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Regulatory Matters - Summary of Additional Significant Virginia Power Electric Transmission Projects Approved and Applied (Detail) - Virginia Electric and Power Company - Virginia Regulation
$ in Millions
 1 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
kV
Dec. 31, 2018
USD ($)
kV
mi
Public Utilities General Disclosures [Line Items]    
Capacity of transmission line (kV)   115
Transmission Lines Between Possum Point Switching Station And Novec's Smoketown Delivery Point [Member]    
Public Utilities General Disclosures [Line Items]    
Application Date   2017-06
Approval Date   2018-02
Capacity of transmission line (kV)   230
Length of kV line (miles) | mi   9.0
Total estimated cost of project | $   $ 20
Transmission Line Between Dooms Substation And Valley Substation [Member]    
Public Utilities General Disclosures [Line Items]    
Application Date   2017-09
Approval Date   2018-09
Capacity of transmission line (kV)   500
Length of kV line (miles) | mi   18.0
Total estimated cost of project | $   $ 65
Transmission Line Between Idylwood and Tysons Substations    
Public Utilities General Disclosures [Line Items]    
Application Date   2017-11
Approval Date   2018-09
Capacity of transmission line (kV) 230 230
Length of kV line (miles) | mi   4.0
Total estimated cost of project | $ $ 30 $ 125
Transmission Lines Between Chesterfield and Hopewell Substations    
Public Utilities General Disclosures [Line Items]    
Application Date   2018-05
Approval Date   2018-11
Capacity of transmission line (kV)   230
Length of kV line (miles) | mi   8.0
Total estimated cost of project | $   $ 30
Transmission Lines Between Chesterfield and Lakeside Substations    
Public Utilities General Disclosures [Line Items]    
Application Date   2018-05
Approval Date   2018-12
Capacity of transmission line (kV)   230
Length of kV line (miles) | mi   21.0
Total estimated cost of project | $   $ 35
Transmission Lines Between Landstown and Thrasher Substations    
Public Utilities General Disclosures [Line Items]    
Application Date   2018-06
Approval Date   2018-12
Capacity of transmission line (kV)   230
Length of kV line (miles) | mi   8.5
Total estimated cost of project | $   $ 20
Transmission Line In Alleghany Country Virginia And Covington Virginia    
Public Utilities General Disclosures [Line Items]    
Application Date   2018-08
Capacity of transmission line (kV)   138
Length of kV line (miles) | mi   5.0
Total estimated cost of project | $   $ 15
Build a new substation and connect three existing transmission lines thereto in Fluvanna County, Virginia    
Public Utilities General Disclosures [Line Items]    
Application Date   2018-10
Capacity of transmission line (kV)   230
Total estimated cost of project | $   $ 30
Maximum | Build a new substation and connect three existing transmission lines thereto in Fluvanna County, Virginia    
Public Utilities General Disclosures [Line Items]    
Length of kV line (miles) | mi   1