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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2020
USD ($)
Subsidiary
mi
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Significant Accounting Policies [Line Items]      
Length Of Pipeline | mi 86    
Gain on divestiture of business, pretax   $ 13.4  
Gain on divestiture of business, net $ 9.8 9.8  
Provision for income taxes   3.6  
Cost of removal obligation 98.8 93.7 $ 96.0
Regulatory assets 133.7 127.3 147.8
Investments in trading securities 5.5 5.1 5.6
Allowance for doubtful accounts 1.8 1.7 1.0
Unbilled Revenues      
Significant Accounting Policies [Line Items]      
Allowance for doubtful accounts 0.1    
Accounts Receivable      
Significant Accounting Policies [Line Items]      
Allowance for doubtful accounts $ 1.7 1.7 1.0
Utilities      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary 3    
Unitil Service; Unitil Realty; and Unitil Resources      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary 3    
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years      
Significant Accounting Policies [Line Items]      
Regulatory assets $ 7.5    
ISO-NE Obligations      
Significant Accounting Policies [Line Items]      
Cash Deposits $ 2.4 3.0 1.9
Lease term 12 months    
Deferred Compensation Plan [Member]      
Significant Accounting Policies [Line Items]      
Investments in trading securities $ 0.4 $ 0.1 $ 0.2