Income Taxes (Unrecognized Tax Benefits) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 175	$ 44		$ 38
Acquired unrecognized tax benefits	0	129	[1]	0
Increases-prior period positions	18	0		10
Decreases-prior period positions	(19)	0		0
Increases-current period positions	1	9		10
Settlements with tax authorities	0	(7)		(6)
Expiration of statutes of limitations	(8)	0		(8)
Balance at December 31	167	175		44
Virginia Electric and Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	0	2		4
Acquired unrecognized tax benefits	0	0		0
Increases-prior period positions	0	0		0
Decreases-prior period positions	0	0		0
Increases-current period positions	0	0		0
Settlements with tax authorities	0	(2)		(1)
Expiration of statutes of limitations	0	0		(1)
Balance at December 31	$ 0	$ 0		$ 2

[1] Acquired unrecognized tax benefits reflect $106 million plus increases in prior period positions of $76 million and decreases in prior period positions of $53 million that were recorded through purchase accounting.