Rate Matters (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended						12 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2010	Oct. 31, 2014	Jun. 30, 2012	Apr. 30, 2012	Aug. 31, 2013	Nov. 30, 2014	Jul. 31, 2014	Dec. 31, 2011	Jan. 23, 2015
Rate Matters [Abstract]
Regulatory Liability, Noncurrent	$ 464,421,000	$ 424,690,000		$ 464,421,000
Pretax charge for project costs	0	0	10,666,000
PSCo [Member] | CPUC Proceeding - Colorado 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount	136,000,000
Public Utilities, Requested Rate Increase (Decrease), Percentage	4.83%
Public Utilities, Requested Return on Equity, Percentage	10.35%
Public Utilities, Requested Rate Base, Amount	6,390,000,000
Public Utilities, Requested Equity Capital Structure, Percentage	56.00%
Public Utilities, Rider costs to be recovered through base rates, amount	19,900,000
PSCo [Member] | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount	136,000,000
Public Utilities, Portion of requested rate increase (decrease) attributable to CACJA recovery	100,900,000
Public Utilities, Portion of requested rate increase (decrease) attributable to CACJA recovery, Amended	98,700,000
Public Utilities, Rider costs to be recovered through base rates, amount	19,900,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				107,200,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage				3.79%
Public Utilities, Requested Return on Equity, Revised, Percentage				10.25%
Public Utilities, Revenue deficiency based on a forecast test year	155,900,000
Public Utilities, Adjustment to requested rate increase (decrease) related to return on equity				(6,200,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to change in capital structure				0
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments				0
Public Utilities, Adjustment to requested rate increase (decrease) related to adjusting to a historic test year				0
Public Utilities, Adjustment to requested rate increase (decrease) related to use of a 13-month average rate base				0
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments for known and measurable plant				0
Public Utilities, Adjustment to requested rate increase (decrease) related to operations and maintenance expenses				0
Public Utilities, Adjustment to requested rate increase (decrease) related to depreciation expense				0
Public Utilities, Adjustment to requested rate increase (decrease) related to property taxes				(5,300,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to removing CACJA rider from base rates				(98,700,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to an updated sales forecast				(15,200,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to prepaid pension amortization				0
Public Utilities, Adjustment to requested rate increase (decrease) related to non-specified settlement adjustments				0
Public Utilities, Adjustment to requested rate increase (decrease) related to other costs				(2,100,000)
Public Utilities, Net recommended base rate increase (decrease)				28,400,000
Public Utilities, Revenue impact of CACJA rider mechanism				98,700,000
Public Utilities, Revenue impact of 2015 TCA rider mechanism				0
Public Utilities, Rider costs not to be recovered through base rates, amount				(19,900,000)
Public Utilities, Total recommended revenue increase (decrease)				107,200,000
PSCo [Member] | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2016
Rate Matters [Abstract]
Public Utilities, Requested increase (decrease) to rider revenue	34,200,000
PSCo [Member] | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2017
Rate Matters [Abstract]
Public Utilities, Requested increase (decrease) to rider revenue	29,900,000
PSCo [Member] | CPUC Proceeding - Manufacturer's Sales Tax Refund
Rate Matters [Abstract]
Public Utilities, Maximum annual amount of property taxes not subject to deferral	76,700,000
Public Utilities, Reduction in property tax deferral	10,000,000
PSCo [Member] | CPUC Proceeding - Annual Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Return on equity threshold for earnings sharing	10.00%
PSCo [Member] | CPUC Proceeding - 2014 Electric Earnings Test
Rate Matters [Abstract]
Regulatory Liability, Noncurrent	74,000,000			74,000,000
PSCo [Member] | CPUC Proceeding - Smart Grid City Cost Recovery
Rate Matters [Abstract]
Public Utilities, Estimated cost of project					45,000,000
Public Utilities, Annual operating and maintenance costs associated with project					4,000,000
Pretax charge for project costs			10,700,000
PSCo [Member] | CPUC Proceeding - Demand Side Management Cost Adjustment
Rate Matters [Abstract]
Public Utilities, Incentive award upon achieving savings goal	5,000,000
Public Utilities, Percentage of net economic benefits on which incentive is earned	5.00%
Public Utilities, Maximum annual incentive	30,000,000
PSCo [Member] | CPUC Proceeding - Demand Side Management Cost Adjustment, 2015
Rate Matters [Abstract]
Public Utilities, Proposed electric demand side management budget						81,600,000
Public Utilities, Proposed gas demand side management budget						13,100,000
PSCo [Member] | CPUC Proceeding - Demand Side Management Cost Adjustment, 2016
Rate Matters [Abstract]
Public Utilities, Proposed electric demand side management budget						78,700,000
Public Utilities, Proposed gas demand side management budget						13,600,000
PSCo [Member] | CPUC Proceeding - Renewable Energy Credit Sharing
Rate Matters [Abstract]
Public Utilities, Customers share of margins credited against RESA regulatory asset balance	600,000	21,700,000
Public Utilities, Cumulative credit against RESA regulatory asset balance	105,100,000	104,500,000		105,100,000
