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Environmental Obligations Recognized by Company (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Site Contingency [Line Items]
Beginning Balance	$ 5.5	$ 14.8
Additions	1.9	6.6
Less: Payments / Reductions	1.9	14.1
Ending Balance	5.5	7.3
Less: Current Portion	5.0	5.3	$ 3.5
Ending Balance	0.5	2.0	$ 2.0
Fitchburg Gas and Electric Light Company
Site Contingency [Line Items]
Beginning Balance	1.9	12.0
Additions		5.5
Less: Payments / Reductions	0.3	13.4
Ending Balance	1.6	4.1
Less: Current Portion	1.6	4.1
Northern Utilities Inc
Site Contingency [Line Items]
Beginning Balance	3.6	2.8
Additions	1.9	1.1
Less: Payments / Reductions	1.6	0.7
Ending Balance	3.9	3.2
Less: Current Portion	3.4	1.2
Ending Balance	$ 0.5	$ 2.0

X

- Definition

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

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The aggregate amount disbursed during the reporting period to settle environmental loss contingencies.

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The amount added to the accrual for newly incurred environmental loss contingencies. These additions include remediation expense and interest accretion. These additions do not relate to changes in the environmental accrual related to a business acquisition or divestiture, to currency movements, or revisions in previously existing estimates.

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Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer.

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