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Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) (USD $)
 1 Months Ended 12 Months Ended
Jun. 30, 2014
Aug. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]        
Deferred Finance Costs, Noncurrent, Net     $ 10,900,000us-gaap_DeferredFinanceCostsNoncurrentNet $ 10,300,000us-gaap_DeferredFinanceCostsNoncurrentNet
Equity to total capitalization ratio (excluding short-term debt), low end of range     45.00%sps_EquityToTotalCapitalizationRatioExcludingShort-TermDebtLowEndOfRange  
Equity to total capitalization ratio (excluding short-term debt), high end of range     55.00%sps_EquityToTotalCapitalizationRatioExcludingShort-TermDebtHighEndOfRange  
Equity to total capitalization ratio (excluding short-term debt)     53.60%sps_EquityToTotalCapitalizationRatioExcludingShort-TermDebt  
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions     396,000,000sps_UnrestrictedRetainedEarningsPerStateRegulatoryCommissionsDividendRestrictions  
First Mortgage Bonds | Series No. 3 Due June 15, 2024        
Debt Instrument [Line Items]        
Debt instrument, Face Amount 150,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeriesNo.3DueJune152024Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		      
Debt instrument, Interest Rate, Stated Percentage 3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeriesNo.3DueJune152024Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		      
Debt Instrument, Maturity Date Jun. 15, 2024      
First Mortgage Bonds | Issued Collateral on Series G Senior Note Due Dec. 1, 2018        
Debt Instrument [Line Items]        
Debt instrument, Face Amount 250,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= sps_IssuedCollateralonSeriesGSeniorNoteDueDec.12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		      
Debt instrument, Interest Rate, Stated Percentage 8.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_IssuedCollateralonSeriesGSeniorNoteDueDec.12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		      
Debt Instrument, Maturity Date Dec. 01, 2018      
First Mortgage Bonds | Senior C and D Due Oct. 1, 2033        
Debt Instrument [Line Items]        
Principal outstanding     100,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorCAndDDueOct12033Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		  
First Mortgage Bonds | Series Due Aug. 15, 2041        
Debt Instrument [Line Items]        
Debt instrument, Interest Rate, Stated Percentage   4.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeriesDueAug152041Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		 4.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeriesDueAug152041Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		  
Debt Instrument, Maturity Date   Aug. 15, 2041 Aug. 15, 2041  
Long term debt previously issued, gross       300,000,000sps_Longtermdebtpreviouslyissuedgross
/ us-gaap_DebtInstrumentAxis
= sps_SeriesDueAug152041Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember
Principal outstanding     400,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeriesDueAug152041Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember		 400,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeriesDueAug152041Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_BondsMember
Unsecured Debt [Member] | Senior C and D Due Oct. 1, 2033        
Debt Instrument [Line Items]        
Debt instrument, Interest Rate, Stated Percentage     6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeniorCAndDDueOct12033Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		  
Debt Instrument, Maturity Date     Oct. 01, 2033  
Principal outstanding     100,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorCAndDDueOct12033Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		 100,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorCAndDDueOct12033Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Unsecured Debt [Member] | Senior E Due Oct. 1, 2016        
Debt Instrument [Line Items]        
Debt instrument, Interest Rate, Stated Percentage     5.60%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeniorEDueOct12016Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		  
Debt Instrument, Maturity Date     Oct. 01, 2016  
Principal outstanding     200,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorEDueOct12016Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		 200,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorEDueOct12016Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Unsecured Debt [Member] | Senior G Due Dec. 1, 2018        
Debt Instrument [Line Items]        
Debt instrument, Interest Rate, Stated Percentage 8.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeniorGDueDec12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		   8.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= sps_SeniorGDueDec12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		  
Debt Instrument, Maturity Date Dec. 01, 2018   Dec. 01, 2018  
Principal outstanding     $ 250,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorGDueDec12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember		 $ 250,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= sps_SeniorGDueDec12018Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember