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Commitments and Contingencies - Additional Information (Detail) (USD $)
 12 Months Ended 1 Months Ended 1 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Other Restructuring
Dec. 31, 2013
New Hampshire
Environmental Restoration Costs
Dec. 31, 2013
Maine
Environmental Restoration Costs
Dec. 31, 2013
Environmental Matters
Dec. 27, 2013
Northern Utilities Inc
Apr. 30, 2013
Northern Utilities Inc
Dec. 31, 2013
Northern Utilities Inc
Dec. 31, 2013
Northern Utilities Inc
Other Current Liabilities
Apr. 30, 2013
Northern Utilities Inc
New Hampshire
Dec. 27, 2013
Northern Utilities Inc
Maine
Jul. 31, 2013
Fitchburg Gas and Electric Light Company
Oct. 31, 2012
Fitchburg Gas and Electric Light Company
May 01, 2012
Fitchburg Gas and Electric Light Company
Jul. 31, 2013
Fitchburg Gas and Electric Light Company
Storm Costs
Dec. 31, 2013
Fitchburg Gas and Electric Light Company
Environmental Clean Up Costs
Dec. 31, 2013
Fitchburg Gas and Electric Light Company
Environmental Restoration Costs
Oct. 31, 2012
Unitil Energy Systems Inc
Dec. 31, 2013
Unitil Energy Systems Inc
Apr. 26, 2011
Unitil Energy Systems Inc
Year One Policy
Apr. 26, 2011
Unitil Energy Systems Inc
Year Two Policy
Apr. 25, 2013
Unitil Energy Systems Inc
Storm Costs
Jun. 30, 2013
Granite State Gas Transmission Inc
Gas Transportation and Storage
Regulatory Assets [Line Items]                                                
Regulatory Assets, Remaining Balance $ 12,700,000                                              
Accrued Revenue 56,600,000 63,400,000 10,200,000                                          
Increase in annual revenue               2,500,000     5,200,000 3,800,000 6,700,000     2,100,000         5,200,000 5,000,000    
Amendment effective date               July 1, 2013       January 1, 2014                 July 1, 2010 May 1, 2011    
TIRA initial term             Four years                                  
Percentage of elligibilty for earnings             10.00%                                  
Percentage of earnings shared equally             11.00%                                  
Service quality plan penalties impossed             500,000                                  
Percent of increase in annual revenue                     9.40%   11.50%                      
Settlement agreement date                                       May 01, 2016        
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program                                       2,800,000        
Annual revenue increase in rate adjustments                                               400,000
Storm recovery adjustment factor amount                         2,800,000                      
Recovery of damage of electrical system                           1,100,000         2,700,000          
Construction expenditures                           300,000         400,000          
Storm expenditures                             4,300,000                  
Recovery of damage of electrical system                                             2,300,000  
Cost recovery period, years                                             5 years  
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments                                             4.52%  
Income (loss) incurred in excess of recorded amount           0                                    
Amortization period for environmental costs       7 years 5 years                         7 years            
Environmental obligations $ 14,800,000 $ 14,800,000             $ 1,800,000 $ 1,000,000             $ 12,000,000