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Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
$ / DTH
$ / MWh
Dec. 01, 2019
USD ($)
Dec. 31, 2018
USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets $ 93   $ 294
Fair Value of Derivative Liabilities 740   279
Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets 24   93
Fair Value of Derivative Liabilities 466   103
Level 3 | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Total Assets 19    
Fair Value, Measurements, Recurring      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Total Assets 5,963   5,156
Total liabilities 740   279
Fair Value, Measurements, Recurring | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Total Assets 2,748   2,297
Total liabilities   $ 466 103
Fair Value, Measurements, Recurring | Commodity      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets 74   250
Fair Value of Derivative Liabilities 131   135
Fair Value, Measurements, Recurring | Commodity | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets 22   90
Fair Value of Derivative Liabilities 103   15
Fair Value, Measurements, Recurring | Level 3      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Total Assets 19   70
Total liabilities 56   6
Fair Value, Measurements, Recurring | Level 3 | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Liabilities 0    
Total Assets 19   66
Total liabilities 56   6
Fair Value, Measurements, Recurring | Level 3 | Commodity      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets 19   70
Fair Value of Derivative Liabilities 56   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 19   66
Fair Value of Derivative Liabilities 56   $ 6
Fair Value, Measurements, Recurring | Level 3 | Natural Gas | Assets      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets [1] 13    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | FTRs [Member] | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Liabilities 5    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | FTRs [Member] | Commodity      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets 6    
Fair Value of Derivative Liabilities 5    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | FTRs [Member] | Commodity | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets $ 6    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Minimum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Assets | FTRs [Member] | Commodity | Market Price [Member] | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 5    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Assets | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Minimum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Assets | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Maximum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 5    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Assets | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Weighted Average | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2],[3] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Assets | FTRs [Member] | Physical Options [Member] | Market Price [Member] | Weighted Average      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2],[3] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets [1] $ 13    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Liabilities | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Liabilities [1] $ 43    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Minimum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 2    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Minimum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 2    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Maximum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Weighted Average      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2],[3] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Commodity | Market Price [Member] | Liabilities | Weighted Average | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2],[3] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | FTRs [Member] | Commodity | Market Price [Member] | Liabilities | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Assets | Commodity | Market Price [Member] | Minimum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Assets | Commodity | Market Price [Member] | Minimum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH 1    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Assets | Commodity | Market Price [Member] | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / MWh [2] 4    
Fair Value, Measurements, Recurring | Discounted Cash Flow [Member] | Level 3 | Natural Gas | Assets | Commodity | Market Price [Member] | Maximum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH 4    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Electricity | Physical Options [Member] | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Assets $ 8    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member]      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Fair Value of Derivative Liabilities $ 8    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Minimum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 1    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Minimum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 1    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 4    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Maximum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2] 4    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Weighted Average      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2],[3] 3    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Market Price [Member] | Liabilities | Weighted Average | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Market Price | $ / DTH [2],[3] 3    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Minimum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [4] 24.00%    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Minimum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [4] 24.00%    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Maximum      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [4] 66.00%    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Maximum | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [4] 66.00%    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Weighted Average      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [3],[4] 37.00%    
Fair Value, Measurements, Recurring | Option Model [Member] | Level 3 | Natural Gas | Physical Options [Member] | Price Volatility [Member] | Liabilities | Weighted Average | Virginia Electric and Power Company      
Fair Value, Option, Quantitative Disclosures [Line Items]      
Price volatility (percentage) [3],[4] 37.00%    
[1] Includes basis.
[2] Represents market prices beyond defined terms for Levels 1 and 2.
[3] Averages weighted by volume.
[4] Represents volatilities unrepresented in published markets.