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Commitments and Contingencies, Leases (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating Leases [Abstract]      
Total expenses under operating lease obligations $ 1.3us-gaap_LeaseAndRentalExpense $ 1.4us-gaap_LeaseAndRentalExpense $ 1.1us-gaap_LeaseAndRentalExpense
Other [Member]      
Operating Leases, Future Minimum Payments Due [Abstract]      
2015 0.9us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
2016 0.9us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
2017 1.0us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
2018 1.0us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
2019 1.0us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
Thereafter 7.9us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember
   
Total $ 12.7us-gaap_OperatingLeasesFutureMinimumPaymentsDue
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nspw_OfficeSpaceAndOtherEquipmentMember