Schedule of Acquisitions of Solar Projects (Detail) $ in Millions		1 Months Ended	12 Months Ended
		Mar. 31, 2019 MW	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Property Plant And Equipment [Line Items]
Project Cost			$ 311	$ 341	$ 151
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]		$ 127
Date of Commercial Operations			December 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]		$ 32
Date of Commercial Operations			June 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
Acquisition of Solar Project Blackville in South Carolina
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-05
Project Cost	[1]		$ 12
Date of Commercial Operations			December 2020
MW Capacity | MW			7
Acquisition of Solar Project Denmark in South Carolina
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-05
Project Cost	[1]		$ 14
Date of Commercial Operations			December 2020
MW Capacity | MW			6
Acquisition of Solar Project Yemassee in South Carolina
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-08
Project Cost	[1]		$ 17
Date of Commercial Operations			January 2021
MW Capacity | MW			10
Acquisition of Solar Project Trask in South Carolina
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-10
Project Cost	[1]		$ 25
Date of Commercial Operations			Expected 2021
MW Capacity | MW			12
Acquisition of Solar Project Hardin I in Ohio
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-06
Date Agreement Closed			2020-06
Project Cost	[1]		$ 240
Date of Commercial Operations	[2]		Split
MW Capacity | MW			150
Acquisition of Solar Project Madison in Virginia
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-07
Date Agreement Closed			2020-07
Project Cost	[1]		$ 125
Date of Commercial Operations			Expected 2022
MW Capacity | MW			62
Acquisition of Solar Project Hardin II in Ohio
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-08
Project Cost	[1]		$ 295
Date of Commercial Operations			Expected 2023
MW Capacity | MW			150
Virginia Electric and Power Company
Property Plant And Equipment [Line Items]
Project Cost			$ 35	$ 182	$ 141
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]		$ 128
Date of Commercial Operations			October 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]		$ 127
Date of Commercial Operations			January 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]		$ 165
Date of Commercial Operations			Expected 2021
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]		$ 185
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
Virginia Electric and Power Company | Acquisition of Solar Project Pumpkinseed in Virginia
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-05
Project Cost	[1]		$ 130
Date of Commercial Operations			Expected 2022
MW Capacity | MW			60

[1] Includes acquisition costs.

[2] In December 2020 and January 2021, 97 MW and 53 MW of the project commenced commercial operations, respectively.