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Restricted Cash and Other Deposits (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Special Deposits [Line Items]    
Restricted Cash And Cash Equivalents $ 3.2us-gaap_RestrictedCashAndCashEquivalents $ 1.7us-gaap_RestrictedCashAndCashEquivalents
Cash Collateral Deposited 9.9nu_CashCollateralDeposited 17.9nu_CashCollateralDeposited
The Connecticut Light And Power Company [Member]    
Special Deposits [Line Items]    
Restricted Cash And Cash Equivalents 2.1us-gaap_RestrictedCashAndCashEquivalents
/ dei_LegalEntityAxis
= nu_TheConnecticutLightAndPowerCompanyMember		 1.4us-gaap_RestrictedCashAndCashEquivalents
/ dei_LegalEntityAxis
= nu_TheConnecticutLightAndPowerCompanyMember
Cash Collateral Deposited 1.2nu_CashCollateralDeposited
/ dei_LegalEntityAxis
= nu_TheConnecticutLightAndPowerCompanyMember		  
NSTAR Electric Company [Member]    
Special Deposits [Line Items]    
Cash Collateral Deposited   9.0nu_CashCollateralDeposited
/ dei_LegalEntityAxis
= nu_NstarElectricCompanyMember
Public Service Company Of New Hampshire [Member]    
Special Deposits [Line Items]    
Restricted Cash And Cash Equivalents 1.0us-gaap_RestrictedCashAndCashEquivalents
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember		  
Cash Collateral Deposited $ 2.5nu_CashCollateralDeposited
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember