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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) Subsidiary mi | Sep. 30, 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 |  |  |
Provision for income taxes |  |  | 3.6 |
Cost of removal obligation |  | $ 107.1 | 98.4 | $ 96.0
Regulatory assets |  | 136.1 | 123.6 | 147.8
Allowance for doubtful accounts |  | 1.6 | 1.0 | 1.0
Power Supply Contract Divestitures |  | 0.0 | 0.4 | 0.3
Unbilled Revenues |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Allowance for doubtful accounts |  | 0.1 |  |
Accounts Receivable |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Allowance for doubtful accounts |  | $ 1.5 | 1.0 | 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 |  |
Power Supply Contract Divestitures |  | $ 0.3 |  |
Annual Electric Sales Volume |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  | 27.00% |  |
Annual Natural Gas Sales Volume |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  | 11.00% |  |
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 |  | $ 4.7 | $ 3.5 | $ 1.9
Lease term |  | 12 months |  |

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