Commitments and Contingencies (Nuclear Operations) (Narrative) (Detail) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
NRC minimum requirement for nuclear power plant licensees				$ 1,060,000,000.00
Maximum assessment for premiums on insurance policy				33,000,000
Maximum assessment for insurance policy				99,000,000
Spent Nuclear Fuel
Loss Contingencies [Line Items]
Receivables				46,000,000	$ 52,000,000
Maximum
Loss Contingencies [Line Items]
Amount that could be assessed for each licensed reactor				138,000,000
Amount that could be assessed for each licensed reactor per reactor				21,000,000
Nuclear Obligations
Loss Contingencies [Line Items]
Maximum liability protection per nuclear incident amount				13,800,000,000
Nuclear Obligations | Secondary Financial Protection Program
Loss Contingencies [Line Items]
Maximum liability protection per nuclear incident amount		$ 13,800,000,000	$ 13,900,000,000
Virginia Electric and Power Company
Loss Contingencies [Line Items]
Maximum assessment for premiums on insurance policy				48,000,000
Maximum assessment for insurance policy				9,000,000
Virginia Electric and Power Company | Spent Nuclear Fuel
Loss Contingencies [Line Items]
Receivables				35,000,000	35,000,000
DESC | Non-nuclear Obligation | European Mutual Association
Loss Contingencies [Line Items]
Property insurance coverage				415,000,000
Annual Premium				2,000,000
Millstone Unit 1
Loss Contingencies [Line Items]
Minimum financial assurance				2,800,000,000
Kewaunee
Loss Contingencies [Line Items]
Maximum assessment for premiums on insurance policy	$ 50,000,000
Kewaunee | Nuclear Obligations
Loss Contingencies [Line Items]
Maximum liability protection per nuclear incident amount				100,000,000
Kewaunee | Virginia Electric and Power Company
Loss Contingencies [Line Items]
Minimum financial assurance				1,800,000,000
Millstone, Summer, Surry and North Anna
Loss Contingencies [Line Items]
Amount of coverage purchased from commercial insurance pools				450,000,000
Surry and North Anna
Loss Contingencies [Line Items]
Settlement amount awarded from other party				24,000,000	15,000,000	$ 16,000,000
Millstone
Loss Contingencies [Line Items]
Settlement amount awarded from other party				11,000,000	11,000,000	$ 13,000,000
Summer
Loss Contingencies [Line Items]
Settlement amount awarded from other party				$ 4,000,000	$ 3,000,000