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EQUITY - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 28, 2025 | Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024 | Dec. 31, 2024
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, shares outstanding |  | 16,291,395 | 16,173,132 | 16,291,395 | 16,173,132 | 16,192,345
Common stock, shares issued |  | 32,838 | 5,245 | 38,311 | 11,048 |
Proceeds from Issuance of Common Stock |  |  |  | $ 2,000 | $ 600 |
Share based compensation expense |  |  |  |  | 1,200 |
Preferred Stock |  | $ 200 | $ 200 | 200 | 200 | $ 200
Share Sold |  | 1,700 | 300 | 2,000 | 600 |
Maximum |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Dividend declared |  | 100 | $ 100 | 100 | $ 100 |
Unitil Corporation |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock offering price |  | $ 50,000 |  | $ 50,000 |  |
Restricted Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Restricted stock vesting period |  |  |  | 4 years |  |
Restricted stock non-vested |  | 25,170 | 59,163 | 25,170 | 59,163 |
Restricted Stock Units Granted |  |  |  | $ 50.85 | $ 47.72 |
Share based compensation expense |  |  |  | $ 1,300 |  |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  | $ 600 | $ 800 | 600 | $ 800 | $ 900
Unrecognized share based compensation |  | $ 1,400 |  | $ 1,400 |  |
Share compensation recognition period |  |  |  | 2 years 8 months 12 days |  |
Restricted Stock Units Granted | 26,430 |  |  |  |  |
Aggregate Market Value | $ 1,400 |  |  |  |  |
Forfeitures under the stock plan |  |  |  | 396 |  |
Cancellations under the stock plan |  |  |  | 0 |  |
Restricted Stock | Maximum |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Restricted stock available for awards |  | 1,027,500 |  | 1,027,500 |  |
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] |  |  |  |  |  |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |  |  |  | 70.00% |  |
Restricted Stock Units Granted |  |  |  | 0 |  |
Restricted stock units outstanding |  |  | 33,921 |  | 33,921 |
Weighted-Average Stock Price |  |  | $ 42.89 |  | $ 42.89 |
Percentage of fully-vested restricted stock units that directors will receive in cash when settled |  |  |  | 30.00% |  |
Performance Shares [Member] |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Restricted stock non-vested |  | 67,380 | 43,453 | 67,380 | 43,453 |
Restricted Stock Units Granted |  |  |  | $ 51.21 | $ 50.35 |
Share based compensation expense |  |  |  | $ 800 | $ 500 |
Unrecognized share based compensation |  | $ 2,500 |  | $ 2,500 |  |
Forfeited |  |  |  | 250 |  |
Share compensation recognition period |  |  |  | 1 year 10 months 24 days |  |
Restricted Stock Units Granted | 26,430 |  |  |  |  |
Aggregate Market Value | $ 1,400 |  |  |  |  |
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 | $ 200 | $ 200 | $ 200 | $ 200
Dividend rate |  |  |  | 6.00% | 6.00% | 6.00%
At-the-Market Equity Offering Program |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common Stock Available For Future Sales |  |  |  | $ 48,500 |  |
At-the-Market Equity Offering Program | Common Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Share sold under ATM program |  | 27,620 |  | 27,620 |  |
Sale of stock, Average price per share |  | $ 53 |  | $ 53 |  |
Net Proceeds |  | $ 1,400 |  | $ 1,400 |  |
Gross proceeds |  | $ 1,500 |  | 1,500 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Proceeds from Issuance of Common Stock |  |  |  | $ 590,600 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock price per share |  | $ 55.25 |  | $ 55.25 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, shares issued |  |  |  | 10,691 |  |

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