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Commitments and Contingencies, Guarantees and Indemnifications (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Guarantor Obligations [Line Items]    
Assets Held As Collateral $ 0nspw_AssetsHeldAsCollateralForGuarantorObligations $ 0nspw_AssetsHeldAsCollateralForGuarantorObligations
Payment or Performance Guarantee | Customer Loans for Farm Rewiring Program    
Guarantees [Abstract]    
Guarantees issued and outstanding 1,000,000us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= nspw_PaymentOrPerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember		 1,000,000us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= nspw_PaymentOrPerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember
Current exposure under these guarantees $ 100,000nspw_GuaranteeCurrentExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= nspw_PaymentOrPerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember		 $ 200,000nspw_GuaranteeCurrentExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= nspw_PaymentOrPerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember