Allowance for Funds Used During Construction (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	$ 3.3	$ 2.3	$ 6.5	$ 4.2
Public Utilities Allowance For Funds Used During Construction Additions	6.3	3.8	11.9	6.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	9.6	6.1	18.4	11.1
Public Utilities Allowance For Funds Used During Construction Rate	7.80%	7.00%	7.40%	6.80%
The Connecticut Light and Power Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.7	0.7	1.5	1.4
Public Utilities Allowance For Funds Used During Construction Additions	1.2	1.2	2.7	2.5
Public Utilities Allowance For Funds Used During Construction Capitalized Total	1.9	1.9	4.2	3.9
Public Utilities Allowance For Funds Used During Construction Rate	7.90%	8.00%	8.00%	8.00%
Public Service Company of New Hampshire [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	2.3	1.5	4.4	2.6
Public Utilities Allowance For Funds Used During Construction Additions	4.4	2.4	7.8	4.2
Public Utilities Allowance For Funds Used During Construction Capitalized Total	6.7	3.9	12.2	6.8
Public Utilities Allowance For Funds Used During Construction Rate	7.70%	6.70%	7.30%	6.50%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.1	0.1	0.1	0.1
Public Utilities Allowance For Funds Used During Construction Additions	0.1	0.2	0.2	0.2
Public Utilities Allowance For Funds Used During Construction Capitalized Total	$ 0.2	$ 0.3	$ 0.3	$ 0.3
Public Utilities Allowance For Funds Used During Construction Rate	6.80%	6.70%	7.00%	4.10%

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Total value of the public utilities allowance for funds used during construction capitalized portions, including both the equity and debt portions

+ Details

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- Definition

The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 71
-Paragraph 15

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- Definition

The component of the allowance for funds used during construction during the period comprised of interest on borrowed funds used in financing the construction of regulated assets, which may be reflected as a reduction of interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 71
-Paragraph 15

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- Definition

Disclosure of the effective rate for the period representing (a) total additions to qualifying regulated assets for allowance for funds used during construction, (b) divided by the sum of debt and equity funds used for construction.

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