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Summary of Significant Accounting Policies - Additional Information (Detail)
Bcf in Billions
 3 Months Ended
Dec. 31, 2015
USD ($)
Bcf
Mar. 31, 2015
USD ($)
Bcf
Mar. 31, 2016
USD ($)
Subsidiary
mi
Bcf
Significant Accounting Policies [Line Items]      
Length Of Pipeline | mi     86
Cost of Removal Obligations $ 70,100,000 $ 65,700,000 $ 72,300,000
Regulatory Assets $ 126,400,000 $ 139,900,000 $ 131,900,000
Number of Natural Gas Storage Outstanding | Bcf 2.5 2.0 1.8
Investments in trading securities $ 700,000 $ 0 $ 700,000
Reduction of other assets (2,900,000) (3,100,000)  
Reduction of long-term debt (2,900,000) (3,100,000)  
Maximum      
Significant Accounting Policies [Line Items]      
Cash equivalents maturity period     3 months
ISO-NE Obligations      
Significant Accounting Policies [Line Items]      
Cash Deposits 2,300,000 1,500,000 $ 2,100,000
Natural Gas Hedging Program      
Significant Accounting Policies [Line Items]      
Cash Deposits 0 0 0
Not Designated as Hedging Instruments | Maximum      
Significant Accounting Policies [Line Items]      
Derivative Assets, not designated as hedging instruments   100,000 100,000
Deferred Storm Charges      
Significant Accounting Policies [Line Items]      
Regulatory Assets $ 15,400,000 $ 20,100,000 13,800,000
Deferred Storm Charges | Recovered over the next two and a half years      
Significant Accounting Policies [Line Items]      
Regulatory Assets     5,800,000
Environmental Costs Rate Case Costs And Other Expenditures | Recovered over the next seven and a half years      
Significant Accounting Policies [Line Items]      
Regulatory Assets     $ 8,300,000
Unitil Service; Unitil Realty; and Unitil Resources      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary     3
Utilities      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary     3