Allowance for Funds Used During Construction (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	$ 5.3	$ 11.8	$ 10.2
Public Utilities Allowance For Funds Used During Construction Additions	6.8	22.5	16.7
Public Utilities Allowance For Funds Used During Construction Capitalized Total	12.1	34.3	26.9
Public Utilities Allowance For Funds Used During Construction Rate	3.70%	7.30%	7.10%
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	2.5	3.3	2.7
Public Utilities Allowance For Funds Used During Construction Additions	1.9	6.0	4.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	4.4	9.3	7.6
Public Utilities Allowance For Funds Used During Construction Rate	3.60%	8.30%	8.30%
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	1.6	7.1	6.6
Public Utilities Allowance For Funds Used During Construction Additions	1.9	13.2	10.4
Public Utilities Allowance For Funds Used During Construction Capitalized Total	3.5	20.3	17.0
Public Utilities Allowance For Funds Used During Construction Rate	5.90%	7.10%	6.80%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.5	0.5	0.3
Public Utilities Allowance For Funds Used During Construction Additions	1.0	1.0	0.6
Public Utilities Allowance For Funds Used During Construction Capitalized Total	1.5	1.5	0.9
Public Utilities Allowance For Funds Used During Construction Rate	6.80%	7.40%	6.40%
NSTAR Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.3	0.2	0.1
Public Utilities Allowance For Funds Used During Construction Capitalized Total	$ 0.3	$ 0.2	$ 0.1
Public Utilities Allowance For Funds Used During Construction Rate	0.40%	0.30%	0.30%

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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

+ References

No definition available.

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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 Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 15

-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 Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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- Definition

Amount capitalized of allowance for funds used during construction.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 15

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

+ Details

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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.

+ References

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