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EQUITY - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 28, 2025
Jan. 30, 2024
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Class of Stock [Line Items]          
Common stock, shares outstanding     16,250,032 16,167,887 16,192,345
Common stock, shares issued     5,473 5,803  
Proceeds from Issuance of Common Stock     $ 300,000 $ 300,000  
Share based compensation expense       1,000,000  
Preferred Stock     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     25,170 59,163  
Restricted Stock Units Granted     $ 50.85 $ 47.72  
Share based compensation expense     $ 1,200,000    
Fair value of liabilities associated with fully vested RSUs that will be settled in cash     900,000 $ 800,000 $ 900,000
Unrecognized share based compensation     $ 1,600,000    
Share compensation recognition period     3 years    
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    
Restricted stock that may be awarded in any one calendar year to any one participant     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       33,653  
Weighted-Average Stock Price       $ 42.8  
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  
Restricted Stock Units Granted     $ 51.21 $ 50.35  
Share based compensation expense     $ 400,000 $ 200,000  
Unrecognized share based compensation     $ 2,900,000    
Forfeited     250    
Share compensation recognition period     2 years 2 months 12 days    
Restricted Stock Units Granted 26,430        
Aggregate Market Value $ 1,400,000        
Cancellations under the stock plan     0    
Series 6 | Unitil Energy Systems Inc          
Class of Stock [Line Items]          
Preferred stock, outstanding     1,727   1,727
Preferred Stock     $ 200,000   $ 200,000
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     $ 305,900,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average          
Class of Stock [Line Items]          
Common stock price per share     $ 55.9    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock          
Class of Stock [Line Items]          
Common stock, shares issued     5,473