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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jan. 31, 2022 $ / shares | Dec. 31, 2021 USD ($) Subsidiary mi $ / shares | Dec. 31, 2020 USD ($) $ / shares | Dec. 31, 2019 USD ($) $ / shares | Dec. 31, 2018 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 |  | $ 107.5 | $ 105.2 |  |
Regulatory assets |  | 156.3 | 164.7 |  |
Investments in trading securities |  | $ 5.7 | 5.7 |  |
Number of Subsidiaries | Subsidiary |  | 3 |  |  |
Allowance for doubtful accounts |  | $ 3.3 | $ 3.3 | $ 1.0 | $ 1.3
Depreciation rate based on average depreciable property balance |  | 3.29% | 3.34% | 3.41% |
Common stock dividend per share, declared | $ / shares |  | $ 1.52 | $ 1.50 | $ 1.48 |
Increase in dividend declared amount per share | $ / shares |  | $ 1.52 |  |  |
Inventory of materials and supplies |  | $ 1.0 | $ 0.6 |  |
Average Interest Rate On Debt |  | 1.71% | 3.12% | 3.90% |
Other Deferred Charges [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Regulatory assets |  | $ 15.3 | $ 14.2 |  |
Materials And Supplies [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Inventory of materials and supplies |  | 8.6 | 8.5 |  |
Subsequent Event [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 1.56 |  |  |  |
Unbilled Revenues |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Allowance for doubtful accounts |  | 0.2 | 0.2 |  |
Accounts Receivable |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Allowance for doubtful accounts |  | $ 3.1 | 3.1 |  |
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 [Member] | Other Deferred Charges [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Hardship accounts in regulatory assets |  | $ 7.9 | $ 6.8 |  |
Quarterly Payment [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Common stock dividend paid per share | $ / shares |  |  | $ 0.375 | $ 0.37 |
Common stock dividend per share, declared | $ / shares |  | $ 0.38 |  |  |
Quarterly Payment [Member] | Subsequent Event [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Common stock dividend paid per share | $ / shares | 0.39 |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 0.01 |  |  |  |
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 |  | $ 8.5 |  |  |
ISO-NE Obligations |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Cash Deposits |  | $ 2.7 | $ 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 |  |

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