Items]
Short term demand note	24,000,000			24,000,000			18,000,000
Dominion Energy Gas Holdings, LLC | Maximum
Related Party Transaction [Line Items]
Interest charges on borrowings	1,000,000		$ 1,000,000	1,000,000		$ 1,000,000
Dominion Energy Gas Holdings, LLC | Pension Benefits | Amounts Associated with the Dominion Pension Plan
Related Party Transaction [Line Items]
Amounts due from Dominion, noncurrent	763,000,000			763,000,000			734,000,000
Dominion Energy Gas Holdings, LLC | Medical Coverage for Local retirees | Amounts Associated with the Dominion Retiree Health and Welfare Plan
Related Party Transaction [Line Items]
Amounts due from Dominion, noncurrent	$ 12,000,000			$ 12,000,000			$ 7,000,000

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

[2] Virginia Power’s Consolidated Balance Sheet at December 31, 2017 has been derived from the audited Consolidated Balance Sheet at that date.

[3] Dominion Energy Gas’ Consolidated Balance Sheet at December 31, 2017 has been derived from the audited Consolidated Balance Sheet at that date.