Goodwill and Intangible Assets (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Amortization expense for intangible assets	$ 69	$ 95	$ 72
Acquisition of intangible assets	$ 152
Weighted-average amortization period (years)	11 years
Virginia Electric and Power Company
Goodwill [Line Items]
Amortization expense for intangible assets	$ 28	$ 30	$ 31
Acquisition of intangible assets	$ 95
Weighted-average amortization period (years)	11 years