XML 58 R53.htm IDEA: XBRL DOCUMENT v2.4.1.9
Income Taxes (Details) (USD $)
 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Tax Increase Prevention Act of 2014 [Abstract]        
Bonus depreciation rate 50.00%nspw_BonusDepreciationRate 50.00%nspw_BonusDepreciationRate    
Number of years bonus depreciation was extended 1 year 1 year    
American Taxpayer Relief Act of 2012 [Abstract]        
Original top tax rate for dividends   15.00%nspw_CurrentTopTaxRateForDividends    
New top tax rate for dividends   20.00%nspw_NewTopTaxRateForDividends    
Bonus depreciation rate 50.00%nspw_BonusDepreciationRate 50.00%nspw_BonusDepreciationRate    
Number of years bonus depreciation was extended 1 year 1 year    
Unrecognized Tax Benefits [Abstract]        
Unrecognized tax benefit — Permanent tax positions $ 100,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate $ 100,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate    
Unrecognized tax benefit — Temporary tax positions 2,900,000nspw_UnrecognizedTaxBenefitsThatWouldNotImpactEffectiveTaxRate 1,400,000nspw_UnrecognizedTaxBenefitsThatWouldNotImpactEffectiveTaxRate    
Total unrecognized tax benefit 3,000,000us-gaap_UnrecognizedTaxBenefits 1,500,000us-gaap_UnrecognizedTaxBenefits 1,300,000us-gaap_UnrecognizedTaxBenefits  
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Balance at Jan. 1 1,500,000us-gaap_UnrecognizedTaxBenefits 1,300,000us-gaap_UnrecognizedTaxBenefits 1,500,000us-gaap_UnrecognizedTaxBenefits  
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions 1,900,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 700,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 500,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions  
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions (200,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions (200,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions  
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions 100,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 500,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 300,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions  
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions (200,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions (800,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions  
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities (100,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities (1,000,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities  
Balance at Dec. 31 3,000,000us-gaap_UnrecognizedTaxBenefits 1,500,000us-gaap_UnrecognizedTaxBenefits 1,300,000us-gaap_UnrecognizedTaxBenefits  
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]        
NOL and tax credit carryforwards (900,000)nspw_TaxBenefitsAssociatedWithNolAndTaxCreditCarryforwards (400,000)nspw_TaxBenefitsAssociatedWithNolAndTaxCreditCarryforwards    
Amounts accrued for penalties related to unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued  
Effective Income Tax Rate Reconciliation, Percent [Abstract]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 4.90%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 5.00%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 3.40%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes  
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (0.70%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (0.90%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (0.90%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits  
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items (1.60%)nspw_EffectiveIncomeTaxRateReconciliationRegulatoryDifferencesUtilityPlantItems (0.90%)nspw_EffectiveIncomeTaxRateReconciliationRegulatoryDifferencesUtilityPlantItems (0.30%)nspw_EffectiveIncomeTaxRateReconciliationRegulatoryDifferencesUtilityPlantItems  
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits 0.00%nspw_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefits 0.00%nspw_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefits 0.10%nspw_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefits  
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.20%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments  
Effective Income Tax Rate Reconciliation, Percent 37.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations 38.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]        
Current Federal Tax Expense (Benefit) (3,932,000)us-gaap_CurrentFederalTaxExpenseBenefit 5,902,000us-gaap_CurrentFederalTaxExpenseBenefit 930,000us-gaap_CurrentFederalTaxExpenseBenefit  
Current State and Local Tax Expense (Benefit) 453,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 4,628,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 2,216,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit  
Current Change In Unrecognized Tax Expense (Benefit) 1,013,000nspw_CurrentChangeInUnrecognizedTaxExpenseBenefit 754,000nspw_CurrentChangeInUnrecognizedTaxExpenseBenefit (69,000)nspw_CurrentChangeInUnrecognizedTaxExpenseBenefit  
Deferred Federal Income Tax Expense (Benefit) 38,321,000us-gaap_DeferredFederalIncomeTaxExpenseBenefit 23,794,000us-gaap_DeferredFederalIncomeTaxExpenseBenefit 25,089,000us-gaap_DeferredFederalIncomeTaxExpenseBenefit  
