Employee Benefit Plans (Narrative) (Detail) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions to qualified defined benefit pension plans and OPEB plans					$ 0
Voluntary Retirement Program
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				$ 423,000,000
Voluntary retirement program related charges net of tax				316,000,000
Voluntary Retirement Program | Other Operations and Maintenance
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				247,000,000
Voluntary Retirement Program | Other Taxes
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				21,000,000
Voluntary Retirement Program | Other Income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				111,000,000
Voluntary Retirement Program | Net Income Including Noncontrolling Interest From Discontinued Operations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				44,000,000
Voluntary Retirement Program | Virginia Electric and Power Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				194,000,000
Voluntary retirement program related charges net of tax				144,000,000
Voluntary Retirement Program | Virginia Electric and Power Company | Other Operations and Maintenance
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				186,000,000
Voluntary Retirement Program | Virginia Electric and Power Company | Other Taxes
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary retirement program related charges				$ 8,000,000
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 45,000,000	$ 41,000,000		131,000,000	$ 121,000,000
Remeasurement resulted in increase in pension benefit obligation		497,000,000
Increase in fair value of plan assets		87,000,000
Net periodic benefit (credit) cost		(10,000,000)	5,000,000		(28,000,000)	87,000,000
Pension Benefits | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate percentage								3.11%
Pension Benefits | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate percentage								3.16%
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		8,000,000	7,000,000		22,000,000	20,000,000
Net periodic benefit (credit) cost		(27,000,000)	(24,000,000)		(83,000,000)	(22,000,000)
Scenario Forecast
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits to retirees expected charge	$ 25,000,000
Benefits to retirees expected charge after tax	19,000,000
Scenario Forecast | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit (credit) cost							$ 4,000,000
Expected contribution defined benefit pension plans and OPEB plans through VEBAs for the remainder of 2020	$ 250,000,000						250,000,000
Scenario Forecast | Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution to voluntary employees beneficiary association							$ 0
Discontinued Operations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 3,000,000	$ 4,000,000		$ 12,000,000	$ 12,000,000