Fair Value Measurements (Financial Instruments' Carrying Amounts and Fair Values) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1],[2]	$ 37,045	$ 32,055
Supplemental 364-Day credit facility borrowings		225	0
Junior subordinated notes	[3]	3,410	4,797
Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1],[2],[4]	43,872	36,155
Supplemental 364-Day credit facility borrowings	[4]	225	0
Junior subordinated notes	[3],[4]	3,589	4,953
Virginia Electric and Power Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[3]	12,328	12,326
Virginia Electric and Power Company | Estimate of Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[3],[4]	$ 15,410	$ 14,281

[1] Carrying amount includes current portions included in securities due within one year and amounts which represent the unamortized debt issuance costs, discount or premium and foreign currency remeasurement adjustments. At September 30, 2020 and December 31, 2019, includes the valuation of certain fair value hedges associated with fixed rate debt of $3 million and $4 million, respectively.

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

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

[4] 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.