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Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) (Pension Plans, USD $)
12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31 $ 1,249,739,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation $ 1,134,184,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation    
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 1,152,836,000us-gaap_DefinedBenefitPlanBenefitObligation 1,194,371,000us-gaap_DefinedBenefitPlanBenefitObligation    
Service cost 23,939,000us-gaap_DefinedBenefitPlanServiceCost 25,206,000us-gaap_DefinedBenefitPlanServiceCost 22,719,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 53,277,000us-gaap_DefinedBenefitPlanInterestCost 46,160,000us-gaap_DefinedBenefitPlanInterestCost 51,192,000us-gaap_DefinedBenefitPlanInterestCost  
Transfer (to) from other plan (13,404,000)psco_DefinedBenefitPlanObligationTransferToFromOtherPlan 11,306,000psco_DefinedBenefitPlanObligationTransferToFromOtherPlan    
Actuarial (gain) loss 133,215,000us-gaap_DefinedBenefitPlanActuarialGainLoss (49,384,000)us-gaap_DefinedBenefitPlanActuarialGainLoss    
Benefit payments (71,906,000)us-gaap_DefinedBenefitPlanBenefitsPaid (74,823,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Obligation at Dec. 31 1,277,957,000us-gaap_DefinedBenefitPlanBenefitObligation 1,152,836,000us-gaap_DefinedBenefitPlanBenefitObligation 1,194,371,000us-gaap_DefinedBenefitPlanBenefitObligation  
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 1,067,057,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 1,055,308,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets    
Actual return (loss) on plan assets 84,871,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 30,684,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets    
Employer contributions 35,156,000us-gaap_DefinedBenefitPlanContributionsByEmployer 44,582,000us-gaap_DefinedBenefitPlanContributionsByEmployer    
Transfer (to) from other plan (11,313,000)psco_DefinedBenefitPlanAssetTransferToFromOtherPlan 11,306,000psco_DefinedBenefitPlanAssetTransferToFromOtherPlan    
Benefit payments (71,906,000)us-gaap_DefinedBenefitPlanBenefitsPaid (74,823,000)us-gaap_DefinedBenefitPlanBenefitsPaid    
Fair value of plan assets at Dec. 31 1,103,865,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 1,067,057,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 1,055,308,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status (174,092,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1] (85,779,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan [1]    
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss 530,674,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized 450,202,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized    
Prior service (credit) cost (18,708,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized (21,800,000)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized    
Total 511,966,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 428,402,000psco_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 31,774,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets 36,271,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsCurrentRegulatoryAssets    
Noncurrent regulatory assets 480,192,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets 392,131,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostRecordedAsNoncurrentRegulatoryAssets    
Total 511,966,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal 428,402,000psco_AmountsNotYetRecognizedAsComponentsOfNetPeriodicBenefitCostTotal    
Significant Assumptions Used to Measure Benefit Obligations [Abstract]        
Measurement date 2014-12-31 2013-12-31    
Discount rate for year-end valuation (in hundredths) 4.11%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate    
Expected average long-term increase in compensation level (in hundredths) 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease    
Mortality table RP 2014 RP 2000    
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period 35,200,000us-gaap_PensionContributions 44,600,000us-gaap_PensionContributions 41,800,000us-gaap_PensionContributions  
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost 23,939,000us-gaap_DefinedBenefitPlanServiceCost 25,206,000us-gaap_DefinedBenefitPlanServiceCost 22,719,000us-gaap_DefinedBenefitPlanServiceCost  
Interest cost 53,277,000us-gaap_DefinedBenefitPlanInterestCost 46,160,000us-gaap_DefinedBenefitPlanInterestCost 51,192,000us-gaap_DefinedBenefitPlanInterestCost  
Expected return on plan assets (70,709,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (63,821,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (65,302,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets  
Amortization of prior service (credit) cost (3,092,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit (1,064,000)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 228,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit  
Amortization of net loss 33,892,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 43,418,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 34,332,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses  
Net benefit cost recognized for financial reporting 37,307,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 49,899,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 43,169,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost  
Significant Assumptions Used to Measure Costs [Abstract]        
Discount rate (in hundredths) 4.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate  
Expected average long-term increase in compensation level (in hundredths) 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease  
Expected average long-term rate of return on assets (in hundredths) 6.81%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.47%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets  
Allocated costs for pension plans sponsored by Xcel Energy Inc. 9,400,000psco_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 11,600,000psco_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent 9,600,000psco_DefinedBenefitPlansAllocatedCostsForPensionPlansSponsoredByParent  
Expected average long-term rate of return on assets for next fiscal year (in hundredths) 6.81%psco_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongtermReturnonAssetsForNextFiscalYear      
Number of years fair market value of plan assets is adjusted using calculated value method (in years) 5 years      
Annual adjustment rate used in calculated value method (in hundredths) 20.00%psco_AnnualAdjustmentRateUsedInCalculatedValueMethod      
Xcel Energy Inc.        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period 130,600,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
192,400,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
198,100,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Number of pension plans to which contributions were made 4psco_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4psco_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
4psco_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Subsequent Event        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period       20,000,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Subsequent Event | Xcel Energy Inc.        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period       $ 90,000,000us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Number of pension plans to which contributions were made       4psco_DefinedBenefitPlanNumberOfPensionPlansToWhichContributionsWereMade
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
[1] Amounts are recognized in noncurrent liabilities on PSCo’s consolidated balance sheets.