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Long Term Debt, Credit Arrangements and Guarantees - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 1 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Sep. 30, 2013
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
Unitil Realty Corp
Dec. 31, 2012
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
Unitil Realty Corp
Sep. 30, 2012
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
Unitil Realty Corp
Sep. 30, 2013
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
Debt Outstanding, Principal Amount
Unitil Realty Corp
Sep. 30, 2013
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
Granite State Gas Transmission Inc
Dec. 31, 2012
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
Granite State Gas Transmission Inc
Sep. 30, 2012
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
Granite State Gas Transmission Inc
Sep. 30, 2013
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
Debt Outstanding, Principal Amount
Granite State Gas Transmission Inc
Sep. 30, 2013
Revolving Credit Facility
Dec. 31, 2012
Revolving Credit Facility
Sep. 30, 2012
Revolving Credit Facility
Oct. 04, 2013
Revolving Credit Facility
Subsequent Event
Oct. 04, 2013
Revolving Credit Facility
Subsequent Event
London Interbank Offered Rate
Sep. 30, 2013
Northern Utilities Inc
Dec. 31, 2012
Northern Utilities Inc
Sep. 30, 2012
Northern Utilities Inc
Line of Credit Facility [Line Items]                                      
Revolving credit facility expiration date                             Oct. 04, 2018        
Revolving credit facility                       $ 60.0 [1] $ 60.0 $ 60.0 $ 120.0        
Sublimit for the issuance of standby letters of credit                             25        
Credit facility, daily fluctuating rate of interest                               1.375%      
Credit Facility by an aggregate additional amount                             30        
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 September 30, 2013, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.              
Outstanding inventory                                 11.8 10.7 10.6
Accounts Payable 21.5 32.7 19.2                           0 2.1 0
Guarantee outstanding 18.5                                    
Guarantee end date Oct. 31, 2014                                    
Senior Secured Notes       2.5 2.8 3.0 2.5                        
Senior Notes               $ 10.0 $ 10.0 $ 10.0 $ 10.0                
[1] Effective October 4, 2013, the Credit Facility borrowing limit was increased to $120 million.