Fair Value Measurements (Financial Instruments' Carrying Amounts and Fair Values) (Detail) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1],[2]	$ 35,140	$ 31,996
Supplemental credit facility borrowings	[3]	900	225
Junior subordinated notes	[4]	1,386	3,411
Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1],[2],[5]	40,471	38,773
Supplemental credit facility borrowings	[3],[5]	900	225
Junior subordinated notes	[4],[5]	1,503	3,633
Virginia Electric and Power Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[4]	13,212	13,207
Virginia Electric and Power Company | Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[4],[5]	$ 15,570	$ 16,455

[1] Carrying amount includes current portions included in securities due within one year and amounts which represent the unamortized debt issuance costs and discount or premium. At September 30, 2021 and December 31, 2020, the carrying amount includes the valuation of certain fair value hedges associated with fixed rate debt of $2 million and $3 million, respectively.

[2] Includes amounts classified as held for sale, see Note 3.

[3] Also includes Supplemental 364-Day credit facility borrowings.

[4] Carrying amount includes current portions included in securities due within one year and amounts which represent the unamortized debt issuance costs, discount or premium.

[5] Fair value is estimated using market prices, where available, and interest rates currently available for issuance of debt with similar terms and remaining maturities. All fair value measurements are classified as Level 2. The carrying amount of debt issuances with short-term maturities and variable rates refinanced at current market rates is a reasonable estimate of their fair value.