Commitments and Contingencies (Narrative) (Detail) $ in Millions	1 Months Ended													6 Months Ended
	Sep. 30, 2017 petition	Jul. 31, 2017	Jun. 30, 2017	May 31, 2017 count	Apr. 30, 2017 petition station	Mar. 31, 2017 count	Aug. 31, 2016 T	Jul. 31, 2016 party	Oct. 31, 2015 ppb	Jun. 30, 2015 party	Apr. 30, 2015 facility	Oct. 31, 2014 facility gal	Jun. 30, 2014	Jun. 30, 2018 USD ($) facility site party	Apr. 30, 2016 USD ($)	Jul. 31, 2013 count
Virginia Electric and Power Company | Future Ash Pond And Landfill Closure Costs
Loss Contingencies [Line Items]
ARO incurred														$ 131
Virginia Electric and Power Company | Increase In Property Plant And Equipment
Loss Contingencies [Line Items]
ARO incurred														46
Virginia Electric and Power Company | Increase In Regulatory Assets
Loss Contingencies [Line Items]
ARO incurred														4
Virginia Electric and Power Company | Other operations and maintenance
Loss Contingencies [Line Items]
ARO incurred														81
ARO incurred after tax														60
CWA | Virginia Electric and Power Company | Minimum | Possum Point Power Station | Judicial Ruling [Member]
Loss Contingencies [Line Items]
Additional specific sediment, water and aquatic life monitoring period		2 years
CWA | EPA | Virginia Electric and Power Company | Final Rule to Revise Effluent Limitations Guidelines for Steam Electric Power Generating Category
Loss Contingencies [Line Items]
Compliance date	2023-12
CWA | EPA | Virginia Electric and Power Company | 2015 Effluent Limitations Guidelines Rule for Steam Electric Power Generating Category
Loss Contingencies [Line Items]
Compliance date	2020-11
CCR | EPA
Loss Contingencies [Line Items]
Number of petition agreed for reconsideration | petition	2
Unfavorable Regulatory Action | EPA
Loss Contingencies [Line Items]
Significant emission rate per rear CO2 equivalent | T							75,000
Public Utilities Electricity generating facilitties Minimum regulatory Threshold per day | gal												2,000,000
Unfavorable Regulatory Action | EPA | Minimum
Loss Contingencies [Line Items]
Electric generating station facilities heightened entrainment analysis per day | gal												125,000,000
Unfavorable Regulatory Action | MATS | EPA
Loss Contingencies [Line Items]
Requested compliance extension under EPA Administrative Order									1 year
Extension period for MATS compliance													1 year
Unfavorable Regulatory Action | MATS | EPA | Virginia Electric and Power Company
Loss Contingencies [Line Items]
Number of effective days to operate power station units			90 days
Period to be in service following receipt of all required permits			23 months
Unfavorable Regulatory Action | Ozone Standards | EPA
Loss Contingencies [Line Items]
Standard emission level prior to revision | ppb									75
Revised emission standard level | ppb									70
Unfavorable Regulatory Action | Ozone Standards | EPA | Virginia Electric and Power Company | Minimum
Loss Contingencies [Line Items]
Estimated compliance costs														25
Unfavorable Regulatory Action | Ozone Standards | EPA | Virginia Electric and Power Company | Maximum
Loss Contingencies [Line Items]
Estimated compliance costs														$ 35
Unfavorable Regulatory Action | Regulations to Reduce Nitrogen Oxide and Volatile Organic Compounds Emissions [Member] | Pennsylvania Department Of Environmental Protection [Member] | Dominion Gas Holdings L L C
Loss Contingencies [Line Items]
Estimated compliance costs															$ 35
Unfavorable Regulatory Action | Oil And Gas New Source Performance Standards
Loss Contingencies [Line Items]
Number of period court appeals DC vacated stay		90 days
Unfavorable Regulatory Action | Oil And Gas New Source Performance Standards | EPA
Loss Contingencies [Line Items]
Requested compliance extension under EPA Administrative Order			2 years
EPA notice issued reviewing month and year														2017-04
Number of period EPA published notice of reconsideration and partial stay			90 days
Unfavorable Regulatory Action | CWA
Loss Contingencies [Line Items]
Number of facilities that may be subject to final regulations | facility												13
Unfavorable Regulatory Action | CWA | Virginia Electric and Power Company
Loss Contingencies [Line Items]
Number of facilities that may be subject to final regulations | facility												11
Unfavorable Regulatory Action | CWA | EPA | Virginia Electric and Power Company | Final Rule to Revise Effluent Limitations Guidelines for Steam Electric Power Generating Category
Loss Contingencies [Line Items]
Number of facilities that maybe subject to additional wastewater treatment requirements | facility														8
Number of separate petitions for reconsideration granted | petition					2
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA
Loss Contingencies [Line Items]
Number of former manufactured gas plant sites | site														19
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Former Gas Plant Site with Post Closure Groundwater Monitoring Program
Loss Contingencies [Line Items]
Number of former sites | site														2
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Virginia Electric and Power Company
Loss Contingencies [Line Items]
Estimated compliance costs														$ 18
Number of former manufactured gas plant sites | site														3
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Virginia Electric and Power Company | Other operations and maintenance
Loss Contingencies [Line Items]
Loss contingency loss in period														$ 16
Loss contingency loss in period after tax														$ 12
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Dominion Energy Gas Holdings, LLC
Loss Contingencies [Line Items]
Number of former manufactured gas plant sites | site														12
Unfavorable Regulatory Action | CCR | EPA | Virginia Electric and Power Company | Environmental Protection Agency Final Rule Regulating Management of CCRs | Facilities Subject to Coal Combustion Residual Final Rule
Loss Contingencies [Line Items]
Number of facilities with CCR | facility											8
Number of stations | station					4
Breach of Contract Lawsuit | DETI | Pending Litigation [Member]
Loss Contingencies [Line Items]
Total number of counts in complaint | count																8
Breach of Contract Lawsuit | DETI | Pending Litigation [Member] | Current Liabilities
Loss Contingencies [Line Items]
Accrued liability for legal matter														$ 14
Breach of Contract Lawsuit | DETI | Judicial Ruling [Member]
Loss Contingencies [Line Items]
Number of counts dismissed | count						3
Breach of Contract Lawsuit | DETI | Settled Litigation
Loss Contingencies [Line Items]
Number of claims withdrew | count				1
Liquefaction Project | Federal Energy Regulatory Commission | Cove Point | Pending Litigation [Member]
Loss Contingencies [Line Items]
Number of parties | party														2
Liquefaction Project | Federal Energy Regulatory Commission | Cove Point | Judicial Ruling [Member]
Loss Contingencies [Line Items]
Number of parties | party								1		1