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Environmental Matters - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 6 Months Ended
Jun. 30, 2014
Dec. 31, 2013
Jun. 30, 2013
Site Contingency [Line Items]      
Environmental obligations, Current $ 6.4 $ 1.0 $ 1.0
Environmental obligations, Non-current 2.0 13.8 13.8
Northern Utilities Inc
      
Site Contingency [Line Items]      
Environmental obligations, Current 1.2 1.0 1.0
Environmental obligations, Non-current 2.0 1.8 1.8
Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs
      
Site Contingency [Line Items]      
Environmental obligations, Current 5.2 0 0
Environmental obligations, Non-current 0 12.0 12.0
Environmental obligations 12.0    
Environmental obligations, spent during the year 0.3    
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs
      
Site Contingency [Line Items]      
Amortization period for environmental costs 7 years    
New Hampshire | Environmental Restoration Costs
      
Site Contingency [Line Items]      
Amortization period for environmental costs 7 years    
Maine | Environmental Restoration Costs
      
Site Contingency [Line Items]      
Amortization period for environmental costs 5 years    
Environmental Matters
      
Site Contingency [Line Items]      
Income (loss) incurred in excess of recorded amount 0    
Revised Estimate | Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs
      
Site Contingency [Line Items]      
Environmental obligations $ 5.5