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Regulatory Matters - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 30, 2016
Jun. 16, 2015
May. 01, 2015
Apr. 15, 2015
May. 30, 2014
Apr. 25, 2013
Mar. 31, 2016
Jun. 30, 2015
Dec. 31, 2014
Dec. 31, 2015
Mar. 31, 2015
Regulatory Assets [Line Items]                      
Current Portion of Regulatory Liabilities             $ 18.8     $ 15.6 $ 39.4
Regulatory Liabilities - non current             $ 3.3     $ 8.1  
Fitchburg Gas and Electric Light Company                      
Regulatory Assets [Line Items]                      
Amendment effective date             May 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]                      
Increase in annual revenue   $ 3.8                  
Percentage of approved return on equity   5.60%                  
Fitchburg Gas and Electric Light Company | Gas base                      
Regulatory Assets [Line Items]                      
Increase in annual revenue   $ 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 $ 6.0                    
Percent of increase in total revenue 4.00%                    
Unitil Energy Systems Inc | Storm Costs                      
Regulatory Assets [Line Items]                      
Cost recovery period, years           5 years          
Recovery amount           $ 2.3          
Northern Utilities Inc                      
Regulatory Assets [Line Items]                      
TIRA initial term             Four years        
Pipeline refund received       $ 22.0              
Current Portion of Regulatory Liabilities             $ 6.8        
Regulatory Liabilities - non current             $ 3.3        
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 | 2014 Step Adjustments                      
Regulatory Assets [Line Items]                      
Increase in annual revenue             $ 1.4        
Amendment effective date             May 01, 2014        
Northern Utilities Inc | New Hampshire                      
Regulatory Assets [Line Items]                      
Increase in annual revenue     $ 1.8                
Amendment effective date             May 01, 2015        
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]                      
Grace period to receive refund             3 years        
Date to receive a refund as one-time lump sum payment             Oct. 05, 2015        
Granite State Gas Transmission Inc | Gas Transportation and Storage                      
Regulatory Assets [Line Items]                      
Increase in annual revenue               $ 0.4      
Amendment effective date               Aug. 01, 2015