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v3.20.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 USD ($) Subsidiary mi | Mar. 31, 2019 USD ($) | Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | mi | 86 |  |
Gain on divestiture of business, pretax |  | $ 13.4 |
Gain on divestiture of business, net | $ 9.8 | 9.8 |
Provision for income taxes |  | 3.6 |
Cost of removal obligation | 98.8 | 93.7 | $ 96.0
Regulatory assets | 133.7 | 127.3 | 147.8
Investments in trading securities | 5.5 | 5.1 | 5.6
Allowance for doubtful accounts | 1.8 | 1.7 | 1.0
Unbilled Revenues |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | 0.1 |  |
Accounts Receivable |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | $ 1.7 | 1.7 | 1.0
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 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | $ 7.5 |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 2.4 | 3.0 | 1.9
Lease term | 12 months |  |
Deferred Compensation Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Investments in trading securities | $ 0.4 | $ 0.1 | $ 0.2

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