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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 $ 508,971,000us-gaap_DefinedBenefitPlanBenefitObligation $ 599,831,000us-gaap_DefinedBenefitPlanBenefitObligation  
Service cost 1,915,000us-gaap_DefinedBenefitPlanServiceCost 2,564,000us-gaap_DefinedBenefitPlanServiceCost 2,825,000us-gaap_DefinedBenefitPlanServiceCost
Interest cost 23,704,000us-gaap_DefinedBenefitPlanInterestCost 22,210,000us-gaap_DefinedBenefitPlanInterestCost 24,527,000us-gaap_DefinedBenefitPlanInterestCost
Medicare subsidy reimbursements 1,753,000us-gaap_DefinedBenefitPlanGrossPrescriptionDrugSubsidyReceiptsReceived 923,000us-gaap_DefinedBenefitPlanGrossPrescriptionDrugSubsidyReceiptsReceived  
Plan amendments 0us-gaap_DefinedBenefitPlanPlanAmendments (14,571,000)us-gaap_DefinedBenefitPlanPlanAmendments  
Plan participants' contributions 4,625,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants 4,589,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants  
Actuarial (gain) loss (63,130,000)us-gaap_DefinedBenefitPlanActuarialGainLoss (76,889,000)us-gaap_DefinedBenefitPlanActuarialGainLoss  
Benefit payments (34,085,000)us-gaap_DefinedBenefitPlanBenefitsPaid (29,686,000)us-gaap_DefinedBenefitPlanBenefitsPaid  
Obligation at Dec. 31 443,753,000us-gaap_DefinedBenefitPlanBenefitObligation 508,971,000us-gaap_DefinedBenefitPlanBenefitObligation 599,831,000us-gaap_DefinedBenefitPlanBenefitObligation
Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at Jan. 1 438,193,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 425,917,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Actual return (loss) on plan assets 11,060,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 30,390,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets  
Plan participants' contributions 4,625,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants 4,589,000us-gaap_DefinedBenefitPlanContributionsByPlanParticipants  
Employer contributions 5,489,000us-gaap_DefinedBenefitPlanContributionsByEmployer 6,983,000us-gaap_DefinedBenefitPlanContributionsByEmployer  
Benefit payments (34,085,000)us-gaap_DefinedBenefitPlanBenefitsPaid (29,686,000)us-gaap_DefinedBenefitPlanBenefitsPaid  
Fair value of plan assets at Dec. 31 425,282,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 438,193,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 425,917,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Funded Status of Plans at Dec. 31 [Abstract]      
Funded status (18,471,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1] (70,778,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1]  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]      
Net loss 56,823,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized 107,232,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized  
Prior service (credit) cost (40,189,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized (46,436,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized  
Total 16,634,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 60,796,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]      
Current regulatory assets 0psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets 8,798,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets  
Noncurrent regulatory assets 16,634,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets 51,998,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets  
Total 16,634,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 60,796,000psco_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 45,581,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation    
One-percent decrease in APBO (38,371,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1    
One-percent increase in service and interest components 3,028,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents    
One-percent decrease in service and interest components (2,488,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1    
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 5,500,000us-gaap_OtherPostretirementBenefitsPayments 7,000,000us-gaap_OtherPostretirementBenefitsPayments 27,500,000us-gaap_OtherPostretirementBenefitsPayments
Expected contribution to postretirement health care plans during 2015 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 1,915,000us-gaap_DefinedBenefitPlanServiceCost 2,564,000us-gaap_DefinedBenefitPlanServiceCost 2,825,000us-gaap_DefinedBenefitPlanServiceCost
Interest cost 23,704,000us-gaap_DefinedBenefitPlanInterestCost 22,210,000us-gaap_DefinedBenefitPlanInterestCost 24,527,000us-gaap_DefinedBenefitPlanInterestCost
Expected return on plan assets (30,214,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (29,227,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (25,056,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets
Amortization of transition obligation 0us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets 785,000us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets 11,004,000us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssets
Amortization of prior service (credit) cost (6,247,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit (7,666,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit (5,150,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
Amortization of net loss 6,434,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 13,699,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 10,930,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
Net periodic benefit cost (4,408,000)psco_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 2,365,000psco_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments 19,080,000psco_DefinedBenefitPlanNetBenefitCostCreditRecognizedBeforeRegulatoryAdjustments
Additional cost recognized due to effects of regulation 0psco_DefinedBenefitPlanAdditionalCostRecognizedDueToEffectsOfRegulation 0psco_DefinedBenefitPlanAdditionalCostRecognizedDueToEffectsOfRegulation 3,891,000psco_DefinedBenefitPlanAdditionalCostRecognizedDueToEffectsOfRegulation
Net benefit cost recognized for financial reporting (4,408,000)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 2,365,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 22,971,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
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) 7.18%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 liabilities on PSCo’s consolidated balance sheets.