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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 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 | $ 49.8 | $ 55.3 | $ 56.0 | $ 55.3 | $ 49.8 |  |  |
Regulatory liability, expected flow back to customers |  |  |  |  | 47.1 |  |  | $ 48.9
Regulatory liability, expected pass back to ratepayers |  |  | $ 0.3 |  | 0.3 | $ 1.7 |  | 1.8
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  |  |  | 5.4 |  |  |
Percentage of employment retention credit |  |  | 50.00% |  |  |  |  |
Employment retention duties capacity |  |  | 100.00% |  |  |  |  |
Employment tax expense |  | 0.6 | $ 0.4 |  |  |  |  |
Consolidated Appropriations Act 2021 [Member] |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Percentage of employment retention credit | 70.00% |  |  |  |  |  |  |
Employment retention duties capacity | 100.00% |  |  |  |  |  |  |
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 |  |  |  |  |
Income Tax Related Liabilities |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Regulatory Liability | $ 45.5 | $ 45.8 | $ 44.6 | $ 45.8 | $ 45.5 |  |  | $ 48.9
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  |  | $ 2.7 |  |  |  |  |
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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