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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 25, 2022
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Class of Stock [Line Items]            
Common stock, shares outstanding   16,039,141 15,971,962 16,039,141 15,971,962 15,977,766
Common stock, shares issued   4,506 925,493 14,369 936,512  
Proceeds from Issuance of Common Stock       $ 800,000 $ 45,300,000  
Restricted Stock Units Granted       $ 47.46 $ 49.61  
Share based compensation expense       $ 1,900,000 $ 1,400,000  
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   $ 800,000 $ 800,000 $ 800,000 800,000 $ 1,000,000
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 $ 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   72,428 57,408 72,428 57,408  
Unrecognized share based compensation   $ 900,000   $ 900,000    
Share compensation recognition period   2 years 8 months 12 days        
Restricted Stock Units Granted 36,770          
Aggregate Market Value $ 1,700,000          
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   677,500    
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]            
Restricted Stock Units Granted       0    
Restricted stock units outstanding     44,224   44,224  
Weighted-Average Stock Price   $ 41.49   $ 41.49    
Series 6 | Unitil Energy Systems Inc            
Class of Stock [Line Items]            
Preferred stock, outstanding   1,861 1,861 1,861 1,861 1,861
Preferred Stock   $ 200,000 $ 200,000 $ 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       $ 752,200    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average            
Class of Stock [Line Items]            
Common stock price per share   $ 52.35   $ 52.35    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock            
Class of Stock [Line Items]            
Common stock, shares issued       14,369