Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014 Greenhouse Gas New Source Performance Standard for Modified and Reconstructed Power Plants	Apr. 30, 2014 Cross-State Air Pollution Rule	Dec. 31, 2010 PSCo Regional Haze Rules Boiler Group Kiln	Jun. 30, 2014 Capital Commitments Federal Clean Water Act Section 316 (b)	Jun. 30, 2014 Capital Commitments PSCo Regional Haze Rules	Jun. 30, 2014 Capital Commitments NSP-Minnesota Regional Haze Rules
Environmental Requirements [Abstract]
Number of issues on which the D.C. Circuit overturned the CSAPR		2
Liability for estimated cost to comply with regulation				$ 48.0	$ 359.7	$ 50.0
Percentage of a comparable new plant's capital cost which would have to be exceeded to consider a project as a reconstruction under the proposed GHG NSPS for Modified and Reconstructed Power Plants (in hundredths)	50.00%
Number of environmental groups who petitioned the U.S. Department of the Interior			2
Number of coal-fired boilers in Colorado			12
Number of coal-fired cement kilns in Colorado			1
Estimated amount spent on projects to reduce NOx emissions on Sherco Units 1 and 2						$ 44.4