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Environmental Matters - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2013
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
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs
  
Site Contingency [Line Items]  
Amortization period for environmental costs 7 years
Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs
  
Site Contingency [Line Items]  
Environmental obligations $ 12.0
Northern Utilities Inc
  
Site Contingency [Line Items]  
Environmental obligations 1.8
Northern Utilities Inc | Other Current Liabilities
  
Site Contingency [Line Items]  
Environmental obligations 1.1
Environmental Matters
  
Site Contingency [Line Items]  
Income (loss) incurred in excess of recorded amount $ 0