Rate Matters, PSCo (Details) - PSCo $ in Thousands		1 Months Ended						12 Months Ended
	Jan. 27, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2015 USD ($)	Jul. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015	Dec. 31, 2015 USD ($) MWh	Dec. 31, 2014 USD ($) MWh	Dec. 31, 2012 USD ($)	Dec. 31, 2011
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Original Request - Gas Rates PSIA Rider 2016
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider						$ 21,700
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Original Request - Gas Rates PSIA Rider 2017
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider						21,200
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 66,200
Public Utilities, Number Of Years Rate Case Is Applicable For						3 years
Public Utilities, Requested Equity Capital Structure, Percentage						56.00%
Public Utilities, Requested Return on Equity, Percentage						10.10%
Public Utilities, Requested Rate Base, Amount						$ 1,260
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						40,500
Public Utilities, Increase in Request By Shift in O&M Expenses Between Rider And Base Rates					$ 0
Public Utilities, Decrease in Request by Corrections And Adjustments Based On Rebuttal Testimony					0
Public Utilities, Revised Base Rate Request From Rebuttal Testimony					40,500
Public Utilities, Total Revenue Impact From Rebuttal Testimony					40,400
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 7,600
Public Utilities, Increase in Request By Shift in O&M Expenses Between Rider And Base Rates					7,000
Public Utilities, Decrease in Request by Corrections And Adjustments Based On Rebuttal Testimony					0
Public Utilities, Revised Base Rate Request From Rebuttal Testimony					14,600
Public Utilities, Total Revenue Impact From Rebuttal Testimony					29,300
Public Utilities, Requested Return on Equity, Percentage						10.10%
Public Utilities, Requested Rate Base, Amount						$ 1,310
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 18,100
Public Utilities, Increase in Request By Shift in O&M Expenses Between Rider And Base Rates					6,400
Public Utilities, Decrease in Request by Corrections And Adjustments Based On Rebuttal Testimony					(7,700)
Public Utilities, Revised Base Rate Request From Rebuttal Testimony					16,800
Public Utilities, Total Revenue Impact From Rebuttal Testimony					38,500
Public Utilities, Requested Return on Equity, Percentage						10.30%
Public Utilities, Requested Rate Base, Amount						$ 1,360
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2015
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider					(100)
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider
Rate Matters [Abstract]
Public Utilities, Number Of Years Rate Case Is Applicable For						3 years
Public Utilities, Revenue Impact Of Requested Rider						$ 42,800
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2016
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider					14,700
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2017
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider					21,700
Colorado 2015 Steam Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 3,500
Colorado 2015 Steam Rate Case, Phase 1 - Jan. 1, 2016
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 2,800
Colorado 2015 Steam Rate Case, Phase 2 - Nov. 1, 2016
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount			700
CPUC Proceeding - Annual Electric Earnings Test 2015 through 2017
Rate Matters [Abstract]
Public Utilities, Return on Equity Threshold for Earnings Sharing							9.83%
2015 Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Refund to customers due to annual earnings test								$ 15,000
CPUC Proceeding - Demand Side Management Cost Adjustment
Rate Matters [Abstract]
Public Utilities, Incentive Award Upon Achieving Savings Goal								$ 5,000
Public Utilities, Percentage of Net Economic Benefits on Which Incentive is Earned								5.00%
Public Utilities, Maximum Annual Incentive								$ 30,000
CPUC Proceeding - Demand Side Management Cost Adjustment, 2015
Rate Matters [Abstract]
Public Utilities, Approved Electric Demand Side Management Budget					81,600
Public Utilities, Approved Gas Demand Side Management Budget					13,100
CPUC Proceeding - Demand Side Management Cost Adjustment, 2016
Rate Matters [Abstract]
Public Utilities, Approved Electric Demand Side Management Budget					78,700
Public Utilities, Approved Gas Demand Side Management Budget					$ 13,600
CPUC Proceeding - Renewable Energy Credit Sharing
Rate Matters [Abstract]
Public Utilities, Customers Share of Margins Credited Against RESA Regulatory Asset Balance								5,500	$ 600
Public Utilities, Cumulative Credit Against RESA Regulatory Asset Balance			$ 110,600					$ 110,600	$ 105,100
Administrative Law Judge | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case
Rate Matters [Abstract]
Public Utilities, Recommended Length Of Average Rate Base By Third Parties (in Months)				13
Public utilities, ROE Recommended By Third Parties				9.50%
Public Utilities, Recommended Equity Capital Structure, Percentage				56.51%
Public Utilities, Proposed Number of Years Extending PSIA rider Recommended by Third Parties				3 years
Administrative Law Judge | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Decrease to Requested Return on Equity				$ (7,800)
