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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |  |
Corporate federal income tax |  | 21.00% | 21.00% | 21.00% |
Regulatory Liability | $ 49.8 | $ 49.8 | $ 54.0 |  | $ 48.9
Regulatory liability, expected flow back to customers |  | 47.1 |  |  | 48.9
Regulatory liability, expected pass back to ratepayers |  | 0.3 | 1.7 |  | $ 1.8
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  | $ 8.2 |  |
Deferred tax assets, operating loss carryforwards, federal | 2.7 | 2.7 |  |  |
Alternative minimum tax credit carryforwards | 0.9 | $ 0.9 |  |  |
Percentage of employment retention credit |  | 50.00% |  |  |
Employment retention duties capacity |  | 100.00% |  |  |
Employment tax expense |  | $ 0.6 |  |  |
Minimum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 15 years |  |  |
Maximum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 20 years |  |  |
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Regulatory liability, expected flow back to customers | $ 1.9 |  |  |  |
Net Operating Loss Carryforward Assets |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  | $ 2.0 |
Tax Year 2018 |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Corporate federal income tax |  | 21.00% |  | 21.00% | 21.00%

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