Allowance for Funds Used During Construction (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	$ 5.8	$ 4.1	$ 5.3
Public Utilities Allowance For Funds Used During Construction Additions	13.7	7.1	6.8
Public Utilities Allowance For Funds Used During Construction Capitalized Total	19.5	11.2	12.1
Public Utilities Allowance For Funds Used During Construction Rate	3.40%	2.70%	3.70%
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	1.9	2.2	2.5
Public Utilities Allowance For Funds Used During Construction Additions	2.9	2.9	1.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	4.8	5.1	4.4
Public Utilities Allowance For Funds Used During Construction Rate	3.40%	3.70%	3.60%
NSTAR Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	2.0	0.5	0.3
Public Utilities Allowance For Funds Used During Construction Additions	3.8
Public Utilities Allowance For Funds Used During Construction Capitalized Total	5.8	0.5	0.3
Public Utilities Allowance For Funds Used During Construction Rate	2.50%	0.50%	0.40%
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	0.6	0.5	1.6
Public Utilities Allowance For Funds Used During Construction Additions	0.6	0.2	1.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	1.2	0.7	3.5
Public Utilities Allowance For Funds Used During Construction Rate	1.80%	1.10%	5.90%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.9	0.5	0.5
Public Utilities Allowance For Funds Used During Construction Additions	1.7	1.0	1.0
Public Utilities Allowance For Funds Used During Construction Capitalized Total	$ 2.6	$ 1.5	$ 1.5
Public Utilities Allowance For Funds Used During Construction Rate	5.60%	6.10%	6.80%

Name:	nu_AllowanceForFundsUsedDuringConstructionLineItems
Namespace Prefix:	nu_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

+ Details

Name:	nu_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedTotal
Namespace Prefix:	nu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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 45

-Paragraph 1

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

Reference 2: 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

+ Details

Name:	us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount capitalized of allowance for funds used during construction.

+ References

Reference 1: 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 2: 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

Name:	us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

+ References

+ Details

Name:	us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=nu_NstarElectricCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=nu_PublicServiceCompanyOfNewHampshireMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=nu_TheConnecticutLightAndPowerCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=nu_WesternMassachusettsElectricCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: