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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Jan. 24, 2023
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Class of Stock [Line Items]        
Common stock, shares outstanding   16,086,230 16,020,047 16,043,355
Common stock, shares issued   5,335 5,511  
Proceeds from Issuance of Common Stock   $ 300,000 $ 300,000  
Restricted Stock Units Granted   $ 48.02 $ 47.44  
Share based compensation expense   $ 1,100,000 $ 1,700,000  
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   64,243 73,178  
Fair value of liabilities associated with fully vested RSUs that will be settled in cash   $ 1,100,000 $ 1,100,000 $ 1,000,000.0
Unrecognized share based compensation   $ 1,100,000    
Share compensation recognition period   2 years 9 months 18 days    
Restricted Stock Units Granted 18,770      
Aggregate Market Value $ 1,000,000.0      
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    
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     49,559  
Weighted-Average Stock Price     $ 41.74  
Performance Shares [Member]        
Class of Stock [Line Items]        
Restricted stock non-vested   18,770    
Restricted Stock Units Granted   $ 51.83    
Share based compensation expense   $ 100,000    
Unrecognized share based compensation   $ 1,200,000    
Share compensation recognition period   2 years 9 months 18 days    
Restricted Stock Units Granted 18,770      
Aggregate Market Value $ 1,000,000.0      
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   1,861 1,861 1,861
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   $ 289,700    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average        
Class of Stock [Line Items]        
Common stock price per share   $ 54.31    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock        
Class of Stock [Line Items]        
Common stock, shares issued   5,335