Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2021 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2020 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 849	$ 397
Fair Value of Derivative Liabilities		264	486
Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		359	181
Fair Value of Derivative Liabilities		178	405
Fair Value, Measurements, Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		7,613	6,959
Total liabilities		264	486
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		3,508	3,223
Total liabilities		178	405
Fair Value, Measurements, Recurring | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		109	167
Fair Value of Derivative Liabilities		78	55
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		76	115
Fair Value of Derivative Liabilities		28	29
Fair Value, Measurements, Recurring | Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		58	110
Total liabilities		2	7
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		58	110
Total liabilities		2	7
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		58	110
Fair Value of Derivative Liabilities		2	7
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		58	110
Fair Value of Derivative Liabilities		2	$ 7
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		2
Fair Value of Derivative Liabilities		2
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		2
Fair Value of Derivative Liabilities		$ 2
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Liabilities | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Maximum | Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	2
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Maximum | Liabilities | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	2
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	3
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1]	3
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Liabilities | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[3]	$ 56
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[3]	$ 56
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	3
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	3
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	(1)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	(1)

[1] Represents market prices beyond defined terms for Levels 1 and 2.

[2] Averages weighted by volume.

[3] Includes basis.