Fair Value Measurements (Financial Instruments' Carrying Amounts and Fair Values) (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[1]	$ 29,362	$ 28,666
Junior subordinated notes	[2]	3,980	3,981
Remarketable subordinated notes	[2]	1,381	1,379
Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[1],[3]	31,044	31,233
Junior subordinated notes	[2],[3]	4,017	4,102
Remarketable subordinated notes	[2],[3]	1,299	1,446
Virginia Electric and Power Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[2]	11,690	11,346
Virginia Electric and Power Company | Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[2],[3]	12,702	12,842
Dominion Energy Gas Holdings, LLC | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[4]	3,579	3,570
Dominion Energy Gas Holdings, LLC | Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[3],[4]	$ 3,657	$ 3,719

[1] Carrying amount includes amounts which represent the unamortized debt issuance costs, discount or premium, and foreign currency remeasurement adjustments. At March 31, 2018 and December 31, 2017, includes the valuation of certain fair value hedges associated with fixed rate debt of $(54) million and $(22) million, respectively.

[2] Carrying amount includes amounts which represent the unamortized debt issuance costs, discount or premium.

[3] Fair value is estimated using market prices, where available, and interest rates currently available for issuance of debt with similar terms and remaining maturities. All fair value measurements are classified as Level 2. The carrying amount of debt issues with short-term maturities and variable rates refinanced at current market rates is a reasonable estimate of their fair value.

[4] Carrying amount includes amounts which represent the unamortized debt issuance costs, discount or premium, and foreign currency remeasurement adjustments.