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Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 24, 2023 | Aug. 06, 2021 | Jan. 31, 2024 | Jan. 30, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Class of Stock [Line Items] |  |  |  |  |  |  |
Common Stock, Shares, Outstanding |  |  |  |  | 16,116,724 | 16,043,355 |
Common stock, shares issued |  |  |  |  | 21,321 | 18,853 | 942,316
Common stock, shares authorized |  |  |  |  | 25,000,000 | 25,000,000 |
Proceeds from Issuance of Common Stock |  |  |  |  | $ 1.1 | $ 1.0 | $ 45.5
Restricted stock weighted average grant date fair value |  |  |  |  | $ 46.94 | $ 46.45 |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |  |  |  |  | 70.00% |  |
Common stock shares repurchase |  |  |  |  | 14,680 | 9,449 | 8,012
Share based compensation expense |  |  |  |  | $ 1.4 | $ 2.1 | $ 1.4
Percentage of fully-vested restricted stock units that directors will receive in cash when settled |  |  |  |  | 30.00% |  |
Preferred Stock |  |  |  |  | $ 0.2 | $ 0.2 |
Restricted Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted stock vesting period |  |  |  |  | 4 years |  |
Restricted stock non-vested |  |  |  |  | 36,483 | 45,473 |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  |  |  |  | $ 0.8 | $ 1.0 |
Unrecognized share based compensation |  |  |  |  | $ 0.7 |  |
Share compensation recognition period |  |  |  |  | 2 years 2 months 12 days |  |
Aggregate Market Value |  |  |  | $ 1.1 |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value |  |  |  | $ 1.1 |  |  |
Cancellations under the stock plan |  |  |  |  | 0 |  |
Restricted Stock | Subsequent Event |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  | 22,680 |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  | 22,680 |  |  |  |
Restricted Stock | Vesting Annually |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted stock vesting percentage annually |  |  |  |  | 25.00% |  |
Restricted Stock | Period One [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  |  |  | 23,140 |  |
Aggregate Market Value |  |  |  |  | $ 0.9 |  |
Share Issued Date |  |  |  |  | Jan. 26, 2021 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  | 23,140 |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value |  |  |  |  | $ 0.9 |  |
Restricted Stock | Period Two [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  |  |  | 36,770 |  |
Aggregate Market Value |  |  |  |  | $ 1.7 |  |
Share Issued Date |  |  |  |  | Jan. 25, 2022 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  | 36,770 |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value |  |  |  |  | $ 1.7 |  |
Restricted Stock | Period Three [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  |  |  | 18,770 |  |
Aggregate Market Value |  |  |  |  | $ 1.0 |  |
Share Issued Date |  |  |  |  | Jan. 24, 2023 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  | 18,770 |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value |  |  |  |  | $ 1.0 |  |
Performance Restricted Shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted stock non-vested |  |  |  |  | 18,770 |  |
Restricted stock weighted average grant date fair value |  |  |  |  | $ 51.83 |  |
Share based compensation expense |  |  |  |  | $ 0.5 |  |
Unrecognized share based compensation |  |  |  |  | $ 0.9 |  |
Share compensation recognition period |  |  |  |  | 2 years |  |
Restricted Stock Units Granted | 18,770 |  |  |  |  |  |
Aggregate Market Value | $ 1.0 |  |  |  |  |  |
Share Issued Date | Jan. 24, 2023 |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period | 18,770 |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value | $ 1.0 |  |  |  |  |  |
Performance Restricted Shares | Subsequent Event |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  |  | 22,680 |  |  |
Aggregate Market Value |  |  |  | $ 1.1 |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  | 22,680 |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value |  |  |  | $ 1.1 |  |  |
Common Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common stock, shares issued |  | 800,000 |  |  |  |  |
Common stock price per share |  | $ 50.8 |  |  |  |  |
Proceeds from Issuance Initial Public Offering |  | $ 38.6 |  |  |  |  |
Unitil Energy Systems Inc | Series 6 |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, outstanding |  |  |  |  | 1,727 | 1,861 |
Preferred Stock |  |  |  |  | $ 0.2 | $ 0.2 |
Dividend rate |  |  |  |  | 6.00% | 6.00% |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Proceeds from Issuance of Common Stock |  |  |  |  | $ 1.1 | $ 1.0 | $ 1.0
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common stock, shares issued |  |  |  |  | 21,321 | 18,853 | 22,316
Over-Allotment Option [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Number Of Shares Granted Overallotment Option To Purchase Additional Shares |  | 30 days |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction |  | 120,000 |  |  |  |  |
Proceeds from Issuance of Private Placement |  | $ 5.9 |  |  |  |  |
Maximum |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Dividend declared |  |  |  |  | $ 0.1 | $ 0.1 |
Repurchase expense |  |  |  |  | $ 0.6 | $ 0.4 | $ 0.4
Maximum | Restricted Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Restricted stock available for awards |  |  |  |  | 677,500 |  |
Restricted stock that may be awarded in any one calendar year to any one participant |  |  |  |  | 20,000 |  |
Average | Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common stock price per share |  |  |  |  | $ 51.58 |  |

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