Public Utilities, Decrease to Cost of Debt and Capital Structure				(500)
Public Utilities, Increase (Decrease) Related to Pipeline Adjustment				4,100
Public Utilities, Decrease in Move to Historical Test Year				(14,100)
Public Utilities, Decrease to O&M Expenses				(3,000)
Public Utilities, Decrease related to other, net				(1,100)
Public Utilities, Recommended Rate Increase (Decrease), Amount				18,100
Administrative Law Judge | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016
Rate Matters [Abstract]
Public Utilities, Recommended Rider Costs to be Recovered Through Base Rates, Amount				20,000
Administrative Law Judge | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2016
Rate Matters [Abstract]
Public Utilities, Decrease in Request by Transfer Of Costs To Base Rates				(20,500)
Public Utilities, Decrease in Request By Cost Recovery Remaining In Base				(4,300)
Public Utilities, Decrease In Request For Projects Not Recovered Through The Rider				(3,600)
Public Utilities, Decrease to Return on Equity and Capital Structure				(300)
Public Utilities, Recommended Rider Increase (Decrease)				(7,000)
Administrative Law Judge | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2017
Rate Matters [Abstract]
Public Utilities, Decrease in Request by Transfer Of Costs To Base Rates				0
Public Utilities, Decrease in Request By Cost Recovery Remaining In Base				0
Public Utilities, Decrease In Request For Projects Not Recovered Through The Rider				(2,000)
Public Utilities, Decrease to Return on Equity and Capital Structure				(1,600)
Public Utilities, Recommended Rider Increase (Decrease)				$ 17,600
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Demand Side Management Cost Adjustment, 2014
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (in MWh) | MWh									384
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Demand Side Management Cost Adjustment, 2015
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (in MWh) | MWh								400
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Demand Side Management Cost Adjustment, 2016 through 2020
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (in MWh) | MWh								400
Public Utilities, Annual Spending Limit								$ 84,300
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
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 REC Margin Limit Approved										20.00%
Public Utilities, Percentage of Margin in Excess of REC Margin Limit Approved										10.00%
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 REC Margin Limit Approved										80.00%
Public Utilities, Percentage of Margin in Excess of REC Margin Limit Approved										90.00%
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case
Rate Matters [Abstract]
Public Utilities, Approved Length Of Average Rate Base By Third Parties (in Months)		13
Public Utilities, Approved Return on Equity, Percentage		9.50%
Public Utilities, Approved Equity Capital Structure, Percentage		56.51%
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Decrease to Approved Return on Equity		$ (7,800)
Public Utilities, Decrease to Cost of Debt and Capital Structure		(500)
Public Utilities, Increase (Decrease) Related to Pipeline Adjustment		4,100
Public Utilities, Decrease in Move to Historical Test Year		(14,100)
Public Utilities, Decrease to O&M Expenses		(2,400)
Public Utilities, Decrease related to other, net		(1,100)
Public Utilities, Approved Rate Increase (Decrease), Amount		18,700
Public Utilities, Increase (Decrease) In Estimated Pre-Tax Impact For Expense Deferrals, Net Amortization		(3,600)
Public Utilities, Estimated Pre-Tax Impact		14,900
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount		19,700
Public Utilities, Approved Rider Costs to be Recovered Through Base Rates, Amount	$ 20,000
Public Utilities, Increase (Decrease) In Estimated Pre-Tax Impact For Expense Deferrals, Net Amortization		1,500
Public Utilities, Estimated Pre-Tax Impact		14,500
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount		0
Public Utilities, Increase (Decrease) In Estimated Pre-Tax Impact For Expense Deferrals, Net Amortization		5,200
Public Utilities, Estimated Pre-Tax Impact		24,000
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2015
Rate Matters [Abstract]
Public Utilities, Approved Rider Increase (Decrease)		(200)
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2016
Rate Matters [Abstract]
Public Utilities, Decrease To Written Order By Transfer Of Costs To Base Rates		(20,500)
Public Utilities, Decrease to Written Order By Cost Recovery Remaining In Base		(4,300)
Public Utilities, Decrease To Written Order For Projects Not Recovered Through The Rider		(3,300)
Public Utilities, Decrease To Written Order For Approved Return on Equity and Capital Structure		(300)
Public Utilities, Approved Rider Increase (Decrease)		(6,700)
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2017
Rate Matters [Abstract]
Public Utilities, Decrease To Written Order By Transfer Of Costs To Base Rates		0
Public Utilities, Decrease to Written Order By Cost Recovery Remaining In Base		0
Public Utilities, Decrease To Written Order For Projects Not Recovered Through The Rider		(800)
Public Utilities, Decrease To Written Order For Approved Return on Equity and Capital Structure		(1,600)
Public Utilities, Approved Rider Increase (Decrease)		$ 18,800