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Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Oct. 02, 2017
Jan. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Class of Stock [Line Items]              
Common stock, shares outstanding     14,119,893 14,060,147 14,119,893 14,060,147 14,065,230
Common stock, shares issued         20,564 24,697  
Proceeds from Issuance of Common Stock         $ 1,000,000 $ 1,000,000  
Share based compensation expense         $ 2,600,000 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     $ 900,000 $ 600,000 $ 900,000 600,000 $ 800,000
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         $ 967,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share     $ 47.02   $ 47.02    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued         20,564    
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     89,326 95,506 89,326 95,506  
Restricted stock weighted average grant date fair value         $ 39.54 $ 35.30  
Unrecognized share based compensation     $ 1,000,000   $ 1,000,000    
Share compensation recognition period         2 years 4 months 24 days    
Forfeitures under the stock plan         831    
Cancellations under the stock plan         0    
Restricted Stock Units Granted   34,930          
Aggregate Market Value   $ 1,600,000          
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       34,498   34,498  
Weighted-Average Stock Price       $ 32.03   $ 32.03  
Restricted Stock Units (RSUs) | Subsequent Event              
Class of Stock [Line Items]              
Restricted Stock Units Granted 10,746