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Environmental Matters - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Site Contingency [Line Items]
Environmental obligations, spent during the year	$ 200,000	$ 1,600,000
Maine \| Environmental Restoration Costs
Site Contingency [Line Items]
Amortization period for environmental costs	5 years
New Hampshire \| Environmental Restoration Costs
Site Contingency [Line Items]
Amortization period for environmental costs	7 years
Environmental Matters
Site Contingency [Line Items]
Income (loss) incurred in excess of recorded amount	0
Fitchburg Gas and Electric Light Company
Site Contingency [Line Items]
Environmental obligations, spent during the year	100,000	1,000,000
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	12,000,000
Environmental obligations, spent during the year	3,700,000
Fitchburg Gas and Electric Light Company \| Revised Estimate \| Environmental Clean Up Costs
Site Contingency [Line Items]
Environmental obligations	$ 5,500,000