Deferred State and Local Income Tax Expense (Benefit) 8,042,000us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 2,720,000us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 1,890,000us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit  
Deferred Change In Unrecognized Tax Expense (Benefit) (967,000)nspw_DeferredChangeInUnrecognizedTaxExpenseBenefit (725,000)nspw_DeferredChangeInUnrecognizedTaxExpenseBenefit 128,000nspw_DeferredChangeInUnrecognizedTaxExpenseBenefit  
Deferred investment tax credits (527,000)nspw_DeferredInvestmentTaxCredits (664,000)nspw_DeferredInvestmentTaxCredits (626,000)nspw_DeferredInvestmentTaxCredits  
Income Tax Expense (Benefit) 42,403,000us-gaap_IncomeTaxExpenseBenefit 36,409,000us-gaap_IncomeTaxExpenseBenefit 29,558,000us-gaap_IncomeTaxExpenseBenefit  
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]        
Deferred tax expense (benefit) excluding selected items 49,793,000nspw_DeferredTaxExpenseBenefitExcludingSelectedItems 27,516,000nspw_DeferredTaxExpenseBenefitExcludingSelectedItems 27,995,000nspw_DeferredTaxExpenseBenefitExcludingSelectedItems  
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities (4,346,000)nspw_AmortizationAndAdjustmentsToDeferredIncomeTaxesOnIncomeTaxRegulatoryAssetsAndLiabilities (1,676,000)nspw_AmortizationAndAdjustmentsToDeferredIncomeTaxesOnIncomeTaxRegulatoryAssetsAndLiabilities (837,000)nspw_AmortizationAndAdjustmentsToDeferredIncomeTaxesOnIncomeTaxRegulatoryAssetsAndLiabilities  
Other Comprehensive Income (Loss), Tax (51,000)us-gaap_OtherComprehensiveIncomeLossTax (51,000)us-gaap_OtherComprehensiveIncomeLossTax (51,000)us-gaap_OtherComprehensiveIncomeLossTax  
Deferred Income Tax Expense (Benefit) 45,396,000us-gaap_DeferredIncomeTaxExpenseBenefit 25,789,000us-gaap_DeferredIncomeTaxExpenseBenefit 27,107,000us-gaap_DeferredIncomeTaxExpenseBenefit  
Deferred Tax Liabilities, Gross [Abstract]        
Deferred Tax Liabilities, Property, Plant and Equipment 319,265,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 287,121,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment    
Deferred Tax Liabilities, Regulatory Assets 72,670,000us-gaap_DeferredTaxLiabilitiesRegulatoryAssets 57,296,000us-gaap_DeferredTaxLiabilitiesRegulatoryAssets    
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits 18,691,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits 16,953,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits    
Deferred Tax Liabilities, Other 14,453,000us-gaap_DeferredTaxLiabilitiesOther 10,193,000us-gaap_DeferredTaxLiabilitiesOther    
Deferred Tax Liabilities, Net 425,079,000us-gaap_DeferredTaxLiabilities 371,563,000us-gaap_DeferredTaxLiabilities    
Deferred Tax Assets, Gross [Abstract]        
Deferred Tax Assets Environmental Remediation 43,207,000nspw_DeferredTaxAssetsEnvironmentalRemediation 43,501,000nspw_DeferredTaxAssetsEnvironmentalRemediation    
Deferred Tax Assets, Operating Loss Carryforwards 18,283,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 17,384,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards    
Deferred Tax Assets Regulatory Liabilities 10,460,000nspw_DeferredTaxAssetsRegulatoryLiabilities 6,205,000nspw_DeferredTaxAssetsRegulatoryLiabilities    
Deferred Tax Assets Deferred Investment Tax Credits 5,628,000nspw_DeferredTaxAssetsDeferredInvestmentTaxCredits 5,976,000nspw_DeferredTaxAssetsDeferredInvestmentTaxCredits    
Deferred Tax Assets Tax credit carryforward 4,515,000nspw_DeferredTaxAssetsTaxCreditCarryforward 4,440,000nspw_DeferredTaxAssetsTaxCreditCarryforward    
Deferred Tax Assets, Other 3,007,000us-gaap_DeferredTaxAssetsOther 3,871,000us-gaap_DeferredTaxAssetsOther    
Deferred Tax Assets, Net of Valuation Allowance 85,100,000us-gaap_DeferredTaxAssetsNet 81,377,000us-gaap_DeferredTaxAssetsNet    
Deferred Tax Assets, Net 339,979,000us-gaap_DeferredTaxAssetsLiabilitiesNet 290,186,000us-gaap_DeferredTaxAssetsLiabilitiesNet    
Internal Revenue Service (IRS)        
Tax Audits [Abstract]        
Year(s) no longer subject to audit as statute of limitations has expired 2008      
Earliest year subject to examination 2009      
Year(s) under examination       2010 and 2011
Tax Adjustments, Settlements, and Unusual Provisions 12,000,000us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		      
Operating Loss Carryforwards 48,500,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		 46,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		    
Tax Credit Carryforward, Amount 4,500,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		 4,400,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		    
Carryforward expiration date range, low 2021      
Carryforward expiration date range, high 2034      
State and Local Jurisdiction        
Tax Audits [Abstract]        
Earliest year subject to examination 2010      
Year(s) under examination 2009 through 2011      
Operating Loss Carryforwards $ 3,400,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		 $ 6,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Carryforward expiration date range, low 2022      
Carryforward expiration date range, high 2031