XML 37 R35.htm IDEA: XBRL DOCUMENT v2.4.1.9
Income Taxes (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Sep. 30, 2012
Dec. 31, 2014
Unrecognized Tax Benefits [Abstract]      
Unrecognized tax benefit — Permanent tax positions $ 200,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate   $ 100,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Unrecognized tax benefit — Temporary tax positions 2,200,000nspw_UnrecognizedTaxBenefitsThatWouldNotImpactEffectiveTaxRate   2,900,000nspw_UnrecognizedTaxBenefitsThatWouldNotImpactEffectiveTaxRate
Total unrecognized tax benefit 2,400,000us-gaap_UnrecognizedTaxBenefits   3,000,000us-gaap_UnrecognizedTaxBenefits
NOL and tax credit carryforwards (600,000)nspw_TaxBenefitsAssociatedWithNolAndTaxCreditCarryforwards   (900,000)nspw_TaxBenefitsAssociatedWithNolAndTaxCreditCarryforwards
Amounts accrued for penalties related to unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued   0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued
Internal Revenue Service (IRS)      
Tax Audits [Abstract]      
Earliest year subject to examination 2009    
Year(s) under examination   2010 and 2011  
Year of carryback claim under examination   2009  
Potential Tax Adjustments $ 12,000,000nspw_PotentialTaxAdjustments
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
State Jurisdiction (Wisconsin)      
Tax Audits [Abstract]      
Earliest year subject to examination 2010