Schedule of Acquisitions of Solar Projects (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2021 USD ($) MW	Mar. 31, 2020 USD ($)
Property Plant And Equipment [Line Items]
Project Cost		$ 23	$ 8
Acquisition of Nonregulated 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]	$ 22
Date of Commercial Operations		March 2021
MW Capacity | MW		12
Acquisition of Nonregulated Solar Project Hardin II in Ohio
Property Plant And Equipment [Line Items]
Date Agreement Entered		2020-08
Project Cost	[1]	$ 300
Date of Commercial Operations		Expected 2023
MW Capacity | MW		150
Virginia Electric and Power Company
Property Plant And Equipment [Line Items]
Project Cost		$ 10	$ 6
Virginia Electric and Power Company | Acquisition of Nonregulated Solar Project Bookers Mill in Virginia
Property Plant And Equipment [Line Items]
Date Agreement Entered		2021-02
Project Cost	[1]	$ 200
Date of Commercial Operations		Expected 2023
MW Capacity | MW		127

[1] Includes acquisition cost