Rate Matters (Details) $ in Millions	1 Months Ended				9 Months Ended	12 Months Ended
	Jul. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015	Sep. 30, 2015 USD ($) GWh	Dec. 31, 2014 GWh
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Equity Capital Structure, Percentage			56.00%
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 40.5
Public Utilities, Requested Rate Increase (Decrease), Percentage			3.50%
Public Utilities, Requested Return on Equity, Percentage			10.10%
Public Utilities, Requested Rate Base, Amount			$ 1,260.0
Public Utilities, Total Revenue Impact			40.4
Public Utilities, Increase In Request By Shift In O&M Expenses Between Rider And Base Rates	$ 0.0
Public Utilities, Decrease In Request By Corrections And Adjustments Based On Rebuttal Testimony	0.0
Public Utilities, Increase To Base Rate Request From Rebuttal Testimony	40.5
Public Utilities, Total Revenue Impact From Rebuttal Testimony	40.4
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 7.6
Public Utilities, Requested Rate Increase (Decrease), Percentage			0.70%
Public Utilities, Requested Return on Equity, Percentage			10.10%
Public Utilities, Requested Rate Base, Amount			$ 1,310.0
Public Utilities, Total Revenue Impact			29.3
Public Utilities, Increase In Request By Shift In O&M Expenses Between Rider And Base Rates	7.0
Public Utilities, Decrease In Request By Corrections And Adjustments Based On Rebuttal Testimony	0.0
Public Utilities, Increase To Base Rate Request From Rebuttal Testimony	14.6
Public Utilities, Total Revenue Impact From Rebuttal Testimony	29.3
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 18.1
Public Utilities, Requested Rate Increase (Decrease), Percentage			1.50%
Public Utilities, Requested Return on Equity, Percentage			10.30%
Public Utilities, Requested Rate Base, Amount			$ 1,360.0
Public Utilities, Total Revenue Impact			39.3
Public Utilities, Increase In Request By Shift In O&M Expenses Between Rider And Base Rates	6.4
Public Utilities, Decrease In Request By Corrections And Adjustments Based On Rebuttal Testimony	(7.7)
Public Utilities, Increase To Base Rate Request From Rebuttal Testimony	16.8
Public Utilities, Total Revenue Impact From Rebuttal Testimony	38.5
Demand Side Management Cost Adjustment 2014 [Member]
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (In GWh) | GWh						384
Demand Side Management Cost Adjustment, 2015 [Member]
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (In GWh) | GWh					400
Public Utilities, Approved Electric Demand Side Management Budget	81.6
Public Utilities, Approved Gas Demand Side Management Budget	13.1
Demand Side Management Cost Adjustment [Member]
Rate Matters [Abstract]
Public Utilities, Incentive Award Upon Achieving Savings Goal					$ 5.0
Public Utilities, Percentage Of Net Economic Benefits On Which Incentive Is Earned					5.00%
Public Utilities, Maximum Annual Incentive					$ 30.0
Demand Side Management Cost Adjustment, 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Approved Electric Demand Side Management Budget	78.7
Public Utilities, Approved Gas Demand Side Management Budget	13.6
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2015 [Member]
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider			(0.1)
Public Utilities, Increase (Decrease) To Rider Revenues Request Related To Rebuttal Testimony	(0.1)
Public Utilities, Revenue Projection Of Requested Rider			67.0
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider			21.7
Public Utilities, Decrease In Request By Transfer Of Operating And Maintenance Expense To Base Rates		$ (24.1)
Public Utilities, Decrease To Return On Equity And Capital Structure		(8.2)
Public Utilities, Increase in Request By Transfer meter replacement program from base rates to rider		1.7
Public Utilities, Recommended Rider Increase (Decrease)		(8.9)
Public Utilities, Increase (Decrease) To Rider Revenues Request Related To Rebuttal Testimony	14.7
Public Utilities, Revenue Projection Of Requested Rider			81.7
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates PSIA Rider 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Revenue Impact Of Requested Rider			21.2
Public Utilities, Decrease In Request By Transfer Of Operating And Maintenance Expense To Base Rates		(2.0)
Public Utilities, Decrease To Return On Equity And Capital Structure		(3.6)
Public Utilities, Increase in Request By Transfer meter replacement program from base rates to rider		1.7
Public Utilities, Recommended Rider Increase (Decrease)		17.3
Public Utilities, Increase (Decrease) To Rider Revenues Request Related To Rebuttal Testimony	$ 21.7
Public Utilities, Revenue Projection Of Requested Rider			103.4
CPUC Proceeding - Annual Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Return On Equity Threshold For Earnings Sharing For 2015 Through 2017				9.83%
Colorado Public Utilities Commission Staff (CPUC) | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			40.5
Public Utilities, Decrease To Requested Return On Equity		(12.8)
Public Utilities, Decrease To Cost Of Debt And Capital Structure		(12.8)
Public Utilities, Increase (Decrease) Related to Pipeline Adjustment		(11.2)
Public Utilities, Decrease In Request To Move To Historical Test Year		(10.5)
Public Utilities, Decrease To O&M Expenses		(3.5)
Public Utilities, Decrease Related To Other, Net		(4.4)
Public Utilities, Total Decrease Adjustment Related To Requested Rate Increase		(55.2)
Public Utilities, Recommended Rate Increase (Decrease), Amount		(14.7)
Colorado Public Utilities Commission Staff (CPUC) | Demand Side Management Cost Adjustment, 2015 through 2020 [Member]
Rate Matters [Abstract]
Public Utilities, Maximum Savings Goal (In GWh) | GWh					400
Public Utilities, Annual Earnings Limit					$ 84.3
Office of Consumer Counsel [Member] | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 40.5
Public Utilities, Decrease To Requested Return On Equity		(13.7)
Public Utilities, Decrease To Cost Of Debt And Capital Structure		(4.8)
Public Utilities, Increase (Decrease) Related to Pipeline Adjustment		4.8
Public Utilities, Decrease In Request To Move To Historical Test Year		(16.4)
Public Utilities, Decrease To O&M Expenses		(2.7)
Public Utilities, Decrease Related To Other, Net		(1.9)
Public Utilities, Total Decrease Adjustment Related To Requested Rate Increase		(34.7)
Public Utilities, Recommended Rate Increase (Decrease), Amount		$ 5.8