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Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Apr. 19, 2016
Jan. 26, 2016
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Class of Stock [Line Items]              
Common stock, shares outstanding     14,052,096 13,974,576 14,052,096 13,974,576 13,991,430
Common stock, shares issued         16,646 19,411  
Proceeds from Issuance of Common Stock         $ 700,000 $ 600,000  
Share based compensation expense         $ 1,800,000 1,500,000  
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     $ 600,000 $ 300,000 $ 600,000 300,000 $ 500,000
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         $ 659,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average              
Class of Stock [Line Items]              
Common stock price per share     $ 39.58   $ 39.58    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock              
Class of Stock [Line Items]              
Common stock, shares issued         16,646    
Unitil Energy Systems Inc | Series 6              
Class of Stock [Line Items]              
Preferred stock, outstanding     1,898 2,059 1,898 2,059 1,898
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     95,506 80,588 95,506 80,588  
Restricted stock weighted average grant date fair value         $ 35.30 $ 33.14  
Unrecognized share based compensation     $ 1,600,000   $ 1,600,000    
Share compensation recognition period         2 years 8 months 12 days    
Forfeitures or cancellations under the stock plan         0    
Restricted Shares issued 800 43,220          
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    
Aggregate Market Value $ 100,000            
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 Shares issued         0    
Restricted stock units outstanding       24,062   24,062  
Weighted-Average Stock Price       $ 29.98   $ 29.98