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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified				6 Months Ended				6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2011 Fitchburg Gas and Electric Light Company Environmental Issue	Jun. 30, 2012 Fitchburg Gas and Electric Light Company Environmental Issue	Jun. 30, 2012 Northern Utilities Inc	Jun. 30, 2012 Northern Utilities Inc Other Current Liabilities	Jun. 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	$ 14.5			$ 2.5	$ 0.3

X

- Definition

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

+ References

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-Publisher FASB

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Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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