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Long Term Debt, Credit Arrangements and Guarantees - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Line of Credit Facility [Line Items]      
Estimated Fair Value of Long-Term Debt $ 344    
Accounts Payable 25.8 30.9 18.7
Guarantee outstanding 13.5    
Guarantee end date Feb. 28, 2014    
Northern Utilities Inc
      
Line of Credit Facility [Line Items]      
Outstanding Inventory 3.3 10.7 6.3
Accounts Payable 1.7 2.1 0.3
Revolving Facility
      
Line of Credit Facility [Line Items]      
Revolving credit facility 60.0 60.0 115.0
Percentage of capitalization The revolving credit agreement also contains a covenant restricting the Company's ability to permit funded debt to exceed 65% of capitalization at the end of each fiscal quarter. As of March 31, 2013, March 31, 2012 and December 31, 2012, the Company was in compliance with the financial covenants contained in the revolving credit agreement.    
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen | Unitil Realty Corp
      
Line of Credit Facility [Line Items]      
Senior Secured Notes 2.7 2.8 3.2
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen | Debt Outstanding, Principal Amount | Unitil Realty Corp
      
Line of Credit Facility [Line Items]      
Senior Secured Notes 2.7    
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen | Granite State Gas Transmission Inc
      
Line of Credit Facility [Line Items]      
Senior Notes 10.0 10.0 10.0
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen | Debt Outstanding, Principal Amount | Granite State Gas Transmission Inc
      
Line of Credit Facility [Line Items]      
Senior Notes 10.0    
Unsecured Revolving Credit Facility
      
Line of Credit Facility [Line Items]      
Revolving credit facility expiration date Oct. 08, 2013    
Revolving credit facility $ 60.0