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) (Parenthetical) (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 770
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	$ 67	[1]
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	[1]
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	[1]
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	[1]
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	$ 50	[1]
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 466	[1]
Revenue, expected to be recognized on multi-year contracts, period
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

[1] Includes $1 million for Virginia Power in 2021.