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v3.25.0.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2024 USD ($) Megawatts | Dec. 31, 2025 $ / shares | Dec. 31, 2024 USD ($) Megawatts Subsidiary mi $ / shares | Dec. 31, 2023 USD ($) $ / shares | Dec. 31, 2022 USD ($) $ / shares | Dec. 31, 2021 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Length Of Pipeline | mi |  |  | 85 |  |  |
Cost of removal obligation | $ 139.2 |  | $ 139.2 | $ 126.3 |  |
Regulatory assets | 112.0 |  | 112.0 | 109.6 |  |
Investments in trading securities | $ 6.3 |  | $ 6.3 | 6.0 |  |
Long-Term Renewable Contracts |  |  | 30.00% |  |  |
Energy storage systems | Megawatts | 5,000 |  |  |  |  |
Mid-Duration Storage | Megawatts | 1,500 |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  | 3 |  |  |
Allowance for doubtful accounts | $ 2.4 |  | $ 2.4 | $ 2.4 | $ 2.6 | $ 3.3
Depreciation rate based on average depreciable property balance |  |  | 3.49% | 3.33% | 3.26% |
Dividends per Common Share | $ / shares |  |  | $ 1.7 | $ 1.62 | $ 1.56 |
Increase in dividend declared amount per share | $ / shares |  |  | $ 1.7 |  |  |
Weighted average cost inventory amount | 1.1 |  | $ 1.1 | $ 1.0 |  |
Average Interest Rate On Debt |  |  | 6.22% | 5.48% | 2.50% |
Other Deferred Charges |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory assets | 14.1 |  | $ 14.1 | $ 11.0 |  |
Electric and Gas Division |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Business Combination, Price of Acquisition, Expected |  |  | 70.9 |  |  |
Materials And Supplies |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Weighted average cost inventory amount | 14.2 |  | 14.2 | 13.5 |  |
Subsequent Event |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Increase in dividend declared amount per share | $ / shares |  | $ 1.8 |  |  |  |
Unbilled Revenues |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Allowance for doubtful accounts | 0.1 |  | 0.1 | 0.1 |  |
Accounts Receivable |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Allowance for doubtful accounts | 2.3 |  | $ 2.3 | 2.3 |  |
Utilities |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  | 3 |  |  |
Unitil Service; Unitil Realty; and Unitil Resources |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  | 3 |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Energy storage systems | Megawatts |  |  | 5,000 |  |  |
Mid-Duration Storage | Megawatts |  |  | 1,500 |  |  |
Fitchburg Gas and Electric Light Company | Electric and Gas Division | Other Deferred Charges |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Hardship accounts in regulatory assets | 7.9 |  | $ 7.9 | $ 6.0 |  |
Quarterly Payment |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Dividends per Common Share | $ / shares |  |  | $ 0.425 | $ 0.405 | $ 0.39 |
Quarterly Payment | Subsequent Event |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Common stock dividend paid per share | $ / shares |  | 0.45 |  |  |  |
Increase in dividend declared amount per share | $ / shares |  | $ 0.025 |  |  |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory assets | 7.1 |  | $ 7.1 |  |  |
ISO-NE Obligations |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash Deposits | $ 5.0 |  | $ 5.0 | $ 3.3 |  |
Maximum |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Lease term | 12 months |  | 12 months |  |  |
Deferred Compensation Plan |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Investments in trading securities | $ 2.2 |  | $ 2.2 | $ 1.3 |  |

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