Fair Value Measurements (Net Change in the Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category) (Detail) - Commodity - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	$ 62	$ 123	$ 103	$ (37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	88	4	49	164
Settlements	(29)	0	(12)	26
Ending balance	144	127	144	127
Virginia Electric and Power Company
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	74	123	103	(37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	41	4	12	164
Settlements	(27)	0	(10)	26
Ending balance	115	127	115	127
Operating Revenue
Total realized and unrealized gains (losses):
Included in earnings	(6)	0	(8)	0
Electric Fuel and Other Energy-Related Purchases
Total realized and unrealized gains (losses):
Included in earnings	29	0	12	(26)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company
Total realized and unrealized gains (losses):
Included in earnings	$ 27	$ 0	$ 10	$ (26)