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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified				9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs	Sep. 30, 2014 Fitchburg Gas and Electric Light Company Environmental Restoration Costs	Sep. 30, 2014 New Hampshire Environmental Restoration Costs	Sep. 30, 2014 Maine Environmental Restoration Costs	Sep. 30, 2014 Environmental Matters	Sep. 30, 2014 Revised Estimate Fitchburg Gas and Electric Light Company Environmental Clean Up Costs
Site Contingency [Line Items]
Income (loss) incurred in excess of recorded amount								$ 0
Amortization period for environmental costs					7 years	7 years	5 years
Environmental obligations	7.3	14.8	14.9	12.0					5.5
Environmental obligations, spent during the year				$ 1.4