Fair Value Measurements (Narrative) (Detail) - USD ($)	3 Months Ended					6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets and other charges (benefit)	$ 321,000,000			$ 44,000,000		$ 416,000,000	$ 812,000,000
Unrealized gains or losses included in operating revenue in Level 3 fair value category	(2,000,000)			0		(2,000,000)	0
Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets and other charges (benefit)	12,000,000			44,000,000		(39,000,000)	808,000,000
Unrealized gains or losses included in operating revenue in Level 3 fair value category	0			$ 0		0	$ 0
Nonregulated Retail Software Development | Level 2 | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets, estimated fair value	1,000,000					$ 1,000,000
Impairment of Assets and Other Charges | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets and other charges (benefit)		$ 76,000,000	$ 127,000,000		$ 754,000,000
Asset impairment charges after tax		$ 56,000,000			$ 561,000,000
Impairment of Assets and Other Charges | Nonregulated Retail Software Development
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of assets and other charges (benefit)	20,000,000
Asset impairment charges after tax	$ 15,000,000