Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail) $ in Millions		9 Months Ended
		Sep. 30, 2018 USD ($) $ / MMBTU $ / MWh		Dec. 31, 2017 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 297		$ 307
Fair Value of Derivative Liabilities		145		284
Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		170	[1]	166
Fair Value of Derivative Liabilities		8		66
Fair Value, Measurements, Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets	[2]	5,650		5,379
Total liabilities		145		284
Fair Value, Measurements, Recurring | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		2,593		2,466
Total liabilities		8		66
Fair Value, Measurements, Recurring | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		148		258
Fair Value of Derivative Liabilities		100		197
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		105		166
Fair Value of Derivative Liabilities		8		9
Fair Value, Measurements, Recurring | Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets	[2]	98		157
Total liabilities		6		7
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Total assets		91		152
Total liabilities		5		5
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		98		157
Fair Value of Derivative Liabilities		6		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		91		152
Fair Value of Derivative Liabilities		5		$ 5
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		12
Fair Value of Derivative Liabilities		6
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		12
Fair Value of Derivative Liabilities		$ 5
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Minimum | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	(1)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Minimum | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	(1)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	(1)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	(1)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Maximum | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	6
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Maximum | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	6
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	7
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	7
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Weighted Average | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	0
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Weighted Average | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	0
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	1
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Level 3 | FTRs [Member] | Commodity | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	1
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[5]	$ 65
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[5]	$ 58
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	(2)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	(2)
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	7
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	7
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3],[4]	0
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Commodity | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3],[4]	(1)
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Physical Options [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 19
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 19
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	27
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	27
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Minimum | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	4.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Minimum | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	4.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	57
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3]	57
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Maximum | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	61.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Maximum | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	61.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	42
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[3],[4]	42
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Weighted Average | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4],[6]	37.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Level 3 | Electricity | Electricity | Weighted Average | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4],[6]	37.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 2
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 2
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Minimum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	1
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Minimum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	1
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Minimum | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	13.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Minimum | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	13.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Maximum | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	7
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Maximum | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3]	7
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Maximum | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	44.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Maximum | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[6]	44.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Weighted Average | Assets | Market Price [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3],[4]	3
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Weighted Average | Assets | Market Price [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[3],[4]	3
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Weighted Average | Assets | Price Volatility [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4],[6]	23.00%
Option Model [Member] | Fair Value, Measurements, Recurring | Natural Gas | Level 3 | Physical Options [Member] | Weighted Average | Assets | Price Volatility [Member] | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Price volatility (percentage)	[4],[6]	23.00%

[1] Noncurrent derivative assets are presented in other deferred charges and other assets in Virginia Power’s Consolidated Balance Sheets.

[2] Includes investments held in the nuclear decommissioning and rabbi trusts. Excludes $217 million and $88 million of assets at September 30, 2018 and December 31, 2017, 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.

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

[4] Averages weighted by volume.

[5] Includes basis.

[6] Represents volatilities unrepresented in published markets.