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Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) (Pension Plans, USD $)
12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31 $ 153,590,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation $ 153,894,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation    
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 163,930,000us-gaap_DefinedBenefitPlanBenefitObligation 179,995,000us-gaap_DefinedBenefitPlanBenefitObligation    
Service cost 4,527,000us-gaap_DefinedBenefitPlanServiceCost 5,682,000us-gaap_DefinedBenefitPlanServiceCost 4,568,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 7,257,000us-gaap_DefinedBenefitPlanInterestCost 6,924,000us-gaap_DefinedBenefitPlanInterestCost 7,765,000us-gaap_DefinedBenefitPlanInterestCost  
Plan amendments 0us-gaap_DefinedBenefitPlanPlanAmendments (1,109,000)us-gaap_DefinedBenefitPlanPlanAmendments    
Actuarial (gain) loss 9,126,000us-gaap_DefinedBenefitPlanActuarialGainLoss (11,097,000)us-gaap_DefinedBenefitPlanActuarialGainLoss    
Benefit payments (19,171,000)us-gaap_DefinedBenefitPlanBenefitsPaid (16,465,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Obligation at Dec. 31 165,669,000us-gaap_DefinedBenefitPlanBenefitObligation 163,930,000us-gaap_DefinedBenefitPlanBenefitObligation 179,995,000us-gaap_DefinedBenefitPlanBenefitObligation  
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 136,935,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 136,546,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets    
Actual return (loss) on plan assets 6,916,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 5,525,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets    
Employer contributions 8,033,000us-gaap_DefinedBenefitPlanContributionsByEmployer 11,329,000us-gaap_DefinedBenefitPlanContributionsByEmployer    
Benefit payments (19,171,000)us-gaap_DefinedBenefitPlanBenefitsPaid (16,465,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Fair value of plan assets at Dec. 31 132,713,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 136,935,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 136,546,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status (32,956,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1] (26,995,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1]    
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss 90,007,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized 84,773,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized    
Prior service (credit) cost 667,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized 778,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized    
Total 90,674,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 85,551,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal    
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]        
Current regulatory assets 6,728,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets 7,631,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets    
Noncurrent regulatory assets 83,946,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets 77,920,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets    
Total 90,674,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 85,551,000nspw_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal    
Significant Assumptions Used to Measure Benefit Obligations [Abstract]        
Measurement date 12/31/2014 12/31/2013    
Discount rate for year-end valuation (in hundredths) 4.11%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate    
Expected average long-term increase in compensation level (in hundredths) 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease    
Mortality table RP 2014 RP 2000    
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period 8,000,000us-gaap_PensionContributions 11,300,000us-gaap_PensionContributions 12,500,000us-gaap_PensionContributions  
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost 4,527,000us-gaap_DefinedBenefitPlanServiceCost 5,682,000us-gaap_DefinedBenefitPlanServiceCost 4,568,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 7,257,000us-gaap_DefinedBenefitPlanInterestCost 6,924,000us-gaap_DefinedBenefitPlanInterestCost 7,765,000us-gaap_DefinedBenefitPlanInterestCost  
Expected return on plan assets (9,642,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (9,995,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (10,489,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets  
Amortization of prior service cost (credit) 111,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 417,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 1,771,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit  
Amortization of net loss 6,617,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 7,924,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 6,004,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses  
Net periodic benefit cost 8,870,000nspw_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 10,952,000nspw_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 9,619,000nspw_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments  
Significant Assumptions Used to Measure Costs [Abstract]        
Discount rate (in hundredths) 4.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate  
Expected average long-term increase in compensation level (in hundredths) 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease  
Expected average long-term rate of return on assets (in hundredths) 7.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 7.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets  
Allocated costs for pension plans sponsored by Xcel Energy Inc. 1,700,000nspw_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 2,200,000nspw_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 1,800,000nspw_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent  
Expected average long-term rate of return on assets for next fiscal year (in hundredths) 7.25%nspw_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongtermReturnonAssetsForNextFiscalYear      
Number of years fair market value of plan assets is adjusted using calculated value method (in years) 5 years      
Annual adjustment rate used in calculated value method (in hundredths) 20.00%nspw_AnnualAdjustmentRateUsedInCalculatedValueMethod      
Xcel Energy Inc.        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period 130,600,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
192,400,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
198,100,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Number of pension plans to which contributions were made 4nspw_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4nspw_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4nspw_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Subsequent Event        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period       4,900,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Subsequent Event | Xcel Energy Inc.        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period       $ 90,000,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Number of pension plans to which contributions were made       4nspw_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
[1] Amounts are recognized in noncurrent liabilities on NSP-Wisconsin’s consolidated balance sheets.