XML 52 R43.htm IDEA: XBRL DOCUMENT v3.24.3
Debt and Financing Arrangements - Additional Information (Detail) - USD ($)
shares in Millions, $ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 21, 2024
Jul. 06, 2023
Sep. 29, 2022
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Line of Credit Facility [Line Items]                
Weighted average interest rate on short term borrowings       6.60% 6.40% 6.60% 6.20% 6.40%
Capital lease obligation, current       $ 0.1 $ 0.1 $ 0.1 $ 0.1 $ 0.1
Capital lease obligation, noncurrent       0.3 0.4 0.3 0.4 0.4
Accounts Payable       36.2 35.6 36.2 35.6 47.7
Total rental expense under operating leases       0.5 0.6 1.6 1.6  
Net Utility Plant       1,502.4 1,390.1 1,502.4 1,390.1 1,420.9
Operating lease obligations           1.6 1.6  
Other current operating lease obligation       1.6 1.8 1.6 1.8 1.9
Other noncurrent operating lease obligation       $ 3.0 $ 3.7 $ 3.0 $ 3.7 3.7
Operating lease, weighted average remaining lease term       3 years 6 months 3 years 8 months 12 days 3 years 6 months 3 years 8 months 12 days  
Operating lease, weighted average discount rate percentage       4.60% 4.40% 4.60% 4.40%  
Repayments of debt           $ 1.0    
Assets under Capital Leases [Member]                
Line of Credit Facility [Line Items]                
Net Utility Plant       $ 0.7 $ 0.7 0.7 $ 0.7 0.7
Net Utility Plant, accumulated amortization       0.3 0.2 0.3 0.2 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           229.9    
Repayments of lines of credit           327.6    
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       $ 8.8   $ 8.8    
Notes due date   Jul. 02, 2053            
Cost of issue           $ 0.2    
Issued note   13.0            
Note issue percentage 5.99% 5.96%            
Note issued 12.5              
Credit Facility | Revolving Credit Facility | Fitchburg                
Line of Credit Facility [Line Items]                
Notes due date   Jul. 02, 2033            
Note issue percentage 5.54% 5.70%            
Note issued 12.5 12.0            
Credit Facility | Revolving Credit Facility | Northern Utilities Inc                
Line of Credit Facility [Line Items]                
Note issue percentage 5.54%              
Note issued 25.0              
Credit Facility | Revolving Credit Facility | Northern Utilities Inc One                
Line of Credit Facility [Line Items]                
Note issue percentage 5.74%              
Note issued 15.0              
Credit Facility | Revolving Credit Facility | Unitil Corporation                
Line of Credit Facility [Line Items]                
Note issue percentage 5.99%              
Note issued 20.0              
Credit Facility | Revolving Credit Facility | Unitil Energy                
Line of Credit Facility [Line Items]                
Note issue percentage 0.69%              
Note issued 40.0              
Credit Facility | Revolving Credit Facility | Granite State                
Line of Credit Facility [Line Items]                
Note issue percentage 5.74%              
Note issued 10.0              
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%