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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 31, 2021 $ / shares | Dec. 31, 2020 USD ($) $ / shares | Sep. 30, 2020 $ / shares | Jun. 30, 2020 $ / shares | Mar. 31, 2020 $ / shares | Dec. 31, 2019 USD ($) $ / shares | Sep. 30, 2019 $ / shares | Jun. 30, 2019 $ / shares | Mar. 31, 2019 USD ($) $ / shares | Dec. 31, 2020 USD ($) mi Subsidiary Entity $ / shares | Dec. 31, 2019 USD ($) $ / shares | Dec. 31, 2018 USD ($) $ / shares | Dec. 31, 2017 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 |  | $ 105.2 |  |  |  | $ 96.0 |  |  |  | $ 105.2 | 96.0 |  |
Regulatory assets |  | 164.7 |  |  |  | 147.8 |  |  |  | 164.7 | 147.8 |  |
Investments in trading securities |  | 5.7 |  |  |  | 5.6 |  |  |  | $ 5.7 | 5.6 |  |
Number of Subsidiaries | Entity |  |  |  |  |  |  |  |  |  | 3 |  |  |
Allowance for doubtful accounts |  | $ 3.3 |  |  |  | 1.0 |  |  |  | $ 3.3 | 1.0 | $ 1.3 | $ 1.6
Power Supply Contract Divestitures |  |  |  |  |  | $ 0.3 |  |  |  |  | $ 0.3 |  |
Depreciation rate based on average depreciable property balance |  |  |  |  |  |  |  |  |  | 3.34% | 3.41% | 3.38% |
Common stock dividend paid per share | $ / shares |  | $ 0.375 | $ 0.375 | $ 0.375 | $ 0.375 | $ 0.37 | $ 0.37 | $ 0.37 | $ 0.37 |  |  |  |
Common stock dividend per share, declared | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.50 | $ 1.48 | $ 1.46 |
Increase in dividend declared amount per share | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.50 |  |  |
Inventory of materials and supplies |  | $ 0.6 |  |  |  | $ 0.8 |  |  |  | $ 0.6 | $ 0.8 |  |
Average Interest Rate On Debt |  |  |  |  |  |  |  |  |  | 3.12% | 3.90% | 2.70% |
Materials And Supplies [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Inventory of materials and supplies |  | 8.5 |  |  |  | 7.9 |  |  |  | $ 8.5 | $ 7.9 |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 1.52 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  | 1.0 |  |  |  | $ 3.1 | $ 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 |  |  |  |  |  |  |  | $ 0.3 |  |  |
Quarterly Payment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend paid per share | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 0.37 | $ 0.365 |
Common stock dividend per share, declared | $ / shares |  |  |  |  |  |  |  |  |  | $ 0.375 |  |  |
Quarterly Payment [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend paid per share | $ / shares | 0.38 |  |  |  |  |  |  |  |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 0.005 |  |  |  |  |  |  |  |  |  |  |  |
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 |  | 8.0 |  |  |  |  |  |  |  | $ 8.0 |  |  |
ISO-NE Obligations |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash Deposits |  | $ 2.4 |  |  |  | 1.9 |  |  |  | $ 2.4 | $ 1.9 |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Lease term |  | 12 months |  |  |  |  |  |  |  | 12 months |  |  |
Deferred Compensation Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investments in trading securities |  | $ 0.5 |  |  |  | $ 0.2 |  |  |  | $ 0.5 | $ 0.2 |  |

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