Rate Matters (Details) (USD $)	0 Months Ended	1 Months Ended														9 Months Ended	1 Months Ended	3 Months Ended			1 Months Ended
	Aug. 30, 2013 Production Formula Rate ROE Complaint [Member]	Apr. 30, 2013 Colorado Public Utilities Commission [Member] 2013 Gas Rate Case [Member] Counterparty	Dec. 31, 2012 Colorado Public Utilities Commission [Member] 2013 Gas Rate Case [Member]	Apr. 30, 2013 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2013 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2013 [Member]	Apr. 30, 2013 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2014 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2014 [Member]	Apr. 30, 2013 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2015 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Gas Rates 2015 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Pipeline System Integrity Adjustment 2014 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Gas Rate Case 2013, Pipeline System Integrity Adjustment 2015 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Steam Rate Case 2013 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Steam Rate Case 2013, Steam Rates 2013 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Steam Rate Case 2013, Steam Rates 2014 [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Steam Rate Case 2013, Steam Rates 2015 [Member]	Sep. 30, 2013 Colorado Public Utilities Commission [Member] Annual Electric Earnings Test [Member]	Mar. 31, 2012 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member]	Sep. 30, 2013 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member]	Sep. 30, 2012 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member]	Dec. 31, 2012 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member]	Mar. 31, 2012 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member] Shareholders [Member]	May 31, 2011 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member] Shareholders [Member]	Mar. 31, 2012 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member] Customers [Member]	May 31, 2011 Colorado Public Utilities Commission [Member] Renewable Energy Credit Sharing [Member] Customers [Member]	Jul. 31, 2013 Colorado Public Utilities Commission [Member] Electric Commodity Adjustment / RESA Adjustment [Member]	Apr. 30, 2013 Office of Consumer Counsel [Member] 2013 Gas Rate Case [Member]	Oct. 31, 2013 Subsequent Event [Member] Colorado Public Utilities Commission [Member] Steam Rate Case 2013, Steam Rates 2014 [Member]	Oct. 31, 2013 Subsequent Event [Member] Administrative Law Judge [Member] 2013 Gas Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 48,500,000		$ 9,900,000		$ 12,100,000	$ 26,800,000	$ 24,700,000		$ 1,600,000	$ 900,000	$ 2,300,000
Public Utilities, Requested Return on Equity, Percentage		10.30%	10.50%									10.50%
Public Utilities, Requested Rate Base, Amount			1,300,000,000									21,000,000
Public Utilities, Requested Equity Capital Structure, Percentage			56.00%									56.00%
Number of days subject to refund																											60
Public Utilities, Approved Rate Increase (Decrease), Amount																											1,200,000
Public Utilities, Number of intervenors filing testimony		4
Public Utilities, Incremental Base Revenue increase (decrease) recommended by third party		(1,100,000)																								15,400,000		15,000,000
Public Utilities, Return on equity recommended by third parties		9.00%																								9.00%		9.72%
Public Utilities, Equity capital structure recommended by third parties		52.00%																								51.03%		56.00%
Public Utilities, Adjustment requested by third parties related to return on equity and capital structure adjustments		(20,800,000)																								(20,000,000)		(7,700,000)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to use of 13 month average from year end rate base		(5,700,000)																								(3,200,000)		(3,300,000)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to removal of pension asset		(5,900,000)																								0		0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to incentive compensation		(3,500,000)																								(200,000)		(200,000)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to challenge of known and measurable		0																								(9,000,000)		0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to elimination of depreciation annualization		0																								(1,800,000)		0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to revenue adjustments		(4,100,000)																								(1,400,000)		(1,400,000)
Public Utilities, Tax impact of adjustments to requested rate increase (decrease) requested by third parties		1,500,000																								4,700,000		(200,000)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to other costs		(4,200,000)																								3,100,000		(1,200,000)
Public Utilities, Rate increase (decrease) recommended by third parties		(15,300,000)																								1,200,000		29,200,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				44,800,000		9,000,000		10,900,000
Public Utilities, Return on equity used in weather normalized earnings test																10.00%
Public Utilities, Refund to customers due to weather normalized earnings test																8,200,000
Public Utilities, Initial percentage of margin associated with stand alone REC transactions																						20.00%		80.00%
Public Utilities, Ultimate percentage of margin associated with stand alone REC transactions																						10.00%		90.00%
Public Utilities, Margin threshold determining percentage of margin sharing																	20,000,000
Public Utilities, Percentage of margin on hybrid REC approved for first 20 million of margins																					20.00%		80.00%
Public Utilities, Percentage of margin on hybrid REC approved for margins in excess of 20 million																					10.00%		90.00%
Public Utilities, Customers share of margins credited against RESA regulatory asset balance																		6,100,000	6,200,000
Public Utilities, Cumulative credit against RESA regulatory asset balance																		99,400,000		82,800,000
Return on equity for third parties, lower bound	10.10%
Return on equity for third parties, upper bound	10.40%
Proposed return on equity recommended by third parties	9.04%
Potential prospective annual revenue increase (decrease)	(2,000,000)
Proposed transfer between ECA and RESA deferred accounts																									26,200,000
Proposed Amortization Period For Recovery Of Deferred Costs																									12
Interest Expense, Other																									2,600,000
Public Utilities, Adjustment for historic test year		(1,600,000)																								(1,600,000)		(1,600,000)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to pension expense net of corrections		(1,600,000)																								0		0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to removal of PSIA from base rates		(14,200,000)																								(14,200,000)		0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to neutralization of PSIA - base rate transfer		14,200,000																								14,200,000		(14,200,000)
Public Utilities, Revenue deficiency based on a forecast test year				$ 30,600,000