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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | May 01, 2024 | Mar. 31, 2024
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common Stock, Shares, Outstanding |  | 16,177,314 | 16,097,182 | 16,177,314 | 16,097,182 | 16,116,724 |  |
Common stock, shares issued |  | 4,182 | 5,763 | 15,230 | 16,287 |  |  |
Proceeds from Issuance of Common Stock |  |  |  | $ 700,000 | $ 800,000 |  |  |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |  |  |  | 70.00% |  |  |  |
Percentage of fully-vested restricted stock units that directors will receive in cash when settled |  |  |  | 30.00% |  |  |  |
Preferred Stock |  | $ 200,000 | $ 200,000 | $ 200,000 | 200,000 | $ 200,000 |  |
Maximum |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Dividend declared |  |  |  | $ 100,000 | $ 100,000 |  |  |
Restricted Stock |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Restricted stock vesting period |  |  |  | 4 years |  |  |  |
Restricted stock non-vested |  | 58,413 | 60,271 | 58,413 | 60,271 |  |  |
Restricted Stock Units Granted |  |  |  | $ 47.75 | $ 48.03 |  |  |
Share based compensation expense |  |  |  | $ 1,300,000 | $ 1,400,000 |  |  |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  | $ 900,000 | $ 800,000 | 900,000 | $ 800,000 | $ 800,000 |  |
Unrecognized share based compensation |  | $ 900,000 |  | $ 900,000 |  |  |  |
Share compensation recognition period |  |  |  | 2 years 6 months |  |  |  |
Restricted Stock Units Granted | 22,680 |  |  |  |  |  |  |
Aggregate Market Value | $ 1,100,000 |  |  |  |  |  |  |
Forfeitures under the stock plan |  |  |  | 0 |  |  |  |
Cancellations under the stock plan |  |  |  | 0 |  |  |  |
Restricted Stock | Maximum |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Restricted stock available for awards |  |  |  |  |  |  | 1,027,500 | 677,500
Restricted stock that may be awarded in any one calendar year to any one participant |  | 20,000 |  | 20,000 |  |  |  |
Restricted Stock | Vesting Annually |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Restricted stock vesting percentage annually |  |  |  | 25.00% |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Restricted Stock Units Granted |  |  |  | 0 |  |  |  |
Restricted stock units outstanding |  |  | 44,848 |  | 44,848 |  |  |
Weighted-Average Stock Price |  |  | $ 40.43 |  | $ 40.43 |  |  |
Performance Shares |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Restricted stock non-vested |  | 43,453 | 18,770 | 43,453 | 18,770 |  |  |
Restricted Stock Units Granted |  |  |  | $ 50.35 | $ 51.83 |  |  |
Share based compensation expense |  |  |  | $ 700,000 | $ 300,000 |  |  |
Unrecognized share based compensation |  | $ 1,700,000 |  | $ 1,700,000 |  |  |  |
Share compensation recognition period |  |  |  | 1 year 10 months 24 days |  |  |  |
Restricted Stock Units Granted | 22,680 |  |  |  |  |  |  |
Aggregate Market Value | $ 1,100,000 |  |  |  |  |  |  |
Forfeitures under the stock plan |  |  |  | 0 |  |  |  |
Cancellations under the stock plan |  |  |  | 0 |  |  |  |
Series 6 | Unitil Energy Systems Inc |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Preferred stock, outstanding |  | 1,727 | 1,727 | 1,727 | 1,727 | 1,727 |  |
Preferred Stock |  | $ 200,000 | $ 200,000 | $ 200,000 | $ 200,000 | $ 200,000 |  |
Dividend rate |  |  |  | 6.00% | 6.00% | 6.00% |  |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Proceeds from Issuance of Common Stock |  |  |  | $ 807,400 |  |  |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common stock price per share |  | $ 53.01 |  | $ 53.01 |  |  |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common stock, shares issued |  |  |  | 15,230 |  |  |  |

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