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EQUITY - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 28, 2025
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Jun. 03, 2025
Class of Stock [Line Items]              
Common stock, shares outstanding   17,899,410 16,177,314 17,899,410 16,177,314 16,192,345  
Common stock, shares issued   1,608,015 4,182 1,646,326 15,230    
Proceeds from Issuance of Common Stock       $ 73,400,000 $ 700,000    
Share based compensation expense         1,300,000    
Preferred Stock   $ 200,000 $ 200,000 200,000 200,000 $ 200,000  
Share Sold   71,500,000 200,000 73,400,000 700,000    
Maximum              
Class of Stock [Line Items]              
Dividend declared   $ 100,000 $ 100,000 $ 100,000 $ 100,000    
Unitil Corporation              
Class of Stock [Line Items]              
Common stock offering price             $ 50,000,000
Restricted Stock              
Class of Stock [Line Items]              
Restricted stock vesting period       4 years      
Restricted stock non-vested   17,883 58,413 17,883 58,413    
Restricted Stock Units Granted       $ 50.94 $ 47.75    
Share based compensation expense       $ 1,900,000      
Fair value of liabilities associated with fully vested RSUs that will be settled in cash   $ 600,000 $ 900,000 600,000 $ 900,000 $ 900,000  
Unrecognized share based compensation   $ 900,000   $ 900,000      
Share compensation recognition period       2 years 9 months 18 days      
Restricted Stock Units Granted 26,430            
Aggregate Market Value $ 1,400,000            
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     34,164   34,164    
Weighted-Average Stock Price     $ 43   $ 43    
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       $ 900,000 $ 700,000    
Unrecognized share based compensation   $ 2,100,000   $ 2,100,000      
Forfeited       250      
Share compensation recognition period       1 year 10 months 24 days      
Restricted Stock Units Granted 26,430            
Aggregate Market Value $ 1,400,000            
Cancellations under the stock plan       0      
Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued       1,602,358      
Common stock price per share   $ 46.65   $ 46.65      
Proceeds From Issuance Initial Public Offering       $ 71,800,000      
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%  
At-the-Market Equity Offering Program              
Class of Stock [Line Items]              
Future sales       $ 48,500,000      
At-the-Market Equity Offering Program | Common Stock              
Class of Stock [Line Items]              
Share sold under ATM program       27,620      
Sale of stock, Average price per share   $ 53   $ 53      
Net Proceeds       $ 1,400,000      
Gross proceeds       1,500,000      
Dividend and Distribution Reinvestment and Share Purchase Plan              
Class of Stock [Line Items]              
Proceeds from Issuance of Common Stock       $ 861,000      
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share   $ 52.67   $ 52.67      
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued       16,348