Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2019 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2018 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 173	$ 294
Fair Value of Derivative Liabilities		369	279
Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		66	93
Fair Value of Derivative Liabilities		190	103
Fair Value, Measurements, Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		5,488	5,156
Total liabilities		369	279
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		2,502	2,297
Total liabilities		190	103
Fair Value, Measurements, Recurring | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		157	250
Fair Value of Derivative Liabilities		45	135
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		66	90
Fair Value of Derivative Liabilities		9	15
Fair Value, Measurements, Recurring | Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		63	70
Total liabilities		10	6
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		62	66
Total liabilities		3	6
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		63	70
Fair Value of Derivative Liabilities		10	6
Fair Value, Measurements, Recurring | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		62	66
Fair Value of Derivative Liabilities		3	$ 6
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		3
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		$ 3
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Minimum | Derivative Financial Instruments, Liabilities
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 | Derivative Financial Instruments, Liabilities | 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 | 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 [Member] | Derivative Financial Instruments, Liabilities
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 [Member] | Derivative Financial Instruments, Liabilities | 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 [Member] | 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 [Member] | 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
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	1
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | FTRs [Member] | Commodity | Weighted Average | Derivative Financial Instruments, Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MWh	[1],[2]	1
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]	$ 60
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]	$ 59
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 [Member] | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	7
Fair Value, Measurements, Recurring | Level 3 | Discounted Cash Flow [Member] | Natural Gas | Commodity | Maximum [Member] | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	7
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 Assets		$ 1
Fair Value of Derivative Liabilities		7
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 Assets		$ 1
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]	2
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]	23.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Assets | 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] | 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 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	10.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Minimum | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	10.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Derivative Financial Instruments, Liabilities | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	16
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Derivative Financial Instruments, Liabilities | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	492.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	7
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1]	7
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	492.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Maximum [Member] | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4]	491.00%
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]	5
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]	82.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	3
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price | $ / MMBTU	[1],[2]	2
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[2],[4]	66.00%
Fair Value, Measurements, Recurring | Level 3 | Option Model [Member] | Natural Gas | Physical Options [Member] | Weighted Average | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[2],[4]	54.00%

[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.