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Commitments and Contingencies, Guarantees (Details) (Payment or Performance Guarantee, Customer Loans for Farm Rewiring Program, USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Payment or Performance Guarantee | Customer Loans for Farm Rewiring Program
  
Guarantee [Abstract]  
Assets held as collateral $ 0nspw_AssetsHeldAsCollateralForGuarantorObligations
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember
Guarantees issued and outstanding 1.0us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember		 [1]
Current exposure under guarantees $ 0.2nspw_GuaranteeCurrentExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_LoansPayableMember		 [1]
Guarantee Expiration Date (year) 2018 [1]
[1] The debtor becomes the subject of bankruptcy or other insolvency proceedings.