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v3.24.0.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jan. 31, 2024 $ / shares | Dec. 31, 2023 USD ($) Subsidiary mi $ / shares | Dec. 31, 2022 USD ($) $ / shares | Dec. 31, 2021 USD ($) $ / shares | Dec. 31, 2020 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |
Length Of Pipeline | mi |  | 85 |  |  |
Cost of removal obligation |  | $ 126.3 | $ 116.1 |  |
Regulatory assets |  | 109.6 | 114.3 |  |
Investments in trading securities |  | $ 6.0 | 5.8 |  |
Number of Subsidiaries | Subsidiary |  | 3 |  |  |
Allowance for doubtful accounts |  | $ 2.4 | $ 2.6 | $ 3.3 | $ 3.3
Depreciation rate based on average depreciable property balance |  | 3.33% | 3.26% | 3.29% |
Dividends per Common Share | $ / shares |  | $ 1.62 | $ 1.56 | $ 1.52 |
Increase in dividend declared amount per share | $ / shares |  | $ 1.62 |  |  |
Weighted average cost inventory amount |  | $ 1.0 | $ 1.8 |  |
Average Interest Rate On Debt |  | 5.48% | 2.50% | 1.71% |
Other Deferred Charges |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Regulatory assets |  | $ 11.0 | $ 11.1 |  |
Materials And Supplies |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Weighted average cost inventory amount |  | 13.5 | 11.4 |  |
Subsequent Event |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 1.7 |  |  |  |
Unbilled Revenues |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Allowance for doubtful accounts |  | 0.1 | 0.1 | $ 0.1 |
Accounts Receivable |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Allowance for doubtful accounts |  | $ 2.3 | 2.5 |  |
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 |  |  |
Fitchburg Gas and Electric Light Company | Electric and Gas Division | Other Deferred Charges |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Hardship accounts in regulatory assets |  | $ 6.0 | $ 5.8 |  |
Quarterly Payment |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Dividends per Common Share | $ / shares |  | $ 0.405 | $ 0.39 | $ 0.38 |
Quarterly Payment | Subsequent Event |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Common stock dividend paid per share | $ / shares | 0.425 |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 0.02 |  |  |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Regulatory assets |  | $ 7.2 |  |  |
ISO-NE Obligations |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Cash Deposits |  | $ 3.3 | $ 6.0 |  |
Maximum |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Lease term |  | 12 months |  |  |
Deferred Compensation Plan [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Investments in trading securities |  | $ 1.3 | $ 0.6 |  |

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