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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Jan. 29, 2019
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Class of Stock [Line Items]        
Common stock, shares outstanding   14,916,044 14,860,123 14,876,955
Common stock, shares issued   5,939 7,812  
Proceeds from Issuance of Common Stock   $ 300,000 $ 300,000  
Share based compensation expense   $ 1,700,000 1,800,000  
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   $ 1,400,000 1,000,000 $ 1,300,000
Preferred Stock   200,000 200,000 $ 200,000
Maximum        
Class of Stock [Line Items]        
Dividend declared   100,000 $ 100,000  
Dividend and Distribution Reinvestment and Share Purchase Plan        
Class of Stock [Line Items]        
Proceeds from Issuance of Common Stock   $ 314,700    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average        
Class of Stock [Line Items]        
Common stock price per share   $ 52.98    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock        
Class of Stock [Line Items]        
Common stock, shares issued   5,939    
Unitil Energy Systems Inc | Series 6        
Class of Stock [Line Items]        
Preferred stock, outstanding   1,893 1,893 1,893
Preferred Stock   $ 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   60,496 90,882  
Restricted stock weighted average grant date fair value   $ 46.23 $ 41.93  
Unrecognized share based compensation   $ 1,300,000    
Share compensation recognition period   3 years    
Restricted Stock Units Granted 33,150      
Aggregate Market Value $ 1,600,000      
Forfeitures under the stock plan   0    
Cancellations under the stock plan   0    
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%    
Restricted Stock Units (RSUs)        
Class of Stock [Line Items]        
Restricted Stock Units Granted   0    
Restricted stock units outstanding     52,677  
Weighted-Average Stock Price     $ 36.27