Credit Risk (Narrative) (Detail)	9 Months Ended
Sep. 30, 2020 USD ($) counterparty	Dec. 31, 2019 USD ($)
Concentration Risk and Guarantor Obligations [Line Items]
Gross credit exposure	$ 160,000,000
Additional collateral to be posted if the credit related contingent features were triggered	13,000,000	$ 10,000,000
Collateral derivatives with credit-related contingent provision in a liability position	1,000,000	0
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position	14,000,000	10,000,000
Virginia Electric and Power Company
Concentration Risk and Guarantor Obligations [Line Items]
Gross credit exposure	10,000,000
Additional collateral to be posted if the credit related contingent features were triggered	1,000,000	$ 8,000,000
Credit Concentration Risk
Concentration Risk and Guarantor Obligations [Line Items]
Gross credit exposure	$ 42,000,000
Number of counterparties \| counterparty	0
Credit Concentration Risk \| Wholesale Customers \| Sales Revenue, Net \| Virginia Electric and Power Company
Concentration Risk and Guarantor Obligations [Line Items]
Gross credit exposure	$ 3,000,000
Number of counterparties \| counterparty	0
Credit Concentration Risk \| Investment Grade \| Investment Grade Counterparty
Concentration Risk and Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	94.00%
Credit Concentration Risk \| Investment Grade \| Investment Grade Counterparty \| Virginia Electric and Power Company
Concentration Risk and Guarantor Obligations [Line Items]
Concentration risk, percentage (percentage)	93.00%

Concentration Risk and Guarantor Obligations [Line Items]

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Concentration risk credit risk number of counterparties.

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The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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