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Equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Jan. 26, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Class of Stock [Line Items]        
Common stock, shares outstanding   13,991,430 13,916,026  
Common stock, share authorized   25,000,000 25,000,000  
Common stock, shares issued   36,265 38,020 39,559
Proceeds from Issuance of Common Stock   $ 1,300,000 $ 1,200,000 $ 1,100,000
Common stock shares repurchase   1,981 2,763 2,969
Repurchase expense   $ 100,000 $ 100,000 $ 100,000
Share based compensation expense   $ 1,900,000 1,400,000 $ 700,000
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   $ 500,000 400,000  
Preferred Stock   200,000 200,000  
Maximum        
Class of Stock [Line Items]        
Dividend declared   $ 100,000 $ 100,000  
Non Vested Restricted Stock        
Class of Stock [Line Items]        
Antidilutive securities excluded from computation of earnings per share   36,941 0 4,481
Dividend and Distribution Reinvestment and Share Purchase Plan        
Class of Stock [Line Items]        
Proceeds from Issuance of Common Stock   $ 1,300,000 $ 1,200,000 $ 1,100,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average        
Class of Stock [Line Items]        
Common stock price per share   $ 34.77    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock        
Class of Stock [Line Items]        
Common stock, shares issued   36,265 38,020 39,559
Unitil Energy Systems Inc | Series 6        
Class of Stock [Line Items]        
Preferred stock, outstanding   1,898 2,250  
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   70,761 67,334  
Restricted stock weighted average grant date fair value   $ 32.56 $ 28.51  
Unrecognized share based compensation   $ 1,100,000    
Share compensation recognition period   2 years 4 months 24 days    
Forfeitures or cancellations under the stock plan   871    
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%    
Subsequent Event | Restricted Stock        
Class of Stock [Line Items]        
Shares 43,220      
Aggregate Market Value $ 1,600,000