Rate Matters (Details) (USD $)	1 Months Ended				6 Months Ended		1 Months Ended			3 Months Ended	12 Months Ended			12 Months Ended				1 Months Ended
	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Electric Rate Case 2014	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Electric Rate Case 2014, Electric Rates 2015	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Electric Rate Case 2014, Electric Rates 2016	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Electric Rate Case 2014, Electric Rates 2017	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Manufacturer's Sales Tax Refund	Jun. 30, 2014 Colorado Public Utilities Commission (CPUC) Annual Electric Earnings Test	Apr. 30, 2014 Colorado Public Utilities Commission (CPUC) 2013 Electric Earnings Test	Apr. 30, 2013 Colorado Public Utilities Commission (CPUC) 2012 PSIA Report	May 31, 2014 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Jun. 30, 2013 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Dec. 31, 2012 Colorado Public Utilities Commission (CPUC) Renewable Energy Credit Sharing	Jun. 30, 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	Aug. 31, 2013 Federal Energy Regulatory Commission (FERC) Production Formula Rate ROE Complaint	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	Jul. 31, 2014 Subsequent Event Colorado Public Utilities Commission (CPUC) 2012 PSIA Report
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 137,700,000																				$ 2,000,000
Public Utilities, Requested Rate Increase (Decrease), Percentage	4.89%
Public Utilities, Requested increase (decrease) to rider revenue		95,000,000	40,000,000	36,000,000				43,500,000
Public Utilities, Requested Return on Equity, Percentage	10.35%																			10.25%
Public Utilities, Requested Rate Base, Amount	6,390,000,000
Public Utilities, Requested Equity Capital Structure, Percentage	56.00%
Public Utilities, Rider costs to be recovered through base rates, amount	19,900,000
Public Utilities, Maximum annual amount of property taxes not subject to deferral					76,700,000
Public Utilities, Reduction in current year property tax deferral					10,000,000
Public Utilities, Return on equity threshold triggering earnings sharing						10.00%
Public Utilities, Proposed refund to customers due to annual earnings test							45,700,000
Public Utilities, Approved increase (decrease) to rider revenue																						43,400,000
Public Utilities, Approved disallowance																						100,000
Public Utilities, Ultimate margin sharing 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										6,500,000
Public Utilities, Cumulative credit against RESA regulatory asset balance												104,600,000	104,500,000
Public Utilities, Margin threshold proposed to be removed from sharing mechanism on hybrid REC trades									$ 20,000,000
Public Utilities, Return on equity charged to third parties, lower bound																		10.10%
Public Utilities, Return on equity charged to third parties, upper bound																		10.40%
Public Utilities, Return on equity requested by third parties																		9.04%		9.15%
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%