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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2024 USD ($) Subsidiary mi | Dec. 31, 2023 USD ($) | Mar. 31, 2023 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Length of Pipeline | mi | 85 |  |
Cost of removal obligation | $ 130.6 | $ 126.3 | $ 120.0
Investments in trading securities | 5.9 | 6.0 | 5.7
Allowance for doubtful accounts | 2.8 | 2.4 | 3.1
Regulatory assets | 117.2 | 109.6 | 118.0
Other Deferred Charges [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 12.6 | 11.0 | 11.2
Unbilled Revenues [member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | 0.1 | 0.1 | 0.1
Accounts Receivable [member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | $ 2.7 | 2.3 | 3.0
Utilities |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Unitil Service; Unitil Realty; and Unitil Resources |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges [Member] | Electric And Gas Division [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Hardship accounts in regulatory assets | $ 6.9 | 6.0 | 5.8
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 6.9 |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 3.1 | 3.3 | 4.2
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Lease term | 12 months |  |
Deferred Compensation Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Investments in trading securities | $ 1.8 | $ 1.3 | $ 0.9

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