v3.26.1
Debt and Financing Arrangements - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Oct. 31, 2025
Sep. 29, 2022
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Apr. 30, 2026
Jul. 08, 2025
Jan. 31, 2025
Jan. 29, 2025
Line of Credit Facility [Line Items]                      
Weighted average interest rate on short term borrowings         4.90% 5.60% 5.40%        
Capital lease obligation, current     $ 0.2 $ 0.1 $ 0.2 $ 0.1 $ 0.2        
Capital lease obligation, noncurrent     0.3 0.3 0.3 0.3 0.4        
Total rental expense under operating leases     0.5 0.6 $ 1.1 1.1          
Debt instrument maturity date         Oct. 31, 2026            
Net Utility Plant     1,904.8 1,644.4 $ 1,904.8 1,644.4 1,798.5        
Operating lease obligations         1.1 1.1          
Operating Lease Obligations     2.1 1.7 2.1 1.7 2.1        
Operating Lease Obligations     $ 4.3 $ 4.0 $ 4.3 $ 4.0 4.9        
Operating lease, weighted average remaining lease term     3 years 4 months 24 days 3 years 9 months 18 days 3 years 4 months 24 days 3 years 9 months 18 days          
Operating lease, weighted average discount rate percentage     5.40% 5.00% 5.40% 5.00%          
Guarantees outstanding     $ 50.3   $ 50.3            
Assets Under Capital Leases [Member]                      
Line of Credit Facility [Line Items]                      
Net Utility Plant     0.9 $ 0.6 0.9 $ 0.6 0.9        
Net Utility Plant, accumulated amortization     0.4 0.2 $ 0.4 0.2 0.3        
Base Rate [Member]                      
Line of Credit Facility [Line Items]                      
Debt instrument variable interest rate additional spread 0.25%                    
Federal Funds Rate                      
Line of Credit Facility [Line Items]                      
Credit facility, daily fluctuating rate of interest 0.50%                    
SOFR                      
Line of Credit Facility [Line Items]                      
Credit facility, daily fluctuating rate of interest 0.10%                    
Debt instrument variable interest rate additional spread 1.25%                    
Bangor Natural Gas [Member]                      
Line of Credit Facility [Line Items]                      
Net Utility Plant                   $ 66.7  
Northern Utilities Inc [Member]                      
Line of Credit Facility [Line Items]                      
Total funded indebtedness as percentage of capitalization         65.00%            
Abenaki [Member]                      
Line of Credit Facility [Line Items]                      
Total Outstanding Amount     2.7   $ 2.7            
Revolving Credit Facility                      
Line of Credit Facility [Line Items]                      
Revolving credit facility     275.0 $ 275.0 275.0 $ 275.0 275.0        
Proceeds from lines of credit         174.5            
Repayments of lines of credit         238.0            
Senior unsecured delayed draw term loan facility                      
Line of Credit Facility [Line Items]                      
Acquisition amount $ 86.0                    
Tranche A [Member] | Senior unsecured delayed draw term loan facility                      
Line of Credit Facility [Line Items]                      
Borrowings outstanding     86.0   86.0            
Tranche B [Member] | Senior unsecured delayed draw term loan facility                      
Line of Credit Facility [Line Items]                      
Borrowings outstanding     42.6   42.6            
Acquisition amount         $ 50.0            
Credit Facility                      
Line of Credit Facility [Line Items]                      
Revolving credit facility   $ 75.0                  
Description of variable rate basis         The facility provides that the Company has an option for determining whether interest on loans under the facility will bear interest based on a Base Rate plus an applicable margin of 0.25% or based on a one-month Term SOFR plus a SOFR adjustment of 0.10% plus an applicable margin of 1.25%. The Base Rate is equal to the highest of the (a) Federal Funds Rate plus 0.50%, (b) the rate of interest in effect for such day as publicly announced from time to time by The Bank of Nova Scotia as its "prime rate", or (c) one-month Term SOFR plus a SOFR adjustment of 0.10% plus 1.00%.            
Credit Facility | Third Amendment Credit Facility                      
Line of Credit Facility [Line Items]                      
Sublimit for the issuance of standby letters of credit   $ 25.0                  
Revolving credit facility             200.0       $ 275.0
Credit Facility | Revolving Credit Facility                      
Line of Credit Facility [Line Items]                      
Cost of issue     0.3       $ 0.2        
Natural gas storage inventory     $ 8.3   $ 8.3            
Debt Instrument, Covenant Description         The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full (or, with respect to letters of credit, they are cash-collateralized). The only financial covenant in the Credit Facility provides that Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis.            
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility                      
Line of Credit Facility [Line Items]                      
Credit facility, daily fluctuating rate of interest   0.10%                  
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Maximum [Member]                      
Line of Credit Facility [Line Items]                      
Debt instrument variable interest rate additional spread   1.375%                  
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Minimum [Member]                      
Line of Credit Facility [Line Items]                      
Debt instrument variable interest rate additional spread   1.125%                  
5.70 % Senior Note Due to 2030 | Bangor Natural Gas [Member]                      
Line of Credit Facility [Line Items]                      
Long-term debt, aggregate principal amount                 $ 14.0    
Long-term debt, stated interest rate                 5.70%    
6.31 % Senior Note Due to 2035 | Bangor Natural Gas [Member]                      
Line of Credit Facility [Line Items]                      
Long-term debt, aggregate principal amount                 $ 18.0    
Long-term debt, stated interest rate                 6.31%    
5.62 % Senior Note Due to 2036 | Fitchburg                      
Line of Credit Facility [Line Items]                      
Long-term debt, aggregate principal amount               $ 23.0      
Long-term debt, stated interest rate               5.62%      
5.87 % Senior Note Due to 2041 | Fitchburg                      
Line of Credit Facility [Line Items]                      
Long-term debt, aggregate principal amount               $ 17.0      
Long-term debt, stated interest rate               5.87%