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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2022 USD ($) Subsidiary mi | Dec. 31, 2021 USD ($) | Jun. 30, 2021 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Length Of Pipeline | mi | 86 |  |
Cost of removal obligation | $ 112.4 | $ 107.5 | $ 105.1
Investments in trading securities | 5.4 | 5.7 | 5.5
Allowance for doubtful accounts | 2.7 | 3.3 | 4.6
Regulatory assets | 148.0 | 156.3 | 156.1
Other Deferred Charges [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 13.3 | 15.3 | 17.8
Unbilled Revenues [member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | 0.1 | 0.2 | 0.1
Accounts Receivable [member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | $ 2.6 | 3.1 | 4.5
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 | $ 7.6 | 7.9 | 9.8
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 6.6 |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 1.7 | 2.7 | 1.5
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Lease term | 12 months |  |
Deferred Compensation Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Investments in trading securities | $ 0.8 | $ 0.6 | $ 0.7

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