The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

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-Paragraph 147, 148, 149, 150

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Publisher AICPA

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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+ Details

Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Site Contingency Line Items
Environmental Site Quantity		33		33
Accrual For Environmental Loss Contingencies		$ 26.0		$ 30.3
Accrual For Environmental Loss Contingencies Component Amount		2.6
Cumulative Environmental Remediation Expense		19.5
Cumulative Payments Environmental Loss Contingencies		16.9
Environmental Remediation Expense	1.6		2.6
The Connecticut Light and Power Company [Member]
Site Contingency Line Items
Environmental Site Quantity		6		6
Accrual For Environmental Loss Contingencies		0.9		0.8
Public Service Company of New Hampshire [Member]
Site Contingency Line Items
Environmental Site Quantity		12		12
Accrual For Environmental Loss Contingencies		6.3		8.8
Western Massachusetts Electric Company [Member]
Site Contingency Line Items
Environmental Site Quantity		8		8
Accrual For Environmental Loss Contingencies		$ 0.2		$ 0.2