Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Millions		Sep. 30, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		$ 297		$ 307
Derivative Liabilities		145		284
Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		170	[1]	166
Derivative Liabilities		8		66
Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		40		32
Derivative Liabilities		13		8
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	[2]	5,650		5,379
Total Liabilities		145		284
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		2,593		2,466
Total Liabilities		8		66
Fair Value, Measurements, Recurring | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		40		32
Total Liabilities		13		8
Fair Value, Measurements, Recurring | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	3,785		3,493
Fair Value, Measurements, Recurring | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	1,699		1,566
Fair Value, Measurements, Recurring | Fixed Income [Member] | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	437		444
Fair Value, Measurements, Recurring | Fixed Income [Member] | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	218		224
Fair Value, Measurements, Recurring | Fixed Income [Member] | Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	1,118		1,101
Fair Value, Measurements, Recurring | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	506		494
Fair Value, Measurements, Recurring | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	13		34
Fair Value, Measurements, Recurring | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]			16
Fair Value, Measurements, Recurring | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		148		258
Derivative Liabilities		100		197
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		105		166
Derivative Liabilities		8		9
Fair Value, Measurements, Recurring | Commodity | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities		9		6
Fair Value, Measurements, Recurring | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		117		17
Derivative Liabilities		42		85
Fair Value, Measurements, Recurring | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		65
Derivative Liabilities				57
Fair Value, Measurements, Recurring | Interest rate | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		8
Derivative Liabilities		1
Fair Value, Measurements, Recurring | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		32		32
Derivative Liabilities		3		2
Fair Value, Measurements, Recurring | Foreign currency | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		32		32
Derivative Liabilities		3		2
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	[2]	4,148		3,834
Fair Value, Measurements, Recurring | Level 1 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		1,857		1,750
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	3,785		3,493
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	1,699		1,566
Fair Value, Measurements, Recurring | Level 1 | Fixed Income [Member] | Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	350		307
Fair Value, Measurements, Recurring | Level 1 | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	158		168
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	13		34
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]			16
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	[2]	1,404		1,388
Total Liabilities		139		277
Fair Value, Measurements, Recurring | Level 2 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		645		564
Total Liabilities		3		61
Fair Value, Measurements, Recurring | Level 2 | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		40		32
Total Liabilities		13		6
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	437		444
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	218		224
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	768		794
Fair Value, Measurements, Recurring | Level 2 | Fixed Income [Member] | Government Securities [Member] | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[3]	348		326
Fair Value, Measurements, Recurring | Level 2 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		50		101
Derivative Liabilities		94		190
Fair Value, Measurements, Recurring | Level 2 | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		14		14
Derivative Liabilities		3		4
Fair Value, Measurements, Recurring | Level 2 | Commodity | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities		9		4
Fair Value, Measurements, Recurring | Level 2 | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		117		17
Derivative Liabilities		42		85
Fair Value, Measurements, Recurring | Level 2 | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		65
Derivative Liabilities				57
Fair Value, Measurements, Recurring | Level 2 | Interest rate | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		8
Derivative Liabilities		1
Fair Value, Measurements, Recurring | Level 2 | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		32		32
Derivative Liabilities		3		2
Fair Value, Measurements, Recurring | Level 2 | Foreign currency | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		32		32
Derivative Liabilities		3		2
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	[2]	98		157
Total Liabilities		6		7
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		91		152
Total Liabilities		5		5
Fair Value, Measurements, Recurring | Level 3 | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Liabilities				2
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		98		157
Derivative Liabilities		6		7
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		91		152
Derivative Liabilities		$ 5		5
Fair Value, Measurements, Recurring | Level 3 | Commodity | Dominion Energy Gas Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities				$ 2

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

[2] Includes investments held in the nuclear decommissioning and rabbi trusts. Excludes $217 million and $88 million of assets at September 30, 2018 and December 31, 2017, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.

[3] Includes investments held in the nuclear decommissioning trusts. Excludes $151 million and $27 million of assets at September 30, 2018 and December 31, 2017, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.