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Long-Term Debt, Credit Arrangements, Leases and Guarantees - Additional Information (Detail) (USD $)
 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Revolving Facility
Jul. 24, 2012
Revolving Facility
Dec. 31, 2011
Revolving Facility
Dec. 31, 2012
Bonds
Dec. 31, 2011
Bonds
Dec. 31, 2010
Bonds
Dec. 31, 2012
Northern Utilities Inc
Dec. 31, 2011
Northern Utilities Inc
Dec. 31, 2012
Fitchburg Gas and Electric Light Company
Dec. 31, 2012
Granite State Gas Transmission Inc
Dec. 31, 2012
Maximum
Unitil Corporation
Dec. 31, 2012
Unitil Realty Corp
Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
Dec. 31, 2012
Granite State Gas Transmission Inc
Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
Line of Credit Facility [Line Items]                                
Total funded indebtedness as percentage of capitalization                   65.00%   65.00% 65.00% 70.00%    
Long term debt repayments $ 500,000 $ 500,000 $ 500,000       $ 500,405 $ 462,055 $ 426,643              
2013             541,938                  
2014             2,486,919                  
2015             4,035,633                  
2016             17,421,724                  
2017             17,160,985                  
Revolving credit facility expiration date       Oct. 08, 2013                        
Revolving credit facility       60,000,000 115,000,000 115,000,000                    
Reduced Borrowing Limit         60,000,000                      
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 December 31, 2012 and December 31, 2011, the Company was in compliance with the financial covenants contained in the revolving credit agreement.                        
Weighted average interest rate on short term borrowings 2.00% 2.20% 2.30%                          
Outstanding Inventory                   10,700,000 14,900,000          
Accounts Payable 30,900,000 26,400,000               2,100,000 2,500,000          
Total rental expense under operating leases 1,300,000 1,400,000 1,000,000                          
Guarantee outstanding 12,300,000                              
Guarantee end date Feb. 28, 2014                              
Principal amount of notes outstanding                             $ 2,800,000 $ 10,000,000