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Debt and Financing Arrangements - Additional Information (Detail) - USD ($)
shares in Millions, $ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jul. 06, 2023
Sep. 29, 2022
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Dec. 31, 2023
Line of Credit Facility [Line Items]              
Weighted average interest rate on short term borrowings     6.70% 6.30% 6.70% 6.10% 6.40%
Capital lease obligation, current     $ 0.1 $ 0.1 $ 0.1 $ 0.1 $ 0.1
Capital lease obligation, noncurrent     0.3 0.3 0.3 0.3 0.4
Accounts Payable     37.2 36.7 37.2 36.7 47.7
Total rental expense under operating leases     0.5 0.5 1.1 1.0  
Net Utility Plant     1,454.8 1,357.8 1,454.8 1,357.8 1,420.9
Operating lease obligations         1.1 1.0  
Other current operating lease obligation     1.7 1.7 1.7 1.7 1.9
Other noncurrent operating lease obligation     $ 3.4 $ 3.5 $ 3.4 $ 3.5 3.7
Operating lease, weighted average remaining lease term     3 years 7 months 6 days 3 years 8 months 12 days 3 years 7 months 6 days 3 years 8 months 12 days  
Operating lease, weighted average discount rate percentage     4.60% 4.50% 4.60% 4.50%  
Assets under Capital Leases [Member]              
Line of Credit Facility [Line Items]              
Net Utility Plant     $ 0.7 $ 0.8 $ 0.7 $ 0.8 0.7
Net Utility Plant, accumulated amortization     0.2 0.4 0.2 0.4 0.2
Revolving Credit Facility              
Line of Credit Facility [Line Items]              
Revolving credit facility     200.0 $ 200.0 200.0 $ 200.0 $ 200.0
Proceeds from lines of credit     133.2        
Repayments of lines of credit     137.4        
Accounts Payable     0.1   $ 0.1    
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 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              
Line of Credit Facility [Line Items]              
Revolving credit facility   $ 75.0          
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          
Credit Facility | Revolving Credit Facility              
Line of Credit Facility [Line Items]              
Natural gas storage inventory     $ 5.5   $ 5.5    
Notes due date Jul. 02, 2053            
Cost of issue         $ 0.2    
Issued note 13.0            
Note issue percentage 5.96%            
Credit Facility | Revolving Credit Facility | Fitchburg [Member]              
Line of Credit Facility [Line Items]              
Notes due date Jul. 02, 2033            
Note issue percentage 5.70%            
Note issued 12.0            
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member]              
Line of Credit Facility [Line Items]              
Credit facility, daily fluctuating rate of interest   0.10%          
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member] | 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 | Secured Overnight Financing Rate [Member] | Minimum [Member]              
Line of Credit Facility [Line Items]              
Debt instrument variable interest rate additional spread   1.125%