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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended
Jan. 28, 2020
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Class of Stock [Line Items]        
Common stock, shares outstanding   14,963,444 14,916,044 14,930,170
Common stock, shares issued   4,644 5,939  
Proceeds from Issuance of Common Stock   $ 300,000 $ 300,000  
Restricted stock weighted average grant date fair value   $ 54.88 $ 46.23  
Share based compensation expense   $ 1,800,000 $ 1,700,000  
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   $ 1,600,000 1,400,000 $ 1,900,000
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
Maximum        
Class of Stock [Line Items]        
Dividend declared   $ 100,000    
Restricted Stock        
Class of Stock [Line Items]        
Restricted stock vesting period   4 years    
Restricted stock non-vested   56,813 60,496  
Unrecognized share based compensation   $ 1,100,000    
Share compensation recognition period   3 years    
Restricted Stock Units Granted 28,630      
Aggregate Market Value   $ 1,800,000    
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    
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     62,206  
Weighted-Average Stock Price     $ 38.36  
Series 6 | Unitil Energy Systems Inc        
Class of Stock [Line Items]        
Preferred stock, outstanding   1,887 1,893  
Preferred Stock   $ 200,000 $ 200,000  
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   $ 284,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average        
Class of Stock [Line Items]        
Common stock price per share   $ 61.16    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock        
Class of Stock [Line Items]        
Common stock, shares issued   4,644