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Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) (Postretirement Benefit Plan, USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Change in Projected Benefit Obligation [Roll Forward]      
Obligation at Jan. 1 $ 54,982,000us-gaap_DefinedBenefitPlanBenefitObligation $ 59,260,000us-gaap_DefinedBenefitPlanBenefitObligation  
Service cost 1,246,000us-gaap_DefinedBenefitPlanServiceCost 1,368,000us-gaap_DefinedBenefitPlanServiceCost 1,259,000us-gaap_DefinedBenefitPlanServiceCost
Interest cost 2,572,000us-gaap_DefinedBenefitPlanInterestCost 2,352,000us-gaap_DefinedBenefitPlanInterestCost 2,831,000us-gaap_DefinedBenefitPlanInterestCost
Medicare subsidy reimbursements 18,000us-gaap_DefinedBenefitPlanGrossPrescriptionDrugSubsidyReceiptsReceived 63,000us-gaap_DefinedBenefitPlanGrossPrescriptionDrugSubsidyReceiptsReceived  
Plan participants' contributions 728,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants 698,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants  
Actuarial (gain) loss (11,828,000)us-gaap_DefinedBenefitPlanActuarialGainLoss (5,215,000)us-gaap_DefinedBenefitPlanActuarialGainLoss  
Benefit payments (3,376,000)us-gaap_DefinedBenefitPlanBenefitsPaid (3,544,000)us-gaap_DefinedBenefitPlanBenefitsPaid  
Obligation at Dec. 31 44,342,000us-gaap_DefinedBenefitPlanBenefitObligation 54,982,000us-gaap_DefinedBenefitPlanBenefitObligation 59,260,000us-gaap_DefinedBenefitPlanBenefitObligation
Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at Jan. 1 46,738,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 46,222,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Actual return (loss) on plan assets 1,073,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 3,228,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets  
Plan participants' contributions 728,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants 698,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants  
Employer contributions 193,000us-gaap_DefinedBenefitPlanContributionsByEmployer 134,000us-gaap_DefinedBenefitPlanContributionsByEmployer  
Benefit payments (3,376,000)us-gaap_DefinedBenefitPlanBenefitsPaid (3,544,000)us-gaap_DefinedBenefitPlanBenefitsPaid  
Fair value of plan assets at Dec. 31 45,356,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 46,738,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 46,222,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Funded Status of Plans at Dec. 31 [Abstract]      
Funded status 1,014,000us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1] (8,244,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1]  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Net loss (14,677,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized (5,344,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized  
Prior service (credit) cost (3,432,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized (3,833,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized  
Total (18,109,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal (9,177,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]      
Current regulatory liabilities (892,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryLiabilities (319,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryLiabilities  
Noncurrent regulatory liabilities (17,217,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryLiabilities (8,858,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryLiabilities  
Total (18,109,000)sps_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal (9,177,000)sps_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) 4.08%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.82%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate  
Mortality table RP 2014 RP 2000  
Health care costs trend rate - initial (in hundredths) 6.50%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear 7.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear  
Ultimate health care trend assumption rate (in hundredths) 4.50%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate 4.50%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate  
Period until ultimate trend rate is reached (in years) 4 years    
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]      
One-percent increase in APBO 4,555,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation    
One-percent decrease in APBO (3,834,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1    
One-percent increase in service and interest components 451,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents    
One-percent decrease in service and interest components (371,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1    
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 200,000us-gaap_OtherPostretirementBenefitsPayments 100,000us-gaap_OtherPostretirementBenefitsPayments 4,400,000us-gaap_OtherPostretirementBenefitsPayments
Expected contribution to postretirement health care plans during 2015 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 1,246,000us-gaap_DefinedBenefitPlanServiceCost 1,368,000us-gaap_DefinedBenefitPlanServiceCost 1,259,000us-gaap_DefinedBenefitPlanServiceCost
Interest cost 2,572,000us-gaap_DefinedBenefitPlanInterestCost 2,352,000us-gaap_DefinedBenefitPlanInterestCost 2,831,000us-gaap_DefinedBenefitPlanInterestCost
Expected return on plan assets (3,247,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (3,183,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (2,701,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets
Amortization of transition obligation 0us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets 0us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets 1,545,000us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets
Amortization of prior service cost (credit) (401,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit (484,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit (148,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
Amortization of net (gain) loss (321,000)us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses (6,000)us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 1,256,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
Net periodic benefit cost (credit) (151,000)sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 47,000sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 4,042,000sps_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments
Significant Assumptions Used to Measure Costs [Abstract]      
Discount rate (in hundredths) 4.82%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Expected average long-term rate of return on assets (in hundredths) 7.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 7.11%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Xcel Energy Inc.      
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 17,100,000us-gaap_OtherPostretirementBenefitsPayments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
17,600,000us-gaap_OtherPostretirementBenefitsPayments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
47,100,000us-gaap_OtherPostretirementBenefitsPayments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Expected contribution to postretirement health care plans during 2015 $ 12,800,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
   
[1] Amounts are recognized in noncurrent assets and noncurrent liabilities on SPS’ balance sheet as of Dec. 31, 2014 and 2013, respectively.