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Debt and Financing Arrangements - Additional Information (Detail) - USD ($)
 3 Months Ended
Jul. 24, 2015
Oct. 04, 2013
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Oct. 30, 2015
Line of Credit Facility [Line Items]            
Weighted average interest rate on short term borrowings     1.70% 1.60% 1.50%  
Capital lease obligation, current fact already tagged     $ 3,100,000 $ 500,000 $ 3,100,000  
Capital lease obligation, noncurrent fact already tagged     10,400,000 8,200,000 11,000,000  
Accounts Payable     27,000,000 34,800,000 33,300,000  
Guarantee outstanding     18,600,000      
Northern Utilities Inc            
Line of Credit Facility [Line Items]            
Natural gas storage inventory     6,700,000 5,600,000 10,800,000  
Accounts Payable     $ 400,000 2,000,000 600,000  
Unitil Service Corp.            
Line of Credit Facility [Line Items]            
Capital lease obligation, total capitalized cost           $ 13,400,000
Capital lease obligation, maturity period     Sep. 30, 2020      
Capital lease obligation, current fact already tagged     $ 2,600,000      
Capital lease obligation, noncurrent fact already tagged     9,800,000      
Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Revolving credit facility     120,000,000 120,000,000 120,000,000  
Proceeds from lines of credit     55,900,000 70,800,000    
Repayments of lines of credit     $ 50,000,000 68,100,000    
Credit Facility | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Revolving credit facility expiration date   Oct. 04, 2018        
Revolving credit facility   $ 120,000,000        
Sublimit for the issuance of standby letters of credit   25,000,000        
Credit Facility by an aggregate additional amount   $ 30,000,000        
Percentage of capitalization     The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full (or with respect to letters of credit, they are cash collateralized). The only financial covenant in the Credit Facility provides that Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At March 31, 2016, March 31, 2015 and December 31, 2015, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. (See also “Credit Arrangements” in Note 5.)      
Credit Facility | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)            
Line of Credit Facility [Line Items]            
Credit facility, daily fluctuating rate of interest   1.375%        
Credit Facility | Amended Credit Facility            
Line of Credit Facility [Line Items]            
Revolving credit facility expiration date Oct. 04, 2020          
Credit Facility | Amended Credit Facility | London Interbank Offered Rate (LIBOR)            
Line of Credit Facility [Line Items]            
Credit facility, daily fluctuating rate of interest 1.25%          
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp            
Line of Credit Facility [Line Items]            
Senior Secured Notes     $ 900,000 1,500,000 1,100,000  
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc            
Line of Credit Facility [Line Items]            
Senior Notes     $ 10,000,000 $ 10,000,000 $ 10,000,000