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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
mi
Sep. 30, 2019
USD ($)
Subsidiary
Jan. 01, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
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      
Provision for income taxes   3.6      
Cost of removal obligation $ 98.4 98.4   $ 90.7 $ 91.2
Recognition of lease assets     $ 4.2    
Recognition of lease liabilities     $ 4.2    
Regulatory assets 123.6 123.6   140.3 138.0
Investments in trading securities 5.9 $ 5.9   4.8 5.3
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      
Annual Electric Sales Volume          
Significant Accounting Policies [Line Items]          
Percentage of total sales volumes revenue subject to RDM   27.00%      
Annual Natural Gas Sales Volume          
Significant Accounting Policies [Line Items]          
Percentage of total sales volumes revenue subject to RDM   11.00%      
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years          
Significant Accounting Policies [Line Items]          
Regulatory assets 5.8 $ 5.8      
ISO-NE Obligations          
Significant Accounting Policies [Line Items]          
Cash Deposits $ 3.5 $ 3.5   3.5 3.2
Maximum          
Significant Accounting Policies [Line Items]          
Cash equivalents maturity period   3 months      
Lease term 12 months 12 months      
Deferred Compensation Plan [Member]          
Significant Accounting Policies [Line Items]          
Investments in trading securities $ 0.1 $ 0.1   $ 0.0 $ 0.0