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Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
MWh
Dec. 31, 2014
MWh
Credit Concentration Risk    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 8sps_Numberofmostsignificantcounterpartiesforwholesaletradingandnontradingcommodityactivitieswithcreditexposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember		  
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 1sps_Numberofmostsignificantcounterpartiesforwholesaletradingandnontradingcommodityactivitieswithcreditexposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 7.8sps_WholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 9.00%sps_PercentageOfWholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		  
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 6sps_Numberofmostsignificantcounterpartiesforwholesaletradingandnontradingcommodityactivitieswithcreditexposure
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 44.7sps_WholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 50.00%sps_PercentageOfWholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= sps_NoInvestmentGradeRatingsFromExternalCreditRatingAgenciesMember		  
Credit Concentration Risk | Credit Quality Less Than Investment Grade [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties 1.0sps_WholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= sps_CreditQualityLessThanInvestmentGradeMember		  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 1.00%sps_PercentageOfWholesaleTradingAndNonTradingCommodityCreditExposureForMostSignificantCounterparties
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_ExternalCreditRatingByGroupingAxis
= sps_CreditQualityLessThanInvestmentGradeMember		  
Interest Rate Swap    
Interest Rate Derivatives [Abstract]    
Amount of accumulated other comprehensive 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 (in megawatt hours)    
Gross Notional Amounts of Commodity FTRs [Abstract]    
Notional Amount 2,954,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= sps_ElectricCommodityMember		 [1] 6,930,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= sps_ElectricCommodityMember		 [1]
[1] (a) Amounts are not reflective of net positions in the underlying commodities.