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Regulatory Matters - Additional Information (Detail)
$ in Millions
 9 Months Ended 12 Months Ended
Aug. 19, 2016
MW
Jun. 28, 2016
USD ($)
Jun. 24, 2016
USD ($)
Apr. 29, 2016
USD ($)
Jun. 16, 2015
USD ($)
May 01, 2015
USD ($)
Apr. 15, 2015
USD ($)
Sep. 30, 2016
USD ($)
State
Customer
Dec. 31, 2014
USD ($)
Jun. 30, 2027
MW
Dec. 31, 2022
MWh
Jun. 30, 2017
MW
Dec. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Regulatory Assets [Line Items]                            
Current Portion of Regulatory Liabilities               $ 13.2         $ 15.6 $ 28.1
Regulatory Liabilities - non current               $ 3.7         $ 8.1 $ 11.0
Scenario Forecast                            
Regulatory Assets [Line Items]                            
Power generation capacity | MWh                     9,450,000      
Fitchburg-Solar Generation                            
Regulatory Assets [Line Items]                            
Power generation facility | MW 1.3                          
Expected completion date of facility 2017-11                          
Offshore Wind Energy | Scenario Forecast                            
Regulatory Assets [Line Items]                            
Power generation facility | MW                   1,600   400    
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                    
Number of states with electric distribution companies | State               3            
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%                  
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               $ 3.5            
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]                            
Amendment effective date               May 01, 2014            
Increase in annual revenue               $ 1.4            
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            
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            
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                      
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