v3.26.1
Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 1 Months Ended 3 Months Ended 6 Months Ended
Oct. 31, 2025
USD ($)
Customer
Jan. 31, 2025
USD ($)
Dec. 31, 2024
Megawatts
Jun. 30, 2026
USD ($)
Jun. 30, 2026
USD ($)
Subsidiary
Customer
MW
mi
Jun. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
Significant Accounting Policies [Line Items]              
Length of Pipeline | mi         85    
Cost of removal obligation       $ 160.6 $ 160.6 $ 147.4 $ 153.0
Investments in trading securities       6.4 6.4 6.1 6.7
Goodwill       6.8 6.8 1.6 4.3
Increase in Accounts Receivable         (24.0) (12.8)  
Regulatory Liabilities         7.4 3.8  
Deductible portion of goodwill $ 1.9     $ 0.4 $ 0.4    
Amortization period 15 years     15 years 15 years    
Business Acquisition Liabilities Assumed       $ 30.0 $ 30.0    
Cash And Cash Equivalents Acquired         70.0    
Acquisition, Net of Cash Acquired         42.6 71.4  
Asset Acquisition Consideration Transferred         100.0    
Net Utility Plant       1,904.8 1,904.8 1,644.4 1,798.5
Allowance for doubtful accounts       2.1 2.1 1.9 2.2
Regulatory assets       103.9 103.9 99.2 116.9
Regulatory Liabilities       99.5 99.5 73.5 80.8
Profit Loss         $ 37.9 31.5  
Number of Customer | Customer         10,700    
Transition Services Agreement Cost         $ 0.6    
Acquired issued and outstanding shares         42.6    
Acquired working capital         62.0    
Acquisition of working capital       0.4 0.4    
Acquisition of goodwill         1.2    
Acquisition noncurrent assets       1.8 1.8    
Acquisition long-term debt       13.5 13.5    
Acquisition noncurrent liabilities       9.3 $ 9.3    
Cost of services margin         5.00%    
Electric And Gas Division [Member]              
Significant Accounting Policies [Line Items]              
Business Combination Price Of Acquisition Expected   $ 71.4          
Other Deferred Charges              
Significant Accounting Policies [Line Items]              
Regulatory assets       16.7 $ 16.7 14.7 12.8
Unbilled Revenues [member]              
Significant Accounting Policies [Line Items]              
Allowance for doubtful accounts       0.1 0.1 0.1 0.1
Accounts Receivable [member]              
Significant Accounting Policies [Line Items]              
Allowance for doubtful accounts       2.1 2.1 1.9 2.1
AWC-NH And Abenaki [Member]              
Significant Accounting Policies [Line Items]              
Goodwill       1.2 $ 1.2    
Gas And Electric Utilities [Member]              
Significant Accounting Policies [Line Items]              
Number of Subsidiaries | Entity | Subsidiary         5    
Unitil Service; Unitil Realty; and Unitil Resources              
Significant Accounting Policies [Line Items]              
Number of Subsidiaries | Entity | Subsidiary         4    
Fitchburg Gas And Electric Light Company [Member]              
Significant Accounting Policies [Line Items]              
Energy Storage Systems 1     5,000   1,268    
Mid Duration Storage | Megawatts     1,500        
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges | Electric And Gas Division [Member]              
Significant Accounting Policies [Line Items]              
Hardship accounts in regulatory assets       9.5 $ 9.5 8.3 8.9
Bangor Natural Gas              
Significant Accounting Policies [Line Items]              
Goodwill   1.6   1.6 1.6    
Net Utility Plant   66.7          
Working Capital   3.8          
Noncurrent assets   0.2          
Regulatory Liabilities   $ 0.9          
Maine Natural Gas Corporation              
Significant Accounting Policies [Line Items]              
Goodwill $ 4.0     4.0 4.0    
Increase in goodwill       1.3      
Acquisition, Net of Cash Acquired 91.1            
Net Utility Plant 81.9            
Working Capital 8.4            
Noncurrent assets 1.4            
Regulatory Liabilities $ 4.6            
Number of Customer | Customer 6,300            
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years              
Significant Accounting Policies [Line Items]              
Regulatory assets       8.3 8.3    
ISO-NE Obligations              
Significant Accounting Policies [Line Items]              
Cash Deposits       $ 6.0 $ 6.0 6.0 8.5
Maximum              
Significant Accounting Policies [Line Items]              
Lease term       12 months 12 months    
Deferred Compensation Plan [Member]              
Significant Accounting Policies [Line Items]              
Investments in trading securities       $ 4.5 $ 4.5 $ 2.9 $ 3.5