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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 $ 458,793,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation $ 402,509,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation    
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 434,307,000us-gaap_DefinedBenefitPlanBenefitObligation 454,184,000us-gaap_DefinedBenefitPlanBenefitObligation    
Service cost 9,184,000us-gaap_DefinedBenefitPlanServiceCost 9,615,000us-gaap_DefinedBenefitPlanServiceCost 8,520,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 20,444,000us-gaap_DefinedBenefitPlanInterestCost 17,908,000us-gaap_DefinedBenefitPlanInterestCost 19,697,000us-gaap_DefinedBenefitPlanInterestCost  
Actuarial (gain) loss 63,209,000us-gaap_DefinedBenefitPlanActuarialGainLoss (27,185,000)us-gaap_DefinedBenefitPlanActuarialGainLoss    
Transfer (to) from other plan (1,939,000)sps_DefinedBenefitPlanObligationTransferToFromOtherPlan 3,625,000sps_DefinedBenefitPlanObligationTransferToFromOtherPlan    
Benefit payments (24,515,000)us-gaap_DefinedBenefitPlanBenefitsPaid (23,840,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Obligation at Dec. 31 500,690,000us-gaap_DefinedBenefitPlanBenefitObligation 434,307,000us-gaap_DefinedBenefitPlanBenefitObligation 454,184,000us-gaap_DefinedBenefitPlanBenefitObligation  
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 393,393,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 376,138,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets    
Actual return (loss) on plan assets 30,159,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 15,455,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets    
Employer contributions 4,869,000us-gaap_DefinedBenefitPlanContributionsByEmployer 22,015,000us-gaap_DefinedBenefitPlanContributionsByEmployer    
Transfer (to) from other plan (1,637,000)us-gaap_DefinedBenefitPlanAssetsTransferredToFromPlan 3,625,000us-gaap_DefinedBenefitPlanAssetsTransferredToFromPlan    
Benefit payments (24,515,000)us-gaap_DefinedBenefitPlanBenefitsPaid (23,840,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Fair value of plan assets at Dec. 31 402,269,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 393,393,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 376,138,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status (98,421,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1] (40,914,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1]    
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss 252,063,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized 208,594,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized    
Prior service (credit) cost 39,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized 93,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized    
Total 252,102,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 208,687,000sps_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 14,437,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets 15,843,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets    
Noncurrent regulatory assets 237,665,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets 192,844,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets    
Total 252,102,000sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 208,687,000sps_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 4,900,000us-gaap_PensionContributions 22,000,000us-gaap_PensionContributions 13,100,000us-gaap_PensionContributions  
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost 9,184,000us-gaap_DefinedBenefitPlanServiceCost 9,615,000us-gaap_DefinedBenefitPlanServiceCost 8,520,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 20,444,000us-gaap_DefinedBenefitPlanInterestCost 17,908,000us-gaap_DefinedBenefitPlanInterestCost 19,697,000us-gaap_DefinedBenefitPlanInterestCost  
Expected return on plan assets (26,179,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (23,970,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (24,928,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets  
Amortization of prior service cost (credit) 54,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 870,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 1,438,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit  
Amortization of net (gain) loss 13,326,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 17,148,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 12,897,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses  
Net periodic benefit cost (credit) 16,829,000sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 21,571,000sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 17,624,000sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments  
(Costs) credits not recognized due to effects of regulation 3,170,000sps_DefinedBenefitPlanCostsCreditNotRecognizedDueToEffectsOfRegulation (1,269,000)sps_DefinedBenefitPlanCostsCreditNotRecognizedDueToEffectsOfRegulation (4,300,000)sps_DefinedBenefitPlanCostsCreditNotRecognizedDueToEffectsOfRegulation  
Net benefit cost recognized for financial reporting 19,999,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 20,302,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 13,324,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost  
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) 6.90%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.49%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.68%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets  
Allocated costs for pension plans sponsored by Xcel Energy Services Inc. 4,100,000sps_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 4,900,000sps_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 4,100,000sps_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent  
Expected average long-term rate of return on assets for next fiscal year (in hundredths) 7.22%sps_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%sps_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 4sps_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4sps_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4sps_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       11,600,000us-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       4sps_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 SPS’ balance sheets.