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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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