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Summary of Significant Accounting Policies - Additional Information (Detail)
Bcf in Billions
 3 Months Ended 6 Months Ended
Jul. 25, 2018
USD ($)
Jun. 30, 2018
USD ($)
Bcf
Jun. 30, 2017
USD ($)
Bcf
Jun. 30, 2018
USD ($)
Subsidiary
mi
Bcf
Jun. 30, 2017
USD ($)
Bcf
Dec. 31, 2017
USD ($)
Bcf
Significant Accounting Policies [Line Items]            
Length Of Pipeline | mi       86    
Revenue sharing payments   $ 200,000 $ 300,000 $ 500,000 $ 500,000  
Other   83,400,000 79,400,000 227,900,000 203,700,000  
Operation and Maintenance Expense   17,800,000 16,500,000 35,100,000 32,500,000  
Cost of removal obligation   87,800,000 82,300,000 87,800,000 82,300,000 $ 84,300,000
Regulatory Assets   $ 131,900,000 $ 132,000,000 $ 131,900,000 $ 132,000,000 $ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf   0.0 1.2 0.0 1.2 0.6
Investments in trading securities   $ 5,200,000 $ 3,400,000 $ 5,200,000 $ 3,400,000 $ 3,600,000
Subsequent Event | Second Amended Credit Facility [Member]            
Significant Accounting Policies [Line Items]            
Borrowing limit $ 120,000,000          
Increase in borrowing limit $ 50,000,000          
ASU 2014-09            
Significant Accounting Policies [Line Items]            
Revenue sharing payments     300,000   500,000  
Operation and Maintenance Expense     16,200,000   32,000,000  
Non Service            
Significant Accounting Policies [Line Items]            
Non-service cost net periodic benefit costs   1,300,000 1,100,000 2,900,000 2,700,000  
Not Designated as Hedging Instruments            
Significant Accounting Policies [Line Items]            
Derivative Assets, not designated as hedging instruments   $ 0 100,000 $ 0 100,000 100,000
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%    
Maximum            
Significant Accounting Policies [Line Items]            
Cash equivalents maturity period   3 months        
Maximum | Subsequent Event | Second Amended Credit Facility [Member]            
Significant Accounting Policies [Line Items]            
Applicable margin rate 1.25%          
Minimum | Subsequent Event | Second Amended Credit Facility [Member]            
Significant Accounting Policies [Line Items]            
Applicable margin rate 1.125%          
ISO-NE Obligations            
Significant Accounting Policies [Line Items]            
Cash Deposits   $ 2,200,000 2,000,000 $ 2,200,000 2,000,000 2,900,000
Natural Gas Hedging Program            
Significant Accounting Policies [Line Items]            
Cash Deposits   0 0 0 0 0
Deferred Storm Charges            
Significant Accounting Policies [Line Items]            
Regulatory Assets   6,800,000 7,700,000 6,800,000 7,700,000 $ 7,200,000
Deferred Storm Charges | Recovered over Next Year            
Significant Accounting Policies [Line Items]            
Regulatory Assets   100,000   100,000    
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years            
Significant Accounting Policies [Line Items]            
Regulatory Assets   $ 6,300,000   $ 6,300,000    
Restatement Adjustment [Member] | ASU 2014-09            
Significant Accounting Policies [Line Items]            
Other     (300,000)   (500,000)  
Operation and Maintenance Expense     $ (300,000)   $ (500,000)  
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