Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		$ 252	$ 93
Derivative Liabilities		776	740
Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		145	24
Derivative Liabilities		491	466
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		6,303	5,963
Total Liabilities		776	740
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		2,925	2,748
Total Liabilities		491	466
Fair Value, Measurements, Recurring | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	4,146	4,195
Fair Value, Measurements, Recurring | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	1,928	1,920
Fair Value, Measurements, Recurring | Fixed Income | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	617	463
Fair Value, Measurements, Recurring | Fixed Income | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	352	256
Fair Value, Measurements, Recurring | Fixed Income | Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	1,245	1,192
Fair Value, Measurements, Recurring | Fixed Income | Government Securities | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	486	547
Fair Value, Measurements, Recurring | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	43	20
Fair Value, Measurements, Recurring | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		14	1
Fair Value, Measurements, Recurring | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		186	74
Derivative Liabilities		51	131
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		145	22
Derivative Liabilities		16	103
Fair Value, Measurements, Recurring | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		60	11
Derivative Liabilities		721	606
Fair Value, Measurements, Recurring | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			2
Derivative Liabilities		475	363
Fair Value, Measurements, Recurring | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		6	8
Derivative Liabilities		4	3
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		4,676	4,687
Total Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		2,118	2,106
Total Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	4,146	4,195
Fair Value, Measurements, Recurring | Level 1 | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	1,928	1,920
Fair Value, Measurements, Recurring | Level 1 | Fixed Income | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 1 | Fixed Income | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	0	0
Fair Value, Measurements, Recurring | Level 1 | Fixed Income | Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	499	473
Fair Value, Measurements, Recurring | Level 1 | Fixed Income | Government Securities | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	176	186
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	31	19
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		14	0
Fair Value, Measurements, Recurring | Level 1 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		1,490	1,257
Total Liabilities		766	684
Fair Value, Measurements, Recurring | Level 2 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		670	623
Total Liabilities		481	410
Fair Value, Measurements, Recurring | Level 2 | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 2 | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	0	0
Fair Value, Measurements, Recurring | Level 2 | Fixed Income | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	617	463
Fair Value, Measurements, Recurring | Level 2 | Fixed Income | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	352	256
Fair Value, Measurements, Recurring | Level 2 | Fixed Income | Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	746	719
Fair Value, Measurements, Recurring | Level 2 | Fixed Income | Government Securities | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	310	361
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	12	1
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0	1
Fair Value, Measurements, Recurring | Level 2 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		49	55
Derivative Liabilities		41	75
Fair Value, Measurements, Recurring | Level 2 | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		8	3
Derivative Liabilities		6	47
Fair Value, Measurements, Recurring | Level 2 | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		60	11
Derivative Liabilities		721	606
Fair Value, Measurements, Recurring | Level 2 | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			2
Derivative Liabilities		475	363
Fair Value, Measurements, Recurring | Level 2 | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		6	8
Derivative Liabilities		4	3
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		137	19
Total Liabilities		10	56
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		137	19
Total Liabilities		10	56
Fair Value, Measurements, Recurring | Level 3 | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 3 | Equity securities: | U.S. | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	0	0
Fair Value, Measurements, Recurring | Level 3 | Fixed Income | Corporate debt instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 3 | Fixed Income | Corporate debt instruments | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	0	0
Fair Value, Measurements, Recurring | Level 3 | Fixed Income | Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 3 | Fixed Income | Government Securities | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[2]	0	0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	0	0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents and other | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0	0
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		137	19
Derivative Liabilities		10	56
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		137	19
Derivative Liabilities		10	56
Fair Value, Measurements, Recurring | Level 3 | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 3 | Interest rate | Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			0
Derivative Liabilities		0	0
Fair Value, Measurements, Recurring | Level 3 | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0	0
Derivative Liabilities		$ 0	$ 0

[1] Includes investments held in the nuclear decommissioning and rabbi trusts. Excludes $297 million and $274 million of assets at September 30, 2020 and December 31, 2019, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.

[2] Includes investments held in the nuclear decommissioning trusts. Excludes $150 million and $159 million of assets at September 30, 2020 and December 31, 2019, respectively, measured at fair value using NAV (or its equivalent) as a practical expedient which are not required to be categorized in the fair value hierarchy.