Schedule of Acquisitions of Solar Projects (Detail) $ in Millions		6 Months Ended
		Jun. 30, 2021 USD ($) MW	Jun. 30, 2020 USD ($)
Property Plant And Equipment [Line Items]
Project Cost		$ 36	$ 187
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]	$ 305
Date of Commercial Operations		Expected 2022
MW Capacity | MW		150
Virginia Electric and Power Company
Property Plant And Equipment [Line Items]
Project Cost		$ 19	$ 19
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
Date Agreement Closed		2021-06
Project Cost	[1]	$ 200
Date of Commercial Operations		Expected 2023
MW Capacity | MW		127
Virginia Electric and Power Company | Acquisition of Nonregulated Solar Project Belcher in Virginia
Property Plant And Equipment [Line Items]
Date Agreement Entered		2019-06
Date Agreement Closed		2019-08
Project Cost	[1]	$ 164
Date of Commercial Operations		June 2021
MW Capacity | MW		88

[1] Includes acquisition cost