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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |
Corporate federal income tax | 21.00% | 21.00% |  |
Regulatory Liability | $ 70.6 | $ 68.7 | $ 64.0 |
Regulatory liability, expected flow back to customers |  |  |  | $ 47.1
State Taxes Credit to Offset Future Taxes |  |  | 1.6 |
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  | 5.6 |
Federal tax credit carryforward |  |  | 1.0 |
Deferred tax assets, operating loss carryforwards, federal |  |  | 1.7 |
Federal Tax Credits to Offset Current Taxes |  |  | 0.6 |
Gas Ratepayers | Massachusetts And Maine [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Regulatory liability, expected flow back to customers | 13.3 |  |  |
Income Tax Related Liabilities |  |  |  |
Income Taxes [Line Items] |  |  |  |
Regulatory Liability | $ 48.8 | $ 50.0 | $ 50.4 | $ 48.9
Tax Year 2018 |  |  |  |
Income Taxes [Line Items] |  |  |  |
Corporate federal income tax |  |  |  | 21.00%

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