Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified		12 Months Ended	1 Months Ended
	Dec. 31, 2013 Federal Clean Water Act Regulation	Dec. 31, 2013 EPA GHG Regulation	Apr. 30, 2012 Electric Generating Unit Mercury And Air Toxics Standards Rule MW	Dec. 31, 2010 Regional Haze Rules Group Boiler Kiln	Dec. 31, 2013 Capital Commitments Regional Haze Rules	Dec. 31, 2013 Other MGP Sites Site	Dec. 31, 2012 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						$ 1.2	$ 0.4
Environmental Requirements [Abstract]
Number of potential regulatory options under the proposed Effluent Limitations Guidelines rule	4
Number of issues granted review by the U.S. Supreme Court		1
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
Liability for estimated cost to comply with regulation					$ 359.5
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