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Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
MWh
Dec. 31, 2013
MWh
Credit Concentration Risk [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale activities with credit exposure 8sps_NumberOfMostSignificantCounterpartiesForWholesaleActivitiesWithCreditExposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember		  
Credit Concentration Risk [Member] | Investment Grade Ratings from Standard & Poor's, Moody's, or Fitch Ratings [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale activities with credit exposure 1sps_NumberOfMostSignificantCounterpartiesForWholesaleActivitiesWithCreditExposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_InvestmentGradeRatingsFromStandardPoorsMoodysOrFitchRatingsMember		  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 15.2sps_WholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_InvestmentGradeRatingsFromStandardPoorsMoodysOrFitchRatingsMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 16.00%sps_PercentageOfWholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_InvestmentGradeRatingsFromStandardPoorsMoodysOrFitchRatingsMember		  
Credit Concentration Risk [Member] | No Investment Grade Ratings from External Credit Rating Agencies [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale activities with credit exposure 6sps_NumberOfMostSignificantCounterpartiesForWholesaleActivitiesWithCreditExposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 44.4sps_WholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 47.00%sps_PercentageOfWholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Credit Concentration Risk [Member] | Credit Quality Less Than Investment Grade [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale activities with credit exposure 1sps_NumberOfMostSignificantCounterpartiesForWholesaleActivitiesWithCreditExposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_CreditQualityLessThanInvestmentGradeMember		  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 1.7sps_WholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_CreditQualityLessThanInvestmentGradeMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 2.00%sps_PercentageOfWholesaleCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ sps_CreditQualityIndicatorAxis
= sps_CreditQualityLessThanInvestmentGradeMember		  
Interest Rate Swap    
Interest Rate Derivatives [Abstract]    
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months (0.2)us-gaap_InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet
/ us-gaap_DerivativeByNatureAxis
= us-gaap_InterestRateSwapMember		  
Electric Commodity [Member]    
Derivative [Line Items]    
Derivative, Nonmonetary Notional Amount 6,930,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= sps_ElectricCommodityMember		 [1] 5,989,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= sps_ElectricCommodityMember		 [1]
[1] Amounts are not reflective of net positions in the underlying commodities.