Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	1 Months Ended	9 Months Ended		18 Months Ended				9 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2012 plaintiff MW	Dec. 31, 2005 Elk River Windfarm	Sep. 30, 2012 Elk River Windfarm MW	Sep. 30, 2012 Southern Star Central Pipeline, Inc. Electric	Sep. 30, 2012 Southern Star Central Pipeline, Inc. Electric Dth	Sep. 30, 2012 Iatan	Sep. 30, 2012 Physical gas and transportation contracts	Sep. 30, 2012 Coal and coal transportation contracts	Sep. 30, 2012 Purchased power Plum Point Energy Station MW	Dec. 31, 2008 Purchased power Cloud County Windfarm	Sep. 30, 2012 Purchased power Cloud County Windfarm MW	Sep. 30, 2012 Leases Electric train	Sep. 30, 2012 Leases Gas office	Sep. 30, 2012 Leases Plum Point Energy Station railcar
Commitments and Contingencies
Amount of total power capacity of generating facility (in megawatts)	18
Proportionate ownership interest (as a percent)						12.00%
October 1, 2012 through December 31, 2012							$ 9.4	$ 10.8
January 1, 2013 through December 31, 2014							45.9	39.9
January 1, 2015 through December 31, 2016							24.5	22.8
January 1, 2017 and beyond							17.8	34.3
Term of long-term contract agreement		20 years							30 years
Agreement to purchase firm gas storage capacity (in Dths)					1,000,000
Term of long-term contract agreement					5 years
Amount of annual storage capacity reservation				1.1
Amount of long-term contract obligation									$ 309.0
Amount of energy generated (in megawatts)			150						665		105
Amount of energy capacity under option to purchase (in megawatts)									50
Period of the average cost that is used as a basis for determining the maximum annual payments										20 years
Number of unit trains under short-term operating leases												2
Number of office facilities under short-term operating leases													1
Number of railcars under capital leases														108
Number of plaintiffs	3