PSCo [Member] | FERC Proceeding - Transmission Formula Rate Cases
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount								2,000,000
Public Utilities, Requested Return on Equity, Percentage							10.25%
Public Utilities, Return on equity requested by third parties, Percentage							9.15%
PSCo [Member] | FERC Proceeding - Production Formula Rate ROE Complaint
Rate Matters [Abstract]
Public Utilities, Return on equity requested by third parties, Percentage									9.04%
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Return on equity recommended by third parties										9.11%
Public Utilities, Equity capital structure recommended by third parties										51.24%
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Adjustment to requested rate increase (decrease) related to return on equity										(69,100,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to change in capital structure										(20,900,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments										(20,800,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to adjusting to a historic test year										(82,500,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to use of a 13-month average rate base										(26,100,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments for known and measurable plant										21,900,000
Public Utilities, Adjustment to requested rate increase (decrease) related to operations and maintenance expenses										(7,200,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to depreciation expense										0
Public Utilities, Adjustment to requested rate increase (decrease) related to property taxes										0
Public Utilities, Adjustment to requested rate increase (decrease) related to removing CACJA rider from base rates										(62,400,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to an updated sales forecast										0
Public Utilities, Adjustment to requested rate increase (decrease) related to prepaid pension amortization										0
Public Utilities, Adjustment to requested rate increase (decrease) related to non-specified settlement adjustments										0
Public Utilities, Adjustment to requested rate increase (decrease) related to other costs										100,000
Public Utilities, Net recommended base rate increase (decrease)										(111,100,000)
Public Utilities, Revenue impact of CACJA rider mechanism										54,200,000
Public Utilities, Revenue impact of 2015 TCA rider mechanism										0
Public Utilities, Rider costs not to be recovered through base rates, amount										(19,900,000)
Public Utilities, Total recommended revenue increase (decrease)										(76,800,000)
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - 2013 Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Refund Approved, Amount											45,700,000
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Smart Grid City Cost Recovery
Rate Matters [Abstract]
Public Utilities, Cost of project allowed for recovery												28,000,000
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Demand Side Management Cost Adjustment, 2014
Rate Matters [Abstract]
Public Utilities, Maximum savings goal (in MWh)	384,000
Public Utilities, Electric demand side management budget	87,800,000
Public Utilities, Gas demand side management budget	12,300,000
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Demand Side Management Cost Adjustment, 2015
Rate Matters [Abstract]
Public Utilities, Maximum savings goal (in MWh)	400,000
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Renewable Energy Credit Sharing
Rate Matters [Abstract]
Public Utilities, Margin threshold determining percentage of margin sharing			20,000,000
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Renewable Energy Credit Sharing | Shareholders
Rate Matters [Abstract]
Public Utilities, Ultimate margin sharing associated with stand alone REC transactions												10.00%
Public Utilities, Percentage of margin on hybrid REC approved for first 20 million of margins			20.00%
Public Utilities, Percentage of margin on hybrid REC approved for margins in excess of 20 million			10.00%
PSCo [Member] | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Renewable Energy Credit Sharing | Customers
Rate Matters [Abstract]
Public Utilities, Ultimate margin sharing associated with stand alone REC transactions												90.00%
Public Utilities, Percentage of margin on hybrid REC approved for first 20 million of margins			80.00%
Public Utilities, Percentage of margin on hybrid REC approved for margins in excess of 20 million			90.00%
PSCo [Member] | Office of Consumer Counsel (OCC) | CPUC Proceeding - Colorado 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Return on equity recommended by third parties										9.10%
Public Utilities, Equity capital structure recommended by third parties										52.70%
PSCo [Member] | Office of Consumer Counsel (OCC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Adjustment to requested rate increase (decrease) related to return on equity										(66,500,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to change in capital structure										(23,700,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments										2,300,000
Public Utilities, Adjustment to requested rate increase (decrease) related to adjusting to a historic test year										(82,500,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to use of a 13-month average rate base										(22,000,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments for known and measurable plant										0
