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Commitments and Contingencies (Nuclear Operations) (Narrative) (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2015 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Loss Contingencies [Line Items] |  |  |  |  |  |
NRC minimum requirement for nuclear power plant licensees |  |  |  | $ 1,060,000,000.00 |  |
Maximum assessment for premiums on insurance policy |  |  |  | 33,000,000 |  |
Maximum assessment for insurance policy |  |  |  | 99,000,000 |  |
Spent Nuclear Fuel |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Receivables |  |  |  | 46,000,000 | $ 52,000,000 |
Maximum |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Amount that could be assessed for each licensed reactor |  |  |  | 138,000,000 |  |
Amount that could be assessed for each licensed reactor per reactor |  |  |  | 21,000,000 |  |
Nuclear Obligations |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Maximum liability protection per nuclear incident amount |  |  |  | 13,800,000,000 |  |
Nuclear Obligations | Secondary Financial Protection Program |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Maximum liability protection per nuclear incident amount |  | $ 13,800,000,000 | $ 13,900,000,000 |  |  |
Virginia Electric and Power Company |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Maximum assessment for premiums on insurance policy |  |  |  | 48,000,000 |  |
Maximum assessment for insurance policy |  |  |  | 9,000,000 |  |
Virginia Electric and Power Company | Spent Nuclear Fuel |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Receivables |  |  |  | 35,000,000 | 35,000,000 |
DESC | Non-nuclear Obligation | European Mutual Association |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Property insurance coverage |  |  |  | 415,000,000 |  |
Annual Premium |  |  |  | 2,000,000 |  |
Millstone Unit 1 |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Minimum financial assurance |  |  |  | 2,800,000,000 |  |
Kewaunee |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Maximum assessment for premiums on insurance policy | $ 50,000,000 |  |  |  |  |
Kewaunee | Nuclear Obligations |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Maximum liability protection per nuclear incident amount |  |  |  | 100,000,000 |  |
Kewaunee | Virginia Electric and Power Company |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Minimum financial assurance |  |  |  | 1,800,000,000 |  |
Millstone, Summer, Surry and North Anna |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Amount of coverage purchased from commercial insurance pools |  |  |  | 450,000,000 |  |
Surry and North Anna |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Settlement amount awarded from other party |  |  |  | 24,000,000 | 15,000,000 | $ 16,000,000
Millstone |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Settlement amount awarded from other party |  |  |  | 11,000,000 | 11,000,000 | $ 13,000,000
Summer |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Settlement amount awarded from other party |  |  |  | $ 4,000,000 | $ 3,000,000 |

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