Rate Matters (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended														3 Months Ended	1 Months Ended		3 Months Ended	12 Months Ended			12 Months Ended				1 Months Ended	3 Months Ended	1 Months Ended
	Dec. 31, 2013 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2013	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2013	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2014	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2014	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2015	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013, Gas Rates 2015	Feb. 28, 2014 Colorado Public Utilities Commission (CPUC) Steam Rate Case 2013	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Steam Rate Case 2013	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Steam Rate Case 2013, Steam Rates 2013	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Steam Rate Case 2013, Steam Rates 2014	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Steam Rate Case 2013, Steam Rates 2015	Mar. 31, 2014 Colorado Public Utilities Commission (CPUC) Annual Electric Earnings Test	Feb. 28, 2014 Colorado Public Utilities Commission (CPUC) 2012 PSIA Report	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) 2012 PSIA Report	Mar. 31, 2013 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Mar. 31, 2014 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Dec. 31, 2013 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing Shareholders	Dec. 31, 2011 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing Shareholders	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing Customers	Dec. 31, 2011 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing Customers	Jul. 31, 2013 Colorado Public Utilities Commission (CPUC) Electric Commodity Adjustment / RESA Adjustment	Mar. 31, 2014 Colorado Public Utilities Commission (CPUC) Electric Commodity Adjustment / RESA Adjustment	Mar. 31, 2014 Federal Energy Regulatory Commission (FERC) Transmission Formula Rate Case	Jun. 30, 2012 Federal Energy Regulatory Commission (FERC) Transmission Formula Rate Case	Apr. 30, 2012 Federal Energy Regulatory Commission (FERC) Transmission Formula Rate Case	Apr. 30, 2014 Subsequent Event Colorado Public Utilities Commission (CPUC) Gas Rate Case 2013	Apr. 30, 2014 Subsequent Event Colorado Public Utilities Commission (CPUC) Annual Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 48.5		$ 9.9		$ 12.1			$ 1.6	$ 0.9	$ 2.3																$ 2.0
Public Utilities, Requested Return on Equity, Percentage		10.30%	10.50%								10.50%																		10.25%
Public Utilities, Requested Rate Base, Amount			1,300								21
Public Utilities, Requested Equity Capital Structure, Percentage			56.00%								56.00%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				44.8		9.0		10.9
Public Utilities, Approved Rate Increase (Decrease), Amount	15.8									2.3
Public Utilities, Approved Return on Equity, Percentage	9.72%
Public Utilities, Approved Equity Capital Structure, Percentage	56.00%
Public Utilities, Adjustment for historic test year	(5.4)
Public Utilities, Adjustment to requested rate increase (decrease) approved by third parties related to return on equity and capital structure adjustments	(8.3)
Public Utilities, Adjustment to requested rate increase (decrease) approved by third parties related to revenue adjustments	(1.4)
Public Utilities, Adjustment to requested rate increase (decrease) approved by third parties related to other costs	(0.1)
Public Utilities, Rate increase (decrease) approved by third parties excluding PSIA adjustment	29.6
Public Utilities, Adjustment to requested rate increase (decrease) approved by third parties related to neutralization of PSIA - base rate transfer	(13.8)
Interim Rate Refund, Amount																															6.6
Public Utilities, Return on equity used in weather normalized earnings test															10.00%
Public Utilities, Refund to customers due to annual earnings test																																45.7
Public Utilities, Requested increase (decrease) to rider revenue																	43.5
Public Utilities, proposed increase (decrease) to rider revenue pursuant to settlement agreement																43.4
Public Utilities, total proposed disallowance pursuant to settlement agreement																0.1
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
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																		4.0
Public Utilities, Cumulative credit against RESA regulatory asset balance																				104.6	104.5
Proposed transfer between ECA and RESA deferred accounts																										26.2
Proposed Amortization Period For Recovery Of Deferred Costs (in months)																										12 months
Approved transfer between ECA and RESA deferred accounts																											26.2
Approved Amortization Period For Recovery Of Deferred Costs (in months)																											12 months
Approved increase (decrease) in interest rate																											(7.40%)
Approved increase (decrease) in interest income resulting from a change in interest rates, Amount																											(4.3)
Public Utilities, Return on equity requested by third parties																													9.15%
Public Utilities, Rate increase (decrease) requested by third parties																													$ (1.8)
Public Utilities, Return on equity requested by third parties, July 2012 through November 2012																												8.91%
Public Utilities, Return on equity requested by third parties, after November 2012																												8.70%