Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Apr. 30, 2012 MW	Jun. 30, 2014	Dec. 31, 2010 Kiln Group Boiler	Dec. 31, 2014 Parts-per-million	Dec. 31, 2013
Greenhouse Gas New Source Performance Standard for Modified and Reconstructed Power Plants
Environmental Requirements [Abstract]
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%
Electric Generating Unit Mercury And Air Toxics Standards Rule
Environmental Requirements [Abstract]
Generating capacity (in MW)	25
Number of years before affected facilities must demonstrate compliance, low end of range	3 years
Number of years before affected facilities must demonstrate compliance, high end of range	4 years
Regional Haze Rules
Environmental Requirements [Abstract]
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
National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of hours the NAAQS for Ozone is based upon (in hours)				8
Current level of air quality concentrations (in parts per million)				0.075
Proposed level of air quality concentrations (in parts per million)				0.070
Lowest proposed level of air quality concentrations that the EPA is taking comment on (in parts per million)				0.060
Capital Commitments | Regional Haze Rules
Environmental Requirements [Abstract]
Amount spent on installation of emission controls				272.6
Liability for estimated cost to comply with regulation				84.6
Minimum | National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Proposed level of air quality concentrations (in parts per million)				0.065
Maximum [Member] | National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Proposed level of air quality concentrations (in parts per million)				0.070
Other MGP Sites
Site Contingency [Line Items]
Number of identified MGP sites under current investigation and/or remediation				2
Liability for estimated cost of remediating sites				1.8	$ 1.2