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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2020 | Mar. 31, 2021 | Sep. 30, 2020 | Mar. 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% |  |  |  |
Regulatory Liability | $ 49,800 | $ 58,200 |  | $ 55,600 | $ 49,800 |  |  |
Regulatory liability, expected flow back to customers |  |  |  |  | 47,100 |  |  | $ 48,900
Regulatory liability, expected pass back to ratepayers |  |  |  |  | 300 | $ 1,700 |  | 1,800
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  |  |  |  | $ 8,200 |  |
Deferred tax assets, operating loss carryforwards, federal | 2,700 |  |  |  | 2,700 |  |  |
Alternative minimum tax credit carryforwards | $ 900 |  |  |  | 900 |  |  |
Percentage of employment retention credit |  | 50.00% |  |  |  |  |  |
Employment retention duties capacity |  | 100.00% |  |  |  |  |  |
Employment tax expense |  |  | $ 600 |  |  |  |  |
Consolidated Appropriations Act 2021 [Member] |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Percentage of employment retention credit | 70.00% |  |  |  |  |  |  |
Employment retention duties capacity | 100.00% |  |  |  |  |  |  |
Employment tax expense |  | $ 350 |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 15 months |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 20 years |  |  |  |  |  |
Income Tax Related Liabilities |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Regulatory Liability | $ 45,500 | $ 45,100 |  | $ 46,100 | $ 45,500 |  |  | $ 48,900
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  | $ 2,300 |  |  |  |  |  |
Net Operating Loss Carryforward Assets |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  |  |  |  | $ 2,000 |
Tax Year 2018 |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Corporate federal income tax |  | 21.00% |  |  | 21.00% |  |  | 21.00%

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