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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jan. 26, 2021
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Class of Stock [Line Items]            
Common stock, shares outstanding   15,046,469 14,996,231 15,046,469 14,996,231 15,012,310
Common stock, shares issued   4,840 5,555 11,019 10,199  
Proceeds from Issuance of Common Stock       $ 500,000 $ 500,000  
Restricted Stock Units Granted       $ 49.44 $ 54.88  
Share based compensation expense       $ 1,300,000 $ 1,900,000  
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   $ 1,000,000.0 $ 700,000 $ 1,000,000.0 700,000 $ 800,000
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 $ 100,000 $ 100,000  
Restricted Stock            
Class of Stock [Line Items]            
Restricted stock vesting period       4 years    
Restricted stock non-vested   62,566 56,813 62,566 56,813  
Unrecognized share based compensation   $ 800,000   $ 800,000    
Share compensation recognition period       2 years 10 months 24 days    
Restricted Stock Units Granted 23,140          
Aggregate Market Value $ 900,000          
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     38,710   38,710  
Weighted-Average Stock Price     $ 41.54   $ 41.54  
Series 6 | Unitil Energy Systems Inc            
Class of Stock [Line Items]            
Preferred stock, outstanding   1,887 1,887 1,887 1,887 1,887
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       $ 527,800    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average            
Class of Stock [Line Items]            
Common stock price per share   $ 47.90   $ 47.90    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock            
Class of Stock [Line Items]            
Common stock, shares issued       11,019