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Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Jan. 24, 2023
Aug. 06, 2021
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Class of Stock [Line Items]          
Common Stock, Shares, Outstanding     16,043,355 15,977,766  
Common stock, shares issued     18,853 942,316 23,658
Common stock, shares authorized     25,000,000 25,000,000  
Proceeds from Issuance of Common Stock     $ 1.0 $ 45.5 $ 1.1
Restricted stock weighted average grant date fair value     $ 46.45 $ 49.72  
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled     70.00%    
Common stock shares repurchase     9,449 8,012 13,194
Share based compensation expense     $ 2.1 $ 1.4 $ 2.2
Percentage of fully-vested restricted stock units that directors will receive in cash when settled     30.00%    
Preferred Stock     $ 0.2 $ 0.2  
Restricted Stock          
Class of Stock [Line Items]          
Restricted stock vesting period     4 years    
Restricted stock non-vested     45,473 37,621  
Fair value of liabilities associated with fully vested RSUs that will be settled in cash     $ 1.0 $ 1.0  
Unrecognized share based compensation     $ 0.8    
Share compensation recognition period     2 years 6 months    
Cancellations under the stock plan     0    
Restricted forfeited shares     270    
Restricted Stock | Subsequent Event          
Class of Stock [Line Items]          
Restricted Stock Units Granted 18,770        
Aggregate Market Value $ 1.0        
Restricted Stock | Vesting Annually          
Class of Stock [Line Items]          
Restricted stock vesting percentage annually     25.00%    
Performance Restricted Shares | Subsequent Event          
Class of Stock [Line Items]          
Restricted Stock Units Granted 18,770        
Aggregate Market Value $ 1.0        
Unitil Energy Systems Inc | Series 6          
Class of Stock [Line Items]          
Preferred stock, outstanding     1,861 1,861  
Preferred Stock     $ 0.2 $ 0.2  
Dividend rate     6.00% 6.00%  
Dividend and Distribution Reinvestment and Share Purchase Plan          
Class of Stock [Line Items]          
Proceeds from Issuance of Common Stock     $ 1.0 $ 1.0 $ 1.1
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock          
Class of Stock [Line Items]          
Common stock, shares issued     18,853 22,316 23,658
Over-Allotment Option [Member]          
Class of Stock [Line Items]          
Number Of Shares Granted Overallotment Option To Purchase Additional Shares   30 days      
Sale of Stock, Number of Shares Issued in Transaction   120,000      
Proceeds from Issuance of Private Placement   $ 5.9      
Public Offering [Member] | Common Stock          
Class of Stock [Line Items]          
Common stock, shares issued   800,000      
Common stock price per share   $ 50.80      
Proceeds from Issuance Initial Public Offering   $ 38.6      
Maximum          
Class of Stock [Line Items]          
Dividend declared     $ 0.1    
Repurchase expense     $ 0.4 $ 0.4 $ 0.5
Maximum | Restricted Stock          
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    
Average | Dividend and Distribution Reinvestment and Share Purchase Plan          
Class of Stock [Line Items]          
Common stock price per share     $ 52.18