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Operating Expenses (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fuel [Line Items]      
Fuel Power Costs $ 599.4nu_FuelPowerCosts $ 466.5nu_FuelPowerCosts $ 346.8nu_FuelPowerCosts
Public Service Company Of New Hampshire [Member]      
Fuel [Line Items]      
Fuel Power Costs $ 113.4nu_FuelPowerCosts
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember		 $ 104.8nu_FuelPowerCosts
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember		 $ 103.4nu_FuelPowerCosts
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember