Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) $ / MMBTU $ / MWh		Dec. 31, 2017 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 244		$ 307
Fair Value of Derivative Liabilities		90		284
Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		129	[1]	166
Fair Value of Derivative Liabilities		23		66
Fair Value, Measurements, Recurring | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		156		258
Fair Value of Derivative Liabilities		31		197
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		123		166
Fair Value of Derivative Liabilities		9		9
Fair Value, Measurements, Recurring | Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		122
Fair Value of Derivative Liabilities		2
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		119
Fair Value of Derivative Liabilities		2
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		122		157
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		119		152
Fair Value of Derivative Liabilities		2		$ 5
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		9
Fair Value of Derivative Liabilities		2
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Liabilities		2
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 8
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Minimum | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	(5)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Minimum | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	(5)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Minimum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	(1)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Minimum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	(1)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Maximum | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Maximum | Liabilities | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Maximum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Maximum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Weighted Average | Liabilities
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2],[3]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | 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	[2],[3]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Weighted Average | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2],[3]	1
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | FTRs | Commodity | Weighted Average | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2],[3]	1
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[4]	$ 83
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	[4]	$ 81
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Minimum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Minimum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	(2)
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Maximum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Maximum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	6
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Weighted Average | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2],[3]	0
Fair Value, Measurements, Recurring | Level 3 | Discounted cash flow | Natural Gas | Commodity | Weighted Average | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2],[3]	(1)
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 29
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 29
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Minimum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	23
Price volatility (percentage)	[5]	7.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Minimum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	23
Price volatility (percentage)	[5]	7.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Maximum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	49
Price volatility (percentage)	[5]	63.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Maximum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2]	49
Price volatility (percentage)	[5]	63.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Weighted Average | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2],[3]	35
Price volatility (percentage)	[3],[5]	27.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Electricity | Electricity | Weighted Average | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MWh	[2],[3]	35
Price volatility (percentage)	[3],[5]	27.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets		$ 1
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Assets | 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 | Natural Gas | Physical options | Minimum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	2
Price volatility (percentage)	[5]	14.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Minimum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	2
Price volatility (percentage)	[5]	14.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Maximum | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	6
Price volatility (percentage)	[5]	29.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Maximum | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2]	6
Price volatility (percentage)	[5]	29.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Weighted Average | Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2],[3]	3
Price volatility (percentage)	[3],[5]	23.00%
Fair Value, Measurements, Recurring | Level 3 | Option model | Natural Gas | Physical options | Weighted Average | Assets | Virginia Electric and Power Company
Fair Value, Option, Quantitative Disclosures [Line Items]
Market price (per Dth, per MWh, per Gal) | $ / MMBTU	[2],[3]	3
Price volatility (percentage)	[3],[5]	23.00%

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

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

[3] Averages weighted by volume.

[4] Includes basis.

[5] Represents volatilities unrepresented in published markets.