Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury And Air Toxics Standards (MATS) Rule [Member] MW	Mar. 31, 2010 Regional Haze Rules [Member]	Sep. 30, 2012 Capital Commitments [Member] Regional Haze Rules [Member]
Environmental Requirements [Abstract]
Generating capacity (in MW)	25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)	3Y
Number of years before affected facilities must demonstrate compliance, high end of range (in years)	4Y
Liability for estimated cost to comply with regulation			$ 334.2
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