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Regulatory Assets and Liabilities, Regulatory Assets (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Regulatory Assets [Line Items]    
Regulatory Asset, Current $ 52,006,000us-gaap_RegulatoryAssetsCurrent $ 27,595,000us-gaap_RegulatoryAssetsCurrent
Regulatory Asset, Noncurrent 323,305,000us-gaap_RegulatoryAssetsNoncurrent 290,415,000us-gaap_RegulatoryAssetsNoncurrent
Past expenditures not currently earning a return 53,000,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided 30,000,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
Pension and Retiree Medical Obligations    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 17,256,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_PensionAndRetireeMedicalObligationsMember		 [1] 17,382,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_PensionAndRetireeMedicalObligationsMember		 [1]
Regulatory Asset, Noncurrent 240,980,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_PensionAndRetireeMedicalObligationsMember		 [1] 200,158,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_PensionAndRetireeMedicalObligationsMember		 [1]
Regulatory asset, remaining amortization period Various  
Recoverable Deferred Taxes on AFUDC Recorded in Plant    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RecoverableDeferredTaxesOnAfudcRecordedInPlantMember		 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RecoverableDeferredTaxesOnAfudcRecordedInPlantMember
Regulatory Asset, Noncurrent 36,878,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RecoverableDeferredTaxesOnAfudcRecordedInPlantMember		 31,362,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RecoverableDeferredTaxesOnAfudcRecordedInPlantMember
Regulatory asset, remaining amortization period Plant lives  
Net AROs    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember		 [2] 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember		 [2]
Regulatory Asset, Noncurrent 21,689,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember		 [2] 21,382,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember		 [2]
Regulatory asset, remaining amortization period Plant lives  
Conservation Programs    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 3,451,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_ConservationProgramsMember		 [3] 1,951,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_ConservationProgramsMember		 [3]
Regulatory Asset, Noncurrent 5,020,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_ConservationProgramsMember		 [3] 7,753,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_ConservationProgramsMember		 [3]
Regulatory asset, remaining amortization period, minimum 1 year  
Regulatory asset, remaining amortization period, maximum 6 years  
Renewable Resources and Environmental Initiatives    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 6,726,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RenewableResourcesAndEnvironmentalInitiativesMember		 3,428,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RenewableResourcesAndEnvironmentalInitiativesMember
Regulatory Asset, Noncurrent 5,124,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RenewableResourcesAndEnvironmentalInitiativesMember		 17,671,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_RenewableResourcesAndEnvironmentalInitiativesMember
Regulatory asset, remaining amortization period, minimum 1 year  
Regulatory asset, remaining amortization period, maximum 4 years  
Losses on Reacquired Debt    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 1,104,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember		 1,225,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember
Regulatory Asset, Noncurrent 2,594,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember		 3,697,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember
Regulatory asset, remaining amortization period Term of related debt  
Deferred Income Tax Adjustment    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember		 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember
Regulatory Asset, Noncurrent 302,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember		 3,375,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember
Regulatory asset, remaining amortization period Typically plant lives  
Recoverable Electric Energy Costs    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 513,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember		 491,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember
Regulatory Asset, Noncurrent 0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember		 0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember
Regulatory asset, remaining amortization period Less than one year  
Texas Surcharge [Member]    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 15,388,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_TexasSurchargeMember		 0us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_TexasSurchargeMember
Regulatory Asset, Noncurrent 0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_TexasSurchargeMember		 0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_TexasSurchargeMember
Regulatory asset, remaining amortization period Less than one year  
Other Regulatory Assets    
Regulatory Assets [Line Items]    
Regulatory Asset, Current 7,568,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_OtherRegulatoryAssetsMember		 3,118,000us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= sps_OtherRegulatoryAssetsMember
Regulatory Asset, Noncurrent $ 10,718,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_OtherRegulatoryAssetsMember		 $ 5,017,000us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= sps_OtherRegulatoryAssetsMember
Regulatory asset, remaining amortization period Various  
[1] Includes the non-qualified pension plan.
[2] Includes amounts recorded for future recovery of AROs.
[3] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.