v3.26.1
Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 27, 2026
Jan. 26, 2026
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Jun. 03, 2025
Class of Stock [Line Items]                
Common stock, shares outstanding     18,225,964 16,291,395 18,225,964 16,291,395 17,919,191  
Common stock, shares issued     217,576 32,838 223,699 38,311    
Proceeds from Issuance of Common Stock         $ 11,300 $ 2,000    
Share based compensation expense           1,300    
Preferred Stock     $ 200 $ 200 200 200 $ 200  
Share Sold     $ 11,000 $ 1,700 11,300 2,000    
Maximum                
Class of Stock [Line Items]                
Dividend declared         $ 100 $ 100    
Unitil Corporation                
Class of Stock [Line Items]                
Common stock offering price               $ 50,000
Restricted Stock                
Class of Stock [Line Items]                
Restricted stock vesting period         4 years      
Restricted stock non-vested     24,703 25,170 24,703 25,170    
Restricted Stock Units Granted         $ 50.71 $ 50.85    
Share based compensation expense         $ 1,600      
Fair value of liabilities associated with fully vested RSUs that will be settled in cash     $ 400 $ 600 400 $ 600 $ 700  
Unrecognized share based compensation     $ 1,400   $ 1,400      
Share compensation recognition period         3 years      
Restricted Stock Units Granted 32,330              
Aggregate Market Value $ 1,600              
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     1,027,500   1,027,500      
Restricted stock that may be awarded in any one calendar year to any one participant     20,000   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]                
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled         70.00%      
Restricted Stock Units Granted         0      
Restricted stock units outstanding       28,638   28,638    
Weighted-Average Stock Price       $ 44.61   $ 44.61    
Percentage of fully-vested restricted stock units that directors will receive in cash when settled         30.00%      
Performance Shares [Member]                
Class of Stock [Line Items]                
Restricted stock non-vested     81,180 67,380 81,180 67,380    
Restricted Stock Units Granted         $ 50.58 $ 51.21    
Share based compensation expense         $ 1,000 $ 800    
Unrecognized share based compensation     $ 2,900   $ 2,900      
Forfeited         0      
Share compensation recognition period         2 years      
Performance restricted shares, shares awarded         820      
Performance restricted shares, shares forfeited         931      
Restricted Stock Units Granted   32,330            
Aggregate Market Value   $ 1,600            
Cancellations under the stock plan         0      
Performance Shares [Member] | Maximum                
Class of Stock [Line Items]                
Share based compensation expense         $ 100      
Common Stock                
Class of Stock [Line Items]                
Common stock, shares issued           1,602,358    
Common stock price per share     $ 46.65   $ 46.65      
Proceeds From Issuance Initial Public Offering         $ 71,800      
Series 6 | Unitil Energy Systems Inc                
Class of Stock [Line Items]                
Preferred stock, outstanding     1,727 1,727 1,727 1,727 1,727  
Preferred Stock     $ 200 $ 200 $ 200 $ 200 $ 200  
Dividend rate         6.00% 6.00% 6.00%  
At-the-Market Equity Offering Program                
Class of Stock [Line Items]                
Future sales         $ 37,500      
At-the-Market Equity Offering Program | Common Stock                
Class of Stock [Line Items]                
Share sold under ATM program         212,042   27,620  
Sale of stock, Average price per share     $ 52.14   $ 52.14   $ 53  
Net Proceeds             $ 1,400  
Gross proceeds         $ 10,800   $ 1,500  
Dividend and Distribution Reinvestment and Share Purchase Plan                
Class of Stock [Line Items]                
Proceeds from Issuance of Common Stock         $ 602,300      
Dividend and Distribution Reinvestment and Share Purchase Plan | Average                
Class of Stock [Line Items]                
Common stock price per share     $ 51.67   $ 51.67      
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock                
Class of Stock [Line Items]                
Common stock, shares issued         11,657