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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 29, 2018
Dec. 14, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Class of Stock [Line Items]              
Common stock, shares outstanding     14,866,588 14,114,551 14,866,588 14,114,551 14,815,585
Common stock, shares issued         14,277 14,391  
Proceeds from Issuance of Common Stock         $ 600,000 $ 700,000  
Share based compensation expense         $ 1,900,000 2,000,000  
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     $ 1,200,000 $ 900,000 $ 1,200,000 900,000 $ 1,000,000
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  
Dividend and Distribution Reinvestment and Share Purchase Plan              
Class of Stock [Line Items]              
Proceeds from Issuance of Common Stock         $ 649,600    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share     $ 45.50   $ 45.50    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued         14,277    
Registered Public Offering              
Class of Stock [Line Items]              
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 1,893 1,893 1,893
Preferred Stock     $ 200,000 $ 200,000 $ 200,000 $ 200,000 $ 200,000
Dividend rate         6.00% 6.00% 6.00%
Restricted Stock              
Class of Stock [Line Items]              
Restricted stock vesting period         4 years    
Restricted stock non-vested     90,882 89,705 90,882 89,705  
Restricted stock weighted average grant date fair value         $ 41.93 $ 39.55  
Unrecognized share based compensation     $ 1,200,000   $ 1,200,000    
Share compensation recognition period         2 years 8 months 12 days    
Restricted Stock Units Granted 37,510            
Aggregate Market Value $ 1,600,000            
Forfeitures under the stock plan         784    
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       44,028   44,028  
Weighted-Average Stock Price       $ 33.60   $ 33.60