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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Jul. 28, 2020
Jan. 28, 2020
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Class of Stock [Line Items]              
Common stock, shares outstanding     15,005,881 14,925,898 15,005,881 14,925,898 14,930,170
Common stock, shares issued     6,650 4,727 16,849 15,793  
Proceeds from Issuance of Common Stock         $ 800,000 $ 900,000  
Restricted stock weighted average grant date fair value         $ 54.41 $ 46.41  
Share based compensation expense         $ 2,100,000 $ 2,200,000  
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     $ 600,000 $ 1,700,000 $ 600,000 1,700,000 $ 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 $ 200,000 200,000 $ 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     59,813 50,917 59,813 50,917  
Unrecognized share based compensation     $ 1,000,000.0   $ 1,000,000.0    
Share compensation recognition period         3 years    
Restricted Stock Units Granted 3,000 28,630          
Aggregate Market Value $ 100,000 $ 1,800,000          
Forfeitures under the stock plan         5,190    
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       62,999   62,999  
Weighted-Average Stock Price       $ 38.61   $ 38.61  
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         $ 821,500    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share     $ 48.75   $ 48.75    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued         16,849