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v3.21.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2021 USD ($) Subsidiary mi | Dec. 31, 2020 USD ($) | Mar. 31, 2020 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | mi | 86 |  |
Cost of removal obligation | $ 102.5 | $ 105.2 | $ 98.8
Power Supply Contract Divestitures |  |  | 0.2
Investments in trading securities | 5.6 | 5.7 | 5.5
Allowance for doubtful accounts | 4.4 | 3.3 | 1.8
Regulatory assets | 152.7 | 164.7 | 133.7
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 | $ 4.3 | 3.1 | 1.7
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 |  |
Power Supply Contract Divestitures | $ 0.2 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 8.5 |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 2.2 | 2.4 | 2.4
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Lease term | 12 months |  |
Deferred Compensation Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Investments in trading securities | $ 0.6 | $ 0.5 | $ 0.4

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