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
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	$ 103	$ (37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	(47)	80
Settlements	21	22
Ending balance	56	43
Virginia Electric and Power Company
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	103	(37)
Total realized and unrealized gains (losses):
Included in regulatory assets/liabilities	(47)	80
Settlements	21	22
Ending balance	56	43
Electric Fuel and Other Energy-Related Purchases
Total realized and unrealized gains (losses):
Included in earnings	(21)	(22)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company
Total realized and unrealized gains (losses):
Included in earnings	$ (21)	$ (22)