Schedule of Acquisitions of Solar Projects (Detail) $ in Millions		1 Months Ended	9 Months Ended
		Mar. 31, 2019 MW	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)
Property Plant And Equipment [Line Items]
Project Cost			$ 245	$ 183
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 Blackville in South Carolina
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-05
Date Agreement Closed			2020-05
Project Cost	[1]		$ 15
Date of Commercial Operations			Expected 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]		$ 15
Date of Commercial Operations			Expected 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]		$ 20
Date of Commercial Operations			Expected 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]		$ 255
Date of Commercial Operations			Expected 2020
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			63
Acquisition of Solar Project Hardin II in Ohio
Property Plant And Equipment [Line Items]
Date Agreement Entered			2020-08
Date Agreement Closed			Expected 2022
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			$ 26	$ 169
MW Capacity | MW		1,292
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 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 2021
MW Capacity | MW			88
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 cost