Rate Matters (Details) $ in Thousands	1 Months Ended						12 Months Ended
	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
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 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 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 - 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%
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
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