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Commitments and Contingencies (Narrative) (Detail) | 1 Months Ended | 3 Months Ended |  |
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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 | Sep. 30, 2013 Bcfe / d | Mar. 31, 2018 USD ($) facility site party | Apr. 30, 2016 USD ($) | Jul. 31, 2013 count
Ozone Standards | EPA | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum period to develop plans to address new standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |
CWA | Virginia Electric and Power Company | Minimum | Chesapeake Power Station | Judicial Ruling |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional specific sediment, water and aquatic life monitoring period |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CWA | EPA | Final Rule to Revise Effluent Limitations Guidelines for Steam Electric Power Generating Category |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance date | 2023-12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CWA | EPA | 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 | Maximum | Non-FTA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorization export approval of natural gas per day | Bcfe / d |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.77 |  |  |
Unfavorable Regulatory Action | EPA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Significant emission rate per rear CO2 equivalent | T |  |  |  |  |  |  | 75,000 |  |  |  |  |  |  |  |  |  |
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 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000,000 |  |
Unfavorable Regulatory Action | Ozone Standards | EPA | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 35,000,000 |  |
Unfavorable Regulatory Action | Regulationsto Reduce Nitrogen Oxide And Volatile Organic Compounds Emissions | Pennsylvania Department Of Environmental Protection | Dominion Gas Holdings L L C |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 35,000,000 |
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 | 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Virginia Electric and Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000 |  |
Unfavorable Regulatory Action | Coal Tar and Other Potentially Harmful Materials | EPA | Virginia Electric and Power Company | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 22,000,000 |  |
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 CCR Final Rule |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of facilities with CCR | facility |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |
Number of stations | station |  |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |
Unfavorable Regulatory Action | CCR | EPA | Virginia Electric and Power Company | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |  |
Unfavorable Regulatory Action | CCR | EPA | Virginia Electric and Power Company | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated compliance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |
Breach of Contract Lawsuit | DETI | Pending Litigation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total number of counts in complaint | count |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8
Accrued liability for legal matter |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,000,000 |  |
Breach of Contract Lawsuit | DETI | Judicial Ruling |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | FERC-regulated | Cove Point | Pending Litigation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of parties | party |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |
Liquefaction Project | FERC-regulated | Cove Point | Judicial Ruling |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of parties | party |  |  |  |  |  |  |  | 1 |  | 1 |  |  |  |  |  |  |

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