Income Taxes (Unrecognized Tax Benefits) (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1		$ 44		$ 38	$ 64
Acquired unrecognized tax benefits		129	[1]	0	0
Increases-prior period positions		0		10	1
Decreases-prior period positions		0		0	(9)
Increases-current period positions		9		10	5
Settlements with tax authorities		(7)		(6)	(23)
Expiration of statutes of limitations		0		(8)	0
Balance at December 31		175		44	38
Virginia Electric and Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1		2		4	13
Acquired unrecognized tax benefits		0		0	0
Increases-prior period positions		0		0	0
Decreases-prior period positions		0		0	(1)
Increases-current period positions		0		0	0
Settlements with tax authorities	$ (8)	(2)		(1)	(8)
Expiration of statutes of limitations		0		(1)	0
Balance at December 31		0		2	4
Dominion Energy Gas Holdings, LLC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1		2		2	9
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		0	(7)
Expiration of statutes of limitations		0		0	0
Balance at December 31		$ 2		$ 2	$ 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.