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EQUITY - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 30, 2024
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
May 01, 2024
Class of Stock [Line Items]          
Common stock, shares outstanding   16,167,887 16,086,230 16,116,724  
Common stock, shares issued   5,803 5,335    
Proceeds from Issuance of Common Stock   $ 300,000 $ 300,000    
Restricted Stock Units Granted   $ 47.72 $ 48.02    
Share based compensation expense   $ 1,000,000 $ 1,100,000    
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  
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   59,163 64,243    
Fair value of liabilities associated with fully vested RSUs that will be settled in cash   $ 800,000 $ 1,100,000 $ 800,000  
Unrecognized share based compensation   $ 1,200,000      
Share compensation recognition period   2 years 9 months 18 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      
Restricted Stock | Maximum          
Class of Stock [Line Items]          
Restricted stock available for awards   677,500     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]          
Restricted Stock Units Granted   0      
Restricted stock units outstanding     44,125    
Weighted-Average Stock Price     $ 40.27    
Performance Shares [Member]          
Class of Stock [Line Items]          
Restricted stock non-vested   43,453      
Restricted Stock Units Granted   $ 50.35      
Share based compensation expense   $ 200,000 $ 100,000    
Unrecognized share based compensation   $ 2,200,000      
Share compensation recognition period   2 years 3 months 18 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 1,861 1,727  
Preferred Stock   $ 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   $ 291,200      
Dividend and Distribution Reinvestment and Share Purchase Plan | Average          
Class of Stock [Line Items]          
Common stock price per share   $ 50.17      
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock          
Class of Stock [Line Items]          
Common stock, shares issued   5,803