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Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 26, 2015
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Class of Stock [Line Items]            
Common stock, shares outstanding   13,974,576 13,895,777 13,974,576 13,895,777 13,916,026
Common stock, shares issued       19,411 18,877  
Proceeds from Issuance of Common Stock       $ 600,000 $ 600,000  
Aggregate Market Value $ 1,500,000          
Share based compensation expense       $ 1,500,000 1,200,000  
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   $ 300,000 $ 200,000 $ 300,000 200,000 $ 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       $ 661,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average            
Class of Stock [Line Items]            
Common stock price per share   $ 34.04   $ 34.04    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock            
Class of Stock [Line Items]            
Common stock, shares issued       19,411    
Restricted Stock            
Class of Stock [Line Items]            
Restricted stock vesting period       4 years    
Restricted stock vesting percentage annually       25.00%    
Restricted Stock Units Granted 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   $ 1,300,000   $ 1,300,000    
Share compensation recognition period       2 years 9 months 18 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 Units (RSUs)            
Class of Stock [Line Items]            
Restricted Stock Units Granted       0    
Restricted stock units outstanding     15,225   15,225  
Weighted-Average Stock Price     $ 28.97   $ 28.97  
Unitil Energy Systems Inc | Series 6            
Class of Stock [Line Items]            
Preferred stock, outstanding   2,059 2,250 2,059 2,250 2,250
Preferred Stock   $ 200,000 $ 200,000 $ 200,000 $ 200,000 $ 200,000
Dividend rate       6.00% 6.00% 6.00%