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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2022 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |  |
Statutory Federal Income Tax Rate |  | 21.00% | 21.00% | 21.00% |
Regulatory Liability |  | $ 64.0 | $ 47.9 |  |
Regulatory liability, expected flow back to customers |  |  |  |  | $ 47.1
Offset future years income |  |  | 7.3 |  |
Regulatory liability, expected pass back to ratepayers |  | 1.6 |  |  |
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  | 2.9 |  |
Deferred tax assets, operating loss carryforwards, federal |  | 6.6 |  |  |
Income Tax Expense (Benefit) |  | 14.0 | 13.2 | $ 11.2 |
Percentage of alternate minimum tax | 15.00% |  |  |  |
Number of years used for calculating adjusted financial statement income | 3 years |  |  |  |
Adjustment financial statement income | $ 1,000.0 |  |  |  |
FederalTaxCreditsToOffsetCurrentTaxes |  | 1.3 |  |  |
Income Tax Related Liabilities |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Regulatory Liability |  | 50.4 | $ 38.6 |  | $ 48.9
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Regulatory liability, expected flow back to customers |  | $ 11.2 |  |  |
Tax Year 2018 |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Statutory Federal Income Tax Rate |  |  |  |  | 21.00%

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