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 Issue	Dec. 31, 2010 PSCo Regional Haze Rules Kiln Boiler Group	Sep. 30, 2014 SPS Cross-State Air Pollution Rule MW	Sep. 30, 2014 Capital Commitments Federal Clean Water Act Section 316(b)	Sep. 30, 2014 Capital Commitments PSCo Regional Haze Rules	Sep. 30, 2014 Capital Commitments NSP-Minnesota Federal Clean Water Act Section 316(b) Plant	Sep. 30, 2014 Capital Commitments NSP-Minnesota Regional Haze Rules	Sep. 30, 2014 Capital Commitments SPS Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation					$ 46.0	$ 360.5		$ 50.0	$ 7.0
Minimum number of plants which could be required to make improvements to reduce entrainment							4
Liability for estimated cost to comply with entrainment regulation							180
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 issues on which the D.C. Circuit overturned the CSAPR		2
Generating capacity (in MW)				700
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								$ 45.8