Long-Term and Short-Term Debt and Liquidity (Narrative) (Detail) - USD ($)		1 Months Ended			3 Months Ended		9 Months Ended
		Aug. 31, 2019	May 31, 2019	Apr. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019	Feb. 28, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Aggregate purchase price of first mortgage bonds		$ 552,000,000								$ 1,200,000,000
Loss on reacquired debt		$ 83,000,000					$ 187,000,000
Redemption of remaining principal outstanding plus accrued interest							1,890,000,000	$ 824,000,000
Equity capital contribution returned to parent							$ 20,000,000	0
Line of credit facility maturity date							2023-03
Weighted average interest rate, outstanding commercial paper											3.82%
Commercial paper borrowing limit				$ 2,200,000,000
Commercial paper maturity period				2020-04
Interest charges	[1]				$ 66,000,000	$ 79,000,000	$ 202,000,000	232,000,000
Interest income from money pool transactions					1,000,000	1,000,000	7,000,000	2,000,000
Interest expense from money pool transactions					1,000,000	$ 1,000,000	7,000,000	$ 2,000,000
Money pool borrowings due to affiliates					222,000,000		222,000,000				$ 282,000,000
Investments due from affiliates					17,000,000		17,000,000				$ 353,000,000
Maximum [Member]
Debt Instrument [Line Items]
Short term commercial paper maturity period				1 year
Dominion Energy
Debt Instrument [Line Items]
Short-term borrowings outstanding, maximum				$ 900,000,000
Short-term borrowings outstanding					501,000,000		501,000,000
Interest charges					1,000,000		1,000,000
Current Joint Revolving Credit Facility
Debt Instrument [Line Items]
Facility limit					1,000,000,000		1,000,000,000
Current Joint Revolving Credit Facility | Dominion Energy
Debt Instrument [Line Items]
Facility limit									$ 6,000,000,000
Line of Credit Facility
Debt Instrument [Line Items]
Facility limit					500,000,000		500,000,000
Letter of Credit
Debt Instrument [Line Items]
Debt instrument, face amount					68,000,000		68,000,000
Facility limit					$ 1,000,000,000.0		$ 1,000,000,000.0
Genco
Debt Instrument [Line Items]
Commercial paper borrowing limit				$ 200,000,000
Genco | Maximum [Member]
Debt Instrument [Line Items]
Short term commercial paper maturity period				1 year
3.05% Promissory Note due in May 2024 | Genco
Debt Instrument [Line Items]
Debt instrument, face amount			$ 230,000,000
Debt instrument, interest rate			3.05%
Debt instrument, maturity date			2024-05
5.49% Senior Secured Note Due 2024 | Genco
Debt Instrument [Line Items]
Debt instrument, interest rate			5.49%
Debt instrument, maturity year			2024
Redemption of remaining principal outstanding plus accrued interest			$ 33,000,000
Equity capital contribution returned to parent			$ 20,000,000

[1] See Note 14 for amounts attributable to affiliates.