Variable Interest Entities - (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Generator MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Virginia Electric and Power Company
Variable Interest Entity [Line Items]
Long-term debt	$ 13,207	$ 12,325
Payment for contract termination		135
Payment for contract termination after tax		100
Payables to affiliates	266	210
Variable Interest Entity, Primary Beneficiary | SBL Holdco
Variable Interest Entity [Line Items]
Securities due within one year	32
Long-term debt	$ 239
Variable Interest Entity, Primary Beneficiary | Partnership Interest | Merchant Solar Projects
Variable Interest Entity [Line Items]
Initial membership interest percentage	67.00%
Variable Interest Entity Not Primary Beneficiary | Distribution | Atlantic Coast Pipeline | Pipelines | Jointly Owned Natural Gas Pipeline
Variable Interest Entity [Line Items]
Initial membership interest percentage	53.00%
Variable Interest Entity Not Primary Beneficiary | Virginia Electric and Power Company | DES
Variable Interest Entity [Line Items]
Shared Services Purchased	$ 349	387	$ 335
Payables to affiliates	$ 175	102
Variable Interest Entity Not Primary Beneficiary | Partnership Interest | Cove Point
Variable Interest Entity [Line Items]
Initial membership interest percentage	50.00%
Variable Interest Entity V I E And Potential V I E Information Unavailability | Virginia Electric and Power Company
Variable Interest Entity [Line Items]
Long term capacity contract non utility generators (generators) | Generator	1
Aggregate generation capacity from long-term power and capacity contracts (MW) | MW	218
Payment for contract termination	$ 135
Payment for contract termination after tax	$ 100
Payment for electric capacity		13	1
Payment for electric energy		$ 50	$ 18