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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	$ 56	$ 43	$ 103	$ (37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	8	80	(39)	160
Settlements	(4)	4	17	26
Ending balance	62	123	62	123
Virginia Electric and Power Company
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	56	43	103	(37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	18	80	(29)	160
Settlements	(4)	4	17	26
Ending balance	74	123	74	123
Operating Revenue
Total realized and unrealized gains (losses):
Included in earnings	(2)	0	(2)	0
Electric Fuel and Other Energy-Related Purchases
Total realized and unrealized gains (losses):
Included in earnings	4	(4)	(17)	(26)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company
Total realized and unrealized gains (losses):
Included in earnings	$ 4	$ (4)	$ (17)	$ (26)