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Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 01, 2015
Jan. 26, 2015
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Class of Stock [Line Items]              
Common stock, shares outstanding     13,982,941 13,906,964 13,982,941 13,906,964 13,916,026
Common stock, shares issued         27,776 28,958  
Proceeds from Issuance of Common Stock         $ 1,000,000 $ 900,000  
Aggregate Market Value   $ 1,500,000          
Share based compensation expense         $ 1,600,000 1,300,000  
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     $ 400,000 $ 200,000 $ 400,000 200,000 $ 400,000
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         $ 960,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share     $ 34.56   $ 34.56    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued         27,776    
Restricted Stock              
Class of Stock [Line Items]              
Restricted stock vesting period         4 years    
Fully-vested restricted stock units issued   40,010          
Restricted stock non-vested     80,588 67,334 80,588 67,334  
Restricted stock weighted average grant date fair value     $ 33.14 $ 28.51 $ 33.14 $ 28.51  
Unrecognized share based compensation     $ 1,200,000   $ 1,200,000    
Share compensation recognition period         2 years 7 months 6 days    
Forfeitures under the stock plan         871    
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]              
Fully-vested restricted stock units issued         0    
Restricted stock units outstanding       14,270   14,270  
Weighted-Average Stock Price       $ 28.96   $ 28.96  
Restricted Stock Units (RSUs) | Subsequent Event              
Class of Stock [Line Items]              
Fully-vested restricted stock units issued 12,807            
Unitil Energy Systems Inc | Series 6              
Class of Stock [Line Items]              
Preferred stock, outstanding     2,009 2,250 2,009 2,250 2,250
Preferred Stock     $ 200,000 $ 200,000 $ 200,000 $ 200,000 $ 200,000
Dividend rate         6.00% 6.00% 6.00%