Variable Interest Entities (Narrative) (Detail) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	May 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) MW Generator
Virginia Electric and Power Company
Variable Interest Entity [Line Items]
Long term debt		$ 12,328			$ 12,328		$ 12,325	[1]
Payables to affiliates		328			328		210	[1]
Variable Interest Entity, Primary Beneficiary | SBL Holdco
Variable Interest Entity [Line Items]
Securities due within one year		31			31		31
Long term debt		260			260		$ 267
Variable Interest Entity (VIE) or Potential VIE, 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			$ 135		$ 135
Payment for contract termination after tax				$ 100		100
Payment for electric capacity						13
Payment for electric energy						1
Variable Interest Entity, Not Primary Beneficiary | Virginia Electric and Power Company | DES
Variable Interest Entity [Line Items]
Shared services purchased		81	$ 83		260	$ 301
Payables to affiliates		$ 171			$ 171		$ 102

[1] Virginia Power’s Consolidated Balance Sheet at December 31, 2019 has been derived from the audited Consolidated Balance Sheet at that date.