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Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
$ / MMBTU
$ / MWh
Dec. 31, 2017
USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value of Derivative Assets $ 250 $ 307
Fair Value of Derivative Liabilities 118 284
Virginia Electric and Power Company    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value of Derivative Assets 147 [1] 166
Fair Value of Derivative Liabilities 14 66
Fair Value, Measurements, Recurring    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Total assets [2] 5,345 5,379
Total liabilities 118 284
Fair Value, Measurements, Recurring | Virginia Electric and Power Company    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Total assets 2,453 2,466
Total liabilities 14 66
Fair Value, Measurements, Recurring | Commodity    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value of Derivative Assets 159 258
Fair Value of Derivative Liabilities 55 197
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value of Derivative Assets 126 166
Fair Value of Derivative Liabilities 8 9
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Total assets [2] 122 157
Total liabilities 3 7
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company    
Fair Value, Option, Quantitative Disclosures [Line Items]    
Total assets 118 152
Total liabilities 3 5
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 3 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 118 152
Fair Value of Derivative Liabilities 3 $ 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 7  
Fair Value of Derivative Liabilities 3  
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 7  
Fair Value of Derivative Liabilities $ 3  
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] 6  
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] 6  
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] 1  
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] 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] 0  
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] $ 90  
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] $ 86  
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] (1)  
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] (1)  
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] 6  
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] 6  
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 $ 24  
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 $ 24  
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] 24  
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] 24  
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] 51  
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] 51  
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] 62.00%  
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] 62.00%  
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] 37  
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] 37  
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] 29.00%  
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] 29.00%  
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 $ 1  
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 $ 1  
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] 2  
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] 2  
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] 6  
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] 6  
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] 35.00%  
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] 35.00%  
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 $211 million and $88 million of assets at June 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.