Schedule of Acquisitions of Solar Projects (Detail) $ in Millions		1 Months Ended	12 Months Ended
		Mar. 31, 2019 MW	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Project Cost			$ 341	$ 151	$ 405
Acquisition of Solar Project Greensville in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-08
Date Agreement Closed			2019-08
Project Cost	[1]		$ 130
Date of Commercial Operations			Expected 2020
MW capacity | MW			80
Acquisition of Solar Project Myrtle in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-08
Date Agreement Closed			2019-08
Project Cost	[1]		$ 35
Date of Commercial Operations			Expected 2020
MW capacity | MW			15
Acquisition of Solar Project Seabrook in South Carolina
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-09
Date Agreement Closed			2019-09
Project Cost	[1]		$ 103
Date of Commercial Operations			December 2019
MW capacity | MW			72
Acquisition of Solar Project Wilkinson in North Carolina
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-11
Date Agreement Closed			2019-11
Project Cost	[1]		$ 153
Date of Commercial Operations			December 2019
MW capacity | MW			74
Virginia Electric and Power Company
Property, Plant and Equipment [Line Items]
Project Cost			$ 182	$ 141	$ 41
MW capacity | MW		1,292
Virginia Electric and Power Company | Acquisition Of Solar Project Pecan In North Carolina
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2017-09
Date Agreement Closed			2018-10
Project Cost	[1]		$ 140
Date of Commercial Operations			December 2018
MW capacity | MW			75
Virginia Electric and Power Company | Acquisition of Solar Project Gutenberg in North Carolina
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2017-09
Date Agreement Closed			2019-06
Project Cost	[1]		$ 142
Date of Commercial Operations			September 2019
MW capacity | MW			80
Virginia Electric and Power Company | Acquisition of Solar Project Gloucester in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2018-06
Date Agreement Closed			2019-02
Project Cost	[1]		$ 37
Date of Commercial Operations			April 2019
MW capacity | MW			20
Virginia Electric and Power Company | Acquisition of Solar Project Grasshopper in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2018-08
Date Agreement Closed			2019-05
Project Cost	[1]		$ 130
Date of Commercial Operations			Expected 2020
MW capacity | MW			80
Virginia Electric and Power Company | Acquisition of Solar Project Chestnut in North Carolina
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2018-08
Date Agreement Closed			2019-05
Project Cost	[1]		$ 130
Date of Commercial Operations			Expected 2020
MW capacity | MW			75
Virginia Electric and Power Company | Acquisition of Solar Project Ft. Powhatan in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-06
Date Agreement Closed			2019-06
Project Cost	[1]		$ 270
Date of Commercial Operations			Expected 2021
MW capacity | MW			150
Virginia Electric and Power Company | Acquisition of Solar Project Belcher in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-06
Date Agreement Closed			2019-08
Project Cost	[1]		$ 160
Date of Commercial Operations			Expected 2020
MW capacity | MW			88
Virginia Electric and Power Company | Acquisition of Solar Project Bedford in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-08
Date Agreement Closed			2019-11
Project Cost	[1]		$ 110
Date of Commercial Operations			Expected 2021
MW capacity | MW			70
Virginia Electric and Power Company | Acquisition of Solar Project Maplewood in Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-10
Date Agreement Closed			2019-10
Project Cost	[1]		$ 190
Date of Commercial Operations			Expected 2022
MW capacity | MW			120
Virginia Electric and Power Company | Acquisition Of Solar Project Rochambeau In Virginia
Property, Plant and Equipment [Line Items]
Date Agreement Entered			2019-12
Date Agreement Closed			2020-01
Project Cost	[1]		$ 35
Date of Commercial Operations			Expected 2021
MW capacity | MW			20

[1] Includes acquisition costs.