Public Utilities, Adjustment to requested rate increase (decrease) related to operations and maintenance expenses										(16,600,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to depreciation expense										(3,800,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to property taxes										(12,100,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to removing CACJA rider from base rates										0
Public Utilities, Adjustment to requested rate increase (decrease) related to an updated sales forecast										0
Public Utilities, Adjustment to requested rate increase (decrease) related to prepaid pension amortization										0
Public Utilities, Adjustment to requested rate increase (decrease) related to non-specified settlement adjustments										0
Public Utilities, Adjustment to requested rate increase (decrease) related to other costs										100,000
Public Utilities, Net recommended base rate increase (decrease)										(68,900,000)
Public Utilities, Revenue impact of CACJA rider mechanism										0
Public Utilities, Revenue impact of 2015 TCA rider mechanism										0
Public Utilities, Rider costs not to be recovered through base rates, amount										(19,900,000)
Public Utilities, Total recommended revenue increase (decrease)										(88,800,000)
PSCo [Member] | Federal Energy Regulatory Commission (FERC) | FERC Proceeding - Transmission Formula Rate Cases
Rate Matters [Abstract]
Public Utilities, Approved Return on Equity, Percentage						9.72%
PSCo [Member] | Federal Energy Regulatory Commission (FERC) | FERC Proceeding - Production Formula Rate ROE Complaint
Rate Matters [Abstract]
Public Utilities, Approved Return on Equity, Percentage						9.72%
Subsequent Event | PSCo [Member] | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Estimated expense deferral from settlement													3,100,000
Subsequent Event | PSCo [Member] | Parties to Settlement Agreement | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Equity Capital Structure, Percentage													56.00%
Public Utilities, Settlement Agreement, Requested Rate Increase (Decrease), Amount													53,300,000
Public Utilities, Settlement Agreement, Requested Rate Increase (Decrease), Percentage													1.87%
Public Utilities, Settlement Agreement, Requested Return on Equity, Percentage													9.83%
Public Utilities, Adjustment to requested rate increase (decrease) related to return on equity													(27,900,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to change in capital structure													0
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments													0
Public Utilities, Adjustment to requested rate increase (decrease) related to adjusting to a historic test year													(23,900,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to use of a 13-month average rate base													0
Public Utilities, Adjustment to requested rate increase (decrease) related to rate base adjustments for known and measurable plant													0
Public Utilities, Adjustment to requested rate increase (decrease) related to operations and maintenance expenses													0
Public Utilities, Adjustment to requested rate increase (decrease) related to depreciation expense													0
Public Utilities, Adjustment to requested rate increase (decrease) related to property taxes													(5,300,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to removing CACJA rider from base rates													(98,700,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to an updated sales forecast													(15,200,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to prepaid pension amortization													9,500,000
Public Utilities, Adjustment to requested rate increase (decrease) related to non-specified settlement adjustments													(31,700,000)
Public Utilities, Adjustment to requested rate increase (decrease) related to other costs													(2,100,000)
Public Utilities, Net recommended base rate increase (decrease)													(39,400,000)
Public Utilities, Revenue impact of CACJA rider mechanism													97,000,000
Public Utilities, Revenue impact of 2015 TCA rider mechanism													15,600,000
Public Utilities, Rider costs not to be recovered through base rates, amount													(19,900,000)
Public Utilities, Total recommended revenue increase (decrease)													$ 53,300,000
Subsequent Event | PSCo [Member] | Parties to Settlement Agreement | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015 | Shareholders
Rate Matters [Abstract]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds													50.00%
Subsequent Event | PSCo [Member] | Parties to Settlement Agreement | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015 | Customers
Rate Matters [Abstract]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds													50.00%
Minimum | PSCo [Member] | FERC Proceeding - Production Formula Rate ROE Complaint
Rate Matters [Abstract]
Public Utilities, Base return on equity charged to customers through production formula rates, Percentage									10.10%
Minimum | Subsequent Event | PSCo [Member] | Parties to Settlement Agreement | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Return on equity threshold for earnings sharing													9.84%
Maximum [Member] | PSCo [Member] | FERC Proceeding - Production Formula Rate ROE Complaint
Rate Matters [Abstract]
Public Utilities, Base return on equity charged to customers through production formula rates, Percentage									10.40%
Maximum [Member] | Subsequent Event | PSCo [Member] | Parties to Settlement Agreement | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Return on equity threshold for earnings sharing													10.48%