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Equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Jan. 29, 2019
Dec. 14, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]          
Common stock, shares outstanding     14,876,955 14,815,585  
Common stock, shares authorized     25,000,000 25,000,000  
Common Stock, Shares issued     25,932   32,095
Proceeds from Issuance of Common Stock     $ 1,200,000 $ 33,000,000 $ 1,300,000
Common stock shares repurchase     791 1,686 1,949
Share based compensation expense     $ 2,200,000 $ 2,700,000 $ 2,200,000
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%    
Fair value of liabilities associated with fully vested RSUs that will be settled in cash     $ 1,300,000 1,000,000  
Preferred Stock     200,000 200,000  
Maximum          
Class of Stock [Line Items]          
Repurchase expense     100,000 100,000 100,000
Dividend declared     100,000 $ 100,000  
Dividend and Distribution Reinvestment and Share Purchase Plan          
Class of Stock [Line Items]          
Common Stock, Shares issued       26,256  
Proceeds from Issuance of Common Stock     $ 1,200,000 $ 1,300,000 $ 1,300,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average          
Class of Stock [Line Items]          
Common stock price per share     $ 47.78    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock          
Class of Stock [Line Items]          
Common Stock, Shares issued     25,932 26,256 32,095
Registered Public Offering          
Class of Stock [Line Items]          
Common Stock, Shares issued       690,000  
Proceeds from Issuance of Common Stock   $ 31,700,000      
Registered Public Offering | Common Stock          
Class of Stock [Line Items]          
Common Stock, Shares issued   690,000      
Common stock price per share   $ 48.30      
Unitil Energy Systems Inc | Series 6          
Class of Stock [Line Items]          
Preferred stock, outstanding     1,893 1,893  
Preferred Stock     $ 200,000 $ 200,000  
Dividend rate     6.00% 6.00%  
Restricted Stock          
Class of Stock [Line Items]          
Restricted stock vesting period     4 years    
Restricted stock non-vested     29,252 89,326  
Restricted stock weighted average grant date fair value     $ 42.86 $ 39.54  
Unrecognized share based compensation     $ 800,000    
Share compensation recognition period     2 years 3 months 19 days    
Forfeitures under the stock plan     2,072    
Cancellations under the stock plan     0    
Restricted Stock | Subsequent Event          
Class of Stock [Line Items]          
Aggregate Market Value $ 1,600,000        
Restricted Stock Units Granted 33,150        
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%