Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		$ 397	$ 93
Derivative Liabilities		486	740
Virginia Electric and Power Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		181	24
Derivative Liabilities		405	466
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		6,959	5,963
Total liabilities		486	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		3,223	2,748
Total liabilities		405	466
Fair Value, Measurements, Recurring | Equity securities: | U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	4,648	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]	2,171	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]	629	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]	348	256
Fair Value, Measurements, Recurring | Fixed Income | Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	1,238	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]	510	547
Fair Value, Measurements, Recurring | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	[1]	47	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	[2]	13	1
Fair Value, Measurements, Recurring | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		167	74
Derivative Liabilities		55	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		115	22
Derivative Liabilities		29	103
Fair Value, Measurements, Recurring | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		230	11
Derivative Liabilities		431	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		66	2
Derivative Liabilities		376	363
Fair Value, Measurements, Recurring | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			8
Derivative Liabilities			3
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		5,188	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		2,385	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]	4,648	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]	2,171	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]	508	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]	201	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]	32	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	[2]	13	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	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
Derivative Liabilities			0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		1,661	1,257
Total liabilities		479	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		728	623
Total liabilities		398	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]	629	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]	348	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]	730	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]	309	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]	15	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	[2]	0	1
Fair Value, Measurements, Recurring | Level 2 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		57	55
Derivative Liabilities		48	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		5	3
Derivative Liabilities		22	47
Fair Value, Measurements, Recurring | Level 2 | Interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		230	11
Derivative Liabilities		431	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		66	2
Derivative Liabilities		376	363
Fair Value, Measurements, Recurring | Level 2 | Foreign currency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset			8
Derivative Liabilities			3
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		110	19
Total liabilities		7	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		110	19
Total liabilities		7	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	[2]	0	0
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		110	19
Derivative Liabilities		7	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		110	19
Derivative Liabilities		7	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	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
Derivative Liabilities			$ 0

[1] Includes investments held in the nuclear decommissioning trusts and rabbi trusts. Excludes $340 million and $274 million of assets at December 31, 2020 and 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 $167 million and $159 million of assets at December 31, 2020 and 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.