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Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified											3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Mar. 31, 2014 Northern Utilities Inc	Dec. 31, 2013 Northern Utilities Inc	Mar. 31, 2014 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs	Dec. 31, 2013 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs	Mar. 31, 2014 Fitchburg Gas and Electric Light Company Environmental Restoration Costs	Mar. 31, 2014 New Hampshire Environmental Restoration Costs	Mar. 31, 2014 Maine Environmental Restoration Costs	Mar. 31, 2014 Environmental Matters
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, Current	1.4	1.0	1.0	0.4	1.0	1.0	0
Environmental obligations, Non-current	$ 12.8	$ 13.8	$ 13.8	$ 1.8	$ 1.8	$ 11.0	$ 12.0

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.

+ References+ Details

- Definition

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

+ References+ Details