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Regulatory Matters - Additional Information (Detail)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 28, 2016
USD ($)
Jun. 24, 2016
USD ($)
Apr. 29, 2016
USD ($)
Jun. 16, 2015
USD ($)
May 01, 2015
USD ($)
Apr. 15, 2015
USD ($)
May 30, 2014
USD ($)
Apr. 25, 2013
USD ($)
Jun. 30, 2016
USD ($)
Customer
Dec. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
Jun. 30, 2015
USD ($)
Regulatory Assets [Line Items]                        
Current Portion of Regulatory Liabilities                 $ 15.9   $ 15.6 $ 31.2
Regulatory Liabilities - non current                 $ 3.7   $ 8.1 $ 11.0
Northern Utilities Inc                        
Regulatory Assets [Line Items]                        
TIRA initial term                 Four years      
Amendment approved date                 Apr. 28, 2016      
Pipeline refund received           $ 22.0            
Current Portion of Regulatory Liabilities                 $ 5.0      
Regulatory Liabilities - non current                 $ 3.6      
Northern Utilities Inc | 2014 Step Adjustments                        
Regulatory Assets [Line Items]                        
Amendment effective date                 May 01, 2014      
Increase in annual revenue                 $ 1.4      
Northern Utilities Inc | First Annual Adjustment                        
Regulatory Assets [Line Items]                        
Annual TIRA Adjustment                   $ 1.5    
Annual TIRA Adjustment, effective date                 May 01, 2016      
Northern Utilities Inc | New Hampshire                        
Regulatory Assets [Line Items]                        
Amendment effective date                 May 01, 2015      
Increase in annual revenue         $ 1.8              
Grace period to receive refund                 3 years      
Northern Utilities Inc | New Hampshire | Natural Gas Distribution                        
Regulatory Assets [Line Items]                        
Increase in annual revenue                 $ 4.6      
Northern Utilities Inc | Maine                        
Regulatory Assets [Line Items]                        
Construction completion period                 3 years      
Number of new customers | Customer                 1,000      
Potential regulated operating revenue                 $ 1.0      
Grace period to receive refund                 3 years      
Date to receive a refund as one-time lump sum payment                 Oct. 05, 2015      
Fitchburg Gas and Electric Light Company                        
Regulatory Assets [Line Items]                        
Amendment effective date     May 01, 2016                  
Revenue requirement reconciled rates effective date     Nov. 01, 2016                  
Fitchburg Gas and Electric Light Company | Cumulative capital investments for 2015 and 2016                        
Regulatory Assets [Line Items]                        
Recovery amount     $ 0.9                  
Fitchburg Gas and Electric Light Company | Electric base                        
Regulatory Assets [Line Items]                        
Amendment effective date     May 01, 2016                  
Increase in annual revenue     $ 2.1 $ 3.8                
Percentage of approved return on equity       5.60%                
Fitchburg Gas and Electric Light Company | Gas base                        
Regulatory Assets [Line Items]                        
Amendment effective date     May 01, 2016                  
Increase in annual revenue     $ 1.6 $ 3.0                
Percentage of approved return on equity       8.30%                
Fitchburg Gas and Electric Light Company | Storm Costs                        
Regulatory Assets [Line Items]                        
Storm expenditures             $ 5.0          
Cost recovery period, years             3 years          
Unitil Energy Systems Inc                        
Regulatory Assets [Line Items]                        
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments               4.52%        
Recovery of damage of electrical system               $ 1.7        
Unitil Energy Systems Inc | New Hampshire                        
Regulatory Assets [Line Items]                        
Increase in base rates, amount     $ 6.3                  
Percent of increase in total revenue     3.60%                  
Temporary rate increase $ 2.4                      
Amendment effective date                 Jul. 01, 2016      
Unitil Energy Systems Inc | Storm Costs                        
Regulatory Assets [Line Items]                        
Recovery amount               $ 2.3        
Cost recovery period, years               5 years        
Granite State Gas Transmission Inc | Gas Transportation and Storage                        
Regulatory Assets [Line Items]                        
Amendment effective date   Aug. 01, 2016                    
Increase in annual revenue   $ 0.3                    
Amendment approved date   Jul. 13, 2017