Significant Accounting Policies (Narrative) (Detail) $ / shares in Units, $ in Millions			3 Months Ended		6 Months Ended
		Jan. 03, 2018 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Payments for sales and services due period					1 month
Cash provided by (used in) other investing activities					$ (3)	$ (4)
Other adjustments					42	64
Decrease in depreciation expense			$ 30
Decrease in depreciation expense, after tax			23
Purchased gas			64	$ 112	404	417
Unrealized gains reclassified from AOCI to retained earnings			1,100		1,100
Unrealized gains reclassified from AOCI to retained earnings after-tax					734
Unrealized gain (loss) on securities recorded in other income, before tax	[1]		$ 73		(11)
Gain per share from unrealized loss on securities | $ / shares			$ 0.06
Other operations and maintenance			$ 1,007	827	1,803	1,611
Other income			185	108	285	270
Tax reform, reclassification of tax expense (benefit) from AOCI to retained earnings					(289)
Accounting Standards Update 2014-09 [Member] | Performing Processing And Fractionation Services
Significant Accounting Policies [Line Items]
Operating revenue			25	0	50	0
Other energy-related purchases			25	0	50	0
Accounting Standards Update 2014-09 [Member] | Transportation And Storage Arrangements
Significant Accounting Policies [Line Items]
Operating revenue				31		63
Purchased gas				31		63
Accounting Standards Update 2016-01 [Member]
Significant Accounting Policies [Line Items]
Unrealized gain (loss) on securities recorded in other income, before tax			45		(2)
Unrealized gain (loss) on securities recorded in other income, net of tax			36		(2)
Cumulative Effect Of Changes In Accounting Principle [Member] | Accounting Standards Update 2014-09 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle to retained earnings and membership interest			3		3
Cumulative Effect Of Changes In Accounting Principle [Member] | Accounting Standards Update 2017-05 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle to retained earnings and membership interest			$ 127		$ 127
Previously Reported
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities						4
Other adjustments						(72)
Other operations and maintenance				779		1,500
Other income				60		176
Merchant Solar Projects | Terra Nova Renewable Partners
Significant Accounting Policies [Line Items]
Percentage of equity interest sold to noncontrolling interest owners			33.00%		33.00%
Merchant Solar Projects | Terra Nova Renewable Partners | Call Option
Significant Accounting Policies [Line Items]
Percentage ownership in total units			67.00%		67.00%
Four Brothers and Three Cedars
Significant Accounting Policies [Line Items]
Percentage ownership in total units			50.00%		50.00%
Dominion Energy Midstream Partners, LP | Common Units
Significant Accounting Policies [Line Items]
Ownership interest percentage of limited partner interests					60.90%
Dominion Energy Midstream Partners, LP | Convertible Preferred Interests
Significant Accounting Policies [Line Items]
Ownership interest percentage of limited partner interests					37.50%
NRG Energy, Inc | Four Brothers and Three Cedars
Significant Accounting Policies [Line Items]
Percentage of equity interest sold to noncontrolling interest owners			33.00%		33.00%
Dominion Energy Gas Holdings, LLC
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities					$ (6)	(1)
Other adjustments					(4)	6
Purchased gas	[2]			38	29	81
Other income			$ 32	27	65	52
Tax reform, reclassification of tax expense (benefit) from AOCI to membership interests					(26)
Dominion Energy Gas Holdings, LLC | Accounting Standards Update 2014-09 [Member] | Performing Processing And Fractionation Services
Significant Accounting Policies [Line Items]
Operating revenue			25	0	50	0
Other energy-related purchases			25	0	50	0
Dominion Energy Gas Holdings, LLC | Accounting Standards Update 2014-09 [Member] | Transportation And Storage Arrangements
Significant Accounting Policies [Line Items]
Operating revenue				24		48
Purchased gas				24		48
Dominion Energy Gas Holdings, LLC | Cumulative Effect Of Changes In Accounting Principle [Member] | Accounting Standards Update 2014-09 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle to retained earnings and membership interest			3		3
Dominion Energy Gas Holdings, LLC | Previously Reported
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities						(11)
Other operations and maintenance				126		259
Other income				5		10
Virginia Electric and Power Company
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities					4	(17)
Other adjustments					19	20
Decrease in depreciation expense			46
Decrease in depreciation expense, after tax			36
Unrealized gains reclassified from AOCI to retained earnings							$ 119
Unrealized gains reclassified from AOCI to retained earnings after-tax		$ 73
Net unrealized gains on equity securities previously classified as cost method investments			36		36
Net unrealized gains on equity securities previously classified as cost method investments, after tax			22		22
Unrealized gain (loss) on securities recorded in other income, before tax	[1]		36		(11)
Other income			21	$ 13	24	44
Tax reform, reclassification of tax expense (benefit) from AOCI to membership interests					3
Virginia Electric and Power Company | Retained Earnings
Significant Accounting Policies [Line Items]
Net unrealized gains on equity securities previously classified as cost method investments			3		3
Virginia Electric and Power Company | Current Regulatory Liabilities [Member]
Significant Accounting Policies [Line Items]
Net unrealized gains on equity securities previously classified as cost method investments			33		33
Virginia Electric and Power Company | Accounting Standards Update 2016-01 [Member]
Significant Accounting Policies [Line Items]
Unrealized gain (loss) on securities recorded in other income, before tax			6		(2)
Unrealized gain (loss) on securities recorded in other income, net of tax			$ 5		$ (2)
Virginia Electric and Power Company | Previously Reported
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities						$ 0
DETI | Cumulative Effect Of Changes In Accounting Principle [Member]
Significant Accounting Policies [Line Items]
Number of existing contracts | Contract					3

[1] Included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

[2] See Note 18 for amounts attributable to related parties.