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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2022
USD ($)
Subsidiary
mi
Dec. 31, 2021
USD ($)
Mar. 31, 2021
USD ($)
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Entity | Subsidiary 3    
Length Of Pipeline | mi 86    
Cost of removal obligation $ 109.8 $ 107.5 $ 102.5
Investments in trading securities 5.5 5.7 5.6
Allowance for doubtful accounts 3.4 3.3 4.4
Regulatory assets 153.3 156.3 152.7
Other Deferred Charges [Member]      
Significant Accounting Policies [Line Items]      
Regulatory assets 15.1 15.3 16.1
Unbilled Revenues [member]      
Significant Accounting Policies [Line Items]      
Allowance for doubtful accounts 0.1 0.2 0.1
Accounts Receivable [member]      
Significant Accounting Policies [Line Items]      
Allowance for doubtful accounts $ 3.3 3.1 4.3
Utilities      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Entity | Subsidiary 3    
Unitil Service; Unitil Realty; and Unitil Resources      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Entity | Subsidiary 3    
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges [Member] | Electric And Gas Division [Member]      
Significant Accounting Policies [Line Items]      
Hardship accounts in regulatory assets $ 7.9 7.9 8.0
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years      
Significant Accounting Policies [Line Items]      
Regulatory assets 7.8    
ISO-NE Obligations      
Significant Accounting Policies [Line Items]      
Cash Deposits $ 2.9 2.7 2.2
Maximum      
Significant Accounting Policies [Line Items]      
Lease term 12 months    
Deferred Compensation Plan [Member]      
Significant Accounting Policies [Line Items]      
Investments in trading securities $ 0.8 $ 0.6 $ 0.6