Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2020 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2019 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 150	$ 93
Fair Value of Derivative Liabilities		1,646	740
Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		51	24
Fair Value of Derivative Liabilities		1,015	466
Fair Value, Measurements, Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		5,155	5,963
Total liabilities		1,646	740
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		2,361	2,748
Total liabilities		1,015	466
Fair Value, Measurements, Recurring | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		121	74
Fair Value of Derivative Liabilities		49	131
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		51	22
Fair Value of Derivative Liabilities		24	103
Fair Value, Measurements, Recurring | Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		49	19
Total liabilities		6	56
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		49	19
Total liabilities		6	56
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		49	19
Fair Value of Derivative Liabilities		6	56
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		49	19
Fair Value of Derivative Liabilities		6	$ 56
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		1
Fair Value of Derivative Liabilities		4
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		1
Fair Value of Derivative Liabilities		$ 4
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Derivative Financial Instruments, 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 | Derivative Financial Instruments, 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]	(1)
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]	(1)
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Maximum | Derivative Financial Instruments, 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 | Derivative Financial Instruments, 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]	2
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]	2
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Derivative Financial Instruments, 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 | Derivative Financial Instruments, 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] | 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]	$ 48
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]	$ 48
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]	2
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)
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Liabilities		$ 2
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Liabilities		$ 2
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Derivative Financial Instruments, Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	1
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Derivative Financial Instruments, Liabilities | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	1
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Derivative Financial Instruments, Liabilities | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	45.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Derivative Financial Instruments, Liabilities | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	45.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum | Derivative Financial Instruments, Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	2
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum | Derivative Financial Instruments, Liabilities | 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 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum | Derivative Financial Instruments, Liabilities | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	83.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum | Derivative Financial Instruments, Liabilities | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	83.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Derivative Financial Instruments, Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	1
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Derivative Financial Instruments, Liabilities | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	1
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Derivative Financial Instruments, Liabilities | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[2],[4]	57.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Derivative Financial Instruments, Liabilities | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[2],[4]	57.00%

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

[2] Averages weighted by volume.

[3] Includes basis.

[4] Represents volatilities unrepresented in published markets.