Significant Accounting Policies (Narrative) (Detail) $ / shares in Units, $ in Millions			3 Months Ended		9 Months Ended
		Jan. 03, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Contract $ / shares	Sep. 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					$ 5	$ 6
Other adjustments					(10)	(14)
Decrease in depreciation expense					30
Decrease in depreciation expense, after tax					23
Purchased gas			$ 5	$ 24	409	441
Unrealized gains reclassified from AOCI to retained earnings							$ 1,100
Unrealized gains reclassified from AOCI to retained earnings after-tax		$ 734
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			$ 236		$ 225
Gain per share from unrealized loss on securities | $ / shares			$ 0.18		$ 0.18
Other operations and maintenance			$ 782	697	$ 2,585	2,308
Other income			373	121	658	391
Tax reform, reclassification of tax expense (benefit) from AOCI to retained earnings					(289)
Retained Earnings
Significant Accounting Policies [Line Items]
Net unrealized gains on equity securities previously classified as cost method investments			3		3
Current Regulatory Liabilities [Member]
Significant Accounting Policies [Line Items]
Net unrealized gains on equity securities previously classified as cost method investments			33		33
Accounting Standards Update 2014-09 [Member] | Performing Processing And Fractionation Services
Significant Accounting Policies [Line Items]
Operating revenue			24	0	74	0
Other energy-related purchases			24	0	74	0
Accounting Standards Update 2014-09 [Member] | Transportation And Storage Arrangements
Significant Accounting Policies [Line Items]
Operating revenue				20		83
Purchased gas				20		83
Accounting Standards Update 2016-01 [Member]
Significant Accounting Policies [Line Items]
Unrealized gain (loss) on securities recorded in other income, before tax			148		146
Unrealized gain (loss) on securities recorded in other income, net of tax			116		116
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						(6)
Other adjustments						(95)
Other operations and maintenance				649		2,200
Other income				73		249
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%
GIP | 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					$ 14	7
Other adjustments					(5)	9
Purchased gas	[1]		$ (7)	19	22	100
Other income			34	27	99	79
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			21	0	71	0
Other energy-related purchases			21	0	71	0
Dominion Energy Gas Holdings, LLC | Accounting Standards Update 2014-09 [Member] | Transportation And Storage Arrangements
Significant Accounting Policies [Line Items]
Operating revenue				16		64
Purchased gas				16		64
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						(16)
Other operations and maintenance				53		312
Other income				6		16
Virginia Electric and Power Company
Significant Accounting Policies [Line Items]
Cash provided by (used in) other investing activities					47	29
Other adjustments					48	28
Decrease in depreciation expense			7		53
Decrease in depreciation expense, after tax			5		41
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			103		92
Other income			25	$ 13	49	57
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			15		13
Unrealized gain (loss) on securities recorded in other income, net of tax			$ 12		$ 11
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] See Note 18 for amounts attributable to related parties.