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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |  |  |
Statutory Federal Income Tax Rate |  | 21.00% | 21.00% | 21.00% |  |
Regulatory Liability |  | $ 47.9 | $ 51.9 |  |  |
Regulatory liability, expected flow back to customers |  |  |  |  | $ 47.1 |
Regulatory liability, expected pass back to ratepayers |  | 1.3 |  |  |  | $ 1.8
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  | 1.4 |  |  |
Deferred tax assets, operating loss carryforwards, federal |  | 4.4 |  |  |  |
Percentage of alternate minimum tax | 15.00% |  |  |  |  |
Number of years used for calculating adjusted financial statement income | 3 years |  |  |  |  |
Adjustment financial statement income | $ 1.0 |  |  |  |  |
FederalTaxCreditsToOffsetCurrentTaxes |  |  | 1.7 |  |  |
Income Tax Related Liabilities |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory Liability |  | 38.6 | $ 41.0 |  |  | $ 48.9
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  | $ 9.3 |  |  |  |
Tax Year 2018 |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Statutory Federal Income Tax Rate |  |  |  |  |  | 21.00%

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