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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified				9 Months Ended				9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2011 Fitchburg Gas and Electric Light Company Environmental Issue	Sep. 30, 2012 Fitchburg Gas and Electric Light Company Environmental Issue	Sep. 30, 2012 Northern Utilities Inc	Sep. 30, 2012 Northern Utilities Inc Other Current Liabilities	Sep. 30, 2012 Environmental Matters
Site Contingency [Line Items]
Income (loss) incurred in excess of recorded amount								$ 0
Accrued Liabilities					12.0
Settlement insurance				2.0
Amortization period for environmental costs					7 years
Accrued Liabilities	$ 14.5	$ 14.5	$ 14.4			$ 2.5	$ 0.3

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- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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