Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Site Contingency Line Items
Environmental Site Quantity	33	33
Accrual For Environmental Loss Contingencies	$ 28.4	$ 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
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	8.2	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

The cumulative to-date charge against earnings for known or estimated future HWP costs arising from requirements to perform environmental remediation activities.

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Namespace Prefix:	nu
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Balance Type:	debit
Period Type:	duration

- Definition

Cumulative to-date payments made to settle HWP environmental loss contingencies.

+ Details

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Namespace Prefix:	nu
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of environmental sites by NU Entity in the remediation or long-term monitoring phase

+ Details

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Namespace Prefix:	nu
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Statement of Position (SOP)
-Number 96-1
-Paragraph 161
-Subparagraph a

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 9

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Recorded amount of the accrual for a material component of an environmental loss contingency.

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Statement of Position (SOP)
-Number 96-1
-Paragraph 147, 148, 149, 150

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 143
-Paragraph 2

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 90-8

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Statement of Position (SOP)
-Number 96-1
-Paragraph 170

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 9, 10

+ Details

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Balance Type:	debit
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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