Operating Revenue (Schedule of Aggregate Amount of Transaction Price Allocated To Fixed-price Performance Obligations That Unsatisfied At End of Reporting Period And Expected To be Recognized) (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 787
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 18
Revenue, expected to be recognized on multi-year contracts, period	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 66
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 66
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 64
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 57
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 516
Revenue, expected to be recognized on multi-year contracts, period
Virginia Electric and Power Company
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 2
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 1
Revenue, expected to be recognized on multi-year contracts, period	3 months
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 1
Revenue, expected to be recognized on multi-year contracts, period	1 year
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 0
Revenue, expected to be recognized on multi-year contracts, period	1 year
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 0
Revenue, expected to be recognized on multi-year contracts, period	1 year
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 0
Revenue, expected to be recognized on multi-year contracts, period	1 year
Virginia Electric and Power Company | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 0
Revenue, expected to be recognized on multi-year contracts, period