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Fuel, Materials and Supplies and Allowance Inventory (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
fuelcosts $ 164.3us-gaap_FuelCosts $ 139.5us-gaap_FuelCosts
Public Service Company Of New Hampshire [Member]    
fuelcosts 95.1us-gaap_FuelCosts
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember		 74.2us-gaap_FuelCosts
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember
Public Service Company Of New Hampshire [Member] | Emission Allowances [Member]    
Other Inventory, Noncurrent $ 20.1us-gaap_OtherInventoryNoncurrent
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember
/ nu_TypeOfInventoryByMajorAssetClassAxis
= nu_EmissionAllowancesMember		 $ 19.4us-gaap_OtherInventoryNoncurrent
/ dei_LegalEntityAxis
= nu_PublicServiceCompanyOfNewHampshireMember
/ nu_TypeOfInventoryByMajorAssetClassAxis
= nu_EmissionAllowancesMember