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v3.23.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 1 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | ---
Aug. 31, 2022 | Mar. 31, 2023 USD ($) Subsidiary mi | Dec. 31, 2022 USD ($) | Mar. 31, 2022 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |
Number of Subsidiaries | Entity | Subsidiary |  | 3 |  |
Length of Pipeline | mi |  | 86 |  |
Percentage of total sales volumes revenue subject to RDM | 43.00% |  |  |
Cost of removal obligation |  | $ 120.0 | $ 116.1 | $ 109.8
Investments in trading securities |  | 5.7 | 5.8 | 5.5
Allowance for doubtful accounts |  | 3.1 | 2.6 | 3.4
Regulatory assets |  | 118.0 | 114.3 | 153.3
Other Deferred Charges [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Regulatory assets |  | 11.2 | 11.1 | 15.1
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 |  | $ 3.0 | 2.5 | 3.3
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 |  | $ 5.8 | 5.8 | 7.9
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Regulatory assets |  | 6.1 |  |
ISO-NE Obligations |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Cash Deposits |  | $ 4.2 | 6.0 | 2.9
Maximum |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Lease term |  | 12 months |  |
Deferred Compensation Plan [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Investments in trading securities |  | $ 0.9 | $ 0.6 | $ 0.8

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