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Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Jan. 28, 2025
Jan. 30, 2024
Jan. 24, 2023
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
May 01, 2024
Mar. 31, 2024
Class of Stock [Line Items]                
Common Equity Outstanding       16,192,345 16,116,724      
Common stock, shares issued       19,510 21,321 18,853    
Common stock, shares authorized       25,000,000 25,000,000      
Proceeds from Issuance of Common Stock       $ 1.1 $ 1.1 $ 1.0    
Common stock shares repurchase       0 14,680 9,449    
Preferred Stock       $ 0.2 $ 0.2      
Restricted Stock                
Class of Stock [Line Items]                
Restricted stock vesting period       4 years        
Restricted stock non-vested       34,417 36,483      
Restricted stock weighted average grant date fair value       $ 48.84 $ 46.94      
Share based compensation expense       $ 1.4 $ 1.4 $ 2.1    
Unrecognized share based compensation       $ 0.8        
Share compensation recognition period       2 years 4 months 24 days        
Forfeited under the stock plan       354        
Cancellations under the stock plan       0        
Restricted Stock | Subsequent Event                
Class of Stock [Line Items]                
Restricted Stock Units Granted 26,430              
Aggregate Market Value $ 1.4              
Restricted Stock | Vesting Annually                
Class of Stock [Line Items]                
Restricted stock vesting percentage annually       25.00%        
Restricted Stock | Period One [Member]                
Class of Stock [Line Items]                
Restricted Stock Units Granted       36,770        
Aggregate Market Value       $ 1.7        
Restricted Stock | Period Two [Member]                
Class of Stock [Line Items]                
Restricted Stock Units Granted       18,770        
Aggregate Market Value       $ 1.0        
Restricted Stock | Period Three [Member]                
Class of Stock [Line Items]                
Restricted Stock Units Granted       22,680        
Aggregate Market Value       $ 1.1        
Performance Restricted Shares                
Class of Stock [Line Items]                
Restricted stock non-vested       44,449        
Restricted stock weighted average grant date fair value       $ 50.26        
Share based compensation expense       $ 1.0 0.5      
Unrecognized share based compensation       $ 1.5        
Share compensation recognition period       1 year 8 months 12 days        
Restricted Stock Units Granted   22,680 18,770     0    
Forfeited under the stock plan       250        
Cancellations under the stock plan       0        
Aggregate Market Value   $ 1.1 $ 1.0          
Performance Restricted Shares | Subsequent Event                
Class of Stock [Line Items]                
Restricted Stock Units Granted 26,430              
Aggregate Market Value $ 1.4              
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%        
Fair value of liabilities associated with fully vested RSUs that will be settled in cash       $ 0.9 $ 0.8      
Restricted Stock Units Granted       2,215 2,646      
Percentage of fully-vested restricted stock units that directors will receive in cash when settled       30.00%        
Unitil Energy Systems Inc | Series 6                
Class of Stock [Line Items]                
Preferred stock, outstanding       1,727 1,727      
Preferred Stock       $ 0.2 $ 0.2      
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       $ 1.1 $ 1.1 $ 1.0    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock                
Class of Stock [Line Items]                
Common stock, shares issued       19,510 21,321 18,853    
Maximum                
Class of Stock [Line Items]                
Dividend declared       $ 0.1 $ 0.1      
Repurchase expense       $ 0.0 $ 0.6 $ 0.4    
Maximum | Restricted Stock                
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        
Average | Dividend and Distribution Reinvestment and Share Purchase Plan                
Class of Stock [Line Items]                
Common stock price per share       $ 54.4