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Commitments and Contingencies (Nuclear Operations) (Narrative) (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Loss Contingencies [Line Items] |  |  |
Amount of coverage purchased from commercial insurance pools | $ 450,000,000 |  |
Maximum assessment for premiums on insurance policy | 86,000,000 |  |
Maximum assessment for insurance policy | 22,000,000 |  |
Spent Nuclear Fuel |  |  |
Loss Contingencies [Line Items] |  |  |
Receivables | 46,000,000 | $ 56,000,000 |
Nuclear Obligations |  |  |
Loss Contingencies [Line Items] |  |  |
Maximum liability protection per nuclear incident amount | $ 13,440,000,000 |  |
Inflationary provision adjustment period | 5 years |  |
Maximum |  |  |
Loss Contingencies [Line Items] |  |  |
Amount that could be assessed for each licensed reactor | $ 127,000,000 |  |
Amount that could be assessed for each licensed reactor per reactor | 19,000,000 |  |
Millstone Unit 1 |  |  |
Loss Contingencies [Line Items] |  |  |
Minimum financial assurance | 2,700,000,000 |  |
Millstone |  |  |
Loss Contingencies [Line Items] |  |  |
Settlement amount | 14,000,000 | 22,000,000 | $ 17,000,000
Surry and North Anna |  |  |
Loss Contingencies [Line Items] |  |  |
Settlement amount | 22,000,000 | 30,000,000 | $ 8,000,000
Virginia Electric and Power Company |  |  |
Loss Contingencies [Line Items] |  |  |
Maximum assessment for premiums on insurance policy | 50,000,000 |  |
Maximum assessment for insurance policy | 10,000,000 |  |
Virginia Electric and Power Company | Spent Nuclear Fuel |  |  |
Loss Contingencies [Line Items] |  |  |
Receivables | 30,000,000 | $ 37,000,000 |
Virginia Electric and Power Company | Kewaunee |  |  |
Loss Contingencies [Line Items] |  |  |
Minimum financial assurance | $ 1,800,000,000 |  |

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